Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 124 23. GEOCHEMISTRY AND ISOTOPIC DATING OF CENOZOIC VOLCANIC ARC SEQUENCES AROUND THE CELEBES AND SULU SEAS 1 H. Bellon 2 and C. Rangin 3 ABSTRACT Geochemical data and whole-rock ^K-^Ar isotopic ages are presented for more than 50 igneous rocks (a majority of lavas and some plutonic bodies) sampled onshore (Philippine Archipelago: Tablas, Panay, Masbate, Mindanao, northern Borneo (Sabah), and north Sulawesi) around the Celebes and Sulu Seas. These data are compared with the ^K-^Ar ages obtained on drilled lavas along the Cagayan Ridge at ODP Sites 769 and 771 and with the major pyroclastic and tephras events recorded in the basins. Onshore ages range from 32 Ma to near 0 Ma for these rocks of generally calc-alkaline affinity with some shoshonitic high-K basalts. On the basis of geological data and kinematic reconstructions, two types of island arcs can be differentiated: those related to the progressive closing of the Celebes and Sulu marginal basins and those belonging to the Philippine Sea Plate. The combined age and chemistry for these two magmatic belts allow us to decipher the Neogene evolution of the complex zone of interaction of the Eurasian, Philippine Sea, and Australian plates. INTRODUCTION The Celebes and Sulu Seas marginal basins lie southeast of the South China Sea within the complex zone of junction of Philippine Sea, Indian, and Pacific plates. These two re- stricted northeast-trending basins are bounded (Fig. 1) along their eastern margins by the Philippine Archipelago, on their western edge by the island of Borneo, and on the south by the north arm of Sulawesi. From northwest to southeast, three ridges, the emerged Palawan Ridge, the submerged Cagayan Ridge, and Sulu Ridge are northeast-southwest trending. This last one is the locus of historical arc volcanism (Sulu Archi- pelago). This paper has two main objectives: (1) A presentation of new data (isotopic ages and geochemical compositions) ob- tained from onshore magmatism around these marginal basins and (2) a comparison of these data and those obtained by similar methods from the offshore magmatism studied during Leg 124 along the Cagayan Ridge (Rangin, Silver, von Brey- mann, et al., 1990; Pubellier et al., this volume) and during previous dredging along this ridge (Kudrass et al., 1986 and 1989) to ascertain the timing of the onshore and offshore magmatism. METHODS AND DATA REDUCTION Forty-seven lavas and ten plutonic rocks collected onshore around the two basins, and five lavas drilled at Sites 769 and 771 during Leg 124 were dated by 40 K- 40 Ar method. Results are listed in Table 1. Forty-three geochemical analyses are reported in Table 2. Sampling locations, types of rocks, and resumed isotopic age relationships are given in Table 3. A sketch map of the studied areas with the sample loca- tions (Fig. 2) shows that the offshore sampling is repre- sentative of the various magmatic arc assemblages surround- ing the Celebes and Sulu basins. 1 Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991. Proc. ODP, Sci. Results, 124: College Station, TX (Ocean Drilling Program). 2 Laboratoire de Géochimie et de Géochronologie, URA 1278 CNRS et GDR 910, Université de Bretagne Occidentale, 29287 Brest Cedex, France. 3 Laboratoire de Géologie Stucturale, URA 1315 CNRS T 26-01, Université Pierre et Marie Curie 4, place Jussieu, 75252 Paris Cedex, France. The Cagayan Ridge can be traced to Panay Island and probably also to Tablas. The Sulu Ridge extends to Sabah and the Zamboanga Peninsula. The north arm of Sulawesi is also the site of still-active volcanism in its eastern segment that started at least 5 m.y. ago. This segment can be extended northward to the Sangihe and Kawio islands. Several active volcanoes occur in the Sangihe Islands. Isotopic ages of 15.6, 5.7, and 2-0.9 Ma are reported by Morrice et al. (1983) for the Kawio Islands. This activity is related to the final stage of subduction of the Molucca Plate (Silver et al., 1983, Moore et al., 1982). A new subduction zone was recently developed along the northern side of the north arm of Sulawesi, initiating subduction of the Celebes Basin floor toward the south (Silver et al., 1983). Mindanao and Panay islands are mainly composed of Cainozoic island arc assemblages forming the backbone of the Philippine Mobile Belt (Gervaiso, 1971). This belt is composed of fragments of the Cagayan, Sulu, and north Sulawesi island arc terranes but mainly of exotic volcanic arc terranes inter- preted as parts of the Philippine volcanic arc (Rangin and Pubellier, 1990). This arc was formerly attached to the Phil- ippine Sea Plate before incipient subduction along the Philip- pine Trench occurred in early Pliocene time (Rangin et al., 1990a, Aurelio et al., 1990). The Philippine Mobile Belt is not composed not only of the fragments of the Philippine Sea Plate but also of minor fragments of the Cagayan-Sulu and north Sulawesi island arcs (Rangin et al., 1990a). Isotopic Ages: Experimental Procedures and Age Calculations Analyses were performed on whole-rock samples (except one from Panay, for which separated biotites and whole rock were both analyzed). The 0.5- to 0.16-mm size fraction was selected after crushing and sieving, and was carefully cleaned with distilled water. Potassium was determined by atomic absorption techniques from solutions of powders made from splits from this grain fraction. Argon extraction from an average 1-g sample wrapped in an aluminum foil with an average weight of 0.125 g was performed by induction heating under high vacuum. Cleaning of active gases was achieved by a series of titanium furnaces 321
Transcript
Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 124
23. GEOCHEMISTRY AND ISOTOPIC DATING OF CENOZOIC VOLCANIC ARC SEQUENCESAROUND THE CELEBES AND SULU SEAS1
H. Bellon2 and C. Rangin3
ABSTRACT
Geochemical data and whole-rock ^K-^Ar isotopic ages are presented for more than 50 igneous rocks (amajority of lavas and some plutonic bodies) sampled onshore (Philippine Archipelago: Tablas, Panay, Masbate,Mindanao, northern Borneo (Sabah), and north Sulawesi) around the Celebes and Sulu Seas. These data arecompared with the ^K-^Ar ages obtained on drilled lavas along the Cagayan Ridge at ODP Sites 769 and 771 andwith the major pyroclastic and tephras events recorded in the basins. Onshore ages range from 32 Ma to near 0 Mafor these rocks of generally calc-alkaline affinity with some shoshonitic high-K basalts. On the basis of geologicaldata and kinematic reconstructions, two types of island arcs can be differentiated: those related to the progressiveclosing of the Celebes and Sulu marginal basins and those belonging to the Philippine Sea Plate. The combined ageand chemistry for these two magmatic belts allow us to decipher the Neogene evolution of the complex zone ofinteraction of the Eurasian, Philippine Sea, and Australian plates.
INTRODUCTION
The Celebes and Sulu Seas marginal basins lie southeast ofthe South China Sea within the complex zone of junction ofPhilippine Sea, Indian, and Pacific plates. These two re-stricted northeast-trending basins are bounded (Fig. 1) alongtheir eastern margins by the Philippine Archipelago, on theirwestern edge by the island of Borneo, and on the south by thenorth arm of Sulawesi. From northwest to southeast, threeridges, the emerged Palawan Ridge, the submerged CagayanRidge, and Sulu Ridge are northeast-southwest trending. Thislast one is the locus of historical arc volcanism (Sulu Archi-pelago).
This paper has two main objectives: (1) A presentation ofnew data (isotopic ages and geochemical compositions) ob-tained from onshore magmatism around these marginal basinsand (2) a comparison of these data and those obtained bysimilar methods from the offshore magmatism studied duringLeg 124 along the Cagayan Ridge (Rangin, Silver, von Brey-mann, et al., 1990; Pubellier et al., this volume) and duringprevious dredging along this ridge (Kudrass et al., 1986 and1989) to ascertain the timing of the onshore and offshoremagmatism.
METHODS AND DATA REDUCTIONForty-seven lavas and ten plutonic rocks collected onshore
around the two basins, and five lavas drilled at Sites 769 and771 during Leg 124 were dated by 40K-40Ar method. Resultsare listed in Table 1. Forty-three geochemical analyses arereported in Table 2. Sampling locations, types of rocks, andresumed isotopic age relationships are given in Table 3.
A sketch map of the studied areas with the sample loca-tions (Fig. 2) shows that the offshore sampling is repre-sentative of the various magmatic arc assemblages surround-ing the Celebes and Sulu basins.
1 Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991. Proc. ODP,Sci. Results, 124: College Station, TX (Ocean Drilling Program).
2 Laboratoire de Géochimie et de Géochronologie, URA 1278 CNRS etGDR 910, Université de Bretagne Occidentale, 29287 Brest Cedex, France.
3 Laboratoire de Géologie Stucturale, URA 1315 CNRS T 26-01, UniversitéPierre et Marie Curie 4, place Jussieu, 75252 Paris Cedex, France.
The Cagayan Ridge can be traced to Panay Island andprobably also to Tablas. The Sulu Ridge extends to Sabah andthe Zamboanga Peninsula.
The north arm of Sulawesi is also the site of still-activevolcanism in its eastern segment that started at least 5 m.y.ago. This segment can be extended northward to the Sangiheand Kawio islands. Several active volcanoes occur in theSangihe Islands. Isotopic ages of 15.6, 5.7, and 2-0.9 Ma arereported by Morrice et al. (1983) for the Kawio Islands. Thisactivity is related to the final stage of subduction of theMolucca Plate (Silver et al., 1983, Moore et al., 1982). A newsubduction zone was recently developed along the northernside of the north arm of Sulawesi, initiating subduction of theCelebes Basin floor toward the south (Silver et al., 1983).
Mindanao and Panay islands are mainly composed ofCainozoic island arc assemblages forming the backbone of thePhilippine Mobile Belt (Gervaiso, 1971). This belt is composedof fragments of the Cagayan, Sulu, and north Sulawesi islandarc terranes but mainly of exotic volcanic arc terranes inter-preted as parts of the Philippine volcanic arc (Rangin andPubellier, 1990). This arc was formerly attached to the Phil-ippine Sea Plate before incipient subduction along the Philip-pine Trench occurred in early Pliocene time (Rangin et al.,1990a, Aurelio et al., 1990).
The Philippine Mobile Belt is not composed not only of thefragments of the Philippine Sea Plate but also of minorfragments of the Cagayan-Sulu and north Sulawesi island arcs(Rangin et al., 1990a).
Isotopic Ages: Experimental Procedures and AgeCalculations
Analyses were performed on whole-rock samples (exceptone from Panay, for which separated biotites and whole rockwere both analyzed). The 0.5- to 0.16-mm size fraction wasselected after crushing and sieving, and was carefully cleanedwith distilled water. Potassium was determined by atomicabsorption techniques from solutions of powders made fromsplits from this grain fraction.
Argon extraction from an average 1-g sample wrapped inan aluminum foil with an average weight of 0.125 g wasperformed by induction heating under high vacuum. Cleaningof active gases was achieved by a series of titanium furnaces
321
H. BELLON, C. RANGIN
- 2 0 "
Marginal oceanic basinsOpened Within the Eurasian plate
Fragments of Eurasian affinity
Fragments of possibleEurasian affinity
Fragments of Philippinesea plate affinity
Active convergent zone orintraplatβ Thrusting
Uπactive Thrust zone
Major strike slip fault zone
Leg 124 Sites
Philippine Sea
05•
Figure 1. Simplified structural map of the Philippines and of the Sulu and Celebes oceanic basins opened within the Eurasian Plate. Suspectedorigins for the structural units of the Philippine Mobile Belt are distinguished: Eurasian- or Philippine Sea Plate affinities (after Rangin et al.,1990a). The study area is outlined. P = Panay, and in Mindanao: S = Surigao, Co = Eastern cordillera, CCM = central cordillera, D = DagumaRange, Zb = Zamboanga.
322
CENOZOIC VOLCANIC ARC SEQUENCES
and a final gettering by two Al-Zr getters. The argon isotopiccomposition was measured by mass spectrometry with areference to air-argon composition measured in the same wayafter each sample to correct mass discrimination effects.
Radiogenic 40Ar was determined by isotopic dilution usingan original procedure described in (Bellon et al., 1981), wherea 38Ar spike is buried as ions in an aluminum foil target. Eachtarget, with a precise 38Ar volume regularly calibrated withstandard samples (Glauconite Gl-O)(Cassignol et al., 1977)was added to the sample at the time of weighing. Conse-quently, isotope dilution was achieved during fusion of thesample.
Isotopic ages reported in Table 1 are calculated using theconstants recommended by Steiger and Jàger (1977). Errorsare estimates of the standard deviation of precision and arecalculated using the extension of the Cox-Dairymple (1967)error equation proposed by Mahood and Drake (1982). Theextended equation includes an additional term (1%) for thespectrometer mass discrimination. Analytical parameters(40ArR(radiogenic); % 40ArR; 36Ar) are only related to thesample, i.e. each value is corrected for nonradiogenic Ar (40Arand 36Ar) of atmospheric composition linked to the wrappingaluminum foil (36Ar = 3.5× 10~10 cm3) and to the blank line thatvaries for a set of nine samples from 36Ar = 7× 10~10 cm3 (inthe first extraction) to 36Ar = 1.2<241010 cm3 (in the ninthextraction). Repeated blank analyses were processed in thesame way as for geologic samples, using only an aluminum foiltarget with buried 38Ar ions.
Geochemistry of Igneous RocksData from Table 2 are displayed on a K2O vs. SiO2 diagram
(Fig. 3) with the superimposed fields of arc tholeiitic, calc-alkaline, high-K calc-alkaline, and shoshonitic suites of Pec-cerillo and Taylor (1976), modified after Maury (1984). Notethat in both diagrams (3A and 3B), full symbols mark rocksyounger than 8 Ma.
The lavas span a range from basalts to rhyodacites. InFigure 3A are plotted dated lavas and plutonic rocks fromwestern Panay, Tablas, and Sabah, and the drilled lavas alongthe Cagayan Ridge. Many of the rocks are located within thecalc-alkaline field or slightly overlap its upper limit.
Onshore rocks from Eastern Panay, Mindanao, and Mas-bate are plotted in Figure 3B.
Rocks that plot within the arc tholeiitic field have beenexamined on the basis of their FeO2*/MgO (FeO2* = total ironas FeO). According to Gill (1981), arc tholeiitic rocks are lyingabove aline FeO*/MgO = 0.1562 × SiO2 - 6.685. Only a fewanalyzed rocks belong to an arc tholeiitic suite: a young basaltfrom Sabah (Sample S 129), a gabbro into a melange fromDent Peninsula (S 87-34) and a diorite (MNO 89-26A) fromDaguma Range in Mindanao. Other rocks from this fieldbelong to a low-K calc-alkaline suite.
Some shoshonitic basalts are associated in space and timewith calc-alkaline suite as in western Panay (Fig. 3A), easternPanay, and among the young volcanoes in Mindanao (Fig.3B).
Figure 4 shows the normalized incompatible elementspatterns for some selected basalts (4A) and andesites (4B).Large variations in Rb, Ba, K, and Sr concentrations (from 10to 300 times the chondritic values) are evidenced in Figure 4A.P and Ti element abundances also show a large range (8 to 70times the chondritic values for phosphorus and 7 to 25 timesfor titanium).
Arc tholeiitic basalt from Sabah (Sample S 129) has atypical flat pattern, normalized values being 15 to 25 times thechondritic values.
Incompatible elements patterns for andesites span a nar-row range in comparison with those for basalts; normalizedabundances of Rb, Ba, and K are between 40 and 150 timesthe chondritic values; those of Sr, P, and Ti are respectively25 to 60, 15 to 35, and 6 to 15 times the chondritic values. Thepatterns for Rb, Ba, and K are almost flat.
PANAY
General Geological OverviewsPanay Island (Fig. 5) belongs to the Philippine Mobile Belt
and to the Negros arc. In western Panay, the Antique Range isformed by northeast-elongated island arc terranes with ophio-lites and melanges, which are thrusting over the Cuyo shelf. Thisarc outcrops in the Buruanga Peninsula. The Antique Range isthe northernmost extension of the Negros Trench inner wall.The Iloilo Basin onlaps westward on the Antique Range upper-most unit and eastward on the Eastern Range.
A detailed stratigraphic and structural study of the differentunits completed by isotopic dates of volcanics (Rangin et al., inpress) shows that this sequence has formed by the collision ofseveral Tertiary volcanic arcs with the North Palawan block, acontinental rifted fragment of the Eurasian continental margin.
Field Identification of the Arc UnitsFrom north to south, the Antique Range is composed of:
1. The unit at the northwestern tip of the BuruangaPeninsula, that is made of a mainly pre-Tertiary metamorphicbasement intruded by later plutonic bodies (Patria and east ofPandan);
2. A northeast-trending, mainly volcanic terrane, the Baloyunit extending from Naba to Cubay River formed by volcanicrocks overlain by a detrital formation with interlayered basalts(Mayos Fm);
3. Another northeast-trending volcanic terrane, the Valder-rama unit extending from Cubay River to Pampanan thrustwas subdivided in two series: a volcanic and volcano-sedi-mentary one (Lagdo Fm) and its basement made of Creta-ceous ophiolites (Lombonero Ridge) and a late Paleogenemelange (Panicuan).
The Iloilo unit consists of a clastic basinal sequence (IloiloBasin) that rests on a volcanic basement. This basement isexposed east and west of this north-south-trending elongatedundeformed basin. On its eastern edge, the Iloilo Basin restson the Eastern Range formations including volcanic andsedimentary sequences intruded by plutonic bodies (Saradiorite). Along its western edge, in the Antique Range, sedi-ments are conformably deposited on volcanic rocks thrustedwestward on the Valderrama unit. The basin ends as a "cul desac" in the Dumalog area (Fig. 5) in central Panay.
Isotopic AgesFifteen selected samples (Tables 1 and 3) for 40K-40Ar
whole-rock dating yield ages ranging between 30 and 12.4 Ma.According to their field locations (Fig. 5), the data may beseparated into three groups.
The oldest dates (30 to 21.5 Ma) were obtained on thevolcanic rocks from the Iloilo Basin basement. This includeslavas from the Dumalog area that were all dated at 30 to 26 Ma.Ages from volcanics collected along the western flank of thebasin are constrained by biostratigraphic data; in the TipuluanRiver up to a thousand meters of volcanic breccias with interca-lated flows and argilitic layers were dated lower Miocene.Between San Jose and Tigmarabo, several hundred meters ofagglomerates and flows cap 1200 m of graywackes, siltstones,
323
H. BELLON, C. RANGIN
Table 1. Conventional 40K-40Ar Age Data on Samples from onshore magmatism (Panay, Tablas,Sabah, Mindanao, Masbate) and offshore volcanism (Cagayan Ridge, Leg 124, Sites 769 and 771).
(a)Sample
P 83-57P 83-91P 83-93P 83-94P 83-97
P 87-39aP 87-43a
P 83-114 (RT)*P 83-114 (B)P 83-117 *
P 83-47eP 83-29P 83-46P 83-19
P 83-22a
P 87-67b *
TA31TA32TA 130TA43TA 134
SSUDE
S 129
S 87-92
S 84-11 *S 84-16 *
S 87-34S 264-1S 264-3S267S266S268
S 87-91S 87-90S 285-2S 138-1
MNO 89-16BMNO 16AMNO 37MNO 89-17
(b)Calculated Age
± error
13.1112.4013.3413.7213.83
14.9822.87
19.4920.8020.85
21.5123.6725.3526.20 :25.75 :30.96 :29.20 :
29.69 :
3.78 j
± 0.91± 0.84± 0.76± 0.82± 1.16
± 0.52± 0.75
± 0.47± 0.61± 0.87
± 0.55± 1.33± 0.70± 0.54± 0.51± 0.57± 0.52
± 0.58
: 0.284.39 ± 0.4718.01 :18.48 :19.86 :
0.01 i0.06 i2.79 i3.11 ±2.94 i3.29 i
6.43 ±6.84 ±
11.47 :
t 0.41t 0.36t 0.50
: 0.40: 0.40: 0.58: 1.11: 1.30: 1.06
(c and d)4 0Ar R (a) 3°Ar
(I0"7 cm3/g) (HT* cπrVg)
PANAY
Valderrama13.193.493.248.485.86
Baloy Mount4.618.90
Buruanga17.7552.4111.02
Iloilo Basement14.656.14
12.0228.5028.0134.6332.64
Eastern Range16.89
TABLAS
1.101.436.547.248.30
SABAH
Young Volcanics0.000.010.270.301.501.68
11.582.992.256.286.34
1.692.95
3.2614.245.17
4.124.212.943.082.441.540.99
1.29
1.052.011.040.521.70
0.620.810.761.479.057.38
Outer Belt : Kinabalu Intrusives: 0.30: 0.26
6.678.90
3.633.72
Inner Belt: Dent Peninsulat 0.81
11.53 ± 0.4712.58 it 0.3311.07 ± 0.3312.69 ± 0.5611.69 i
a: Asterisk that follows the sample code refers to its plutonic origin, b: Ages are calculated using constantsrecommended by Steiger and Jüger (1977).c: Calculated concentrations only refer to sample. Blank valuesare deduced (see text for explanations), d: Subscript R means radiogenic Ar.
tuffs and conglomerates with calcareous levels bearing Foramin-ifers lenses of upper Oligocene to late lower Miocene. Underly-ing volcanic rocks range in age from 25.3 to 21.5 Ma. Lavaswithin the Eastern Range are probably older than 30 Ma as theyare intruded by dioritic bodies (Sara) dated 29.7 Ma.
The second set of ages, which ranges from 23 to 15 Ma,includes:
1. The intrusive of the Buruanga unit (Patria and TimbabanRiver near Pandan). Ages are from 20.9 to 19.5 Ma, compa-rable with the age (20.8 Ma) of Tapian Mine tonalite onMarinduque island (Walther et al., 1981);
2. Volcanics of the Mt. Baloy unit were collected into twodistinct thrust slices: one from the Tibiao River (22.9 Ma), andthe other younger (15 Ma) in the Dalanas River.
3. The third group of ages (13.9 to 12.4 Ma) concernsvolcanics from the Valderrama unit: 1. In the CangarananRiver, 5 km upstream of Valderrama; 2. In the Inyaman River,
that grooves Kipott Mount, made of volcanic breccias over-lying limestones containing Zone NN9 nannofossils.
Geochemical CompositionsSeveral dated samples have been analyzed for their major
elements and some trace elements (Table 2). Except threebasalts (two from Iloilo Basin and one from the Valderramavolcanic sequence), the lavas are andesitic and plot in a K2Ovs. SiO2 diagram (Figs. 3A and 3B) in the medium-K calc-alkaline field. Plutonic bodies of dioritic to granodioriticcomposition fall in the lower part of the high-K calc-alkalinefield as does the Marinduque tonalite. All the rocks have atypical orogenic parentage.
Basalts are of special interest because they plot in theshoshonitic field. They have K20/Na20 ratios ranging from 1.6to 3 and high Cr contents (150-230 ppm). Those from Iloilobasement have low TiO2 contents (0.67%) and high Ba con-tents (> 1000 ppm) and can be tentatively related to the
325
H. BELLON, C. RANGIN
Table 2. Major- and trace-element concentrations of 43 samples from onshore magmatism and 4 offshore lavas drilled at Sites 769and 771 during Leg 124. Analyses were performed at UBO by J. Cotten, analyst, using atomic absorption. Geochemical data arepresented following the different geographical origins of sampling. LOI = Loss on ignition.
(a)Sample
(wt%)SiO2
TiO2
A12O3
Fe2O3
MnOMgOCaONa2OK2OP2O5
TOTAL
LOI(ppm)SrRbBaNiCrV
14P
83-91
54.900.61
17.368.350.173.898.642.780.890.35
99.74
1.81
63818
1584027
238
-Valderram
P
83-92 !
61.500.61
15.624.030.120.877.503.082.490.35
99.39
3.22
26061
2551550
165
15P
$3-57
50.001.27
19.577.230.124.839.432.832.650.40
99.25
0.92
75195
3331523
368
10P
87-39A
53.000.66
18.087.080.165.758.802.201.040.15
99.59
3.80
32911
1955444
275
9P
87-43A
54.800.68
17.807.340.133.558.422.921.200.15
100.17
3.28
44315
2751013
190
PANAY
P
D
8P
7P
87-48A 83-114* 83-117*
47.201.70
15.8811.300.178.109.032.890.940.35
100.02
2.46
30722
245170230200
64.550.60
15.964.840.092.384.053.212.810.40
99.68
0.79
309100417
1550
117
59.300.58
17.095.920.103.376.863.701.650.35
99.38
1.06
52134
1941548
205
2P
83- 29
58.000.98
16.627.890.163.307.343.350.790.30
99.94
1.21
25927
1362530
206
Iloilo6P
83-19
50.050.66
12.8310.320.177.59
10.071.963.150.45
100.02
2.77
75167
124853
233300
5P
83-22A
49.500.61
12.5510.100.207.96
10.631.543.610.50
99.48
2.28
160581
101246
148271
E.Range1P
87-67B
55.500.64
17.378.330.163.747.913.001.750.20
99.%
1.36
59734
472206
240
SABAH - MASBATE
(wt%)SiO2
TiO2
A12O3
Fe2O3
MnOMgOCaONa2OK2OP2O5
TOTAL
LOI(ppm)SrRbBaNiCrV
Young Vole13S
87-92
58.401.06
16.197.560.153.394.473.481.580.25
99.22
0.69
35055
371406880
12S
129
52.002.27
14.1213.100.156.076.603.060.250.15
98.57
0.80
1629
77150232167
Kinabalu11S
84-11
60.200.64
16.655.920.142.395.512.982.850.20
99.17
1.69
46612
4098
13153
10
s87-34
48.500.69
18.928.060.175.32
10.032.920.480.05
100.05
4.91
3266
711633
280
D<9S
264-1
53.100.98
17.588.260.143.758.922.541.520.30
99.20
2.11
45761
3351910
271
*nt5S
268
62.200.57
15.855.310.102.645.133.212.300.20
99.95
2.44
482100439
1930
121
6S
266
62.200.64
16.695.440.081.926.513.012.110.25
99.72
0.47
42384
3651724
167
4S
87-91
62.350.50
15.985.880.112.466.002.732.310.12
100.16
1.72
27081
3128
10140
Semporna3S
87-90
64.300.54
15.535.260.102.234.843.062.530.10
100.23
1.74
24590
3498
14120
1S
138-1
59.500.77
16.006.660.142.956.052.891.660.10
98.63
1.91
21056
43093
182
2MAS
88-69
61.350.82
16.353.350.201.013.424.263.860.30
100.07
2.15
43673
76043
110
b<itci
MAS88-79A
54.000.94
18.448.980.181.987.213.782.170.35
99.34
1.31
50039
4606
10250
326
CENOZOIC VOLCANIC ARC SEQUENCES
Table 2 (continued).
TABLAS
Y((a)
Sample
(wt%)SiO2
TiO2
A12O3
Fe2O3
MnOMgOCaONa2OK2OP2O5
TOTAL
LOI(ppm)SrRbBaNiCrV
>ung arc-5
TA31
60.900.54
15.775.020.113.975.583.950.900.08
97.64
0.82
42930
24848
135-
4TA32
65.500.49
15.934.360.102.794.724.301.010.05
99.78
0.53
42528
2881831
-
3TA130
60.000.49
17.465.170.093.435.673.371.120.01
99.78
2.97
26726
1641325
-
-Old arc2
TA43
61.350.57
16.085.560.143.315.503.131.210.05
99.91
3.21
26818
3273
17-
1TA134
69.500.43
13.753.350.080.754.103.491.290.15
98.49
1.60
23222
15606-
SULU SEA Sites 769C and 771A
(wt%)SiO2
TiO2
A12O3
Fe2O3
MnOMgOCaONa2OK2OP 2 O 5
TOTAL
LOI(ppm)SrRbBaNiCrV
769C7R-1
118-120 cm
55.700.99
17.017.570.163.857.703.231.220.30
99.51
1.78
27328
1583147
223
769C9R-3
110-113 cm
58.400.97
16.317.460.163.366.813.251.450.25
100.06
1.64
24832
1452016
228
769C9R-4
49-50 cm
52.400.98
19.357.050.153.978.273.521.780.40
100.30
2.43
30817
1453541
238
771A11R-2
91-94 cm
46.900.68
14.669.590.238.56
12.351.560.180.10
99.72
4.91
2933
35105221278
MINDANAO
(a)Sample
(wt%)SiO2
TiO2
A12O3
Fe2O3
MnOMgOCaONa2OK2OP2O5
TOTAL
LOI(ppm)SrRbBaNiCrV
14MNO
89-16B
49.801.07
15.228.550.149.608.553.091.300.30
98.95
1.33
57820
215236426140
13MNO
89-16A
52.501.01
18.897.820.143.017.463.653.060.75
98.97
0.68
64372
6351735
230
olcanoes —12
MNO89-37
54.000.93
17.389.670.194.068.233.111.780.40
99.77
0.02
64034
3921316
260
11MNO89-17
53.501.42
16.388.550.135.877.503.600.980.30
99.22
0.99
54016
18483
142175
10MNO88-46
65.650.42
16.203.080.062.053.465.302.770.15
100.06
0.92
94642
624314850
o _
9MNO88-49
58.000.53
18.266.600.162.457.004.081.310.15
99.80
1.26
50622
36744
140
4MNO88-54
49.500.80
17.509.690.184.10
10.182.611.320.25
99.83
3.66
58311
1091541
270
S. D8
MNO89-21A
63.900.30
16.163.650.080.982.624.863.520.15
99.20
2.98
30372
725
60
7MNO
89-2 IB
60.700.42
17.164.660.101.993.874.862.380.25
99.43
3.04
55542
527142885
—C.Cordill<
6MNO89-12
55.300.87
18.187.460.163.858.243.040.650.30
99.65
1.60
32020
1322339
180
*m3
MNO89-20
60.850.69
16.725.290.072.114.693.662.420.30
99.22
2.42
41149
580
120
5MNO89-25
49.501.14
17.7610.190.193.826.984.370.540.08
99.13
4.66
3039
4553
260
-Daguma Range2
MNO89-26A
55.351.06
16.679.900.203.397.504.110.670.05
99.49
0.59
19810
12243
260
1MNO
89-26B
60.200.82
16.576.180.172.317.243.100.160.05
99.01
2.21
2512
66--
140
327
H. BELLON, C. RANGIN
Table 3. Summary of data on onshore magmatism including: rock-types deduced from Table 2and Figures 3A and 3B, isotopic ages from Table 1, field locations and site sampling numbersfrom Figure 2.
MK C A andesiteMK C A daciteLK CA andesiteHK C A granodioriteHK C A graniteHK C A granodiorite
Age(Ma)
19.9
16.729.931.9
9.710.6
4.14.46.98.8
18.522.2
Field Location
Tangkulang Mt
SalonicTalubTalub
Along theSibuyan Sea fault
North of BilungalaEast of BilungalaWest of GorontaloBetween Marizaand GorontaloSoutheast of Soronga
SamplingSite
(Figure 2)
3
521
21
654321
Abbreviations for rock-types: A.T., arc tholeiitic series; CA., calc-alkaline series with LK: low-Kcalc-alkaline rock; MK, medium-K calc-alkaline; HK, high-K calc- alkaline; SH, shoshonitic series.
Serawagan pillow basalts (from southwestern Panay) de-scribed by Santa Cruz et al. (1989). The shoshonitic basaltsample from Valderrama has a higher TiO2 content (1.27%)and a lower Ba content (333 ppm).
Conclusions
The isotopic ages obtained on orogenic lavas and plutonicbodies on Panay Island allow one to distinguish three maindiachronous episodes of arc activity:
1. The oldest ages belong to the Iloilo volcanic and plutonicbasement with a magmatic activity ranging in age from 30 Ma(or even before) to 21.5 Ma.
2. The second episode is concentrated in the Buruanga andthe Mt. Baloy units of northern Panay. It stops around 15 Ma.The oldest ages for the Mt. Baloy unit slightly overlap theyoungest ages for the Iloilo basement.
3. These volcanic units are spatially and tectonically sep-arated by the Valderrama unit that appears as a younger(13.9-12.4 Ma) and a distinct volcanic episode.
TABLAS
Recent field work on Tablas Island (Marchadier, 1988;Marchadier and Rangin, 1989) has shown that a large and thickvolcaniclastic series overlies a metamorphic basement. Itforms a north-south-trending dome across the whole islandand it consists of a lower volcanic sequence conformablyoverlain by a volcaniclastic sequence and pillow basalts, andminor volcaniclastic sediments containing Zones NN6-NN9calcareous nannoplankton. Two acid andesites and a daciterecovered at the base of the lower level of the central part ofthe island (Tugis River) yield ages of 19.9-18 Ma. A secondvolcaniclastic sequence overlies disconformably the metamor-phic basement in the southern part of the island, south toAlcantara Village. An andesite and a dacite from this se-quence have yielded more recent ages of 4.4 and 3.8 Ma.Isotopic dates clearly show that two arc episodes have oc-curred in Tablas.
One can note the overall similarity in geochemistry of thecalc-alkaline acid andesites of both episodes (Table 2 and Fig.3A).
The oldest volcanic episodes that ceased between 18 and 14Ma (Zone NN6) can be compared with the Mt. Baloy volcanicsequence in Panay.
The early Miocene volcanic belt extending from Tablas tonorthern Panay Island can be traced offshore in the Sulu Seaalong the Cagayan Ridge (Fig. 2).
ISOTOPIC AGES OF VOLCANIC ROCKS DRILLEDALONG THE CAGAYAN RIDGE
The east-northeast submerged Cagayan Ridge within theSulu Sea separates the outer basin from the inner one. Thisridge was the locus of volcanic activity that has emittedbasalts and porphyritic andesites. Their chemical composi-tions indicate that this ridge originated as a volcanic arc.Isotopic dates were determined on dredged volcanic rocks(Kudrass et al., 1986; Kudrass et al., 1990) and have yieldedthe following results:
1. At the southern end of the ridge, near Meander Reef, abasaltic andesite is dated 14.7 ± 0.6 Mα
2. Volcanic rocks at its northern end are dated 20-14 Ma(Site SO 49-55) or much older (SO 49-59); whole-rock ages ofa porphyritic andesite and a dacite range from 50.5 to 22.0 Maand separated minerals yield ages as old as 158, 60, and 36 Mafor amphiboles and 25.8-23.9 Ma for Plagioclase. If results foramphibole are interpreted as proving their xenolithic charac-ter (metamorphic basement mechanically mobilized by volca-nism) (Kudrass et al., 1990), one may remark that Site SO49-59 is not far from the axis of a large canyon flowing fromsouthern Panay (Rangin and Silver, this volume). Conse-quently, these dredged lavas can belong to southern Panayisland where older volcanic rocks outcrop.
These data have been completed with ages determined on fivedrilled lavas during Leg 124, at ODP Sites 769 and 771, and listedin Table 1. Four lavas have been analyzed for major and traceelements (Table 2). Results are plotted in the K2O vs. SiO2
diagram (Fig. 3A) and two rocks have been selected for theirnormalized incompatible elements patterns (Figs. 4A and 4B).
Site 769 was drilled in the southeastern flank of the CagayanRidge. Volcaniclastic material was encountered at 285 mbsf.Upper sediments that rest on it are early Miocene to early middleMiocene in age. Volcaniclastic material consists of massive and
329
H. BELLON, C. RANGIN
123°E 128°E
10°N
10°N
5°N
118°E 123°E
Figure 2. Map of the study area around the Sulu and Celebes basins with the location of the sampling (first number) and the K-Arisotopic ages (second number) taken from Table 1. Asterisks indicate isotopic dates previously published: by Wolfe (1981) onBantayan (B) and Mindanao; by Walther et al. (1981) on Marinduque (Ma) and Negros (N); by Morrice et al. (1983) on the KawioIslands and Sulawesi.
Figure 3. Graph of K2O vs. SiO2 for 43 dated samples. Superimposed fields are from Peccerillo and Taylor (1976) modified after Maury (1984).A: samples from western Panay, Tablas, Sabah, and Cagayan Ridge. B: samples from central and eastern Panay, Mindanao, and Masbate. Fullsymbols represent samples younger than 8 Ma.
Figure 4. Comparison of normalized incompatible elements (Rb, Ba, K, Sr, P and Ti) patterns forselected basalts (A) and andesites (B). Abundances are normalized using chondritic abundancesfrom Wakita et al. (1971) and from Sun et al. (1979): Rb, 0.35; Ba, 3.51; K, 120; Sr, 11; P, 46; andTi, 600 ppm.
332
CENOZOIC VOLCANIC ARC SEQUENCES
Sample locationand age (Ma)
Pliocene andQuaternary sediments
Olistostrome
Ophiolites
Intrusives
Buruanga Unit
Thrust
Figure 5. Schematic geologic map of Panay (from Rangin et al., in press) with indications of sampling localities andK-Ar isotopic dates taken from Table 1. It shows both the structural and age relationships between the arc units inwestern Panay and the Iloilo basement in central Panay.
unstratified lapillistone and- coarse tuff. Volcanic clasts aremainly porphyritic andesites with Plagioclase, clinopyroxene,and olivine or orthopyroxene. Rock aliquotes were sampledbetween 319 and 328.6 mbsf (Core 124-769-7R-1), and between338.2 and 347.9 mbsf (Cores 124-769-9R-3 and -9R-4).
Among the three analyzed lavas, the basaltic andesites(Sample 124-769-9R-4, 49-50 cm) is more altered than theandesites (Sample 124-769-7R-1, 118-120 cm, and -9R-3,
110-113 cm). It has a significantly higher LOI (loss on ignition)(2.43%) and exhibits a brown oxidized glass and a largerdeveloped secondary mineralogy.
These facts increase confidence in the mean isotopic datesof 20.6 and 19.8 Ma for the andesites, and of 14.7 Ma for thebasaltic andesites as a minimum age.
Site 771 was drilled in the Cagayan Ridge, on the largeplateau along its southern flank, 50 km southwest of Site 769.
333
H. BELLON, C. RANGIN
117°E 119°E
1 -12,9 Sample locationand age (Ma)
SABAH
V •£'£•• CROCKER elastics
j V " - KALUMPANG-TUNGKU
Volcanic arc
£2^4-*f*
Figure 6. Schematic geologic map of eastern Sabah and location of Kinabalu intrusives (cartridge). K-Ar isotopic dates are taken from Table 1.
The hole has penetrated, between 233.9 and 303 mbsf, avolcanic unit consisting of massive lapillistone, tuff, andprobable basaltic flows. The upper unit is middle Miocene tolate Pliocene in age and is made of sediments (clays and marl)with interbedded altered ash layers.
Sample 124-771-11R-2, 91-94 cm, from a basaltic flowseated between 232 and 241.7 mbsf, is a low K2O (0.22%) andlow TiO2 LOI (1050°C) (0.68%) vesicular basalt of orogenicparentage (arc tholeiitic basalt) but unfortunately has highLOI (4.91%). Vesicles are partly filled with silica.
Sample 124-771- 18R-7, 142-144 cm (294.4-304.1 mbsf), isfrom an andesitic block among andesitic angular clasts in amassive lapillistone. Its LOI was not measured but its undervac-uum outgassing spectrum is two times lower than that of the arctholeiitic basalt from section 11R-2, suggests its lower LOI.
These two lavas yield discordant isotopic ages (Table 1);respectively 19.9 Ma for the basalt and 14 Ma for the andesite.Taking account of the better preservation of the andesite, we aremore certain of a 14-Ma age for the activity at this site where theoldest mudstone layer within the pyroclastics was dated upperZone NN3 and the youngest pyroclastics, Zone NN5 (Ranginand Silver, this volume). This last age is in accordance with theyoungest isotopic age determined at this site.
In conclusion, the set of isotopic dates compared with thestratigraphic ages allows us to propose an activity for theCagayan Ridge that spans 20-14 Ma. Two stages can betentatively distinguished.
SABAH
Geological OverviewSabah (northern Boraeo)(Fig. 6) is bordered to the east by
the Sulu Sea and to the north by an active subduction orcollision zone, the Palawan-north Borneo Trench (Hamilton,1979). Its Tertiary Belt is considered to be part of an accre-tionary prism related to the Palawan Trench. This orogenicbelt consists of an outer belt (Crocker Range) and an inner beltmade of terranes of central and southern Sabah that over-thrusts the outer belt.
The Crocker Range is dominantly sedimentary; calcareousnannofossils from limestones give late middle Eocene toOligocene ages; i.e., Zones NP17-NP20 in Kudat Peninsula,Zone NP24 in Banggi Island and in central eastern Sabah,north of the Kinabatangan River. Sediments with Zones NN5and NN6 nannoplankton of lower middle Miocene were foundin Kudat Peninsula and in the northern islands.
334
CENOZOIC VOLCANIC ARC SEQUENCES
The inner belt consists of large deformed volcanic andsedimentary sequences that outcrop in the Semporna andDent Peninsulas. They are imbricated with ophiolites andchaotic formations. The different volcanic facies are irregu-larly distributed in the Semporna Peninsula; tuffs and pyro-clastites are mainly present in the northern slices, and massiveflows and breccias are widely distributed around Tawau(Rangin et al., 1990b). The sequence is folded with a generalnortheast trend that curves to the east-southeast at the DentPeninsula. Nannofossil ages from the sedimentary levelsrange from late Oligocene, Zone NP24, to early middleMiocene, Zone NN5. A similar folded volcanic assemblage ispresent in the Dent Peninsula.
Isotopic AgesMassive lava flows were selected for isotopic dating along
the southern coast of Dent Peninsula and in Semporna Penin-sula south of Kunak and close to Tawau (Fig. 6 and Table 3).A gabbro from the inner belt melange was also selected.Among ten dates, nine of them fall within the interval 12.9-11.1 Ma. All these rocks are within folded sequences. Only theyoungest sample (9 Ma) is distinct from these sequences andwas taken from a plug near Tawau. To these results may beadded younger ages obtained on Mount Kinabalu intrusives(6.8 and 6.4 Ma), the highest peak in Borneo, on volcanisminjected along N 130 E-trending faults in the Kunak area (Ma)and on volcanics that outcrop at the northern end of Sabah(Sude with a near 0-Ma age). Ages of 3 Ma (with a large error)remain suspect because of the high amount of 36Ar isotope inthese fresh lavas without alteration. Such a concentration mayreflect strong fluid circulations within the volcanic pile, the^Ar/^Ar isotopic ratio of it being possibly greater than theatmospheric one. In this case, the applied atmospheric cor-rection is incorrect and leads to ages older than the geologicalage. From these results, we conclude that these injectedvolcanics along N 130 E faults are probably very young and,in consequence, the N 130 E faults are still active.
GeochemistryAndesites and dacites dated 12.9-9 Ma plot into the
K-2O-SiO2 diagram (Fig. 3A) along the limit of the calc-alkaline and the high-K calc-alkaline field and within thehigh-K field. The normalized incompatible elements pattern(Fig. 4B) for one andesite (S 264-1) shows its high Rb, Ba, andK concentrations (> 100 times the chondritic values).
The gabbro from the melange has a typical low-K arctholeiitic basalt composition using the FeO*/MgO ratio crite-rion (Gill, 1981). Arc tholeiitic and calc-alkaline suites areassociated in space and time.
Younger rocks display large variations of composition.Kinabalu intrusive plots in the high-K calc-alkaline field. Aprevious detailed study (Vogt and Flower, 1989) pointed outthat the high-K character was developed during high-pressuresialic contamination of a low-K type melt that forms the smallcentral core of the batholit.
Volcanoes younger than 3 Ma are of arc tholeiitic and ofcalc-alkaline parentages.
MINDANAOUntil now, dates on samples from Mindanao were few and
poorly located (Wolfe, 1981). Except for the surprising agesranging from Ordovician to Early Cretaceous of the Bisligquartz diorite (Surigao del Sur), all the other ages are younger.One is 60 Ma (diorite dike at 7°30'N, 126°13'E) and the othersare 21, 11, and 6.7 Ma, so in the range of the new agesreported here (Table 1). The 11-Ma date remains unlocated,the 21-Ma date was tentatively attributed to volcanics located
east of Compostela (7°42'N, 126°10'E) in the Pacific cordillera,and the 6.7-Ma age is that of a probable andesite at thesoutheastern tip of the island.
Fourteen new isotopic dates (Tables 1 and 3) rangebetween 32 and 0.2 Ma. These results are preliminarybecause field work is in progress, but they fall into five maingroups: 32-30 Ma; 20-16 Ma; 11-9 Ma; 4.5-2.3 Ma; and lessthan 1.2 Ma. Oldest ages are restricted to the DagumaRange, where, near Talub, dioritic to granodioritic blocks ofarc tholeiitic parentage (MNO 89-26A) and of low-K calc-alkaline parentage (MNO 89-26B) (Table 2, Fig. 3B) andassociated with altered "andesites", yield ages of 31.9 and30 Ma. They geochemically resemble the Sipalay Minetonalite in western Negros dated at 30.2 Ma (Walther et al.,1981).
Four samples, one from the Daguma Range, two from thecentral cordillera, and the last from Surigao del Sur, yield agesthat fall between 20 and 16 Ma. Lavas dated 16.7 and 16.2 Maare from a low-K calc-alkaline series. Oldest lavas (20 Ma) arefrom a middle-K calc-alkaline series. Two different stages ofarc activity can be ascertained here.
An andesite and a dacite with ages of 10.6 and 9.3 Maoutcrop in South Davao. These lavas belong to a high-Kcalc-alkaline series as the andesites dated 10.6 and 9.7 Ma,which outcrop along the Sibuyan Sea Fault on the northeast-ern coast of Masbate (Table 1).
As in Tablas Island, activity renewed around 4.5 Ma inSurigao del Sur and perhaps earlier (6.7 Ma) at the southeast-ern tip of Mindanao, if we include the date reported by Wolfe(1981). This arc produced medium-K andesites to high-Kcalc-alkaline dacites (Fig. 3B).
A last volcanic event, active since 1.15 Ma, is responsiblefor all the volcanoes (among those, the still-active Apo Vol-cano) that outcrop at the western edge of the central cordilleraand eastward of Lanao. One can observe from Figures 3B and4A the diversity of chemical compositions of basalts andbasaltic andesites (calc-alkaline, high-K calc-alkaline, andshoshonitic) that erupted during this recent stage.
NORTH SULAWESIThe first preliminary isotopic ages (Tables 2 and 3) together
with geochemical compositions (not listed here) of plutonicbodies and lavas sampled (Fig. 2) in September 1989 aroundGorontalo lead to the following interpretation: the oldestplutonic bodies are 22 to 18.5 Ma (a period similar to that ofCagayan Ridge activity and, in Panay, of the Baloy arcsequence). This arc basement is intruded by plutonic bodies at8.8 Ma and covered by arc volcanics, the ages of which rangefrom 6.9 to 4.1 Ma. At least two different arcs can be specifiedbetween 22 and 4 Ma in North Sulawesi.
CONCLUSIONS: AGE CORRELATIONS ANDGEODYNAMIC IMPLICATIONS
A synthesis of all isotopic dates from this work togetherwith previous published ages is reported in Figure 7 and iscompared with volcanic episodes in evidence offshore duringLeg 124 drilling. The drilled volcanics of Cagayan Ridge,air-fall ash, and pyroclastic flows in the Sulu and Celebes Seasare considered to be a good record for the volcanic arc activityaround these basins (Rangin, Silver, von Breymann, et al.,1990; Pubellier et al., this volume).
The period 32-0 Ma was only considered because theproblem of preservation of older tephras in the basins remainsdifficult. 32 Ma is also the oldest ^K-^Ar age obtainedonshore on this volcanic arc sequences. Figure 7 reveals thatages are randomly distributed between 32 and 0 Ma. How-ever, six distinct stages can be tentatively differentiated.
335
H. BELLON, C. RANGIN
10 15Mα
20 25 30 35
TABLAS -MARINDUQUE M*
III
PANAYI Ii II I
SABAH
MASBATEBANTAYAN
B* BB•
NEGROS
MINDANAO
I
NORTH SULAWESI
SULU SEA
CELEBES SEACαgαyαn R i d g e -
Figure 7. Chronological diagram of the onshore magmatic activities around the Sulu and Celebes basins and offshore volcanic activity (CagayanRidge). Data are taken from Table 1, and data with (*) are from Walther et al. (1981) for Marinduque (M) and for Negros; from Wolfe (1981)for drilled igneous rocks in the Kabak oil well in Bantayan Island (B) (location in Fig. 2) and for Mindanao volcanics. Asterisk at the top ofvertical bar marks the date taken from literature. Solid vertical bar refers to the age of volcanic rocks. Dotted vertical bar refers to the age ofplutonic rocks. The lower array in the diagram gives three sets of data. Graphic symbol a: Tentatively distinct stages of activity. Graphic symbolb: Timing of air-fall ash and pyroclastic events in the Sulu and Celebes basins according to Pubellier et al. (this volume). Graphic symbol c:Ages of arc activity along the Cagayan Ridge (data taken from Table 1).
336
CENOZOIC VOLCANIC ARC SEQUENCES
Stage 1 (older than 28 Ma) is only present in the PhilippineMobile Belt (Mindanao, Negros, and the Iloilo basement inPanay). No continental basement is known for these volcanicsequences, suggesting that they belong to the intraoceanicPhilippine arc.
Stages 2 (28-22 Ma) and 3 (22-16 Ma) are both present inthe Philippine arc (Masbate, Mindanao), but also along theCagayan volcanic arc (Panay, Cagayan Ridge, Tablas) and thenorth arm of Sulawesi.
Stage 3 was probably one of the most volcanically activeperiod of Southeast Asia. Collisions of the Cagayan arc withthe Palawan block to the north, and of the north Sulawesi arcwith the Sula block to the south, are both marked by cessationof volcanic activity around 18-17 Ma. A second phase ofvolcanic activity (16-14 Ma) along the Cagayan Ridge iscoeval with this collision and is marked by emplacement of thelarge pyroclastic aprons drilled at Sites 769 and 771.
Stage 4, dated 16-11 Ma, was marked by medium-Kcalc-alkaline volcanism in Panay and Sabah and can be easilyexplained by incipient subduction of the Sulu Sea along theNegros and Sulu Trenches after complete collision of thePhilippine arc with the Cagayan arc in the central part of thePhilippine Archipelago (Tablas, Marinduque).
The 11- to 8-Ma stage (denoted 5 in Fig. 7) occurred alongmajor strike-slip faults such as the Philippine Fault in Mind-anao (north Davao) and Masbate or Tawau Fault zone inSabah. The high-K character of this calc-alkaline magmatismsuggests that it could be the product of volcanism sourced intometasomatized mantle after cessation of subduction.
Activities younger than 8 Ma are developed within thecomplex tectonic framework where the three major plates of thisarea are strongly imbricated. Volcanism in Tablas can be easilylinked with the termination of the Luzon arc; in the central partof north Sulawesi, young volcanic activity can be related to theHolocene southward subduction of the Celebes Sea. Similarly,young volcanism in Mindanao can be the result of incipientsubduction along the Philippine and Cotobato Trenches. Young-est ages in Sabah are either related to fault activity (Tawau) orcooling and unroofing of major plutons (Kinabalu Mt.).
This first tentative correlation of isotopic ages for volcanicrocks collected around the Sulu and Celebes basins, withmarkers of this activity in the basins, allows one to understandthe complex magmatic- and geodynamic-related evolutions ofthis area. Although preliminary, this approach needs to bereinforced by much more onshore data.
ACKNOWLEDGMENTS
We thank J. C. Philippet and J. Cotten for their respectivetechnical assistance in mass spectrometry and analyticalchemistry. We greatly appreciate the helpful manuscript re-views of R. A. Duncan and M. J. Defant.
REFERENCES
Aurelio, M. A., Rangin, C , Barrier, E., and Muller, C , 1990.Tectonique du segment central de la faille Philippine (Region deBondoc-Masbate): un dcrochement très recent. C. R. Acad. Sci.Ser. 2, 310:403-410.
Bellon, H., Quoc Buu, N., Chaumont, J., and Philippet, J. C , 1981.Implantation ionique d'argon dans une cible support: applicationau traéage isotopique de 1'argon contenu dans les minéraux et lesroches. C. R. Acad. Sci. Ser. 2, 292:977-980.
Cassignol, C , David, B., and Gillot, P. Y., 1977. Contribution audosage de 1'argon dans 1'échantillon de glauconite Gl-O. Geo-stand. Newsl., 1:105-106.
Cox, A., and Dalrymple, G. B., 1967. Statistical analysis of geomag-netic reversal data and the precision of potassium-argon dating. J.Geophys. Res., 72:2603-2614.
Gervaiso, F. C , 1971. Geotectonic developments of the Philippines.J. Geol. Soc. Philipp., 25:18-38.
Gill, J. B., 1981. Orogenic Andesites and Plate Tectonics: New York(Springer-Verlag).
Kudrass, H. R., Heidicke, M., Cepek, P., Kreuzer, H., and Muller,P., 1986. Mesozoic and Cenozoic rocks dredged from the SouthChina Sea (Reed Bank area) and Sulu Sea, and their significancefor plate tectonic reconstructions. Mar. Pet. Geol., 3:19-30.
Kudrass, H. R., Muller, P., Kreuzer, H., and Weiss, W., 1990.Volcanic rocks and tertiary carbonates dredged from the CagayanRidge and the Southwest Sulu Sea, Philippines. In Rangin, C ,Silver, E. A., von Breymann, M. T., et al., Proc. ODP, Init.Repts., 124: College Station, TX (Ocean Drilling Program), 93-100.
Mahood, G. A., and Drake, R. E., 1982. K-Ar dating young rhyoliticrocks: a case study of the Sierra La Primavera, Jalisco, Mexico.Geol. Soc. Am. Bull, 93:1232-1241.
Marchadier, Y., 1988. La terminaison de la fosse de Manille endomaine continental. Etude stratigraphique et tectonique des tiesde Mindoro-Tablas (Philippines) [thesis]. Univ. of Paris VI.
Marchadier, Y., and Rangin, C , 1989. Passage subduction-collision ettectoniques superposées à 1'extrémité méridionale de la fosse deManille (Mindoro - Tablas, Philippines). C. R. Acad. Sci. Ser. 2,308:1715-1720.
Maury, R. C , 1984. Les consequences volcaniques de la subduction.Bull. Soc. Geol. Fr., 26:489-500.
Moore, G. F., and Silver E. A., 1982. Collision processes in theNorthern Molucca Sea. In Te Tectonic and Evolution of SoutheastAsian Seas and Islands (Pt. 2). Geophys. Monogr. Ser., 27:360-372.
Morrice, M. G., Jezek, P. A., Gill, J. B., Whitford, D. G., andMonoarfa, M., 1983. An introduction to the Sangihe arc: volca-nism accompanying arc-arc collision in the Molucca Sea, Indone-sia. J. Volcanol. Geotherm. Res., 19:135-165.
Peccerillo, A., and Taylor, S. R., 1976. Geochemistry of Eocenecalc-alkaline volcanic rocks from the Kastamonu area, northernTurkey. Contrib. Mineral. Petrol, 58:63-81.
Rangin, C., Jolivet, L., Pubellier, M., and Tethys Pacific WorkingGroup, 1990a. A simple model for the tectonic evolution ofSoutheast Asia and Indonesia regions for the past 43 Ma. BullSoc. Geol. Fr., 6:887-905.
Rangin, C , Bellon, H., Benard, F., Letouzey, J., Muller, C , andSanudin, T., 1990b. Neogene arc-continent collision in Sabah,Northern Borneo (Malaysia). In Angelier, J. (Ed.), GeodynamicEvolution of the Eurasian Margin. Tectonophysics, 183:305-319.
Rangin, C , Pubellier, M., 1990. Subduction and accretion of oceanicfragments along the Eurasian margin: southern Japan-Philippineregion. Some constraints for continental growth. In Aubouin, J.,and Bourgois, J. (Eds.), Tectonics of Circum Pacific ContinentalMargins, (V.S.P International Publ.), 139-144.
Rangin, C , Silver, E. A., von Breymann, M. T., et al., 1990. Proc.ODP, Init. Repts., 124: College Station, TX (Ocean DrillingProgram).
Rangin, C , Stephan, J. F., Butterlin, J., Bellon, H., Muller, C ,Chorowicz, J., Baladad, D., in press. Collision néogène d'arcsvolcaniques dans le centre des Philippines. Stratigraphic et struc-ture de la chine d'Antique: ile de Panay. Bull. Soc. Geol. Fr. SantaCruz, J. R., Letargo, M.R.R., and Samaniego, C. L., 1989.Petrography and chemistry of high Mg, biotite-rich pillow-lavasfrom Southwestern Panay, Philippines. Tectonophysics, 168:137—149.
Silver, E. A., McCaffrey, R., Smith, R. B., 1983. Collision, rotation,and the initiation of subduction in the evolution of Sulawesi,Indonesia. J. Geophys. Res., 88:9407-9418.
Steiger, R. H., and Jàger, E., 1977. Subcommission on geochronol-ogy: convention on the use of decay constants in geo- andcosmochronology. Earth Planet. Sci. Lett., 26:359-362.
Sun, S.-S., Nesbitt, R. W., and Sharaskin, A. Y., 1979. Geochemicalcharacteristics of mid-ocean ridge basalts. Earth Planet. Sci.Lett., 44:119-138.
Vogt, E. T., and Flower, F. J., 1989. Genesis of the Kinabalu (Sabah)granitoid at a subduction-collision junction. Contrib. Mineral.Petrol, 103:493-509.
337
H. BELLON, C. RANGIN
Wakita, H., Rey, P., and Schmitt, R. A., 1971. Abundances of the 14 Wolfe, J. A., 1981. Philippine geochronology. J. Soc. Geol. Phil.rare-earth elements and 12 other trace-elements in Apollo 12 35:1-30.samples: five igneous and one breccia rocks and four soils. Proc.2nd Lunar Sci. Conf., 1319-1329.
Walther, H. W., Förster, H., Harre, W., Kreuzer, H., Lenz, H.,Muller, P., and Raschka, H., 1981. Early Cretaceous porphyry D a t e of initial receipt: 3 July 1990copper mineralization on Cebu Island, Philippines dated with Date of acceptance: 28 February 1991K-Ar and Rb-Sr methods. Geol. Jahrb., D48:21-35. Ms 124B-163
