Mesozoic Ophiolites Research Articles

Juxtaposition of Cenozoic and Mesozoic ophiolites in Palawan island, Philippines: New insights on the evolution of the Proto-South China Sea

The proto-South China Sea (proto-SCS) is a fully consumed Mesozoic plate that once existed between the South China-Eurasian margin and Borneo. Remnants of this oceanic domain are believed to be emplaced as ophiolitic lithologies in Palawan and Zambales, Philippines and Borneo. Cretaceous and Eocene ophiolitic lithologies are exposed in the central and southern portions of Palawan island. The Eocene central Palawan Ophiolite (CPO) is composed of mantle peridotites, isotropic and layered gabbros, and massive and pillow lavas. In lieu of a sheeted dike complex, mafic dikes with distinct bake-and-chill margins cut the CPO, from the metamorphic sole to the crustal section. The Cretaceous southern Palawan Ophiolite (SPO) comprises of peridotite and pillow basalt exposures. Isotropic olivine gabbros are mainly preserved as floats. Troctolite and olivine gabbro dikes also cut the peridotites in the SPO.The CPO crustal rocks and mafic dikes have dominantly back-arc basin basalt affinities. The SPO lithologies exhibit ocean island basalt-like signatures with a mafic dike showing island arc basalt-like compositions. A plagiogranite sample from Puerto Princesa indicates an Eocene age (40.01 ± 0.54 Ma) for the CPO. U-Pb dating of olivine gabbro and syenite floats yielded a minimum age of Early Cretaceous (100.73 ± 1.07 and 102.97 ± 1.07 Ma, respectively) for the SPO. This is the first reported radiometric age for the now completely subducted Mesozoic proto-SCS exposed in southern Palawan, which complements existing paleontological dates. Geochemical and geochronological characteristics of the SPO and CPO are similar to those of the Darvel Bay and Telupid Ophiolites in Borneo, which are interpreted to originate from the proto-SCS. The CPO and SPO are thus construed to represent fragments of the proto-SCS emplaced in Palawan. Complex tectonic processes in the region during the Cretaceous to Eocene led to the inception of multiple subduction and rifting events involving the proto-SCS.

Petrological and geochemical characterization of metamorphic sole rocks beneath the RasKoh ophiolite, western Pakistan: Implication for a Late Cretaceous supra‐subduction zone‐type forearc

Small, isolated outcrops of metamorphic sole rocks to the RasKoh ophiolite are exposed south‐east of the RasKoh Arc range, western Pakistan. These crop out as thin slices beneath the sheared serpentinites, and consist of amphibolites, epidote‐bearing amphibolites, and muscovite‐rich amphibolites with granoblastic, grano‐nematoblastic, and porphyroblastic textures. Geochemically, amphibolites present two compositional groups. Group‐I amphibolites are alkaline and enriched in light rare‐earth elements (LREE; LaN/YbN = 11.47–11.60) with Sr‐Nd isotopic compositions of (87Sr/86Sr)i = 0.705481–0.706128, (143Nd/144Nd)i = 0.512668–0.512672, and lower values of εNd(t) (+2.24 to +2.32). While, Group‐II amphibolites are tholeiitic with LREE‐depleted signatures (LaN/YbN = 0.88–1.31), (87Sr/86Sr)i = 0.706103–0.706303, (143Nd/144Nd)i = 0.512851–0.512936, and higher values of εNd(t) (+5.81 to +7.47). Primitive mantle‐normalized and other geochemical diagrams indicate that the protoliths for Group‐I amphibolites are oceanic island basalts (OIB). Conversely, Group‐II amphibolites are more akin to subduction‐related arc tholeiites with geochemical signatures, slightly enriched in large‐ion lithophile elements and with negative Nb‐Ta anomalies. The geothermobarometric results estimate that these rocks were metamorphosed at amphibolite facies at 690°C and 6.5 kbar. Alkaline volcanism is well documented from the other Mesozoic ophiolites of Neo‐Tethyan suture zones. We suggest that the OIB accreted to the base of the obducted plate at an early stage, whereas the island arc tholeiites (IAT) basalts metamorphosed and accreted to the base of the RasKoh ophiolite at a later stage. The OIB‐ and IAT‐type magmas as protolith of amphibolites suggest that the RasKoh ophiolite was emplaced in a supra‐subduction zone‐type forearc in Late Cretaceous (ca. 68–64 Ma).

Petrogenesis of the chromitite body from the Cerro Colorado ophiolite, Paraguaná Peninsula, Venezuela

Ultramafic-mafic rocks of ophiolitic affinity crop out along the Venezuelan Caribbean region. They have been interpreted as remnants of the oceanic lithosphere of the Caribbean volcanic arc (135-70 Ma) as well as relicts of proto-Caribbean oceanic lithosphere (Upper Jurassic-Lower Cretaceous) related to Pangea’s break-up. The Cerro Colorado ophiolite, located in the Paraguaná Peninsula, together with the case of the Cordillera de la Costa in north-central Venezuela, are a unique case of these Venezuelan ophiolites containing chromitite bodies. However, the petrogenesis of such a mafic-ultramafic complex and associated chromite ore remains are unknown to date. To advance our understanding of chromite ores in the Caribbean region, the genesis of the Cerro Colorado chromitite body is challenged. The Cerro Colorado chromitite body is characterized by a low-Cr content [Cr# =Cr/ Cr+Al= 0.44-0.60] and a distribution of trace elements in chromite as is typical of high-Al chromitites found in the shallower portions of the petrological Moho Transition Zone of Mesozoic ophiolites. The calculated melts in equilibrium with chromite forming this high-Al chromitite body are back-arc basin basalts. These melts were extracted after ~20 % partial melting of moderately depleted peridotites, which resulted in the precipitation of high-Al chromitite relatively impoverished in PGE (≤ 100 ppb total PGE). A comparison of the geochemical signatures of minor and trace elements in chromite and bulk-PGE contents of the Cerro Colorado chromitite with those of other known chromitites in the peri-Caribbean ophiolites show certain similitude with those high-Al described in the Moa-Baracoa ophiolite in eastern Cuba. The obtained results allow us to suggest that the ultramafic rocks of the Cerro Colorado and the chromitite body associated with it are closely related to the formation of a back-arc basin developed between ca. 125-120 Ma in the rear of the Great Antilles Arc.

Metallogeny of Serpentinite‐hosted Magnetite Deposits: Hydrothermal Overgrowth on Chromite or Metamorphic Transformation of Chromite?

AbstractPeculiar and rare occurrences of serpentinite‐hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece (Skyros), Iran (Nain and Sabzevar) and Oman (Aniba). These deposits have diverse thickness (from a few centimeters up to 50 m) and length (2 to >500 m). Magnetite ores show variable textures, including massive, nodular and banded ores, veins, net and fine‐grained disseminations in serpentinites. Intriguingly, the investigated magnetite deposits can be mistaken for chromitite pods. Serpentinite‐hosted magnetite deposits show three modes of occurrences including: (i) boulders strewn across the serpentinites (i.e. Skyros Island); (ii) ore bodies along the nonconformity contacts between serpentinites and limestones (i.e. Aniba); (iii) irregular and discontinuous trails of massive and semi‐massive ore bodies within highly sheared serpentinite masses (i.e. Nain; Sabzevar). In all of these magnetite ore bodies, relicts of chromian spinel grains are occasionally enclosed in magnetite crystals. The chemistry of Cr‐spinel relics found in these magnetite bodies are comparable to those of accessory Cr‐spinels in the surrounding serpentinized peridotites. BSE images and elemental mapping revealed that magnetite occurs as a nucleation on chromian spinels but not being involved in reaction either with chromite or ferritchromite. Low‐grade metamorphic transformation of chromite into Fe‐chromite is documented along the cracks and fractures of a few chromite grains. Generally, magnetite has typical hydrothermal compositions, characterized by low Cr, V and Ti and high Mg and Mn. It is crucial to note that a few magnetite grains with metamorphic origin are characterized by high Cr and low Ti and Ni. The potential source of iron is essentially the Fe‐rich olivine. We believe that multi‐episodic serpentinization of peridotite systems at high fluid‐rock ratios is the main process responsible for precipitation of magnetite at ore levels whereas low‐grade metamorphic transformation of chromite to magnetite has minor contribution. Cumulative factors in generation of these deposits are modal volume of mantle olivine, peridotite composition, fluid chemistry, fluid‐rock ratio, mechanisms of transportation and precipitation, structural controls such as cracks and shear zones.

Heavy metal contamination in groundwater and surface water due to active Pb-Zn-Cu mine tails and water-rock interactions: A case study from the Küre mine area (Turkey)

This study aimed at elucidating the groundwater and surface water pollution resulting from water-rock interaction, mine tailings, and mining activities at Küre (Kastamonu) sulfidic Pb-Zn-Cu mine area and its vicinity. The study area has a surface area of approximately 2990 km2. The Küre River drainage site covering the study area is approximately 440 km2. The concentrations of SO4, Al, As, Ba, Mn, Ni, Sb, and Pb in some groundwater (K10, K20, K21, K27, K28, K29, K30), and surface water (KR3) exceed the maximum limit values with respect to the Turkish Standards for Water Intended For Human Consumption and World Health Organization Standards. The most polluted waters were K10, K27, and KR3. The K10 well was drilled for observation at the edge of the liquid mine tailing pond of the Küre Pb-Zn-Cu mine area. The K27 well was opened in alluvium outside of the study area and it cuts the clastic units and Mesozoic ophiolites. The KR3 measurement point is located on the Ersizler River, which drains the Küre Pb-Zn-Cu mining site and its dumps. The SO4, Al, As, Pb, and Sb concentrations of the K10, K27, and KR3 waters were 29.3, 0.8, and 15.1 meq/L; 1135, 1112, and 1186 ppb; 284, 255, and 271 ppb; 19.1, 19, and 18.5 ppb; and 18.7, 17.4, and 19.3 ppb, respectively, in the dry period. As revealed by the analysis, the study area had dual pollution source natural pollution caused by water-rock interactions (K27 and others) and anthropogenic pollution (K10 and KR3) caused by the mine tailings. Furthermore, the K10 water had high tritium (3H)-high electrical conductivity (EC) values that likely indicated anthropogenic contamination. The K27 water had low 3H-high EC values, presumably referring to geogenic contamination. The current study also demonstrated that there was leakage from the liquid mine tailing pond into the groundwater (K10), possibly implying discharge of the liquid mine tailings into the river (KR3).

Mesozoic rock suites along western Philippines: Exposed proto-South China Sea fragments?

An ancient oceanic crustal leading edge east of mainland Asia, the proto-South China Sea crust, must have existed during the Mesozoic based on tectonic reconstructions that accounted for the presence of subducted slabs in the lower mantle and the exposed oceanic lithospheric fragments strewn in the Philippine and Bornean regions. Along the western seaboard of the Philippine archipelago, numerous Mesozoic ophiolites and associated lithologies do not appear to be genetically associated with the younger Paleogene-Neogene ocean basins that currently surround the islands. New sedimentological, paleomagnetic, paleontological, and isotopic age data that we generated are presented here, in combination with our previous results and those of others, to reassess the geological make-up of the western Philippine island arc system. We believe that the oceanic lithospheric fragments, associated melanges, and sedimentary rocks in this region are exhumed slivers of the proto-South China Sea ocean plate.

Magnetic anisotropy reveals Acadian transpressional fabrics in an Appalachian ophiolite (Thetford Mines, Canada)

SUMMARY Magnetic anisotropy has proved effective in characterizing primary, spreading-related magmatic fabrics in Mesozoic (Tethyan) ophiolites, for example in documenting lower oceanic crustal flow. The potential for preservation of primary magnetic fabrics has not been tested, however, in older Palaeozoic ophiolites, where anisotropy may record regional strain during polyphase deformation. Here, we present anisotropy of magnetic susceptibility results from the Ordovician Thetford Mines ophiolite (Canada) that experienced two major phases of post-accretion deformation, during the Taconian and Acadian orogenic events. Magnetic fabrics consistent with modal layering in gabbros are observed at one locality, suggesting that primary fabrics may survive deformation locally in low strain zones. However, at remaining sites rocks with different magmatic origins have consistent magnetic fabrics, reflecting structurally controlled shape preferred orientations of iron-rich phases. Subhorizontal NW-SE-oriented minimum principal susceptibility axes correlate with poles to cleavage observed in overlying post-obduction, pre-Acadian sedimentary formations, indicating that the magnetic foliation in the ophiolite formed during regional NW-SE Acadian shortening. Maximum principal susceptibility axes plunging steeply to the NE are orthogonal to the orientation of regional Acadian fold axes, and are consistent with subvertical tectonic stretching. This magnetic lineation is parallel to the shape preferred orientation of secondary amphibole crystals and is interpreted to reflect grain growth during Acadian dextral transpression. This structural style has been widely reported along the Appalachian orogen, but the magnetic fabric data presented here provide the first evidence for transpression recorded in an Appalachian ophiolite.

Origin of Mesozoic ophiolitic mélanges in the western Yarlung Zangbo suture zone, SW Tibet

The petrogenesis and tectonic evolution of the Mesozoic ophiolitic mélanges in the western section of the Yarlung Zangbo suture zone (YZSZ) remain controversial. In this paper, we present the results of whole-rock geochemical and SrNd isotope analyses, zircon UPb ages and in situ LuHf isotopic data obtained from mafic rocks of the northern and southern sub-belts of the western YZSZ Mesozoic ophiolitic mélanges to help us understand these controversial issues. Diabases and dolerites from the northern sub-belt and gabbros from the southern sub-belt exhibit variable fore-arc basalt (FAB)-like geochemical compositions and have zircon UPb ages of ∼126.4–120.3 Ma. In addition, gabbro-diabases from the northern sub-belt have boninite series affinities and yield a zircon UPb age of ∼125.7 Ma. These results, along with previous studies on the YZSZ Mesozoic ophiolitic mélanges and the Gangdese arc, reveal that the western YZSZ Mesozoic ophiolites were likely generated over multiple stages in the epicontinental Gangdese fore-arc basin as the Yarlung Zangbo Neo-Tethyan Ocean subducted northward in front of the Lhasa terrane. The Early Cretaceous FAB-like and boninite series mafic rocks were formed by the reinitiation of subduction, which was followed by a retreat of the subduction zone and the creation of the fore-arc basin and strong hyperextension, accompanied by asthenosphere upwelling at ∼130–120 Ma. During this process, the upwelling asthenosphere underwent decompressional melting with limited penetration of slab-derived fluids and gave rise to the N-MORB (normal mid-ocean ridge basalt)-like basaltic magmas that intruded the overlying, previously generated depleted mantle as FAB-like gabbro, diabase and dolerite sills or dykes. Then, boninitic magmas represented by boninitic gabbro-diabases were generated by remelting the extremely depleted residual mantle source, which was metasomatized by a small amount of slab-derived fluids, following previous extractions of FAB-like magma.

Geochemistry and origin of a Mesozoic ophiolite: the Pounamu Ultramafics, Westland, New Zealand

ABSTRACTThe Pounamu Ultramafic Belt is an elongate zone of serpentinites and associated rocks within the Alpine Schist of north Westland, New Zealand. It includes metamorphosed pyroxenite–peridotite or boninite, gabbro and a breccia dominated by clasts of basalt and gabbro, overlain by pelagic sediments, a mid-Cretaceous tuff (108–105 Ma), and quartzofeldspathic turbidites with Cretaceous (< 105 Ma) detrital zircon minimum age populations. Rare earth element geochemistry indicates metabasites are similar to fractionated normal mid-ocean ridge basalt (N-MORBs), with minor enriched mid-ocean ridge basalt (E-MORB). However, because high field strength element concentrations of these metabasites are depleted relative to N-MORB, they have affinities to fore-arc magmas. The Pounamu ophiolite is likely, therefore, to have formed in a supra-subduction zone setting. Formation of oceanic crust and sedimentation during the Cretaceous is most likely in the re-entrant plate boundary zone between northwest and southeast Zealandia. Subsequent (c. 70 Ma) metamorphism occurred during accretion, subduction and interfolding of this exotic Pounamu Terrane with Rakaia Terrane-derived Otago Schist beneath western southeast Zealandia.

Metallogeny of the peridotite-hosted magnetite ores of the Nain ophiolite, Central Iran: Implications for Fe concentration processes during multi-episodic serpentinization

Magnetite bodies of the Late Cretaceous Nain ophiolite mélange (Central Iran) are hosted by a small volume of a serpentinized peridotite nappe. These ore bodies and their host peridotites have been studied in detail with respect to their mode of occurrence, petrography and mineral chemistry. The investigated ore deposits consist of m-scale lenses and pods of massive magnetite rocks exposed along a semi-brittle shear zone between pervasively serpentinized harzburgites, upper one with higher-Cr# [=Cr/(Cr + Al), 0.6–0.7] spinel and lower one with lower Cr# (0.5). Silicate mesostasis of the ore bodies is composed of stringy serpentine and chlorite. Cr-spinel grains are occasionally found dispersed within the magnetite rocks. Serpentines of the host serpentinized harzburgites show mesh and bastite textures, and were in part replaced by fibrous serpentine showing an interpenetrating texture. The Cr-spinels show a wide range of Cr# (0.5–0.7) and Mg# [=Mg/(Mg + Fe2+), 0.5–0.7], low TiO2 contents (<0.1 wt%), and relatively high contents of ZnO (0.11–0.26 wt%) and MnO (0.33–0.43 wt%). The magnetites are close in chemistry to the end-member Fe3O4, but some grains show high-SiO2 (up to 1.5 wt%) cores. They strongly resemble accessory Cr-spinels in the host serpentinized harzburgites in chemistry. The magnetite rocks have total precious-metal abundances (ΣPGE + Au = 66–262 ppb) analogous to those of mantle peridotites from other Mesozoic ophiolites. They are poor in PGE (21–67 ppb) relative to Au (45–195 ppb). Our analyses and observations are inconsistent with an igneous origin for the Nain magnetite deposits, but indicate a replacement origin from harzburgite, not from chromitite, via hydrothermal fluids. Overall data indicate that the Nain magnetite bodies were generated by multi-episodic serpentinization of harzburgites. The magnetite ore deformation style itself indicates syn-kinematic emplacement of the ore bodies within the semi-brittle shear zone, where the serpentinization-derived fluid/(altered) peridotite ratio became so high that eventually caused precipitation of magnetite at the ore level.

Nitrogen Concentrations and Isotopic Compositions of Seafloor-Altered Terrestrial Basaltic Glass: Implications for Astrobiology.

Observed enrichments of N (and the δ15N of this N) in volcanic glasses altered on Earth's modern and ancient seafloor are relevant in considerations of modern global N subduction fluxes and ancient life on Earth, and similarly altered glasses on Mars and other extraterrestrial bodies could serve as valuable tracers of biogeochemical processes. Palagonitized glasses and whole-rock samples of volcanic rocks on the modern seafloor (ODP Site 1256D) contain 3–18 ppm N with δ15Nair values of up to +4.5‰. Variably altered glasses from Mesozoic ophiolites (Troodos, Cyprus; Stonyford volcanics, USA) contain 2–53 ppm N with δ15N of −6.3 to +7‰. All of the more altered glasses have N concentrations higher than those of fresh volcanic glass (for MORB, <2 ppm N), reflecting significant N enrichment, and most of the altered glasses have δ15N considerably higher than that of their unaltered glass equivalents (for MORB, −5 ± 2‰). Circulation of hydrothermal fluids, in part induced by nearby spreading-center magmatism, could have leached NH4+ from sediments then fixed this NH4+ in altering volcanic glasses. Glasses from each site contain possible textural evidence for microbial activity in the form of microtubules, but any role of microbes in producing the N enrichments and elevated δ15N remains uncertain. Petrographic analysis, and imaging and chemical analyses by scanning electron microscopy and scanning transmission electron microscopy, indicate the presence of phyllosilicates (smectite, illite) in both the palagonitized cracks and the microtubules. These phyllosilicates (particularly illite), and possibly also zeolites, are the likely hosts for N in these glasses. Key Words: Nitrogen—Nitrogen isotope—Palagonite—Volcanic glass—Mars. Astrobiology 18, 330–342.

Geochemical and zircon U–Pb age constraints on the origin of the Mesozoic Xigaze ophiolite, Yarlung Zangbo suture zone, SW China

ABSTRACTThe Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U–Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174–149 Ma and 137–123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive εNd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle–Late Jurassic ophiolitic massifs (174–149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150–137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137–123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle.

Kalaymyo Peridotite Massif in the Indo‐Myanmar Ranges (Western Myanmar): Its Mineralogy and Petrology

Mesozoic ophiolites crop out discontinuously in the Indo‐Myanmar Ranges in NE India and Myanmar, and represent the remnants of the Neotethyan oceanic lithosphere (Sengupta et al., 1990; Mitchell, 1993). These ophiolites in the Indo‐Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet farther northwest (Mitchell, 1993; Fareeduddin and Dilek, 2015), as indicated by their coeval crystallization ages and geochemical compositions (Yang et al., 2012; Liu et al., 2016). The Kalaymyo ophiolite is located in the central part of the eastern Indo‐Myanmar Ranges (Fig. 1).composition of these ophiolites from the central Tibetan Plateau (CTP) is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs, which is inconsistent with most ophiolites worldwide (Robinson and Zhou, 2008; Zhang et al., 2008). But the generation and tectonic nature of these ophiolites are still controversial.The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse‐grained equigranular textures. They are composed of olivine (Fo = 89.8–90.5), orthopyroxene (En86‐91Wo1‐4Fs8‐10; Mg# = 89.6–91.9), clinopyroxene (En46‐49Wo47‐50Fs3‐5; Mg# = 90.9–93.6) and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole‐rock compositions with Mg#s of 90.1–90.8 and SiO2 (41.5–43.65 wt.%), Al2O3 (1.66–2.66 wt.%) and CaO (1.45–2.67 wt.%) contents.They display Light Rare Earth Element (LREE)‐depleted chondrite‐normalized REE patterns with (La/Yb)CN = 0.04–0.21 and (Gd/Yb)CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr)CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd‐enriched chondrite‐normalized PGE patterns with superchondritic (Pd/Ir)CN ratios (1.15–2.36). Their calculated oxygen fugacities range between QFM–0.57 and QFM+0.90.These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5–15%) of partial melting at a mid‐ocean‐ridge (MOR) environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB‐like melts, percolating through these already depleted, residual peridotites. The Kalaymyo and other ophiolites in the Indo‐Myanmar Ranges hence represent mid‐ocean ridge (MOR)–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.

186Os–187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys

Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history.Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356±21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361±20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292±25), and 186Os/188Os of 0.1198388±29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.

Stratigraphic units overlying the Zambales Ophiolite Complex (ZOC) in Luzon, (Philippines): Tectonostratigraphic significance and regional implications

The Zambales Ophiolite Complex (ZOC) on the island of Luzon, Philippines is one of the most well-studied crust-mantle sequences in the region. Several massifs comprise the ZOC, one of which is the Coto Block overlain by clastic sedimentary units previously dated as Eocene. Geochronologic studies from diabase, granodiorites and other late-stage magmatic products similarly yielded the same age. Succeeding tectonic models have therefore all been grounded on the assumption that the entire ZOC is Eocene.Recent investigations, however, revealed the presence of chert blocks within the Early to Middle Miocene clastic formation overlying the Acoje Block in the northern part of the ophiolite complex. Radiolarians extracted from the cherts yielded a stratigraphic range that suggests a Late Jurassic to Early Cretaceous age. The recognition of a much older age than previously reported of the ZOC warrants a re-examination of its actual distribution and genesis. Correlating with other similarly-aged ophiolites, we suggest defining a western Mesozoic ophiolite belt, largely extending from the west-central portion of the archipelago to the northeastern tip of Luzon island. Tentatively, we attribute the Mesozoic ophiolitic and associated rocks in western Luzon to an arc-continent collision involving the Philippine Mobile Belt and the Palawan Microcontinental Block.In addition, differences in the clastic compositions of the Cenozoic sedimentary formations provide material not only for deciphering the ZOC’s unroofing history but also for constraining the timing of province linkage. The intermittent appearance of lithic fragments and detrital minerals from the ophiolite in the units of the Middle Miocene Candelaria Limestone and the Late Miocene to Early Pliocene Sta. Cruz Formation indicates significant but geographically variable contributions from the ophiolite complex. In the northern Zambales Range, the Sta. Cruz Formation caps the Coto Block and the Acoje Block of the ZOC, providing a minimum age for their amalgamation.

Estuarine Lago Mare fauna from the Tertiary Piedmont Basin indicates episodic Atlantic/Mediterranean exchange during the final stage of the Mediterranean Salinity Crisis

The geochemical signature of fossils from Lago Mare deposits at Moncucco Torinese (NW Italy) indicates a temporary marine ingression into the Tertiary Piedmont Basin just before the Miocene/Pliocene boundary. Isotope (δ18O, δ13C, 87Sr/86Sr) and trace element (Sr/Ca, Ba/Ca) data from mollusk shells of multiple species and croaker otoliths indicate an upper estuarine environment with considerable seasonal variability in evaporation and freshwater input. Reconstructed salinities<10psu and low 87Sr/86Sr values indicate the close proximity of a watershed draining off of Mesozoic ophiolites and carbonates of the Alps and/or Apennines. Communication with an open marine environment is supported by otoliths of fully marine fishes such as lanternfishes, codlets, and slimeheads found in the same assemblage that carry a euhaline signature close to open ocean values. A thorough taphonomic evaluation renders reworking of the otoliths from pre-evaporitic deposits unlikely; instead we explain their presence in the Lago Mare assemblage as related to large predatory marine fish that periodically migrated into marginal marine and estuarine environments.The presence of a paralic paleobiotope with thalassogenic water in the Tertiary Piedmont Basin contradicts ideas of the Mediterranean as a brackish lake fed by the Paratethys during the latest stage of the Mediterranean Salinity Crisis. Instead, our data support previous notions of temporary Atlantic/Mediterranean exchange during sea-level highstands preceding the permanent re-connection at the Miocene/Pliocene transition. The establishment of short-lived and disjunct estuarine and lagoonal environments such as in the Tertiary Piedmont Basin during marine ingressions also puts highly variable 87Sr/86Sr values and occurrences of shallow water benthic foraminifera in Lago Mare deposits of other Mediterranean basins into a new perspective.

Platinum group placer minerals in ancient gold artifacts – Geochemistry and osmium isotopes of inclusions in Early Bronze Age gold from Ur/Mesopotamia

One of the most significant characteristics of the gold artifacts from the Early Dynastic Royal Tombs of Ur, Mesopotamia are numerous inclusions consisting of the platinum group elements (PGE) osmium–iridium–ruthenium. In nature, minerals of PGE (PGM) are enriched along with gold and other heavy minerals in placer deposits. During metallurgical gold extraction from placer material and subsequent production of artifacts, PGMs were incorporated in the gold artifacts due to their refractoriness almost unmodified. In order to evaluate their potential for provenance studies of gold, the PGE inclusions were analyzed for their chemical and Os-isotope compositions. They contain highly variable concentrations of Os (26–70 wt.%), Ir (14–62 wt.%) and Ru (0.4–45 wt.%). 187Os/188Os isotope ratios vary between 0.118 und 0.178. Due to the high Ru content of the alloys, the chemical composition point to a geological context of ophiolite complexes. Os isotope ratios are a powerful tool to narrow down the potential ore sources for the gold. However, the interpretation of calculated model ages is difficult due to the unknown genesis of the parental magma. Calculated ages (290–610 Ma) for measured 187Os/188Os of 0.125 using different reference values could indicate placers close to Paleozoic ophiolites like Samti (Takhar) in Northern Afghanistan and Zarshouran (Western Azerbaijan) in Iran, but need to be confirmed by additional measurements of their Os isotope signature in the future. Other archaeological relevant sources of PGM and gold could be excluded by direct comparison of their Os isotope data: 1.) old Neoproterozoic ophiolites from the Eastern Desert type (750–800 Ma), Egypt, 2.) young Mesozoic ophiolites from the Samail complex (96 Ma) in Oman. Thus, in combination with other tracers the Os isotope ratio is a valuable source for provenance studies.

Paleomagnetism of the Samar Ophiolite: Implications for the Cretaceous sub-equatorial position of the Philippine island arc

Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K–Ar dating of two basalt samples yielded an age of 100.2±2.7Ma and 97.9±2.8Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D=340°, I=−24°, k=15, α95=11° and tilt-corrected direction of D=342°, I=−27°, k=15, α95=11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S±6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S.

Middle to Late Cenozoic tectonic events in south and central Palawan (Philippines) and their implications to the evolution of the south-eastern margin of South China Sea: Evidence from onshore structural and offshore seismic data

Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.

Linking the NE Anatolian and Lesser Caucasus ophiolites: evidence for large-scale obduction of oceanic crust and implications for the formation of the Lesser Caucasus-Pontides Arc

In the Lesser Caucasus and NE Anatolia, three domains are distinguished from south to north: (1) Gondwanian-derived continental terranes represented by the South Armenian Block (SAB) and the Tauride–Anatolide Platform (TAP), (2) scattered outcrops of Mesozoic ophiolites, obducted during the Upper Cretaceous times, marking the northern Neotethys suture, and (3) the Eurasian plate, represented by the Eastern Pontides and the Somkheto-Karabagh Arc. At several locations along the northern Neotethyan suture, slivers of preserved unmetamorphozed relics of now-disappeared Northern Neotethys oceanic domain (ophiolite bodies) are obducted over the northern edge of the passive SAB and TAP margins to the south. There is evidence for thrusting of the suture zone ophiolites towards the north; however, we ascribe this to retro-thrusting and accretion onto the active Eurasian margin during the latter stages of obduction. Geodynamic reconstructions of the Lesser Caucasus feature two north dipping subduction zones: (1) one under the Eurasian margin and (2) farther south, an intra-oceanic subduction leading to ophiolite emplacement above the northern margin of SAB. We extend our model for the Lesser Caucasus to NE Anatolia by proposing that the ophiolites of these zones originate from the same oceanic domain, emplaced during a common obduction event. This would correspond to the obduction of non-metamorphic oceanic domain along a lateral distance of more than 500 km and overthrust up to 80 km of passive continental margin. We infer that the missing volcanic arc, formed above the intra-oceanic subduction, was dragged under the obducting ophiolite through scaling by faulting and tectonic erosion. In this scenario part of the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. Distal outcrops of this exceptional object were preserved from latter collision, concentrated along the suture zones.