Contrib Mineral Petrol (1981) 78:379~88 Contributions to Mineralogy and Petrology Springer-Verlag 1981 Geochemistry of Ophiolites from the Chamrousse Complex (Belledonne Massif, Alps) J.L. Bodinier ~, C. Dupuy l, J. Dostal z, and F. Carme 3 1 Centre G6ologique et G6ophysique, Universit6 des Sciences et Techniques, Place Eug6ne Bataillon, F-34060 Montpellier, France 2 Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada 3 D~partement de G6ologie, Universit6 des Sciences, F-88039 Amiens, France Abstract. The ophiolite complex of Chamrousse (Belledonne Massif, Alps), consists of mafic to ultramafic cumulates and non-cumulates metamorphosed to amphibolite facies grade. The non-cumulitic rocks are similar in chemical composition to recent ocean-floor olivine tholeiites (both N-type and enriched P-type). The distribution oflithophile elements shows that the non-cumu- litic rocks represent several magmas of different parentage. The character of the magmas varies according to the time of emplace- ment. Geological and geochemical data suggest that the Cham- rousse complex was formed at a spreading oceanic ridge. Tile dynamic partial melting of an upper mantle diapir generated tholeiitic melt which decreased in amount and in REE contents. The first melt, enriched in light REE, was generated along the axis of the ridge while the second batch of melt, of lesser quantity and slightly depleted in light REE, was emplaced on the flank of the ridge. The third melt formed cross-cutting dikes with REE abundances typical of N-type (strongly light REE depleted) mid-ocean ridgc basalts. Introduction Along the "Ibero-Armorican Arc", the European Variscan belt contains a succession of generally allochthonous mafic and ultra- mafic complexes (Matte and Burg 1981) some of which may have an ophiolitic origin (Montigny and All6gre 1974). From southern Brittany to the eastern edge of the French Massif Cen- tral, these mafic and ultramafic rocks, interbedded with gneisses metamorphosed from amphibolite to granulite facies grade, form a more or less continuous unit (the "leptyno-amphibolitic group" of Forestier 1963 and Lasnier 1977) located along what is considered to be a former suture zone (Carme 1975a, Bard et al. 1980). A continuation of this unit has been recognized in the Alpine external crystalline massifs, especially in the north- ern and middle sections of the Belledonne area. Here, the lep- tyno-amphibolitic rocks consist mainly of tholeiitic rocks inter- bedded with metamorphosed graywackes. The mafic-ultramafic massif of Chamrousse is part of this formation. Among the ophiolitic massifs of the European Variscan belt, the Chamrousse complex (Carme 1975a, b) was selected for study because of the diversity of lithological types it contains and the relatively well-preserved magmatic textures in the rocks. Recent isotopic data obtained from the French Massif Central (Pin 1979) and Brittany (Peucat et al. 1981) suggests, by analogy, a lower Paleozoic to upper Proterozoic age for the Chamrousse rocks. The main purpose of the study is to present geochemical data on the Chamrousse complex and to discuss its geodynamic and petrogenetic evolution. Reprint requests to : J.L. Bodinier Fig. 1. Generalized geological map of the Chamrousse complex. 1 Zone I; 2zones IIa and I[b; 3zones IIIa and IIIb; 4 basement - lower series (pre-Devonian); 5 basement - upper series (upper Devonian to Visean); 6major tectonic contacts; 7minor tectonic contacts. The white area represents Stephanian to Quarternary sediments. Striped area of insert map shows the extent of the Alpine external crystalline massifs 0010-7999/81/0078/0379/$02.00
Transcript
Contrib Mineral Petrol (1981) 78:379~88 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1981
Geochemistry of Ophiolites from the Chamrousse Complex (Belledonne Massif, Alps)
J.L. Bodinier ~, C. Dupuy l, J. Dostal z, and F. Carme 3
1 Centre G6ologique et G6ophysique, Universit6 des Sciences et Techniques, Place Eug6ne Bataillon, F-34060 Montpellier, France 2 Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada 3 D~partement de G6ologie, Universit6 des Sciences, F-88039 Amiens, France
Abstract. The ophiolite complex of Chamrousse (Belledonne Massif, Alps), consists of mafic to ultramafic cumulates and non-cumulates metamorphosed to amphibolite facies grade. The non-cumulitic rocks are similar in chemical composition to recent ocean-floor olivine tholeiites (both N-type and enriched P-type). The distribution ofli thophile elements shows that the non-cumu- litic rocks represent several magmas of different parentage. The character of the magmas varies according to the time of emplace- ment.
Geological and geochemical data suggest that the Cham- rousse complex was formed at a spreading oceanic ridge. Tile dynamic partial melting of an upper mantle diapir generated tholeiitic melt which decreased in amount and in REE contents. The first melt, enriched in light REE, was generated along the axis of the ridge while the second batch of melt, of lesser quantity and slightly depleted in light REE, was emplaced on the flank of the ridge. The third melt formed cross-cutting dikes with REE abundances typical of N-type (strongly light REE depleted) mid-ocean ridgc basalts.
Introduction
Along the "Ibero-Armorican Arc" , the European Variscan belt contains a succession of generally allochthonous mafic and ultra- mafic complexes (Matte and Burg 1981) some of which may have an ophiolitic origin (Montigny and All6gre 1974). From southern Brittany to the eastern edge of the French Massif Cen- tral, these mafic and ultramafic rocks, interbedded with gneisses metamorphosed from amphibolite to granulite facies grade, form a more or less continuous unit (the "leptyno-amphibolit ic group" of Forestier 1963 and Lasnier 1977) located along what is considered to be a former suture zone (Carme 1975a, Bard et al. 1980). A continuation of this unit has been recognized in the Alpine external crystalline massifs, especially in the north- ern and middle sections of the Belledonne area. Here, the lep- tyno-amphibolitic rocks consist mainly of tholeiitic rocks inter- bedded with metamorphosed graywackes. The mafic-ultramafic massif of Chamrousse is part of this formation.
Among the ophiolitic massifs of the European Variscan belt, the Chamrousse complex (Carme 1975a, b) was selected for study because of the diversity of lithological types it contains and the relatively well-preserved magmatic textures in the rocks. Recent isotopic data obtained from the French Massif Central (Pin 1979) and Brittany (Peucat et al. 1981) suggests, by analogy, a lower Paleozoic to upper Proterozoic age for the Chamrousse rocks. The main purpose of the study is to present geochemical data on the Chamrousse complex and to discuss its geodynamic and petrogenetic evolution.
Reprint requests to : J.L. Bodinier
Fig. 1. Generalized geological map of the Chamrousse complex. 1 Zone I; 2zones IIa and I[b; 3zones IIIa and IIIb; 4 basement - lower series (pre-Devonian); 5 basement - upper series (upper Devonian to Visean); 6major tectonic contacts; 7minor tectonic contacts. The white area represents Stephanian to Quarternary sediments. Striped
area o f insert map shows the extent of the Alpine external crystalline massifs
0010-7999/81/0078/0379/$02.00
380
Table 1. The composition of several representative non-cumulitic rocks
[Mg]= Mg/Mg+Fe 2+; Fe 3 +/Fe 2+ standardized to 0.15.; amphibolite without relict magmatic textures: 756, 759, 7727, 801, 804; amphibolite with relict magmatic texture: 757, 758, 7728, 802, 8017, 8020; fine-grained gabbro and dolerite gabbro: 8012, 8013, 8014, 8015, 8022, 803
Geological Setting
The Chamrousse complex (Den Tex 1950; Carme 1970, 1973a, 1975 b; Meno t 1979) belongs to the pre-Stephanian metamorphic format ion of the Belledonne belt. The complex, which is exposed over 32 km and is 2 to 5 km wide, is located in the core of a late Variscan syncline (Fig. 1). It consists of a layered sequence including from top to bo t tom (Carme 1975b): - blastomylonit ic ultramafic and mafic banded metacumulates
broadly layered metagabbros and gabbros cut by a locally dense network of metadoleri t ic dikes -massive amphiboli tes (fine-grained metagabbros, metadoler- ites and metabasalts), striped amphiboli tes (metatuffs) and a few bands of metagraywackes
Al though refractory ultramafic rocks (e.g. harzburgite) are absent, the whole sequence has been interpreted as an overturned ophiolitic complex, thrusted over a Variscan tectonic uni t (Carme 1973b).
Petrographic and Structural Notes
In detail, the Chamrousse complex is complicated petrographi- cally and structurally (Carme 1975b). The complexity mainly
reflects the superimposed effect of mineral accumulation, crystal mush flow, solid state deformation and serpentinization. The Alpine deformation was limited to recurrent faulting of late Variscan structures.
On the basis of lithology, five zones (I, IIa, IIb, IIIa, IIIb) can be distinguished (Fig. 2). The serpentinized rocks of Zone I are separated from the underlying mafic cumulates of Zone IIa by a late shear zone. Other zones however grade into one an- other. Al though the thickness of each zone is variable, the maxi- mum thickness of the whole complex is approximately 2,000 m.
Zone I consists of a layered series of ultramafic rocks ( 95%) occasionally cut by dikes ( ~ 5%). Zone IIa is predominant- ly composed of blastomylonites of brown hornblende and/or plagioclase. This zone corresponds to a layered series of pyroxen- ite, gabbro and anorthosi te which underwent intense deforma- tion. Zone IIb differs f rom Zone IIa by less constant banding and more irregular but less intense deformation. Zone IIb is composed of gabbro of several distinct types, including gabbro with typical cumulate layering, broadly layered and heteroge- neous gabbro, and flaser gabbro. The spatial relationship among the various types of gabbro suggests successive episodes of defor- mat ion and magmatic injections. Zone IIIa is characterized by
numerous undeformed dikes cross-cutting bands of metagabbro and gabbro. Occasionally the gabbro shows orthocumuli t ic tex- ture. Some conformable layers of fine-grained gabbro and doter- itic gabbro are present in the banded metagabbro. The fine- grained gabbro occurs in the form of sills which were intruded after the main deformat ion of the layered sequence. Zone IIIb corresponds to the amphiboli t ic base of the complex where most pr imary magmat ic structures have been obliterated by deforma- t ion during the regional mesozonal metamorphism. It may repre- sent a volcanic series of effusive and pyroclastic rocks.
Dikes. All five zones were intruded by dikes. In Zone IIIa, they are particularly abundan t with thicknesses ranging from 0.5 to 3 m. In Zones IIIa and IIIb, the dikes were usually recrys- tallized with relict doleritic texture, and contain plagioclase and green hornblende. Some dikes, which clearly postdate most of the layered cumulates, occur in Zones IIa and IIb. They consist either of amphiboli t ized fine-grained gabbro or of clinopyroxen- ite containing olivine or plagioclase. The fine-grained dikes form several generations, which cut each other.
Relative chronology of the magmatic events : On the basis of field and petrographic observations three
magmat ic evolut ionary stages of the Chamrousse complex have
I
?257 g
Fig. 2. Schematic cross-section through the Chamrousse complex. Ro-
man numerals refer to the corresponding zones of the massif. 1 Meta- morphosed dunite and wehrlite ; 2 banded metapyroxenites ; 3 banded metagabbro and gabbro ; 4 coarse-grained gabbro ; 5 pyroxenite dikes ; 6 anorthositic and Ieucogabbroic dikes ; 7 fine-grained gabbro ; 8 meta- dolerite and metabasalt; 9 porphyritic metadolerite; 10 metagrayw- acke; 11 metatuff
been recognized. Dur ing the initial stage, the banded cumulates of Zones I and IIa, the banded metagabbros of Zone IIIa and massive amphiboli tes of Zone IIIb were formed. The intermedi- ate stage was characterized by the emplacement of most of the rocks of Zone IIb and fine-grained gabbroic sills which are par- ticularly abundan t in Zone IIIa. A narrow shear zone composed of flaser gabbro and mylonitic structures were also formed during this stage. The final stage which postdates the deformat ion under high temperature condit ions is marked by intrusions of cross- cutting dikes mainly in Zone IIIa.
382
Table 2. The composition of several representative dikes
[Mg]= Mg/Mg+FeZ+; Fe a +/Fe 2+ standardized to 0.15; doleritic dike: 8011, 8010, 807, 809, 7514, 805, 806, 808; pyroxenitic dike of zone I: 7510
Analytical Methods
Thirty-seven selected samples were analyzed for major and trace elements. Major elements and Rb, Sr, Ba, V, Cr, Co, Ni, Cu and Zn were determined by plasma emission spectroscopy at C R P G of Nancy. REE, Sc, Hf and Th were determined by instrumental neut ron activation. Based upon 6 determinations of s tandard rock W- l , the precision of the neut ron activation data is better than 5% for most elements except Tb for which it is 10-15%.
Geochemistry
The major and trace element composit ions of the analyzed rocks are given in Tables 1-3 where the samples are grouped according to petrographic type and relative age. In each group, the samples are arranged in order of decreasing [Mg] values (Mg/Mg + Fe 2 + atomic ratio) to illustrate the variat ions trends.
Al though the rocks underwent alteration and metamorphism, the systematic variat ion trends indicate that most elements con- sidered (with the exception of K, Na, Rb and Ba) were not affected by secondary processes.
Major Elements
Most of the rocks are olivine tholeiites. A few samples which contain quartz or nepheline in their norms correspond to differ- entiated rocks and plagioclase cumulates. On the A F M diagram,
the various petrographic types display a typical tholeiitic trend marked by an iron enrichment. A notable geochemical feature of the Chamrousse complex is the large range of the [Mg] ratios. The highest values (0.83-0.85) belong to the ultramafic rocks of Zone I while the lowest (0.3%0.50) are found in the differen- tiated rocks of Zone IIIb. The major element composi t ion of the ultramafic rocks suggests tha t the rocks are cumulitic ra ther than residual. The marie non-cumuli t ic rocks resemble recent ocean-floor tholeiites and other ophiolitic basalts.
As the [Mg] ratio in the rocks of the Chamrousse complex decreases, SiO2 varies only slightly while MgO and CaO sharply decrease and Fe and Ti increase. A1203 tends to decrease with differentiation and from the A1203 - [Mg] plot, two groups of cumulates can be distinguished: plagioclase cumulates having A1203 higher than 17% and ferric cumulates with less than 14% A1203.
Trace Elements"
Rare-ear th elements. The REE patterns normalized to chondrites are shown in Fig. 3 for the non-cumuli t ic rocks and in Fig. 4 for the cumulitic rocks. Regarding the noncumuli t ic rocks, the samples of Zone IIIb (Fig. 3 A) display, with one exception (sam- ple 8020), a sub-parallel REE pat tern characterized by a slight LREE enr ichment and fract ionat ion of HREE. These samples have small but distinct negative Eu anomalies due to the influ- ence of plagioclase. Their (La/Yb)N and (La/Sm)N ratios vary
Table 3. The composition of several representative cumulates
[Mg] -Mg/Mg+Fe2+; Fe 3 +/Fe 2+ standardized to 0.15
respectively f rom 1.8 to 2.7 and from 1.1 to 1.4. Both ratios increase with the differentiation suggesting pyroxene fractiona- t ion as indicated by the major element content. The fine-grained gabbro of Zone IIIa (Fig. 3B) and sample 8020 (Fig. 3A) bo th display lower LREE content and lower (La/Yb)N ratios (1.2 to 1.8). These ratios increase with differentiation but only one analyzed sample f rom this group (sample 8015) has a negative Eu anomaly. Most of the dike samples (Fig. 3 C) are character- ized by a depletion of LREE [ (La/Yb)N-0 .8 to 1.1]; this deple- t ion is ra ther distinct in a pyroxenitic dike of Zone I (sample 7510).
The analyzed samples show large differences in their REE pat terns and their abundances according to the zones and to the chronological emplacement. The rocks of Zone IIIb and to a lesser extent the fine-grained gabbros of Zone IIIa differ from the type N of oceanic tholeiites by their distinctly higher LREE content. In this respect, they resemble the types T and P of mid-ocean ridge basalts (Sun et al. 1979). On the other hand, the dike samples are similar to the N type of oceanic ridge basalts (Sun et al. 1979). Compared to Alpine ophiolites (Montigny 1975; Lewis and Smewing 1980; Sinewing and Potts 1976) most of the studied samples are higher in LREE. They are most similar to the Paleozoic ophiolites of the European Caledonian belt (Furnes et al. 1980) and to the Paleozoic and Proterozoic rocks of Newfoundland (Coish and Church 1979; Strong and Dostal 1980).
The cumulitic rocks also display variable REE patterns and contents (Fig. 4). F r o m the REE pattern, the cumulitic character is clearly shown in the ferrogabbro of Zone IIIa where in addit ion to Ti-magnetite accumulation, the positive Eu anomaly suggests the presence of cumulitic plagioclase. In the gabbro of Zone IIb, the REE pattern indicates the p redominan t accumulat ion either of plagioclase (sample 8019) or of pyroxene (samples 8025 and 751). The cumulitic character is less marked in the other samples because they probably contain intercumulus liquid.
Th, Hf, Rb, Ba and Sr. Th and Hf display a good positive correlation between themselves and with LREE. Their contents are low in cumulitic rocks and increase with the differentiation of the rocks. However, for a given [Mg] value, their content tends to be higher in the rocks of Zone IIIb. Rb and Ba do not correlate with other incompatible elements. Like K, they show large erratic variat ions a l though there is an overall positive intercorrelation. The K / R b ratio is relatively low, ranging in most samples between 290 to 350. The abundances of these relatively mobile elements may be affected by secondary pro- cesses such as metamorphism and alteration.
Transi t ion elements. In equivalent non-cumuli t ic rocks of the various zones, the abundances of transit ion elements (except Cu) are similar and vary systematically with the [Mg] values. Ni and Cr decrease towards the more differentiated rocks; this decrease is more pronounced for Cr than for Ni indicating that pyroxene is the dominan t crystallizing femic mineral. Sc remains
Fig. 3 A - C . R E E a b u n d a n c e s n o r m a l i z e d to c h o n d r i t e s (F rey et al.
1968) fo r n o n - c u m u l i t i c r o c k s o f zone I I Ib (A), zone I I I a (B) a n d
dikes (C)
relatively constant but V increases until [Mg]=0.5 then de- creases, thus corroborating the Ti-magnetite precipitation in the late stage of differentiation.
In the cumulitic rocks, the abundances of transition elements vary according to the proportion of the cumulitic minerals and the [Mg] values of the rocks which reflect the degree of differenti- ation. The main features are summarized on Fig. 5 where selected samples are normalized to ocean-floor basalts. It is noteworthy that ferrogabbro 755, which has a low [Mg] value, is depleted in Cr and Ni and enriched in Ti, V, Mn, Fe and Co. This enrichment is related to the accumulation of Ti-magnetite.
30
z O .I-
..~1o
5
20
10
g ~5
r
u~ 20 I--
z O "I"
.~.1o
5
D
E3~D 8024 D
u u"-~ D
I ~ I yI b Llu La Ce Sm Elu Tab
illl I\ B
, /"
I I I I ; y i b I La Ce Sm Eu b Lu
C
~ . ~
La Ce Sm Eu T b Lu
Fig. 4A42. REE abundances normaIized to chondrites (Frey et al. 1968) for cumulate rocks of zone IIIa (A), zone IIb (B) and zone IIa (C)
,5
lO /',,
, ' \ ,i
t '\ i ~ i '\ i' ',
./! i,,X i/'<\iiii i / , , i , I ' i ; '~ I I . '
i ~,..,~/.~A, _ . - ' \ ,, s * ~s / \ ; \ , i # ' / /~ , ,1,,'{ i , \ Y~
Fig. 5. T r a n s i t i o n e lement a b u n d a n c e s in c u m u l a t e r o c k s n o r m a l i z e d to m i d - o c e a n r idge basa l t s ( M O R B - B o u g a u l t a n d H e k i n i a n 1974)
385
Petrogenesis
Several features indicate that a process of fractional crystalliza- tion has played some role in the genesis of the Chamrousse complex. These features include the presence of cumulates asso- ciated with differentiated basic rocks and the large range of [Mg] values displaying regular variation trends when plotted against several major and transition trace elements. This fraction- al crystallization process involved mainly plagioclase and clino- pyroxene. In addition, olivine played a role in the early stage of differentiation and Ti-magnetite in the last stages. These phases are typical of common cumulitic rocks. However, the distribution of the incompatible elements (especially REE) sug- gests that the petrogenetic relationship among the various non- cumulate rocks is more complex. This is obvious from Fig. 6 where the large La/Yb ratio variation cannot be explained by a simple process of crystal fractionation of the same parental magma even if the differentiation trend had resulted only from the fractionation of clinopyroxene.
In Fig. 6, the samples are separated according to their litho- logical type, and this grouping suggests at least three parental magmas. In fact, a detailed study shows that samples from the same lithological type in many cases cannot be related by a simple fractional crystallization process. For example, the high content of lithophile elements in sample 8020 and their depletion in sample 802 compared to other Zone IIIb rocks of similar [Mg] values suggests a derivation from parental magmas with different compositions. The same seems to be true for the cumu- late rocks. Their large range of REE contents and La/Yb ratios probably indicate derivation from several parental liquids.
Two important aspects of the petrogenesis of the rocks of the Chamrousse complex will be treated in turn. The first aspect deals with the relationship between cumulate and non-cumulate rocks. The second one refers to the relationship among the var- ious parental magmas from which the rocks of the Chamrousse complex were derived.
Relationship Between the Cumulate and Non-Cumulate Rocks
The structural and field observations suggest that the banded cumulates of Zone IIa are genetically related to the amphibolites
of Zone IIIb while a similar relationship exists between the broadly banded cumulates (Zone IIb) and the fine-grained gab- bro of Zone IIIa. This hypothesis may be tested for several elements by calculating the bulk partition coefficients (D) during fractional crystallization of the rocks of Zone IIIb and Zone
C s Ilia. The D obtained were then applied to the equation ~ r = D
(C ~ and C~= concentrations in cumulates and liquids respectively) in order to calculate the composition of a cumulate in equilibri- um with the various liquids (cf. Fig. 7). The results plotted graph- ically for several samples show a good agreement between the calculated and the observed compositions of the cumulates. For the cumulates without typical cumulitic REE patterns, their trace element composition can be matched only by incorporating var- ious proportions of trapped liquids.
Relationship Between the Various Parental Liquids
The composition of the different parental liquids are plotted in Fig. 8. The assumed parental liquids correspond to the ana- lyzed unfractionated samples with [Mg] values > 0.68. These samples display a large range of La/Yb ratios (0.6 to 3.5) and of La content (1.0 to 12.9). Such a large variation cannot be explained by a variable degree of partial melting (F) of the same source. The calculation presented in Fig. 8 shows that liq- uids produced by partial melting (3 30%) of upper mantle peri- dotite having two times chondritic abundances have La/Yb ratios varying between 1.9 and 3.6. This variation range may be in- creased by the presence of garnet in the sources but in this case the REE pattern of the first liquid produced by melting would be significantly more fractionated than those of the ana- lyzed samples.
The large variation in composition encountered in the Cham- rousse massif may be explained either in terms of a heterogeneous upper mantle source or by dynamic partial melting of a homoge- neous source. A model of dynamic anatexis (Langmuir et al. 1977) which assumes melting of a rising mantle source with continuous but incomplete removal of melt was invoked for the origin of basalts from various oceanic environments (Dupuy et al. 1981). Model calculations of dynamic partial melting show that the first liquids produced (F~3-5%) have the composition of the
L a / Y b 0
0 A A
A A
O O
0 0 0
0 0 �9 0 A § a~
J /
.Sy." 5%
3'
7 >/"
/'lO %
/ / _~_--~, CPx
20% ~ *
3
I I 5 10
La p p m
, o ' 2'0
Fig. 6. La/Yb vs La diagram for non-cumulitic rocks. �9 rocks of Zone IIIa; o rocks of Zone IIIB ; z~ dolerite dikes ; �9 amphibolite dikes ; * cumulates of Zone I. The solid line represents the composition of the liquid produced by equilibrium partial melting of an upper mantle source having two times chondritic abundances. The degree of fusion (F) used for the calculation ranged from 3 to 30%. The composition of the parent: ol 0.54, opx 0.31, cpx 0.15; composition of the melt: ol 0.15, opx 0.35, cpx 0.50. The partition coefficients used are from Davis and Condie (1977). The dashed lines represent the variation of the residual liquid after fractional crystallization of clinopyroxene, plagioclase and olivine. Partition coefficients of Schnetzler and Philpotts (1970), Drake and Weill (1975) and Grutzeck et ai. (1975) were used. Two sets of values were used for clinopyroxene and plagioclase
386
20 -
. . . . 2s . . . . . . . . m6
8- l z A
6 ~ t ( 1 )
i I I I r i
2O
z 6 B Q
4 -- i i i r r I
= I i T ib yr b I La Ce Sm Eu Lu
Fig. 7A-C. Calculated composition of cumulates in equilibrium with various non-cumulitic rocks. The calculations were done according to the following equation: C~ = D ~/~ �9 C ~ where C~ = composition of cu- mulate, C~=composition of liquid, D~/~=bulk partition coefficient. In A, C~=sample 7727 (zone IIIb); in B, C~=sample 756 (zone IIIb); in C, Cl=sample 803 (zone IIIa). In A and C, D ~/1 was estimated using interelement correlations among non-cumulate samples from zone IIIb which were assumed to be related by fractional crystalliza- tion. D Th was taken as 0.01 (Wood et al. 1979). In B, D~/I=D pig. In A, 1 =cumulate; 2=cumulate with 10% liquid (Sample 7727). Solid lines represent the composition of the cumulates
rocks from Zone IIIb while the successive liquids are comparable to the fine-grained gabbro of Zone IIIa ( F ~ 9 % ) and dikes (F ~ 11-13 %). The last liquids (F ~ 19%) represent the composi- tion of the pyroxenite veinlets present in Zone I. This succession corresponds to the chronology of emplacement of the various lithological rock-types and is consistent with the dynamic melting model.
Discuss ion and Conclus ion
The Chamrousse complex has structural, lithological and geo- chemical characteristics typical of an ophiolitic sequence (Am- stutz 1980). Three major stages have been recognized during the emplacement of the massif. Stage I is characterized by the formation of banded cumulates (Zones I, IIa, IIia) overlain by effusive rocks. All these rocks are rather abundant in the complex and were probably derived from a parental melt with a REE pattern enriched in LREE. The near absence of the primitive and also evolved rocks in this sequence and the recurrence of olivine in the banded cumulates indicate the existence of an open magma chamber, periodically replenished with new batches of undifferentiated parental magma (O'Hara 1977; Church and Riccio 1977 ; Clarke and Loubat 1977 ; Dostal and Muecke 1978 ; Stern 1979). All the cumulates belonging to this stage were subse- quently affected by a tangential deformation more intense to- wards the bottom and by a hydrous metamorphism producing brown hornblende.
L a / Y b
I ~- '~ \'\'x \\ / / -.
l/j" "'-\ 4 / J
J 3% . /
3 . - - ' ~ , L.
/J'B / / / / 7% //~
13% _- - " " " "
1 ; 5 %
La ppm
O - - f i 10 20
Fig. 8. Composition of melts produced by dynamic partial melting of upper mantle peridotite with initially two times chondritic REE abundances. Initial melt formed by 3% melting. Subsequent liquids produced with 2% melting increment and 2% liquid remaining in the residue. Other parameters are the same as in Fig. 8. Stars with circles represent parental magmas. Samples with [Mg] > 0.68 were taken as the parent. In zone IIIb where the rocks are highly fractionated, the parental magma has been calculated by adding olivine, clinopyrox- ene and plagioclase in the proportions 2 : 1 : 1 in order to get a parent with [Mg]=0.7 and thus obtain F. Co was calculated using this F and the previously estimated D (Fig. 7). Dashed lines A, B, and C delineate the fields of zone IIIb, zone IIIa and dikes respectively
Stage II corresponds to be intrusion of a LREE-depleted liquid which generated cumulates (Zone IIb) and fine-grained gabbros (Zone IIIa). It differs from the previous stage by the following characteristics:
a) higher abundances of primitive rocks are associated with only a limited degree of fractional crystallization; in particular, Ti-magnetite did not crystallize and ferrogabbro is absent.
b) there are only few typical banded rocks among the flow lay- ered gabbros which form the main part of cumulates in Zone IIb.
c) during the tangential deformation which started in stage I, the cumulates were deformed before the complete solidification of the melts.
All these features favor the presence of a small temporary magma chamber.
Stage III is represented by the emplacement of the cross- cutting dikes throughout the entire complex. The dikes are even more depleted in LREE than the rocks of stage II. They corre- spond to primitive liquid in their composition and are not asso- ciated with any cumulates. These dikes are virtually undefonned with sharp contacts suggesting their emplacement at a relatively low temperature into already solidified surrounding rocks. At the end of the process, small amounts of strongly REE depleted melts formed veinlets in the cumulates of Zone I.
All these features suggest that the Chamrousse complex was generated within a spreading oceanic ridge. The dynamic partial melting of an upper mantle diapir generated tholeiitic melts
which successively decreased in amount and in LREE contents and distribution. The first melt, the most abundan t and the highest in LREE corresponds to stage I. I t was generated along the axis of the ridge and produced (by differentiation) cumulates at the bo t tom and magmat ic liquids at the top of the magma chamber. The cumulates were subsequently displaced laterally toward the flank of the ridge by tangential deformation.
The second batch of melt (stage II), limited in volume and with a L R E E depleted pattern, was emplaced in the f lank in a mobile environment . The newly formed magmatic chamber was rapidly destroyed, producing " f low layered" cumulate gab- bro. The associated liquids correspond to fine-grained gabbro which forms sills in the upper levels of the ophiolitic sequences. The emplacement of the melt of stage III is related to fracturing of the newly formed oceanic crust which was particularly inten- sive on the flank of the ridge (cf. Church and Riccio 1974). During stage III, the intrusion of melts into fractures produced cross-cutting dikes with REE abundances typical of the N-type of mid-ocean ridge basalts (Sun et al. 1979). The model described above resembles those invoked for some ophiolitic sequences by Stern and DeWit (1980) and also for the Mid-Atlant ic ridge by Wood et al. (1979) who suggested the presence of a lateral magmatic chamber.
Acknowledgements. This study was initiated by Dr. M. Mattauer and financially supported by Centre G~ologique et G6ophysique, Montpel- lier, the Natural Sciences and Engineering Research Council, Canada (operating grant A3782) and the Universit6 de Amiens. We thank Dr. W.R. Church for critical comments.
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