Petrogenesis of ultramafic-mafic clasts in the Dos Hermanos Mélange, Ilocos Norte: Insights to the evolution of western Luzon, Philippines

Journal of Asian Earth Sciences 184 (2019) 104004

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Petrogenesis of ultramafic-mafic clasts in the Dos Hermanos Mélange, Ilocos Norte: Insights to the evolution of western Luzon, Philippines

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Julius A. Pascoa, , Jesley Mei A. Dycocoa, Gabriel Theophilus V. Valerab, Betchaida D. Payota, Jon Dave B. Pillejeraa, Frances Aleksis Anika E. Uya, Leo T. Armadaa, Carla B. Dimalantaa a b

National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City 1101, Philippines Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Luzon Mélange Peridotite Gabbro Petrology

The clasts of ophiolitic mélanges formed in orogenic margins reflect the tectonomagmatic history of a region and record the petrological signatures of oceanic lithospheres that interacted in the past. Exposed at the northwestern edge of Luzon, Philippines, the highly deformed Dos Hermanos Mélange (DHM) provides new insights on the complex history of western Luzon island. The DHM is a tectonic mélange composed predominantly of ultramafic-mafic clasts set in a sheared serpentinite matrix. The ultramafic clasts are mostly harzburgites with rare occurrences of lherzolite, dunite and chromitite. Petrographic (e.g. protogranular to equigranular texture) and geochemical characteristics (e.g. spinel Cr# = 0.17–0.60, olivine Fo content = 87–91) of the peridotites typify residual mantle peridotites which underwent low to moderately high degrees of partial melting. Mineral chemistry of some dunite and harzburgite samples (e.g. high spinel TiO2 = 0.01–0.64 wt%) further record subsequent modification of the depleted mantle material by arc-related processes (e.g. metasomatism). Most of the mafic clasts classify as gabbros and are composed of highly anorthitic plagioclase (An88–99) and Ti-poor pyroxenes which suggest derivation from arc-related melts. One troctolite clast, however, records the distinct petrographic (e.g. ophitic texture) and geochemical (e.g. low An content of plagioclase = 73–80) signatures of primitive MOR-related magma. These contrasting petrologic signatures in the ultramafic-mafic clasts of the DHM are similar to those observed in the crustal and mantle sections of the Eocene Zambales Ophiolite Complex (ZOC). This suggests that the DHM, like the ZOC, records the complex history of the convergence and emplacement of an ancient oceanic crust onto the Philippine Mobile Belt. Later tectonic processes in the region, which occurred after the emplacement of the ZOC, resulted to the extensive dissection and translation of ophiolitic blocks northwards transforming them into the DHM.

1. Introduction Studying the magmatic processes involved in the genesis of ophiolites has proven to be crucial in understanding how the Earth’s lithosphere evolves (Pearce et al., 1984; Dilek and Robinson, 2003). Petrological signatures of plutonic rocks in an ophiolitic sequence, for example, are used as indices in determining the origin of these fragments of ancient oceanic lithosphere (e.g. Yumul and Dimalanta, 1997; Dilek and Furnes, 2011). These, in turn, provide better constraints in reconstructing the geologic history of a region. In the absence of an ophiolite suite, preserved ophiolitic mélanges are alternative tools for such studies. Mélanges are chaotically mixed and distorted rocks set in a pervasively deformed matrix (e.g. Silver and Beutner, 1980; Festa et al., 2012). The characteristics of mélange units strongly reflect the

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different geologic processes in convergent margins through time (e.g. Suzuki, 1986; Festa et al., 2010, 2012). Through the study of the clasts in a mélange, the geologic history of the stratigraphic units from which these materials are derived can be deduced (e.g. Wakabayashi and Dilek, 2011; Ichiyama et al., 2017; Morishita et al., 2017). In island arcs such as the Philippines, mélanges are typically associated with ophiolites such as those observed in Palawan, Panay, Bohol and Pujada (Tamayo et al., 2004; Yumul et al., 2008; Queaño et al., 2017a). However, very few investigations have been done to characterize these lithodemic units and their implications to the geology of the region (e.g. De Jesus et al., 2000). The Dos Hermanos Mélange (DHM) is made up of ophiolitic clasts set in a highly sheared serpentinite matrix. It is extensively exposed in Ilocos Norte in the northwestern edge of Luzon Island (Queaño et al., 2017a) (Fig. 1a). The

Corresponding author. E-mail address: [email protected] (J.A. Pasco).

https://doi.org/10.1016/j.jseaes.2019.104004 Received 3 March 2019; Received in revised form 30 August 2019; Accepted 30 August 2019 Available online 31 August 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Asian Earth Sciences 184 (2019) 104004

J.A. Pasco, et al.

Fig. 1. Geologic setting of the Dos Hermanos Mélange. (a) Map of northern Luzon showing the different tectonic features in the region and the spatial distribution of the Dos Hermanos Mélange (enclosed in red rectangle) and the Zambales Ophiolite Complex (ZOC). Topographic and bathymetric map for Luzon was generated using Submap 4.2 (Heuret and Lallemand, 2005). The extent of the ZOC was adopted from Yumul and Dimalanta (1997). The Acoje Block of the ZOC is defined by the purple-filled area, while the yellow field corresponds to the Coto Block of the ZOC. SBFZ: Subic Bay Fault Zone. (b) Map showing the distribution of the Dos Hermanos Mélange in the Ilocos Region and other localities discussed in text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

sill complexes and associated volcanic rocks (Rossman et al., 1989; Yumul and Dimalanta, 1997). This ophiolite complex can be further subdivided into three massifs which are referred to as the Masinloc, Cabangan and San Antonio massifs. Petrological investigations revealed that the Masinloc Massif consists of two blocks with distinct signatures: the Coto Block, which is interpreted to have formed in a transitional mid-ocean ridge to island arc, and the Acoje Block which is of island arc origin (Yumul, 1992; Yumul and Dimalanta, 1997). Subsequent studies on the San Antonio and Cabangan Massifs suggest that the Cabangan Massif has similar geochemical signatures with the Coto Block of the Masinloc Massif, while the petrological signatures of the San Antonio Massif suggest formation in an arc environment, similar to the Acoje Block of the Masinloc Massif (Yumul and Dimalanta, 1997). Zircon UPb geochronology of the silicic rocks of the ZOC and the paleontological dating of the fossil assemblages of the overlying Aksitero Formation gave an Eocene age for the ophiolite (Encarnacion et al., 1993; Yumul et al., 1998, 2019). Various models of emplacement have been proposed for the ZOC: (a) the presence of an anticlinorium which exposed the underlying Coto Block, (b) the intrusion of the Coto Block-Cabangan Massif igneous body into the Acoje Block-San Antonio Massif platform, (c) the intrusion of the Acoje Block and San Antonio Massif into the Coto Block-Cabangan Massif and (d) the amalgamation of these units by strike-slip faulting (Yumul et al., 1998). Moreover, it is generally believed that the emplacement of these blocks of the ZOC is largely controlled by translation along the West Luzon Shear-Subic Bay Fault Zone (Yumul et al., 1998) (Fig. 1a). The Eocene age of the ZOC constrains its origin from either the Celebes Sea basin, Molucca Sea basin or the northwest sub-basin of the South China Sea (Yumul, 1994; Tamayo et al., 2004). More recent studies, however, correlate the ZOC with the East Asian Sea, an oceanic crust that borders the Philippine Sea Plate (Wu et al., 2016; Perez et al., 2018).

Eocene Zambales Ophiolite Complex is exposed southwest of the DHM (e.g. Encarnacion et al., 1993; Yumul et al., 2019). Earlier works on the DHM constrained its maximum age to Late Jurassic to Early Cretaceous based on the radiolarian assemblage extracted from the chert clasts in the mélange (Queaño et al., 2017a). Arai and others (1997) suggested that the DHM represents an oceanic lithosphere formed in a suprasubduction zone setting based on the geochemistry of detrital spinels in the region. Information on the origin and evolution of the DHM and its possible association with the proximal ZOC, however, remains lacking. In this study, we present the first detailed petrological characterization of the ophiolitic clasts in the DHM. The petrographic and geochemical signatures of the clasts are then used to evaluate the tectonic and magmatic processes that subsequently shaped the northwest Luzon region.

2. Geologic background The DHM is extensively exposed along the Tulnagan River in Pasuquin, Baruyen River in Bangui, La Paz in Laoag and in its type section at the Dos Hermanos Islands, Pagudpud (Fig. 1b). At the Dos Hermanos Islands, the mélange is composed of isolated mounds of exotic angular blocks of metaperidotite and peridotite within an indurated serpentinite matrix (Fig. 2a–b). Well-preserved exposures of the mélange at Tulnagan River, Pasuquin reveal the characteristic phacoid-shaped clasts surrounded by the sheared serpentinite matrix of the mélange (Fig. 2c). In this locality, megaclasts of highly fractured serpentinized peridotites occur as small hills measuring 200 m across and 150 m high (Fig. 2d). Other exposures such as those in Baruyen River also consist of ophiolite-derived clasts including gabbros, cherts and metamorphic rocks such as greenschists and mica schists (Fig. 2e–f). These exotic blocks range in size from granule- to bouldersized blocks. Volcanic rocks such as basalts and andesites are not observed comprising the mélange. The Zambales Ophiolite Complex (ZOC) is located southwest of DHM. It is a complete ophiolite suite comprised of residual harzburgites and lherzolites, layered and massive ultramafic-mafic cumulates, dike-

3. Analytical procedures Petrographic analysis, mineral chemistry and trace element geochemistry were employed for the ultramafic-mafic clasts of the Dos 2

Journal of Asian Earth Sciences 184 (2019) 104004

J.A. Pasco, et al.

Fig. 2. Field photographs of the Dos Hermanos Mélange. (a) Outcrop of the Dos Hermanos Mélange at its type locality in Dos Hermanos Islands, Pagudpud, Ilocos Norte. (b) Peridotite and metaperidotite clasts are surrounded by an indurated serpentinite matrix in the Dos Hermanos Islands. (c) Classic fabric of a tectonic mélange exhibited by the DHM. A sheared serpentinite matrix envelopes phacoid-shaped peridotite clasts. (d) Megaclast of fractured peridotite along Tulnagan River, Pasuquin. (e) Chert boulders commonly occur with peridotites as clasts of the mélange. (f) Coarse-grained gabbro floats are also present in proximity with the mélange exposures.

Hermanos Mélange. Seven ultramafic and five gabbroic clasts from the Dos Hermanos Mélange were subjected to petrographic analysis at the University of the Philippines - National Institute of Geological Sciences (UP-NIGS). Representative thin sections of these clasts were prepared and point counting of at least two thousand points was done for each sample. The results were then normalized and plotted into their respective International Union of Geological Sciences (IUGS) classification diagrams (Fig. 3a–b). Analyses of major elements of the main mineral phases in the peridotite and gabbro clasts were carried out using a JEOL JXA-8320 electron probe microanalyzer (EPMA) at the University of the Philippines - National Institute of Geological Sciences (UP-NIGS). The compositions of the cores and rims in olivine, clinopyroxene, orthopyroxene and spinel in peridotites and olivine, clinopyroxene, orthopyroxene and plagioclase in gabbros were measured with a 10–20 nA beam current, acceleration voltage of 20 kV, and 3–12 µm probe

diameter. Representative analyses of the peridotites are listed in Table 1. Natural and synthetic standards were used during the analysis with analytical accuracy maintained at < 1%. Microprobe analyses of the gabbros are listed in Table 2. Iron in silicates was assumed to be Fe2+ in the recalculation. Proportions of ferric and ferrous iron were determined using spinel stoichiometry (Droop, 1987). Trace element concentrations of selected clinopyroxenes in the peridotite and gabbro clasts of the DHM were determined using a 193 nm ArF Excimer laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) (Agilent 7500S equipped with MicroLas GeoLas Q-plus; Ishida et al., 2004) at the Department of Earth Science in Kanazawa University, Japan. Spot diameters of 60 μm were ablated at 5–10 Hz. The NIST SRM 612 glass was selected as the primary calibration standard using the concentrations from Pearce et al. (1997). Further data correction was done using Si and Ca as internal standards, the concentrations of which were determined by EPMA following a 3

Journal of Asian Earth Sciences 184 (2019) 104004

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Fig. 3. Modal plots of the main mineral phases in the ultramafic and mafic clasts of the mélange. (a) IUGS classification diagram for ultramafic rocks showing the peridotites of the Dos Hermanos Mélange. (b) IUGS classification diagram for mafic rocks showing the gabbro, olivine gabbro and troctolite clasts of the Dos Hermanos Mélange.

last to crystallize. The other mafic clasts are identified as olivine gabbro and gabbros and are composed of varying amounts of plagioclase (22–37%), olivine (0.5–11%), orthopyroxene (8–23%) and clinopyroxene (11–43%). Olivine is extensively replaced by serpentine while amphibole occurs as replacement to clinopyroxene (Figs. 4f, 5d). Clinopyroxenes in the gabbro exhibit exsolution of orthopyroxenes. The crystallization order of the gabbros and olivine gabbro is as follows: olivine, clinopyroxene, and then plagioclase (Figs. 4f, 5d). Disequilibrium textures (e.g. zoning and resorbed boundaries) are not observed.

protocol outlined by Longerich and others (1996). The NIST SRM 614 was also analyzed as control sample after each measurement. Details on the analytical method and data quality are described in Morishita and others (2005a, 2005b). Representative analyses of trace element and rare earth element concentrations of clinopyroxenes in the ultramafic and mafic rocks are shown in Table 3.

4. Petrography The ultramafic clasts from the Dos Hermanos Mélange include lherzolites, harzburgites, dunite and chromitite (Fig. 3a). In general, these ultramafic rocks show kink banding and bent pyroxene lamellae which suggest plastic deformation prior to the emplacement of the mélange (Figs. 4a–b, 5a–b). Lherzolites (BUR-F1 and BUR-03A) and harzburgites (BUR-4A, BUR-4C and BUR-02A) are moderately serpentinized and composed of olivine (73–76%), clinopyroxene (0.3–7%), orthopyroxene (24–26%) and spinel (1–2%). These peridotites generally exhibit protogranular to porphyroclastic texture. Orthopyroxene porphyroclasts (2–20 mm) are surrounded by fine olivine and spinel neoblasts (Figs. 4a–b, 5a). Dunite (BUR-01) is composed of coarsegrained (2.5–5 mm) olivine (98%), with few interstitial clinopyroxene grains (< 1%) closely associated with spinel (2%). It shows porphyroclastic to equigranular texture although most of the grain boundaries are masked by serpentine (Figs. 4c, 5b). Euhedral spinel grains also occur along olivine boundaries. The chromitite clast is composed of euhedral, opaque to red chromian spinel (> 95%) surrounded by serpentine (Fig. 4d). Gabbroic clasts from the Dos Hermanos Mélange are medium- to coarse-grained and are classified as troctolite (LAO-02), gabbros (BURF2, BUR-F4 and GTV-03) and olivine gabbro (G2-1) (Fig. 3b). The troctolite is composed of plagioclase (67%), olivine (27%), orthopyroxene (1%) and clinopyroxene (5%). It exhibits orthocumulate texture with euhedral to subhedral olivine and plagioclase as the cumulus phases and interstitial clinopyroxene as the intercumulus phase (Figs. 4e, 5c). Orthopyroxenes mostly occur as exsolution from the clinopyroxenes and some of the olivine grains are replaced by serpentine. These petrographic characteristics suggest olivine as the first crystallizing phase followed by plagioclase, and clinopyroxene as the

5. Results 5.1. Mineral chemistry 5.1.1. Peridotites Olivines in the lherzolite (Fo88-90) and harzburgite clasts (Fo89-91) are forsteritic and their spinel Cr# [Cr/(Cr + Al) = 0.17–0.24 and 0.47–0.52, respectively] typify mantle peridotites delineated by the olivine-spinel mantle array (OSMA) (Fig. 6a) (Arai, 1994). In particular, the high Mg# [Mg/(Mg + Fe2+) = 0.63–0.72] and Al content, and low Cr# (=0.17–0.24) of spinel in the lherzolites are similar to that of fertile peridotites (Fig. 6b). Some lherzolites from the Acoje Block of the ZOC also consist of spinel with low Cr# and olivine with low Fo content plotting close to the DHM peridotites (Fig. 6a–c). In contrast, the relatively high Cr# (=0.47–0.55) and low Mg# (=0.55–0.60) of spinel in the harzburgites are similar to more depleted peridotites. Most of the peridotite clasts of the ultramafic section of the Acoje and Coto Blocks of the ZOC also plot around the DHM harzburgites. Olivine in the dunite (Fo87) and harzburgite sample BUR-02 (Fo87-88) clasts are notably less forsteritic. Spinels in the dunite are also characterized by higher TiO2 content (0.45–0.64 wt%, Fig. 6c), lower Mg# (=0.37–0.55) and moderate Fe-enrichment (Fig. 6d). The chromitite sample consists of spinels with high Cr# (=0.71) and TiO2 content similar to ultramafic rocks of the Acoje Block of the ZOC. Clinopyroxenes in the lherzolites of the DHM along with the peridotites from the Acoje Block are characterized by high Al2O3 (Fig. 7a), TiO2 (Fig. 7b) and Na2O (=0.07–0.50 wt%, Table 1) at the same Mg#, 4

Journal of Asian Earth Sciences 184 (2019) 104004

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Table 1 Representative mineral chemistry analyses of the ultramafic clasts of the Dos Hermanos Mélange. (–) means below detection limit.

Sample

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total Mg# Cr# Fo

Sample

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total Mg# Cr# Fo

Lherzolite

Lherzolite

Harzburgite

Harzburgite

BUR-F1

BUR-03A

BUR-4A

BUR-4C

Ol

Sp

Cpx

Opx

Ol

Sp

Cpx

Opx

Ol

Sp

Cpx

Opx

Ol

Sp

Cpx

Opx

40.77 0.02 – 0.02 10.39 0.14 48.47 0.01 – 0.02 0.42 100.27

0.03 0.02 46.30 21.14 14.90 0.21 17.16 – – 0.02 0.29 100.07 0.70 0.23

51.15 0.10 5.40 1.29 2.53 0.06 16.14 22.55 0.15 0.01 0.06 99.44 0.92

55.89 0.03 3.08 0.56 6.71 0.16 33.65 0.28 0.01 0.01 0.12 100.49 0.90

40.35 – – – 10.23 0.12 48.35 – 0.04 – 0.39 99.49

0.02 0.09 47.57 17.46 16.75 0.15 17.70 – 0.04 – 0.30 100.08 0.72 0.20

51.52 0.14 5.03 1.10 2.46 0.09 15.54 23.33 0.47 – 0.10 99.78 0.92

54.44 0.07 4.20 0.55 6.40 0.10 31.69 1.54 0.05 0.01 0.12 99.17 0.90

41.01 – – – 10.00 0.14 49.43 0.01 0.01 – 0.35 100.94

0.11 0.05 24.33 38.85 23.44 0.31 11.82 0.03 0.03 0.01 0.09 99.07 0.55 0.52

53.58 – 1.56 0.39 2.53 0.09 17.42 23.67 0.07 – 0.04 99.34 0.93

56.03 – 2.16 0.59 6.97 0.14 33.32 1.20 – – 0.08 100.48 0.89

40.33 0.00 0.01 0.00 10.07 0.13 49.59 0.01 0.00 0.01 0.39 100.55

0.01 0.05 26.82 38.94 21.66 0.29 12.99 0.06 0.02 – 0.15 100.99 0.58 0.48

54.63 – 1.69 0.37 2.28 0.09 17.56 24.13 0.01 0.02 0.06 100.84 0.93

55.23 0.01 2.28 0.82 6.76 0.12 33.75 1.50 – 0.01 0.08 100.55 0.90

89.3

89.38

89.81

90.62

Harzburgite

Dunite

Chromitite

BUR-02

BUR-01

BUR-CHR

Ol

Sp

Cpx

Opx

Ol

Sp

Cpx

Sp

40.53 0.01 – – 12.41 0.18 47.47 – 0.02 0.01 0.33 100.95

0.01 0.10 22.37 37.80 30.54 0.33 9.51 – 0.03 – 0.08 100.77 0.44 0.53

53.58 0.10 2.20 1.00 2.73 0.07 16.52 22.61 0.15 0.01 0.07 99.03 0.92

56.51 0.02 1.47 0.36 8.63 0.20 33.20 0.42 – – 0.08 100.90 0.87

40.09 – – – 13.01 0.21 47.29 – 0.03 – 0.34 100.97

0.04 0.48 19.70 39.13 30.01 0.38 9.41 – 0.04 – 0.12 99.31 0.45 0.57

53.24 0.21 1.58 0.77 2.65 0.06 17.00 24.46 0.12 – 0.04 100.13 0.92

0.02 0.19 14.60 53.33 16.98 0.26 15.17 – 0.01 – 0.12 100.68 0.70 0.71

87.21

86.63

Ol, olivine; Sp, spinel; Cpx, clinopyroxene; Opx, orthopyroxene; Total iron as FeO*.

0.74–0.81, respectively). Clinopyroxenes also exhibit a progressive increase in Na2O and Al2O3 content with decreasing Mg# (Fig. 8b–c). Orthopyroxenes in the gabbros and olivine gabbro also have lower TiO2 content (< 0.2 wt%). Generally, the troctolite sample exhibits similar characteristics as the crustal section of the Coto Block and other MORderived gabbros, while the gabbro clasts follow the same trends as the gabbros from the Acoje Block and other arc-related gabbros.

which are analogous to abyssal peridotites. Clinopyroxenes in harzburgite clasts meanwhile have lower Al2O3 (~1.0–2.77 wt%), TiO2 (< 0.10 wt%) and Na2O (< 0.15 wt%) concentrations and higher Mg# (=0.92–0.94) similar to other forearc peridotites (Fig. 7a–b, Table 1). Interstitial clinopyroxenes in the dunite are less magnesian and are characterized by higher TiO2 (=0.10–0.21 wt%) and Na2O (=0.12–0.17 wt%) compared to the more depleted peridotites (Table 1). Orthopyroxenes in the lherzolite are marked by higher Al2O3 at the same CaO and Cr2O3 contents compared to those in the harzburgite (Fig. 7c–d).

5.2. Trace element geochemistry Chondrite-normalized trace element patterns of clinopyroxenes in the lherzolites are characterized by depletions in highly incompatible elements such as Sr, Zr and light rare earth elements (LREEs) relative to less incompatible elements (Fig. 9a). These signatures are similar to those observed in abyssal peridotites from other major ridge systems (Fig. 9b) (Johnson et al., 1990; Liu et al., 2008; Warren, 2016). Clinopyroxenes in the harzburgites are even more depleted as shown by the low concentrations of Ti and heavy REEs. Superimposed on this depleted signature in the harzburgites are enrichments in Pb, Sr and light REEs relative to the lherzolites. Similar signatures are observed in the supra-subduction zone peridotites of SW Turkey (Fig. 9b) (Aldamanz et al., 2009; Aldamanz, 2012). Clinopyroxenes in the troctolite are highly enriched in trace elements, i.e. ten times chondrite values, and characterized by negative Sr and Eu anomalies (Fig. 9c–d). The troctolite also exhibits depletion in

5.1.2. Gabbros The minerals comprising the gabbro, olivine gabbro and troctolite clasts do not show compositional zonation from core to rim. Olivines in the troctolite are forsteritic (Fo81-82) and have high NiO (=0.14–0.22 wt%). Plagioclases in the troctolite are less anorthitic (An73-80) with low Or-content (< 1 wt%) (Fig. 8a, Table 2). Their clinopyroxenes (Mg# = 0.83–0.84) and orthopyroxenes (Mg# = 0.82–0.83) are highly magnesian. The Na2O, Al2O3 and TiO2 contents of the pyroxenes in the troctolite are also relatively high (Fig. 8b–d, Table 2). The olivine gabbro and gabbro clasts, on the other hand, are composed of olivine with lower Fo (0.66–0.80) and NiO (0.10–0.18 wt%), and plagioclases that are more anorthitic (An88-99). Clinopyroxenes and orthopyroxenes in these samples (G2-1, BUR-F4, BUR-F2 and GTV-03) have a wide range of Mg# (=0.79–0.90 and 5

Journal of Asian Earth Sciences 184 (2019) 104004

J.A. Pasco, et al.

Table 2 Representative mineral chemistry analyses of the mafic clasts of the Dos Hermanos Mélange. (–) means below detection limit.

Sample

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total An% Mg# Fo

Sample

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total An% Mg# Fo

Troctolite

Olivine gabbro

Gabbro

LAO-02

G2-1

GTV-03

Ol

Plg

Cpx

Opx

Ol

Plg

Cpx

Opx

Ol

Plg

Cpx

Opx

39.59 0.01 – 0.02 16.94 0.25 43.07 0.03 0.10 0.04 0.18 100.22

49.73 0.05 32.20 – – – – 15.44 2.82 0.03 – 100.27 75.01

51.64 1.01 3.08 0.63 6.12 0.16 16.81 21.05 0.39 – 0.04 100.92

53.3 0.42 2.07 0.32 9.98 0.2 26.7 6.82 0.11 – 0.04 99.96

38.12 – – – 21.32 0.27 40.69 0.01 0.02 0.01 0.14 100.57

45.29 – 34.95 – 0.31 – – 17.80 0.93 0.03 – 99.30 91.24

53.34 0.20 1.96 0.24 4.92 0.16 16.19 22.17 0.14 – 0.05 99.37

54.43 0.08 1.70 0.19 12.57 0.28 29.81 0.91 0.02 – 0.05 100.05

38.34 – – – 22.03 0.30 39.87 0.02 0.03 0.01 0.13 100.72

45.04 0.05 34.68 – 0.27 – – 18.62 0.87 0.01 – 99.52 92.19

52.98 0.17 2.20 0.14 5.78 0.14 15.6 21.98 0.16 0.01 0.04 99.21

53.7 0.08 1.55 0.06 13.81 0.27 29.55 1.18 – – 0.05 100.25

0.83

0.83

0.85

0.81

0.83

0.79

81.92

77.27

76.33

Gabbro

Gabbro

BUR-F2

BUR-F4

Ol

Plg

Cpx

Plg

Cpx

Opx

32.93 – 0.01 – 27.74 0.34 38.00 – 0.04 0.02 0.13 99.21

43.74 – 35.66 – 0.23 – – 19.05 0.36 0.03 – 99.08 96.48

53.89 0.07 1.15 0.17 5.69 0.17 16.11 22.94 0.08 0.01 0.05 100.33

44.494 – 34.398 – 0.262 – – 18.283 0.786 0.037 – 98.26 95.57

52.06 0.12 2.06 0.10 7.99 0.20 17.61 19.03 0.12 0.01 0.01 99.30

54.35 0.01 1.48 0.11 15.20 0.32 27.64 0.86 – 0.01 0.05 100.03

0.80

0.76

0.83 70.94

Ol, olivine; Sp, spinel; Cpx, clinopyroxene; Opx, orthopyroxene; Total iron as FeO*.

et al., 2002), respectively. Equilibration temperature values of 941 ± 62 °C were obtained for the lherzolites and 920 ± 54 °C for the harzburgites. These ranges of values are analogous to those calculated for abyssal peridotites (e.g. Hess Deep) and the IBM forearc, respectively (e.g. Okamura et al., 2006). The olivine-orthopyroxene-spinel oxygen barometer of Ballhaus and others (1990) was also used to estimate the oxidation state of the spinel peridotites clasts of the DHM. A pressure of 1.5 GPa was used in the calculations based on the stability field of spinel peridotites from Green and Ringwood (1967). Assuming these pressure conditions, the calculated oxygen fugacity for the lherzolite clasts is relatively low (Δ log (fO2)FMQ = −1.2 to +1.1, where Δ log is a log unit difference in oxygen fugacity from the fayalite-magnetite-quartz buffer). These values are similar to abyssal peridotites [Δ log (fO2)FMQ = −2.5 to +1.5 (Bryndzia et al., 1989; Blatter and Carmichael, 1998)]. The obtained values for the harzburgite clasts on the other hand are significantly higher [Δ log (fO2)FMQ = +0.39 to +1.70]. These values are similar to mantle xenoliths exhumed throughout the western Pacific island arcs (e.g. Arai and Ishimaru, 2007). The oxygen fugacity values obtained for the ultramafic clasts are also in agreement with previous works regarding the redox state of peridotites (Parkinson and Arculus, 1999; Dare et al., 2009).

LREEs (La/Sm = 0.07–0.10). In the REE diagram, the highly magnesian clinopyroxenes of the troctolite (Mg# = 0.83–0.84) exhibit similar patterns as the lower crust cumulates of Talkeetna which are more evolved (clinopyroxene Mg # = 0.75) and have undergone extensive plagioclase fractionation (Fig. 9d) (Greene et al., 2006). Clinopyroxene in the gabbros, on the other hand, contains significantly less trace element and distinct negative Zr and Ti anomalies. Their REE trends do not record a negative Eu anomaly and exhibit a more moderate depletion in LREEs (La/Sm = 0.14–0.24) compared to the troctolite.

5.3. Equilibrium condition and oxygen fugacity Estimation of equilibration temperature in spinel peridotites and gabbro clasts was done using the two-pyroxene geothermometer of Putirka (2008). This thermometer is based on the Brey and Köhler (1990) formula which uses the mutual solubility of Ca and Mg and Fe/ Mg partitioning between coexisting clinopyroxene and orthopyroxene. New global regression data were used to calibrate and increase the precision of the Brey and Köhler (1990) thermometer. The calculated equilibrium temperature for the gabbros and olivine gabbro (=892 ± 50 °C) is lower than those obtained for the troctolite (=1107 ± 75 °C). This temperature reflects the initial crystallization of these pyroxenes from the magma. Nonetheless, these values are similar to high temperature cumulates formed from primitive magmas in arcs (e.g. Alonso-Perez et al., 2009) and in mid-oceanic ridges (e.g. Niu 6

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Table 3 Representative trace element analyses of the clinopyroxenes in the ultramafic and mafic clasts of the Dos Hermanos Mélange. (–) means below detection limit. Lherzolite

Harzburgite

Harzburgite

Troctolite

Olivine gabbro

Sample

BUR-F1

BUR-4C

BUR-4A

LAO-02

G2-1

Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

0.07 – – – 0.07 – – – 0.27 – 0.08 0.03 0.14 0.03 0.11 0.06 642.82 0.42 0.12 1.06 6.76 0.27 0.83 0.13 0.89 0.12

0.19 0.08 – – 0.06 – – – 0.22 – 0.14 0.04 0.10 0.04 0.11 0.07 537.11 0.40 0.12 1.01 6.15 0.25 0.75 0.11 0.87 0.11

– – – – 0.06 – – – 0.09 – 0.10 0.04 0.13 0.03 0.13 0.08 600.57 0.43 0.11 1.07 6.75 0.26 0.85 0.13 0.92 0.14

– – – – 0.08 – 0.01 0.03 – – 0.42 – 0.09 – – – 144.64 – – 0.10 0.83 0.03 0.14 0.02 0.23 0.03

0.13 0.12 – – 0.08 – 0.02 0.03 0.53 – 1.29 – 0.22 – – – 99.86 – – 0.08 0.67 0.03 0.11 0.02 0.21 0.03

0.06 0.03 – – 0.08 – 0.01 0.03 0.23 0.00 0.82 – 0.22 – – – 128.36 – 0.01 0.12 0.98 0.03 0.15 0.03 0.25 0.04

– – – – 0.08 – 0.03 0.06 – 0.01 1.15 – 0.30 – – – 157.23 – 0.01 0.10 0.96 0.04 0.16 0.03 0.31 0.04

0.02 0.09 – – 0.07 – 0.02 0.07 0.10 0.01 2.13 0.03 0.51 – – 0.01 133.77 – 0.01 0.08 0.72 0.03 0.12 0.02 0.19 0.04

6. Discussion

– – – – 0.07 – 0.01 0.02 – – 0.33 – 0.08 – – – 143.98 – 0.01 0.12 1.10 0.04 0.16 0.03 0.26 0.05

– – 0.01 – 0.14 0.02 0.26 2.16 – 0.54 6.74 4.22 34.83 1.26 2.18 0.54 5289.14 3.41 0.64 4.66 25.27 0.98 2.85 0.41 2.75 0.39

0.11 – – – 0.18 0.02 0.27 2.18 – 0.53 6.79 4.11 32.22 0.86 2.14 0.52 4481.47 3.16 0.60 4.45 24.34 0.97 2.80 0.40 2.78 0.38

0.08 – – – 0.12 0.01 0.27 2.12 0.13 0.50 6.58 3.49 25.76 0.75 1.80 0.53 3991.40 2.68 0.52 3.81 20.42 0.81 2.27 0.34 2.32 0.32

– – – – 0.02 – 0.07 0.29 – 0.05 7.51 0.36 1.56 0.09 0.19 0.09 511.46 0.32 0.07 0.53 2.70 0.11 0.33 0.04 0.35 0.05

– 0.08 – – 0.03 – 0.09 0.39 0.19 0.07 8.48 0.48 2.70 0.15 0.26 0.13 773.02 0.55 0.10 0.76 4.08 0.16 0.45 0.08 0.47 0.06

– – – – 0.05 – 0.07 0.34 – 0.08 8.45 0.59 2.46 0.14 0.35 0.19 1572.55 0.69 0.15 1.18 6.19 0.25 0.72 0.11 0.76 0.11

SW Turkey (Fig. 9b) (Parkinson and Pearce, 1998; Aldamanz, 2012; Uysal et al., 2012). Petrographic (e.g. inequigranular to porphyroclastic) and geochemical characteristics (Fig. 5a–b) of the dunite (BUR-01) and harzburgite (BUR-02) also reflect their restitic nature. The presence of interstitial clinopyroxenes characterized by high TiO2 and Al2O3, however, suggests the interaction between these peridotites with percolating primitive melts after partial anatexis. This is further supported by the high spinel TiO2 content and lower spinel Mg# and olivine Fo content in these clasts (Fig. 5a–c). Such signatures suggest Fe-enrichment related to metasomatic processes commonly associated with arcs (e.g. Wang et al., 2007). The presence of chromitite clasts in the DHM is similar to the podiform chromitites of the Acoje Block of the ZOC. The signatures of these chromitites are indicative of melt-rock interaction which occurred in the ancient oceanic lithosphere where the ophiolitic clasts were derived from (e.g. Yumul, 1992; Arai, 1997). Melt-rock interaction between peridotites and melts has been widely documented in the forearc region and is consistent with enrichment due to subduction components (e.g. Ulrich et al., 2010; Birner et al., 2017). These processes must have transpired before the formation of the mélange, as the same trends are not observed in the gabbroic clasts.

6.1. Petrogenesis of the ultramafic clasts The peridotite clasts of the DHM typify residual mantle peridotites albeit reflecting different degrees of partial melting. All of the samples classify as spinel peridotites and exhibit protogranular to porphyroclastic texture (e.g. Mercier and Nicolas, 1975). The low spinel Cr#, high spinel Mg# and olivine Fo content of the lherzolites suggest that they experienced relatively low degrees of partial melting (~10%) (e.g. Arai, 1994; Ohara et al., 2002). This is also reflected by the high TiO2 and Na2O contents of clinopyroxenes and the high Al2O3 concentrations in both pyroxenes. High degrees of partial melting tend to reduce the concentration of these elements in peridotites (Choi et al., 2008). These petrological characteristics of the DHM are akin to abyssal peridotites such as those from the Indian Ocean (e.g. Hamlyn and Bonatti, 1980) and Mid-Atlantic fracture zones (e.g. Dick, 1989). This is further supported by the low oxygen fugacity values of the lherzolites and their high equilibration temperatures. The higher spinel Cr#-Mg# and olivine Fo content of the harzburgite clasts suggest higher degrees of partial melting (~25%) relative to the lherzolite. This is consistent with lower TiO2, Na2O and Al2O3 concentrations in the pyroxenes of the harzburgites (Table 1). The difference in the degree of partial melting experienced by the lherzolite and harzburgite clasts is also apparent in the trace element patterns of their clinopyroxenes. The relatively high concentrations of MREEs and HREEs of clinopyroxenes in the lherzolites compared to the harzburgites coupled by marked depletion in Sr and LREE reflect significantly lower degrees of partial melting underwent by the lherzolites. The trace element patterns of clinopyroxenes in the harzburgites, on the other hand, reflect subsequent enrichment of these depleted peridotites possibly by subduction components (e.g. slab-derived fluids). This is reflected by their enrichments in fluid mobile elements such as Pb, Ba and LREEs relative to the lherzolites. These signatures are consistent with the high oxygen fugacity values and equilibration temperatures obtained for the harzburgite clasts. These harzburgites therefore represent typical forearc peridotites such as those in the Izu-Bonin-Mariana and

6.2. Petrogenesis of the gabbroic clasts The petrological characteristics of the troctolite, gabbros and olivine gabbro also suggest that they are generated from magmas with contrasting characteristics. The crystallization sequence observed in the troctolite wherein olivine and plagioclase extensively crystallized prior to clinopyroxenes typifies cumulates derived from anhydrous basaltic melts (Elthon, 1987; Niu et al., 2002). This is further supported by the low An content of their plagioclases (An73-80) relative to the gabbros and olivine gabbro (An88–99) (Fig. 8a). Additional evidence for a primitive MORB-like parental melt for the troctolite are the notably high concentrations of rare earth elements and trace elements in clinopyroxene (Fig. 9c–d). The distinct negative Sr and Eu anomalies in the trace element patterns of clinopyroxenes also suggest extensive plagioclase crystallization. Furthermore, the highly magnesian olivine (Fo817

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Fig. 4. Representative photomicrographs of the ultramafic and mafic clasts of the Dos Hermanos Mélange. (a) Porphyroclastic texture in the lherzolite clast of the Dos Hermanos Mélange. The coarse-grained orthopyroxene (opx) porphyroclast is surrounded by fine-grained olivine (ol) neoblasts. (b) Orthopyroxene with clinopyroxene (cpx) exsolution lamellae in a harzburgite clast exhibiting bent structure and undulose extinction. (c) Equigranular to porphyroclastic texture in the dunite clast. Clinopyroxene is closely associated with spinel (sp). (d) Photomicrograph of a chromitite clast under reflected light showing chromian spinel and interstitial serpentine (serp). (e) Ophitic texture in the troctolite clast exhibited by well-formed plagioclase (plag) surrounded by anhedral clinopyroxene. (f) Cumulate texture in the gabbro clast. Euhedral clinopyroxene is surrounded by interstitial plagioclase. Portions of the clinopyroxene are altered to amphibole (amp).

plagioclase relative to clinopyroxenes. The inhibited fractionation of plagioclase is further supported by their anorthitic composition (An88–99) and the increasing Al2O3 concentration in clinopyroxenes with decreasing Mg# (Fig. 8c). The absence of negative Sr and Eu anomalies in the trace element patterns of clinopyroxenes in the gabbros and olivine gabbro can also be attributed to late plagioclase formation (Fig. 9b–c). Such petrological signatures likely reflect their genetic link to hydrous parental magmas which suppress plagioclase crystallization (Müntener et al., 2001; Alonso-Perez et al., 2009). The increasing Na2O content of clinopyroxene with decreasing Mg# in the gabbros was also noted in lower crust gabbros of Talkeetna (Greene et al., 2006) and Goksun Ophiolite (Parlak et al., 2019). Ubiquitous in

orthopyroxenes (Mg# = 0.82–0.98) and clinopyroxenes 82), (Mg# = 0.83–0.84) in the troctolite indicate crystallization from a primitive melt. This anhydrous basaltic melt is MORB-like and was likely sourced from a relatively fertile (i.e. enriched in mobile and incompatible elements) mantle material as evidenced by the high concentrations of Na2O (Fig. 8b), Al2O3 (Fig. 8c) and trace elements in clinopyroxenes (Fig. 9c–d) as well as high TiO2 content in orthopyroxenes (Fig. 8d) (Elthon, 1987; Niu et al., 2002). Similar signatures were observed in the gabbros and troctolites of the Coto Block in the ZOC (Fig. 8a–d) (Yumul and Dimalanta, 1997). On the other hand, the petrographic characteristics of the olivine gabbro and gabbro clasts suggest late crystallization of interstitial

8

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Fig. 5. Backscattered electron imagery of the peridotite and gabbro clasts of the mélange. (a) Porphyroclastic texture is observed in harzburgite. A bent orthopyroxene (opx) porphyroclast is surrounded by olivine (ol) neoblasts. Clinopyroxene (cpx) is also observed as exsolution lamellae within the orthopyroxene. (b) Relict orthopyroxene porphyroclast in the dunite clast surrounded by fine-grained olivine. Discrete spinel (sp) grains are observed in the dunite. (c) Coarse-grained plagioclase (plag) is partly surrounded by clinopyroxene in the troctolite clast. (d) Well-formed olivine and clinopyroxene adjacent to interstitial plagioclase in the gabbro clast. The clinopyroxenes are also partly altered to amphibole (amp).

gabbronorite, norite and norite-gabbro, is interpreted to be of island arc origin (e.g. Yumul and Dimalanta, 1997). On the other hand, the Coto Block is made up of harzburgites, gabbros and troctolites and was likely derived from a transitional mid-ocean ridge to an island arc setting (e.g. Yumul and Dimalanta, 1997). As discussed in previous sections, the petrographic characteristics and mineral chemistry of the peridotite clasts of the DHM are restitic in origin and record low to moderately high degrees of partial melting similar to the lherzolites and harzburgites of the ZOC (Fig. 6a–b). The ultramafic rocks of the Acoje Block of the ZOC also share the same geochemical signatures as the chromitite clast of the DHM (Fig. 6c) Their similarity is also apparent in the mafic clasts of the DHM. Moreover, troctolites and gabbros formed from anhydrous magmas and hydrous arc-like melts, respectively, are also found in the crustal section of the ZOC (Yumul, 1990). The similarities in the petrological signature of the crust to mantle section of the ZOC and the ultramafic-mafic clasts of the DHM as well as the equivalent ages of the chert blocks found in both localities imply a genetic link between the two localities despite being approximately 250 km apart at present. Based on available evidence, it is apparent that fragments of the same oceanic lithosphere that was obducted as the Zambales Ophiolite Complex also comprise the Dos Hermanos Mélange. Moreover, uranium-lead dating of zircons from the tonalite and quartz diorite of the ZOC also yielded an Eocene age for the ophiolite (Encarnacion et al., 1993; Yumul et al., 1998). Such scenario while shedding light on the origin of the DHM, also invokes questions on the tectonic events in this region responsible for the intense deformation and possible translation of the DHM with respect to the relatively intact ZOC. Previous petrological studies on the Zambales Ophiolite Complex established it as initially being formed as a marginal basin that was

the island arc setting, these hydrous melts are derived from highly depleted mantle sources which would explain the low trace element concentrations in their clinopyroxenes and the low TiO2 content of their orthopyroxenes. Similar signatures were also observed in the gabbros and gabbronorites of the Acoje Block of the ZOC which was previously interpreted to have formed in an island arc setting (Fig. 8a–d) (Yumul and Dimalanta, 1997).

6.3. Origin of the Dos Hermanos Mélange vis-à-vis ZOC The clasts of the DHM are exclusively ophiolitic in origin and are enveloped by a serpentinite matrix. The DHM is interpreted to be tectonic in origin because of the characteristic scaly fabric of the matrix, the angular and sheared nature of the blocks comprising the mélange and the lack of internal layering characteristic of mélanges of sedimentary origin (Queaño et al., 2017a). Based on the composition of its clasts, the DHM likely represents tectonized sections of an ancient oceanic crust and portions of its upper mantle. The source of these clasts, however, has yet to be determined. Based on paleontological analysis of the chert clasts of the DHM, a maximum age of Uppermost Jurassic to Lower Cretaceous (Queaño et al., 2017a) was given for the source oceanic lithosphere. Paleontological analysis of chert blocks within the Early to Middle Miocene clastic formation overlying the Acoje Block of the Zambales Ophiolite Complex also yielded a Late Jurassic to Early Cretaceous age (Queaño et al., 2017b). Past studies on the crust to mantle section of the ZOC concluded that they record formation in two contrasting terranes which led earlier workers to divide the complex into the Acoje and Coto Blocks (e.g. Yumul, 1992; Yumul and Dimalanta, 1997). The mafic-ultramafic section of the Acoje Block, composed of harzburgites, lherzolites, 9

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Fig. 6. Variation diagrams of olivine and spinel of the ultramafic clasts of the mélange. (a) Spinel Cr# vs olivine forsterite content. Most of the peridotite clasts plot within the olivine spinel mantle array (OSMA) (Arai, 1994). (b) Cr# vs Mg# of chromian spinels in the peridotite clasts. The abyssal peridotite field is from Dick and Bullen (1984) while the forearc peridotite field is adopted from Arai and Ishimaru (2007). (c) Cr# vs TiO2 wt. % of the ultramafic clasts of the DHM. A distinct TiO2 enrichment is observed in the dunite. Field of abyssal peridotites is adopted from Choi et al. (2008). (d) Ternary diagram showing the Cr-Al-Fe3+ content of spinels from the ultramafic clasts. Slight Fe enrichments are observed in the dunite and harzburgite clasts. The forearc peridotite field is from Ishii et al. (1992) while the abyssal peridotite field is from Dick and Bullen (1984). Data for the ZOC are from Yumul (1987, 1992), and Yumul and Dimalanta (1997). Data for abyssal peridotites are from Hamlyn and Bonatti (1980), Dick (1989) and Andal et al. (2005). Forearc peridotite data are from Ishii et al. (1992) and Parkinson and Pearce (1998).

10

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Fig. 7. Variation diagrams of orthopyroxene and clinopyroxene of the ultramafic clasts of the mélange. (a) Al2O3 wt% vs Mg# of clinopyroxene in the peridotite clasts. The lherzolite clasts consistently plot with background data for abyssal peridotites. (b) TiO2 wt% vs Mg# of clinopyroxenes in the peridotite clasts. The clinopyroxenes in the dunite clast plot closely with the clinopyroxenes in the lherzolite clasts at higher TiO2 wt. % compared to the harzburgites. (c) CaO wt% vs Al2O3 wt% of orthopyroxenes in the peridotites from the mélange. The harzburgites plot at lower Al2O3 wt% compared to the lherzolites. (d) Cr2O3 wt% vs Al2O3 wt % of orthopyroxenes in the peridotite clasts. The lherzolite clasts plot closely with the background data for abyssal peridotites while the harzburgites plot with the forearc peridotite data. Data for the ZOC are from Yumul (1987, 1992), and Yumul and Dimalanta (1997). Data for abyssal peridotites are from Hamlyn and Bonatti (1980), Dick (1989) and Andal et al. (2005). Forearc peridotite data are from Ishii et al. (1992) and Parkinson and Pearce (1998).

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Fig. 8. Variation diagrams of the mafic clasts of the mélange. (a) Olivine Fo content vs plagioclase An content of the mafic clasts. Olivine gabbro and gabbros plot within the arc gabbro field, while troctolite falls within the MOR gabbro field (Beard, 1986). (b) Na2O wt% vs Mg# of clinopyroxenes in the gabbroic clasts of the DHM. Higher Na2O wt% is observed in the clinopyroxenes in troctolite relative to the gabbros and olivine gabbro. (c) Al2O3 wt% vs Mg# of clinopyroxene in the gabbroic clasts. See text for discussion. (d) TiO2 wt% vs Mg# of orthopyroxenes in the mafic clasts of the DHM. Lower TiO2 wt% is observed in the olivine gabbro and gabbro compared to the troctolite. Data for the ZOC are from Yumul (1987, 1992), and Yumul and Dimalanta (1997). Data for arc gabbros are from Greene et al. (2006). Data for mid-ocean ridge gabbros are from Elthon (1987), Cannat et al. (1992) and Niu et al. (2002).

Complex was derived. Reconstructions of the evolution of the Southeast Asia region and paleomagnetic data of northwestern Luzon further indicate that the Philippine Mobile Belt was translated northward from a previously subequatorial position in the Cretaceous (Hall, 2002; Queaño et al., 2007). Because of the Eocene magmatic age of the Zambales Ophiolite Complex, the subduction of the East Asian Sea

subsequently altered by subduction zone processes (Hawkins and Evans, 1983; Pearce et al., 1984; Geary et al., 1989; Yumul and Dimalanta, 1997). Unfolding of subducted slabs beneath the Philippine Mobile Belt unveiled the presence of a Cretaceous marginal sea, the East Asian Sea (Wu et al., 2016; Perez et al., 2018). The East Asian Sea is considered as the possible oceanic crust where the Zambales Ophiolite

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Fig. 9. Chondrite-normalized rare earth element and trace element diagrams of clinopyroxene in the ultramafic and mafic clasts of the Dos Hermanos Mélange. (a) Chondrite-normalized extended trace element diagram of clinopyroxenes in the peridotite clasts. The lherzolite is generally more enriched in REEs compared to the harzburgites. (b) Chondrite-normalized REE patterns of clinopyroxenes in the peridotite clasts. Noticeable enrichments in the LREEs are observed in the harzburgites. Abyssal peridotite field is from Johnson et al. (1990) while the forearc peridotite field is from Aldamanz (2012). (c) Chondrite-normalized extended trace element diagram of clinopyroxenes in the gabbroic clasts. See text for discussion. (d) Chondrite-normalized REE patterns of clinopyroxenes in the peridotite clasts. Distinct negative Eu anomaly is present in the clinopyroxenes of the troctolite.

1995b). Portions of the then emplaced ZOC may have been sheared by this strike-slip fault. Regardless of the associated structure, the stress brought about by this strike-slip motion likely caused the shearing of ophiolitic materials within the Dos Hermanos Mélange, transforming the angular fragments into phacoid-shaped blocks with slickensided surfaces and the formation of the highly sheared serpentinite matrix which envelopes these blocks.

beneath the Philippine Mobile Belt which led to the emplacement of the ophiolite must have transpired after the Eocene. As the oceanic crust underplated the island arc, portions of it became accreted to the Philippine Mobile Belt and were later modified by subduction processes (Wu et al., 2016; Perez et al., 2018). The lherzolite clasts of the DHM can therefore be interpreted as evidence of this MOR-derived oceanic lithosphere which experienced subsequent melting and modification resulting to the signatures observed in the harzburgites (e.g. Payot et al., 2018). The two types of magmas that formed the troctolite (anhydrous MOR-like) and the gabbros (hydrous arc-like) also indicate a similar evolution history for its source oceanic lithosphere. The collision and translation of the Philippine Mobile Belt were later responsible for the emplacement of the Zambales Ophiolite Complex (Encarnacion et al., 1993; Yumul et al., 1998) and possibly the initial deformation of the DHM. Subsequent deformations of the DHM likely came from shearing related to the northern segments of the Philippine Fault Zone during the end of the Miocene (Fig. 1a–b) (Pinet and Stephan, 1990). Active segments of the fault possibly transected the oceanic fragments of the ZOC and as the fault underwent left-lateral motion, it deformed these fragments and formed the DHM. Previous works estimated that the total slip of various segments of this predominantly sinistral fault since Middle Miocene ranges from 90 to 300 km (Mitchell et al., 1986; Bischke et al., 1990; Pinet and Stephan, 1990). This amount of slip suggests that aside from shearing, the DHM blocks were likely translated left-laterally with respect to the ZOC which also accounts for the 250-km gap separating the two localities. Another structure that possibly influenced the formation of the DHM is the Manila Trench. During the Eocene to Oligocene, the Manila Trench manifests as a large-scale strike-slip fault which facilitates the northeastward translation of the Philippine Mobile Belt (Hall et al., 1995a,

7. Conclusion Petrological signatures of the ophiolitic clasts of the Dos Hermanos Mélange (DHM) provided new constraints regarding its origin. The lherzolite, harzburgite and dunite clasts are residual mantle peridotites which underwent low to moderately high degrees of partial melting. Subsequent metasomatic interaction between these peridotites and percolating melts and fluids are observed in the harzburgites leading to their secondary enrichment (e.g. Pb and LREEs) based on the trace element geochemistry of their clinopyroxenes. These processes transpired before the emplacement of the mélange since the same geochemical variations are not observed in the mafic clasts. Lherzolites show abyssal affinity whereas the harzburgite and dunite clasts are of supra-subduction zone origin. These contrasting origins are also reflected by the petrological characteristics of the gabbroic clasts. The troctolite was likely derived from primitive anhydrous, MOR-related magmas, while the gabbros and olivine gabbro indicate crystallization from hydrous, arc-related melts. The MOR and island arc signatures recognized in these DHM clasts are similar to the lithologies observed in the Zambales Ophiolite Complex. This suggests that the DHM corresponds to highly sheared fragments of the same oceanic lithosphere preserved as the ZOC. Strike-slip faulting in the region is the probable 13

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cause of the extreme dissection of the DHM.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is funded by the University of the Philippines Office of the Vice Chancellor for Research and Development (UP-OVCRD) through its Outright Research Grants. Initial funding was also provided by the University of the Philippines - National Institute of Geological Sciences (UP-NIGS). The authors would like to express their gratitude to Prof. Shoji Arai, Dr. Tomoaki Morishita and Dr. Akihiro Tamura of the Department of Earth Science, Kanazawa University, Japan for the fruitful discussions, and assistance provided during the analyses of samples. The authors also acknowledge the support provided by the local government units of Burgos, Bangui, Laoag, Pasuquin and Pagudpud, Ilocos Norte. Editorial handling by Prof. Michel Faure, and comments from the reviewers, Prof. René Maury and Dr. Gregory Shellnutt, are also highly appreciated. References Aldamanz, E., 2012. Trace element geochemistry of primary mantle minerals in spinelperidotites from polygenetic MOR-SSZ suites of SW Turkey: Constraints from an LAICP-MS study and implications for mantle metasomatism. Geol. J. 47, 59–76. Aldamanz, E., Schmidt, M.W., Gourgaud, A., Meisel, T., 2009. Mid-ocean ridge and suprasubduction geochemical signatures in spinel-peridotites from the Neotethyan ophiolites in SW Turkey: Implications for upper mantle melting processes. Lithos 113, 691–708. Alonso-Perez, R., Muntener, O., Ulmer, P., 2009. Igneous garnet and amphibole fractionation in the roots of island arcs: Experimental constraints on andesitic liquids. Contrib. Miner. Petrol. 157, 541–558. Andal, E.S., Arai, S., Yumul Jr., G.P., 2005. Complete mantle section of a slow-spreading ridge-derived ophiolite: An example from the Isabela ophiolite in the Philippines. Isl. Arc 14, 272–294. Arai, S., 1994. Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation. Chem. Geol. 113, 191–204. Arai, S., 1997. Origin of podiform chromitites. J. Asian Earth Sci. 15, 303–310. Arai, S., Ishimaru, S., 2007. Insights into petrological characteristics of the lithosphere of mantle wedge beneath arcs through peridotite xenoliths: A review. J. Petrol. 49, 665–695. Arai, S., Kadoshima, K., Manjoorsa, M.V., David, C.P., Kida, M., 1997. Chemistry of detrital chromian spinels as an insight into petrological characteristics of their source peridotites: An example from the Ilocos Norte ophiolite, northern Luzon, Philippines. J. Mineral., Petrol., Econ. Geol. 92, 137–141. Ballhaus, C., Berry, R.F., Green, D.H., 1990. Oxygen fugacity controls in the Earth’s upper mantle. Nature 348, 437–440. Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: Implications for the tectonic setting of gabbroic plutons and for andesite genesis. Geology 14, 848–851. Birner, S.K., Warren, J.M., Cottrell, E., Davis, F.A., Kelley, K.A., Falloon, T.J., 2017. Forearc peridotites from Tonga record heterogeneous oxidation of the mantle following subduction initiation. J. Petrol. 58, 1755–1780. Bischke, R.E., Suppe, J., Del Pilar, R., 1990. A new branch of the Philippine Fault system as observed from aeromagnetic and seismic data. Tectonophysics 183, 243–264. Blatter, D.L., Carmichael, I.S.E., 1998. Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26, 1035–1038. Brey, G.P., Kohler, T., 1990. Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. J. Petrol. 31, 1353–1378. Bryndzia, L.T., Wood, B.J., Dick, H.B., 1989. The oxidation state of the Earth’s suboceanic mantle from oxygen thermobarometry of abyssal spinel peridotites. Nature 341, 526–527. Cannat, M., Bideau, D., Bougault, H., 1992. Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N. Earth Planet. Sci. Lett. 109, 87–106. Choi, S.H., Shervais, J.W., Mukasa, S.B., 2008. Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite, California. Contrib. Miner. Petrol. 156, 551–576. Dare, S.A.S., Pearce, J.A., McDonald, I., Styles, M.T., 2009. Tectonic discrimination of peridotites using fO2-Cr# and Ga-Ti-FeIII systematics in chrome-spinel. Chem. Geol. 261, 199–216. De Jesus, J.V., Yumul Jr., G.P., Faustino, D.V., 2000. The Cansiwang Mélange of

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