Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinental Block, Philippines

Home , Busuanga Island, Calamian Islands, Mindoro, Palawan (island)

῏῕῍ΐῐ῔ῌ (48), pp. 177ῌ199, 2012 ῒ 3 ῎ 28 ῑ Mem. Natl. Mus. Nat. Sci., Tokyo, (48), pp. 177ῌ199, March28, 2012

Kazumi Yokoyama1, ῍, Yukuyasu Tsutsumi1, Tomoki Kase1, Karlo L. Quean˜o 2 and Aguilar Yolanda M.2

1 Department of Geology, National Museum of Nature and Science, Tokyo, 4ῌ1ῌ1 Amakubo, Tsukuba, Ibaraki 305ῌ0005, Japan 2 Mines and Geosciences Bureau, Diliman, Quezon City, Philippines ῍ E-mail: [email protected]

Abstract. The Palawan microcontinental block in the Philippines was separated from the southwestern coast of the Asian continent (i.e. southwestern Taiwan) during the opening of the South China Sea in the Oligocene to Miocene times. In this paper, provenances of the detrital grains in the Jurassic to Early Cretaceous sandstones from the Palawan block were studied to determine whether they were derived from the Asian continent. Age distributions of detrital monazites in the sandstones from the Busuanga, Mindoro, and Panay islands are essentially bimodal with peaks at 140ῌ260 Ma and 1800ῌ2000 Ma. Such a pattern is not recognized from the region of assumed origin before the opening of the South China Sea nor from the Indochina Peninsula. A similar bimodal pattern is observed on the Korean Peninsula, in coastal areas of the Shangdong Peninsula, and in Zhejiang Province in China. These areas are located at the marginal East China Sea. Hence, it is concluded that the Jurassic to Early Cretaceous sandstones of the Palawan microcontinent were parentally deposited on the eastern side of present-day Taiwan. Key words: Palawan, monazite, age, sandstone, tectonics.

et al., 2003). The Palawan microcontinent con- Introduction sists of Upper Paleozoic to Mesozoic rocks The Palawan microcontinent is a small block (Hashimoto and Sato, 1973: Isozaki et al. consisting mainly of the north terrane of 1987). The constituent rocks belong to oceanic Palawan Island, Busuanga Island, the south- plate stratigraphy or a subduction complex western part of Mindoro Island, and the north- composed mainly of Permian to Upper Jurassic western part of Panay Island (Fig. 1). The chert, Middle Jurassic to Cretaceous clastics, tectonic model of the microcontinent during and limestone blocks of various ages (e.g. the Tertiary has been well illustrated. The Isozaki et al. 1987: Zamoras & Matsuoka, 2001 Palawan microcontinent was located in the &2004; Zamoras et al. 2008). Basaltic blocks western part of Taiwan and separated from the and Cretaceous shallow marine sediments Asian continent at the time of the opening of occur locally on the microcontinent (e.g. the South China Sea (Holloway, 1982). A mag- Zamoras and Matsuoka, 2001: Andal et al., netic anomalies in the South China Sea show 1968). that the spreading started from about 32 Ma In this paper we study the provenances of and ended at 17 Ma (Taylor and Hayes, 1980). the sandstones in the Palawan microcontinent After that time period, the microcontinent to elucidate the location of the deposition or began to collide with the Philippine Mobile subduction at the time of the Jurassic to Early Belt (Hamilton, 1979: Holloway, 1982: Yumul Cretaceous. The conventional approach to pro- 178 Kazumi Yokoyama et al.

Fig. 1. Sampling localities of recent sands from Asia and the Palawan microcontinent. Reconstruction of the microcontinent before the opening of the South China Sea is of Zamoras and Matsuoka (2004). Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 179 venance studies of sandstones is based on deter- durated sandstone and shale. Zamoras and mination of chemical compositions of detrital Matsuoka (2001) referred to the BMG (1984) minerals in the sandstones and comparison geological map, but they used the name with data from the Asian continent. The devel- “Guinlo Formation” instead of the King opment of analytical techniques that allow age Ranch Formation and the clastics in the determinations to be made on individual min- Liminangcong Formation were treated as part eral grains has provided a powerful tool in of the “Guinlo Formation”. Except for lime- provenance studies. Many age dating methods stone blocks, Zamoras and Matsuoka (2001 & have been applied to provenance studies of 2004) classified the rocks on the island into zircon, for example the Sensitive High- chert, siliceous mudstone, and terrigenous clas- Resolution Ion Microprobe (SHRIMP) (e.g. tics and divided chert-clastic sequences in the Ireland, 1991; Tsutsumi et al. 2003), fission- Guinlo and Liminangcong formations into track dating (e.g. Garver et al. 1999), in- three belts on the basis of the radiolarian ductively coupled plasma mass spectrometry fossils in the terrigenous clastics: the Northern (ICP-MS) (e.g. Wyck & Norman, 2004; Evans Busuanga (Middle Jurassic), Middle Busuanga et al. 2001), and by monazite data via the (Late Jurassic), and Southern Busuanga electron probe micro-analyzer (EPMA) (e.g. (Early Cretaceous). Zamoras and Matsuoka Suzuki, Adachi & Tanaka, 1991; Fan et al. (2001) described the sandstone in the Guinlo 2004; Yokoyama et al. 2007). Because age data Formation occasionally as arkose sandstones of monazites in the sands from the rivers cut- and have not used the term “tu#aceous” for the ting through the coastal provinces of Eastern clastics. Asia have already been reported (Fig. 1: On the western part of Panay Island, a Yokoyama et al., 2007, 2008, 2010), the bedded chert and clastic sequence occurs. Al- monazite data will be a strong tool for compar- though continuous succession from clastic rock ison between the sandstones on the Palawan to chert has not been confirmed, they are microcontinental block and the sands on the treated as a Jurassic subduction complex con- Asian coastal provinces. sisting of a chert-shale-clastic sequence corre- sponding to the Middle Busuanga belt (Zamoras et al. 2008). The sandstone is ar- Geological setting enitic in composition. In the southwestern part Most of the sandstone samples were collect- of Mindoro Island, Jurassic shallow marine ed from Busuanga Island, north of the Palawan sediments occur. They are composed mainly of block. Geological maps of the island were sandstone and shale with a thin layer of tu#. published by the Bureau of Mines and Geosci- The sediments are rather rich in fossil remains. ences (BMG) in 1984 and radiolarian ages of Late Middle to Early Late Jurassic ammonite, the chert-clastic sequences were later estab- belemnite, and pelecypod were described lished by Zamoras and Matsuoka (2001 & (Andal et al. 1968). Many animal tracks are 2004). According to BMG (1984), the island is also observed in the Jurassic sediments. Some composed of three formations: the King Ranch sandstone layers are frequently intercalated by Formation, the Malajon Limestone, and the black bituminous shale. Liminangcong Formation. The King Ranch Formation is composed predominantly of Sample Description tu#aceous shale and sandstone with intercalated tu# and minor thinly bedded chert, On Busuanga Island, about twenty sand- whereas the Liminangcong Formation is com- stones were collected from the three belts of posed of bedded chert with interbedded in- Zamoras and Matsuoka (2001 & 2004) as 180 Kazumi Yokoyama et al. Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 181 shown in Fig. 2. Some samples were collected Mindoro and Panay islands are indurated cal- from the same route studied in detail by careous quartzose sandstone and arkose sand- Zamoras and Matsuoka (2001). Sandstones stone, respectively (Fig. 3). were also collected from the shallow marine In addition to the sandstone samples from sediment in the southeastern part of Mindoro the Palawan microcontinent, a few river sands, Island and from a clastic sequence in the west- BA019 and BA020, were collected from the ern part of Panay Island. western part of Busuanga Island (Fig. 2). On Busuanga Island, there are two types of Additionally, five sand samples were newly col- sandstones which correspond to those from the lected from the southern coastal provinces of King Ranch Formation and the Liminangcong the Asian continent (Fig. 1). Characteristics of Formation described in the BMG (1984). The the major river sands from the Asian continent sandstones from the King Ranch Formation (Fig. 1) have been reported by Yokoyama et are poor in quartz and feldspar grains (Fig. 3). al. (2007, 2008, & 2010). The present sand Clay minerals are abundant and loosely samples will also provide indicators of probable packed. And, just as described by the BMG provenance areas on the assumption that the (1984), the clay minerals are identified as being Palawan microcontinent was separated from “tu#aceous”. On the other hand, sandstones the Asian continent. from the Liminangcong Formation are in- They durated samples. are also poor in quartz Analytical Procedures and feldspar fragments (Fig. 3). The di#er- ences between them are simply due to later Procedures for the separation of heavy min- stage metamorphism as described later in this erals and their subsequent identification are the paper. Most of the collected samples from same as have been described by Yokoyama et Busuanga Island are tu#aceous, which is di#eral. (1990). Carbonate and micaceous minerals ent from the quartzose or arkose sandstones on were not subjected to examination, and mag- Mindoro and Panay islands (Fig. 3). One netic fractions were removed prior to the sepa- sample, BA017, was collected from an outcrop ration of the heavy minerals. Modal propor- along a newly reconstructed road. The outcrop tions of representative heavy minerals are belongs to the Middle Busuanga Belt and con- shown in Table 1. The light minerals are less sists of sandstone and shale. The shale is bitu- source-diagnostic and therefore are not a major minous and locally contains a thin patch of focus in this provenance study. coal (Fig. 3). Sand pipe, a possible trace fossil, Among the heavy minerals, monazite is the is common in the sandstones from both sides of most important for elucidating the provenance. the bituminous shale. Although we did not The theoretical basis for monazite age calcula- observe a clear transition from sandstone to tions is essentially the same as that developed chert, BMG (1984) described the intercalation by Suzuki et al. (1991). Monazites were of tu#aceous sandstone and chert, and analyzed by the EPMA fitted with a Wave- Zamoras and Matsuoka (2001) treated the length Dispersive Spectrometer (WDS) JXA- sandstone as a part of the chert-siliceous shale- 8800 situated in the National Museum of terrigenous clastic sequence of the subduction Natureand Science. Analytical conditions used complex. Both the sandstone samples from the here have been described by Santosh et al.

Fig. 2. Sampling localities of sandstones on Busuanga, Mindoro, and Panay islands. Three belts, NBB, MBB, and SBB, are abbreviations for the Northern Busuanga, Middle Busuanga, and Southern Busuanga belts of Zamoras and Matsuoka (2004). 182 Kazumi Yokoyama et al.

Fig. 3. A & B: outcrop showing a bituminous shale layer, dark part, with coal patch. Sand pipes develop in the sandstones. Sampling locality BA017. CῌF: photomicrograph of sandstone (crossed polars). C: tu#aceous sandstone, BA005. D: well-solidiﬁed sandstone (possibly tu#aceous) BA002. E: calcareous quartzose sandstone from the Mindoro Island. F: arkose sandstone from the Panay Island.

(2003). Age calibrations were carefully ages obtained from both techniques were found performed by comparing data obtained from to have good consistency. Monazites with ages EPMA dating with those acquired via the of 3020 Ma and 64 Ma that were obtained by SHRIMP technique (e.g. Santosh, et al., SHRIMP zircon and K-Ar mica methods, re- 2006). Apart from minor shifts due to machine spectively, have been used as internal standards drift and variations in standard conditions, the for age calibrations. The standard deviation of rvnneSuyo uasct al rtcosSnsoe rmtePlwnMicrocontinent Palawan the from Sandstones Cretaceous Early to Jurassic of Study Provenance

Table 1. Heavy mineral species in the sandstones from Busuanga, Mindoro, and Panay islands. Numbers show grains counted properly from the heavy

fractions under energy dispersive spectrum. garnet: Fe-rich garnet. gr-and: grossular-andradite series garnet. TiO2:TiO2 polymorphs. epidote: epidote

group minerals. Sandῌ: sample BA019 & BA020

formationῌ1 KR KR LM LM LM LM LM LM KR KR KR KR KR KR Mindoro Is. Panay Is. beltῌ2 NBB NBB NBB NBB NBB MBB MBB MBB MBB MBB MBB MBB SBB SBB sample No. BA007 BA009 BA010 BA011 BA012 BA001 BA002 BA003 BA004 BA005 BA008 BA017 BA014 BA015 garnet 8 19 14 91 51 18 57 1 2 65 14 gr-and 1 2 epidote 14 94 74 24 17 6

TiO2 13 129 29 4 20 8 53 33 11 66 49 52 60 58 56 19 zircon 4768181033537019347992109954215381 titanite 40 47 7 25 apatite 1 2 7 27 13 tourmaline 1 2 1 1 6 1 3 1 5 5 6 allanite 1 1 1 3 1 ilmenite 106 3 7 spinel 2 2 3 2 1 40 35 3 monazite 20 1 preῌ3 pre6 3 22352 29pre xenotime 1 1 2 1 thorite 1 1 1 total 210 203 205 14 192 195 191 92 45 210 144 167 202 148 175 335

ῌ1: formation of KGM(1984) KR: King Ranch, LM: Liminangcong ῌ2: belts of Zamorasu and Matsuoka (2001),NBB: North Busuanga Belt, MBB: Middle Busuanga Belt, SBB: South Busuanga Belt ῌ3: pre:present 183 184 Kazumi Yokoyama et al.

Fig. 4. Back-scattered images of detrital monazites with various ages. A & B: monazites with ages less than 200 Ma. C: monazite with age around 260 Ma. D & E: rare monazites with 430 Ma and 800 Ma, respectively. F: monazite with 1860 Ma. The number in each grain shows monazite age (Ma). the age obtained depends mostly on the PbO The content of the monazite. errors for the age Heavy Minerals are within a few percent for most of the analyzed monazites that were rich in ThO2. Although many sandstone samples are Among the other heavy minerals, we tu#aceous and poor in heavy minerals, four- analyzed chemical compositions of spinel and teen mineral species were observed in the heavy garnet by EPMA. fractions and the abundance of each of the Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 185 mineral species has been determined (Table 1). suggesting a detrital origin (Fig. 4). Angular A restricted number of species is due to the or decomposed monazite occurs in the well common dissolution of some detrital mineral solidified sandstones from the Liminangcong species in the sandstones (e.g. Pettijohn, 1941; Formation. Such monazite texture was Morton, 1984 & 1991). Among the common reported from weakly metamorphosed sand- heavy minerals, zircon, monazite, spinel, tour- stones on the Japanese Islands and Taiwan maline, TiO2 polymorphs, and garnet are con- (Yokoyama & Goto, 2000; Yokoyama et al., sidered to be ultrastable minerals. Apatite and 2007). The monazite is clearly a secondary post xenotime are rare in the sediments and are also depositional mineral, supporting the idea that treated as detrital minerals. the sandstones in the Liminangcong Formation As a result of the dissolution of unstable were more or less metamorphosed. Occasional- heavy minerals, zircon and TiO2 polymorphs ly, rounded monazite is surrounded by angular are predominant in the heavy fractions of most monazite aggregate. In the sandstone BA007 of the sandstones (Table 1). Garnet and spinel and sandstone from Mindoro Island monazite are occasionally abundant. Apatite, tourma- is common; consisting of about 10% of the line, and monazite are sporadic and mostly heavy minerals. Monazite from sandstone found in small quantities. Both epidote and BA007 occurs as an aggregate, showing titanite usually occur in well solidified sand- decomposition by a later stage metamorphic stones belonging to those from the event. On the other hand, the monazite from Liminangcong Formation of the BMG (1984). the Mindoro Island sandstone is usually In addition to grossular-andradite series garnet rounded detrital grain. and allanite, these minerals have been totally Heavy fractions of the river sands from dissolved in Jurassic to Cretaceous sandstones Busuanga Island and the Asian continent were (Yokoyama and Goto, 2000: Yokoyama and collected by panning. Monazite is mostly an Saito, 2001). The presence of such less- angular and fine-grained aggregate in the resistant minerals shows that the sandstones samples from Busuanga Island. However, a su#ered from a weak metamorphism. number of mostly rounded or sub-rounded Monazite is mostly small or scarce in quanti- grains of monazite have been observed in the ty and less than a few percent of the heavy sands recently collected from the Asian conti- fraction of the sandstones (Table 1). Monazite nent. grains are mostly rounded or sub-rounded

Fig. 5. Back-scattered images of secondary monazites with ages around 100 Ma. A & B: decomposed monazite usually ﬁne-grained and forming aggregate. 186 Kazumi Yokoyama et al.

Table 2. Age data of monazites in the sandstones and sands from the Palawan microcontinent and coastal regions of China (Figs. 1 & 2).

Busuanga coastal area of Asian continent Age (Ma) BA03 BA05 BA08 BA09 BA14 BA15 BA17 sandsῌ Age (Ma) Mindoro Panay Age(Ma) Ou R. Jiulong R. Gulong Is. HanR.YiR. 0 00000000 0 0 0 0 0 0 0 00 0.25 00000000 0.25 0 0 0.25 0 0 0 0 0 0.500000000 0.50 0 0.50 0 0 00 0.75 00000002 0.75 0 0 0.75 4 6 6 3 1 1 00000008 1 0 0 1 26454330 1.25 00001020 1.25 0 0 1.25 13 46 12 40 0 1.5194493466 1.5180 1.53222520 1.75 2747107474 1.75 46 0 1.75 3 8 0 15 0 2 00011222 2 682 2 21170190 2.25 00126123 2.25 89 1 2.25 177 26 1 48 0 2.500001033 2.5350 2.5644 0100 2.75 00000000 2.75 0 0 2.75 0 0 0 0 0 3 00000000 3 1 0 3 0 0 0 00 3.25 00000000 3.25 0 0 3.25 0 0 0 0 0 3.500000000 3.50 0 3.50 0 0 00 3.75 00000000 3.75 0 0 3.75 1 0 0 0 0 4 00000000 4 0 0 4 2 8 0 50 4.25 00000010 4.25 0 0 4.25 1 9 1 15 0 4.500000000 4.50 0 4.50 4 0 50 4.75 00000000 4.75 0 0 4.75 0 0 0 0 0 5 00000000 5 0 0 5 0 0 0 00 5.25 00000000 5.25 0 0 5.25 0 0 0 0 0 5.500000000 5.50 0 5.50 0 0 00 5.75 00000000 5.75 0 0 5.75 0 0 0 0 0 6 00000000 6 0 0 6 0 0 0 01 6.25 00000000 6.25 0 0 6.25 0 0 0 0 0 6.500000000 6.50 0 6.50 0 0 00 6.75 00000000 6.75 0 0 6.75 0 0 0 0 0 7 00000000 7 0 0 7 0 0 0 00 7.25 00000000 7.25 0 0 7.25 0 0 0 0 0 7.500100000 7.50 0 7.50 0 0 00 7.75 00000000 7.75 0 0 7.75 0 0 0 0 0 8 00000000 8 0 0 8 0 0 0 00 8.25 00000000 8.25 0 0 8.25 0 0 0 0 0 8.500000000 8.50 0 8.50 0 0 00 8.75 00000000 8.75 0 0 8.75 0 0 0 0 0 9 00000000 9 0 0 9 0 0 0 00 9.25 00000000 9.25 0 0 9.25 0 0 0 0 0 9.500000000 9.50 0 9.50 0 0 10 9.75 00000000 9.75 0 0 9.75 0 0 0 0 0 1000000000 100 0 100 0 0 00 10.25 00000000 10.25 0 0 10.25 0 0 0 0 0 10.5 00000000 10.5 0 0 10.5 0 0 0 0 0 10.75 00000000 10.75 0 0 10.75 0 0 0 0 0 1100000000 110 0 110 0 0 00 11.25 00000000 11.25 0 0 11.25 0 0 0 0 0 11.5 00000000 11.5 0 0 11.5 0 0 0 0 0 11.75 00000000 11.75 0 0 11.75 0 0 0 0 0 1200000000 120 0 120 0 0 00 12.25 00000000 12.25 0 0 12.25 0 0 0 0 0 12.5 00000000 12.5 0 0 12.5 0 0 0 0 0 12.75 00000000 12.75 0 0 12.75 0 0 0 0 0 1300000000 130 0 130 0 0 00 13.25 00000000 13.25 0 0 13.25 0 0 0 0 0 13.5 00000000 13.5 0 0 13.5 0 0 0 0 0 Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 187

Table 2. (continued)

Busuanga coastal area of Asian continent Age (Ma) BA03 BA05 BA08 BA09 BA14 BA15 BA17 sandsῌ Age (Ma) Mindoro Panay Age(Ma) Ou R. Jiulong R. Gulong Is. HanR.YiR. 13.75 00000000 13.75 0 0 13.75 0 0 0 0 0 1400000000 140 0 140 0 0 00 14.25 00000000 14.25 0 0 14.25 0 0 0 0 0 14.5 00000000 14.5 0 0 14.5 0 0 0 0 0 14.75 00000000 14.75 0 0 14.75 0 0 0 0 0 1500000000 150 0 150 0 0 00 15.25 00000000 15.25 0 0 15.25 0 0 0 0 0 15.5 00000000 15.5 0 0 15.5 0 0 0 0 0 15.75 00000000 15.75 0 0 15.75 0 0 0 0 0 1600000000 160 0 160 0 0 00 16.25 00000000 16.25 0 0 16.25 0 0 0 0 0 16.5 00000000 16.5 0 0 16.5 0 0 0 0 0 16.75 00000000 16.75 0 0 16.75 1 0 0 0 0 1700000000 170 0 170 0 0 00 17.25 00000010 17.25 1 0 17.25 1 0 0 0 0 17.5 00000000 17.5 0 0 17.5 0 0 0 0 0 17.75 00000100 17.75 1 0 17.75 0 0 0 0 0 1800010010 18130 183 0 0 00 18.25 01115110 18.25 31 0 18.25 7 2 0 0 3 18.5 00212190 18.5 66 0 18.5 7 0 0 0 1 18.75 02202142 18.75 71 3 18.75 4 0 0 0 1 1900000100 19581 192 2 0 20 19.25 00000000 19.25 14 1 19.25 1 0 0 0 0 19.5 00000000 19.5 1 0 19.5 0 0 0 0 1 19.75 00000000 19.75 0 0 19.75 0 0 0 0 0 2000000000 200 0 200 0 0 00 20.25 00000000 20.25 0 0 20.25 0 0 0 0 0 20.5 00000000 20.5 0 0 20.5 0 0 0 0 0 20.75 00000000 20.75 0 0 20.75 0 0 0 0 0 2100000000 210 0 210 0 0 00 21.25 00000000 21.25 0 0 21.25 0 0 0 0 0 21.5 00000000 21.5 0 0 21.5 0 0 0 0 0 21.75 00000000 21.75 0 0 21.75 0 0 0 0 0 2200000000 220 0 220 0 0 00 22.25 00000000 22.25 0 0 22.25 0 0 0 0 0 22.5 00000000 22.5 0 0 22.5 0 0 0 0 0 22.75 00000000 22.75 0 0 22.75 0 0 0 0 0 2300000000 230 0 230 0 0 00 23.25 00000000 23.25 0 0 23.25 0 0 0 0 0 23.5 00000000 23.5 0 0 23.5 0 0 0 0 0 23.75 00000000 23.75 0 0 23.75 0 0 0 0 0 2400000000 240 0 240 0 0 00 24.25 00000000 24.25 0 0 24.25 0 0 0 0 1 24.5 00000000 24.5 0 0 24.5 0 0 0 0 5 24.75 00000000 24.75 0 0 24.75 0 0 0 0 11 2500000000 250 0 250 0 0 010 25.25 00000000 25.25 0 0 25.25 0 0 0 0 10 25.5 00000000 25.5 0 0 25.5 0 0 0 0 5 25.75 00000000 25.75 0 0 25.75 0 0 0 0 0 2600000000 260 0 260 0 0 00 Total 3 19 15 17 37 18 119 30 513 8 341 199 65 218 50 188 Kazumi Yokoyama et al.

1800 Ma are scarce (Fig. 6). The major peak is at 176ῌ19 Ma. Small peaks are located at 240 Age of Monazite ῌ24 Ma and 1860ῌ34 Ma. Those from Monazite is a rare earth elements (REEs)- Mindoro and Panay islands also exhibit a bearing phosphate mineral occurring as an ac- bimodal distribution similar to those from cessory mineral in granitic and high-grade met- Busuanga Island (Fig. 6). Age data from amorphic rocks and in sands derived from Mindoro Island have major peaks at 230ῌ27 them. All the analytical positions were selected Ma and 1880ῌ43 Ma and a minor peak at 177 from back-scattered electron images and ῌ18 Ma. Considering the standard deviation metamictised areas/zones were avoided. The of each peak position, these peak positions are standard deviation of ages within a single grain similar to those from Busuanga Island. Al- is usually less than a few percent in old though a notably di#erent age distribution pat- monazites (῎ca. 300 Ma) or less than 25 Ma tern is not observed among the sandstones in younger monazites (῍ca. 300 Ma). One from Busuanga, Mindoro, and Panay islands, representative age has been selected from each monazites with 430 Ma and 770 Ma are ob- grain. A list of the age data is shown in Table 2. served in the sandstones from Busuanga Island. A total of 1652 grains have been analyzed in Such an age is totally absent in the monazites this study: 258 grains from Busuanga Island, from Mindoro Island. 513 grains from Mindoro Island and 873 grains In the well-consolidated sandstones from from the Asian continent. Only 8 monazite Liminangcong Formation and sands from the grains were analyzed in the sandstone from western part of the island, monazites are Panay Island, probably due to the later stage mostly angular in shape. Such monazites show metamorphic e#ect. Back-scattered electron an age peak at around 100 Ma (Fig. 6). images of representative detrital and secondary Additional data collected from the coastal grains are shown in Fig. 4 and 5, respectively. zone of the Asian continent have sands with Monazite ages from the Palawan micro- monazite ages that are shown in Fig. 7. The continent and Asian continent are presented as sand samples have distinct age distribution probability diagrams in Fig. 6 and 7, respec- characteristics, reflecting the di#erent rocks in tively. Probability distributions for monazite their drainage basins. In the Lishui samples, ages were calculated with a multi-peak monazite is bimodal with peaks at 100 Ma and Gaussian fitting method (e.g. Williams, 1998). 1900 Ma. Whereas the Changtai and Fengxi As monazite is rare or scarce in the sandstone have di#erent distribution patterns: peaks at 40 samples from Busuanga Island, all the data Ma and 230 Ma. The sand from the Linyi also from the sandstone samples obtained from the has a di#erent pattern that shows a strong peak island are presented in one of the diagrams in at 2500 Ma. The oldest monazites are supplied Fig. 6. Many monazite grains in the sand from an Archean terrane-the Tishan samples from the islands are fine and angular, Mountains-in the drainage basin. which is di#erent from the rounded grains. In southeast Asia (Sundaland), age patterns They are presented in the other diagram in Fig. in the sands are represented by a strong peak at 6. 250 Ma and no clear peak at 1900 Ma Ages of rounded or sub-rounded monazite (Yokoyama et al., 2010). In the Yangtze grains from Busuanga Island range from ca. River, most of the sands are characterized by a 140 Ma to 2200 Ma, apparently showing strong peak at 400 Ma and small peak at 700 bimodal distribution with strong clusters at Ma. Bimodal distribution with peaks at 250 150ῌ270 Ma and 1800ῌ1950 Ma, while and 1900 Ma is observed widely on the Korean monazites with ages ranging from 300 Ma to Peninsula. Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 189

Fig. 6. Probability distribution diagrams of monazite ages in the sandstones and sands from Busuanga, Mindoro, and Panay islands. Numerical value (n) denotes the number of analyzed monazite grains. 190 Kazumi Yokoyama et al. Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 191

Chemical compositions of spinel and garnet in the TiO2ῌMgO diagram by Kamenetsky Chemical compositions of spinel and garnet et al. (2001). Spinels from the Southern are source diagnostic and well summarized by Busuanga Belt are characterized by high TiO2 many authors (e.g. Kamenetsky et al., 2001; content and are mostly plotted in an area of Yokoyama et al., 1990). Spinel is derived from ocean-island basalt (Fig. 9). On the other various types of basalt, gabbro, and peridotite. hand, spinels in the Middle and Northern belts

It is abundant in the sandstones from the South are low in TiO2 showing that they are mostly Busuanga Belt, whereas it is small in quantity derived from peridotite and are rarely from an in the other sandstones. Spinel from the South ocean-island basalt region. Spinels from the

Busuanga Belt commonly contains glass inclu- Mindoro and Panay islands are also TiO2-poor, sions and rarely olivine (Fig. 8). Garnet is and are mainly plotted in a peridotite region. sporadically abundant. There is no correlation Garnet is plotted in CaῌMgῌFe and CaῌMnῌ between the modal proportion of garnet and Fe diagrams (Fig. 10), and is usually derived the classiﬁed belts on Busuanga Island. from metamorphic rocks. Generally speaking, The chemical composition of spinel is plotted Mg content in garnet is related to the metamor-

Fig. 8. Back-scattered images of detrital spinels in BA014 & BA015. A: spinel (sp) with glass inclusions (gl). B: devitriﬁed glass inclusion in spinel. C: spinel with zoning texture. D: olivine inclusion (ol) in zoned spinel.

Fig. 7. Probability distribution diagrams of monazite ages in the sands collected along the East Asian continental margin. Numerical value (n) denotes the number of analyzed monazite grains. 192 Kazumi Yokoyama et al.

Fig. 9. Al2O3 vs TiO2 compositional relationships in detrital spinels from the Palawan microcontinent and the Asian continent. Compositional variation of spinel from oceanic island basalt is enclosed by a solid line (Kamenetsky et al., 2001). TiO content of spinel from peridotite is usually less than 1.0 wt%. phic grade: Mg-rich indicates a high grade of On the other hand, garnets from Panay Island metamorphism while Mg-poor indicates a are Mg-rich and depleted in CaO content lower grade. Ca-rich garnet, more than 10% in which is common in the sandstones from the grossuilar content, is derived from metamor- subduction complex of the Japanese Islands. phic rock with a basaltic composition, whereas This phenomenon is explained by selective dis- Ca-poor one is of a pelitic composition. solution after subduction rather than an ab- Garnets from the Busuanga and Mindoro sence of Ca-rich garnet in the drainage basin islands are relatively poor in Mg content, with (Yokoyama and Saito, 2001). less than a 25% pyrope component. There is no # clear di erence in garnet composition between Discussion those from Busuanga Island and Mindoro Island. It shows that high-grade metamorphic Provenance of detrital monazite terrane was absent in their provenances. It is Age of detrital monazite has produced sig- noteworthy that the Ca-rich garnets from both niﬁcant information on the provenance of the islands are also preserved. Generally, Ca-rich sandstones from the Palawan microcontinent. garnet has been totally dissolved in sandstones The microcontinent is considered to have been from the Jurassic subduction complex of the located at the southwestern part of Taiwan and Japanese Islands (Yokoyama and Saito, 2001). separated from the Asian continent during the Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 193

Fig. 10. Compositional variations of detrital garnets from Busuanga, Mindoro, and Panay islands. Each datum is plotted in CaῌMgῌFe and MnῌMgῌFe diagrams. opening of the South China Sea (e.g. Hollo- detrital monazites in the sandstones from the way, 1982: Zamoras and Matsuoka, 2004). Palawan microcontinent. Hence, comparison Ages of monazites in the sands from the coastal of the Palawan data with equivalent data in zone of the Asian continent have been reported sands from the Asian continent should deduce (Fig. 1: Yokoyama et al., 2007, 2008, 2010). In the provenance of the detrital monazite and the this study, we have determined the ages of original depositional site for the Jurassic to 194 Kazumi Yokoyama et al.

Fig. 11. Frequency and probability distribution diagrams of monazite ages in the sands collected along the East Asian continental margin (Fig. 7 of Yokoyama et al. 2007). Numerical value (n) denotes the number of analyzed monazite grains. Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 195

Early Cretaceous sediments. microcontinenet. It indicates that the Indo- The age patterns from the three islands, china Peninsula and the southern coastal Busuanga, Mindoro, and Panay, on the region of China could not have been the prov- Palawan microcontinent show simple bimodal enance area for sandstones on the Palawan distribution with clusters at 150ῌ270 Ma and microcontinenet. 1800ῌ1950 Ma. As the data from Panay Island Yangtze and Yellow rivers have huge drain- are too minor to discuss a peak position, more age basins. The former has strong peaks at 180 detailed comparisons were done for data from Ma, 410 Ma, and 1870 Ma with a subordinate both the Busuanga and Mindoro islands. peak at 700ῌ800 Ma. The latter has a strong Roughly three peaks are recognized in the data peak at 410 Ma with subordinate peaks at 230 from Busuanga and Mindoro islands. They are Ma and 1880 Ma. As both the rivers are ca. 180 Ma, 240 Ma, and 1870 Ma. In compar- characterized by a strong peak at 410 Ma, their ison of these data with the data from the sands drainage basins are not appropriate candidates from the Asian continent, data younger than for the sandstones in the Palawan sandstones. 150 Ma in the latter are excluded because such The Korean Peninsula has a bimodal pattern young monazites were absent at the time of the with clusters at 150ῌ300 Ma and 1800ῌ2000 deposition of the Jurassic to Early Cretaceous Ma (Fig. 11). Monazite with ages from 300 sandstones. Ma to 1700 Ma is scarce. Peak positions are On the Indochina Peninsula, age patterns are 164ῌ23 Ma, 229ῌ29 Ma, and 1860ῌ39 Ma. characterized by a strong peak at 250 Ma and a The bimodal nature and peak positions are well subordinate peak at 450 Ma (Yokoyama and consistent with those from the sandstones in Tsutsumi, 2008: Yokoyama et al., 2010). Data the Palawan microcontinent. The Jurassic to around 180 Ma and 1870 Ma are negligible. Cretaceous sandstones in the subduction com- The southern coastal area of China- notably plex from the Japanese islands also have a Guangdong and Fujian provinces- is the most bimodal age pattern. Yokoyama et al. (2000) probable candidate from where the Palawan concluded that the detrital monazites in the microcontinent migrated during the Oligocene- Japanese Islands were derived from the Korean Early Miocene. In the Guangdong Province, Peninsula. Zamoras and Matsuoka (2001) con- two rivers, the Zhu and Han rivers, cut cluded that the Jurassic-Early Cretaceous through the coastal area of the continent. Age chert-siliceous shaleῌclastic sequence on patterns of sands collected from eastern Asia Busuanga Island is similar to those from the are reproduced in Figure 11 (Yokoyama et al., Japanese Islands. Andal et al. (1968) also dis- 2007). The Zhu River is characterized by a cussed the similarity in Jurassic fossils from the strong peak at 250 Ma with small clusters at Mindoro Island with those on the Japanese 400ῌ500 Ma and 800ῌ900 Ma. The Han River Islands. Hence, it is not ridiculous to conclude has three peaks at 155 Ma, 230 Ma, and 440 that detrial monazites in the Jurassic to early Ma. In both rivers, no visible peak is Cretaceous sandstones on the Palawan micro- recognized at 1800ῌ1900 Ma. The Min and continent were parentally derived from the Jiulong rivers in the Fujian Province are also Korean Peninsula or surrounding areas. As for characterized by peaks at 430ῌ450 Ma and 230 the other probable candidates, there are coastal Ma. Monazite with 1800ῌ1900 Ma is scarce in areas near the Shandong Peninsula and both the rivers. None of the recent sands col- Zhejiang Province. In the Shandong Peninsula, lected from the Indochina Peninsula and the monazite age is bimodal with peak ages at 118 southern coast of China show an age pattern ῌ18 Ma and 1845ῌ26 Ma. In the Zhejiang similar to those observed in the Jurassic to Province, the age pattern from the Ou River Early Cretaceous sandstones on the Palawan has a strong peak at 242ῌ19 Ma and subordi- 196 Kazumi Yokoyama et al. nate peaks at 117ῌ29 Ma and 1854ῌ36 Ma. stone is generally treated as terrigenous sedi- Although younger monazites in both areas ment. These facies changes are brought by should not be expected for the JurassicῌEarly gradual plate movement from a remote oceanic Cretaceous sandstones, the areas cannot be environment toward the subduction zone. simply excluded as a contributor to the prove- Zamoras and Matsuoka (2001) treated the nance for the sandstones on the Palawan sandstones in Busuanga Island as a part of the microcontinent. The Korean Peninsula, chert-siliceous shale-clastic sequence. How- Shangdong Peninsula, and Zhejian Province ever, the sandstones on the Busuanga Island are located in the marginal part of the East are di#erent from those in the subduction com- China Sea. The sea with a continental crust is plex on the Japanese Islands. They are mostly widely distributed, but there is no datum about tu#aceous and loosely packed, di#erent from monazite from the sea. The drainage systems of arkose or quartzose, and well-solidified in the rivers in the East Asia region have changed normal Jurassic subduction complex. Shallow with geological time. Assuming that giant marine or brackish sediment occurs in the rivers like the present Yangtze and Yellow Middle Busuanga belt, Upper Jurassic zone of rivers were not developed during the Jurassic the Zamoras and Matsuoka (2001). It is to Early Cretaceous, the areas including the characterized by a bituminous shale layer with East China Sea, Korean Peninsula, Shangdong patches of coal, a sandstone layer with sand Peninsula, and Zhejian Province are probable pipe, and possible traces of fossil. Such bitumi- candidates as a provenance for sandstones on nous shale is commonly intercalated with shal- the Palawan microcontinent. As the micro- low marine sandstone on Mindoro Island. continent was located at the southwestern part In the sandstone from the subduction com- of Taiwan before the opening of the South plex, Ca-rich garnets dissolved as did those China Sea, the migration from the probable from Panay Island. Ca-rich garnet on depositional area around the East China Sea Busuanga Island has been preserved similar to will be at least several hundred km. that in the shallow marine sandstone from Secondary monazite showing a texture of Mindoro Island. In contrast with Mg-rich aggregate has a peak age at 101ῌ14 Ma. Plu- garnet on Panay Island, garnets in the sand- tonic age with around 100 Ma has been stones from both Busuanga and Mindoro reported from the coastal region of China (e.g. islands are Mg-poor. It shows that sediments Zhou and Li, 2000; Fig. 7 & 11, this paper). on Panay Island were derived from a region Sandstone with such monazite is a well- including high-grade metamorphic rock, solidified rock and has metamorphic minerals whereas the latter was supplied by a relatively such as epidote and titanite. Hence, it is con- restricted region including only low-grade met- cluded that the age corresponds to the meta- amorphic rock. The samples BA014 and BA morphic event at the coastal zone of East Asia. 015 contain abundant spinel grains, and they As the distribution of such sandstone is sporad- are mostly derived from ocean-island basalt ic on the island, the metamorphic event may (Kamenetsky et al., 2001). As the monazite not be regional but local-possibly hydrothermal grains must be supplied from the continental or contact metamorphism. region only, the spinel grains will be supplied locally from an uplifted or obducted oceanic Depositional conditions island. In spite of some coincidences with the In a subduction complex, the chert-siliceous subduction complex, it is possible that the shale-clastic sequence is a general succession. sandstones on Busuanga Island will be shallow Chert is pelagic, and deep-sea sediment and marine sediments as well as fossiliferous sand- siliceous shale are hemipelagic. Whereas sandstone on Mindoro Island. Some serious prob- Provenance Study of Jurassic to Early Cretaceous Sandstones from the Palawan Microcontinent 197 lems will remain if the sandstones were shallow marine sediment. One is an observation by Acknowledgements BMG (1984) that tu# and tu#aceous sandstone are intercalated with minor thinly bedded The authors are very grateful to Ms. M. chert, i.e. coexistence of shallow marine and Shigeoka for her help with modal and chemical pelagic sediments. The other is the occurrence analyses and the heavy mineral separations of tu# and tu#aceous sandstone in three belts throughout this study. We also thank Mr. W. on Busuanga Island showing continuous vol- Mago for his help of the sampling on the canic eruption throughout the Jurassic to Early islands. Cretaceous. Further field work and analyses may resolve these problems. References

Conclusions Andal, D. R., J. S. Esguerra, W. Hashimoto, B. P. Reyes & T. Sato, 1968. The Jurassic Mansalay For- The heavy minerals in the sandstones from mation, Southern Mindoro, Philipinnes. Geology and Paleontology of South East Asia, 4:179ῌ197. Busuanga, Mindoro, and Panay islands, i.e. the Bureau of Mines and Geosciences (BMG), 1984. Palawan microcontinent, were studied to eluci- Geological maps of Busuanga Island: Sheet nos. 2956 date the provenance of the detrital minerals. I & II, 3056 III & IV. Although more detailed analyses will be neces- Evans, J. A., J. I. Chisholm & M. J. Leng, 2001. How sary to deduce the tectonic reconstruction of U-Pb detrital monazite ages contribute to the inter- the microcontinent, the following conclusions pretation of the Pennine Basin infill. Journal of Geological Society, 158: 741ῌ744. or suggestions are obtained from this study: Fan, D., C. Li, K. Yokoyama, B. Zhou, B. Li, Q. 1: Age pattern of detrital monazite in the Wang, S. Yang, B. Deng & G. Wu, 2004. Study on microcontinent is bimodal with clusters of the age spectrum of monazites in Late Cenozoic 150ῌ270 Ma and 1800ῌ1950 Ma. None of the stratum of the Yangtze River delta and the run- recent sands collected from the Indochina Pen- through time of the Yangtze River. Science in China (D), 34: 1015ῌ1022 (in Chinese). insula and southern coast of China shows an Garver, J. I., M. T. Brandon, M. Roden-Tice & P. J. J. age pattern similar to those observed on the Kamp, 1999. Exhumation history of orogenic high- microcontinent. As the bimodal pattern is ob- lands determined by detrital fission-track thermo- served around the East China Sea, it is con- chronology. In: Ring, U., Brandon, M. T., Lister, G. cluded that the sandstones were parentally S. and Willett, S. D. (eds.), Exhumation processes: ῌ formed near the sea. Before the opening of the Normal Faulting, ductile flow and erosion, pp. 283 304. Geological Society, London. South China Sea, the microcontinent had Hamilton, W., 1979. Tectonics of the Indonesian moved for several hundred km from northeast region. U.S. Geological Survey Professional paper to southwest of Taiwan. 1078:1ῌ345. 2: The sandstones on Busuanga Island are Hashimoto, W. & T. Sato, 1973. Geologic structure of mostly tu#aceous and loosely packed. Dissolu- North Palawan and its bearing on the geological history of the Philippines. In: Toriyama R. & tion of garnet is not as severe as that in the Kobayashi T. (eds.), Geology and Paleontology of subduction complex. In one outcrop, bitumi- Southeast Asia, 13:145ῌ61. nous shale with a coal patch occurs and is Holloway, N. H., 1982. North Palawan Block, Philip- surrounded by sandstones with abundant sand pines, its relation to Asian mainland and role in pipes. These evidences show that the sand- evolution of South China Sea. American Association ῌ stones on Busuanga Island were probably of Petroleum Geologists Buletin,. 66: 1355 83. Ireland, T. R., 1991. Crustal evolution of New Zealand: formed under shallow marine conditions as Evidence from age distributions of detrital zircons in well as the well-documented conditions on Western Province paragneisses and Torlesse Mindoro Island. greywacke. Geochimica et Cosmochimica Acta, 56: 198 Kazumi Yokoyama et al.

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