Characterization of the proto-Philippine Sea Plate: Evidence from the emplaced oceanic lithospheric fragments along eastern Philippines

Abstract

The proto-Philippine Sea Plate (pPSP) has been proposed by several authors to account for the origin of the Mesozoic supra-subduction ophiolites along the Philippine archipelago. In this paper, a comprehensive review of the ophiolites in the eastern portion of the Philippines is undertaken. Available data on the geology, ages and geochemical signatures of the oceanic lithospheric fragments in Luzon (Isabela, Lagonoy in Camarines Norte, and Rapu-Rapu island), Central Philippines (Samar, Tacloban, Malitbog and Southeast Bohol), and eastern Mindanao (Dinagat and Pujada) are presented. Characteristics of the Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison. Nearly all of the crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages. The geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and supra-subduction signatures. Although paleomagnetic information is currently limited to the Samar Ophiolite, results indicate a near-equatorial Mesozoic supra-subduction zone origin. In general, correlation of the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are fragments of the Mesozoic pPSP. © 2019 China University of Geosciences (Beijing) and Peking University

Full text

Research Paper
Characterization of the proto-Philippine Sea Plate: Evidence from the
emplaced oceanic lithospheric fragments along eastern Philippines
Carla B. Dimalanta
a
,
*
, Decibel V. Faustino-Eslava
b
, Jillian Aira S. Gabo-Ratio
a
,
Edanjarlo J. Marquez
c
, Jenielyn T. Padrones
d
, Betchaida D. Payot
a
, Karlo L. Queaño
e
,
Noelynna T. Ramos
a
, Graciano P. Yumul Jr.
f
,
g
a
Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines
b
School of Environmental Science and Management, University of the Philippines, Los Baños, Laguna, Philippines
c
Department of Physical Sciences and Mathematics, University of the Philippines - Manila, Padre Faura, Manila, Philippines
d
Institute of Renewable Natural Resources, University of the Philippines, Los Baños, Laguna, Philippines
e
Department of Environmental Science, Ateneo de Manila University, Quezon City, Philippines
f
Itogon-Suyoc Resources Inc., Ortigas, Pasig City, Philippines
g
Apex Mining Company Inc., Ortigas, Pasig City, Philippines
article info
Article history:
Received 26 May 2018
Received in revised form
28 November 2018
Accepted 14 January 2019
Available online 23 February 2019
Keywords:
Eastern Philippines
Ophiolites
Proto-Philippine Sea Plate
abstract
The proto-Philippine Sea Plate (pPSP) has been proposed by several authors to account for the origin of
the Mesozoic supra-subduction ophiolites along the Philippine archipelago. In this paper, a compre-
hensive review of the ophiolites in the eastern portion of the Philippines is undertaken. Available data on
the geology, ages and geochemical signatures of the oceanic lithospheric fragments in Luzon (Isabela,
Lagonoy in Camarines Norte, and Rapu-Rapu island), Central Philippines (Samar, Tacloban, Malitbog and
Southeast Bohol), and eastern Mindanao (Dinagat and Pujada) are presented. Characteristics of the
Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison. Nearly all of the
crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages. The
geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and supra-
subduction signatures. Although paleomagnetic information is currently limited to the Samar Ophio-
lite, results indicate a near-equatorial Mesozoic supra-subduction zone origin. In general, correlation of
the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are frag-
ments of the Mesozoic pPSP.
Ó2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by
Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
licenses/by-nc-nd/4.0/).
1. Introduction
One of the most enigmatic aspects of Philippine tectonics is
the origin of Mesozoic ophiolitic bodies that are scattered along
the western boundary of the Eocene Philippine Sea Plate (PSP).
Most ophiolites in the Philippines are of supra-subduction origins
(Mitchell et al., 1986; Geary et al., 1988; Yumul et al., 1997;
Tamayo, 2001; Tamayo et al., 2004) but do not appear to have
come from ocean basins that presently surround the region.
Whereas current ocean basins around the archipelago are of
Cenozoic ages, Mesozoic and pre-Mesozoic oceanic fragments
have been documented from the northern Philippines all the way
south to Indonesia (e.g., Billedo et al., 1996; David et al., 1996;
Monnier, 1996; Tamayo et al., 2004; Yumul, 2007). Furthermore,
the age of formation of pre-Mesozoic arcs in eastern Philippines
does not agree with younger ocean basins and subduction sys-
tems that now straddle the archipelago. To account for these
tectonic features, an older subduction system that involved a now
totally consumed proto-Philippine Sea Plate (pPSP) has been
suggested (e.g., Wolfe, 1983; Billedo, 1994; David, 1994a, b;
Sajona, 1995; Tamayo et al., 2004; Lallemand, 2016).
The concept of a pPSP is derived from studies of the Philippine
Sea Plate (PSP) itself. While most of the PSP consists of sedi-
mentary and igneous rocks of Eocene or younger ages, some re-
gions within this oceanic basin, particularly the Daito Ridges
region or Amami-Daito-Oki Daito (ADO) (Fig. 1A), are older at
Jurassic or Early Cretaceous ages (e.g., Okino and Kato, 1992;
*Corresponding author.
E-mail address: cbdimalanta@up.edu.ph (C.B. Dimalanta).
Peer-review under responsibility of China University of Geosciences (Beijing).
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ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Geoscience Frontiers 11 (2020) 3e21

Hickey-Vargas, 2005; Ishizuka et al., 2011; Tani et al., 2012; Hall,
2018). Hence, the central section of the PSP is considered as
distinct from the rest of the oceanic plate that likely represents a
trapped portion of the pPSP (e.g., Ingle et al., 1975; Matsuda et al.,
1975; Mizuno et al., 1978; Hussong and Uyeda, 1981; Taylor and
Goodliffe, 2004).
The occurrence of these bodies necessitates a better under-
standing of what the pPSP was. It had been suggested that the pPSP
consisted of Mesozoic terranes of various origins including island
arcs as those trapped in the Philippines, and the ADO ridges (Hall,
2002; Hickey-Vargas et al., 2008; Tani et al., 2015; Ishizuka et al.,
2018). However, Philippine ophiolites are relatively poorly pre-
served, highly dismembered, and in some places, accessibility is-
sues have prevented thorough sampling. On the other hand, the
ADO ridges also have ease of access issues, being underwater. To
establish some consistency in describing the pPSP, therefore, is
quite challenging but is not necessarily impossible.
In order to better define the nature of the pPSP, this work
compiles published information on ophiolites in the Philippines
that are believed to represent pieces of the pPSP. These ophiolites
include those exposed in Luzon (Isabela, Lagonoy in Camarines
Norte, and Rapu-Rapu island), Central Philippines (Samar, Taclo-
ban, Malitbog and Southeast Bohol), and eastern Mindanao
(Dinagat and Pujada) (Fig. 1B). Details on their geology, stratig-
raphy, geochronology, and geochemistry are presented and
compared to help determine distinguishing characteristics of the
pPSP. Halmahera Ophiolite in the south is reviewed and discussed
for a more regional consideration of the pPSP (Fig. 1A). This,
hopefully, can give us an idea on the geological character of the
pPSP, which can help us in understanding the evolution of this
part of the western Pacific.
2. Regional geology
The geologic history and tectonic evolution of the Philippines
result from complex tectonic processes involving continental rift-
ing, oceanic spreading, continent-arc collision, and oblique sub-
duction (Quebral et al., 1996; Aurelio et al., 2013; Yan et al., 2018).
Accreted ophiolites and ophiolitic complexes, magmatic arcs, and
sedimentary formations comprise a large part of the archipelago,
which is referred to as the Philippine Mobile Belt (PMB) ea com-
plex and largely deformed terrane bounded by oppositely dipping
subduction zones (Gervasio, 1967; Rangin et al.,1991; Galgana et al.,
2007). West of the archipelago, the Manila Trench, Negros-Sulu
Trench system, and Cotabato Trench form the East-dipping sub-
duction zones along which oceanic lithospheres of the South China
Sea, Sulu Sea, and Celebes Sea subduct, respectively (Cardwell et al.,
1980; Yumul et al., 2008). To the east, oblique convergence between
the PMB and the Philippine Sea Plate (PSP) occurs along the West-
dipping Philippine Trench, where shear partitioning led to the
formation of the Philippine Fault (Fitch, 1972; Aurelio, 2000). The
East Luzon Trough is a site of incipient subduction and is a reju-
venation of the proto-East Luzon Trough responsible for the
intrusion of Eocene to Oligocene magmatic rocks exposed in the
Northern Sierra Madre range. The trough is separated from the
Philippine Trench by a transform fault (Cardwell et al., 1980;
Hamburger et al., 1983; Bautista et al., 2001; Queaño et al., 2007).
The PMB is generally underlain by Mesozoic basement com-
plexes, often ophiolites and ophiolitic complexes, which formed
from varied tectonic settings and later emplaced by collision pro-
cesses. Extensive volcanism, basin formation, and carbonate build-
up are geological events that mark the Cenozoic era (Florendo,
1994; Pacle et al., 2017; Dimalanta et al., 2018). Eocene and
Figure 1. (A) Mesozoic terranes present east of the Philippine archipelago. (B) Map showing the ophiolites along the eastern portion of the Philippine archipelago. Gray areas show
the occurrence of ophiolites in the western portion of the Philippines, while the black portions indicate the location of ophiolites alongeastern Philippines as discussed in this study.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e214

Oligocene lithologies consist of volcanic complexes, diorite in-
trusions, clastic rocks, and limestone while Miocene formations are
mostly clastic rocks and limestone with subordinate basalt,
andesite, and pyroclastic deposits. From Pleistocene to present day,
subduction is prevalent and results in the emplacement and
deposition of volcanic rocks and pyroclastic deposits throughout
the Philippines. Episodes of marine transgression and regression
since the late Quaternary also allowed the formation and emer-
gence of thick and extensive deposits of marine clastic rocks and
limestone (e.g., Ramos and Tsutsumi, 2010; Ramos et al., 2012;
Gabo et al., 2015).
3. Ophiolites in eastern Philippines
Most Philippine ophiolites are characterized by volcanic se-
quences displaying geochemical compositions similar to mid-ocean
ridge basalts (MORB), transitional MORB-island arc tholeiites, and
arc volcanic rocks originating from modern Pacific-type oceans,
back-arc basins, and island arcs (Pubellier et al., 2004; Tamayo et al.,
2004). Along the archipelago’s eastern margin, ophiolites and
ophiolitic complexes include, from north to south, Isabela, Camar-
ines Norte, Lagonoy, Rapu-Rapu, Samar, Tacloban, Malitbog, Dina-
gat, and Pujada (Fig. 1B). The characteristics of each of these oceanic
lithosphere fragments, along with the Halmahera ophiolite to the
south of the Philippine archipelago, are discussed in detail in the
succeeding sections.
3.1. Geology of Isabela Ophiolite
The Isabela Ophiolite is a NeS-trending complete ophiolite
sequence that forms the Cretaceous basement of northeastern
Luzon Island in the Philippines (Fig. 1B) (Andal et al., 2005). Spinel
lherzolites, clinopyroxene-rich harzburgites, depleted harzbur-
gites, and dunites comprise the mantle section while layered gab-
bros and basalts comprise the crustal section (Fig. 2). A lherzolite-
dominated ultramafic section with alternating occurrences of
peridotites, isotropic gabbros, and volcanic rocks suggests forma-
tion of the Isabela Ophiolite at a slow-spreading ridge. Major-
element geochemistry of spinel and olivine, along with the rare
earth element composition of clinopyroxene, in the Isabela
Ophiolite peridotites indicates similarity to abyssal peridotites from
modern mid-oceanic ridges (Andal et al., 2005). The spinel chem-
istry covers the entire range of abyssal peridotites, where Cr# (¼Cr/
(Cr þAl)) ranges from 0.08 to 0.77, while the REE content of the
clinopyroxenes steadily decreases from the fertile lherzolites to the
residual harzburgites. Forsterite (Fo) content (¼100 Mg/
(Mg þFe
total
)) in olivine ranges from 89.4% to 91.9% for all peri-
dotites. A study by Morishita et al. (2006) on the podiform chro-
mitites of the lherzolite-dominant upper mantle section similarly
suggests that the ophiolite massif initially formed beneath a slow-
spreading mid-ocean ridge. A transition from abyssal to arc setting
is also reflected in the high-Cr# spinels of late-stage dunites and
chromitites. Bedded cherts, overlying the Isabela Ophiolite, reveal
an Early Cretaceous age based on radiolarian fossil dating (MMAJ-
JICA, 1987). A metamorphosed equivalent of the Isabela Ophiolite
is found further south and is referred to as the Dibut Bay Meta-
ophiolite (also known as the Baler Ophiolite; Billedo et al., 1996;
Ishida et al., 2012). This ophiolite is made up of volcanic rocks,
gabbros, pyroxenites and dunites that have undergone ocean floor
metamorphism. It also has well-exposed amphibolites, serpentin-
ites and garnet gneiss making them different from the Isabela
Ophiolite in terms of metamorphism. The occurrence of Mictyoditra
sp. in the chert blocks within the phyllite cover overlying the meta-
ophiolite indicates a Late Jurassic to Early Cretaceous age (Ishida
et al., 2012).
3.2. Geology of Camarines Norte Ophiolite Complex
The Camarines Norte Ophiolite Complex (CNOC) (Geary et al.,
1988; Geary and Kay, 1989; Tamayo et al., 1998) (also known as
the Calaguas Islands Ophiolite; Encarnacion, 2004) includes peri-
dotites exposed in the northeastern part of Camarines Norte (Figs. 2
Figure 2. Stratigraphic columns of the ophiolite and ophiolitic units along eastern Philippines.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 5

and 3A) and the complete mantle-crust sequence in the Calaguas
Island Group. Low grade greenschist facies metamorphism is noted
and believed to be associated with ocean floor metamorphism.
Minor amphibolitization of the gabbros and serpentinization of
ultramafic rocks are also observed. Residual peridotites are
composed of harzburgite with lherzolite and pyroxenite pods
observed in the CNOC (Geary and Kay, 1989). The spinels from the
mantle harzburgites show relatively low Cr# (0.35e0.44), which
indicate back-arc basin (BAB) affinity (Tamayo et al.,1998). Gabbros
are composed of both isotropic and layered units. In some areas,
gabbros occur in fault contact with peridotites and are intermixed
with diabases, basalts and amphibolites as exposed in other islands.
Dikes are composed of sheeted basalt, diabase and microgabbro.
Basalts occur as pillows and flows. The mafic units show a crys-
tallization order of olivine (spinel), plagioclase, and clinopyroxene
(orthopyroxene), which indicate MORB and BABB affinity
(Beccaluva et al., 1983). This is affirmed by the geochemistry of the
pillow basalts and diabase dikes, which show LREE depletion, high
FeO*/MgO (1.5e2.5), and strong Fe-enrichment exhibiting tholeiitic
trend (Geary and Kay, 1989). Available data indicate that the CNOC
is a fragment of an evolved oceanic crust formed at a back-arc basin
spreading center. Encarnacion (2004) concluded that the CNOC
Figure 3. Representative photos of some outcrops from the eastern Philippines ophiolites and ophiolitic units. (A) Layered peridotite from the Camarines Norte Ophiolite Complex
along the coast of Paracale; (B) harzburgites from theSamar Ophiolite in Manicani Island; (C) pillow basalts of the Samar Ophiolite in Marabut, Eastern Samar; (D) harzburgites thrust
onto massive to layered gabbros of the Malitbog Ophiolite; (E) mantle peridotites of the Dinagat Ophiolite in Dinagat Island; (F) serpentinized peridotites of the Pujada Ophiolite.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e216

may have been a back-arc basin of the Lagonoy Ophiolite. Geary and
Kay (1989) noted that there are no overlying sediments in the
volcanic rocks although a sequence of Late Cretaceous sedimentary
rocks is inferred to unconformably overlie the ophiolite unit in
Camarines Norte (Encarnacion, 2004). The minimum age for the
ophiolite emplacement was inferred from the
40
Ar/
39
Ar dating of
amphibole from the amphibolite taken from Calaguas Islands,
which yielded a Late Cretaceous age (99.9 7 Ma) (Geary, 1986).
3.3. Geology of Lagonoy Ophiolite
The Lagonoy Ophiolite rock units, which form part of the
basement of Caramoan Peninsula are exposed on the western side
of the Caramoan Peninsula in southeastern Luzon (Fig. 1B). The
ophiolite units and the sedimentary carapace are deformed and
metamorphosed, thus it was previously mapped as the “Lagonoy
Schists”(Encarnacion, 2004). The suite is composed of ultramafic
rocks, layered gabbros, and pillow lavas capped by arc volcaniclastic
sedimentary rocks (Fig. 2). The mantle section is composed of
serpentinized harzburgites that are associated with dunites and
chromitite (David et al., 1997). Geochemical analyses indicate a
supra-subduction origin and island arc affinity for the Lagonoy
Ophiolite (Geary, 1986). A hornblende gabbro sample analyzed by
David et al. (1996) shows low Ti/V, high field strength element
(HFSE) depletion and large ion lithophile element (LILE) enrich-
ment, which are characteristics typical of supra-subduction zone
ophiolites (Pearce, 1982). Pillow basalts and breccias with inter-
pillows composed of reddish calcareous sediments were also
observed. Field evidence reveals the association of tuff and grey-
wacke with the pillow lavas, which suggests island arc formational
setting. Available ages for the Lagonoy Ophiolite are from the
40
Ar/
39
Ar dating of amphibole from meta-leucodiabase and meta-
gabbro units, which yielded Jurassic ages, 156 2 Ma and
151 3 Ma, respectively (Geary, 1986).
3.4. Geology of Rapu-Rapu Ophiolite
The Rapu-Rapu Ophiolite (Fig. 1B) is composed of dismembered
but complete crust-mantle sequence observed in the Pitogo block
located at the western side of Rapu-Rapu island in southeastern
Luzon (Yumul et al., 2006)(Fig. 2). Layered ultramafic cumulate
rocks are composed of wehrlite, olivine websterite, and harzbur-
gite. The order of crystallization is olivine (spinel), clinopyroxene,
orthopyroxene, and plagioclase (Yumul et al., 2006). Spinel Cr#
values (0.06e0.56) indicate mid-oceanic ridge (MOR) derivation.
Isotropic and layered gabbros are both present; the isotropic variety
is observed to overlie a sequence of layered ultramafic cumulate
and residual peridotite. The crystallization order for the gabbro is
plagioclase followed by clinopyroxene. Dikes are composed of
andesite and diorite that are affected by low-grade metamorphism.
The pillow basalts contain manganiferous chert carapace in some
areas while metamorphosed volcaniclastic sediments, brought
about by combined ocean floor metamorphism and hydrothermal
alteration, are observed in the Malobago block (located on the
eastern side of the island containing different varieties of schists).
The basalts and gabbros are LREE-depleted and show flat MREE to
HREE patterns. However, the diorite samples show slight LREE
enrichment. REE ratios, i.e. (La/Yb)
N
and (La/Sm)
N
, also show similar
low values for the basalts (0.38e0.40 and 0.3e0.32) and gabbros
(0.31e0.48 and 0.36e0.44) whereas the diorites are characterized
by relatively high ratios (1.12e1.17 and 0.87e0.91) (Yumul et al.,
2006). These characteristics are typical of MOR setting. A meta-
morphic sole, located at the boundary of the two blocks, was
observed to be in thrust contact with harzburgite, gabbro, and
chlorite quartz schists (Yumul et al., 2006). A minimum age of Late
Cretaceous (77.1 4.6 Ma) was reported from
40
Ar/
39
Ar dating of
diorite that intruded into the harzburgite (David et al., 1996).
3.5. Geology of Samar Ophiolite
The Samar Ophiolite is distributed along the eastern and
southern parts of Samar Island (Fig. 1B). The exposure in southern
Samar, which spans w40 km in width, is composed, from east to
west, of ultramafic rocks, gabbros, sheeted dikes and pillow basalts.
The mantle section comprising of harzburgites and dunites are best
exposed in Manicani Island (Figs. 2 and 3B). Harzburgites have
porphyroclastic texture defined by large grains of olivine, ortho-
pyroxene and clinopyroxene. Dunites, which are made up of olivine
with minor orthopyroxenes and clinopyroxenes, are slightly to
intensely serpentinized (Guotana et al., 2017). The Fo content of
olivine in the harzburgites is from 90.3% to 90.9% whereas in the
dunites, the Fo content varies from 89.8% to 92.0%. The NiO content
falls within a narrow range of 0.34e0.41 wt.% in harzburgites and
0.33e0.44 wt.% in dunites. The spinel Cr# of the dunites
(0.68e0.85) is higher than that of the harzburgites (0.62e0.72)
(Guotana et al., 2018). The geochemical characteristics of the
harzburgites and dunites suggest formation in a forearc region that
exposes both MORB-like and island arc-like affinities (e.g., Stern
et al., 2012; Dilek and Furnes, 2014). Limited exposures of domi-
nantly isotropic gabbros are thrust against serpentinized dunites.
Petrographic examination of the gabbros shows euhedral clino-
pyroxenes surrounded by plagioclase laths. Massive and pillowed
lava flows of diabasic and basaltic andesite composition crop out in
the central to western portions of southern Samar Island (Fig. 3C).
The basalts are characterized by porphyritic, glomeroporphyritic
and trachytic textures. Cherts and siliceous mudstones are inter-
calated with the upright pillow basalts (Guotana et al., 2017). The
Samar Ophiolite had been previously assigned an Early to Late
Cretaceous age based on the Cretaceous radiolarian forms extracted
from the cherts intercalated with the pillow basalts (Dimalanta
et al., 2006). This age was later affirmed by the whole rock K-Ar
dating done on two basalt samples, which yielded a 100.2 2.7 Ma
to 97.9 2.8 Ma age (Balmater et al., 2015).
3.6. Geology of Tacloban Ophiolite
There are two ophiolite sequences, which have been mapped in
Leyte IslandeTacloban Ophiolite and Malitbog Ophiolite. The
Tacloban Ophiolite is exposed in the northeastern part of the island
and extends for a distance of w24 km to the southeast (Fig. 1B). It is
dominated by crustal rocks with isolated small patches of mantle
rocks (Fig. 2). The mantle section is made up of extensively ser-
pentinized harzburgite with dunite lenses. The harzburgite exhibits
a porphyroclastic texture with isolated clinopyroxene grains sur-
rounded by finer-grained olivine neoblasts. The orthopyroxene is
altered to bastite and the olivine neoblasts are altered to fibrous
serpentine (Suerte et al., 2005). The olivine Fo in the harzburgites
ranges from 90.4% to 91.2% (Tamayo, 2001; Tamayo et al., 2004).
The spinel Cr# of the harzburgites is from 0.42 to 0.52 (Tamayo,
2001; Dimalanta et al., 2006, 2009). The crustal section of the
Tacloban Ophiolite is dominantly made up of isotropic gabbros. The
gabbros are orthocumulates where the cumulus clinopyroxene is
surrounded by plagioclase crystals. Some samples display sub-
ophitic to ophitic textures where the plagioclase laths are partly
to wholly enclosed by clinopyroxenes. The sheeted diabase and
basalt dike complex outcrops as an isolated hill made up of light
gray, highly jointed parallel dikes where each dike is <1 m thick.
Similar to the gabbros, the diabase is composed of plagioclase and
clinopyroxene crystals displaying an intergranular and sub-ophitic
to ophitic texture. Both gabbros and diabases show contrasting
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 7

crystallization orders ein some samples, plagioclase crystallizes
before clinopyroxene; in other samples, the reverse is observed
(Suerte et al., 2005). The pillowed and massive lava flow deposits
outcrop at the northern and southern ends of the Babatngon Range.
Most of the pillow lavas are seen with their cusps pointing down-
wards suggesting that these are in the upright position. The basalts,
on the other hand, exhibit aphanitic and, less commonly, porphy-
ritic and glomeroporphyritic textures. Previous whole rock KeAr
dating of a basalt and two gabbro samples gave an age of
47.21 2.15 Ma, 50.00 3.9 Ma and 55.23 1.2 Ma (Sajona et al.,
1997). However, SHRIMP UePb dating zircon separates from a
gabbro sample revealed two ages e124.7 3.3 Ma and
145.1 3.2 Ma (Suerte et al., 2005). The age of the Tacloban
Ophiolite thus had to be revised from Eocene to Early Cretaceous.
3.7. Geology of Malitbog Ophiolite
Another sequence of ophiolitic units is exposed in Southern
Leyte, the majority of which are found in the easternpart of Maasin
Peninsula (Fig. 1B). The Malitbog Ophiolite is composed of mantle
peridotites, layered ultramafic cumulate sequence, layered and
isotropic gabbros, diabase dikes, massive to pillowed lava deposits
(Fig. 2). The ophiolitic units are distributed in a NeS trend along
northeast-trending thrust faults. The mantle peridotites composed
of harzburgite, lherzolite and dunite are variably serpentinized. In
one exposure, it was observed that the peridotite is thrust on top of
massive to layered gabbros (Fig. 3D) (Dimalanta et al., 2009). In
Panaon Island, the peridotite exhibits an equigranular mosaic
texture with olivine, orthopyroxene, clinopyroxene and spinel
(KOICA-KIGAM, 1993). The layered ultramafic cumulate sequence is
made up of dunite, pyroxenite and harzburgite. The observed
attitude of cumulate layering is N25

W, 10

SW. Despite being
intensely altered (>75%), layering is still preserved as seen from the
varying percentages of olivine and orthopyroxene. The layered
mafic cumulate rocks are made up of gabbro and anorthosite.
Isotropic gabbros are relatively fresh and exhibit a phaneritic
texture consisting of medium- to coarse-grained olivine, ortho-
pyroxene and plagioclase. The dike complex is made up wholly of
diabase dikes. Massive and pillowed lava flows are basaltic in
composition. Stretched ferruginous pillows with associated chert
were also encountered.
3.8. Geology of Southeast Bohol Ophiolite Complex
The dismembered w2 km thick Southeast Bohol Ophiolite
Complex (SEBOC) in central Philippines (Fig. 1B) has been referred
to previously as the Boctol Serpentinite (Arco, 1962), after the
highly brecciated and pervasively serpentinized units exposed at
Boctol Hills in Jagna, Bohol. Further field investigations (e.g., Diegor
et al., 1995; Yumul et al., 1995) led to the interpretation that the
different mafic and ultramafic rocks in southeastern Bohol are part
of an ophiolite they referred to as the SEBOC. Peña (2008) referred
to the SEBOC as the “Bohol Ophiolite”. Thin layers of clinopyrox-
enites, wehrlites and dunites, extensive harzburgites with rare
lherzolites, layered to massive gabbros and norites, diabases and
microgabbros, basalt dike complex, sheet flows and pillow lavas
make up the complete ophiolite sequence (Fig. 2)(Militante-Matias
et al., 2000; Faustino et al., 2006). Yumul et al. (2001) noted that the
harzburgite spinels have relatively low Cr# (0.25e0.66), whereas,
the dunite spinels range from aluminian (Cr# ¼0.34e0.39) to
chromian (Cr# ¼0.78e0.80). The lherzolites and wehrlites have
spinels with low Cr# values (0.16e0.24). Geochemical analyses of
diabases from the dike complex indicate IAT affinity (Faustino et al.,
2006). Whole-rock analysis of the sheet flows and pillow lavas
show transitional MORB-IAT characteristics, pointing to its supra-
subduction zone affinity (Barretto et al., 2000; Faustino et al.,
2006). The pelagic chert carapace, which is either intercalated or
capped by tuffs and tuffites, contains upper Albian radiolarian and
foraminiferal assemblages, giving an early Cretaceous age. Under-
thrust beneath the SEBOC is the Cansiwang Mélange. The mélange
contains centimeter-wide to hundreds-of-meter-wide clasts of
harzburgite, microgabbro, basalt, tuffaceous rocks and chert
embedded in a serpentinite matrix (Yumul, 2003). Dimalanta et al.
(2006) opined that the association of this mélange and SEBOC
suggested that subduction kneading was a major process during
the emplacement of these oceanic fragments. Yumul et al. (2001)
noted that the lithological characteristics and the absence of sig-
nificant listric faults in the volcanic-hypabyssal section of the
SEBOC indicate generation in a fast-spreading center. The chert-
tuffaceous rock cover plus the MORB-IAT signature of the sheet
flows and pillow lavas, together with the geochemistry of the
cumulate and residual rocks suggest formation in a small land-
bounded, subduction-related ocean basin.
3.9. Geology of Dinagat Ophiolite Complex
The Dinagat Ophiolite Complex (DOC) comprises most of the
Dinagat Island group, extending to Surigao del Sur and along the
Mainit Range in northeastern Mindanao (Fig. 1B). From bottom to
top, the ophiolite consists of harzburgites with lenses of dunites
and chromitites, alternating layers of orthopyroxenites, harzbur-
gites and dunites, massive and layered gabbros, sheeted dike
complex, pillow basalts and basalt breccias (Figs. 2 and 3E) (David,
1994a, b; Peña, 2008; MGB, 2010). Occasionally, tuffs, tuffaceous
sandstones, siltstones and shales were observed to be intercalated
with the basalt flows. Geochemical analyses of Tamayo et al.
(2004) indicate transitional island arc tholeiite (IAT) to MORB af-
finities from the harzburgite spinels with Cr# values ranging from
0.48 to 0.69 and the dunite spinel Cr# values of 0.59e0.68. The Fo
content of the olivine from the harzburgites is of 90.4%e91.5%
(Tamayo, 2001). Chromite deposits are sporadically distributed
throughout the island. They occur as pods, layers and irregular
bodies within the ultramafic units of the ophiolite (David, 1994a, b;
MGB, 2010). Santos (2014) reported that the layered chromite is
dominantly chromian, with Cr# ranging from 0.68 to 0.86.
Geochemical analyses of the volcanic units favor a supra-
subduction zone origin for this ophiolite (Yumul et al., 1997).
Potassium-argon dating (KeAr) of the harzburgite yielded an
84 Ma age (Late Cretaceous) (Sunga and Palaganas, 1986; MMAJ-
JICA, 1990). Pubellier et al. (2004) pegged the age of formation of
the ophiolite as Cretaceous to Eocene. Isotopic age dating using the
Re-Os isotope system yielded various ages. Santos (1997) obtained
130 Ma (Early Cretaceous) and 180 Ma (Early Jurassic) ages for the
harzburgite and chromitite, respectively. Another ReeOs dating
yielded a 235 Ma (Late Triassic) age, which Santos (2014) opined
makes the DOC comparable in age to most accreted terranes in the
Philippine Sea Plate. The metamorphic sole is represented by the
Nueva Estrella Schist. It is composed mainly of amphibolite schist,
garnet-amphibolite, quartzo-feldspathic schist, and biotite-quartz
schist with minor metamorphosed chert and other metasediments
(Peña, 2008; Santos, 2014). A Cretaceous age has been assigned to
this unit (Wright et al., 1958; Sunga and Palaganas, 1986; MMAJ-
JICA, 1990).
3.10. Geology of Pujada Ophiolite
The Pujada Ophiolite is located in the southeasternmost tip of
Mindanao island and serves as the basement of the Pujada Penin-
sula. It is comprised of mantle peridotites, isotropic gabbros,
sheeted dike complex and pillow basalts (Figs. 2 and 3F) (Villamor
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e218

et al., 1984; Hawkins et al., 1985; Yumul et al., 2003). The mantle
peridotites in this ophiolite have been previously classified into the
Nagas and Surop belts. However, recent work by Olfindo et al.
(2017) revealed that these mantle peridotites show a coherent
geochemical trend (spinel Cr#<0.60; olivine Fo ¼89.6%e91.4%)
suggesting that these peridotites are genetically related to each
other. Hawkins et al. (1985) further reported the presence of arc
tholeiite rocks in the Mati area while back-arc basin basalts were
recognized in the Pujada Peninsula. The Iba Formation, which is
composed of chert, limestone and red pelagic mudstones and
exposed in Mati is interpreted as the sedimentary carapace of the
Pujada Ophiolite (Yumul et al., 2003). On the basis of radiolarians in
the capping chert (Quebral et al., 1996) and due to the presence of
Globotruncana sp. in a limestone sample (de Leon in Yumul et al.,
2003), a Late Cretaceous age had been assigned to the Pujada
Ophiolite. During a recent field mapping in the Pujada Ophiolite, it
was observed that the mantle peridotites were thrust over the
sedimentary rocks of the Iba Formation. In addition, initial whole
rock geochemical data on the volcanic rocks of the Iba Formation in
Mati reveal enriched MORB (EMORB) signatures while the volcanic
rocks and diabase-gabbro intrusives within the Pujada Peninsula
show a clear MORB character (Olfindo et al., 2019). These results
cast uncertainties on the previous interpretation of the Iba For-
mation as the sedimentary carapace of the Pujada Ophiolite. The
Late Cretaceous age of the Pujada Ophiolite remains indisputable as
it is confirmed by the UePb dating of isotropic gabbros and diabasic
dikes from the peninsula, which yielded an age of w92 Ma (Olfindo
et al., 2019). Previously recognized as the northernmost extension
of the Molucca Sea Collision Complex, the Pujada Ophiolite has
been interpreted as continuous to either the Sangihe arc (Rangin
et al., 1996; Lallemand et al., 1998; Bader et al., 1999;
Widiwijayanti et al., 2003) or the Halmahera arc (Hawkins et al.,
1985; Yumul et al., 2003). Some workers also suggested that the
ophiolite may be an uplifted fragment of the Molucca Sea Plate
(Hamilton, 1979; McCaffrey, 1982) or the proto-Molucca Sea Plate
(Yumul et al., 2003).
3.11. Geology of Halmahera Ophiolite
The Halmahera Ophiolite is a NEeSW-trending basement
complex that underlies Halmahera Island in eastern Indonesia (Hall
and Nichols, 1990). The tectonically-dismembered ophiolite shows
all the elements of a complete ophiolite with the exception of a
sheeted dike complex (Ballantyne, 1992). Peridotites are predomi-
nantly harzburgites with rare occurrences of lherzolites while cu-
mulates are mostly dunites, olivine clinopyroxenites, wehrlites, and
olivine gabbro-norites. Volcanics in the ophiolite complex include
boninitic rocks interpreted as cogenetic with the cumulates, arc
tholeiites, and amygdaloidal calc-alkaline basalts that are chemi-
cally similar to ocean island and seamount volcanic rocks
(Ballantyne, 1991). Arc-related plutonic rocks intrude the ophiolite
and are reported to be co-magmatic with the Halmahera Ophiolite
volcanic rocks (Ballantyne, 1992). The chemistry of spinels in the
peridotites reveals a multi-stage melting history for the Halmahera
Ophiolite. The Cr-rich harzburgite spinels generally cluster outside
the abyssal peridotite field, indicating that their host rocks have
undergone a higher degree of partial melting. The Al-rich lherzolitic
spinels, meanwhile, plot within the abyssal peridotite field and
suggest a mid-oceanic spreading ridge origin for their host rocks.
Furthermore, depletion in TiO
2
,Al
2
O
3
, and CaO while enrichment in
Cr and Ni of the Halmahera lherzolites do not represent a pristine
upper mantle origin. Petrological, geochemical, and geochrono-
logical data from the Halmahera Ophiolite indicate formation in a
supra-subduction zone setting before the Late Cretaceous along the
western edge of the Philippine Sea Plate (Harris, 2003; Pubellier
et al., 2004). Rock units of the Halmahera Ophiolite are further
reported to be similar to rocks dredged from the Mariana Trench,
which is situated at the eastern margin of the Philippine Sea Plate
(Bloomer, 1983; Ballantyne, 1992; Milsom et al., 1996).
4. Geochemistry
4.1. Mineral chemistry data
Spinel Cr# is commonly considered a good parameter in
assessing the degree of partial melting experienced by mantle pe-
ridotites. Low spinel Cr# in mantle peridotites (<0.60) is commonly
associated with low to moderate degrees of partial melting at the
abyssal setting whereas high Cr# (>0.60) is often observed in
mantle peridotites, which underwent higher degrees of partial
melting in supra-subduction zones (Dick and Bullen, 1984; Pearce
et al., 1984; Arai, 1994; Gaetani and Grove, 1998; Zhou et al.,
2014). The mantle peridotites of the ophiolites in the eastern
margin of the Philippines show a wide range of spinel Cr# values
(Fig. 4). Lherzolites, harzburgites and dunites with spinel Cr#
values less than 0.60 occur in the Isabela, Camarines Norte, Rapu-
Rapu, Tacloban, Malitbog, Panaon, SE Bohol, Dinagat and Pujada
ophiolites. Depleted harzburgites, dunites and chromitites with
generally high spinel Cr# values (>0.60) comprise the mantle
sections of the Camarines Norte, Samar, SE Bohol and Dinagat
ophiolites. Olivine Fo content plotted against spinel Cr# values
(Fig. 5) shows that these peridotites fall with the olivine spinel
mantle array (OSMA; Arai, 1994) indicating mantle origin. The TiO
2
vs. Al
2
O
3
of the spinels in the mantle peridotites (Fig. 6) further
suggests derivation from both MOR and SSZ settings. The variable
amounts of Cr and Al in the examined spinels are clearly shown in
Fig. 7.
The mantle sections of the different ophiolite and ophiolitic
complexes in eastern Philippines have been subjected to variable
degrees of partial melting as indicated by spinel geochemistry. Very
fertile peridotites with Cr# (<0.20) occur in the Isabela, Rapu-Rapu,
SE Bohol and Tacloban (Figs. 4 and 5). The different localities also
reflect both MOR and SSZ signatures (Tamayo et al., 2004; Yumul,
2007) except for the Samar Ophiolite, which shows a clear SSZ af-
finity (Guotana et al., 2017, 2018).
4.2. Whole rock geochemical data
The geochemical compositions of the volcanic or crustal sec-
tions of ophiolites can also reveal clues on their tectonic origin and
geodynamic setting (Shervais, 1982; Dilek and Furnes, 2011, 2014;
Saccani et al., 2017). Studies on the basaltic and gabbroic rocks
allowed the characterization of several ophiolite complexes along
the eastern portion of the Philippines. The volcanic and crustal
sections of the eastern Philippine ophiolites have wide variations of
FeO*/MgO compared with the different major oxides (Fig. 8). To
compare the fractionation trends of supra-subduction zone type
and mid-oceanic ridge ophiolite-related volcanic rocks, fields from
the major element data of the Troodos Ophiolite Complex (Pearce
and Robinson, 2010) and the global database of mid-oceanic ridge
basalts by Arevalo and McDonough (2008) are also plotted.
In the SiO
2
and alkali diagram by Irvine and Baragar (1971), most
of the crustal section rocks of the different ophiolites plot in the
subalkaline field, except for several pillow basalts from SE Bohol
and Samar (Fig. 9A). In the (Na
2
OþK
2
O)eFeO*eMgO ternary
(AFM) diagram, crustal rocks from Rapu-Rapu, SE Bohol and Samar
plot in the calc-alkaline field whereas those from Camarines Norte
and Tacloban plot just above the tholeiitic field (Fig. 9B).
In the V vs. Ti/1000 diagram (Fig. 10A), most eastern seaboard
ophiolites in the Philippines plot in the island arc tholeiite (IAT)
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 9

field, with Rapu-Rapu and Tacloban plotting in the MORB/BABB
fields and SE Bohol samples plotting in the alkaline field. Similarly,
in the Cr vs. Y diagram (Fig. 10B), most samples plot in the volcanic
arc basalt (VAB) field, except for the Tacloban, Rapu-Rapu and SE
Bohol samples. The Nb/Yb vs. Th/Yb diagram (Johnson and Plank,
1999) was used to distinguish subduction-related rocks from
those with subduction-unrelated MORB-OIB affinities (Fig. 10C).
The volcanic rocks and gabbros from Samar, Tacloban and Dinagat
reveal subduction affinities, whereas those from Isabela, Rapu-
Rapu and Camarines Norte plot near the N-MORB. The Th/Yb
values of crustal rocks from SE Bohol vary widely.
5. Paleomagnetic data in the Philippines and surrounding
regions
To date, very few studies have been conducted on pre-Cenozoic
rocks of the Philippines that are interpreted to represent fragments
of the proto-Philippine Sea Plate (pPSP). These include the work of
Queaño et al. (2009) who reported paleomagnetic data
(Dec ¼159.3

, Inc ¼e12.5

;
a
95
¼6.0

,k¼162.5) from the Eocene
to Cretaceous Chico River pillow basalts in the Luzon Central
Cordillera, suggesting formation at subequatorial region (6.3


3.1

)N. Interestingly, Ali et al. (2001) obtained paleomagnetic data
(Dec ¼219.4

, Inc ¼12.1

, angular separation of 20.1

) from a
Mesozoic ophiolite on Obi Island in eastern Indonesia. Their results
suggest formation at sub-equatorial latitudes. In a more recent
work by Balmater et al. (2015) from the pPSP, the Late Cretaceous
Samar Ophiolite in Central Philippines reported characteristic
remanent magnetization tilt-corrected direction of Dec ¼342

,
Inc ¼e27

,k¼15,
a
95
¼11

. These values suggest that the
ophiolitic basement rocks of Samar formed in the Late Cretaceous
at a paleolatitude of (14

6

)S. By considering previous paleo-
magnetic, geochemical and age-dating results from other pPSP
ophiolites (including the Halmahera Ophiolite) from the region,
Balmater et al. (2015) suggested that the ophiolites originated from
a Mesozoic supra-subduction zone setting that spanned a few de-
grees north of the equator to around 15

S.
Previous paleomagnetic data (e.g., McCabe et al., 1987; Almasco
et al., 2000) from the Mesozoic rocks of the Philippines, predating
the work of Queaño et al. (2009) and Balmater et al. (2015) ,were
Figure 5. Olivine forsterite content vs. spinel Cr# (Arai, 1994) in peridotites from: (A) Isabela, Camarines Norte, Lagonoy, Rapu-Rapu; (B) Tacloban, Panaon, Malitbog, Samar,
Southeast Bohol (SEBOC) and (C) Dinagat and Pujada.
Figure 4. Spinel Cr# vs. Mg# for peridotites from the mantle sequences of the ophiolites from: (A) Isabela, Camarines Norte, Lagonoy, Rapu-Rapu; (B) Tacloban, Panaon, Malitbog,
Samar, Southeast Bohol (SEBOC) and (C) Dinagat and Pujada. Diagrams and fields were modified from Dick and Bullen (1984) and Van der Laan et al. (1992).
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e2110

mainly from the terranes of Palawan, Mindoro and Panay. However,
these terranes are mostly of continental affinity, having been
derived from southern mainland Asia (Taylor and Hayes, 1980,
1983; Yumul et al., 2003; Padrones et al., 2017; Dimalanta et al.,
2018).
Fuller et al. (1983, 1989) and McCabe et al. (1987) provided
extensive paleomagnetic data collection for Luzon and adjacent
regions, similarly concentrating on Cenozoic rocks. Based on the
paleomagnetic results, Fuller et al. (1983) proposed a model that
shows the counterclockwise rotation of the Early and Middle
Miocene sites in northern Philippines as resulting from the north-
westward motion of the Philippine Sea Plate and the pinning of
northern Philippines at its southern end by the Palawan continental
fragment. Plio-Pleistocene paleomagnetic data set from Mindoro,
Panay, Negros and Mindanao reflect an almost zero declination
offset (e.g. McCabe et al., 1982; Fuller et al., 1983; McCabe, 1984).
McCabe et al. (1987) interpreted this result as suggesting that: (1)
the Philippine arc behaved as a single tectonic unit during the past
5 Myr or; (2) deformation occurred at levels not detectable given
the resolution of the paleomagnetic data. They reported a mean
inclination value (16.2

) for Plio-Pleistocene rocks that is appre-
ciably shallower than the expected for the axial dipole inclination
(30

) for this latitude, similar to that obtained by Fuller et al. (1983).
McCabe et al. (1987) reported results from five sites on the clastic
rocks of the Jurassic Mansalay Formation (part of the North Pala-
wan Block) in Mindoro. Their results (mean Dec ¼75.7

;
Inc ¼27.7

;
a
95
¼19.4

) support earlier suggestions (e.g., Hamilton,
1979; Taylor and Hayes, 1980) that the North Palawan Block was a
rifted continental fragment derived from southern China. By
comparing inclination data from the Cenozoic rocks of the Philip-
pine arc and those from the Philippine Sea Plate reported by pre-
vious workers (e.g., Louden, 1977), McCabe et al. (1987) concluded
that the Philippine arc shared a common Cenozoic northward drift
with the Philippine Sea Plate. They proposed a pre-late Miocene
collision of the west Philippine arc with the North Palawan Block to
account for the differential rotation of the region (i.e., counter-
clockwise in Marinduque and clockwise in Panay, Cebu and
Mindanao) as supported by the data set from the Lower to Middle
Miocene rocks.
Clearly, the quantitative information provided by the paleo-
magnetic data from the Philippines is crucial for delineating ter-
ranes of pPSP and NPB origin. Inclination data from the pPSP imply
formation at relatively lower latitudes when compared to the more
northerly latitude of formation for the NPB. With a likely pPSP
origin also for the eastern seaboard Mesozoic ophiolites of the
Philippines, all having subduction-related affinities and similar
ages, such low latitudinal formation for these ophiolites is also
suggested.
6. Discussion
6.1. The proto-Philippine Sea Plate
Quite a number of literature on Mesozoic ophiolites in the
Western Pacific region look to a pPSP source for the emplaced
crust-mantle sequences. Samples from some parts of the present-
day PSP, which include the ADO region and Huatung Basin have
been collected by diving, dredging, or coring, and yielded Mesozoic
ages and can be found in Lallemand (2016). These materials have
become the bases for the idea that an older ocean basin compared
to that of the present-day PSP is needed to account for the tectonic
reconstruction of this region. The earliest suggestions of a pPSP
were made after ages derived from studies of the aseismic ridges of
the WPB revealed older materials than were expected for the PSP
Figure 6. Spinel Al
2
O
3
vs. TiO
2
(modified from Kamenetsky et al., 2001) for peridotites from: (A) Isabela, Camarines Norte, Lagonoy, Rapu-Rapu; (B) Tacloban, Panaon, Malitbog,
Samar, Southeast Bohol (SEBOC) and (C) Dinagat and Pujada.
Figure 7. Ternary diagrams showing CreAleFe
3þ
content in spinels of the peridotites from: (A) Isabela, Camarines Norte, Lagonoy, Rapu-Rapu; (B) Tacloban, Panaon, Malitbog,
Samar, Southest Bohol (SEBOC) and (C) Dinagat and Pujada. Fields of arc-related and ocean floor peridotites from Dick and Bullen (1984). See text for details.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 11

Figure 8. Major elements (in wt.%) plotted against FeO*/MgO as the fractionating index. Field for MORB from Arevalo and McDonough (2008) and field for the Troodos Ophiolite
Complex from Pearce and Robinson (2010). Symbols same as in Fig. 9.
Figure 9. Discrimination diagrams from Irvine and Baragar (1971) for SiO
2
vs. Na
2
OþK
2
O and (Na
2
OþK
2
O)‒FeO
*
‒MgO.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e2112

(e.g., Ingle et al., 1975; Matsuda et al., 1975; Hussong and Uyeda,
1981). In the northern WPB, tonalites and basalts dredged from
the Amami Plateu were first reported to be Late Cretaceous based
on whole-rock KeAr dating (Matsuda et al., 1975). Subsequent
studies on the same materials indicated older ages (w115 e118 Ma)
likely due to Ar-loss but still well within the Cretaceous period
(Hickey-Vargas, 2005). More recent work on andesites dredged
from the Daito Ridge returned 116e119 Ma ages (Ishizuka et al.,
2011). Slightly older ages were reported from UePb dating of zir-
cons of plutonic rocks (Tani et al., 2012) that were collected from a
2010 diving cruise. At the westernmost edge of the PSP, sandwiched
between Taiwan and the Gagua Ridge, is the Huatung Basin.
Different workers have regarded the origin of this basin differently.
Some models consider the basin to have been part of the pPSP (e.g.,
Deng et al., 2015). On the other hand, Lallemand (2016) considered
the Huatung Basin as likely derived from a Mesozoic oceanic crust
distinct from the pPSP, possibly the New Guinea ophiolite back-arc
basin (Hall, 2002; Pubellier et al., 2003). However, the same work
by Lallemand (2016) illustrates the Huatung Basin to be contiguous
with the pPSP at 60 Ma, separated only by a transform boundary
that was later converted to a subduction zone. The subduction
eventually failed and resulted in the formation of the Gagua Ridge
at about 50 Ma. Hence, in the interest of accounting for more
possible components of the pPSP in the region, the Huatung Basin is
herein considered as also a likely remnant of the pPSP. Dredged
gabbros from this area similarly yielded Early Cretaceous
40
Ar/
39
Ar
ages of w131e115 M a ( Deschamps et al., 2000). Chert float samples
believed to have come from the Huatung Basin that were collected
on Lanyu Island also provided w113e117 Ma radiolarian ages
(Deschamps et al., 2000). These, however, are not the only ages
derived from this area, hence, the controversy to the basin’s true
nature. Northern Huatung Basin has yielded samples with ages
ranging from 52 Ma to 43 Ma while the southern section is dated at
Early Cretaceous (Sibuet et al., 2002). Other studies have also
published younger ages for dredged materials from within the
basin (e.g., 40e33 Ma, Hilde and Lee (1984);42e33 Ma, Doo et al.
(2015);30e15 Ma, Kuo et al. (2009)). A more extensive consider-
ation of these issues is discussed in Eakin et al. (2015). Nonetheless,
the recognition of Cretaceous materials from the basin cannot be
ignored as it correlates very closely to the ages derived from the
ADO region.
6.1.1. Geochemistry of the pPSP
By inference, if the Mesozoic sections of the PSP are to be the
basis for defining what the pPSP was, then the range of geochemical
compositions of the pPSP must closely mimic those of the ADO
ridges and the Huatung Basin. Figs. 10 and 11 compare the whole
rock major and trace element signatures of ophiolites from the
Figure 10. Tectonic discrimination diagrams for the eastern Philippine ophiolites: (A) Ti/1000 vs. V (Shervais, 1982) and (B) Y vs. Cr plot (Pearce, 1982). Samples from Amami
Plateau, Huatung basin and East Halmahera were plotted for comparison. Data are from Ballantyne (1992), Hickey-Vargas (2005) and Ishizuka et al. (2011). (C) The Nb/Yb vs. Th/Yb
(Saccani, 2015) and (D) Nb/Yb vs. Ba/Yb (Pearce, 2008) diagrams show mid-oceanic ridge signatures with significant subduction contributions of the crustal sections of the eastern
Philippine and East Halmahera ophiolites.
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 13

eastern Philippines, and the dredged samples from the ADO region
and the Huatung basin. The island arc characteristics of the Amami
Plateau basalts and tonalites have been established earlier on by
Shiki (1985) and concurred later by Hickey-Vargas (2005). High-K
and high-Al typify the basalts with pronounced enrichment of
LILE, Th and U, and depletion of Nb and Ta compared with REE. The
tonalites share similar signatures but with higher contents of
incompatible trace element, particularly La, Nb, and Th. In contrast,
the work by Hickey-Vargas et al. (2008) demonstrated that diabases
and gabbros from the Huatung Basin are most similar in compo-
sition to low to medium-K tholeiitic ocean ridge and intraplate
basalts. Compared to typical island arc materials, they are
characterized by lower Al
2
O
3
and Ba/Yb (Figs. 8 and 10d). They
display relatively enriched Nb and Ta compared with La on primi-
tive mantle normalized (Sun and McDonough, 1989) incompatible
element diagrams and ratios (Fig. 11A) with La/Nb ¼0.51 and 0.43
and Th/Nb ¼0.056 and 0.050. Their REE ratios (e.g., Nb/U of 56e59
and Ce/Pb of 28e34) are within the ranges of modern oceanic ridge
and intraplate basalts. Although some variations are displayed by
the diabases, such as a flat HREE pattern and negative Eu anomaly
for one and a sloping trend and no Eu anomaly for the other, what is
common among them is the strong LREE enrichment resulting in a
humped pattern similar to enriched-MORB (Fig. 11A). These sig-
natures, coupled with the low Ba/Yb, low La/Nb and high Ce/Pb
Figure 11. Plots showing primitive mantle normalized trace element of igneous rocks from (A) Huatung Basin and Amami Plateau (data from Hickey-Vargas, 2005 and Hickey-
Vargas et al., 2008), (B) Samar, Dinagat and Pujada, (C) Rapu-Rapu, Isabela and Tacloban and (D) SE Bohol and East Halmahera. Normalizing values are from Sun and
McDonough (1989).
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e2114

ratios distinguish Huatung Basin rocks from the products of island
arc magmatism. Based on Sr, Pb, Nd and Hf isotopes, the Amami
Plateau rocks have been shown to be of island arc origin associated
with a subducted Pacific-type MORB slab (Hickey-Vargas et al.,
2008). Specifically, the pre-subduction mantle of the plateau is of
Indian Hf and Nd isotopic signature, but with a clear PacificPb
subduction component. The same work showed that Huatung basin
gabbros exhibit a close affinity to Indian MORB based on their Hf-
Nd isotopic signatures. However, their Pb isotope signatures are
intermediate between Pacific MORBs and Indian Pb isotope
signature.
6.1.2. Location of the pPSP
Different palinspatic reconstructions place the Philippine ar-
chipelago quite differently during the Cretaceous period, and
therefore, also place the pPSP in relatively different paleopositions
(e.g., Queaño et al., 2007; Zahirovic et al., 2014; Wu et al., 2016).
Some works have suggested that during the Late Cretaceous, the
Philippines was either in the southern Pacific or at the boundary
between the Pacific and the North New Guinea plates (Maruyama
et al., 1989). Therefore, the pPSP was likely positioned further
south. From models that suggest the pPSP to have developed from
the opening of the New Guinea Basin during the Middle Jurassic-
Early Cretaceous (e.g., Monnier et al., 1995; Pubellier et al., 2003),
the pPSP would have been located along the northern margin of
Australia. These differences in paleo-latitudes have been taken to
indicate that the Huatung and the New Guinea Basins could not
have been part of the same basin (Deschamps and Lallemand,
2002). However, it may also be likely that the pPSP is similar to
present-day marginal basins that are elongate and cut across wide
latitudinal domains, such as the current back-arc basins of the PSP
(e.g., Sasaki et al., 2014; Deng et al., 2015; Hall, 2018). Thus,
combining these contrasting opinions, it can be surmised that the
Huatung and New Guinea basins represented different sections of
the single ocean basin of the pPSP that straddled the south and the
north hemispheres but was likely confined to low latitudes
(Deschamps and Lallemand, 2002). Additional support to this
paleoposition is derived from onland exposures of ophiolitic ma-
terials, including those in the Philippines, as will be discussed in
section 6.2. From studies of different sections of the PSP, it can be
deduced that the pPSP was a Cretaceous oceanic crust that is very
much like present-day marginal basins made up of both island arc
subduction-related (exemplified by the ADO region) and enriched
MORB sections (e.g., Huatung Basin). It was formed near the
equator and spanned across both the north and south hemispheres
but was limited in extent to low latitude regions.
6.2. Eastern seaboard of the Philippines and Halmahera island were
part of the proto-Philippine Sea Plate
There are many similarities in the modes of possible emplace-
ment, field characteristics, ages, and geochemical characteristics of
ophiolites that are preserved in the central and eastern islands of
the Philippines and the southern Halmahera Island in Indonesia. To
help establish the extent of occurrence of preserved pieces of the
pPSP, Table 1 summarizes the characteristics of Mesozoic ophiolites
in the central and eastern Philippines, the Halmahera Ophiolite in
Indonesia, and the ADO ridges. Excluding the ADO ridges, all the
ophiolitic bodies presented here lie along the western boundary of
the present PSP. However, their origins cannot be directly ascribed
to the latter or any other nearby present-day oceanic basins pri-
marily because of age and paleomagnetic considerations. This has
been strongly supported by previous work (e.g. Tamayo et al., 1996;
Hall, 2002; Faustino et al., 2003, 2006; Hickey-Vargas et al., 2008;
Balmater et al., 2015; Guotana et al., 2018) and therefore leaves
little doubt as to the null relationship between these ophiolites and
the Eocene PSP as their source lithospheric fragment.
Previous works have established that the basement of the
Philippine region is an amalgamation of various terranes that
include ophiolites of mostly supra-subduction origin (e.g., Manalo
et al., 2015; Queaño et al., 2017; Perez et al., 2018). Examination
of the crustal sections of these ophiolites, especially those along the
eastern seaboard of the Philippines, and the Halmahera Ophiolite
reveals a wide variation in terms of their geochemical composi-
tions. In terms of their major element compositions (Fig. 8), the
Isabela, Camarines Norte, Rapu-Rapu and Tacloban ophiolites plot
closer to MORB trends, which deviates from those of the Samar and
Tacloban ophiolites that are more similar to trends from the Troo-
dos Ophiolite. Trace element signatures similarly show the same
clustering of ophiolitic bodies (e.g., Fig. 10A and B), with the
Camarines Norte Ophiolite Complex plotting clearly in the WPB
field, the Tacloban Ophiolite in the transitional MORB-WPB, the
Halmahera and SEBOC plotting across the VAB to the MORB, and
the rest in the VAB or transitional fields. Fig. 10c (Nb/Yb vs. Th/Yb)
and 10d (Nb/Yb vs. Ba/Yb) illustrate that although most of these
ophiolites have source regions with closer compositional affinity to
MORB sources, their plot scatter indicates moderate to significant
contributions from subduction inputs. The same is indicated in
normalized REE plots (Fig. 11), where SSZ indicators such as nega-
tive anomalies at Ti and Nb are evident in most sample populations,
regardless of whether their LILE are elevated or not, or if their HREE
are flat or sloping. Hence, in general, all ophiolites along the eastern
seaboard of the Philippines are SSZ ophiolites that more specifically
fall into three geochemical groups: (1) those with stronger MORB-
BABB affinities that include Isabela, Camarines Norte, Rapu-Rapu,
Tacloban and Pujada ophiolites, (2) those with more pronounced
IAT signatures that include Samar and Dinagat ophiolites, and (3)
the SEBOC and Halmahera ophiolites that reflect both MORB/BABB
and IAT characteristics.
In comparison to the WPB therefore, where the Early Cretaceous
ADO region preserves features more akin to volcanic arcs and the
Huatung Basin comprising MORB/BABB rocks, the ophiolites of the
Eastern Philippines also preserve such variable compositions. This
indicates that although they may have belonged to a single oceanic
crust, these ophiolites represented different regions of the pPSP
basin. Although this needs further analysis, it also appears that the
northerly ophiolites, along with the Tacloban ophiolite, were
formed in more mature spreading regions compared to other
ophiolites that have stronger SSZ signatures. This is most analogous
to how present-day marginal basins are configured, with the pe-
ripheral regions receiving more subduction inputs compared to the
central basins that have well-developed spreading centers.
Alternatively, the pPSP may have been part of a larger Mesozoic
ocean basin associated with the Mesotethys and the Paleo-Pacific
that subducted toward the Eurasian-Sundaland continent along
the great Circum-Southeast Asia subdution-accretion zone that
formed in the Middle or Late Cretaceous (Zhou et al., 2008).
Although numerous models have been proposed for the recon-
struction of the Southeast Asian region (e.g., Metcalfe, 2006, 2011;
Hall, 2009, 2012; Queaño et al., 2009; Zahirovic et al., 2014), the
paleo-position of the Philippine archipelago, and therefore of the
pPSP as well, need to be better constrained. The work by Balmater
et al. (2015), which places the Samar Ophiolite at 14

S6

S, pro-
vides the only paleomagnetic information for these eastern Phil-
ippine ophiolites.
6.3. Mineralization potential
Mineralization associated with ophiolitic suites in the
Philippines mainly occurs in the form of nickel laterites, bauxites,
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 15

Table 1
Summary of the chracteristics of the eastern Philippine ophiolites. Data from Halmahera Ophiolite and Amami-Oki-Daito are included. The different ophiolite units are arranged from north to south based on their present-day
location.
Location Spinel Cr# Ol Fo (%) Volcanic rocks Tectonic setting Crystallization order Age of volcanic
rocks
Age of intrusives Age of sedimentary
carapace
References
Amami-Oki-
Daito
IA 85.1‒82.4 Ma;
whole rock
40
K/
40
Ar
dating; basalts
71.5‒69.5 Ma; whole rock
40
K/
40
Ar dating; tonalites
Matsuda et al.
(1975); Hickey-
Vargas (2005);
Ishizuka et al.
(2011)
116.9‒
118.9 Ma;
40
Ar/
39
Ar
dating; basalts
117‒115.8 Ma;
40
Ar/
39
Ar dating
of hornblende; tonalites
Isabela Lz ¼0.08‒0.95
Hz ¼0.36‒0.40
Du ¼0.49‒0.77
Lz ¼89.5‒90.0
Du ¼89.9‒91.9
Hz ¼89.4‒91.4
BAB/N-MORB MOR/BAB/SSZ 87.2 Ma; whole
rock
40
K/
40
Ar
dating; basalt
92 Ma;
40
K/
40
Ar dating of
amphibole; amphibolite
Early Cretaceous;
radiolarians from
chert
Billedo et al.
(1996); Tamayo
(2001); Tamayo
et al. (2001, 2004);
Andal et al. (2005)
Camarines
Norte
Hz ¼0.35‒0.44 Hz ¼89.3‒91.1 BABB MOR/BAB gabbros: ol /pl /
cpx opx; mafic rocks:
ol sp /pl, cpx opx
100 Ma;
40
Ar/
39
Ar dating of
amphibolite associated with
gabbro
Geary et al. (1988);
Tamayo et al. (1996,
2004); Tamayo
(2001)
Lagonoy IA MOR 156‒151 Ma;
40
Ar/
39
Ar dating of
amphibole; meta-leucodiabase
and meta-gabbro
Geary (1986);
David et al. (1997);
Tamayo et al.
(1998)112.7 Ma;
40
K/
40
Ar dating;
gabbro
Rapu-Rapu Hz ¼0.06‒0.52
Dun ¼0.41‒0.42
Wer ¼0.55
Ol Wb ¼0.56
Hz ¼89.5‒91
Wer ¼89.2
Ol Wb ¼90.5
BABB MOR/BAB layered ultramafic
cumulate sequence:
ol sp /cpx /opx
/pl; gabbros: pl/
cpx
77.1 Ma;
40
K/
40
Ar dating; diorite David et al. (1996);
Yumul et al. (2006)
Tacloban Hz ¼0.42‒0.52 Hz ¼90.4‒91.2 BABB to IA tholeiite SSZ/MOR gabbros and diabases:
pl /cpx and cpx /pl
145.1‒124.7 Ma; UePb dating
of zircon; gabbro
Late Miocene to
Pliocene ages;
foraminifera and
calcareous
nannofossil
assemblages from
shale and
mudstone
Tamayo (2001);
Tamayo et al.
(2004); Suerte et al.
(2005); Dimalanta
et al. (2006)
Samar Hz ¼0.62‒0.72
Du ¼0.68‒0.85
Lz ¼0.59‒0.82
Hz ¼90.3‒90.9
Du ¼89.8‒92.0
Lz ¼89.5‒90.0
SSZ/Forearc (IAT,
transitional IAT-
MORB, CA)
SSZ volcanic rocks: ol /
cpx /pl /opaque
min.; gabbros: cpx /
pl
100.2‒97.9 Ma;
whole rock
40
K/
40
Ar
dating; basalts
Late Cretaceous;
radiolarians from
chert
Tamayo (2001);
Tamayo et al.
(2004); Dimalanta
et al. (2006);Yumul
(2007);Balmater
et al. (2015);
Guotana et al.
(2017, 2018)
Malitbog Lz ¼0.11‒0.40
Hz ¼0.12‒0.50
Lz ¼89.5‒90.7
Hz ¼90.4‒91.3
IA tholeiite SSZ Late Cretaceous;
foraminiferal
assemblage from
limestones
Florendo (1987);
Quebral (1994);
Tamayo (2001)
Late Cretaceous;
radiolarians from
chert
Southeast
Bohol
Hz ¼0.16‒0.70
Lz ¼0.11‒0.24
Wer ¼0.16
Du ¼0.37‒0.79
90.1‒91.4 BABB to IA tholeiite
Boninite present
(MOR and
transitional IAT-
MOR)
SSZ gabbros: pl/cpx and
pl/cpx /hbl;
norites: cpx /opx/
pl
Early Cretaceous;
radiolarians from
chert
Yumul et al. (2001);
Faustino et al.
(2003); Tamayo
et al. (2004);
Dimalanta et al.
(2006)
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e2116

volcanogenic massive sulfide (VMS) deposits, and podiform chro-
mitites with associated platinum group metals (PGM). Nickel
laterite deposits are tropical weathering products of ultramafic
rocks associated with ophiolite complexes with primary Ni con-
tents of 0.2%e0.4% (Brand et al., 1998; Golightly, 2010). The
weathering of ophiolite complexes in the Philippines resulted in
extensive nickel laterite deposits (MGB, 2004a). The Philippines
was the top producer of mined nickel in 2016 with 347,000 tons,
but slid in second place beneath Indonesia in 2017 with decreased
production of 230,000 tons (USGS, 2018). The nickel laterite from
the Dinagat Ophiolite Complex in Nonoc and Dinagat islands and in
Surigao del Norte and Surigao del Sur were the first nickel deposits
mined from the 1940s and still have extensive deposits being
mined until today (MGB, 2004a). Other ophiolite complexes in the
eastern margin of the Philippines with known nickel ore reserves
include those from Pujada, Samar, Malitbog and Camarines Norte.
Despite the abundance of nickel laterite resources, there are very
limited published studies on this deposit type in the Philippines.
Aside from nickel, these laterite deposits are also important sources
of Fe, Co and Sc as by-products (Elias, 2002; Golightly, 2010; Butt
and Cluzel, 2013).
Bauxite deposits are only found in the west-central portion of
Samar Island. The deposits are reddish to yellowish brown, loosely
compacted soil occurring as pockets in sinkholes or as blanket
deposits (Pacis, 1981). These deposits are related to the weathering
of the mantle and crustal sections of the Samar Ophiolite (Garcia
and Mercado, 1981).
Besshi-type, metamorphic rock-hosted volcanogenic massive
sulfide deposits have been mined in Rapu-Rapu (Sherlock et al.,
2003). Kuroko-type deposits, on the other hand, have been re-
ported and developed in Samar (Balce and Esguerra, 1974; Muyco,
1979). Both mineralized areas are believed to be related to the
upper part of their respective ophiolite complexes.
Podiform chromitite deposits are primarily associated with
dunites and harzburgites in ophiolite complexes (Dickey, 1975;
Gonzalez-Jimenez et al., 2014). Most of the studies on podiform
chromitites in the Philippines are focused on Zambales (e.g. Stoll,
1958; Hock et al., 1986; Bacuta et al., 1990; Yumul, 1992; Payot
et al., 2013), as it is the primary producer of chromitite in the
country. Aside from the Zambales Ophiolite Complex, another
historical producer of chromitite is the Dinagat Ophiolite Complex,
which started production in 1938 (MGB, 2004b). The chromitites in
Dinagat occur as pods or irregular bodies with massive, layered,
disseminated or nodular textures (David, 1994a, b). Mossbauer
spectroscopy of chromite samples from the Dinagat Ophiolite
Complex revealed high oxygen fugacities for its formation (Kuno
et al., 2000). Other chromitite sources of economic importance
include the Pujada Ophiolite, Lagonoy Ophiolite and the Samar and
Malitbog Ophiolites (MGB, 2004b). Studies on podiform chromitite
deposits along the eastern portion of the Philippines remain scarce,
and further exploration may reveal more chromitite deposits
associated with the dunite and harzburgite sections in these
ophiolite complexes. In particular, supra-subduction zone ophio-
lites are the main host for chromite deposits (Yumul and Balce,
1994; Arai, 1997; Xiong et al., 2017). A by-product of podiform
chromitite deposits are platinum group elements in minor
amounts, although their potential have yet to be explored espe-
cially in the eastern ophiolite complexes of the Philippines.
7. Conclusions
The geology and geochemistry of the different lithospheric frag-
ments along the eastern seaboard of the Philippines reveal several
things. These are: (1) Almost all are of Mesozoic age, within the
CretaceouseJurassic range; (2) the geochemistry of the ultramaficto
Dinagat Hz ¼0.48‒0.69
Du ¼0.59‒0.68
Hz ¼90.4‒91.5 BABB to IA tholeiite
(transitional IAT-
MORB)
SSZ gabbros: opx /cpx/
pl
MMAJ-JICA(1986);
Santos (1997);
Tamayo (2001);
Tamayo et al.
(2004); Yumul
(2007)
Pujada Hz, Lz, Du ¼0.14‒
0.55
Hz, Lz, Du ¼89.6‒
91.4
BABB to IA tholeiite MORB 92 Ma; UePb dating of zircon;
gabbro and diabase
Late Cretaceous;
foraminiferal
assemblage from
pelagic mudstones
and limestones
Hawkins et al.
(1985); Quebral
(1994); Quebral
et al.(1996); Yumul
et al. (2003);
Olfindo et al. (2019)
Halmahera
Ophiolite
Lz ¼14.60‒24.30
Hz ¼45.40‒75.80
Ol-rich
cumulates ¼18.70‒
57.40
Lz ¼90.1‒91
Hz ¼90‒92.1
Ol-rich
cumulates ¼85.3‒
89.8
SSZ Ballantyne (1991,
1992)
C.B. Dimalanta et al. / Geoscience Frontiers 11 (2020) 3e21 17

mafic rocks reveals multiple stages of magmatism and degrees of
partial melting; (3) although almost all have been formed in mar-
ginal basins, some would have higher subduction imprints similar to
other reported supra-subduction ophiolites elsewhere; (4) some
ophiolites are characterized by a lherzolite mantle sequence indi-
cating low degrees of partial melting and the involvement of slow-
spreading ridges; (5) available paleomagnetic data suggest that the
eastern seaboard ophiolites of the Philippines formed at low lati-
tudes, more specifically a few degrees north of the equator to around
15

S; and (6) major mineralizations associated with these ophiolites
range from nickel laterite and volcanogenic massive sulfide deposits
to chromitite deposits. Considering that most of these complexes are
believed to have been generated from the pPSP, these data and in-
formation give us a good idea about how the pPSP looked like and
how it could have evolved through space and time.
Acknowledgements
Funding support from the Department of Science and Technol-
ogy, University of the Philippines-Diliman, National Institute of
Geological Sciences and National Research Council of the
Philippines are acknowledged. Most of the analyses were done
through our international collaborations with different academic
and research institutions. We also thank the two anonymous re-
viewers for their constructive comments. We acknowledge with
gratitude the Guest Editors of this Special Issue for inviting us to
publish.
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