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Geosphere
doi: 10.1130/GES00679.12011;7;1392-1418Geosphere
SantacruzBernal, Elizard Gonzlez Becuar, Floyd Gray, Margarita Lpez Martnez and Rufino LozanoCarlos M. Gonzlez-Len, Luigi Solari, Jess Sol, Mihai N. Ducea, Timothy F. Lawton, Juan Pabloin north-central Sonora, MexicoStratigraphy, geochronology, and geochemistry of the Laramide magmatic arc
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ABSTRACT
The Laramide magmatic arc in theArizpe-Mazocahui quadrangle of north-central Sonora, Mexico, is composed ofvolcanic rocks assigned to the TarahumaraFormation and several granitic plutons thatintrude it. The arc was built over juxta-posed crustal basements of the Caborca andMazatzal provinces. A basal conglomerateof the >4-km-thick Tarahumara Forma-tion overlies deformed Proterozoic igneous
rocks and Neoproterozoic to Early Creta-ceous strata, thus constraining the age of acontractional tectonic event that occurredbetween Cenomanian and early Campaniantime. The lower part of the TarahumaraFormation is composed of rhyolitic ignim-brite and ash-fall tuffs, andesite flows, andinterbedded volcaniclastic strata, and itsupper part consists of rhyolitic to daciticignimbrites, ash-fall tuffs, and volcaniclasticrocks. The Tarahumara Formation showsmarked lateral facies change within the studyarea, and further to the north it grades intothe coeval fluvial and lacustrine Cabullona
Group. The age of the Tarahumara Forma-tion is between ca. 79 and 59 Ma; the mon-zonitic to granitic plutons have ages of ca.7150 Ma. The informally named El Babizoand Hupac granites, La Aurora and LaAlamedita tonalities, and the Puerta del Solgranodiorite compose the El Jaralito batho-lith in the southern part of the area.
Major and trace element composition of theLaramide igneous rocks shows calc-alkaline
differentiation trends typical of continentalmagmatic arcs, and the isotope geochemistryindicates strong contribution from a maturecontinental crust. Initial 87Sr/86Sr values rangefrom 0.70589 to 0.71369, and Nd valuesrange from 6.2 to 13.6, except for the ElGueriguito quartz monzonite value, 0.5. TheNd, Sr, and Pb isotopic values of the studiedLaramide rocks permit comparison with thepreviously defined Laramide isotopic provincesof Sonora and Arizona. The El Gueriguitopluton and Bella Esperanza granodiorite in
the northeastern part of the study area alongwith plutons and mineralization of neighbor-ing northern Sonora have isotopic values thatcorrespond with those of the southeasternArizona province formed over the Mazat-zal basement (Lang and Titley, 1998; Bouseet al., 1999). Isotopic values of the otherLaramide rocks throughout the study areaare similar to values of provinces A and B ofSonora (Housh and McDowell, 2005) and tothose of the Laramide Pb boundary zone ofwestern Arizona, while the Rancho Vaqueraand La Cubana plutons in the northernmostpart of the area have the isotopic composi-
tion of the Proterozoic Mojave province ofthe southwestern United States. These datapermit us to infer that a covered crustalboundary, between the Caborca block witha basement of the Mojave or boundary zoneand the Mazatzal province, crosses throughthe northeastern part of the area. The bound-ary may be placed between outcrops of theEl Gueriguito and Rancho Vaquera plutons,probably following a reactivated Cretaceous
thrust fault located north of the hypothesizedMojave-Sonora megashear, proposed to crossthrough the central part of the area.
INTRODUCTION
The Late Cretaceousearly Cenozoic Laramide
magmatic arc of Sonora, Mexico (Damon et al.,
1983a, 1983b; McDowell et al., 2001), is com-
posed of a thick and geographically extensive
volcanic succession and nearly contempora-
neous, mostly granitic, plutons that are part of
the Sonoran batholith (Damon et al., 1983a)(Fig. 1). These rocks formed in a continental,
Andean-type magmatic arc related to subduc-
tion of the Farallon plate beneath North America
during Late Cretaceous and early Cenozoic time
(Coney and Reynolds, 1977; Dickinson, 1981,
1989; Damon et al., 1983a; Engebretsen et al.,
1985; Stock and Molnar, 1988).
Based mostly on their own data set of pre-
dominantly K-Ar ages of plutonic rocks,
Damon et al. (1983a) proposed that the
Laramide arc in Sonora spanned 9040 Ma, a
range mostly accepted by others and broadly
supported by subsequent geochronologic stud-
ies (Anderson and Silver, 1977; Anderson
et al., 1980; Valencia-Moreno et al., 2001,
2003, 2006; McDowell et al., 2001; Housh and
McDowell, 2005; Prez-Segura et al., 2009;
Roldn-Quintana et al., 2009). In contrast, the
study of the arcs cogenetic volcanic succession
has long been neglected despite extensive expo-
sures, although some ages were published from
northern and central Sonora (Supplemental
Table 11). The name Mesa formation (Valentine,
For permission to copy, contact editing@geosociety.org
2011 Geological Society of America
Geosphere; December 2011; v. 7; no. 6; p. 13921418; doi: 10.1130/GES00679.1; 14 figures; 2 tables; 5 supplemental tables.
Stratigraphy, geochronology, and geochemistry of the
Laramide magmatic arc in north-central Sonora, Mexico
Carlos M. Gonzlez-Len1, Luigi Solari2, Jess Sol3, Mihai N. Ducea4, Timothy F. Lawton5, Juan Pablo Bernal2,Elizard Gonzlez Becuar6, Floyd Gray7, Margarita Lpez Martnez8, and Rufino Lozano Santacruz31Instituto de Geologa, Estacin Regional del Noroeste, Universidad Nacional Autnoma de Mxico, Apartado Postal 1039,
Hermosillo, Sonora, Mxico 830002Centro de Geociencias, Universidad Nacional Autnoma de Mxico, Campus Juriquilla, Quertaro, QRO, Mxico 760013Instituto de Geologa, Ciudad Universitaria, Universidad Nacional Autnoma de Mxico, Mxico, Distrito Federal 045104Geosciences Department, University of Arizona, Tucson, Arizona 85721, USA5Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA6Departamento de Geologa, Universidad de Sonora, Hermosillo, Sonora, Mxico 830007U.S. Geological Survey, GeologyEcosystem Analysis, 520 North Park Avenue, Suite 355, Tucson, Arizona 85719, USA8Departamento de Geologa, Centro de Investigacin Cientfica y de Educacin Superior de Ensenada (CICESE),
Km 107 Carretera Ensenada-Tijuana No. 3918, 22860 Ensenada, Baja California, Mxico
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1393
1936) was first assigned to outcrops of this
succession near the town of Cananea (Fig. 1),
where it consists of a basal conglomerate and
overlying bedded tuffs, andesite flows, agglom-
erates, and subordinate rhyolite flows >1600 m
thick. Wilson and Rocha (1949) applied the
term Tarahumara Formation to the Laramide
volcanic succession and it became a commonly
used name for extensive volcanic and volcani-
clastic outcrops in Sonora; they described it as
consisting of highly altered, aphanitic interme-
diate volcanic rocks unconformably overlying
Triassic strata in its type section near the town
of Tecoripa, in central Sonora (Fig. 1). From
that locality, McDowell et al. (2001) reported
an age range of 9070 Ma for an ~2.5-km-thick
rhyolitic, dacitic, andesitic, and volcaniclastic
succession.
Other studies of partial sections of these
rocks in northern Sonora assigned local infor-
mal names, including Alcaparros formation and
Arroyo Alcaparros andesitic rocks (Gonzlez-
Len et al., 2000) and El Tuli formation (Rodr-
guez-Castaeda, 1994). To avoid confusion and
to provide uniformity in the terminology, we
follow other authors in applying the formal,
more commonly used name Tarahumara Forma-
tion to this succession throughout its region o
exposure (McDowell et al., 2001).
In this paper we provide new constraints on
the stratigraphy and geochronology of the Tara
humara volcanic succession and the petrology
and geochronology of associated plutonic bod
ies that represent the Laramide magmatic arc
within the Arizpe-Mazocahui area in northcentral Sonora (Fig. 1). The area is located
in a position that is transitional between the
classic localities of the Tarahumara Forma
tion to the south, and the Mesa formation to
the north. It includes the 15 20, 1:50,000scale topographic quadrangles named Arizpe
Nacozari (part), Santa Ana (part), Banmichi
Agua Caliente, Aconchi, Cumpas, Bavicora
and El Rodeo (part) (Fig. 2), published by
Instituto Nacional de Estadstica, Geografa e
Informtica of the Mexican government. Ou
work is based on geologic mapping conducted
at that scale and illustrated here by a general
ized geologic map (Fig. 2). This cartographyimproves and in many instances correct
some of the previous geologic maps of the
Arizpe, Banmichi, Bavicora, and Santa
Ana quadrangles (Gonzlez Gallegos et al.
2003; Quevedo Len and Ramrez Lpez
2008; Servicios Geolgicos y Cartogrfico
del Noroeste, S.A. de C.V., 1999; Corra
Gastelum and Hernndez Morales, 2008
respectively). Six measured stratigraphic col
umns and seven accompanying structural sec
tions are included to illustrate the stratigraphic
and tectonic relationships of the Laramide
rocks with older and younger geologic unitsThe ages of the Laramide volcanic succession
and the plutonic bodies are constrained by 28
U-Pb dates, one 40Ar/39Ar date, and 9 K/A
dates, none of which have been reported pre
viously. Another new 40Ar/39Ar date reported
here helps to constrain the age of the younge
unit of the basement over which the Laramide
succession was deposited as well as the age of
the younger tectonic event that deformed tha
basement in the region. We also dated detrita
zircons from four sandstone units to constrain
their maximum deposition ages. Two of these
come from the Proterozoic basement and two
others are from sandstone beds within the
Laramide succession.
The study also incorporates 35 whole rock
geochemical analyses and 12 isotope analyse
of Sm/Nd, Rb/Sr, and Pb/Pb of the Laramide
plutonic and volcanic rocks, and 2 from the
Proterozoic plutons. All of the analyzed sam
ples have age and stratigraphic control. The
field and analytical data help to document the
stratigraphic, tectonic, and temporal frame
work of the Laramide magmatic arc for thi
part of the Cordillera, and the new cartography
11100
3100
11400
Puerto
Penasco
Guaymas
Ures
SantaAna
Agua Prieta
Sonoita
SONORA
A RIZONA
Gulf
of
Calif
ornia
11200
+
+
Nogales
A
B C
D
Cananea
Arizpe
Nacozari
MEXICO
Laramide volcanic rocks(Tarahumara Formation)
Laramide plutonic rocks(Sonoran batholith of Damon et al., 1983a)
Cabullona Group
A ierra Los Ajos
B a Lmina
C erro Prieto
D
S
L
C
El Crestn
Trace of Mojave-Sonora megashear
10900
U S A
SONORA
2700
N
C. Obregon
Navojoa
Sahuaripa
100 km
Mazatn
MAZATZALPROVINCE
CABORCABLOCK
Caborca
HERMOSILLO
Bacoachi
Moctezuma
Studyquadrangle
Figure 2
Ycora
3100
2800
Cucurpe
Tecoripa
Mazochui
Figure 1. Map outcrop distribution of plutonic and volcanic rocks of the Laramide magmaticarc in Sonora. The distribution of outcrops of the Late Cretaceous Cabullona Group, locali-ties mentioned in the text, and inset map for location of Figure 2 are shown. The boundarybetween the Mazatzal province and Caborca blocks is indicated by the trace of the hypo-thetical Mojave-Sonora megashear as presented by Anderson and Silver (2005, their fig. 4).
1Supplemental Table 1. Bibliographic compila-tion of geochronologic ages of Laramide volcanicand plutonic rocks of Sonora between isochronesof 70 and 50 Ma, as indicated in Figure 14. Histo-gram of plutonic ages in Figure 13 was drawn withthis information. If you are viewing the PDF of thispaper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S1 or the full-text article onwww.gsapubs.org to view Supplemental Table 1.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1395
and isotope data place constraints on possible
delimitations of the Caborca and Mazatzal
crustal blocks.
ANALYTICAL METHODS
U-Pb ages by laser ablationmulticollector
inductively coupled plasmamass spectrometry(LA-MC-ICP-MS) were determined at the Ari-
zona Laser ChronCenter of the Geosciences
Department of the University of Arizona (results
are reported in Supplemental Table 22) and at
Centro de Geociencias, Universidad Nacional
Autnoma de Mxico (UNAM) (Supplemental
Table 33) (procedures are described in Appen-
dices 1 and 2, respectively). K-Ar analyses on
biotites from igneous rocks were conducted at
the Instituto de Geologa, UNAM. Procedures
are described in Appendix 3 and results are in
Supplemental Table 44. The 40Ar/39Ar dating
was performed at the Geochronology Labora-
tory of the Departmento de Geologa, Centrode Investigacin Cientfica y de Educacin
Superior de Ensenada (CICESE), and proce-
dures were described in Gonzlez-Len et al.
(2010). A hornblende separate from sample
04ES-8 was analyzed by the resistance-furnace
incremental-heating age spectrum method at
the New Mexico Geochronology Research
Laboratory. (Details of the method and over-
all operation of the laboratory are provided at
http://www.ees.nmt.edu/Geol/labs/Argon_Lab
/Methods/Methods.) Geochemical analyses for
major and trace elements were done by X-ray
fluorescence and with a SIEMENS SRS 3000spectrometer in the Laboratorio Universitario
de Geoqumica Isotpica, UNAM, and by high
resolution ICP-MS at the Department of Geol-
ogy, University of WisconsinEau Claire, USA.
Radiogenic isotopic and select trace element
concentrations were determined at the Isotopic
Laboratory of the Geosciences Department of
the University of Arizona following procedures
reported in Appendix 4.
REGIONAL GEOLOGIC SETTING
The Proterozoic basement of Sonora wasrecognized as part of crustal southwestern
North America by Damon et al. (1962) on the
basis of geochronology data. Silver and Ander-
son (1974) noted that it could be divided into
a northern block with ages between 1.7 and
1.6 Ga and a southern block with ages from 1.8
to 1.7 Ga that were later assigned to the North
America (or Mazatzal) and Caborca terranes,
respectively (Campa and Coney, 1983) (Fig. 1).
However, the nature, age, and location of the
crustal boundary are debatable. On one side,
the Caborca block is interpreted as a piece of
crustal southwestern USA translated to its pres-
ent position by a major left-lateral fault assignedeither to the Jurassic Mojave-Sonora megashear
(Silver and Anderson, 1974; Anderson and Sil-
ver, 2005) (Fig. 1) or to the PermianTriassic
California-Coahuila transform fault (Dickinson
and Lawton, 2001). On the contrary, Poole et al.
(2005) argued that the Mojave basement and its
Neoproterozoic sedimentary cover wrap around
the Laurentian margin in the southwestern USA
to continue southeast into the Caborca block,
without major structural displacement. Simi-
larly, Arvizu et al. (2009) depicted the Mojave
and Mazatzal blocks in Sonora separated by a
Nd isotope Yavapai crustal province.The Pinal Schist, the basement of the Mazatzal
province, crops out in Sierra Los Ajos (Fig. 1)
and nearby areas of northern Sonora, where it
is dated as 1.69 Ga (Anderson and Silver, 2005)
and 1.64 Ga (U-Pb, zircon) (Page et al., 2010).
Proterozoic granites with ages near 1.4 Ga
intrude the Pinal Schist (Anderson and Silver,
2005), and the nearest outcrop of this granite
to the study area is in the town of Bacoachi
(Fig. 1; our own observations). Other granites
and gabbros that are assigned to the basement
of the Caborca block because of their isotopic
signatures and ages of ca. 1.7 Ga crop out in
near localities of Cerro Prieto (Anderson and
Silver, 2005), Rancho La Lmina (Amato et al.,
2009), and El Crestn (Valenzuela-Navarro
et al., 2005) (Fig. 1). Based on the occurrence
of Caborcan Proterozoic granites in Rancho
La Lmina, Amato et al. (2009) inferred that if
present, the trace of the hypothetical Mojave-
Sonora megashear might be located north of that
locality (Fig. 1).
Proterozoic, mostly clastic, strata assigned to
the Las Vboras and El Aguila Groups by Stewart
et al. (2002) crop out within a few kilometers to
the west of the study area. These units exceed a
combined thickness of 3.5 km and overlie the
igneous basement of the Caborca block. Super
jacent Paleozoic clastic and carbonate strat
(Stewart et al., 1997, 1999) are >4 km thick
This ProterozoicPaleozoic succession of the
Caborca block is lithologically different from
the CambrianPermian sedimentary successionthat unconformably overlies the basement of the
Mazatzal province and that correlates with and
resembles the Paleozoic formations of south
eastern Arizona (e.g., Hayes and Landis, 1965)
The nearby outcrops of this Paleozoic succes
sion occur in the town of Bacoachi (Stewart and
Poole, 2002) and in the vicinity of Cananea
where the successtion is 1.2 km thick (Gonzlez
Len, 1986; Page et al., 2010).
Mesozoic rocks of the neighboring region to
the north in Sonora include the Lower Jurassic
Basomar Formation (Legget et al., 2007) and
the Middle Jurassic Rancho San Martn (Mauel
2008), Elenita (Valentine, 1936; Wodzicki1994), and Lily (McAnulty, 1970, Gonzlez
Len et al., 2009) Formations, and a few dated
Middle Jurassic granites (Anderson et al.
2005). These formations make up a clastic and
volcanic succession that was deposited within
a continental magmatic arc that developed in
northern Sonora (Riggs et al., 1993, Anderson
et al., 2005). Marine strata of the Upper Juras
sic Cucurpe Formation (Villaseor et al., 2005
Mauel et al., 2011) overlie the Basomari and
Rancho San Martn successions near the town
of Cucurpe. The combined thickness of the
Jurassic formations is at least 3.5 km, and theyare unconformably overlain by strata of the Bis
bee Group, which in this region is documented
to be Early Cretaceous in age and ~3 km thick
(Peryam, 2006; Peryam et al., 2011).
A major Middle to Late Jurassic tectonic
event of extensional deformation formed a rif
basin, termed the Altar-Cucurpe Basin, where
the Cucurpe Formation was deposited (Peryam
2006; Mauel, 2008; Mauel et al., 2011). Alterna
tively, formation of this basin has been assigned
to development of the left-lateral displacemen
of the Mojave-Sonora megashear (Anderson
and Nourse, 2005; Anderson and Silver, 2005)
A younger, contractional tectonic event affected
the Bisbee Group and older strata during early
Late Cretaceous time (Rangin, 1986). The age
of the clastic continental succession of the
Cocspera Formation (Gilmont, 1978) depos
ited during this tectonic event (Gonzlez-Len
et al., 2000) is constrained by a 40Ar/39Ar age o
93.3 0.7 Ma (Fig. 3) obtained from an inter
bedded andesite in outcrops ~2 km northwest o
the study area (Lawton et al., 2009). Following
a period of uplift and erosion, deposition of the
Laramide volcanic arc succession commenced.
2Supplemental Table 2. U-Pb data of detrital zirconfrom sandstone and tuffaceous sandstone samples,and Laramide volcanic and plutonic rocks of thestudy area. Analyses were done at the Arizona LaserChron Center of the University of Arizona by LA-ICP-MS method. If you are viewing the PDF of thispaper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S2 or the full-text article on
www.gsapubs.org to view Supplemental Table 2.3Supplemental Table 3. U-Pb data for volcanicand plutonic rocks of the study area done at the Cen-tro de Geociencias, Universidad Nacional Autnomade Mxico by the LA-ICP-MS method. If you areviewing the PDF of this paper or reading it offline,please visit http://dx.doi.org/10.1130/GES00679.S3or the full-text article on www.gsapubs.org to viewSupplemental Table 3.
4Supplemental Table 4. K-Ar age data of datedsamples of the study area. If you are viewing the PDFof this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S4 or the full-textarticle on www.gsapubs.org to view SupplementalTable 4.
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Gonzlez-Len et al.
1396 Geosphere, December 2011
Regionally, the Laramide magmatic arc ofSonora has been documented mostly through a
large geochronology database and geochemi-
cal and isotopic studies of the plutonic rocks
(Anderson and Silver, 1977; Anderson et al.,
1980; Damon et al., 1983a, 1983b; Wodzicki,
1994; McDowell et al., 2001; Valencia-Moreno
et al., 2001, 2003, 2006; Valencia et al., 2005;
Roldn-Quintana et al., 2009; Prez-Segura
et al., 2009; Prez-Segura and Gonzlez-Partida,
2010), but far fewer data are available for the
volcanic rocks (McDowell et al., 2001). Some
ages are published for the Laramide volcanic
rocks north of the study area, in the Cananea-
Nacozari region (Wodzicki, 1994; Valencia et al.,
2005; Cox et al., 2006; Page et al., 2010), and 640Ar/39Ar ages between 73 and 66 Ma are listed
in Supplemental Table 1 (see footnote 1). Based
on Sr and Nd isotopic variations and trace ele-
ment compositions, Laramide granites of north-
ern and central Sonora with ages between 57
and 68 Ma were assigned to northern and central
granites by Valencia-Moreno et al. (2001). On
the basis of Sr, Nd, and Pb isotope geochemistry,
other granites with ages between 59 and 67 Ma
were considered as belonging to provinces A
and B by Housh and McDowell (2005), whoalso included isotopic characteristics of Oligo-
cene and Miocene volcanic rocks to define their
provinces. Geographically, the Laramide gran-
ites of provinces A and B roughly occupy the
same region as the northern and central granites
of Valencia-Moreno et al. (2001), while rocks of
the study area are within the geographic domains
of the northern granites and province A.
Younger regional events consist of Late
OligoceneMiocene magmatism, core complex
formation, and basin-fill continental sedimen-
tation of the Bucarit Formation that occurred
associated with Basin and Range extensional
deformation. Basin and Range deformation
structurally dismembered the Laramide arc and
older basement (Nourse et al., 1994; Wong
and Gans, 2008; Gonzlez-Len et al., 2010;
Wong et al., 2010).
GEOLOGY OF THE STUDY AREA ANDPREVIOUS STUDIES
Within the study area the older rocks are El
Jacaln diorite and the Santa Margarita gran-
ite (Rodrguez-Castaeda, 1994) that crop
out in the Santa Ana quadrangle (Fig. 2). The
diorite and granite have U-Pb (zircon) ages of
1702 Ma and 1104 Ma, respectively (Anderson
and Silver, 2005). A gneissic zone developed
in the El Jacaln diorite is spatially associated
with the El Jucaral normal fault of postEarly
Cretaceous age (Fig. 2) and is not a separate
Proterozoic lithostratigraphic unit, as previouslyinterpreted (the El Alamito unit of Rodrguez-
Castaeda, 1994). A thick Proterozoic succes-
sion (>1 km thick) of mostly sandstone that
locally overlies the igneous basement crops out
in the Banmichi and Santa Ana quadrangles. It
was named the Los Changos orthoquarzite, of
supposed Paleozoic age by Rodrguez-Casta-
eda (1994) and later reassigned to the Protero-
zoic Las Vboras Group by Stewart et al. (2002)
(Fig. 2). Detrital zircons from a sample of the
middle part of this succession in the Banmichi
quadrangle yielded U-Pb peak ages at 1.2, 1.47,
1.67, and 1.87 Ga (Figs. 2 and 4A) (Plascencia
Corrales, 2008), similar to a sample collectedfrom this succession that unconformably over-
lies the El Jacaln granodiorite in the Santa Ana
quadrangle (Figs. 2 and 4B).
Isolated outcrops of schist, recrystallized
limestone, and quartz-rich sandstone that occur
as roof pendants in the Laramide plutons in the
Sierra El Jaralito may be Proterozoic and/or
Paleozoic in age (Peabody, 1979, in Roldn-
Quintana, 1989; Mead et al., 1988) (Fig. 2).
Cambrian to Permian formations in the Naco-
zari and Agua Caliente quadrangles (Fig. 2)
are typical of Paleozoic strata that overlie the
Mazatzal basement in northeastern Sonora.The Mesozoic rocks in the study area include
incomplete sedimentary successions of the Lily
(Gonzlez-Len et al., 2009) and Cucurpe For-
mations of Jurassic age, the Morita, Mural and
Cintura Formations of the Early Cretaceous
Bisbee Group, and the previously undated
Cocspera Formation (Fig. 2). Assignment of
a Jurassic age by Rodrguez-Castaeda (1994)
and Corral Gastlum and Hernndez Morales
(2008) to widespread outcrops of the Bisbee
Group and the Tarahumara Formation in the
Santa Ana quadrangle are herein corrected
on the basis of our geochronology. However,
most of the area is occupied by outcrops of the
volcanic and plutonic rocks of the Laramide
magmatic arc; volcanic rocks, sedimentary
strata, and a few rhyolitic domes of Oligo-
ceneMiocene age; and by younger alluvial
deposits (Fig. 2).
Previously published geochronologic infor-
mation of the Laramide volcanic rocks in the
study area include a U-Pb (zircon) age of 76 Ma
from the Santa Ana quadrangle (McDowell
et al., 2001), a 40Ar/39Ar (biotite) age of 58.67
0.17 Ma from the Arizpe quadrangle (Gonzlez-
93.3 0.7 Ma(MSWD = 1.7)
%
Radiogenic
1
K/Ca
1
0.1
0.01
0
Cumulative % Ar released39
k
Integrated age = 90.9 0.7
10 20 30 40 50 807060 10090
50
60
70
80
90
100
110
0
80
40
Apparent
age
(Ma)
1125
F1080
D
1105
E 1155
G
1200
H
1300
I
1050
C
A
1
700
J
L# 55854: 04ES-8, Hornblende
965
B
120
Figure 3. 40Ar/39Ar age spectrum of hornblende separate of sample 04ES-8 from an andesiteinterbedded with conglomerate of the Cocspera Formation. This sample was collected nearRancho San Antonio, ~2 km northwest of the study quadrangle (Universal Transverse Mer-cator locality 12R 562410E 3382580N). MSWDmean square of weighted deviates.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1397
Len et al., 2000), and a K-Ar (whole rock)
age of 40.6 1.1 Ma from the El Rodeo quad-
rangle (Damon et al., 1983a). Freshwater,
non-age-diagnostic fossils including plants
(Hernndez-Castillo and Cevallos-Ferriz, 1999),
algae (Beraldi-Campesi et al., 2004), diatoms
(Chacn-Baca et al., 2002), and other micro-
fossils (Beraldi-Campesi and Cevallos-Ferriz,
2005) were reported from local, interbedded
lacustrine strata of the Tarahumara Formation in
the Aconchi quadrangle.
Ages reported for the plutonic rocks include
a 40Ar/39Ar (biotite) age of 56.7 Ma for the
Rancho Vaquera pluton (pluton names derive
from our informal terminology described in the
following; Fig. 2) (Gonzlez-Len et al., 2000),
a K-Ar (biotite) age of 56.4 Ma (Mead et al.,
1988), and a 40Ar/39Ar (biotite) age of 57.3 Ma
(Zuiga Hernndez, 2010) for the Las Cabecitas
granodiorite (Fig. 2), and K-Ar ages reported by
Damon et al. (1983b) of 51 Ma (orthoclase) for
the San Felipe porphyry and 55.9 Ma (biotite)
for the Bella Esperanza granodiorite (Fig. 2).
The Bella Esperanza granodiorite was also
dated as 56.9 Ma (K-Ar, whole rock) by Housh
and McDowell (2005). Stocks of monzonite
and diorite in the Cumobabi Mine (Fig. 2
yielded K-Ar ages between 63 and 56 Ma on
biotite (Scherkenbach et al., 1985) and 51 Ma
(40Ar/39Ar; Zuiga Hernndez, 2010).
Mead et al. (1988) first reported 40Ar/39A
ages of 46.6 Ma (hornblende) and ca. 37.1 Ma
(biotite), and a K-Ar age of ca. 39.5 Ma (bio
tite) for a granodiorite pluton in the Sierra E
Jaralito. The El Jaralito and the Aconchi batho
liths are names assigned by Roldn-Quintana
(1991) to plutonic rocks that crop out in the
Sierra El Jaralito and the Sierra de Aconchi
respectively (Fig. 2). Roldn-Quintana (1991
A 11-22-07-1
C 11-21-07-1
B 3-25-09-4
D 12-5-08-3
1000
1400
1800
2200
0.1
0.2
0.3
0.4
0 2 4 6 8
data-point error ellipses are 2
500
1500
2500
0.0
0.2
0.4
0.6
0 4 8 12 16
data-point error ellipses are 2
0
5
10
15
20
25
0 500 1000 1500 2000 2500 3000
Relativeprobability
Number
Age, Ma
~1260 Ma
~1470 Ma
~1670 Ma
~1870 Ma
0
2
4
6
8
10
12
14
0 500 1000 1500 2000 2500
Relativeprobability
Number
Age, Ma
70
74
78
82
0.0106
0.0110
0.0114
0.0118
0.0122
0.0126
0.0130
0.064 0.068 0.072 0.076
0.088
0.092
ConcordiaAge = 76.39 0.67 Ma
(2, decay-constant errors included)
MSWD(of concordance) = 2.5,
Probability(of concordance) = 0.11
data-point error ellipses are 2
~1218 Ma
~1460 Ma
~1670 Ma
~1870 Ma
0
1
2
3
4
5
6
0 500 1000 1500 2000 2500 3000
Relativeprobability
Number
Age, Ma
1000
1400
1800
2200
2600
0.05
0.15
0.25
0.35
0.45
0.55
0.65
0 4 8 12 16
data-point error ellipses are 2
1500
2500
0.0
0.2
0.4
0.6
0.8
0 10 20 30
data-point error ellipses are 2
~78 Ma
~152 Ma
~192 Ma ~1060 Ma
~1744 Ma
0
2
4
6
8
10
12
14
0 500 1000 1500 2000 2500 3000
Relativeprobability
Number
Age, Ma
20
60
100
140
180
220
260
300
0.00
0.01
0.02
0.03
0.04
0.05
0.0 0.1
0.2
0.3
207Pb/235U
206Pb/238U
207Pb/235U
206Pb/238U
207Pb/235U
206Pb/238U
207Pb/235U
206Pb/238U
Figure 4. Detrital zircon age distribution for sandstone and tuffaceous sandstone samples of the Arizpe-Mazocahui quadrangle. (A) Sample11-22-07-1 from the Proterozoic Las Vboras Group, Banmichi quadrangle. (B) Proterozoic sandstone from the Las Vboras Group in theSanta Ana quadrangle. (C) Tuffaceous sandstone from the lower part of the Tarahumara Formation in structural section DD . (D) Tuffaceous sandstone from the Tarahumara Formation in the Santa Ana quadrangle. Approximate locations of these samples are indicated inFigure 2. MSWDmean square of weighted deviates.
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Gonzlez-Len et al.
1398 Geosphere, December 2011
noted that the El Jaralito batholith is composed
of granitic, granodioritic, quartz dioritic, and
quartz monzonite facies with ages between
ca. 69 and ca. 51 Ma, whereas he referred to
the Aconchi batholith as a two-mica, alkaline
granite with red garnet with an age of 35.96
0.7 Ma. Radelli et al. (1991) and Macias Valdz
(1992) later renamed this two-mica pluton theHupac granite. In this work we follow the sub-
sequent authors to include the Aconchi batholith
as part of the El Jaralito batholith and based on
our geochronology identify granite outcrops to
the south in the Sierra El Jaralito as the Hupac
granite. Furthermore, in this work and based
mostly on geochronology and geochemistry, we
recognize the El Jaralito batholith as a plutonic
suite that includes previously unrecognized plu-
tons (Fig. 2) (described herein).
Abundant centimeter- to meter-thick peg-
matite dikes composed of K-feldspar, quartz,
plagioclase, muscovite, biotite, garnet, and
accessory minerals that cut through the ElJaralito batholith were studied in detail by
Roldn-Quintana et al. (1989) and Macias-
Valdez (1992). Several published K-Ar and40Ar/39Ar ages obtained from K-feldspar, horn-
blende, muscovite, and biotite from the two-
mica Hupac granite, from pegmatite dikes,
and from skarn rocks range from 41.6 Ma to ca.
18 Ma. The older ages are interpreted as cooling
ages of the plutons (Mead et al., 1988), while
ages between ca. 28 to ca. 18 Ma were inter-
preted by Lugo Zazueta (2006) and Wong et al.
(2010) as cooling ages of the exhumed footwall
of the core complex that forms the Sierra deAconchi (Fig. 2).
The Laramide and older rocks of the study
area are intruded by rhyolitic and dacitic domes
with ages between 23 and 25 Ma (Fig. 2;
Gonzlez-Len et al., 2010; our data) and by
basaltic dikes with ages near 23 Ma (Wong and
Gans, 2008). These rocks are deformed by nor-
mal faults of the Basin and Range extensional
event that formed the north-south elongated
basins of the Sonora and Moctezuma Rivers,
where thick, Late Oligocene to Miocene vol-
canic and terrigenous strata of the Bucarit
Formation accumulated (Gonzlez-Len et al.,
2010) (Fig. 2).
Stratigraphy and Structural Relationshipof the Tarahumara Formation
Structural relationships of the Tarahumara
Formation with older and younger units are
illustrated along seven cross sections (Figs. 2
and 5). Tarahumara Formation stratigraphy is
described by means of six measured columns
(Fig. 6) and from its estimated thickness along
structural sections F-F and G-G (Fig. 5). Sam-
ples from different stratigraphic levels of the
Tarahumara sections were collected during field
work for petrographic, geochronologic, and
geochemical studies. A summary of the U-Pb,
K-Ar, and 40Ar/39Ar geochronology is presented
in Table 1. Geochemical and isotope analyses
were performed from samples collected at same
stratigraphic levels of the dated samples andresults are presented in Supplemental Table 55
and Table 2, respectively. The main structural
and the stratigraphy characteristics of the Tara-
humara Formation along the cross sections are
described next.
Cross-section A-A located in the Arizpe quad-
rangle crosses through the Picacho de Arizpe
peak (Figs. 2 and 5). On the eastern flank of the
Picacho, the Tarahumara Formation unconform-
ably overlies deformed strata of the tectonically
juxtaposed Mural and Cocspera Formations.
It dips homoclinally to the northeast and is
unconformably overlain by Oligocene volcanic
rocks (Gonzlez-Len et al., 2000). In the west-ern flank of the Picacho de Arizpe, the Mural
Formation is part of a block of Bisbee Group
strata that thrusts over the deformed Cocspera
Formation. The measured thickness of the Tara-
humara is 1260 m (Fig. 6A). Its lowermost part
consists of crystal-poor rhyodacitic welded
tuff and crystal-rich porphyritic dacite. Zircons
from the rhyodacite gave a U-Pb age of 75.70
+0.30/0.70 Ma (Fig. 7A; mean 206Pb/238U age,
97.3% confidence, n = 21). The remainder of
the section is composed of well-bedded rhyolite
ash-fall tuff and brown to reddish volcaniclastic
sandstone and siltstone.Along cross-section B-B, the Tarahumara
Formation unconformably overlies the deformed
Late Jurassic Cucurpe Formation, which crops
out in an erosional window of an open anticline
of the Tarahumara Formation in the El Tegua-
chi ranch area (Figs. 2 and 5). In the southern
part of this section the Cintura and Tarahumara
Formations are normally faulted against each
other across the Los Alisos fault. Between the
El Teguachi ranch and the Sierra El Juparo, the
Tarahumara Formation dips to the north and its
section is offset by the normal fault of Caada
El Potrerito. The composite stratigraphic col-
umn of the Tarahumara starts north of the El
Teguachi ranch and ends in Sierra El Juparo
(Fig. 6B). It has an incomplete thickness of
1000 m and its lower part is occupied by a
basal, 29-m-thick conglomerate that grades
upward to interbedded volcaniclastic strata,
rhyolitic to dacitic ignimbrite, ash-fall tuff and
lacustrine limestone. The overlying unit con-
sists of andesitic breccia, rhyolitic ash-fall tuff,
and ignimbrite. A normal fault at the top of this
unit omits part of the stratigraphic column, and
its upper part was measured in the southern
flank of Sierra Los Juparos. It is composed ofbedded ash-fall tuff, rhyolitic breccias, sand-
stone, and conglomerate, and its upper part is
composed of ignimbritic rhyolite and subor-
dinate interbedded rhyolitic ash-fall tuff. Zir-
cons from the upper ignimbritic rhyolite gave
a U-Pb age of 71.7 1.7 Ma (Fig. 7B; mean206Pb/238U age, n = 16).
In cross-section C-C (Fig. 5) the Tarahu-
mara Formation unconformably overlies the
El Jacaln diorite and the Morita Formation of
the Bisbee Group, both of which are juxtaposed
across the El Jucaral fault (Fig. 2). Tarahumara
rocks dip homoclinally 30NE and at Caada
La Nopalosa are unconformably overlain byCenozoic dacite, rhyolite, and conglomerate of
the Sierra Las Guijas (Fig. 5). The 880-m-thick
Caada Motepori stratigraphic column (Fig.
6C) was measured between the creek of same
name and Caada La Nopalosa. Its basal unit is
a 100-m-thick conglomerate with subordinate
coarse-grained sandstone and ash-fall tuff beds
that grade upward to volcaniclastic sandstone
and ash-fall tuffs. Its middle part is composed
of dacite to rhyolite ignimbrite and subordinate
ash-fall tuffs and its upper part consists of vol-
caniclastic sandstone, siltstone, andesite brec-
cia, fiamme-rich rhyolite ignimbrite, and daciteignimbrite. A tuff bed from the lower part of this
column did not yield zircons, but McDowell
et al. (2001) reported a U-Pb zircon age of
76 Ma for the volcanic section near El Tuli
ranch (Fig. 2) that we believe probably belongs
to our measured section.
At cross-section D-D (Figs. 2 and 5), theProterozoic Las Vboras Group thrusts over
the Early Cretaceous Mural Formation of the
Bisbee Group and the Tarahumara Formation
overlies both faulted units. Tarahumara beds
homoclinally dip to the northeast with angles
as steep as 50. The 1040-m-thick stratigraphic
column El Salmn (Fig. 6D) has in its lower
part an 80-m-thick conglomerate that is over-
lain by conglomerate, sandstone, and siltstone
beds arranged in upward-fining successions
with intercalations of rhyolitic and andesitic
ash-fall tuffs. Concordant detrital zircons sepa-
rated from a tuffaceous sandstone collected
133 m above the base of the formation yielded
a younger U-Pb peak age of 76.39 0.67 Ma,
interpreted as the maximum age of deposition
(Fig. 4C, inset; concordia age, n = 10), and
abundant Proterozoic grains.
5Supplemental Table 5. Summary of geochemi-cal analyses for the Laramide rocks and Proterozoicgranites of the study area. If you are viewing the PDFof this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S5 or the full-textarticle on www.gsapubs.org to view SupplementalTable 5.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1399
The middle part of the column is composed
of andesite flows and breccia, while the upper
part that crops out between Cerros El Cuervo
and Caada El Cuervo (Fig. 5) is composed of
rhyolite ignimbrite, ash-fall tuff, and andesitic to
dacitic breccia and agglomerate. A rhyolite from
this section yielded a 70.02 1.5 Ma age (Fig.
7C; mean 206Pb/238U age, n = 24). The upper-
most part of this section between Caada El
Cuervo and Arroyo El Aliso (Fig. 5) has an esti-
mated thickness of 700 m and is composed in its
lower part of bedded dacitic ignimbrite, dacitic
agglomerate and subordinate volcaniclastic
sandstone with an uppermost quartz porphyritic
rhyolite ignimbrite.
Cross-section E-E preserves the most com
plete stratigraphic succession of the Tarahumara
Formation (Figs. 2 and 5), which unconformably
Picachode Arizpe
1000
1300
Arroyo ElCumarito
ArroyoLa Galera
A 75.7 MaTahuichopafault
750
1250
Arroyo LasHigueras
CerrosEl Cuervo
CerroEl Vigia
ArroyoEl Aliso
D 76 Ma 70 Ma
2 km
CaadaEl Salmn
CaadaEl Cuervo
Rhyolite,brecciaand agglomerate
Conglomerateand sandstone
TIRACUAB
Fm
)enecoi
M/enecogil
O(
Dacite flow
1000
1500 Sierra Las GuijasC
76 Ma
CaadaMotepori
CaadaEl Picacho
CaadaLa Nopalosa
1 km
1000
CerroEl Volantin
PuertoLos Mojones
LasCabecitas
150068.6 Ma
63 MaF
Sierra LaCieneguita
1500
750
CerroColorado
ArroyoMalpaso
ArroyoLos Nogales
Sierra El OsoEl Saucitofault
E74 Ma 74.8 Ma 75 Ma
900
1500
Arroyo Los
Alisos
Cerro Cajon
de Enmedio
Puerto El
Teguachi
El Teguachi
ranch
Sierra El
Juparo
B
73 MaLos Alisos
fault62 Ma
Caada
El Potrerito
Cocospera formation
Cintura Formation
Mural Formation
Morita Formation
Cucurpe Formation
Proterozoic sedimentaryrocksEl Jacalon diorite (1.7 Ga
ee
bsi
B G r o u p
aramu
hara
T-er
P
basemen
t
Angular unconformity
Granitic to dioriticplutons
Rhyolitic ignimbriteand ash-fall tuff
Dacite
Andesite
Agglomerateand breccia
Limestone
Conglomerate
Volcanosedimentaryrocks
AMGAMEDIMARAL
CRACIT
noitamro
Faramu
ha
raT
Downthrown blockin normal faultThrust fault
Mine
Unconformity
FrontRanges
fault
500
1000
MoctezumaRanchoAgua ZarcaEl Rodeo
fault
70 Ma 5 km78.7 Ma 76 Ma
72 MaG
59 Ma
A
B
C
D
E
F
G
Figure 5. Structural cross sections from the study quadrangle showing relationships of the Tarahumara Formation with other older andyounger lithologic units. Rock ages shown as reported in this contribution, except for cross-section CC , taken from McDowell et al. (2001)Locations of cross sections indicated in Figure 2. Vertical scale is in meters. Horizontal scale of 2 km is for cross-sections AA to EE .
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Gonzlez-Len et al.
1400 Geosphere, December 2011
overlies the Proterozoic Las Vboras Group and
dips homoclinally to the east. In the western
foothills of the Sierra El Oso, the Tarahumara
is offset by the northwest-southeast El Saucito
normal fault that dips steeply to the southwest.
In the eastern part of the section the Tarahumara
is unconformably overlain by conglomerate and
rhyolite of the Bucarit Formation. Because
of the El Saucito fault offset, we measured the
stratigraphy of the Tarahumara in two columns,
named Cerro Colorado and Sierra El Oso,
located west and east of the El Saucito fault,
respectively.
The Cerro Colorado stratigraphic column
(Fig. 6E) is 1865 m thick. Its basal unit is a
50-m-thick conglomerate composed of quartz-
rich sandstone clasts that grades upward to
a volcaniclastic succession with interbedded
rhyolite and dacite ignimbrite. Zircons from a
20-m-thick, fiamme-rich rhyolite tuff located
120 m above the base of the formation yielded
an age of 74.30 1.3 Ma (Fig. 7D; mean206Pb/238 age, n = 39). Upward, the succession
is composed of rhyolite ignimbrite and ash and
lapilli tuff, whereas its middle part consists of
andesite flows and breccia, rhyolitic and dacitic
ignimbrite, volcaniclastic sandstone, and well-
bedded ash-fall tuff. Its upper part is composed
of rhyolitic and dacitic ignimbrite, interbedded
volcaniclastic sandstone, conglomerate, and
andesite ash-fall tuff. The lower part of this col-
umn located between the measured section and
the Santa Elena Mine (Fig. 2) is intruded by a
quartz phenocrystbearing rhyolite dome that
we dated as 73.56 1.3 Ma (Figs. 6E and 7E;
mean 206Pb/238U age, n = 59).
The exposed lower part of the 930-m-thick
Sierra El Oso stratigraphic column (Fig. 6F)
is a rhyolitic ignimbrite flow that was dated as
74.64 1.5 Ma (Fig. 7F; mean 206Pb/238U age,
n = 43). Most of the lower part of this succes-
sion is composed of rhyolitic to dacitic ignim-
brite and interbedded ash-fall tuffs, whereas its
middle part consists of volcaniclastic strata,
minor ash-fall tuffs, dacite, and andesitic brec-
cia. Its upper part is andesitic and rhyolitic
ash-fall tuff, trachyandesite flows, and rhyolite.
Zircons from a rhyolitic tuff of this upper part
yielded a U-Pb age of 75.10 1.2 Ma (Fig. 7G;
mean 206Pb/238U age, n = 41).
Structural cross-sections F-F and G-G in thesouthern part of the area traverse thick succes-
sions of the upper Tarahumara Formation as its
lower part is offset by normal faults and cut by
plutonic intrusions (Figs. 2 and 5). Its estimated
thickness along section F-F is more than 2 km
dipping homoclinally to the northeast, except in
its upper part where it forms an open syncline,
the northwest-southeast axis of which follows
the upper part of the Sierra Las Palomas (Fig. 2).
Outcrops of its exposed lower part are inter-
bedded andesitic volcaniclastic sandstone, con-
glomerate, and andesite flows that are overlain
by crystal- and lithic-rhyolite ignimbrite and an
interbedded porphyritic dacite that was dated as
68.50 2.0 Ma (Fig. 7H; mean 206Pb/238U age,
n = 14). The middle part of the Tarahumara For-
mation is occupied by andesitic flows and its
upper part is composed of stratified, crystal- to
Proterozoic El Jacaln diorite
500
0
880
Basal, massive to poorly bedded, clast-supported, cobble conglomerate withclasts of subrounded sandstone,quartzarenite, andesite and gneiss.Upper part composed of coarse- tom e d iu m -g r a i ne d v o l ca n i cl a s ti csandstone and bedded ash-fall tuff.
Dacite to rhyolite ignimbrite in beds up to12 m thick, and subordinate, interbedded
ash-fall tuffs. Petrographic compositionof a dacitic ignimbrite is crystal-rich withaltered plagioclase and hornblende ing lo me ro po rp hyr it ic t ex tu re , i n ahyalopilitic groundmass.
Bedded volcaniclast ic sandstone,conglomerate, siltstone and andesitebreccia in the lower part. A 40-m-thick,f iamme-r ich rhyoli te ignimbrite in themiddle part is overlain by reddishsandstone. The upper part is composedof reddish rhyoli tic ignimbrite,subordinate ash-fall tuffs and a top 50-m-thick welded dacite tuff composed offeldspar, b io ti te and a ltered mafi cminerals.
C. Caada Motepori
500
0
1000
Massive, clast-supported, angularto sub-rounded, poorly sor tedcobble- to pebble-conglomeratewi th c la st s o f q ua rt za re ni te ,andesite, sandstone and siltstone.In terbedded, coarse-gra inedvolcaniclas tic sands tone andrhyolite to dacite ignimbrite flows,andesitic ash-fall tuff and mediumbedded, dark gray, stromatolitic,oolitic and oncolitic, dolomitizedlacustrine limestone.
Andesitic breccia and ash-fall tuff,rh yo li te i gn imbr it e fl ows a ndrhyoliticash-falltuffs.
Lithic, bedded ash-fall rhyolitic tuff,r hy ol it ic b recc ia , s ands tone ,siltstone and subordinate pebbleconglomerate lenses.
Ignimbritic rhyolite flows composedof quartzphenocrysts, feldspar andfiamme, with interbedded rhyoliticash-fall tuff.
Crystal-rich ignimbritic rhyolite
flows composed of fragmented andp ar ti al ly r es or be d q ua rt zphenocrystals, feldspar and alteredbiot i te in a vitr ic eutaxit icgroundmass. Interbedded rhyoliticash-falltuff.
LateJurassicCucurpe Formation
B. Rancho Teguachi
500
0
1260
1000
Crystal-poor rhyodacitewith crystalsof quartz, feldspar, biotite andvolcanic rock fragments in a vitricgroundmass.Crystal-richporphyriticdacite with feldspar and volcanicrock fragments in a fluidal vitricmatrix.
C o ng l om e ra t e, s a nd s to n e,
stromatolitic limestone, dacitic torhyoliticbrecciaand ash-fall tuff.Thetuff contains plagioclase,biotiteandpumice fragments altered tochlorite and clay minerals, and thegroundmass consists of devitrifiedmicrocrystal line quartz and claymaterial.
Middle and upper parts of the
column are composed of rhyolitica sh - fa l l t uf f, v ol ca ni c la st i csandstone and siltstone. The tuffoccursas bedded intervalsup to 58m thick and typically has quartz,altered feldspar and biotite crystalsin a groundmass of devitrified glassshards. The volcaniclastic strata arebrown,reddish andgreen and occurasmassivebeds upto 5 m thick thatbecomemoreabundant upwards inthesection.
EarlyCretaceousBisbeeGroup
A. Picacho de Arizpe
Figure 6 (on this and following page). Measured stratigraphic columns of the Tarahumara Formation in the study area with lithologicdescription. Thickness is in meters.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 140
lithic-rich rhyolite ignimbrite with subordinate
rhyolitic and andesitic ash-fall tuff and breccias.
A rhyolite from the uppermost part of this suc-
cession was dated as 63.5 1.4 (Fig. 7I; mean206Pb/238U age, n = 19).
Along section G-G the Tarahumara Forma-tion dips homoclinally to the east and is offset
by normal faults that bound north-trending
Cenozoic basins (Fig. 5). At the southwestern
end of this section, the Tarahumara Formation is
intruded by the La Aurora tonalite (discussed in
the following). The faulted section of the Tara-
humara Formation between the El Rodeo ranch
and the Range Front fault (Deen and Atkinson,
1988) (Figs. 2 and 5) is ~1 km thick and com-
posed of interbedded rhyolitic ignimbrite and
pebble-cobble conglomerate, and its upper part
is composed of stratified ash-fall tuff, rhyolitic
ignimbrite, conglomerate, and sandstone. A
rhyolite from the lower part of this succession
was dated as 78.7 1.3 (K-Ar, biotite, sample
1126094; Table 1). The normal-faulted Tara-
humara succession that crops out from Sierra La
Cieneguita to the town of Moctezuma (Figs.
2 and 5) consists in its lower part of interbed-
ded rhyolitic and dacitic flows and subordinate
ash-fall tuff, volcaniclastic sandstone, and con-
glomerate; the upper part, near Moctezuma, it is
composed of stratified red ignimbrite rhyolite.
A dacitic ignimbrite from the lower part of this
section yielded an age of 75.75 0.55 Ma (Fig.
8A; mean 206Pb/238U age, 94.3% confidence, n =
14) and a rhyolite from its upper part yielded an
age of 72.20 1.6 Ma (Fig. 8B; mean 206Pb/238U
age, 96.1% confidence, n = 12). The thickness
of the Tarahumara Formation in this section
may be >1.5 km.
Other U-Pb ages of the Tarahumara succes-
sion in the study area come from samples that
are not located on a measured section. From the
Santa Ana quadrangle (Fig. 2) we dated detri-
tal zircons from a volcaniclastic sandstone (Fig.
4D, n = 85) that yielded a dominant age peak
of 152 Ma and subordinate peaks of 78, 192,
1060, and 1700 Ma, and two other rhyolites that
yielded ages of 76.0 2.7 Ma (Fig. 8C; mean206Pb/238U age, n = 13) and 73.8 1.6 Ma (Fig.
8D; mean 206Pb/238U age, n = 23). A reworked,
tuffaceous rhyolite from the Aconchi quadran
gle (Fig. 2) yielded an age of 69.1 2.4 Ma (Fig
8E; mean 206Pb/238U age, n = 17).
Laramide Plutonic Rocks
Laramide plutons in the northern part of the
study area crop out as small exposures of a few
square kilometers, but they form larger outcrop
in Sierra El Jaralito batholith in the southern par
(Roldn-Quintana, 1989). Based on field obser
vation of well-exposed outcrops, most of the
plutons are apparently fresh and homogeneou
in texture and mineralogy, although detailed
examination could reveal subtle variations in
these characteristics. As also observed by othe
authors from the Laramide plutons of Sonora
(Richard et al., 1989; Roldn-Quintana, 1991)
mineralogy of the study plutons is simple with
varying proportions of plagioclase, alkali feld
spar, quartz, biotite, scarce amphibole and pyrox
ene, and accessory minerals including titanite
opaque minerals, and zircon. Feldspar, biotite
and hornblende may present slight to moderate
Basefaulted
Rhyol i te ign imbr i te w i thplagioclase, sanidine, biotiteand rock fragments in a vitricand eutaxitic groundmass withfiammefragments.
930
Crystal-rich rhyolitic, daciticand rhyodacitic ignimbrite andinterbedded dacitic to rhyoliticash-fall tuff.
Stratified, fine- to coarse-grained, thin- to medium-b e d d e d v o l c a n i c l a s t i clitharenite, ash-fall tuffs, lensesof pebble conglomerate withclasts of volcanic rocks, daciteandandesiticbreccia.
Andesitic and rhyolitic ash-falltuffand trachyandesiteflows.
500
0
F. Sierra El Oso
0
1000
1800
200
Massive, poorly sorted, clast-supported,pebble- to cobble-conglomerate.
Bedded, arkosic to lithic sandstone,siltstone and interbedded ash-fall tuff beds,pebble-conglomerate and rhyolitic to daciticash-flow tuffs. Intruded by a quartzphenocryst-bearing rhyolite dome.
Proterozoic Las Vboras Group.
Crystal- and lithic-rich rhyolite ignimbriteand interbedded ash- and lapilli-tuff beds.Crystals in rhyolite are feldspar, quartz andbiotite; lithic grains are altered volcanicrocks. Mesostasis is vitric and eutaxitic.Flatfiamme fragmentsare common.
Aphanitic andesite flows, well-bedded ash-fall tuff and minor volcaniclastic sandstone.The upper part of this interval is massive,aphaniticandesiteflowsand beds up to 5 m
thickof andesiteandrhyolitebreccias.
Fiamme-rich rhyolite ignimbrite. Crystalsare fragmented and resorbed quartzcrystals, feldspar and biotite set in aeutaxitic, vitric mesostasis.
Porphyritic dacite ignimbrite, hornblendeandesite, breccia, and volcaniclasticsandstone. Dacite in cooling units up to 40m thick is crystal-rich with plagioclase,hornblende, andesite lithics and fiammefragments in a hyalopilitic mesotasis.An de si te has hy al op il it ic te xt ur e,phenocrystsof plagioclaseand hornblende.Volcanicbreccia in massive unitsup to 50 mthickwithblocks of andesiteandrhyolite.
Rhyolite and crystal-rich dacite ignimbriteinunits up to 50 m thick, sandstone, pebbleconglomerate and andesitic tuff. Typicallyrhyolite has abundant fiamme, up to 25%volume of feldspar and biotite crystals in avitric mesostasis. Dacite consists of up to35% crystals of feldspar, andesite lithicfragments, and biotite in a eutaxitic vitricmatrix.
E. Cerro Colorado
1040
Massive, clast-supported, poorlysorted cobble conglomerate withclasts of quartzarenite. It finesupwards topebble-conglomerate.
R e d d is h b r o w n t o p u r p l evolcaniclastic, lenticular, pebbleconglomerate, sandstone andsiltstone forming upward-finingcycles up to 45 m thick. Interbeddedyellow rhyolitic ash-fall tuff beds andsubordinateandesitetuffs.
Massive, light gray porphyritichornblende andesite and andesiticbreccia.
Lithic-, and crystal-rhyolite ignimbritein beds up to 50 m thick, subordinateinterbeds of ash-fall tuff, daciteignimbrite and andesitic to daciticbreccia and agglomerate. A typical,crystal-rich rhyolite tuff from thisinterval is composed of fragmentedand resorbed phenocrysts of quartz,fe ldspar, b io ti te and f iammefragmentsin a fluidalvitricmatrix.
500
0
ProterozoicLas Vboras Group.
D. El Salmon
Volcaniclasticrocks
Conglomerate
Andesitic brecciaand agglomerate
Rhyoliteignimbrite
AndesitictuffAndesiteflows
Dacite
Ash-falltuff
Rhyolitic brecciaand agglomerate
Limestone
Unconformity
Rhyolitic dome
Pre-Tarahumarabasement rocks
Covered
Figure 6 (continued).
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Gonzlez-Len et al.
1402 Geosphere, December 2011
TABLE 1. SUMMARY OF U-Pb, K-Ar AND 39Ar/40Ar GEOCHRONOLOGY FOR THE LARAMIDE ROCKS IN THE STUDY AREA
Sample
Age(Ma)
U-Pb (zr)
Age(Ma)
K-Ar (bi)
Age(Ma)
Ar/Ar (bi) snoitavresbOedutitla,noitacolMTU
dna,esalcoigalpcidos,ztrauqhtiwetiloyhrcitiryhproPm7741,N9605033E002795R214.15.365-90-32-4chloritized grains of biotite in a eutaxitic vitric mesostasismoderately altered to sericite.
,etitoib,esalcoigalphtiweticadcitiryhprop,hcir-latsyrCm8121,N0098923E228395R2125.8621-90-32-4and hornblende strongly altered to chlorite. Hyalopiliticmesostasis recrystallized to microcrystalline quartz,chlorite, and iron oxides.
ztrauqdetnemgarfdnadebroserhtiwetiloyhrcitiryhproPm968,N0183333E830775R215.120.072-80-5-3crystals to 3 mm long, broken plagioclase crystals,biotite altered to chlorite and scarce volcanic rockfragments. Hyalopilitic mesostasis and glass shards.
dnadedorrochtiwetirbmingicitiloyhrhcir-latsyrCm4031,N5545433E435165R217.17.172-90-82-1fragmented quartz crystals to 3 mm long, K-feldspar andsodic plagioclase and subordinate biotite in a hyalopiliticmesostasis with glass shards.
mm2otpurapsdleffoslatsyrcderetlahtiweticadoyhRm486,N0705923E347526R2102.1/06.1+02.276-90-52-11long, quartz and subordinate laths of biotite altered tochlorite. Groundmass is vitric and fluidal with fragmentsof fiamme recrystallized to quartz and calcite.
mm2otsniargztrauqdebroserhtiwetiloyhrcitiryhproPm709,N6562233E642185R213.165.374-70-61-11long, sodic plagioclase and orthoclase and subordinatechloritized biotite. Vitric mesostasis with incipientrecrystallization.
,esalcoigalp,ztrauqfoslatsyrchtiwffutcitiloyhRm0221,N2778333E244355R216.18.372-80-5-21subordinate lithic fragments, and biotite. Crystals are
broken and angular. Quartz is locally resorbed andplagioclase is altered to calcite. Matrix recrystallized tomicrocrystalline quartz.
htiwetiloyhretirbmingi,)slatsyrc%lov02
7/30/2019 Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico
13/28
http://geosphere.gsapubs.org/7/30/2019 Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico
14/28
Gonzlez-Len et al.
1404 Geosphere, December 2011
Mean206Pb/238Uat
63.51.4
Ma
MSW
D(concordance)=0.2
1
4-23-09-5
200
0.04
0.06
0.08
0.10
0.12
0.14
0
20
40
6
0
80
100
120
data-pointerrorellipsesare2
64
68
72
76
80
84
Mean=74.3
01.3
Ma
MSWD=2.0
(95%confidence)
(errorbarsare2)
207Pb/
206Pb
Interceptsat
70.62.3
&1692140Ma
MSWD=5.4
9-18-07-2
238U/206Pb
D
70
80
0.04
0.06
0.08
0.10
0.12 7
2
76
80
84
88
92
96
100
data-pointerrorellipsesare2
64
68
72
76
80
84
Mean=73.5
61.3
Ma
MSWD=6.3
(95%confidence)
(errorbarsare2)
207Pb/
206Pb
238U/206Pb
Mean206Pb/238Uat
73.51.3
Ma
MSWD(concordance)=0.88
11-16-07-4
E
60
80
100
0.0
4
0.0
6
0.0
8
0.1
0
0.1
2
0.1
4
0.1
6
0.1
8 60
70
80
90
100
110
data-pointerrorellipsesare2
58
62
66
70
74
78
82
86
90
94
207Pb/
206Pb
238U/206Pb
Mean206Pb/238Uat
74.61.3
Ma
MSWD(concordance)=0.7
8
11-19-07-1
F
Mean=74.6
41.5
Ma
MSWD=6.9
(95%confidence)
(errorbarsare2)
MSWD
=0.3
2
4-23-09-12
Interceptsat
7228&25355Ma
400
0.0
4
0.0
6
0.0
8
0.1
0
0.1
2
0.1
4
0.1
6
0.1
80
20
40
60
80
100
data-pointerrorellipsesare2
207Pb/
206Pb
238U/206Pb
G70
72
74
76
78
80
Mean=75.1
01.2
Ma
MSWD=2.0
(95%confidence)
(errorbarsare2)
MSWD(concordance)=0.8
6
2-27-08-4
Interceptsat
74.9
60.6
7&170542Ma
207Pb/
206Pb
238U/206Pb
H
I
207Pb/
206Pb
238U/206Pb
B
Mean206Pb/238Uat
71.71.7
Ma
MSWD(concordance)=0.35
1-28-09-2
207Pb/
206Pb
238U/206Pb
110
90
70
0.0
4
0.0
5
0.0
6
0.0
7
0.0
8
0.0
9
0.1
0 55
65
75
85
95
105
115
data-pointerrorellipsesare
2
55
65
75
85
95
Mean=71.71.7
Ma
MSWD=0.3
69(95%confidence)
(errorbarsare2)
160
0.0
4
0.0
5
0.0
6
0.0
70
20
40
60
80
100
120
data-pointerrorellipsesare
2
80
70
60
50
0.0
4
0.0
5
0.0
6
0.0
7
0.0
8
0.0
975
85
95
105
115
125
data-pointerrorellipsesare2
70
78
86
0.0
7
0.0
6
0.0
5
0.0
4
0.0
8
84
88
80
76
72
238U
/206Pb
207Pb/
206Pb
4-7-08-1
Mean206Pb/238Uage
75.7
00.3
0/-0.7
0Ma
MSWD=2.0
data-pointerrorellipsesare2
Age
boxheights
are2
TuffZirc
Age=75.7
0
+0.3
0
-0.70
Ma
(97.3
%
confidence,
fromcoherentgroup
of21)
A
207Pb/
206Pb
238U/206Pb
M
ean206Pb/238Uat
70.01.5
Ma
MSW
D(concordance)=1.5
3-5-08-2
C280
200
120
0.0
4
0.0
6
0.0
8
0.1
0
0.1
2
0.1
4 20
40
60
80
100
120
data-pointerrorellipsesare2
56
60
64
68
72
76
80
84
Mean=70.0
21.5
Ma
MSWD=0.4
4(95%confidence)
(errorbarsare2)
50
60
70
80
90
Mean=68.52Ma
MSWD=0.9
6(95%confidence)
(errorbarsare2)
48
52
56
60
64
68
72
76
Mean=63.51.4
Ma
MSWD=0.7
2(95%confidence)
(errorbarsare2)
Figure7.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1405
medium grained with plagioclase, quartz, bio-
tite, K-feldspar, hornblende, titanite, apatite,
zircon, and iron oxides. Plagioclase is euhe-
dral oligoclase in crystals as large as 7 mm and
quartz is anhedral. Biotite occurs as euhedral
phenocrysts as large as 8 mm, and K-feldspar
is euhedral orthoclase and microcline as large as
3 mm. Zircons from this pluton yielded a U-Pbage of 61.10 +0.90/0.50 Ma (Fig. 8H; mean206Pb/238U age, 95.1% confidence, n = 17). La
Aurora is the largest pluton of the study area
and varies in composition from diorite in its
western outcrops to tonalite in its eastern out-
crops. It is medium grained, holocrystalline, and
hypidiomorphic with anhedral quartz, euhedral
plagioclase (oligoclase), biotite, orthoclase, and
microcline. Accessory minerals are titanite, apa-
tite, zircon, and iron oxides. La Aurora yielded
a zircon U-Pb age of 69.65 +1.05/0.45 Ma
(Fig. 8I; mean 206Pb/238U age, 95% confidence,
n = 26), and three other samples of different
localities of the pluton gave K-Ar ages (biotite)of 60.4 1.2, 57.2 1.4, and 49.5 1.1 Ma
(Table 1).
Granodioritic plutons include the Rancho
Vaquera, Los Alisos, San Antonio, Las Cabe-
citas, Puerta del Sol, and Bella Esperanza (Figs.
2 and 9A). The Rancho Vaquera is a medium-
grained, phaneritic, hypidiomorphic rock with
plagioclase, quartz, K-feldspar, and biotite and
equal or lesser amounts of hornblende, titanite,
augite, and iron oxide. This pluton gave a U-Pb
age of 55.8 0.9 Ma (Fig. 10A; mean 206Pb/238U
age, n = 27). The Los Alisos granodiorite
(Fig. 2) is medium grained, holocrystalline, andporphyritic with euhedral plagioclase (albite-
oligoclase), subhedral quartz, and euhedral
orthoclase. Secondary minerals are biotite and
magnetite in a glomeroporphyritic texture, and
accessory minerals are titanite, apatite, and zir-
con. This pluton yielded a 40Ar/39Ar plateau age
of 61.76 0.81 Ma in biotite (Fig. 11; Table 1).
The San Antonio granodiorite is medium
grained, holocrystalline, and hypidiomorphic
with euhedral plagioclase (oligoclase-ande-
sine), anhedral quartz, K-feldspar, biotite and
lesser amounts of hornblende, titanite, magne-
tite, apatite and zircon. It yielded a U-Pb age of
67.7 1.6 Ma (Fig. 10B, mean 206Pb/238U age,
n = 22). Las Cabecitas granodiorite is medium
grained, hypidiomorphic, and granular with
commonly zoned plagioclase, quartz, K-feld-
spar, biotite, and lesser amounts of hornblende,
titanite, and magnetite. Two samples collected
at different localities in this pluton yielded ages
of 59.1 1.6 (Fig. 10C; mean 206Pb/238U age,
n = 12) and 56.3 1.2 Ma (Fig. 10D; mean206Pb/238U age, n = 18).
The Puerta del Sol pluton intrudes La Alame-
dita tonalite and the Hupac granite in its outcrop
in the eastern part of the Sierra de Aconchi, but
its more extensive outcrop is in the southwest-
ern part of the study area (Fig. 2). We assume
that these outcrops belong to the same pluton
based on their similar composition and age. The
Puerta del Sol was named by Anderson et al.
(1980) for its outcrops that extend south of the
study area, where U-Pb zircon ages of 57 3 Maand 49.1 Ma were reported by Anderson et al.
(1980) and Gonzlez Becuar (2011), respec-
tively. It is medium to coarse grained and holo-
crystalline, with quartz, plagioclase, K-feldspar,
biotite, titanite, muscovite, zircon, and iron
oxide. Quartz is anhedral, as long as 1.3 cm, and
K-feldspar is euhedral orthoclase and microcline
as long as 1.2 cm; plagioclase is albite-oligo-
clase. Biotite crystals are euhedral and musco-
vite is secondary. Samples from the two different
outcrops of the Puerta del Sol within the study
area yielded U-Pb ages of 51.26 1.0 (Fig.
10E; mean 206Pb/238U age, n = 17) and 49.95
+1.05/0.45 Ma (Fig. 10F; mean 206Pb/238U age,95% confidence, n = 26), but a biotite K-Ar age
was 23.6 1.1 Ma (Table 1). We did not study
the Bella Esperanza granodiorite that was dated
by Housh and McDowell (2005), but their iso-
topic data are referred in the Discussion.
Granites are part of the El Jaralito batho-
lith and include the leucocratic El Babizo and
Hupac granites (Fig. 2). These plutons have
meter-sized enclaves of metasedimentary rocks,
and larger blocks of these rocks are interpreted as
roof pendants. The El Babizo granite is intruded
by the La Alamedita, La Aurora, Hupac, and
Puerta del Sol plutons. It is coarse grained topegmatitic and composed of subhedral quartz,
euhedral orthoclase, microcline and oligo-
clase, biotite, muscovite, hornblende, titanite,
zircon, and iron oxide. Biotite is euhedral and
hornblende is anhedral; anhedral to subhedral
muscovite occurs as late-stage filling of micro-
fractures and along twin planes of plagioclase.
Perthite is common and some plagioclase has
myrmekitic rims. The El Babizo ranges in com-
position from monzogranite to syenogranite
(Fig. 9A). Zircons from two samples collected
at different localities of this granite (Fig. 2;
Table 1) were dated as 71.50 +0.20/0.70 Ma
(Fig. 10G; mean 206Pb/238U age, 94.3% confi
dence, n = 14) and 70.50 + 0.30/0.60 Ma (Fig
10H; sample 102-098, mean 206Pb/238U age
97.7% confidence, n = 24). A biotite separate
from sample 102-098 was also dated by K-A
as 48.7 1.0 Ma (Fig. 2; Table 1).
The Hupac granite is fine to mediumgrained, allotriomorphic-granular to hypidio
morphic-granular, and composed of quartz
K-feldspar, plagioclase, muscovite, biotite, gar
net, zircon, and magnetite. Quartz is subhedra
to euhedral with undulose extinction. K-feldspa
is euhedral to subhedral orthoclase and micro
cline with late-stage inclusions of quartz, plagio
clase, and muscovite. Plagioclase is mostly
euhedral oligoclase and minor albite. Muscovite
is euhedral and biotite is anhedral. Myrmekite
and perthitic intergrowths are present. Two sam
ples from different localities yielded ages of 58
+0.60/0.90 Ma (Fig. 10I; mean 206Pb/238U age
93.5% confidence, n = 11) and 54.95 1.6 Ma(mean 206Pb/238U age, n = 36); two other sam
ples yielded K-Ar biotite ages of 29.5 0.9 and
28.7 1.0 Ma (Table 1).
Other intrusive rocks that crop out in the area
and were not studied are the Rancho Viejo dio
rite, the San Felipe porphyry, local stocks o
diorite and granodiorite in the Cumobabi Mine
(Scherkenbach et al., 1985), a rhyolitic dome
near Rancho Agua Caliente (Fig. 2), and dikes
of pegmatite, microgranite, diorite, and basal
that occur throughout the area.
GEOCHEMISTRY
Elemental Geochemistry
The analyzed volcanic rocks range in composi
tion between 63 and 78 wt% SiO2. They are mostly
high-K calc-alkaline andesites to rhyolites, and
half of the samples are high-silica rhyolites in the
(K2O/SiO
2)
Ndiagram with the fields of Peccerillo
and Taylor (1976) (Fig. 9B). Major elements such
as Al2O
3, Fe
2O
3, and CaO show negative cor
relation trends with SiO2, except for K
2O (Fig
9B), typical calc-alkaline differentiation trends
Figure 8 (on following page). Histogram and concordia diagrams of U-Pb ages of the Tarahumara Formation and plutonic rocks of the Arizpe-Mazocahui area. MSWDmeansquare of weighted deviates. (A) Sample 11-26-09-3 from a dacite tuff in the lower part ofthe Tarahumara section of Sierra La Huerta, cross-section GG . (B) Sample 11-25-09-6from the upper part of the Tarahumara section of Sierra La Huerta. (C) Sample 3-3-09-7from a rhyolite tuff of the Santa Ana quadrangle. (D) Sample 12-5-08-2 from a rhyolite inthe Santa Ana quadrangle. (E) Sample 3-30-09-13 from a rhyolitic tuff from the Aconchiquadrangle. (F) Sample 3-3-08-1 from La Cubana monzonite. (G) Sample 2-27-09-8 fromthe El Gueriguito quartz monzonite. (H) Sample 10-1-09-1 from La Alamedita tonalite(I) Sample 9-30-09-4 from the La Aurora tonalite.
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1406 Geosphere, December 2011
Mean206Pb/238Uat
58.11.7
Ma
MSWD(concordance)=0.2
6
2-27-09-8
ElGueriguito
Mean206Pb/238Uat
52.7
60.9
Ma
M
SWD(concordance)=0.7
3
3-3-08-1
LaCubana
Mean206Pb/238Uat
69.12.4
Ma
MSWD(concordance)=0.7
9
3-30-09-13
MSWD=0.2
3
12-5-08-2
Interceptsat
73.04.1
&144333Ma
200
0.0
4
0.0
6
0.0
8
0.1
00
20
40
60
80
100
120
data-pointerrorellipsesare2
100
9
0
80
70
60
0.04
0.06
0.08
0.10 6
0
70
80
90
100
110
120
data-pointerrorellipsesare2
D G
E
C
56
60
64
68
72
76
80
84
88
92
Mean=73.81.6Ma
MSWD=0.45(95%confidence)
(errorbarsare2)
50
60
70
80
90
Mean=69.12.4Ma
MSWD=0.79(95%confidence)
(errorbarsare2)
200
0.04
0.06
0.08
0.10
0.12 0
40
80
120
160
data-pointerrorellipsesare2
40
50
60
70
80
90
Mean=52.760.9Ma
MSWD=0.73(95%confidence)
(errorbarsare2)
80
60
0.04
0.08
0.12
0.16
0.20
0.24
0.28
70
90
110
130
150
data-pointerrorellipsesare2
42
46
50
54
58
62
66
70
74
78
Mean=58.11.7Ma
MSWD=0.26(95%confidence)
(errorbarsare2)
238U/206Pb
238U/206Pb
207Pb/
206Pb 207
Pb/206
Pb
207Pb/
206Pb
207Pb/
206Pb
207Pb/
206Pb
238U/206Pb
207Pb/
206Pb
data-pointerrorellipsesare2
86
78
70
0.0
40
0.0
44
0.0
48
0.0
52
0.0
56
0.0
60
0.0
64
0.0
68 7
0
74
78
82
86
90
94
MSWD
=4.3
70
72
74
76
78
80
82
84
86
Age
TuffZirc
Age=75.7
5
+0.5
5
-0.8
5Ma
(94.3
%confidence,
fromcoherentgroupof14)
boxheightsare2
11-26-09-3
Mean206Pb/238Uage
75.7
5+0.5
5/-0.8
5Ma
A
238U/206Pb
238U/206Pb
238U/206P
b
238U/206
Pb
207Pb/
206Pb
data-pointerrorellipsesare2
200
0.0
4
0.0
5
0.0
6
0.0
7
0.0
8
0.0
9
0.1
00
20
40
60
80
100
120
Interceptsat
72.21.6
&1089190Ma
MSWD=19
74
64
68
72
76
80
84
88
Age
boxheightsare2
TuffZirc
Age=72.2
0+1.6
0/-1.2
0Ma
(96.1
%
confidence,coherentgroupof12)
11-25-09-6
B
MSWD(concordance)=0.3
8
3-3-09-7
Interceptsat
7
7.34.1
&140235Ma
200
0.04
0.06
0.08
0.10
0
20
40
60
8
0
100
120
data-pointerrorellipsesare2
55
65
75
85
95
Mean=76.02.7
Ma
MSWD=0.6
0(95%confidence)
(errorbarsare2)
238U/206Pb
207Pb/
206Pb
data-pointerrorellipsesare2
78
74
70
66
0.0
40
0.0
44
0.0
48
0.0
52
0.0
56
0.0
60
0.0
64
0.0
68 8
2
86
90
94
98
102
MSWD=2.0
69.6
5+1.0
5/-0.4
5Ma
64
66
68
70
72
74
76
Age
TuffZirc
Age=69.6
5
+1.0
5-0.4
5Ma
(95%
conf,fromcoherentgroupof26)
boxheightsare2
9-30-09-4
LaAurora
238U/206Pb
207Pb/
206Pb
data-pointerrorellipsesare2
80
70
50
0.040
0.044
0.048
0.052
0.056
0.060
0.064
0.068 8
0
90
100
110
120
130
MSWD=6.4
61.1
0+0.9
0/-0.5
0Ma
10-1-09-1
LaAlamedita
52
56
60
64
68
72
76
Age
TuffZir
c
Age=61.10+0.90-0.50Ma
(95.1%
confidence,coherentgroupof17)
boxheightsare2
60
H
IF
Mean206Pb/238Uat
Mean206Pb/238Uat
Figure8.
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Laramide magmatic arc in northern Sonora
Geosphere, December 2011 1407
6
4
2
0
CaO
Fe
O2
3
8
6
4
2
0
PeraluminousMetaluminous
Al O /(CaO + Na O + K O) (molar)2 3 2 2
Al
O
/(Na
O
+K
O)(mo
lar)
2
3
2
2
0.5 0.7 0.9 1.1 1.3 1.5 1.7
0.6
1.0
1.4
1.8
2.2
2.6
Monzonite, diorite and granodiorites
Granites
El Jacaln dioriteSanta Margarita granite
12
14
16
18
Al
O2
3
B
D
Volcanic arc
Y + N b
Rb
1 10 1000
10
100
100
1000
Syn-collisionalWithin plate
Ocean ridge
E
Diorite - tonalite
Granite
500
250
750
1000
1500 2000 2500 3000
Alkali granite
Syenogranite
Monzogranite
Granodiorite
Tonalite
DioriteMonzodiorite
Monzonite
Quartzmonzonite
1250
R1
R2
Granodiorite
Monzonite
Gabbro-Diorite
1500
Santa Margaritagranite
El Jacaln diorite
A
1000
2
4
6
0SiO2
K
O2
Medium-
K
High-K
, calc-a
lkaline
60 70 8050
Nd Dy Ho Er Tm Yb LuSm Eu Gd TbPrCeLa
Volcanic rocks
Monzonite, diorite
and granodiorites
Huepac granite
El Babizo granite C1000
100
10
1.0
0.1
Northern and central granites ofSonora (from Valencia-Morenoet al., 2001)
IAGCAG
CCG
Volcanic rocks
Plutonic rocks
Figure 9. Geochemical discrimination plots for the geochronologically dated Laramide volcanic and plutonic rocks of the studyarea. (A) Chemical classification of the dated Laramide plutons according to the R1-R2 diagram of De la Roche et al. (1980). Rockgrouping used in this paper is derived from this plot. (B) Al2O3 versus SiO2, Fe2O3 versus SiO2, and CaO versus SiO2 Harker dia-grams, as well as (K2O vs. SiO2)N diagram with fields of Peccerillo and Taylor (1976). (C) Rare earth element diagram normalizedto chondrite (McDonough and Sun, 1995) for the volcanic and plutonic rocks of the study area compared with the northern andcentral granites of Sonora reported by Valencia-Moreno et al. (2001). (D) Shands index diagram A/(NK) versus A/(CNK) fromManiar and Piccoli (1989) to characterize geochemical and tectonic environments of the studied plutons, including the ProterozoicEl Jacaln diorite and the Santa Margarita granite. Fields for island arc (IAG), continental collision (CCG), and continental arc(CAG) granites are indicated. (E) Rb versus (Y+Nb) diagram (Pearce et al., 1984) characterizing tectonic environment.
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Gonzlez-Len et al.
1408 Geosphere, December 2011
Chondrite-normalized rare earth element (REE)
patterns are primarily enriched in the light (L)
REEs and have unfractionated heavy (H) REE
patterns. LaN/Lu
Nratios are moderately fraction-
ated,