^{3Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063 USA}

The Capoas intrusion is a metaluminous, high-K calc-alkaline, I-type biotite granite emplaced within Permian-Jurassic sedimentary rocks of the North Palawan Continental Terrane (NPCT) in the western Philippines. The NPCT is a fragment of the Mesozoic Andean-type margin of southeast China that was separated from the mainland during the late Oligocene-early Miocene opening of the South China Sea. Zircons from the granite have xenocrystic cores, and form a discordant array with a lower intercept age of 15 (+3/-4) Ma. Monazites have concordant ²⁰⁷Pb/²³⁵U ages with a mean age of 13.4 (±0.4) Ma. The late middle Miocene age and the location of the pluton in the NPCT uniquely constrain the formation of the Capoas granite in a post-rifting, non-collisional tectonic setting unrelated to any subduction zone. The major and trace element geochemistry of the granite and the presence of apparently Proterozoic xenocrystic zircon indicate that the pluton is composed largely, if not entirely, of older continental crust. The only viable heat source for crustal melting and/or assimilation was widespread basaltic magmatism that occurred in the area following cessation of seafloor spreading in the South China Sea in early Miocene time. The geochemical affinity of the Capoas granite with calc-alkaline magmatic arc and collisional granites is therefore a function of the source rocks that were melted to produce the granite rather than the specific tectonic setting in which the granite was generated. The calc-alkaline source rocks most likely formed in the Mesozoic Andean-type margin of south China and subsequently underwent partial melting in late middle Miocene time in an "anorogenic" setting.

Keywords: granite, geochemistry, geochronology, Southeast Asia, Philippines, igneous petrology

[links to figures are found below with the figure captions]

1. Introduction

Although there is both skepticism and confidence regarding our ability to fingerprint granitic rocks geochemically to infer their tectonic setting of formation, there are actually few cases were the specific tectonic setting of the granitoid is unambiguous enough that a clear connection can be made between geochemistry and tectonic setting. In this paper we describe the geology, geochemistry, and zircon/monazite U-Pb systematics of a granite body that intruded a microcontinental fragment that rifted from China with the opening of the South China Sea. The age of the pluton and its location in the continental fragment uniquely constrain the pluton as having formed in an "anorogenic" within-plate setting unrelated to any subduction zone, thus providing an excellent opportunity to characterize a young I-type granite in a well constrained tectonic setting. The results have implications for the tectonomagmatic history of the South China Sea area and implications for factors that affect granite geochemistry.

2. Geologic setting and petrography of Capoas granite

In the Mesozoic, the southern margin of China was an Andean-type margin along which abundant calc-alkaline plutonic and volcanic rocks were emplaced (e.g., Holloway, 1982). Following this period of convergence, the margin and surrounding areas (Fig. 1) underwent a phase of lithospheric extension that began in the late Cretaceous and culminated with oceanic crust formation and opening of the South China Sea in the late Oligocene to early Miocene (Holloway, 1982; Taylor and Hayes, 1983; Ru and Pigott, 1986). North-south opening of the South China Sea was accomodated by south-directed subduction along the Palawan trench (Holloway, 1982; Encarnación et al., 1995). Several microcontinental fragments broke away and were isolated from the Asian mainland during this rifting event. Following the cessation of seafloor spreading, widespread basaltic volcanism affected the South China Sea and surrounding areas of the Asian mainland (e.g., Barr and MacDonald, 1981).

The Capoas pluton is exposed in northern Palawan (Figs. 1 and 2) where it intrudes a sequence of Permian and Jurassic chert, shale, and limestone, which are part of the North Palawan Continental Terrane (NPCT). The latter is a microcontinent that rifted from southern China with the opening of the South China Sea (Holloway, 1982; Taylor and Hayes, 1983). The pluton was sampled as part of a larger geochronologic project aimed at providing precise age constraints on Philippine basement rocks (Encarnación, 1994). Prior to this work, it was thought that the Capoas granite might have formed as part of the Mesozoic Andean-type arc of southern China, an interpretation that was consistent with the geology of the NPCT.

The Mt. Capoas area was mapped at a scale of 1:50,000 and approximately 20 hand specimens of the granitoid were collected for petrographic work. The Capoas intrusion is a quite uniform medium grained, seriate K-feldspar porphyritic to equigranular, biotite granite with 29% modal quartz, 23% white microperthitic K-feldspar, 33% plagioclase and 15% biotite (mean of 5 thin sections from separate hand specimens of the equigranular variety; the porphyritic variety had slightly more K-feldspar). Accessory phases consist of abundant zircon and subordinate monazite and apatite. No Fe-Ti oxide phase was observed, even under back-scattered electron imaging. A textural continuum exists between a K-feldspar phenocryst-rich (up to 20% phenocrysts) variety and a variety poor in K-feldspar phenocrysts, the central area of the pluton tending to have more K-feldspar phenocrysts. The granite contains occasional centimeter to decimeter size isotropic and foliated elongate enclaves especially near the borders. Both types of enclaves consist of the same mineralogy as the enclosing granite, but they are finer grained and richer in biotite. In some areas, the elongate enclaves and K-feldspar phenocrysts exhibit magmatic flow alignment. Where the enclaves are abundant, the K-feldspar phenocrysts appear rounded, i.e., partly resorbed.

Intrusion of the granite occurred at shallow crustal levels as indicated by the unmetamorphosed chert-shale country rock. At the intrusive contacts the chert country rock is recrystallized to medium-grained quartzite. Samples of chert away from the contacts are fine-grained and preserve relict radiolaria. The radiolaria have been identified as Permian to Jurassic in age, and the rocks are interpreted to be part of an outer arc-trench complex that formed part of the convergent margin of southeast China in the Mesozoic (Isozaki et al., 1987). This interpretation is supported by the presence of pre-Tertiary calc-alkaline volcanic and plutonic rocks encountered by drill holes in the NPCT offshore northwest Palawan (Saldivar-Sali et al., 1981). In the areas around the main pluton, a few porphyritic dikes containing phenocrysts of quartz, plagioclase, biotite, and K-feldspar were observed.

At a few sites, brittle shear zones up to several meters wide cut the granite or the contact of granite with chert-shale country rock. These shear zones have a general northeasterly strike and near vertical or northwesterly dips. No shear sense was obtained from these zones.

3. Samples and analytical methods

The samples used in this study were collected from the south coast of Mt. Capoas (the two porphyritic varieties; samples 1 and 2) and from the large coastal exposure ~15 km south-southeast of Mt. Capoas (the equigranular variety; sample 1) (Fig. 2). The porphyritic and equigranular varieties are end-members to a continuum in texture of the pluton. These two textural varieties probably also represent the extremes in chemical composition within the pluton. Because there is no substantial difference in the major and trace element geochemistry of these two varieties, we suggest that the pluton is fairly uniform in geochemistry and that we have obtained representative analyses of the intrusion.

Zircon and monazite were extracted, using conventional mineral separation techniques, from an approximately 5 kg sample of porphyritic granite (sample 1) that did not contain any visible enclaves. Monazite was separated from zircon magnetically and by final hand-picking. Sample dissolution and chemistry procedures were similar to those described in Parrish et al. (1987), but with both zircons and monazites undergoing a double digestion procedure wherein fresh acid is added to the sample, which is then returned to the oven after drying it down following the first digestion step. This dissolution procedure was adopted because of the potential refractory nature of the young zircon and monazite.

The X-ray fluorescence (XRF) analyses for major elements were determined on a Siemens SRS303 XRF spectrometer at the National Institute of Geological Sciences, University of the Philippines. Instrumental neutron activation analyses (INAA) were obtained at the Phoenix Memorial Laboratory of the University of Michigan. Porphyritic granite sample 1 was irradiated for INAA several months before porphyritic and equigranular samples 2 and 3, respectively, which were irradiated together. Isotope composition and dilution measurements were performed at the Radiogenic Isotope Geochemistry Laboratory of the University of Michigan.

4. U-Pb systematics and age of Capoas granite

Despite careful hand-picking to avoid zircons with obvious inclusions, the five size fractions without visible cores shown in the inset of Fig. 3 fall off concordia along a mixing array with an inherited component having an apparent age of around 2 Ga. Analyses of eight grains and a single zircon, with obvious subrounded and turbid cores (20% to 30% of the volume of the crystals), fall much further off concordia and define a widening mixing envelope with an apparent range in age of inherited components (Fig. 3, Table 1). Although an old zircon component is clearly present, the apparent Proterozoic upper intercept age cannot be directly interpreted as the age of the source, or of the contaminant. First, the older zircon component may be a mixture of several older components, and second, it may be zircon recycled into younger rocks. A regression through the seven zircon fractions, weighted for the degree of concordance (Ludwig, 1994), yields a lower intercept age of 15 (+3/-4) Ma.

The four monazite analyses are inversely discordant (Fig. 4) with ²⁰⁷Pb/²⁰⁶Pb ages that are negative, or lower than the U-Pb ages, a characteristic common in young monazites (e.g., Parrish, 1990). Schärer (1984) explained this phenomenon as being due to the presence of 'excess ²⁰⁶Pb' produced by the decay of ²³⁰Th unsupported by ²³⁸U in the monazite. Monazite preferentially incorporates Th over U (demonstrated by the amount of radiogenic ²⁰⁸Pb; Table 1) and will generally crystallize with a ²³⁰Th/²³⁸U activity ratio greater than one. A correction for the excess ²⁰⁶Pb can be made based on the amount of enrichment of Th over U in monazite relative to the magma in which the ²³⁰Th/²³⁸U activity ratio is assumed to be equal to one (Schärer, 1984; Parrish, 1990). We used the whole rock U and Th concentrations in Table 2 and the U concentration in monazite from Table 1. The monazite Th content was determined from the amount of radiogenic ²⁰⁸Pb and the crystallization age based on the ²⁰⁷Pb/²³⁵U values. The applied correction reduces the ²⁰⁶Pb/²³⁸U ages by about 0.75 Ma and places the data points on, or closer to, the concordia curve (Fig. 4, filled ellipses). A systematic error of about ±0.5 Ma applies to the corrections deriving mainly from uncertainties in the Th content of the rock.

The ²⁰⁷Pb/²³⁵U ages of all monazite fractions are concordant with respect to one another, with an error-weighted mean of 13.4 ± 0.4 Ma, which is within uncertainty of the lower intercept age defined by the zircon discordia at 15 +3/-4 Ma. Parrish and Tirrul (1989) demonstrated age differences of up to 5 Ma between monazite and zircon and differences of up to 2 Ma in ²⁰⁷Pb/²³⁵U ages among monazites from Himalayan leucogranites intruded into high grade metamorphic rocks (650-700° C) that cooled to around 400° C over 6 to 16 Ma. They interpreted the discordant ages as resulting from diffusive Pb loss from monazite during slow cooling. Parrish and Carr (1994) also observed that high U monazites (>4000 ppm) may lose their Pb during post-crystallization cooling over the interval 400-650° C. Because of the relatively large errors on the lower intercept age of the Capoas zircons, it is not possible to say whether there is a similar age difference between the magmatic zircon and monazite of the Capoas granite. However, we suspect that there is unlikely to be a significant time difference between closure of zircon and monazite to Pb in the Capoas granite for the following reasons: (1) the Capoas granite intruded shallow, unmetamorphosed crust and thus probably did not undergo prolonged cooling between the closure temperatures for Pb in zircon and monazite; (2) the U content of the monazites is half the minimum amount in samples for which U-dependent Pb loss was observed by Parrish and Carr (1994); (3) the ²⁰⁷Pb/²³⁵U ages of the monazite fractions are concordant with respect to each other.

Because of the low precision on the lower intercept age of the zircon discordia, and the potentially large systematic errors in the monazite ²⁰⁶Pb/²³⁸U ages, we take the mean monazite ²⁰⁷Pb/²³⁵U age of 13.4 ± 0.4 Ma (late middle Miocene) as the crystallization age of the granite. This age is within uncertainty of the zircon lower intercept age and the corrected monazite ²⁰⁶Pb/²³⁸U ages. It is considerably younger than previous estimates of the age of the Capoas pluton that have ranged from Mesozoic to Oligocene, based on the age of the country rock and broad correlations with other granitoids (e.g., Tamesis et al., 1973; Mitchell et al., 1986; JICA, 1990).

5. Geochemistry and source of Capoas granite

The presence of Proterozoic core components in magmatic zircon of the Capoas granite unequivocally demonstrates the presence of older continental material in the source of the granite, and requires substantial partial melting of that material. The ~2 Ga age of the inherited zircons implies that the Mesozoic convergent margin of south China was partly built on, or was proximal to older crustal material. This is consistent with the 1.35 Ga Nd model age of the granite (Table 3). Phanerozoic granitoids and metasediments from southeast China also exhibit Proterozoic Nd model ages (1.0-2.5 Ga) and have apparent Proterozoic inherited zircons (1.2-2.4 Ga) (Jahn et al., 1990; Li, 1994).

The Pb, Sr, and Nd isotopic compositions of the Capoas granite are compared with potential magma sources and contaminants in Fig. 5. Among these are the South China Sea (SCS) post-spreading lavas, SCS sediment, south China continental crust, and amphibolites beneath the central Palawan ophiolite, which may represent proto-SCS oceanic basalt and sediment. A substantial amount of crustal component is required as an assimilant if the granite formed by an assimilation-fractional crystallization (AFC) process from SCS post-spreading basalts. A simple bulk mixing curve is shown in Fig. 5c, using Nd and Sr isotopic compositions and concentrations for SCS basalts and SCS sediment that would require the least amount of crust to produce the Sr and Nd isotopic composition observed from the Capoas granite. The calculation shows that at least 50% crust is required to produce the granite Nd and Sr isotopic compositions. Mixing of any other composition of SCS post-spreading basalt or crust would require a larger proportion of crustal component.

Fig. 6 shows REE data for the Capoas granite and the same potential sources and contaminants as in Fig. 5. In the absence of REE data for SCS sediment, data from South China Mesozoic granitoids and volcanics have been plotted. Note that the middle and heavy REE abundances of the Capoas granite are lower than those in the post-spreading lavas. Thus, if a simple mixing or AFC process involving the post-spreading lavas (as the fractionating magma) and rocks presently exposed in the upper crust (as the assimilant) is invoked, then this requires fractionation of a phase in which the REEs behave compatibly and in which the heavier REE are more compatible, such as zircon.

Roberts and Clemens (1993) reviewed the available experimental data on compositions of partial melts derived from a variety of crustal rocks and concluded that the only suitable source rocks for high-K calc-alkaline I-type granitic rocks such as the Capoas granite (Fig. 7a) are hydrated, calc-alkaline and high-K calc-alkaline andesites and basaltic andesites. Because the NPCT was formerly part of the Andean-type margin of south China, such source rocks are likely to be available at depth, and indeed, have been encountered beneath the supracrustal sequence by exploratory oil wells (Saldivar-Sali et al., 1981). It has been suggested that potential restitic phases from partial melting of such rocks are K-feldspar, plagioclase, quartz, orthopyroxene, and clinopyroxene (Roberts and Clemens, 1993). The somewhat lower middle and heavy REE concentrations of the Capoas granite compared to the Mesozoic magmatic rocks (Fig. 6) may be due to the increasing compatible behavior of the heavier REE in clinopyroxene, and to a lesser extent orthopyroxene, in more silicic melts (Rollinson, 1993, p. 110, and references therein). Hence, partial melting of source rocks similar to the Mesozoic calc-alkaline South China plutons and volcanics may also account for the REE patterns and isotopic composition of the Capoas granite.

Whether the Capoas granite is the product of an AFC or mixing process between mantle-derived magmas and the crust, or a partial melt of predominantly crustal origin, the young age and the location of the granite in the NPCT places important constraints on possible candidates for the potential mantle-derived magma and mechanisms for crustal melting as discussed below.

6. Tectonic setting and implications for origin of Capoas granite

The late middle Miocene age of the Capoas granite provides an important constraint on its tectonic setting of formation and on the nature of the magma, or heat source, required for assimilation or crustal melting. The Miocene age precludes an origin during formation of the Mesozoic convergent margin of southeast China where abundant arc magmatic rocks were generated (e.g., Jahn et al., 1990; Sun and Chen, 1992; Charvet et al., 1994). Formation above any younger subduction system can also be ruled out because the only nearby subducting slabs - those associated with the Palawan and Manila trenches - dip away from the NPCT (Figs. 1 and 2) (Holloway, 1982; Cardwell et al., 1980). In other words, the NPCT was not above any subduction zone when the Capoas granite formed, and hence should be free of subduction-generated mantle-derived magmas.

Mitchell et al. (1986) reported biotite K-Ar ages of 36.6 ± 1.8 and 37.0 ± 1.9 Ma for the biotite quartz monzonite body in central Palawan about 30 km southwest of Barton (Fig. 2). They interpreted this granitoid and, by correlation, the Capoas granite as anatectic melts formed in the thrust wedge of the Palawan trough. The ⁴⁰Ar/³⁹Ar ages of hornblende and muscovite from the upper amphibolite-lower granulite facies metamorphic sole of the Palawan ophiolite in central Palawan are 34.0 ± 0.6 Ma (Encarnación et al., 1995); therefore, generation of the quartz monzonite dated by Mitchell et al. (1986) may indeed be related to the detachment and obduction of the Palawan ophiolite as subduction along the Palawan trench was initiated, and South China Sea spreading commenced (Encarnación et al., 1995). However, the much younger age of the Capoas granite and its location within the NPCT require a different mechanism and heat source for crustal melting.

A collision-related origin for the granite (e.g., Harris et al., 1986) may be ruled out because collision of the NPCT with the Cagayan arc-Palawan trench system (Fig. 1 and 2) was terminated by early middle Miocene time. This event is constrained by biostratigraphic ages (boundary of zones N8 and N9; 15.2 Ma before present) of undeformed sediment overlying deformed rocks of the collision zone that were penetrated in exploratory wells drilled offshore, NW and SE of Palawan (Rangin and Silver, 1991, and references therein). Furthermore, the geochemistry and geology of the granite is unlike collision-related granites, which are generally peraluminous, S-type granites associated with regional metamorphic rocks and anatexis. Harris et al. (1986) also discussed the possibility of post-collision calc-alkaline magmatism associated with continuing, but dying subduction beneath the collision zone. Again, this cannot be invoked for the Capoas pluton because it is on the northwest side of the Palawan trench where subduction dipped southeast (Figs. 1 and 2).

Following seafloor spreading, widespread basaltic volcanism occurred in the South China Sea area (e.g., Barr and MacDonald, 1981) and on the NPCT (Fig. 1 and 2). Because of the timing of formation of the Capoas granite and its position in the NPCT, this basaltic magmatism provides the only viable heat source for crustal melting or assimilation to produce the Capoas granite. The K-Ar and ⁴⁰Ar/³⁹Ar ages of these basalts range from 14 Ma to 0.5 Ma (Barr and MacDonald, 1981; Tu et al., 1992, and references therein) and are thus partly coeval with emplacement of the Capoas granite. The cause of the widespread post-spreading basaltic magmatism in the South China Sea area is poorly understood, but regional extension and adiabatic upwelling of mantle has been suggested (Tu et al., 1992). It certainly cannot be related to subduction and collision at the Palawan trench system.

7. Regional implications

If the Capoas granite formed as the result of crustal melting induced by basaltic underplating or intraplating (e.g., Huppert and Sparks, 1988) of the NPCT then other areas around the South China Sea (largely submerged, extended continental crust) may have also experienced the same style of basaltic input into the crust, with concomitant granitic magmatism. The zone of basaltic magmatism in the South China Sea area is large, spanning an area greater than 10⁶ km² (Fig. 1), and thus such a process may be an important mechanism for crustal differentiation in the region.

Another implication of the age of the Capoas granite has to do with the timing of the amalgamation of the Luzon arc with the rifted margin of Eurasia, i.e., the NPCT. Because the Capoas granite intruded the NPCT after the NPCT collided with the Palawan trench-Cagayan arc system, the presence of brittle shear zones cutting the granite implies that a tectonic event affected the NPCT after the NPCT-Palawan trench-Cagayan arc system collision. The tectonic event that most likely caused the deformation is the collision of the Luzon arc with the NPCT along the Manila trench (e.g. McCabe et al., 1982). Stratigraphic evidence from the central Philippines suggests that the Luzon arc-NPCT collision occurred some time between the late Oligocene and late Miocene (McCabe et al., 1982). If deformation of the Capoas granite was caused by the Luzon arc-NPCT collision then the age of the granite constrains the collision to post late middle Miocene time.

8. Implications for tectonic discrimination and I-type granite petrogenesis

The Capoas granite formed in an "anorogenic" within plate setting, i.e., in a setting unrelated to contemporaneous collision, rifting, or a subduction zone. It is unusual to find such a young pluton in a relatively unambiguous setting. Given the known tectonic setting, it is instructive to plot the Capoas trace element data on conventional granitoid tectonic discrimination diagrams. Three tectonic discrimination diagrams that can be used with the available trace element data in Table 2 are illustrated in Figs. 7b-8c using the trace elements Rb, Hf, Ta and Yb (Pearce et al., 1984). Although the Capoas granite formed in a "within plate" anorogenic setting, the data straddle the fields for late/post-collisional and volcanic arc granites in Fig. 7b, and straddles the fields for syn-collisional and volcanic arc granites in Figs. 7c and 7d. Hence, like many other I-type granitoids, it appears that the major and trace element characteristics of this granite are controlled more by the nature of the source materials for partial melting rather than the tectonic environment in which the granite formed (e.g., Chappell and Stephens, 1988; Roberts and Clemens, 1993). The arc-like and collisional granite geochemical signatures (Fig. 7) are most likely inherited from source rocks that formed in the Mesozoic Andean type margin of southern China. These Mesozoic source rocks subsequently underwent partial melting in late middle Miocene time in an anorogenic setting unrelated to any subduction, or collision zone.

Acknowledgments

Geologic mapping in northern Palawan was conducted under the supervision of Dr. Helge H. Fischer and was supported by a joint German Technical Cooperation Agency-University of the Philippines project. Joel Aquino, Rene Claveria and Jun. Obille were part of the team that mapped the Mount Capoas area. Phil Simpson aided in the preparation and analysis of samples by INAA. Reviews by Sandra Barr, Jim Hawkins, Ken Ludwig, and Jean-Paul Liégeois improved the manuscript. Supported by NSF grant EAR 90-18967 to S.B.M.

Figure captions

Figure 1. Tectonic setting of Capoas granite. The North Palawan Continental Terrane (NPCT) and Reed Bank (RB) area rifted from China during the opening of the South China Sea Basin (SCSB) in late Oligocene to early Miocene time (23-17 Ma). Seafloor spreading ended with the collision of the NPCT with the Palawan trench (PT)-Cagayan arc (CA)-Sulu Sea back-arc (SSB) system. The collision was over by the early Miocene (15 Ma) (Holloway, 1983; Taylor and Hayes, 1983; Rangin and Silver, 1991). Areas of post-spreading basaltic volcanism are indicated in black (Barr and MacDonald, 1981; Tu et al., 1992). The Capoas granite in northern Palawan (rectangle) formed after the collision, apparently coeval with the earliest post-spreading basaltic lavas. Extinct spreading axis of SCSB is in bold dashed lines; SM-Scarborough seamounts; MT-Manila trench. Continuous and dashed barbed lines are active and extinct subduction systems, respectively, with barbs on the upper plate. Lightly shaded area is submerged continental crust; denser shaded area is underlain by oceanic crust. Philippine islands are dotted. Figure based on Barr and MacDonald (1981), Taylor and Hayes (1983) and Tu et al. (1992). Rectangle outlines area covered by Fig. 2.

Figure 2. Geologic map of northern Palawan (see Fig. 1 for location). A northwest-southeast schematic section is shown at bottom. Offshore features and their ages are based on several exploratory oil wells (Saldivar-Sali et al., 1981; Holloway, 1982; Müller, 1991; Rangin and Silver, 1991, and references therein). The ages of sediment between the unconformities are: a - late Eocene to early middle Miocene, b - early middle to latest Miocene, c - early Pliocene to Recent. The lower and middle unconformities (a and b) are interpreted to have formed during rifting of the NPCT from China and collision of the NPCT with the Cagayan arc, respectively. The youngest unconformity may be related to the collision of the west Philippine/Luzon arc with the NPCT-Cagayan arc amalgam (McCabe et al., 1982). The granite body southwest of Roxas has two biotite K-Ar ages at 37 ± 2 Ma (Mitchell et al., 1986) and is unrelated to the Capoas intrusive. Figure is based on JICA (1990), Letouzey et al. (1988), and mapping by the National Institute of Geological Sciences, University of the Philippines.

Figure 3. Concordia diagram of Capoas granite zircon fractions from sample 1. The fractions form a discordant array, caused by inheritance of older zircon components with an apparent age of ~2 Ga, with a lower intercept at 15 (+3/-4) Ma (Middle Miocene). Small box at lower left indicates area of inset figure. The uncertainty in the data in the inset are smaller than the symbols.

Figure 4. Concordia diagram of Capoas monazites from sample 1. Like other young monazites (e.g., Parrish, 1990) these exhibit reverse discordance. The method of Schärer (1984) was used to correct for “excess 206Pb” generated by decay of 230Th unsupported by 238U. Because of uncertainties in the correction, the externally concordant 207Pb/235U ages are more accurate. The error-weighted mean of the 207Pb/235U ages is 13.4 ± 0.4 Ma (late middle Miocene) and is the best estimate for the crystallization age of the Capoas Granite.

Figure 5. Isotope correlation diagrams comparing the composition of Capoas granite with potential source materials and contaminants. South China crust is represented by South China Sea sediment and south China granitoids for Pb, and South China Sea sediment for Nd and Sr. ‘Palawan basalts’ are late Tertiary terrestrial basalts of northern Palawan in Fig. 2. Amphibolites are from metamorphic sole of central Palawan ophiolite (Encarnación et al., 1995). The ticked curve in (c) is a simple bulk mixing line between SCS post-spreading lavas and sediment with ticks at 10% increments. The Sr and Nd concentrations used for the mixing are 273 and 16 ppm, respectively, for basalt and 258 and 28 ppm, respectively, for crust (data from Tu et al. (1992) and McDermott et al. (1993)). Data sources for isotopic compositions are: Tu et al. (1992) for post-spreading lavas; Mukasa (in preparation) for Palawan basalts; Jahn and Li (1990), Chung et al. (1994), and Mukasa (in preparation) for South China crust/sediment; Encarnación (1994) for Palawan amphibolites. NHRL is the northern hemisphere regression line for oceanic basalts (Hart, 1984).

Figure 6. Chondrite-normalized REE diagram of Capoas granite and potential source materials (see text for discussion). Data for South China crust are Hong Kong granites and Mesozoic arc-related igneous rocks from SE China (Sun and Chen, 1992; Charvet et al., 1994). Data sources for others as in Fig. 5. Chondrite values from Boynton (1984).

Figure 7 Major and trace element data of the Capoas Granite plotted on SiO2-K2O and conventional discrimination diagrams. The granite formed in an ‘anorogenic’ setting, but appears to have inherited the geochemical fingerprint of potential source rocks that formed in a subduction/collisional setting. (a) SiO2 versus K2O diagram illustrating fields for tholeiitic, calc-alkaline, high-K calc-alkaline, and shoshonitic magma series. The average compositions of I- and S-type granites, “i” and “s”, respectively, are also shown (figure modified from Roberts and Clemens, 1993). (b) Rb-Hf-Ta tectonic discrimination diagram (Harris et al., 1986). (c-d) tectonic discrimination diagrams of Pearce et al. (1984). Abbreviations are: VAG - volcanic arc granites, WPG - within plate granites, COLG - collisional granites, ORG - oceanic ridge granites.