What underlies the Philippine island arc? Clues from the Calaton Hill, Tablas island, Romblon (Central Philippines)

Journal of Asian Earth Sciences 36 (2009) 371–389

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What underlies the Philippine island arc? Clues from the Calaton Hill, Tablas island, Romblon (Central Philippines) B.D. Payot a,*, S. Arai b, R.A. Tamayo Jr. c, G.P. Yumul Jr. c,d a

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa City, Ishikawa 920-1192, Japan Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa City, Ishikawa 920-1192, Japan c National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City 1101, Philippines d Department of Science and Technology, Bicutan, Taguig, Metro Manila 1631, Philippines b

a r t i c l e

i n f o

Article history: Received 19 January 2009 Received in revised form 14 May 2009 Accepted 2 July 2009

Keywords: Calaton Hill Metamorphic/plutonic complex Lower crust Arc magma

a b s t r a c t We report here for the first time the occurrence of a high-temperature metamorphic/plutonic complex (amphibolites, metagabbros, hornblende pyroxenites and hornblendites) in Calaton Hill, Tablas island, Romblon, Central Philippines. The mineral assemblages and relic magmatic textures in these rocks imply apparent derivation from arc-related protoliths. Major element and trace element data are also comparable to those of gabbroic rocks in arc-related setting. Subsolidus re-equilibration under granulite to amphibolite facies is documented by the triple junctions between mineral phases in the different lithologies, the recrystallization of plagioclase and the presence of coronas around olivine with mineral assemblage of orthopyroxene + amphibole ± green spinel. The formation of hornblendite and the pervasive occurrence of amphiboles in the different lithologies are being attributed to the infiltration of a younger hydrous arc magma which also caused metamorphism and hybridization on the surrounding rocks. The characteristics of the Calaton Hill samples are comparable with those of the well-studied xenoliths from Ichinomegata, NE Honshu arc, Japan. We therefore interpret the Calaton Hill metamorphic/plutonic complex as representative of the lower crust underlying the Philippine island arc. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Island arcs located along convergent margins are records of the robust magmatism generated during the subduction of a downgoing slab back to the mantle. Most studies of the erupted volcanic products in island arcs often suggest the existence of complementary plutonic sequences and cumulates at depths corresponding to the lower crust (Arculus and Willis, 1980; Kay and Kay, 1985; DeBari and Coleman, 1989; Claeson and Meurer, 2004; Murphy, 2006; Greene et al., 2006). The solidifying unextruded arc magmas in the lower crust also undergoes upper amphibolite to granulite facies metamorphism as a result of multiple magma generation accompanied by large amounts of magmatic underplating (Kushiro, 1987; Yoshino et al., 1998). Exhumed granulite facies terranes are often believed to be representative of what was once deep crustal material, if not the base of the crust (Bohlen, 1987, 1991; Ellis, 1987; Kemp et al., 2007). The occurrence of plutonic sequences is therefore essential in understanding both magmatic and metamorphic processes in the deep crust of island arcs.

* Corresponding author. Tel.: +81 76 264 6513; fax: +81 76 264 6545. E-mail addresses: [email protected], [email protected] (B.D. Payot). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.07.001

In the case of the Philippine archipelago, little is known about the lower crust, and most of the sub-arc studies have been largely dependent on mantle xenoliths hosted by erupted arc magmas (Maury et al., 1992; Arai and Kida, 2000; Arai et al., 2004; Payot et al., 2007; Gregoire et al., 2008). In this work, we report for the first time the occurrence of a metamorphic/plutonic complex composed of amphibolites, metagabbros, hornblende pyroxenites and hornblendites in Calaton Hill, Tablas island, Romblon in Central Philippines. The metabasites contain relics of an igneous mineral assemblage suggestive of derivation from a gabbroic protolith of arc-affinity. Subsequent re-equilibration under granulite to amphibolite facies conditions is recorded by the coronitic mineral assemblage of orthopyroxene, amphibole and green spinel around olivine in the metagabbro. The possible involvement of a hydrous basaltic magma led to the formation of hornblendites and the pervasive occurrence of amphiboles in the other lithologies. The lithologic association, mineral assemblage and geochemistry of the Calaton Hill samples are potentially typical of well-studied subarc middle to lower crust materials comparable with the Ichinomegata xenolith suite (Aoki, 1971; Takahashi, 1978, 1986; Kushiro, 1987) and the intrusive complex from Talkeetna, Alaska (Greene et al., 2006). The Calaton Hill metamorphic/plutonic suite, therefore, provides a unique opportunity to evaluate the nature of the lower crust and understand the metamorphic and magmatic

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processes occurring beneath the Philippine island arc. Moreover, the emplacement of this sliver of the magmatic arc crust is a good marker for understanding the geotectonic evolution of the region which is largely controlled by the arc-continent collision between the North Palawan Block and the Philippine Mobile belt. 2. Geological setting Pre-tertiary metamorphic rocks have been previously recognized in Central Philippines. They are classified into three geographic groups: (1) the Palawan metamorphics in North Palawan and Calamian, (2) the Mindoro metamorphics in East Mindoro, Lubang, Golo and Ambil, and (3) the Romblon metamorphics in Tablas, Romblon, Sibuyan, Carabao and Panay (Gervasio, 1967; Rangin et al., 1985; Faure et al., 1989; Encarnacion et al., 1995; Yumul et al., 2003; Ramos et al., 2005). The inception of these rocks is being associated with the collision of the North Palawan Block (NPB) with the Philippine Mobile Belt (PMB) during Late Oligocene to Middle Miocene (McCabe et al., 1982) or the Early Miocene (Karig, 1983; Marchadier and Rangin, 1990; Yumul, 2007). The continental NPB is postulated to be a rifted fragment from mainland Eurasia which drifted and collided with island arc units of the PMB (Holloway, 1982). The easternmost extent of the arc-continent collision in Central Philippines is believed to be located east of the Romblon Island Group (RIG) (Pineda and Aurelio, 1992; Yumul et al., 2003; Ramos et al., 2005; Dimalanta et al., 2009). The Romblon Island Group is composed of the three main islands namely Romblon, Sibuyan and Tablas and 12 smaller islands. A wide variety of igneous, sedimentary and metamorphic rocks ranging from Paleozoic to Quaternary in age comprise the Romblon Island Group (Fig. 1). Faultbounded slices of peridotites with layered clinopyroxenites and layered and isotropic gabbro, diabase dike swarms and basaltic– andesitic pillow lava deposits in the islands of Tablas and Sibuyan have been collectively termed as the Sibuyan Ophiolite Complex (Yumul et al., 2003; Ramos, 2006). The Romblon Metamorphics which is exposed in the three major islands is composed of

intercalated marbles, phyllites and schists of quartz-feldspar-mica and talc-chlorite varieties. This metamorphic unit had been previously pegged to be of Pre-Eocene age in correlation with metamorphosed sequences in north Palawan (Faure et al., 1989). However, a relatively younger age of 12 Ma (Late Middle Miocene) had also been reported from K–Ar isotopic dating of schists from the Tablas and Sibuyan islands (Ramos, 2006). Periods of arc magmatism are represented by volcanic and dioritic intrusions of the Tablas Volcanics, Calatrava Intrusives and the Banton Volcanics. Deposition of sedimentary units occurred from the Eocene to Pleistocene as evidenced by the clastic and carbonate rocks belonging to the Bailan, Binoog, Anahao and Peliw Formations. 3. Field occurrence and petrography Calaton Hill is an ear-shaped protrusion located at the southeastern portion of Tablas island (Fig. 1). The highest point of the hill reaches up to 245 m which starkly contrasts with the surrounding flat areas. A variety of rock types are sampled from the slopes of Calaton Hill which can be roughly grouped into light-colored granular metagabbros and amphibolites (Fig. 2a–b) and the dark-colored hornblende pyroxenites and hornblendites (Fig. 2c and d). At the outcrop scale, the latter occur as foliated and massive exposures. The hornblende pyroxenites and hornblendites are phaneritic and exhibit granular texture. At times, mineral grains in the hornblendites show distinct flowage pattern. Gradation from hornblende pyroxenite to hornblendite or vice versa was difficult to draw in the field. The granular metagabbros and amphibolites are similarly noted as deformed and weathered outcrops. Relatively fresh hand samples are coarse-grained with a mineral assemblage of plagioclase, olivine, pyroxenes and hornblende. The hornblende pyroxenites are noted to be thrusted unto the granular metagabbros at an inclination of N30°W, 60°SW. The mineral assemblages and petrographical characteristics of the different rock types from the Calaton Hill are shown in Table 1 and Fig. 3, respectively. The granular metagabbros retain relics of an igneous mineral assemblage but show variations with respect

Fig. 1. Locality and tectonic setting of the Calaton Hill, Romblon, the Philippines. (a) Map showing the location of the Romblon Island Group and tectonic elements in the Philippines and surrounding areas. (b) Distribution of the lithologies in the Romblon Island Group (rectangle in (a)) is also shown.

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Fig. 2. Field occurrences of the Calaton Hill metamorphic/plutonic complex. (a) Metagabbro, (b) amphibolite, (c) hornblende pyroxenite and (d) hornblendite.

Table 1 Mineral assemblages of metamorphic/plutonic rocks from Calaton Hill, the Philippines. See text for classification (Types I, II and III) of amphiboles. Rock type

Metagabbro

Amphibolite

Sample no.

Plagioclase

RTER-413-03 BP08-18 BP08-27-1

x x x

BP08-26 BP08-25

x x

Hornblende pyroxenite

BP08-23 BP08-24-1 BP08-27-1

Hornblendite

BP08-19

Olivine

x x

Orthopyroxene

Clinopyroxene

x x x

x x

x x

x

x x

to grain size and the absence or presence of pyroxenes and green spinel. The coarsest-grained granular metagabbro (RTER-413-03) contains plagioclase (76%), olivine (9%), hornblende (5%), coronitic amphibole (6.5%), coronitic orthopyroxene (1.5%) and green spinel (3%). Garnets are totally absent. Polygonal grains of commonly twinned and unzoned plagioclases frequently up to 0.5 mm across are interlocked at a 120° angle (Fig. 3d). Olivines up to 1 mm across are usually dissected by serpentine. Amphiboles occur in two distinct textural types as an interstitial phase and as coronas around the olivine porphyroclasts. The interstitial type is noted as subhedral grains (2 mm) of dusty pale green to brownish hornblende with boundaries showing curvilinear contact with plagioclases. The textural appearance of the interstitial amphibole implies a magmatic origin in contrast to the coronitic amphibole. The most striking textural feature in this granular metagabbro is the occurrence of well-preserved coronitic shells of orthopyroxenes, amphiboles and green spinels around some of the olivine grains (Fig. 3a). Coronitic orthopyroxene usually occurs as thin layers in the inner rim of the olivine whereas the thicker greenish amphibole is noted in the outer margin (Fig. 3a). Green spinels are at times observed as dispersed grains within the coronitic amphiboles. Coronitic rims

x x x

Amphibole

Spinel

Type I

Type II

x x x

x x

Magnetite

Type III x

x x

x x

x x x x

usually preserved in gabbroic rocks are formed due to the disequilibrium between olivine and plagioclase during a prograde or retrograde metamorphic event or the cooling of intrusive bodies from igneous temperatures (Gardner and Robins, 1974; Mongkoltip and Ashworth, 1983; Claeson, 1998; De Haas et al., 2002; Cruciani et al., 2008). Compared to the previous sample, BP08-18 is a mediumgrained granular metagabbro and contains abundant clinopyroxene which was not noted in the earlier sample. Plagioclase is still the most dominant mineral (50%) and is usually twinned and show triple junctions between grains. Porphyroclasts of olivine (11%), clinopyroxene (5%) and orthopyroxene (4%) are embedded within the polygonal aggregates of plagioclases. Olivines are dissected by serpentine and at times rimmed by coronitic orthopyroxenes and amphiboles. However, the coronitic shells are not as defined as in the previous sample. Clinopyroxenes are up to 0.3 mm in size and have dirty appearance. Most of them have been partially converted to amphibole along the rims and in some cases, even in the inner core of the crystal as lamellar replacement. Discrete orthopyroxenes are colorless and subhedral in thin section. Aside from occurring as coronas around olivine, pale brown to greenish

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Fig. 3. Photomicrographs of Calaton Hill and Ichinomegata samples. (a) Corona assemblage composed of orthopyroxene, amphibole and green spinel around olivine in Calaton Hill metagabbro. (b) The corona mineral assemblage in the Ichinomegata hornblende–pyroxene gabbro xenolith. (c) Amphibolite showing foliated texture. Polygonal grains of plagioclase have recrystallized boundaries. Greenish amphibole occurs as overgrowth over the elongated brownish orthopyroxene. (d) Plagioclase showing triple junctions in metagabbro. (e) Clinopyroxene in hornblende pyroxenite is cut and partially altered to amphibole. This clinopyroxene grain shows continuous extinction under crossed polars. (f) Amphibole formed at the expense of clinopyroxene in the Ichinomegata hornblende-pyroxene gabbro xenoliths. (g) Hornblendite from Calaton Hill. (h) Hornblendite from Ichinomegata. All photomicrographs are by plane-polarized light except for (d), which is by crossed-polarized light. Scale shown in all photomicrographs is equal to 0.5 mm. Abbreviations: plag = plagioclase, ol = olivine, opx = orthopyroxene, cpx = clinopyroxene, amp = amphibole, sp = spinel.

amphiboles are also noted as large crystals (3 mm) with very elaborate shape and well-defined contact with plagioclase. No green spinel is observed within this sample. Amphibolites are strongly foliated (Fig. 3c) and essentially comprised of plagioclase (60–62%) and amphibole (27–29%) with

minor amounts of orthopyroxene (1–6%), clinopyroxene (0–9%) and magnetite (2–3%). Plagioclases occur as polygonal grains with twinning and undulose extinction. Though triple junctions between plagioclase grains can still be observed, smaller aggregates of plagioclase have indistinct and recrystallized boundaries.

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Table 2 Selected microprobe analyses of mineral phases from the Calaton Hill metamorphic/plutonic rocks. Abbreviations: plag = plagioclase, ol = olivine, opx = orthopyroxene, cpx = clinopyroxene, amp = amphibole, sp = spinel and mgt = magnetite. FeO* = total iron as FeO. Sample no. Rock type

RTER-413-03 Metagabbro

BP08-25 Amphibolite

Mineral

plag

ol

amp

opx

amp

green sp

plag

opx

cpx

amp

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total

43.72 0.00 37.26 0.00 0.21 0.01 0.04 18.54 0.83 0.00 0.00 100.62

37.85 0.00 0.00 0.00 19.64 0.23 42.70 0.01 0.00 0.00 0.11 100.54

43.73 0.42 15.57 0.00 8.58 0.15 16.48 10.97 2.60 0.19 0.02 98.71

53.45 0.01 3.97 0.00 12.99 0.26 28.97 0.38 0.01 0.01 0.03 100.09

42.64 0.11 16.15 0.00 8.95 0.14 16.23 11.17 2.70 0.18 0.03 98.30

0.75 0.00 60.45 0.00 22.51 0.12 14.58 0.07 0.06 0.01 0.14 98.67

54.64 0.00 28.94 0.00 0.06 0.00 0.02 11.02 5.46 0.04 0.00 100.18

52.09 0.05 1.53 0.00 21.40 0.53 23.64 0.65 0.04 0.00 0.00 99.94

52.53 0.20 2.28 0.00 9.15 0.23 13.83 21.21 0.41 0.00 0.01 99.86

45.80 1.23 10.28 0.00 12.01 0.12 14.09 11.04 2.47 0.18 0.01 97.23

An Ab Or

95.5 7.5 0.02

0.66

0.73

0.64

BP08-23 Hornblende pyroxenite

BP08-19 Hornblendite

mgt

opx

cpx

amp

amp

amp

amp

0.10 1.87 1.73 0.10 86.09 0.16 0.31 0.01 0.00 0.00 0.00 90.37

55.81 0.11 1.07 0.00 12.71 0.30 29.72 0.56 0.02 0.00 0.04 100.35

53.67 0.22 1.35 0.03 5.31 0.15 16.78 22.30 0.20 0.01 0.07 100.06

45.77 1.33 10.57 0.19 7.80 0.10 16.72 12.02 2.12 0.29 0.04 96.94

42.91 1.54 13.33 0.00 10.10 0.11 15.42 11.43 2.42 0.33 0.04 97.63

43.32 1.54 13.20 0.00 9.96 0.13 15.68 11.40 2.40 0.32 0.01 97.97

42.49 1.66 14.00 0.00 9.99 0.13 15.16 11.74 2.44 0.35 0.01 97.97

0.81

0.85

0.79

0.74

0.74

0.73

Corona

Mg# Fo

52.6 47.2 0.25 0.77

0.80

0.76

79.5

Fig. 4. Variation diagrams for orthopyroxenes and clinopyroxenes in the Calaton Hill samples, the Philippines. Data for Ichinomegata pyroxene–hornblende gabbro xenoliths (Takeuchi, 2008) and Talkeetna gabbronorite (Greene et al., 2006) are shown for comparison.

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Orthopyroxenes are light brown in color in thin section and reach up to 2 mm in size. Most of these grains are enclosed, dissected or replaced by dark green amphibole such that only the core of the original orthopyroxene crystals are usually preserved. Subhedral clinopyroxene grains which are also partially replaced by greenish amphibole are observed only in BP08-26. Magnetite grains (0.5 mm) are lobate and irregular in shape. They are usually in close association with the green amphibole replacing the pyroxenes. Hornblende pyroxenites are coarse-grained and composed of clinopyroxene (55%), amphibole (43%) and orthopyroxene (2%). Clinopyroxenes are colorless under plane polars and reach up to 2 mm in size. Most of the clinopyroxenes are partially altered to amphibole (Fig. 3e). Orthopyroxene is less abundant compared to clinopyroxene. They are observed as large (1 mm) subhedral grains with undulose extinction. Similar to clinopyroxene, they are also usually enclosed or in contact with amphibole. Aside from occurring as rims around the pyroxene, amphiboles are also noted as large grains up to 7 mm. They are distinctly brown in color in thin section. Subrounded clinopyroxene and orthopyroxene up to

0.5 mm in size occur as inclusions within the amphibole. Some of the inclusions and the smaller amphibole grains also exhibit subgrain boundaries and undulose extinction. Some other samples display very uneven distribution of amphibole, which is concentrated to form hornblenditic patches. The sharp contact between granular metagabbro and the hornblende pyroxenite was noted in one sample (BP08-27-1). Both lithologies have the same mineralogy as detailed above. However, the minerals in the metagabbro part are elongated and appear to have a preferred magmatic foliation. Along the contact, irregular and small plagioclase grains shoot out of the metagabbro part and inundate the amphiboles of the hornblende pyroxenite. Large amphiboles similar in appearance with amphiboles in the hornblende pyroxenite also occur as isolated patches within the metagabbro. Hornblendite is monomineralic and totally composed of interlocking grains of euhedral to subhedral brown and green amphiboles (Fig. 3g). The amphiboles are coarse-grained reaching up to 7 mm in size. At times, the amphibole form well-defined 120° angle with each other. Most of the crystals show subgrain boundaries.

Fig. 5. Classification of amphiboles (Leake et al., 1997) from the Calaton Hill, the Philippines. Structural formulae for amphibole were calculated on anhydrous basis (O = 23). The Calaton Hill amphiboles are further grouped into three types based on geochemistry combined with mode of occurrence. See text for details.

Fig. 6. Three types of amphiboles according to geochemistry from the Calaton Hill, the Philippines. (a) Mg# vs TiO2 wt% and (b) Mg# vs YbN. N = normalized to chondrite values (Sun and McDonough, 1989).

B.D. Payot et al. / Journal of Asian Earth Sciences 36 (2009) 371–389

4. Mineral chemistry Mineral phases were analyzed using a JEOL electron microprobe (JXA8800) at the Kanazawa University, Japan using the following analytical conditions: 20 kV acceleration voltage, 20 nA probe current and 3.0 lm probe diameter. Clinopyroxene and amphibole trace element abundances were acquired through a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) (Agilent 7500s, Yokogawa Analytical Systems) outfitted with a GeoLas QPlus, MicroLas laser ablation system at the Kanazawa University. Analyses were performed at 5 Hz with energy density of 8 J/cm2

377

by ablating spots of 60 lm in clinopyroxene and amphibole. The National Institute of Standards and Technology (NIST) standard reference materials (SRM) 612 and 614 glasses were used for calibration with Si as an internal standard. 4.1. Major elements Representative electron probe analyses of major phases (olivine, plagioclase, clinopyroxene, orthopyroxene, amphibole and magnetite) and coronitic minerals (orthopyroxene, amphibole and green spinel) in the different rock types are presented in

Fig. 7. REE and trace-element distribution patterns for clinopyroxenes normalized to chondrite (a) and NMORB (b) abundances. Normalizing values are after Sun and McDonough (1989). Gray fields show the range of clinopyroxenes from Ichinomegata hornblende–pyroxene gabbro xenoliths (Takeuchi, 2008).

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Table 3 Major and trace element data of clinopyroxenes and amphiboles from the Calaton Hill metamorphic/plutonic rocks. d.l. means below detection limits. FeO*=total iron as FeO and Mg#=(Mg/Mg+Fe2+). Sample # Rock Type Mineral Texture

RTER-413-03 Metagabbro Amphibole Type I

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total Mg# ppm Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Sm Zr Hf Eu Ti Gd Dy Ho Y Er Tm Yb Lu

43.73 0.42 15.57 0.00 8.58 0.15 16.48 10.97 2.60 0.19 0.02 98.71 0.77

44.04 0.45 15.10 0.00 9.11 0.17 16.08 11.18 2.60 0.19 0.00 98.91 0.76

42.72 0.48 15.38 0.00 8.59 0.15 16.34 10.82 2.66 0.21 0.04 97.37 0.77

42.72 0.48 15.38 0.00 8.59 0.15 16.34 10.82 2.66 0.21 0.04 97.37 0.77

42.69 0.14 16.44 0.00 8.68 0.15 15.95 11.01 2.71 0.20 0.02 98.00 0.77

42.69 0.14 16.44 0.00 8.68 0.15 15.95 11.01 2.71 0.20 0.02 98.00 0.77

42.64 0.11 16.15 0.00 8.95 0.14 16.23 11.17 2.70 0.18 0.03 98.30 0.76

42.42 0.13 16.08 0.00 9.12 0.10 16.21 10.84 2.82 0.18 0.03 97.92 0.76

43.06 0.09 16.41 0.00 8.69 0.09 16.32 10.89 2.74 0.20 0.04 98.52 0.77

42.78 0.09 14.94 0.00 9.86 0.21 15.97 10.37 2.40 0.18 0.03 96.83 0.77

41.76 0.10 15.95 0.00 8.89 0.13 15.96 10.99 2.73 0.19 0.02 96.71 0.76

42.14 0.14 16.05 0.00 8.60 0.12 16.08 11.03 2.74 0.18 0.03 97.11 0.77

1.97 14.12 0.23 0.05 0.38 d.l. 0.91 5.02 0.11 0.95 79.24 5.40 1.57 14.52 0.56 0.65 3192.37 1.68 1.65 0.34 8.35 0.90 0.12 0.89 0.12

2.00 13.90 0.25 0.06 0.46 d.l. 1.00 5.23 0.08 1.03 76.14 6.19 1.75 18.99 0.71 0.72 3055.77 1.85 1.76 0.32 8.90 0.90 0.13 0.89 0.12

1.89 13.43 0.28 0.06 0.37 d.l. 0.98 4.96 d.l. 0.98 76.91 5.55 1.62 17.19 0.61 0.68 2832.21 1.64 1.55 0.31 7.65 0.80 0.10 0.73 0.09

1.86 13.49 0.25 0.05 0.40 d.l. 0.94 4.89 d.l. 0.95 70.77 5.57 1.60 16.81 0.62 0.68 2729.84 1.60 1.48 0.27 7.60 0.81 0.12 0.81 0.11

0.92 10.84 0.15 0.04 0.56 d.l. 1.14 6.32 d.l. 1.27 65.38 7.50 2.31 18.87 0.83 0.72 4106.88 2.30 2.23 0.46 11.49 1.33 0.19 1.16 0.15

1.97 14.23 0.24 0.06 0.35 d.l. 0.96 4.96 d.l. 0.93 78.83 5.30 1.60 14.27 0.54 0.66 2810.53 1.63 1.67 0.33 7.79 0.86 0.12 0.85 0.11

2.24 13.59 0.28 0.05 0.10 d.l. 0.72 4.03 0.13 0.78 69.31 4.27 0.97 9.11 0.20 0.72 909.76 0.75 0.39 0.08 1.98 0.19 0.03 0.18 0.03

2.14 13.29 0.25 0.06 0.11 d.l. 0.75 4.15 0.13 0.76 69.73 4.36 0.98 7.16 0.15 0.77 754.49 0.64 0.40 0.07 1.97 0.19 0.03 0.19 0.04

1.92 11.97 0.36 0.07 0.11 d.l. 0.70 3.60 d.l. 0.64 68.38 3.18 0.62 8.72 0.12 0.58 820.94 0.49 0.32 0.06 1.50 0.15 0.02 0.16 0.04

1.81 11.70 0.29 0.06 0.13 d.l. 0.69 3.30 d.l. 0.56 90.13 2.76 0.57 8.02 0.14 0.55 628.53 0.45 0.26 0.05 1.40 0.12 0.02 0.17 0.03

1.89 13.33 0.30 0.05 0.11 d.l. 0.73 3.98 d.l. 0.77 66.06 4.44 1.03 8.75 0.19 0.69 1003.11 0.80 0.42 0.07 2.21 0.18 0.03 0.22 0.04

1.81 12.66 0.25 0.03 0.14 d.l. 0.78 3.95 d.l. 0.69 65.63 3.86 0.91 7.32 0.13 0.69 883.19 0.59 0.41 0.07 1.96 0.19 0.02 0.22 0.03

Type II

BP08-19 Hornblendite

BP08-18 Metagabbro

Amphibole Type I

Clinopyroxene

BP08-27-1 Hornblende Pyroxenite Amphibole Type I

Amphibole Type I

Metagabbro Clinopyroxene

42.91 1.54 13.33 0.00 10.10 0.11 15.42 11.43 2.42 0.33 0.04 97.63 0.74

43.32 1.54 13.20 0.00 9.96 0.13 15.68 11.40 2.40 0.32 0.01 97.97 0.74

42.49 1.66 14.00 0.00 9.99 0.13 15.16 11.74 2.44 0.35 0.01 97.97 0.79

51.71 0.36 3.15 0.06 4.93 0.13 16.14 22.96 0.21 0.00 0.00 99.65 0.85

52.21 0.42 3.21 0.02 4.80 0.14 16.03 23.09 0.23 0.00 0.02 100.16 0.86

45.09 0.93 12.85 0.13 7.51 0.08 16.99 11.87 2.29 0.12 0.02 97.86 0.80

43.30 1.74 13.21 0.08 8.95 0.09 15.93 11.67 2.43 0.20 0.01 97.61 0.76

43.82 1.79 13.10 0.09 8.60 0.09 16.06 11.69 2.43 0.22 0.01 97.90 0.77

46.12 1.59 10.84 0.00 10.00 0.13 15.09 11.73 2.10 0.23 0.03 97.85 0.73

45.68 1.54 10.96 0.00 9.54 0.11 15.65 11.35 2.24 0.25 0.01 97.32 0.75

53.01 0.29 2.15 0.00 6.59 0.23 15.04 21.97 0.29 0.00 0.04 99.61 0.80

53.16 0.27 2.13 0.00 6.66 0.22 15.32 21.78 0.29 0.01 0.00 99.84 0.80

52.26 0.35 2.06 0.00 7.05 0.19 16.11 20.42 0.27 0.00 0.00 98.71 0.80

1.55 23.82 0.05 d.l. 0.58 d.l. 1.16 5.23 0.36 1.02 175.24 6.81 2.49

0.96 17.31 0.04 d.l. 0.53 d.l. 1.01 4.64 0.36 0.97 171.84 6.26 2.43

1.08 19.13 d.l. d.l. 0.43 d.l. 0.82 4.06 0.32 0.84 176.11 5.43 2.33

d.l. d.l. d.l. d.l. d.l. d.l. 0.31 1.24 d.l. 0.24 24.82 1.48 0.63

d.l. d.l. d.l. d.l. 0.02 d.l. 0.28 1.25 d.l. 0.26 22.34 1.69 0.87

1.07 18.22 0.09 0.02 0.25 d.l. 0.85 3.49 0.19 0.64 126.04 4.10 1.61

1.28 27.80 0.13 0.03 0.40 d.l. 1.20 5.33 0.23 1.05 168.51 6.78 2.61

1.07 21.67 0.10 0.02 0.28 d.l. 0.94 4.02 0.18 0.78 126.54 5.03 1.91

1.40 21.96 0.07 0.03 1.20 d.l. 1.63 7.64 0.26 1.51 109.05 9.73 3.74

1.39 21.13 0.06 0.03 1.32 d.l. 1.44 7.17 0.35 1.38 129.21 8.91 3.50

0.25 1.87 0.05 0.02 0.03 d.l. 0.59 3.03 0.10 0.64 41.50 4.30 1.87

d.l. 0.51 d.l. d.l. 0.01 d.l. 0.45 2.41 0.08 0.52 38.72 3.41 1.44

0.15 0.71 0.08 d.l. 0.01 d.l. 0.47 2.49 0.08 0.54 33.57 3.54 1.66

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B.D. Payot et al. / Journal of Asian Earth Sciences 36 (2009) 371–389 Table 3 (continued) BP08-19 Hornblendite

BP08-18 Metagabbro

Amphibole Type I

Clinopyroxene

14.91 0.73 0.92 10156.59 3.07 3.12 0.61 14.70 1.58 0.20 1.37 0.17

14.73 0.71 0.88 10025.16 2.79 2.63 0.51 12.11 1.34 0.17 1.07 0.14

13.13 0.67 0.81 9941.97 2.85 2.94 0.55 13.29 1.45 0.19 1.22 0.15

6.72 0.47 0.25 1776.70 0.94 1.01 0.20 4.18 0.50 0.06 0.36 0.04

BP08-27-1 Hornblende Pyroxenite Amphibole Type I

Amphibole Type I 9.23 0.60 0.30 2735.12 1.28 1.39 0.26 5.67 0.66 0.08 0.45 0.05

BP08-23 Hornblende Pyroxenite Amphibole Type I

21.19 1.07 0.65 7164.97 2.07 2.25 0.41 10.40 1.14 0.14 0.90 0.12

14.74 0.66 0.88 11513.07 3.11 3.23 0.64 15.30 1.72 0.23 1.37 0.18

12.04 0.55 0.64 8516.81 2.37 2.36 0.47 11.34 1.26 0.16 1.03 0.13

24.89 1.14 0.94 10692.33 4.49 4.30 0.85 20.20 2.12 0.28 1.75 0.24

Metagabbro Clinopyroxene 22.72 1.06 1.02 11008.30 4.25 4.27 0.80 20.31 2.12 0.27 1.69 0.22

12.90 0.67 0.51 2798.36 2.43 2.56 0.52 11.44 1.26 0.17 1.07 0.15

10.02 0.47 0.38 1368.76 1.71 1.68 0.35 8.09 0.91 0.12 0.82 0.11

11.07 0.57 0.45 2194.61 2.31 2.29 0.45 10.49 1.17 0.15 0.96 0.13

BP08-26 Amphibolite Amphibole Type III

Clinopyroxene

45.77 1.33 10.57 0.19 7.80 0.10 16.72 12.02 2.12 0.29 0.04 96.94 0.79

46.39 1.15 9.20 0.25 7.67 0.12 17.09 12.40 1.97 0.23 0.06 96.52 0.80

45.77 1.48 10.70 0.19 8.07 0.10 16.72 11.83 2.20 0.28 0.08 97.40 0.79

52.80 0.28 1.82 0.08 5.46 0.13 16.93 21.12 0.22 0.00 0.01 98.85 0.85

52.69 0.20 1.55 0.09 4.95 0.18 15.99 22.83 0.21 0.01 0.02 98.71 0.85

44.71 0.94 12.17 0.00 13.24 0.14 13.89 10.86 1.99 0.34 0.00 98.28 0.65

45.11 0.89 11.96 0.00 12.93 0.17 13.99 10.87 2.03 0.34 0.03 98.32 0.66

44.34 0.85 11.61 0.00 12.98 0.18 14.18 10.90 1.88 0.34 0.01 97.27 0.66

45.20 0.91 11.68 0.00 12.81 0.17 14.31 11.13 1.98 0.34 0.00 98.54 0.67

0.85 15.15 d.l. d.l. 0.66 d.l. 1.20 5.65 0.26 1.12 177.50 6.75 2.51 14.97 0.78 0.93 9050.01 2.73 2.64 0.52 12.31 1.27 0.18 1.12 0.13

0.95 12.37 0.03 d.l. 0.56 d.l. 1.16 5.78 0.22 1.19 138.03 7.77 2.91 16.60 0.90 0.97 7238.31 3.55 3.38 0.64 15.92 1.67 0.24 1.42 0.19

1.13 16.41 0.05 d.l. 0.81 d.l. 1.30 6.18 0.30 1.28 167.47 8.26 3.03 17.83 0.85 1.02 8865.62 3.51 3.24 0.65 15.85 1.71 0.22 1.40 0.21

d.l. d.l. d.l. d.l. 0.01 d.l. 0.43 2.21 d.l. 0.46 41.11 3.20 1.28 4.88 0.26 0.41 1750.31 1.66 1.71 0.35 7.40 0.84 0.10 0.65 0.09

d.l. d.l. d.l. d.l. 0.01 d.l. 0.40 2.30 d.l. 0.47 48.10 3.08 1.13 6.37 0.38 0.39 1309.65 1.39 1.40 0.27 6.01 0.68 0.08 0.53 0.08

2.59 15.65 d.l. d.l. 0.08 d.l. 1.08 4.03 0.31 0.69 102.76 3.85 1.25 2.46 0.17 0.94 6343.32 1.49 1.51 0.31 7.23 0.82 0.11 0.74 0.09

2.46 13.96 d.l. d.l. 0.06 d.l. 1.05 4.06 0.30 0.65 91.51 3.69 1.18 1.92 0.13 0.84 5633.35 1.43 1.53 0.29 6.67 0.75 0.11 0.66 0.10

2.03 11.68 d.l. d.l. 0.06 d.l. 1.08 4.16 0.29 0.69 95.96 3.76 1.12 2.06 0.11 0.86 5425.99 1.37 1.40 0.27 6.27 0.76 0.11 0.70 0.09

2.24 12.98 d.l. d.l. 0.06 d.l. 1.00 3.86 0.23 0.65 77.44 3.55 1.29 2.13 0.15 0.88 5327.57 1.43 1.49 0.31 6.84 0.77 0.10 0.70 0.09

Table 2. Mg# is Mg/(Mg + total Fe) atomic ratio and all Fe are Fe2+ in silicates. Olivine composition in metagabbros has a narrow range of Fo = 78–80. CaO contents are negligible (<0.04 wt%). MnO contents are at 0.25–0.35 wt% while NiO is from 0.05 to 0.10 wt%. Plagioclase is calcium-rich with CaO content of 11–19 wt%. Anorthite content increases from An52 to An70 in amphibolites and An70 to An96 in metagabbros. They plot within the labradorite, bytownite, and anorthite fields. Clinopyroxene is diopside to augite. Clinopyroxene Mg# = 0.80– 0.86 in metagabbro overlaps with Mg# = 0.83–0.86 in hornblende pyroxenite. The clinopyroxene in amphibolites have distinctly lower

Mg# = 0.72–0.77. All clinopyroxenes have very low Cr2O3 content, <0.2 wt%. A plot of TiO2 and Al2O3 wt% vs Mg# is shown in Fig. 4. Clinopyroxene data from the Ichinomegata hornblende–pyroxene and pyroxene gabbro xenoliths (Takeuchi, 2008) and the Talkeetna gabbronorite (Greene et al., 2006) are also shown in the diagrams. The Ichinomegata samples have higher TiO2 and Al2O3 contents than the Calaton Hill rocks and Talkeetna gabbronorite. Clinopyroxenes in the Calaton Hill amphibolites have almost the same TiO2 and Al2O3 abundances with the clinopyroxenes in the Talkeetna gabbronorite. Orthopyroxene is enstatite with Mg# increasing from amphibolites (0.65–0.70) to metagabbros (0.75–0.81) and hornblende pyroxenites (0.80–0.81). TiO2 (<0.2 wt%) and CaO (<1.5 wt%) com-

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Fig. 8. Chondrite-normalized REE patterns of the three types of amphiboles from the Calaton Hill, Philippines. (a) Type I amphiboles. (b) Coronitic amphiboles (Type II) in metagabbros and amphiboles (Type III) in amphibolites. The patterns of Type II coronitic amphiboles are almost similar to plagioclase in Talkeetna gabbronorite. The gray field represents amphiboles from Ichinomegata hornblende–pyroxene and pyroxene gabbro xenoliths (Takeuchi, 2008). Type III amphiboles have intermediate characteristics between Types I and II amphiboles. Chondrite normalization values after Sun and McDonough (1989).

positions in all Calaton Hill rock types are within the same range of the Ichinomegata pyroxene gabbro xenoliths and the Talkeetna gabbronorite (Fig. 4). In terms of Al2O3, the Calaton Hill rocks generally contain about 1–2 wt%. The coronitic orthopyroxene is distinct from discrete orthopyroxene in metagabbro as it is characterized by lower TiO2 and CaO contents. Moreover, it has higher Al2O3 (4 wt%), more than twice of the amount in the discrete orthopyroxene. Structural formulae for amphibole were calculated on anhydrous basis (O = 23). According to the classification of Leake et al. (1997), the amphiboles are mostly magnesiohornblende and tschermakite (Fig. 5). Several grains have (Na + KA) P 0.5 and plot within the edenite and magnesiohastingsite fields. Correlation of

Mg# and textural occurrence reveals that the amphiboles in the Calaton Hill rocks can be classified into three (Fig. 6a and b). Type 1 is comprised of interstitial large euhedral to subhedral amphiboles observed in metagabbros, hornblende pyroxenites and hornblendites. They have high Mg# ranging from 0.76 to 0.81 in metagabbros and hornblende pyroxenites and slightly lower Mg# (=0.73–0.74) in hornblendites. Coronitic amphiboles occurring around olivine constitute the second type and have intermediate Mg# = 0.76–0.77. In addition, Type II amphiboles have very low TiO2. The third type is the greenish amphibole observed as overgrowths on orthopyroxene in the amphibolites. The Mg# (=0.65–0.68) of Type III amphiboles is distinctly lower than Type I and Type II amphiboles.

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381

Fig. 9. Extended trace element patterns for (a) Type I and (b) Types II and III amphiboles compared with amphiboles from the Ichinomegata hornblende–pyroxene and pyroxene gabbro xenoliths (Takeuchi, 2008). NMORB normalization values are after Sun and McDonough (1989).

The green spinel seen as dispersed grains within coronitic amphibole in the metagabbro belongs to the spinel-hercynite series. Al2O3 and FeO contents are at 60.0–63.5 wt% and 22.0–22.5 wt%, respectively. Magnetite in the amphibolites has FeO contents ranging from 78 to 86 wt%. Cr2O3 is also low at 0.03–0.20 wt% whereas Al2O3 varies from 1.7 to 3.9 wt%. 4.2. Trace element compositions Distinction between the three types of amphiboles is also clearly depicted by the YbN vs Mg# diagram (Fig. 6b) wherein Type I amphiboles have higher YbN abundance over Type II amphiboles

even if both types have almost the same Mg#. Type III amphiboles have intermediate YbN content between the first two types but with lower Mg#. Trace element data for clinopyroxene and amphibole in each rock type are presented in Table 3. Chondrite-normalized patterns for clinopyroxene are uniformly concave downward in shape (Fig. 7a). They are depleted in light and heavy rare earth elements with pronounced negative Eu anomaly. Trace element abundance in clinopyroxene increases from metagabbro through hornblende pyroxenite and metagabbro in contact with hornblendite. The extended NMORB-normalized diagram shows negative anomalies for Nb, Zr and Ti (Fig. 7b). The concave downward patterns as well as the negative anomalies for Nb, Zr and Ti in the clinopyroxenes in

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the Calaton Hill samples are comparable with chondrite-normalized patterns of clinopyroxenes in hornblende-pyroxene and pyroxene gabbro xenoliths from Ichinomegata, NE Japan arc (Fig. 7). Amphiboles in the Calaton Hill samples have elevated trace element abundances compared with clinopyroxene. Trace element abundances decrease from Type I to Type III and Type II amphiboles. Type I amphiboles have chondrite-normalized patterns similar to the concave downward pattern previously observed in clinopyroxenes (Fig. 8a). Their patterns also show depletion in light and heavy rare earth elements with varying negative to negligible Eu anomaly. Type III amphiboles have the same patterns as Type I amphiboles but with lower trace element abundance and pronounced positive Eu anomaly (Fig. 8b). Lastly, coronitic amphiboles of Type II have almost the same light rare earth abundance as Types I and III. However, they have very pronounced positive Eu anomaly and contain extremely depleted heavy rare earth elements. The general trend of the Type II coronitic amphibole mimics the REE pattern of a plagioclase in the Talkeetna gabbronorite (Fig. 8b). In the extended NMORB-normalized diagram, strong depletions in Nb and Zr and slight enrichment in Sr and Ti can be noted in the Calaton Hill amphiboles. Such features are again reminiscent of amphibole patterns from the Ichinomegata hornblende–pyroxene and pyroxene gabbro xenoliths (Fig. 9).

5. Thermobarometry Two-pyroxene thermometers were applied on selected clinopyroxene and orthopyroxene pairs in hornblende pyroxenite and metagabbro. The thermometer of Wells (1977) based on diopside–enstatite miscibility is applicable to Mg-rich pyroxenes over a temperature range of 800–1700 °C ± 70 °C. This thermometer yielded 896–928 °C and 904–1024 °C for the hornblende pyroxenite and metagabbro, respectively. The thermometer of Brey and Kohler (1990) yielded slightly lower values of 842–878 °C for the hornblende pyroxenite and 896–927 °C for the metagabbro. In addition to two-pyroxene thermometry, we also utilized Holland and Blundy’s (1994) geothermometer based on the equation: edenite + albite = richterite + anorthite on selected plagioclase and amphibole pairs in amphibolite and metagabbro. This geothermometer is appropriate for silica-undersaturated and -saturated rocks and magmas in the range of 400–900 °C and 1–15 kbars, with an uncertainty of ±40 °C. The amphibolites yielded 884–905 °C at 5 kbars. Resulting temperature values for the metagabbros are 782– 839 °C and 812–874 °C at 5 and 10 kbars, respectively. Clinopyroxenes and orthopyroxenes in metagabbros and hornblende pyroxenites have high Mg# (dominantly 0.80) comparable with pyroxenes of the Ichinomegata pyroxene–hornblende gabbro xenoliths (Fig. 4). A plot of the distribution of Mg and Fe2+ between coexisting clinopyroxenes and orthopyroxenes (Fig. 11) reveal that the magma temperatures of the Calaton Hill metagabbros and hornblende pyroxenites are slightly lower than those of normal magmatic (=plutonic) rocks (Kretz, 1963).

try, we interpret the Calaton Hill rocks as metamorphic/plutonic lower crustal materials derived from hydrous arc magmas. Major element data of the mineral phases in metagabbros are comparable to those of gabbroic rocks in arc-related setting (Beard, 1986; DeBari and Coleman, 1989; Bonev and Stampfli, 2009). Plagioclase An content plotted against olivine Fo content (Fig. 10) falls within the arc Types I and III olivine gabbro classification of Beard (1986). The coexistence of calcic plagioclase (An85–100) and moderately Fe-rich (Fo60–80) olivine is unique to arc cumulate gabbros. High water pressure in hydrous basaltic magma causes anorthite enrichment in plagioclase (Arculus and Willis, 1980; Sisson and Grove, 1993). Mg-rich olivine is the first or an early crystallizing phase from basaltic magmas. On the basis of the Mg–Fe exchange partitioning between olivine and basaltic liquids (KD = (Fe/Mg)ol/ (Fe/Mg)liq = 0.3 (Roeder and Emslie, 1970; Tatsumi et al., 1983), we determined the composition of the liquid from which the olivine crystallized. Mg# of the calculated melt ranges from 0.54 to 0.57. These values correspond to the Mg# of typical orogenic basic andesites (Gill, 1981). Moreover, depletion in Nb, Zr and Ti in the extended NMORBnormalized diagram is shared by the Calaton Hill and Ichinomegata pyroxenes (Fig. 7b). Strong negative peaks in Nb and Zr and to a lesser extent Ti in clinopyroxene trace element patterns are reminiscent of island arc basalt compositions (Arculus, 1994; Pearce et al., 1994). Calculated basaltic and andesitic melt compositions in equilibrium with the clinopyroxenes of the Calaton Hill rocks are shown in Fig. 12. The general trend of the calculated values is comparable with calc-alkaline andesite and basalt from Mts. Mayon and Arayat (Castillo and Newhall, 2004; Bau and Knittel, 1993). Mafic to intermediate arc rocks typically exhibit jagged MORB-normalized patterns due to the depletion of high field strength elements (HFSE) such as Nb and Ta and the enrichment of large ion lithophile elements (LILE) Rb, Ba, U, Th, K, Pb and Sr (McCulloch and Gamble, 1991; Murphy, 2006). Compared to the latter, the LILEs are readily dissolved in and transported by aqueous fluids released by dehydration during subduction zone magmatism (Kogiso et al., 1997). The pervasive occurrence of hornblende in all Calaton Hill rock types is another strong indication for the involvement of hydrous magma. Beneath an active island arc, a large part of the lower crust may be partially fused and renewed repeatedly by frequent trans-

6. Discussion The Calaton Hill rocks have been previously interpreted as part of the gabbroic section of the Sibuyan Ophiolite Complex (Ramos, 2006; Yumul et al., 2003; Dimalanta et al., 2009). Faure et al. (1989) have also suggested that the Calaton Hill represents the southern section of the Romblon Metamorphics whose thermal metamorphism has been overprinted by granitic intrusion. In this work, four rock types are identified from the Calaton Hill: amphibolites, metagabbros, hornblende pyroxenites and hornblendites. Based on their characteristic mineral assemblages and geochemis-

Fig. 10. Plagioclase An plotted against olivine Fo in metagabbro. Note that all samples plot in the arc Type I and III fields after Beard (1986).

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383

Fig. 11. Mg–Fe distribution between orthopyroxenes and clinopyroxenes in the Calaton Hill rocks and Ichinomegata gabbroic xenoliths. Trends for magmatic and metamorphic pyroxenes are after Kretz (1963).

fer of magmas from the upper mantle towards the surface (Aoki, 1971; Takahashi, 1986; Murphy, 2006). The amphiboles are classified into three types: (a) Type I – primary and interstitial amphiboles in metagabbros, hornblende pyroxenites and hornblendites, (b) Type II – coronitic amphibole in metagabbros and (c) Type III – amphiboles having metamorphic textures in amphibolites. The first two types have Mg# close to the Mg# of the pyroxenes whereas the Type III amphiboles have lower Mg# = 0.65–0.67. A more detailed discussion on the formation of the amphiboles is presented in the succeeding section. We would like to emphasize that the amphiboles in the Calaton Hill rocks are not homogenous but rather show particular characteristics reflective of varying physical and chemical conditions of formation. The mode of occurrence of hornblendite strongly suggests the younger formation of hornblendite replacing pyroxene-rich lithologies. Combining field and laboratory data, a schematic model of the lithologic correlation of the Calaton Hill metamorphic/plutonic complex is shown in Fig. 13. Olivine gabbro and pyroxenite derived from arc magmas possibly andesitic to basaltic andesite in composition are the probable protoliths of the metagabbros and hornblende pyroxenites. Subsequent infiltration of a new batch of hydrous magma formed hornblendite and the pervasive formation of hornblende at the expense of pyroxenes and plagioclase in the pre-existing lithologies. The infiltration of this younger melt could account for the overlapping geochemical characteristics shared by the mineral phases in metagabbros, hornblende pyroxenites and hornblendites. The spatial relationship of amphibolites with the other rock types is not so clear. Amphibolites are composed of dominant plagioclase and amphibole with subordinate amounts of pyroxene and magnetite. Plagioclases in the amphibolites are also calcic (An52–70). Pyroxenes have Al2O3, TiO2 and CaO contents within the range of the metagabbros and hornblende pyroxenites. However, they have lower Mg# = 0.65–0.70. The amphibolites could have also independently and separately originated from a gabbroic protolith located far from the main influx of the infiltrating younger magma. A distal occurrence would mean minimal reaction or alteration compared to the metagabbros and

hornblende pyroxenites which appear to have been strongly affected by the younger magma. In summary, the metagabbros, amphibolites and hornblende pyroxenites are representative of unextruded basaltic magma which was later modified to various extents by the arrival of a new batch of hydrous magma which formed the hornblendite and caused widespread amphibole formation. Cooling from igneous conditions coupled with retrogressive metamorphism (=supply of fluids) is recorded by the metamorphic features in the rocks (e.g. triple junctions between minerals, foliated texture in amphibolites, corona formation in the metagabbros and recrystallization of plagioclase boundaries) (Fig. 3a, c and d). Plagioclase–amphibole geothermometry (Holland and Blundy, 1994) on selected pairs from amphibolites and metagabbros gave temperature values of 884–905 °C at 5 kbars for the amphibolites whereas the metagabbros yielded 782–839 °C at 5 kbars and 812–874 °C at 10 kbars. Moreover, the absence of garnet from the Calaton Hill hornblende granulite provides a pressure constraint and implies that the pressure conditions for these rocks are lower than 8 kbars: two-pyroxene granulite transforms to garnet granulite at about 10 kbars at 1100 °C and about 9 kbars at 900 °C (Irving, 1974; Cruciani et al., 2008). The two-pyroxene temperature, 900–1000 °C, for the Calaton Hill metagabbros is close to the high-temperature limit of the granulite facies (Yardley, 1989; Miyashiro, 1994), and higher than the temperature for ordinary granulite facies metamorphic rocks (Hewins, 1975; Daczko et al., 2001). This temperature range is very close to that of hornblende precipitation from calc-alkaline magmas (Eggler, 1972). This is consistent with our scenario (Fig. 13). The metamorphism including formation of Type II amphibole as well as the precipitation of Type I amphibole in the metagabbro was caused by invasion of calc-alkaline magma into the olivine gabbro protolith (Fig. 13). Considering the slightly lower temperature, 800–900 °C, obtained by the plagioclase–hornblende thermometer for both amphibolites and metagabbros, the Calaton Hill rocks have experienced metamorphism at a high-temperature condition, 800–1000 °C (amphibolite to granulite facies). The hightemperature condition (950–1100 °C) recorded by granulite xeno-

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Fig. 12. Calculated (a) basaltic and (b) andesitic melts in equilibrium with Calaton Hill clinopyroxenes. Gray fields represent Mt. Mayon calc-alkaline andesite and basalt (Castillo and Newhall, 2004) whereas stippled fields are from basaltic lavas of Mt. Arayat (Bau and Knittel, 1993). Clinopyroxene-melt partition coefficients used are as follows: basaltic melt = Hart and Dunn (1993) and Fujimaki et al. (1984), and andesitic melt = Pearce and Norry (1979), Gill (1981), Fujimaki et al. (1984) and Bedard (1999).

liths from Central Mexico was possibly due to underplating in the lower crust beneath an active arc (Hayob et al., 1989). Alternatively and more preferably, it could also be hypothesized that the Calaton Hill rocks have been produced from the direct crystallization of basaltic magma in granulite facies condition at lower crust level. Such process of formation had been suggested by Jahn (1990) in contrast to the commonly known prograde metamorphism from greenschist through amphibolite and ultimately granulite facies conditions. The notion of direct magmatic crystallization in granulite facies P–T conditions had been suggested to account for the short time spans between magmatic emplacement and granulite facies metamorphism observed in other granulite terrains.

7. Amphibole and corona formation Amphibole may form as a late stage magmatic phase, crystallizing from chemically evolved hydrous melts or exsolved magmatic fluids as well as a hydrothermal phase replacing pyroxene and

olivine or filling fractures in oceanic plutonic sequences (Mevel, 1987; Coogan et al., 2001; Gillis and Meyer, 2001). They therefore record the complex evolution from solidification of primary cumulates and magmatic fluids to alteration by seawater-derived fluids. The pervasive amphibole occurrence in the different lithologies from Calaton Hill reflects the derivation of the protoliths from the place where hydrous magmas or aqueous fluids where available. Arc mafic magmas are commonly hydrous and undergo partial fractional crystallization and accumulation at different crustal levels (DeBari and Coleman, 1989; Meurer and Claeson, 2002; Greene et al., 2006; Helmy et al., 2008). Type I amphiboles are clearly noted as large discrete and interstitial euhedral and subhedral grains in metagabbros, hornblende pyroxenites and hornblendites. The Mg# of Type I amphiboles ranges from 0.76 to 0.81 which is significantly low compared to coexisting pyroxenes. The amphiboles in the metagabbros and hornblende pyroxenites are possibly phases precipitated from the invading melt responsible for hornblendite formation or late stage-crystallization products from residual liquid after the crystallization of the protolith olivine gabbro. The general trend of

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Fig. 13. Schematic diagram showing the spatial distribution of the Calaton Hill metamorphic/plutonic complex. (a) Protoliths are composed of olivine gabbros with lenses of amphibolites and pyroxenites. (b) Infiltration of hydrous magma modified the pre-existing arc magma-derived protoliths and triggered pervasive formation of amphiboles. The possible sites of formation for the three amphibole types are also denoted.

the chondrite-normalized patterns of Type I amphiboles also mimics that of clinopyroxene but at higher abundance levels. Such feature had also been reported from titanian pargasite which precipitated from fractional crystallization of interstial fluid in olivine-bearing gabbros from the Northern Appenine, Italy (Tribuzio et al., 1999). The negative Eu anomaly in Type I amphiboles could also have been due to the fractionation of Eu into early crystallizing plagioclase. The similarity of REE patterns of all clinopyroxenes and Type I amphiboles in metagabbros, hornblende pyroxenites and hornblendites possibly indicates strong modification by the newly supplied hydrous melt (basaltic andesite to andesite) to form hornblendite (Arai et al., 2008). Alternatively, hydrous magmas which had been active around the same place shared the similar trace-element characteristics. This resulted in the similar traceelement features for all concerning phases, irrespective of the origin, igneous, metasomatic, or metamorphic. Type II coronitic amphiboles are observed around olivine in the metagabbro. These usually occur in association with secondary orthopyroxene and green spinel (Fig. 3a). Corona textures between plagioclase and olivine are well-documented in mafic rocks (Gardner and Robins, 1974; Ikeda et al., 2007; Cruciani et al., 2008). In most cases, the corona consists of an inner shell of orthopyroxene around olivine, rimmed by clinopyroxene and/or amphibole + spinel symplectite. Disequilibrium between olivine and plagioclase assisted by late stage magmatic fluids (Claeson, 1998; De Haas et al., 2002; Helmy et al., 2008) or through metamorphic processes (Gardner and Robins, 1974; Mongkoltip and Ashworth, 1983; Trolliard et al., 1988; Cruciani et al., 2008). In the first model, coronitic amphiboles are thought to have grown early at the magmatic stage and continued cooling aided by the progressive concentration of hydrous liquid. Slow cooling during metamorphism and redistribution of elements such as Fe and Mg

from olivine and Ca and Al from plagioclase are necessary for amphibole development in metamorphosed gabbros. Though most of these works show that the diversity of mineral assemblages and textures within a corona in mafic rocks depend largely on several factors such as the chemistry of the reactants, pressure, temperature and the presence or absence of liquids, the magmatic versus metamorphic origin of olivine–plagioclase coronas remains to be resolved. Either way, amphibole growth in an otherwise anhydrous mineral assemblage requires the intervention of an aqueous fluid (De Haas et al., 2002). In the case of the Calaton Hill metagabbro, the inner rim of the olivine consists of orthopyroxene whereas the outer rim is largely occupied by brownish to greenish amphibole at times containing green spinel. This corona assemblage in the Calaton Hill metagabbro is exactly similar with the coronas in the Ichinomegata hornblende–pyroxene gabbro xenoliths (Fig. 3a and b). Mongkoltip and Ashworth (1983) have also reported the occurrence of such corona assemblage in the Newer Basic intrusions of NE Scotland. The coronas are interpreted to have formed after this reaction: olivine + calcic plagioclase + H2O M orthopyroxene + amphibole + spinel (Mongkoltip and Ashworth, 1983). The subsequent infiltration of a hydrous magma thought to be the source of the hornblendite could have provided the liquid needed in the reaction. Types I and II amphiboles have the same range of Mg# but have different trace element patterns. LREE abundance in Types I and II are similar but Type II coronitic amphiboles exhibit a positive Eu anomaly and severe depletion in HREE. Type II amphiboles formed at the expense of plagioclase as suggested by the similarities of their REE patterns (Fig. 8b) with contribution from an LREE-enriched hydrous magma. The origin of the Type III amphiboles observed as overgrowths over orthopyroxenes in amphibolites is more difficult to ascertain. Chondrite-normalized patterns of Type III amphiboles show

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LREE-enrichment as Types I and II. They are also characterized by a pronounced positive Eu anomaly as Type III amphiboles. HREE contents are less than Type I amphiboles but significantly higher than Type II amphiboles. Moreover, the Mg# of Type III amphiboles are significantly lower than Types I and II. The Type III amphiboles are only observed in the amphibolites. As discussed in the previous section, the amphibolites may have been formed in an older and separate event with respect to the other rock types. Type III amphiboles in the amphibolites may have been modified by the infiltrating magma since it also has the same LREE-enrichment noted in Types I and II amphiboles. Despite their geochemical differences, LREE-enrichment in the three types of amphiboles suggests that the all the amphiboles may have been affected and have undergone modification by an LREE-enriched melt (Tribuzio et al., 1999; Gillis and Meyer, 2001). Alternatively, they may have been modified by circulating aqueous fluids with the same REE content. Fig. 13 illustrates the possible sites of formation of the three types of amphiboles. In this model, we assume that a younger influx of hydrous magma infiltrated into older lithologies, formed the hornblendite and triggered the widespread formation of amphiboles. Type I amphiboles could have formed near the main influx of the hydrous magma thus the similar geochemical characteristics of Type I amphiboles in the metagabbros, hornblende pyroxenites and the hornblendite. Type

II amphiboles are formed by the reaction of the olivine and plagioclase with liquids migrating away from the cooling hydrous magma. Lastly, Type III amphiboles in the amphibolites located far from the metasomatizing agent experienced the least alteration.

8. Comparison with the Ichinomegata mantle xenoliths The Ichinomegata xenoliths from the NE Honshu arc have been intensively studied and abundant data is available (Aoki, 1971; Takahashi, 1978, 1986; Kushiro, 1987; Abe et al., 1995; Nishimoto et al., 2005; Arai et al., 2007; Takeuchi, 2008). Located along the convergent boundary between the Pacific plate and the North American plate, volcanism along the NE Honshu arc is being attributed to the westward subduction of the Pacific Plate at a 30° angle. The structure of the crust and upper mantle beneath the Northeast Honshu arc has been inferred on the basis of the mafic and ultramafic xenoliths from volcanoes along this arc particularly from Ichinomegata. Xenoliths incorporated within the calc-alkaline andesite host rocks include spinel lherzolite, harzburgite, pyroxenite and mafic rocks such as gabbro, granulite, amphibolite and low-grade metamorphic rocks (Aoki, 1971; Takahashi, 1978, 1986; Kushiro, 1987). Detailed studies on the Ichinomegata xenoliths have revealed that the lower crust of the NE Honshu

Fig. 14. Simplified schematic diagram for the exhumation of the Calaton Hill rocks (modified after Mattauer, 1985).

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arc consist of amphibolite and hornblende gabbros and most likely granulite in the lowermost part of the crust. Recent geophysical data further suggest that the thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition (Nishimoto et al., 2005). Hydration of the Ichinomegata mafic and ultramafic xenoliths by infiltrating hydrous arc magmas forming hydrous phases such as amphiboles and phlogophites have also been reported (Abe et al., 1995; Arai et al., 2007; Takeuchi, 2008). Petrographic analysis of the metagabbros, hornblende pyroxenites and hornblendites from Calaton Hill reveal striking similarities with the Ichinomegata gabbroic xenoliths. The wellpreserved coronitic shells of orthopyroxenes, amphiboles and green spinels around some of the olivine grains in the metagabbros have been observed in hornblende–pyroxene gabbro xenoliths (Fig. 3a and b) from Ichinomegata (Takeuchi, 2008). Amphibole cutting and replacing pre-existing pyroxenes noted in the Calaton Hill hornblende pyroxenites are also common in the hornblende– pyroxene gabbro xenoliths (Fig. 3e and f) (Takeuchi, 2008). Lastly, the Calaton Hill and Ichinomegata hornblendites appears to be almost identical (Fig. 3g and h). The formation of coronas in the metagabbro, the formation of amphibole at the expense of pyroxenes in the other rock types, the presence of hornblendites and the pervasive occurrence of amphiboles in the Calaton Hill rocks suggest that the Calaton Hill samples may have undergone hydration like the Ichinomegata gabbroic xenoliths (Abe et al., 1995; Takahashi, 1986; Takeuchi, 2008). Moreover, some of the geochemical trends in the Calaton Hill rocks are akin to that of the Ichinomegata gabbroic xenoliths. Major oxide contents of the orthopyroxenes fall within the range of orthopyroxenes in the Ichinomegata pyroxene–hornblende gabbros (Fig. 4). Moreover, the Calaton Hill rocks have similar clinopyroxene REE patterns (Fig. 7) to the Ichinomegata xenoliths, NE Japan arc. The chondrite-normalized trace element patterns of the clinopyroxenes in the Calaton Hill rocks share the same depletion in Nb, Zr and Ti observed in the clinopyroxenes of the Ichinomegata hornblende– pyroxene and pyroxene gabbro xenoliths. From these observations, we can clearly see that the Calaton Hill metamorphic/plutonic complex is comparable with the well-studied Ichinomegata gabbroic xenoliths hosted by the volcanic rocks of the NE Honshu arc which are thought to be typical sub-arc lower crust materials.

9. Geodynamic implications The Calaton Hill metamorphic/plutonic complex is representative of the lower crust of the Philippine island arc. Derivation from the middle crust is also possible depending on the thickness and the geothermal gradient of the crust. The lithologic association, mineral assemblages and geochemistry of the Calaton Hill metamorphic/plutonic complex is analogous to the well-established arc-affinity and lower crustal structure of the Ichinomegata, NE Japan arc. Assuming that the Calaton Hill rocks are indeed representative of the lower crust of the Philippine island arc, we reconstruct a scenario for its exhumation. Since no age dating data is available on these rocks, we attempt to constrain the age of the Calaton Hill rocks by looking into pre-existing arc-related lithogies in the area. The most likely candidate would be the Tablas Volcanics which has been suggested to be 18–19 Ma based on 40K–40Ar isotopic dating (Bellon and Rangin, 1991). It is probable that the Calaton Hill rocks were originally lower crustal materials associated with the then active Tablas volcanic arc. Fig. 14 is a simplified schematic diagram for the emplacement of the Calaton Hill rocks (modified after Mattauer, 1985). Arc volcanism was prevalent during the preMiocene resulting to the formation of the Tablas Volcanics. The Calaton Hill rocks were later exhumed due to the collision of the North

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Palawan Block (NPB) with the Philippine Mobile Belt (PMB) during the Early Miocene (Karig, 1983; Marchadier and Rangin, 1990; Yumul, 2007). The age of exhumation of the Tablas Volcanics is probably related to the Romblon Metamorphics which have been reported to be 12.2 ± 0.3 to 12.3 ± 0.3 Ma in age (Ramos, 2006). 10. Conclusions (a) The Calaton Hill metamorphic/plutonic complex is comprised of amphibolites, metagabbros, hornblende pyroxenites and hornblendites. Major and trace element data on these rocks reveal derivation from igneous gabbroic protoliths believed to be derived from arc magma. (b) Infiltration of hydrous andesitic magma into pre-existing lithologies led to the formation of hornblendite and the pervasive occurrence of amphiboles in the different rock types. The three types of amphiboles reflect various formation and degrees of modification. (c) The Calaton Hill metamorphic/plutonic complex has petrographic and geochemical similarities with that of the wellstudied Ichinomegata, NE Honshu arc, Japan. The Calaton Hill metamorphic/plutonic complex is believed to be representative of the lower crust underlying the Philippine island arc.

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