South China continental margin signature for sandstones and

Transcription

Gondwana Research 26 (2014) 699–718
Contents lists available at ScienceDirect
Gondwana Research
journal homepage: www.elsevier.com/locate/gr
South China continental margin signature for sandstones and granites
from Palawan, Philippines
Simon M. Suggate a,⁎, Michael A. Cottam a,b, Robert Hall a, Inga Sevastjanova a, Margaret A. Forster c,
Lloyd T. White a, Richard A. Armstrong c, Andrew Carter d, Edwin Mojares e
a
SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom
BP Exploration Operating Co. Ltd., Wellheads Avenue, Dyce, Aberdeen AB21 7PB, United Kingdom
Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
d
Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, United Kingdom
e
Geosciences Division, Mines and Geosciences Bureau, 1515L & S Bldg., Roxas Boulevard, Manila, Philippines
b
c
a r t i c l e
i n f o
Article history:
Received 17 January 2013
Received in revised form 8 July 2013
Accepted 21 July 2013
Available online 29 July 2013
Handling Editor: M. Santosh
Keywords:
Palawan
North Borneo
Mount Capoas granite
Zircon U–Pb geochronology
Heavy minerals
a b s t r a c t
We report results of heavy mineral analysis and U–Pb dating of detrital zircons from metasediments and
Cenozoic sandstones, and U–Pb dating of zircons from Cenozoic granites of the North Palawan Continental
Terrane (NPCT) and the South Palawan Terrane (SPT). The NPCT metasediments are derived mainly from granitic
and metamorphic rocks of continental character. They contain zircons that indicate a maximum depositional age
of Late Cretaceous and other age populations indicating a South China origin. The sediments were deposited on
the South China margin before rifting of the continental margin during opening of the South China Sea.
Miocene SPT sandstones contain similar heavy mineral assemblages suggesting sources that included NPCT
metasediments, metamorphic basement rocks at the contact between the SPT and the NPCT, South China Sea
rift volcanic and/or minor intrusive rocks, and the Palawan ophiolite complex. The SPT sandstones are very
similar to Lower Miocene Kudat Formation sandstones of northern Borneo suggesting a short-lived episode of
sediment transport from Palawan to Borneo in the Early Miocene following arc-continent collision. U–Pb dating
of zircons shows that the Central Palawan granite is Eocene (42 ± 0.5 Ma). The Capoas granite was intruded
during a single pulse, or as two separate pulses, between 13.8 ± 0.2 Ma and 13.5 ± 0.2 Ma. Inherited zircon
ages from the Capoas granite imply melting of continental crust derived from the South China margin with a
contribution from Cenozoic rift-related and arc material.
© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction
Palawan, the westernmost island of the Philippine archipelago, lies
at the southern margin of the South China Sea, approximately 400 km
to the northeast of Borneo (Fig. 1). Geologically, Palawan can be divided
into two blocks, the North Palawan Continental Terrane (NPCT) and the
South Palawan Terrane (SPT) (e.g. Hamilton, 1979; Taylor and Hayes,
1983; Faure et al., 1989; Yumul et al., 2009). The NPCT is interpreted
as a continental fragment that was derived from the South China margin
(e.g. Holloway, 1982; Taylor and Hayes, 1983; Hall, 1996). This is
supported by previous provenance studies (Suzuki et al., 2000; Walia
et al., 2012) which suggested that Upper Cretaceous to Eocene sandstones of Central Palawan (NPCT) were derived from the Kwangtung
and Fukien regions of South China. The SPT includes a Lower
Cretaceous–Eocene ophiolitic complex (e.g. Yumul et al., 2009) and
Oligocene to Miocene sediments. Almost nothing is known about the
provenance of these sediments from this terrane.
⁎ Corresponding author. Tel.: +44 1784 443592; fax: +44 1784 434716.
E-mail address: [email protected] (S.M. Suggate).
As an area with proven hydrocarbon potential, Palawan has been the
attention of a number of recent studies (e.g. Yumul et al., 2009; Knittel
et al., 2010; Walia et al., 2012). Despite this, many aspects of the tectonic
evolution and geology of this region remain unclear. In particular, there
are still outstanding questions about the ages of igneous, metamorphic
and sedimentary rocks on Palawan. For example, metasedimentary
rocks that were previously considered Palaeozoic have yielded Cretaceous detrital zircons (e.g. Walia et al., 2012). The geology of Palawan
is also similar to that of North Borneo and both include Mesozoic
ophiolitic rocks that are overlain by Mesozoic–Cenozoic sedimentary
rocks and are intruded by granites. Both areas share a strong NE–SW
orientation (Fig. 1). In both cases (e.g. Hutchison, 2010) the onshore
regions are flanked to the west by significant bathymetric troughs
(the NW Borneo and Palawan Troughs) that are in turn flanked by
bathymetric highs (the Dangerous Grounds and Reed Bank). Perhaps
most notably, both areas are intruded by young granite plutons: the
Mt Kinabalu pluton in northern Borneo (Cottam et al., 2010), and the
Mt Capoas intrusion in Palawan (Encarnación and Mukasa, 1997).
K–Ar age determinations on the Kinabalu granite in northern Borneo
by a number of authors (Jacobson, 1970; Rangin et al., 1990; Bellon and
1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.gr.2013.07.006
700
S.M. Suggate et al. / Gondwana Research 26 (2014) 699–718
Rangin, 1991; Swauger et al., 1995; Hutchison et al., 2000) suggested
that the granite may be as old as ~14 Ma. However, U–Pb dating of
zircons by Cottam et al. (2010) showed that the Kinabalu granite is a
Late Miocene pluton emplaced and crystallised in less than
800,000 years between 7.85 ± 0.08 and 7.22 ± 0.07 Ma. Encarnación
and Mukasa (1997) had reported Middle Miocene ages (~14 Ma) for
the Capoas granite based on U–Pb dating of zircon and monazite but
recognised that these were discordant and could be mixtures of older
cores and younger magmatic rims. The new SHRIMP age data for the
Kinabalu granite raised the question of whether the Capoas granite is
possibly of similar age and origin.
Heavy minerals are sensitive provenance indicators, because of the
diversity of common assemblages, restricted parageneses of many common heavy mineral species and their ability to preserve geochemical
characteristics of parental source rocks. Heavy mineral analysis has
been successfully applied in provenance studies across the world
(Morton et al., 1994; Mange et al., 2005; Garzanti and Ando, 2007), including SE Asia (e.g. van Hattum et al., 2006; Clements and Hall, 2011;
Suggate, 2011; Sevastjanova et al., 2012; Witts et al., 2012; van
Hattum et al., in press), in areas where there are sufficient differences
between sediment source areas. Several authors have suggested that
initial heavy mineral assemblages undergo modifications during
sediment generation, transport and storage. The most significant of
these include (a) hydraulic sorting (density fractionation), (b) dissolution during deep burial (diagenetic dissolution) and (c) dissolution
during tropical weathering. It is recognised that these secondary processes change the initial abundances of the minerals (e.g. Garzanti
et al., 2011; Andò et al., 2012) or possibly can selectively remove minerals from the initial assemblage (e.g. Morton and Hallsworth, 2007).
However, minerals that remain in the heavy mineral assemblage still
yield useful information about their source rocks.
Recent provenance studies based on heavy minerals suggest that
during the Early Miocene Palawan shed granitic and metamorphic
detritus to northern Borneo (van Hattum, 2005; Suggate, 2011; van
Hattum et al., in press). Provenance studies of NPCT metasediments
(Suzuki et al., 2000; Walia et al., 2012) concentrated on light minerals
and U–Pb dating of detrital zircons. Detrital heavy minerals were briefly
described from thin section, but interpretations of provenance were
based on limited data, insufficient for detailed characterisation of
heavy mineral assemblages (e.g. Mange and Maurer, 1992).
In order to address these uncertainties, we carried out fieldwork
in Palawan to collect igneous rocks, metasediments and Cenozoic
sandstones from the NPCT and SPT. We report here the results of
heavy mineral analysis, U–Pb dating of detrital zircons and zircons
from Cenozoic granites from Palawan.
2. Geological background
There is general agreement that parts of northern Borneo and
Palawan (Fig. 1), along with areas such as the Dangerous Grounds and
Reed Bank in the South China Sea, are extended and attenuated
continental fragments rifted from the South China margin. The rifted
material has been termed the Palawan Continental Terrane (PCT;
e.g. Holloway, 1982; Taylor and Hayes, 1983), the North Palawan
Block (NPB; e.g. Almasco et al., 2000), and the North Palawan Continental Terrane (NPCT; e.g. Encarnación et al., 1995; Encarnación and
Mukasa, 1997). The term North Palawan Continental Terrane (NPCT)
is used here. This continental crust was originally envisaged to be a single large fragment rifted from the South China margin (Holloway, 1982;
Taylor and Hayes, 1983), but recent studies (Yumul et al., 2009) have
suggested that there are several internal sutures and multiple
fragments. Continental crust has been identified in northern Palawan,
parts of the islands of Mindoro and Panay, and Reed Bank (Holloway,
1982; Taylor and Hayes, 1983; Kudrass et al., 1986; Schluter et al.,
1996; Encarnación and Mukasa, 1997; Yumul et al., 2009; Franke
et al., 2011; Knittel, 2011). The NPCT moved south during subduction
of the proto-South China Sea beneath NW Borneo and the Cagayan
Arc and the opening of the South China Sea (Holloway, 1982; Taylor
and Hayes, 1983; Kudrass et al., 1986; Vogt and Flower, 1989; Rangin
et al., 1990; Hall, 1996; Hutchison, 1996; Schluter et al., 1996;
Encarnación and Mukasa, 1997; Hutchison et al., 2000; Hall, 2002;
Replumaz and Tapponnier, 2003; Cottam et al., 2010; Hutchison,
2010; Franke et al., 2011; Hall, 2012). Subduction of the proto-South
China Sea terminated in the Early Miocene after collision of the NPCT
with the active continental margin of Sabah and the Cagayan Arc
(Holloway, 1982; Rangin et al., 1990; Tan and Lamy, 1990; Hinz et al.,
1991; Hall, 1996; Hall and Wilson, 2000; Hutchison et al, 2000). Oceanic
spreading in the South China Sea ceased in the Early or Middle Miocene
(Taylor and Hayes, 1983; Briais et al., 1993; Barckhausen and Roeser,
2004).
3. Geology and stratigraphy of Palawan Island
The geology of Palawan Island (Fig. 2) has commonly been
interpreted to comprise two discrete tectonic elements (e.g. Hamilton,
1979; Holloway, 1982; Taylor and Hayes, 1983; Mitchell et al., 1986;
Encarnación et al., 1995; Encarnación and Mukasa, 1997; Almasco
et al., 2000). The northern part of the island is made up of the
continental-derived metamorphic and sedimentary rocks of the NPCT.
The southern part of the island comprises ophiolitic rocks and Cenozoic
clastic sediments of the SPT. The NPCT and SPT are in contact along a
broadly north–south trending steep fault that cuts through Ulugan
Bay, in the centre of the island.
3.1. North Palawan Continental Terrane metamorphic and sedimentary
rocks
The NPCT includes a succession of low to medium grade metamorphic rocks and sedimentary rocks related to the pre-, syn- and postrift stages of the opening of the South China Sea (Sales et al., 1997;
Suzuki et al., 2000; Franke et al., 2011) and isolated granite bodies in
central and northern Palawan. Reviewing the stratigraphy of the
NPCT, Sales et al. (1997) classified it on the basis of three distinct tectonic environments: pre-rift and rift; drifting and South China Sea (SCS)
seafloor spreading; collision and post-collision. Based on the region's
offshore seismostratigraphy, Franke et al. (2011) recognised four main
phases: (1) Mesozoic pre-rift sedimentation associated with the margin
of the Asian mainland; (2) Latest Cretaceous–Eocene sedimentation associated with rifting of the South China Sea basin; (3) Oligocene to Early
Miocene sedimentation concurrent with the drifting episode of the
Palawan–Mindoro microcontinental block during South China Sea
seafloor spreading; (4) Late Miocene to Recent sedimentation during
and after the collision between the microcontinental block and the
Philippine Mobile Belt.
The oldest rocks reported from the NPCT (Fig. 3) are a series of
Upper Palaeozoic to Lower Mesozoic metasedimentary rocks (Sales
et al., 1997). They include sandstones, tuffs, slates, phyllites and schists
(Sales et al., 1997; Yumul et al., 2009) that have undergone mediumgrade regional metamorphism (e.g. Suzuki et al., 2000). Metamorphic
rocks from Mindoro in the north of the NPCT have variously been
dated as Late Palaeozoic (Knittel and Daniels, 1987), older than Late
Cretaceous (Sarewitz and Karig, 1986), and Paleocene (Faure et al.,
1989). Dating of igneous and detrital zircons from metamorphic rocks
exposed in southern Mindoro suggests a Late Palaeozoic (Middle to
Late Permian) age for the metamorphic rocks of the NPCT (Knittel
et al., 2010). These rocks were suggested to have formed in association
with a Permian magmatic arc that extended along the south coast of
Asia (Knittel et al., 2010) prior to the opening of the SCS. In places, the
metasediments are overlain by a sequence of cherts, clastic sediments
and carbonates, exposed mainly in the north of Palawan and on the
Calamian Islands (e.g. Sales et al., 1997; Suzuki et al., 2000; Yumul
et al., 2009). All of these rocks belong to the pre-rift succession, and
117°0'0"E
118°0'0"E
701
119°0'0"E
120°0'0"E
NPCT
SOUTH CHINA
MARGIN
11°0'0"N
PHILIPPINES
Capoas
Granite
BORNEO
10°0'0"N
Palawan
South
China
Sea
125°0'0"E
9°0'0"N
20°0'0"N
Puerto
Princesa
120°0'0"E
Luzon
Sulu
Sea
SPT
8°0'0"N
15°0'0"N
Philippines
115°0'0"E
Capoas
Granite
South
China
Sea
Tr
ou
gh
Reed
Bank
Palawan
Panay
Mindanao
Sulu
Sea
N
or
th
W
es
tB
or
ne
o
Tr
ou
Ca
gh
Dangerous
Grounds
ga
ya
n
Ri
dg
e
10°0'0"N
Pa
la
w
an
b
5°0'0"N
Mount
Kinabalu
granite
a
Celebes
Sea
Borneo
Fig. 1. The position of Palawan, Philippines and northern Borneo within SE Asia (a). SRTM digital elevation model and sea floor bathymetry of Palawan showing the North Palawan
Continental Terrane (NPCT), the South Palawan Terrane (SPT), the location of the regional capital Puerto Princesa and the Capoas granites bodies (b).
702
118°0'0"E
119°0'0"E
Quaternary Alluvium
Middle Miocene Capoas
and Bay Peak Granites
Oligocene - Miocene
Siliciclastics
11°0'0"N
Middle Eocene Central
Palawan Granite Bodies
Eocene
Siliciclastics
Mt Beaufort
Metamorphics
Stavely Range
Gabbro
Espina
Basalt
Palawan
SPT - Cretaceous - Eocene
Ophiolite Complex
Capoas
Granite
Bay Peak
Granite
Tumarbong
Semi Schist
Caramay
Schist
NPCT - Cretaceous to
Eocene Meta-sediments
Babuyan River
Turbidites
Ulugan
Fault
9°0'0"N
10°0'0"N
Mesozoic Mélange
South China
Sea
North Palawan
Continental
Terrane (NPCT)
South Palawan
Terrane (SPT)
Sulu Sea
Kilometres
0
25
50
100
Fig. 2. Simplified geological map of Palawan Island, Philippines modified from Almasco et al. (2000) and Mines and Geoscience Bureau (2011). The Ulugan fault (red dashed line) is the
boundary between the North Palawan Continental Terrane (NPCT) and the South Palawan Terrane (SPT).
were deposited along the southern margin of Asia prior to the opening
of the SCS (e.g. Sales et al., 1997; Suzuki et al., 2000; Yumul et al.,
2009; Franke et al., 2011).
These rocks are overlain by a sequence of Upper Cretaceous–Eocene
sedimentary rocks in Central Palawan (Suzuki et al., 2000) that are
thought to represent rift-related sedimentation (Sales et al., 1997;
Franke et al., 2011). They are very low to low grade metamorphosed
sedimentary rocks that are exposed mainly in Central Palawan and
include mudstone, pebbly mudstone, and interbedded sandstone and
mudstone (Suzuki et al., 2000). The succession is divided into three
units: the Caramay Schist, the Concepcion Pebbly Phyllite, which is
also called the Tumarbong Semi Schist (Mines and Geoscience Bureau,
2011), and the Babuyan River Turbidite (Mitchell et al., 1985; Suzuki
et al., 2000) which is variously called the Boayan Formation (Walia
et al., 2012), the Boayan Clastics (Hashimoto and Sato, 1973) and the
Boayan–Caruray Clastics (Wolfart et al., 1986).
3.2. North Palawan Continental Terrane granitic rocks
The Central Palawan granite (Fig. 4) is a difficult-to-access northtrending body with a mapped area of approximately 19 km2. It has
also been called Stripe Peak granite (Mitchell et al., 1985). Float samples
collected during this study were taken from riverbeds approximately
20 km away from the mapped area of the granite. It has been suggested
to be no older than Late Eocene, as it intrudes sedimentary rocks of
probable Eocene age (Mitchell et al., 1985). Two K–Ar dates (36.6 ±
PLIOCENE
L
M I O C E N E
PLEISTOCENE
L
South Palawan
Terrane
North Palawan
Continental Terrane
PlioceneRecent clastics
PlioceneRecent clastics
MA
E
5
10
M
Miocene Sediments
15
E
+
+
+
inc. Isugod Formation
Capoas Granite
Emplacement
20
OLIGOCENE
St Pauls Limestone
25
L
30
Oligocene
Sediments
E
EOCENE
34
L
M
+
37
41
49
E
55
+
+
South Palawan
Ophiolite
Complex
Central Palawan
Granite
Emplacement
Rift Meta-sediments
PALEOCENE
(inc. Babuyan River
Turbidites, Tumarbong Semi
Schist & Caramay Schist
65
K
145
Pre-rift Sediments
(cherts, clastics and
carbonates)
J
200
T
251
P
Metamorphic
Basement
Fig. 3. Onshore stratigraphy of the North Palawan Continental Terrane (NPCT) and the
South Palawan Terrane (SPT) based on data from Mitchell et al. (1985), Almasco et al.
(2000), Franke et al. (2011), Mines and Geoscience Bureau (2011) and this study.
1.8 Ma and 37.0 ± 1.9 Ma) from biotite in biotite–quartz monzonite
(Mitchell et al., 1985) support a Late Eocene age for the intrusion.
The Capoas granite comprises several small bodies that intrude the
basement rocks of the NPCT on the Capoas peninsula in north-central
Palawan (Fig. 4). The largest body (~7 × 7 km) crops out in the
flanks and summit of Mount Capoas. Further south, a second body
(~4 × 7 km) is exposed in coastal outcrops and the flanks of Bay Peak.
Some geological maps show a third, smaller (~3 × 3 km) exposure of
granite around Binga Point. It is unclear if these three bodies represent
703
separate plutons, or if they are linked at depth. Here we subdivide the
Capoas granite into the Mount Capoas granite, the Bay Peak granite
and the Binga Point granite.
Only the Mount Capoas granite has been studied in detail
(Encarnación and Mukasa, 1997). It is an equigranular to porphyritic
biotite granite (29% quartz; 23% K-feldspar; 33% plagioclase; 15%
biotite; abundant accessory zircon; subordinate monazite and
apatite) with a textural continuum between K-feldspar phenocrystrich and phenocryst-poor varieties (Encarnación and Mukasa,
1997). The granite contains enclaves of biotite-rich fine-grained
granite and K-feldspar phenocrysts that show magmatic flow alignment (Encarnación and Mukasa, 1997). Chemically the granite is
classified as metaluminous and high-K calc-alkaline, and plots on
the tectonic discrimination diagrams of Pearce et al. (1984) in the
syn-collisional and volcanic arc granite fields (Encarnación and
Mukasa, 1997). Previous dating studies have suggested a late Middle
Miocene age for crystallisation of the Capoas intrusion (Encarnación
and Mukasa, 1997). Based on the age and regional tectonic models,
Encarnación and Mukasa (1997) suggested that the Mount Capoas
granite formed in a “post-rifting, non-collisional tectonic setting
unrelated to any subduction zone”. They argued that the chemical
affinities with syn-collisional and volcanic arc granites reflect source
rock composition, rather than the tectonic setting of melting. The
meaning of the Late Middle Miocene age is open to question. The
age is based on a small number of zircon and monazite analyses
from a single sample of the Mount Capoas granite. The U–Pb analyses
were carried out using isotope dilution methods that have been
superseded in many ways by newer techniques. The whole-scale
dissolution of grains that was used does not allow distinction
between the ages of magmatic rims and those of any inherited
cores. Encarnación and Mukasa (1997) dated seven fractions of
zircons. Despite attempts to avoid zircons with obvious cores, all of
the analyses fall off the U–Pb concordia forming a mixing array
towards an older component with an estimated Proterozoic age
(Encarnación and Mukasa, 1997). All analyses reflect variable mixing
of magmatic and inherited ages. Based on the lower intercept of a
weighted regression line passing through all the data points
Encarnación and Mukasa (1997) derived a magmatic crystallisation
age of 15 + 3/− 4 Ma for the Mount Capoas granite. However, such
estimates are extremely sensitive to the slope of the regression
line; small changes in data and/or weightings can produce large
variations in intercept age. They also analysed four sub-samples of
monazite from the same sample. The 207Pb/206Pb and 206U/238Pb
ages are inversely discordant (Encarnación and Mukasa, 1997).
However, 207U/235Pb ages are concordant and range between
13.5 ± 0.2 Ma to 12.7 ± 1.3 Ma, with an error weighted mean age
of 13.4 ± 0.4 Ma (Encarnación and Mukasa, 1997).
Direct dating methods such as SIMS (Secondary Ionisation Mass
Spectrometry), of which the SHRIMP (Sensitive High Resolution Ion
MicroProbe) is an example, have been used successfully to independently date cores and rims (e.g. Ireland and Williams, 2003; Ireland
et al., 2008). Such studies can therefore provide information on both
the magmatic (crystallisation) age of the rock, the age(s) of possible
protoliths and subsequent phases of metamorphism. We report new
SHRIMP ages below.
3.3. South Palawan Terrane
The SPT (Fig. 3) is dominated by ophiolitic rocks belonging to the
Palawan Ophiolitic Complex (Mitchell et al., 1986). The complex comprises a full ophiolitic sequence of basal harzburgites, gabbros, pillow
basalts and chert (Encarnación et al., 1995). It is dated as Early
Cretaceous to Eocene based on radiolarians (Raschka et al., 1985),
nannoplankton (Müller, 1991), biostratigraphy (Faure et al., 1989),
and K–Ar dating of basalt (Fuller et al., 1991).
KMm
9°20'0"N
Cretaceous - Eocene
Ophiolite Complex
Quaternary
Mt Beaufort
Alluvium
Metamorphics
Stavely Range
Holocene
Gabbro
Siliciclastics
Espina
Miocene
Basalt
Siliciclastics
a
Eim
119°0'0"E
Esg
Esg
11°0'0"N
704
Ebu
Eocene
Siliciclastics
Mi
Map B
Esg
PLi
Qa
Keb
Keb
Map C
Ebu
Esg
Ma
Ulugan
Fault
KEbp
Mr
53 & 55
9°10'0"N
Keb
Esg
Qa
Esg
Ma
Ebu
Puerto
Princesa
KEbp
Keb
Keb
Keb
Palawan,
Philippines
Esg
118°10'0"E
Qa
d
118°0'0"E
Keb
KEbp
Map D
9°0'0"N
117°0'0"E
Detrital
U-Pb analysis
Middle Miocene Capoas and Bay Peak Granites
South
China
Sea
Oligocene to Pliocene sediments
Middle Eocene Central Palawan Granitic Intrusion
Upper Cretaceous to Eocene meta-sediments
Detrital
HM analysis
Cretaceous-Eocene Ophiolite Complex
Mesozoic mélange
Granite U-Pb
analysis
0
10°50'0"N
Mount
Capoas
granite
35
119°10'0"E
200
31
Eocene Mt Beaufort
Metamorphics
San
Miguel
Quaternary Alluvium
Middle Miocene Capoas
Granite Bodies
Upper Cret to Eocene
Tumarbong Semi Schist
10°40'0"N
150
Holocene Iwahig
Formation
Oligo-Miocene St Paul’s
Limestone
Middle Eocene Central
Palawan Granite
Mesozoic
Mélange
26
27
30
New
Canipo
Bay
Peak
granite
10°10'0"N
10°45'0"N
34
Kilometers
100
Quaternary
Alluvium
Binga
Point
granite
33 37
50
118°50'0"E
119°20'0"E
10°30'0"N
b
25
25
8 & 11
Cretaceous-Eocene
Meta-sediments
Tumarbong
Semi Schist
5
Caramay
Schist
19
119°15'0"E
c
21
24
Babuyan River
Turbidites
Fig. 4. Simplified geological map of Palawan (a). Geological map of the Mount Capoas region showing granite sample locations (b). Geological map of central Palawan showing sample
locations of metasediments and Central Palawan granite (c). Geological map of central southern Palawan showing sample locations of Neogene sandstones (d).
705
Along the contact between the SPT and the NPCT through Ulugan
Bay in central Palawan amphibolites and schists form a sole of highgrade metamorphic rocks (Encarnación et al., 1995). The ophiolitic
rocks are overlain by a sequence of Paleogene and Neogene clastic sedimentary rocks (Almasco et al., 2000). The character of these sedimentary rocks is largely unknown. However, the association of ophiolitic
and continental rocks overlain by Paleogene and Neogene clastic sedimentary rocks has led to comparisons with the stratigraphy of northern
Borneo (e.g. Hamilton, 1979; Müller, 1991; Almasco et al., 2000).
siliciclastic samples (PAL-5, PAL-21, PAL-55) and two granite samples
(PAL-5, PAL-11) from the Central Palawan granite were analysed by
LA-ICPMS at University College London. Zircon separates from four samples from the Mount Capoas granite (PAL-33, PAL-34, PAL-35, PAL-37)
and four samples from the Bay Peak granite (PAL-25, PAL-26, PAL-27,
PAL-30) were analysed by sensitive high-resolution ion microprobe
(SHRIMP) on the SHRIMP II at the Australian National University.
4. Sampling
The U–Th–Pb isotope analyses were performed using a New Wave
213 aperture-imaged frequency-quintupled laser ablation system
(213 nm) coupled to an Agilent 7700 quadrupole-based ICP–MS. Grains
typically were ablated with 40 μm laser spot. Real-time data were
processed using the GLITTER® software package (Griffin et al., 2008).
Plesovice zircon (TIMS reference age 337.13 ± 0.37 Ma; Sláma et al.,
2008) and NIST SRM 612 silicate glass (Pearce et al., 1997) were used
as external standards for correcting mass fractionation and instrumental bias. A 10% cutoff was adopted to reject discordant data. 238U/206Pb
ages are used for zircons b 1000 Ma and the 207Pb/206Pb ages were
used for older zircons.
Siliciclastic sedimentary rock and granite samples were collected
from Palawan (Fig. 4) during a field survey in 2011 conducted in collaboration with the Philippines Mines and Geoscience Bureau.
Siliciclastic sedimentary rock samples were collected from the
Babuyan River Turbidites, Caramay Schist and the Tumarbong Semi
Schist of the NPCT and from the Miocene Isugod Formation of the SPT.
Float samples were collected from the Central Palawan granite.
Sampling was limited to float samples from riverbeds because of the
inaccessibility of the main granite body. Sampling of the Capoas granite
was limited to two of its three bodies: the Bay Peak granite and the
Mount Capoas granite. The Binga granite, the smallest of the three
granite bodies within the Capoas granite, was not sampled. Although
sampling of the Capoas granite was limited to a small number of coastal
outcrops, field observations suggest little lithological variation within
the pluton. In outcrop the granite has a consistent composition and texture. There is a coarse-grained groundmass of quartz, mica, pink feldspar and hornblende with large phenocrysts of feldspar (up to 3 cm)
that display a strong, but variable, preferred orientation. Occasional
even larger megacrysts display both simple twinning and concentric zonation. It was not possible to sample the Capoas granite over a range of
elevations because of the inaccessibility of the mountain. However,
minimal lithological variation was observed within float boulders on
the cobble beach around the base of Mt Capoas. Although not in situ,
the abundance of similar cobbles, likely to be sourced from all elevations
of the pluton, suggests little lithological variation. Field observations
suggest the Bay Peak granite is different from the Mt Capoas granite. It
has a medium-grained groundmass of quartz, mica, feldspar and hornblende. Feldspar phenocrysts are smaller (up to 1.5 cm) than those
seen in the Capoas granite, being only slightly larger than the groundmass, subhedral and with no apparent preferred orientation. The Bay
Peak granite was well exposed in coastal outcrops to the south of Bay
Peak itself, as well as occurring as characteristic knolls of large
sub-rounded boulders both within and along the track that leads
north over the western flanks of Bay Peak. These sites allowed sampling
of the Bay Peak granite over a small elevation range.
5.1. U–Pb isotopic dating — LA–ICP–MS
5.2. U–Pb isotopic dating — SHRIMP
High purity zircon separates were analysed alongside the
417 Ma Temora U–Pb dating standard (Black et al., 2003) and the
SL13 U and Th concentration standard (U = 238 ppm and Th =
20 ppm; Claoué-Long et al., 1995). The grains were sectioned and
polished until exposed through their midsections and Au coated.
The internal zonation and structure of single grains were mapped
using cathodoluminescence and reflected light images, allowing
spot analyses to be targeted on grain areas free of cracks and inclusions. U–Pb analyses were performed following the analytical procedures outlined by Williams (1998). Data reduction and all age
calculations were achieved using the SQUID 1.03 and Isoplot/Ex
2.29 programmes of Ludwig (2001a,b). Young ages are assumed to
be concordant, and were determined solely using the 238U/206Pb
ratio; common Pb was estimated using the 207Pb. Precambrian crystals and cores were corrected for common Pb using the 204Pb/206Pb
ratio, and the age was based on the 206Pb/207Pb ratio. 238U/206Pb
ages are used for zircons b 1000 Ma and the 207Pb/206Pb ages were
used for older zircons. Uncertainties in isotopic ratios and ages
(including data tables and error bars for plotted data) are reported
at the 1σ level. Final, weighted mean ages are reported as 95%
confidence limits, with the uncertainty in the standard calibrations
included.
6. Results
5. Methods
6.1. Heavy minerals and detrital zircon geochronology
Heavy minerals were separated from five metasedimentary samples
from the NPCT and two sandstones from the SPT following the standard
procedure described by Mange and Maurer (1992). Point counting was
performed on the heavy mineral assemblage using an automated
stepping stage. The line point-counting method of Mange and Maurer
(1992) was used in this study. At least 200 non-opaque and nonmicaceous heavy minerals were identified and counted. Different
types of zircon, tourmaline and apatite were counted separately.
Opaque, altered, carbonate, mica group and light minerals were
recorded, but not included into the total heavy mineral count. Selected
heavy mineral grains were analysed with a JEOL Scanning Electron
Microscope (SEM) with the attached energy dispersive system (EDS)
in order to confirm optical identifications.
Zircon separates were separated using standard heavy liquid and
Frantz isodynamic separation techniques. High-purity zircon separates
were handpicked and mounted in epoxy resin. Zircons from three
Heavy mineral assemblages were identified for seven samples (Fig. 5
and Table 1) from the NPCT and the SPT. Five samples were analysed
from the NPCT. The Babuyan River Turbidite sample (PAL-5) contains
predominantly colourless euhedral, subhedral and anhedral zircon
(98.5%), titanite (0.9%), apatite (0.3%), epidote (0.3%), rutile (tr.) and anatase (tr.). A few rounded zircons with frosted surfaces are also present.
The Tumarbong Semi Schist samples (PAL-19 and PAL-31) contain zircon (94.4–100%), amphibole (0–1.6%), clinopyroxene (0–1.2%), epidote
(0–0.8%), tourmaline (0–0.8%), chlorite (0–0.4%), garnet (0–0.4%), rutile
(0–0.4%) and traces (1 grain on the slide) of orthopyroxene. Zircon is
predominantly colourless euhedral and subhedral. Some rutile and
zircon is surrounded by micaceous matrix. Amphibole is fresh and
is pleochroic in shades of green and brown. The Caramay Schist samples
(PAL-21 and PAL-24) contain colourless, euhedral, subhedral, anhedral
and subrounded zircon (93.7–99.0%), apatite (0–5.1%), chlorite
706
NPCT Meta-sediments
Tumarbong Semi
Schist PAL-31
1
Zrtot 100.0 %
Tumarbong Semi
Schist PAL-19
Zr2 41.4 %
1
colourless
euhedral
2
Zr1 34.3 %
Zrtot 94.4 %
Zr1 27.5 %
Zr2 49.4 %
colourless
subhedral
colourless
euhedral
ZrOth 9.6 %
ZrOth 7.6 %
ss
urle
colo unded
ro
sub
colourless
subhedral
Other 1.6 %
Grt 0.4 %
Ep 0.8 %
Cpx 1.2 %
Am 1.6 %
Zr5 4.5 %
Zr3 10.1 %
Zr6 2 %
Zr5 4.4 %
Zr3 3.6 %
Babuyan River
Turbidites PAL-5
1
2
Zrtot 98.5 %
Zr1 25.5 %
Zr2 57.9 %
Caramay Schist
PAL-24
colourless
euhedral
1
colourless
subhedral
Zr6 2.1 %
Other 0.9 %
Ep 0.3 %
Ap 0.3 %
ZrnOth 0.6 %
Zrtot 99.0 %
Zr1 16.8 %
Zr5 9.1 %
Zr2 70.1 %
colourless
subhedral
colourless
euhedral
Other 1 %
ZrOth 0.3 %
Zr6 1.7 %
Zr3 2.7 %
Zr4 0.6 %
Zr5 3.7 %
Zr3 6 %
Miocene Sandstones
PAL-55
1
2
Zrtot 97.4 %
Zr4 0.3 %
Caramay Schist
PAL-21
3
Zr1 28.9 %
1
2
Zr2 44.8 %
Zrtot 93.7 %
colourless
euhedral
colourless
subhedral
ZrOth 3.9 %
Zr1 27 %
Zr2 49.2 %
Other 1.3 %
Sp 0.3 %
Ep 0.3 %
Am 0.6 %
colourless
euhedral
colourless
subhedral
Zr6 2.9 %
Ap 5.1 %
Miocene Sandstones
PAL-53
Zr5 12.3 %
Zr3 4.2 %
1
3
Am 55.6 %
Zr5 10.8 %
Ap 1%
amphibole
ZrOth 0.7%
Zr5 0.3%
Zr2 0.7%
Other 2.6%
Ky 1.6 %
Grt 1 %
Heavy mineral groups
Approx. amount in each sample
Total = 100%
granitic/
metamorphic
volcanic
1
2
ultramafic
(ophiolitic)
2
3
Cpx 2 %
Opx 2.9 %
Sp 14.7 %
Ep 17 %
ZrOth 1.2 %
Zr6 1.2 %
Zr3 4.2 %
Zrtot 1.7%
Zr4 0.3 %
SPT Sandstones
Other 0.3 %
Grt 0.3 %
Cpx 0.6 %
Zrtot - total zircon
Zr1- colourless, euhedral
Zr2- colourless, subhedral
Zr3- colourless, subrounded
Zr4- colourless, rounded
Zr5- colourless, anhedral
Zr6- colourless, elongate
ZrOth- zircon, other
Ap - apatite
Am- amphibole
Cpx - clinopyroxene
Opx - orthopyroxene
Ep - epidote
Sp - Cr spinel
Grt - garnet
St - staurolite
Ky - kyanite
Fig. 5. Detrital heavy mineral compositions of sediments from the NPCT and SPT. Other zircons include purple, brown, yellow, zoned with visible overgrowths and surrounded by matrix.
Other heavy minerals include titanite, tourmaline, monazite, rutile, clinozoisite and chlorite. Geological map key as for Fig. 4.
707
Table 1
Sample locations and compositions of detrital heavy mineral assemblages analysed from Palawan.
North Palawan Continental Terrane meta-sediments
Zrn
Tur
Rt
Ttn
Mnz
Grt
St
Am
Ap
Cpx
Opx
Sp
Ep
Czo
An
Chl
Frg
Fbr
Ky
n
Provenance
group:
South Palawan Terrane
sandstones
Babuyan River Turbidites
Tumarbong Semi Schist
PAL-5
PAL-19
PAL-31
PAL-21
PAL-24
PAL-53
PAL-55
E118.81523 N10.06044
E119.07534 N10.01709
E119.32183 N10.59063
E119.09926 N10.02111
E119.21148 N10.10088
E118.05455
N9.18894
98.5
94.4
0.8
0.4
100
93.7
75.8
1.7
97.4
0.3
0.3
tr.
0.9
Caramay Schist
Isugod Formation
tr.
0.6
0.4
0.3
1
1.6
0.3
55.6
1
2
2.9
14.7
17
tr.
5.1
0.6
1.2
tr.
0.3
0.8
0.3
tr.
0.6
0.3
0.3
tr.
0.4
330
251
Granitic/metamorphic and volcanic
0.5
23.4
0.3
198
333
389
tr.
1.6
306
308
Granitic/metamorphic,
volcanic and ultramafic
(ophiolitic)
Zrn — zircon, Tur — tourmaline, Rt — rutile, Ttn — titanite, Mnz — monazite, Grt — garnet, St — staurolite, Am — amphibole, Ap — apatite, Cpx — clinopyroxene, Opx — orthopyroxene,
Sp — Cr spinel, Ep — epidote, Czo — clinozoizite, An — anatase, Chl — chlorite, Frg — polymineral fragments, Fbr — fibrolite (sillimanite, Ky — kyanite). N — total number of heavy
minerals counted.
(0–0.7%), clinopyroxene (0–0.6%), garnet (0–0.3%), clinozoisite
(0–0.3%) and possibly fibrolite (0–0.3%). Apatite is euhedral (fresh)
and colourless. Some apatite grains show faint pleochroism from
colourless to light brown.
Heavy mineral assemblages of the two Miocene sandstones from the
SPT (PAL-53 and PAL-55) are very different. PAL-55 is composed of
zircon (97.4%), amphibole (0.6%), epidote (0.3%) and Cr spinel (0.3%).
As in all samples analysed from NPCT, colourless euhedral and subhedral
zircon dominate PAL-55 from the SPT. PAL-53 is composed of amphibole
(55.6%), epidote (17%) and Cr spinel (14.7%), orthopyroxene (2.9%),
clinopyroxene (2%), zircon (1.7%), kyanite (1.6%), garnet (1%) and apatite (1%). Amphibole is pleochroic in green-brown and blue-green
shades. SEM–EDS analyses show that SPT amphiboles are of actinolite
composition. Representative field photographs and heavy mineral
photomicrographs of sediments from the NPCT and SPT are presented
in Fig. 6a–j.
Detrital zircons from the Babuyan River Turbidites and the Caramay
Schist from the NPCT yield almost identical age populations (Fig. 7).
There are two Phanerozoic populations: Cretaceous to Jurassic (60 Ma
to 200 Ma) and a smaller Middle Devonian to Middle Ordovician
(380 Ma to 460 Ma). A Paleoproterozoic population is prominent in
both samples. Detrital zircons from the Neogene sandstones from the
SPT contain Cenozoic, Cretaceous, Jurassic, Permo-Triassic, Palaeozoic
and Proterozoic zircons. The dominant age populations are Cretaceous
to Jurassic (60 Ma to 200 Ma). There is a smaller Paleocene to Eocene
(60 Ma to 40 Ma) age population (Fig. 7).
6.2. Granite petrography
Two samples (PAL-8, PAL-11) from the Central Palawan granite have
a similar appearance and mineralogy. PAL-8 comprises feldspar, quartz
and biotite (altered to chlorite), with some minor hornblende. It is
impossible to differentiate between plagioclase and K-feldspar as all of
the feldspar in this sample is sericitised. This sample also shows
extensive granophyric intergrowth textures between quartz and feldspar, which often appear to radiate from a central rounded quartz grains
and indicate simultaneous crystallisation of quartz and feldspar. The
PAL-11 sample shares a similar mineralogy (K-feldspar, plagioclase,
quartz and biotite altered to chlorite), but was deformed after
crystallisation. Quartz exhibits undulose extinction, grain boundary migration and subgrain development, and the feldspar grains show extensive micro-fracturing. Plagioclase exhibits deformation twinning. The
granophyric textures observed in the PAL-8 sample reflect simultaneous
crystallisation of quartz and feldspar. These textures were not observed
in the other sample of this granite (PAL-11), which might indicate that
these textures may have been destroyed due to the localized deformation observed in this sample.
The Capoas granite is a fine-grained granodiorite (Fig. 8a–b). The
samples are predominantly composed of plagioclase, orthoclase, quartz
and biotite. The feldspars commonly show sericitisation and oscillatory
zoning. The sample PAL-37 has undergone some alteration and has
smaller amounts of biotite compared to the other samples. The granodiorite underwent some localized post-crystallisation deformation, as
microfractures and deformation twinning were observed in plagioclase
in PAL-37 and some of the biotite grains were kinked in PAL-30. All samples showed undulose extinction and subgrain development in quartz
grains indicating that regional post-crystallisation strain occurred.
The Bay Peak granite is a medium-grained granodiorite (Fig. 8c–d).
The samples are composed predominantly of plagioclase, orthoclase,
quartz and biotite. PAL-27 contains trace amounts of epidote. Plagioclase and orthoclase show oscillatory zoning indicating local compositional changes in the melt during crystallisation (Vernon, 2004).
Exsolution lamellae intergrowths are seen in PAL-26 and suggest that
there was some instability during crystallisation and unmixing of the
solid solution into two minerals (Vernon, 2004). The granodiorite is
effectively undeformed, but undulose extinction of the feldspars and
quartz, subgrain development in quartz grains indicates that some
post-crystallisation strain occurred (Vernon, 2004).
708
Fig. 6. Selection of field photographs of outcrops, sediment samples and heavy mineral photomicrographs from the sediments on the NPCT and the SPT. Babuyan River Turbidites
(a), titanite (b), Tumurbong Semi Schist (c), amphibole (d), Caramay Schist (e), apatite (f), PAL-53 — SPT Miocene sandstones (g), staurolite (h), PAL-55 — SPT Miocene sandstones
(i), Cr spinel (j). Scale bars are 100 μm.
709
Proterozoic
K
J
T/P
C/D
S/O
Neo- Meso-
Archean
Paleo-
Magmatic age
30
27
24
21
18
15
12
9
6
3
0
n= 50
n= 3
1
Central Palawan
granite
C PALAWAN
Analyses number
Cen
LA-ICPMS
0
Analyses number
2
1
Analyses number
Analyses number
SHRIMP
0
n= 94
n= 6
2
Miocene SPT
Sandstones
(PAL-55)
1
LA-ICPMS
0
n= 225
n= 25
2
Early Miocene Tajau Sandstone
Member Kudat Peninsula,
N Borneo
1
LA-ICPMS
0
n= 98
n= 97
0
100
200
300
400
10
9
8
7
6
5
4
3
2
1
0
n= 21
Caramay Schist
(PAL-21)
NPCT
LA-ICPMS
10
9
8
7
6
5
4
3
2
1
0
500
500
Age,Ma
NPCT
20
18
16
14
12
10
8
6
4
2
0
Inherited zircons from the
Mount Capoas and Bay
Peak granites
3
20
18
16
14
12
10
8
6
4
2
0
50
45
40
35
30
25
20
15
10
5
0
27
24
21
18
15
12
9
6
3
0
n= 18
4
SPT AND N BORNEO
Analyses number
10
9
8
7
6
5
4
3
2
1
0
CAPOAS
Analyses number
5
n= 38
n= 21
Babuyan River Turbidites
(PAL-5)
NPCT
LA-ICPMS
1000
1500
2000
2500
3000
3500
4000
Age,Ma
Fig. 7. Histograms and probability density curves for all zircons analysed from the NPCT metasediments, the Central Palawan granite, the SPT sandstones, Early Miocene sandstones from
northern Borneo (Suggate, 2011) and inherited zircons from the Capoas granite bodies.
The Capoas and Bay Peak granites share a similar granodiorite composition of plagioclase, K-feldspar, quartz and biotite. The only major difference between these is the grain-size, where the Capoas granite samples
are finer grained than the Bay Peak granite. These could therefore represent one pulse of magmatism that crystallised differently, or two distinct
pulses of similar chemistry with different grain-sizes.
6.3. Magmatic ages of granites
U–Pb (LA–ICPMS) dating of zircons from the Central Palawan granite
(Table 2) yielded Middle Eocene ages (42 ± 0.5 Ma) that are
interpreted to represent the magmatic age of the zircons and thus the
crystallisation age of the granite (Fig. 9).
710
Fig. 8. Representative field photographs of outcrops and thin section photomicrographs (ppl and xpl) of the Mount Capoas (a–b) and the Bay Peak (c–d) granites.
U–Pb SHRIMP dating of zircons from the Mount Capoas granite
bodies reveal a group of tightly clustered Middle Miocene ages that
are interpreted to represent zircon magmatic ages and thus the
crystallisation age of the pluton (Fig. 10 and Table 2). There is very little
variation between the ages from the Mount Capoas granite bodies, with
the mean ages plotting within error of each other. Eighty-seven
spot analyses were made on four samples (PAL-33, PAL-34, PAL-35,
PAL-37) from the Capoas granite body. Fifty-eight form a coherent
group interpreted as the magmatic age (Fig. 11). Fifty-seven of the
magmatic ages plot close to concordia, stretched along the mixing line
to common Pb (Fig. 10). These groups define a mean magmatic age of
13.5 ± 0.2 Ma. One hundred and nineteen spot analyses were made
on four samples (PAL-25, PAL-26, PAL-27, PAL-30) from the Bay Peak
granite body. Ninety-two form a coherent group interpreted as
reflecting the magmatic age (Fig. 11). There is a greater variation in
the ages from the Bay Peak granite, but the mean ages still plot within
error of each other. There is no correlation between age and elevation.
All of the magmatic ages plot close to the concordia, stretched along
the mixing line to common Pb (Fig. 10). These groups define a
mean magmatic age of 13.8 ± 0.2 Ma. Representative CL images of
Table 2
Sample locations, descriptions and mean magmatic ages of granites from Palawan.
Sample number
Longitude (decimal degrees)
Latitude (decimal degrees
Elevation (m)
Lithology
Spot analysesa
Magmatic analysesb
Age (Ma)c
Error ± (Ma)d
Zircon SHRIMP ages
Mount Capoas Intrusion
PAL-33
E 119.301248
PAL-34
E 119.290950
PAL-35
E 119.291377
PAL-37
E 119.309682
N 10.774655
N 10.769261
N 10.769329
N 10.772678
4
4
9
6
Granodiorite
Granodiorite
Granodiorite
Granodiorite
21
25
22
19
87
18
20
11
9
58
13.5
13.5
13.5
13.3
Mean = 13.5
0.2
0.2
0.2
0.4
Mean = 0.25
Bay Peak Intrusion
PAL-25
E 119.333292
PAL-26
E 119.336209
PAL-27
E 119.333001
PAL-30
E 119.329835
N 10.652703
N 10.671705
N 10.664068
N 10.654861
32
192
154
1
Granodiorite
Granodiorite
Granodiorite
Granodiorite
41
37
19
22
119
29
29
17
17
92
13.8
14.1
13.6
13.8
Mean = 13.8
0.3
0.2
0.2
0.3
Mean = 0.25
Zircon LA–ICPMS ages
Central Palawan Granitic Intrusion
PAL-8
E 118.874868
PAL-11
E 118.874868
N 10.159678
N 10.159678
25
28
53
14
15
29
42.1
42.0
Mean = 42.05
1.2
1.3
Mean = 1.25
a
b
c
d
Total number of spot analyses made on sample.
Number of analyses giving magmatic ages.
Mean age of all accepted magmatic analyses.
Errors are 1σ for SHRIMP analyses and 2σ for LA-ICPMS analyses.
34.0
34.0
Granite
Granite
711
51
49
47
Central Palawan Granite
PAL-8, PAL-11
Age = 41.96 ± 0.5 Ma
(97.3% conf, from coherent group of 21)
Age
45
43
41
39
37
35
Fig. 9. Zircon age extractor diagram (Ludwig, 2001b) showing magmatic zircon LA–ICPMS U–Pb ages for samples from the Central Palawan granite. Green error boxes indicate analyses
accepted for calculation of the median age. The blue error-boxes indicate analyses rejected for calculation of the median age. Box heights are 2σ error.
magmatic zircons from the Mount Capoas granite bodies are presented
in Fig. 12.
6.4. Inherited zircons
In addition there is a wide range of inherited zircon ages in all the
granites (Fig. 6). The Central Palawan granite contains a small number
of zircons with Cretaceous, Jurassic, Palaeozoic and Proterozoic ages.
The Mount Capoas granite and Bay Peak granite contain inherited zircons with several significant age populations. All are from analyses of
cores. Fifty-six of the 206 spot analyses are ages that are inherited,
with all except one being greater than twice the magmatic age.
There are two major peaks, one of Cretaceous and Jurassic ages
representing 53% of all inherited ages, and a second of Neoproterozoic
ages representing 18% of all inherited ages. The Bay Peak granite
(PAL-25) contains the oldest inherited ages. This sample includes
two zircons with Archaean (2747 ± 26 Ma and 2505 ± 11 Ma)
cores. One other Archaean (2586 ± 6 Ma) core is present in the
Capoas granite sample PAL-35. Both granite bodies have yielded a
range of inherited Proterozoic zircon ages. There is a group of five
Paleoproterozoic and Mesoproterozoic zircon ages from 1810 ±
38 Ma to 1389 ± 28 Ma and a group of ten Neoproterozoic ages
from 955 ± 9 Ma to 662 ± 7 Ma. There is one Devonian age
(396 ± 5 Ma), one Carboniferous age (310 ± 4 Ma) and two Triassic
ages (247 ± 4 Ma and 230 ± 3 Ma). The largest group is of 30 Jurassic and Cretaceous ages (191 ± 4 Ma to 74 ± 1 Ma). The youngest
inherited ages are Paleocene (64 ± 0.9 Ma), Late Eocene (36 ±
0.5 Ma), Early Oligocene (30 ± 0.4 Ma) and Early Miocene (18 ±
1.7 Ma). Representative CL images of inherited zircons from the
Mount Capoas granite bodies are presented in Fig. 12.
7. Discussion
7.1. Provenance of the NPCT and SPT sediments
The heavy mineral species present in the NPCT and SPT sediments
are zircon (73.5–100%), amphibole (0–55.6%), apatite (0–12.3%),
orthopyroxene (0–2.9%) clinopyroxene (0–2%), epidote (0–2.6%), Cr
spinel (0–2.3%) and minor (b1%) titanite, kyanite, tourmaline, monazite, chlorite, garnet, rutile, staurolite, clinozoisite, and fibrolite. The
diversity of zircon and apatite morphological types and the presence
of different amphibole varieties suggest that these minerals are derived
from different sources that are discussed below.
Zircon is ubiquitous in crustal igneous, volcanic and metamorphic
rocks. The abundance of euhedral and subhedral zircon suggests firstcycle provenance from granitic, rhyolitic or metamorphic rocks. Apatite
is a common accessory mineral in almost all igneous rock types. It also
crystallises in carbonatites, hydrothermal and metamorphic (regional
and thermal) rocks or may be of authigenic origin (e.g. Deer et al.,
1966; Mange and Maurer, 1992). The fresh euhedral morphology and
presence of slightly pleochroic grains suggest that apatite was derived
predominantly from volcanic rocks. This is consistent with the association of apatite and clinopyroxene (augite), which is a common accessory of intermediate volcanic rocks (e.g. Mange and Maurer, 1992).
Less common, subhedral and subrounded apatite that is present in
zircon-dominated assemblages was most likely derived from the
granitic sources. Amphibole is common predominantly in igneous and
metamorphic rocks. The most common amphibole in the SPT is actinolite, which forms in contact and regionally metamorphosed rocks.
According to Deer et al. (1966), the tremolite–actinolite association is
characteristic of low-grade regionally metamorphosed ultramafic
rocks, whereas actinolite–epidote–chlorite associations are produced
by low-temperature metamorphism of basaltic rocks (Deer et al.,
1966). Actinolite from the SPT is found in association with epidote, chlorite and Cr spinel, which is an indicator of ultramafic/ophiolitic source
rocks (e.g. Mange and Maurer, 1992). Such heavy mineral associations
suggest that the SPT actinolite was derived from metamorphosed ultramafic rocks. Rutile, kyanite, fibrolite (sillimanite), staurolite and
clinozoisite are metamorphic minerals (e.g. Mange and Maurer, 1992).
Kyanite and fibrolite indicate a contribution from high-grade metamorphic rocks. Garnet may be derived from a variety of source rocks
(e.g. Suggate and Hall, 2013), but most commonly is of metamorphic origin. Titanite and monazite are derived either from acid igneous or
metamorphic rocks (e.g. Mange and Maurer, 1992).
712
0.12
Capoas Granite
0.12
PAL-33
To common Pb
0.10
207Pb/206Pb
0.10
207Pb/206Pb
PAL-34
To common Pb
0.08
206Pb/ 238U
Mean
age:
13.42 ± 0.12 Ma
0.08
Mean 206Pb/ 238U age
13.50 ± 0.11 Ma
MSWD = 1.2, N = 20
MSWD = 0.95, N = 18
0.06
0.06
Data plotted uncorrected for common Pb
4.1
16
15
0.04
400
13
1
440
12
480
520
11
0.04
560
600
16
15
400
14
12
440
480
238Pb/206Pb
11
520
560
600
238Pb/206Pb
0.12
0.12
PAL-35
To common Pb
PAL-37
To common Pb
0.10
207Pb/206Pb
207Pb/206Pb
0.10
0.08
Mean 206Pb/ 238U age:
13.52 ± 0.16 Ma
0.08
13.29 ± 0.21 Ma
MSWD = 0.58, N = 9
0.06
MSWD = 1.3, N = 11
0.06
0.04 16
16
15
0.04
400
14
440
13
480
12
520
14
13
0.02
400
600
440
480
238Pb/206Pb
0.20
12
11
11
560
15
520
560
600
238Pb/206Pb
Bay Peak Granite
0.08
PAL-25
PAL-26
To common Pb
10.1
To common Pb
0.07
13.80 ± 0.12 Ma
MSWD = 0.72, N = 29
207Pb/206Pb
207Pb/206Pb
0.16
0.12
13.81 ± 0.12 Ma
0.06
MSWD = 1.00, N = 29
0.05
16
0.08
20
0.04
300
21.1
18
340
380
15
13
12
420
460
500
0.03
400
540
420
440
460
238Pb/206Pb
PAL-27
520
540
PAL-30
0.10
MSWD = 1.3, N = 17
To common Pb
207Pb/206Pb
207Pb/206Pb
500
To common Pb
13.59 ± 0.13 Ma
0.060
0.056
0.052
0.048
0.044
480
238Pb/206Pb
0.068
0.064
12
0.04
14.8
0.040
430
14.4
13.2
12.8
470
238Pb/206Pb
Mean 206Pb/238U age:
13.82 ± 0.15 Ma
MSWD = 1.4, N = 17
0.06
12.4
16 15.6 15.2 14.8
450
0.08
490
510
0.04
400
420
14.4
44
440
12.8
460
480
500
12.4
520
238Pb/206Pb
Fig. 10. Tera–Wasserburg concordia diagrams (Tera and Wasserburg, 1972) showing zircon SHRIMP U–Pb analyses for samples from the Mount Capoas (red) and Bay Peak (blue) granites.
Data-point error ellipses are 68.3% confidence. Uncertainties are 1σ weighted mean ages and reported as 95% confidence limits.
713
14.8
Mount Capoas Granite
Pal 26
Age = 13.50 ± 0.2 Ma
Pal 25
14.4
Pal 30
Pal 37
Age
14.0
Pal 27
Pal 33
Pal 34
Pal 35
13.6
13.2
Bay Peak Granite
Age = 13.80 ± 0.3 Ma
12.8
12.4
Fig. 11. Zircon age extractor diagrams (Ludwig, 2001b) showing SHRIMP U–Pb ages for magmatic zircon samples from the Mount Capoas and Bay Peak granites. Box heights are 2σ error.
To aid provenance interpretations the dominant heavy mineral
assemblages have been have been assigned to different heavy mineral provenance groups: (1) granitic/metamorphic, (2) volcanic and
(3) ophiolitic/ultramafic.
The NPCT metasediments contain heavy mineral assemblages
typical of granitic and metamorphic rocks with a continental crust character, and a subordinate volcanic component. Cretaceous and Jurassic
are the dominant zircon age populations. The most likely sources are
Mesozoic rocks of the South China continental margin (Fig. 13) where
there are abundant Jurassic and Lower Cretaceous granitic and volcanic
rocks and small Triassic plutons (e.g. Zhou et al., 2008; Sun et al., 2012).
There are also Upper Cretaceous–Paleogene tholeiitic basalts, andesites
and trachytes/rhyolites in the Sanshui, Heyuan and Lienping sedimentary basins in South China (Chung et al., 1997). All these are potential
sources for the granitic/metamorphic and volcanic detritus in the
NPCT sediments which were deposited before the NPCT rifted away
from the South China margin during Oligocene opening of the South
China Sea. This interpretation supports previous provenance interpretations by Suzuki et al. (2000) and Walia et al. (2012) that the NPCT
metasediments were derived from the South China margin, that the
three units (Caramay Schist, Concepcion Pebbly Phyllite/Tumarbong
Semi Schist and Babuyan River Turbidite) are contemporaneous, and
that they do not form part of the basement of north Palawan. The
Caramay Schist was originally interpreted to be of Palaeozoic age
(Mitchell et al., 1985), and the Concepcion Pebbly Phyllite was suggested to be of probable Palaeozoic but possible Early Cenozoic age
(Mitchell et al., 1985). There were no age data to support these interpretations. The Babuyan River Turbidite was dated as Late Cretaceous based
on the presence of the coccolith Prediscophaera cretacea (Wolfart et al.,
1986). Suzuki et al. (2000) suggested all these units were Cretaceous to
Eocene in age. Walia et al. (2012) showed that all these units contain
Cretaceous zircons. The new U–Pb zircon ages from our study (Fig. 13)
indicate that the maximum depositional age for the NPCT metasediments is Late Cretaceous and confirms the conclusion of Walia
et al. (2012). The zircon ages from the Caramay Schist and the Babuyan
River Turbidite are almost identical and suggest that the two are derived
from a protolith of similar age (Late Cretaceous or younger).
The Miocene sandstones of the SPT have heavy mineral assemblages
that indicate they were derived predominantly from a granitic and
metamorphic source and an ultrabasic (ophiolitic) source, with a
minor volcanic contribution. The zircons in the sandstones (PAL-55) of
the SPT (Fig. 13) are predominantly Cretaceous and Jurassic but include
Eocene ages. Probable source areas for the Miocene SPT sandstones are
the NPCT metasediments, metamorphic rocks at the SPT-NPCT contact,
Eocene rift-related volcanic and/or minor intrusive rocks of the South
China Sea margin, and the Palawan Ophiolite Complex. The Middle
Eocene (42 Ma) Central Palawan granite is an example of a South
China Sea rift-related intrusion.
7.2. Palawan and north Borneo sediments
Lower Miocene sandstones from the Tajau Sandstone Member of the
Kudat Formation of Sabah have unusual heavy mineral assemblages
compared to other sandstones, both older and younger, from northern
Borneo (van Hattum, 2005; Suggate, 2011; van Hattum et al., in
press). They were derived predominantly from a granitic and metamorphic source, and contain continental derived accessory minerals that include unabraded zircon, tourmaline, rutile, monazite and apatite as well
as medium- to high-grade metamorphic minerals that include garnet,
epidote, staurolite, sillimanite and kyanite. Heavy mineral assemblages
and compositions (Suggate, 2011; Suggate and Hall, 2013) indicate
that they were not derived from the same sources as other Borneo sandstones whereas they have many similarities to possible Palawan
sources. Garnet and kyanite in the Tajau Sandstone Member are suggested to be derived from high-grade metamorphic rocks exposed
along the contact between the SPT and the NPCT in Ulugan Bay. This
suggestion is supported by similar medium- to high-grade metamorphic minerals in the Neogene SPT sandstones and the Lower Miocene
Tajau Sandstone Member. Both also contain Cr spinel and clinopyroxene
derived from an ultrabasic source area, likely to be the Palawan
Ophiolite Complex.
The zircon age populations of the Neogene SPT sandstones and the
Lower Miocene Tajau Sandstone Member are also similar. Zircons of
Eocene age (36 Ma to 49 Ma) are found in both. Cretaceous zircons
could have a Borneo source since zircons of this age derived from the
Schwaner Mountains (van Hattum et al., 2006; Hall et al., 2008;
Suggate, 2011; van Hattum et al., in press) dominate the Paleogene
and Neogene sediments. However, the presence of Jurassic zircons in
the SPT and Kudat Peninsula suggests a non-Borneo source. Jurassic zircons are largely absent from Paleogene and Neogene sediments and
Jurassic igneous rocks are not abundant in Borneo, except for one granite body in the south Schwaner Mountains of SW Kalimantan (Haile
et al., 1977; L. Davies, pers. comm., 2012). A significant population of
Permo-Triassic zircons would be expected in sediments derived from
Borneo, where zircons of this age are abundant in Paleogene sedimentary rocks (van Hattum et al., 2006). The similarities of heavy mineral
assemblages and detrital zircon ages indicate a short-lived episode of
erosion and transport of sediment from Palawan to northern Borneo
714
Fig. 12. Representative CL images of magmatic and inherited zircons from the Mount Capoas and Bay Peak granites.
in the Early Miocene at about 20 Ma which we interpret to be the result
of collision of the NPCT with the Cagayan Arc.
7.3. Age and crustal inheritance patterns of the Mount Capoas granite
The new U–Pb ages presented here provide a precise age for
the Mount Capoas granite. These new ages broadly confirm earlier
207
Pb/235U mean ages of 13.4 ± 0.4 Ma on monazite from the Mount
Capoas granite body by Encarnación and Mukasa (1997) and show
that the Capoas granite is significantly older (~6.6 myr) than the
Mount Kinabalu granite in northern Borneo. The Bay Peak granite is
slightly older (0.3 myr) than the Mount Capoas granite suggesting either a single magmatic pulse between 13.8 Ma and 13.5 Ma, or two
pulses that lasted no more than 300 ka.
The inherited zircons in the Mount Capoas granite indicate that during magma formation, transport and emplacement the granite sampled
continental crust or sediments that had been reworked from older rocks
that were once part of the South China margin (Fig. 13). Cenozoic
inherited zircon ages probably indicate magmatism associated with
rifting of the South China Sea or subduction of the proto-South China
Sea. Other inherited zircons are predominantly Cretaceous, Jurassic
and Proterozoic, and the oldest grains are Archaean. The crustal inheritance pattern suggests a number of different sources and gives an insight into the NPCT basement. The basement of the NPCT is thought to
be remnants of the Cathaysian block that rifted away from South
China in the Cenozoic. A belt of granitic and metamorphic rocks of Proterozoic, Jurassic and Cretaceous age is known from SE China (Yui et al.,
1996; Li et al., 2007; He et al., 2010; Zhu et al., 2010; Jiang et al., 2011).
PHANEROZOIC
PROTEROZOIC
NEO-
MESO-
715
ARCHEAN
PALEO-
NEO-
MESO-
PALEO-
EO-
CAPOAS
24
Inherited zircons
Capoas granite
n= 56
50
n= 100
140
Tajau Sandstone Mbr
Kudat Formation
SPT AND N BORNEO
Neogene Sandstones
PAL 55
n= 250
70
n= 119
70
NPCT
Number of analyses
Caramay Schist
PAL−21
Babuyan River
Turbidites
PAL-5
n= 118
220
Plutonic Protolith
n= 608
Volcanic Protolith
n= 109
130
Metamorphic Protolith
SOUTH CHINA MARGIN
24
n= 820
190
Detrital Protolith
n= 1133
0
1000
2000
3000
4000
Age, Ma
Fig. 13. Schematic probability density curves that show zircon populations common in the Capoas granite bodies (inherited zircons), the SPT and northern Borneo, the NPCT and zircon
ages that are expected to occur in rocks derived from the South China margin (probability density curves based on data from Duan et al. (2011), Gao et al. (2011), He et al. (2010), Jiang
et al. (2011), Knittel (2011), Knittel et al. (2010), Li et al. (2005), Li et al. (2007), Li et al. (2009), Li et al. (2011), Liu et al. (2009), Shu et al. (2011), Wan et al. (2007), Wang et al. (2007),
Wong et al. (2011), Xu et al. (2005), Yao et al. (2012), Ye et al. (2007), Yui et al. (1996), Yui et al. (2012), Zhang et al. (2006), Zheng et al. (2006), and Zhu et al. (2010)).
716
The source of the small number of Archaean zircons is uncertain. There
is no record of exposed Archaean basement rocks in South China but zircon xenocrysts in Cenozoic and Mesozoic volcanic and plutonic rocks in
South China (Fletcher et al., 2004; Zheng et al., 2011) suggest the presence of unexposed Archaean basement beneath the western Cathaysia
Block, where the oldest exposed rocks are Neoproterozoic in age
(Fletcher et al., 2004; Zheng et al., 2011). This basement has yielded
zircons with age populations of 2900–2500 Ma (Zheng et al., 2011). Alternatively, the Archaean zircons could be reworked from a protolith
further to the north in the North China Craton (Jahn et al., 1987; Liu
et al., 1992) where Archaean rocks are widespread.
Encarnación and Mukasa (1997) suggested that the Mount Capoas
granite formed in a post-rifting, non-collisional tectonic setting
unrelated to any subduction zone. The age of the granite indicates that
it cannot be related to proto-South China Sea subduction which was terminated by Early Miocene collision, and it post-dates collision by about
6 myr. An alternative is magmatism associated with the early stages of
development of the Sulu Arc associated with northwestward subduction of the Celebes Sea (Hall, in press). Collision in the Early Miocene
caused folding and thrusting on Palawan (e.g. Holloway, 1982;
Hutchison, 1996). This elevated much of the region around Palawan
above sea level and sediment from the orogenic belt was transported
south to the Kudat Formation of northern Sabah. South of Palawan
and east of northern Sabah ODP drilling shows that the oldest rocks in
the Sulu Sea were erupted in a backarc basin oceanic before 19 Ma,
but are overlain by rocks of a volcanic arc that emerged rapidly above
sea level and then subsided below the CCD by about 15–14 Ma (Silver
and Rangin, 1991; Silver et al., 1991). This implies an interval of rapid
migration of the active Sulu arc to the southeast (Hutchison, 1992),
collapse of the volcanic arc, and extension of the former orogenic belt
of Palawan. Hall (in press) suggested that trench rollback at about
16 Ma drove Neogene extension in Palawan, and was accompanied by
crustal melting. The syn-collisional and volcanic arc character of the
granite is interpreted to not reflect the tectonic setting of magmatism
but the compositions of the source rocks that were melted (Frost
et al., 2001).
8. Conclusions
NPCT metasediments are no older than Late Cretaceous, they were
formed on the South China margin and were derived from granitic
and metamorphic rocks, all of which rifted away during opening of
the South China Sea. The Miocene SPT sandstones and Lower Miocene
Tajau Sandstone Member of northern Borneo were derived from four
different sources, which include the NPCT metasediments, metamorphic basement rocks at the contact between the SPT and NPCT, South
China Sea rift volcanic and/or minor intrusive rocks, of which the Middle
Eocene (42 ± 0.5 Ma) Central Palawan granite is an example, and the
local ophiolite complex.
The Mount Capoas granite body was intruded during a single pulse
that endured for c.300 ka or as two separate pulses between 13.8 ±
0.2 Ma (Bay Peak granite) and 13.5 ± 0.2 Ma (Mount Capoas granite).
It is significantly older than the Kinabalu granite of northern Borneo.
However, like the Kinabalu granite, inherited zircon ages from the
Mount Capoas granite imply melting of continental crust derived from
the South China margin with a contribution from Cenozoic rift-related
and arc material. Melting is suggested to have occurred in an extensional setting induced by subduction rollback.
Acknowledgements
The project was funded by the SE Asia Research Group at Royal
Holloway University of London, supported by an oil company consortium. We thank Roland de Jesus at the Mines and Geosciences Bureau
(MGB) for the permission to conduct fieldwork in Palawan.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.gr.2013.07.006.
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South China continental margin signature for sandstones and

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