Type Ophiolites Research Articles

Chemical constraints and tectonic setting of the Ginebra ophiolite complex

The occurrence and types of ophiolites in the Central Cordillera of the Colombian Andes have not been identified in terms of their tectonic settings and geochemical features. Therefore, the study of the tectonic setting and subduction zones in the ophiolites’ magmatic evolution becomes a significant issue. The Ginebra Ophiolitic Complex (GOC) is located on the western flank of the Central Cordillera in the Valle del Cauca Department, southwest of Colombia, assigned to the Lower Cretaceous period, as indicated by the contact of the GOC with the Buga Batholith. The GOC is formed by three main lithologies: amphibolite, gabbro, and ultramafic rocks (pyroxenite and peridotite), according to Ossa (2007). The rocks in the Puente Piedra section, located in the northeast of Ginebra municipality, are part of the gabbroic cumulates within the ophiolitic sequence. This sequence comprises interspersed layers of gabbroic cumulates, gabbrodiorites, and diorites; interpreted as a result of crystal accumulation and fractional crystallization processes. The mafic rocks of the GOC are sub-alkaline and correspond to the low potassium (K2O wt% 0.03-0.06) tholeiitic series. Geochemically, they have SiO2 wt% ranging from 49 to 61 and an aluminum oxide saturation index of approximately ~0.4 and ~0.8, indicating a metaluminous type. The geochemistry of the studied rocks from Puente Piedra section indicates that the GOC formed through fractional crystallization and accumulation processes from a single magma source. Cluster analysis, used to compare the geochemistry of GOC rocks and the Amaime Complex basalts, suggests a similar magma source, possibly linked to multiple recharge events that underwent fractional crystallization, melt extraction, and accumulation processes. The geochemical parameters are indicative of a suprasubduction zone ophiolite, characterized by a low potassium tholeiitic series affinity, TiO2 values typically <1.2 wt%, Th enrichment typical of subduction zones, high Pb content, and low values of Ti, Y, Yb, Ta, Nb, Zr, and Hf.

Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone: Implications for the Opening of the Neo‐Tethys Ocean

AbstractIn this study, zircon U‐Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone (YZSZ) in southern Xizang (Tibet) yielded an age of 247 ± 3 Ma. According to whole rock geochemical and Sr‐Nd‐Pb isotopic data, the Early Triassic samples could be divided into two groups: Group 1 with P‐MORB affinity, showing initial 87Sr/86Sr ratios of 0.70253–0.70602, εNd(t) values of 4.2–5.3, (206Pb/204Pb)t ratios of 16.353–18.222, (207Pb/204Pb)t ratios of 15.454–15.564, and (208Pb/204Pb)t ratios of 35.665–38.136; Group 2 with OIB affinity, showing initial 87Sr/86Sr ratios of 0.70249–0.70513, εNd(t) values of 4.4–4.9, (206Pb/204Pb)t ratios of 17.140–18.328, (207Pb/204Pb)t ratios of 15.491–15.575, and (208Pb/204Pb)t ratios of 36.051–38.247. Group 2 rocks formed by partial melting of the mantle source enriched by a former plume, and assimilated continental crustal material during melt ascension. The formation of Group 1 rocks corresponds to the mixing of OIB melts, with the same components as Group 2 and N‐MORBs. The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo‐Tethys in southern Xizang (Tibet).

The ephemeral history of Earth’s youngest supra-subduction zone type ophiolite from Timor

Ophiolites occur widely in orogenic belts, yet their origins remain controversial. Here we present a modern example with a geodynamic model from Timor, eastern Indonesia, where Earth’s youngest supra-subduction zone (SSZ)-type ophiolitic fragments are exposed. Zircon U-Pb ages and geochemical data indicate a short timespan (~10 to 8 Ma) for the magmatic sequence with boninitic and tholeiitic arc compositions. We interpret the Timor ophiolite as part of the infant Banda arc-forearc complex, which formed with the opening of the North Banda Sea and subsequent arc-continent collision along the irregular Australian continental margin. Our study connects the occurrence of small, short-lived ocean basins in the western Pacific with orogens around the globe where ephemeral SSZ-type ophiolites occur. These orogenic ophiolites do not represent preexisting oceanic crust, but result from upper-plate processes in early orogenesis and thus mark the onset of collision zone magmatism.

Development of a Modern-Style Trench-Arc-Backarc System in the Proto-Tethys Ocean (Qilian Orogenic Belt, NW China)

Abstract The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma.

Petrography and Geochemical Relationships of the Ultramafic Rocks in Galalah area within Erbil Governorate, NE Iraq

The current research to study the petrography, mineralogy and geochemical relationships of the ultramafic rocks in the Galalah area within Erbil Governorate, at the Unstable Shelf in the Imbricate Zone, to determine the origin of the ultramafic rocks. Dunite and Harzburgite are the most abundant rock bodies in the study area, and they represent most of the outer surface exposure of the ultramafic rock aggregates. The dunite rocks are characterized by pale brown color on the altered surface that is broken into olive green and dark green lithic. Dunite represents a homogenous lithologic feature that mainly consists predominately of olivine with accessory spinel and traces of orthopyroxene. While the harzburgite appears as a massive rock body consisting of olivine, orthopyroxene, and clinopyroxene. These rocks are mainly characterized by alteration to serpentinite, with accessory chromian spinel, secondary magnetite, chlorite, and talc. The studied ultramafic rocks are predominately rich in ferromagnesian minerals of less silica content, and other major elements affected by serpentinization are causing the deviation of elements. They have a strong decrease in sodium and calcium oxides relative to MgO and verified the silica and alumina with MgO and a slight increase in iron oxide. The low values of TiO2 in dunite and harzburgite are characteristic property of the Alpine type ophiolites during the primary process of the mantle in its upper part, before the tectonic emplacement of the ultramafic rocks, and also due to partial melting degree for the asthenospheric mantle. The MREE depletion relative to HREE and LREE may be attributed to the hornblende absence and the presence of olivine and pyroxenes minerals and/or to the source rock features. The U-shaped patterns of REE and their ratios are typical properties of ophiolitic ultramafic rocks, which are compatible with the supra-subduction zone. During the partial melting, the negative anomalies of Nb in ultramafic rocks with sequestration of the Nb concentration by sphene and ilmenite in various types of the source rocks have also been attributed to the crust contamination. These are also characterized by island arc conditions, and its depletion in the studied rocks may indicate the fore-arc tectonic environments

Coexistence of Carboniferous oceanic island basalts with Permian supra‐subduction zone ophiolites in the Changning–Menglian accretionary wedge: Implication for tectonic reconstruction

This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic.

Formation of Metamorphic Soles Underlying Ophiolites During Subduction Initiation: A Systematic Numerical Study

AbstractThe understanding of subduction initiation (SI) remains ambiguous due to limited geological records. The metamorphic sole, generally considered to be generated by oceanic crustal metamorphism during SI, is characterized by high temperature condition (∼800°C) at shallow depths (<40 km). However, the exact tectonic setting of the metamorphic sole with such a high geothermal gradient is still controversial. The petrological and geochemical signatures of ophiolites and metamorphic soles in nature indicate three different types: (a) supra‐subduction zone (SSZ)‐type ophiolite with mid‐ocean ridge basalt (MORB)‐type metamorphic sole; (b) SSZ‐type ophiolite with SSZ‐type metamorphic sole; and (c) MORB‐type ophiolite with MORB‐type metamorphic sole. To clarify the conditions of metamorphic sole generation in different tectonic settings, a series of numerical models are conducted. The model results indicate that the SI at a (back‐arc) spreading center or spontaneous SI at a transform fault provides the favorable high‐temperature condition for formation of the metamorphic sole underlying the ophiolite. The former regime generates SSZ‐type ophiolite with SSZ‐type sole, whereas the latter generates SSZ‐type ophiolite with MORB‐type sole. The P‐T conditions of natural metamorphic soles may not represent the characteristic subduction channel condition for the majority of ophiolites, but stand for the end‐member high‐temperature regime that facilitates weakening, detachment and further exhumation of metamorphic soles. It thus illustrates the less widely distributed metamorphic soles than ophiolites in nature. The model results are further compared with three present‐day back‐arc basins on the Earth to evaluate the likelihood of future metamorphic sole generation and preservation in these basins.

Supra‐subduction zone ophiolite from the Great Xing'an Range, China: Geochemistry, geochronology, and implication for formation in a back‐arc setting

Ophiolite provides key information on subduction–accretion processes along convergent plate margins. The Wunuer ophiolite in the Great Xing'an Range of NE China was considered to be an accreted fragment of the Palaeo‐Oceanic Plate. Based on field investigation, we evaluate the petrotectonic assemblages, geochemical features, and formation age of the ophiolite, which comprises gabbro, diabase, metabasalt, and radiolarian bedded chert with serpentinized amphibole‐pyroxene peridotites. Zircon U–Pb dating shows ages of 346 ± 6 Ma and 341 ± 6 Ma suggesting the timing of formation of the ophiolite as Early Carboniferous. The ophiolitic mafic rocks in Wunuer can be divided into two types based on their geochemical features. Type I is characterized by high Ti and low Al, with weakly negative Nb‐Ta and positive Pb anomalies and low light rare earth elements (LREE). These rocks display MORB‐like features, and correspond to partial melting product of depleted mantle. Type II has low Ti, high Al, notably negative Nb‐Ta and positive Pb anomalies, and is obviously enriched in LREE, large‐ion lithophile elements (LILE), and depleted in high‐field‐strength elements (HFSE), characterized by IAB‐like features, and may be formed by partial melting of more depleted mantle, which was strongly metasomatized by subduction fluids, resulted in high LILE/HFSE. The two types of ophiolites belong to SSZ type, obviously influenced by subduction, and their tectonic setting is inferred to be within a spreading ridge in a back‐arc basin. Combined with regional geological background, the Wunuer ophiolite probably represents a shrinking ocean basin and may be the late product of the long‐term and mature back‐arc basin, related to the terminal stage of ridge spreading within the back‐arc basin.

Geology of Zagros metamorphosed volcaniclastic sandstones: a key for changing the Mawat Ophiolite Complex to a metamorphic core complex, Kurdistan Region, NE-Iraq

Mawat Ophiolite Complex is located about 36 km to the northeast of Sulaimani city and directly to the east-northeast of Mawat town near the border of Iran in the northeastern Iraq. The complex has about 600-km2 surface area and consists of high mountain terrains that subjected to intense geological investigations from the fiftieth of previous century till now. According to previous studies, the complex contains tens of igneous rocks such as basalt, metabasalt, tuff, diabase, metadiabase, diorite dykes, periodotite, serpentinite, serpentinite-matrix mélange, gabbro, metagabbro, harzbergite, pyroxenite, plagiogranite, pegmatite, granitiod rocks and dunite. They added occurrences of the volcanic and subvolcanic rocks in the form of dykes or basaltic flows. The present study tries to change the petrology and tectonics of whole complex from Ophiolite Complex to Metamorphic Core Complex. The revision includes refusal of all the above igneous rocks, instead they considered as medium grade regional metamorphism of different types of volcaniclastic sandstones (volcanic wackes), arenites and greywackes (impure sandstones) which sourced predominantly from remote volcanic source area inside Iran. The revision depended on several conjugate field and laboratory evidences inside the complex. These evidences such as absence of pillow basalt, volcanic flows, glass shards, volcanic cones, dykes, sills, contact metamorphism, dilatational structures and flow structures. Other evidences are presence of cross beddings, erosional surfaces, lensoidal channel fills, metamorphosed conglomerate, exposures of thousands of laminated planar beds and transition from fresh volcaniclastic sandstones to its medium grade metamorphosed counterparts, which previously considered as igneous rocks of ophiolite types. Another, evidence, in contrast to ophiolite section, the basalt location is at the base of the claimed ophiolite section while plutonic (dunite and peridotite) rocks located at its top. These locations of the two rocks contradict the definition of ophiolites. Accordingly, the present study changed the geological map of the whole Mawat area from igneous outcrops to metamorphosed volcaniclastic sandstones, arenites and greywackes that belong to Walash-Naoperdan Series. The parent rocks of the series transformed to different types of regionally metamorphosed rocks by deep burial during Eocene. During the burial, diageneses and metamorphisms enhanced by complex mixture of materials from different source areas and seawaters environments. Later, they uplifted, unroofed and exhumed during Pliocene as a core complex.

The Middle-Late Cretaceous Zagros ophiolites, Iran: Linking of a 3000 km swath of subduction initiation fore-arc lithosphere from Troodos, Cyprus to Oman

Abstract New trace-element, radiogenic Sr-Nd-Pb isotopic and geochronological data from Middle-Late Cretaceous Zagros ophiolites of Iran give new insights into the tectono-magmatic history of these supra-subduction zone (SSZ)-type ophiolites. The distribution of Middle-Late Cretaceous SSZ-type ophiolites in Iran comprises two parallel belts: (1) the outer Zagros ophiolitic belt and (2) the inner Zagros ophiolitic belt. These Middle-Late Cretaceous ophiolites were generated by seafloor spreading in what became the fore-arc and back-arc during the subduction initiation event and now define a ~3000-km-long belt from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. The Zagros ophiolites contain complete (if disrupted) mantle and crustal sequences. Mantle sequences from both outer-belt and inner-belt ophiolites are dominated by dunites, harzburgites, and lherzolites with minor chromitite lenses. Peridotites are also intruded by gabbros and a variety of mafic to minor felsic (plagiogranite and dacite) dikes. Crustal rocks comprise ultramafic-mafic cumulates as well as isotropic gabbros, sheeted dike complexes, pillowed and massive lavas, and felsic rocks. Our new zircon U-Pb ages indicate that the outer-belt and inner-belt ophiolites formed near coevally during the Middle-Late Cretaceous; 100–96 Ma for the outer belt and 105–94 Ma for the inner belt. Both incompatible-element ratios and isotopic data confirm that depleted mantle and variable contributions of subduction components were involved in the genesis of outer-belt and inner-belt rocks. Our data for the outer belt and inner belt along with those from better-studied ophiolites in Cyprus, Turkey, the UAE, and Oman lead to the conclusion that a broad, ~3000-km-long swath of fore-arc lithosphere was created during Middle-Late Cretaceous time.

U - Pb zircon age of gabbro and plagiogranite in Hiep Duc, Quang Nam and their geological significances

The gabbro and plagiogranite magmas of the Ngoc Hoi and Dieng Bong complexes are mainly distributed in the northern part of the Kon Tum block. They were previously considered parts of the Tam Ky - Phuoc Son ophiolite complex. In this study, 02 samples of gabbro and plagiogranite were collected from the Hiep Duc area. Petrographic characteristics showed that the rocks were highly foliated and weakly metamorphosed; the schist formed after the crystallization of the rocks. U - Pb zircon age dating from the gabbro rocks as 497.7±1.4 Ma, similar to the plagiogranite age of 498.0±1.3 Ma. The available results in the northern Kon Tum block and Laos indicate the existence of magma series formed during the Late Cambrian period that is probably extended from the northern Kon Tum block to the northeastern part of Laos. The research results on the northern Kon Tum block also confirmed two types of magma in the area: island - arc magma complex and ophiolite type magma complex.

Neoproterozoic Trench‐arc System in the Western Segment of Jiangnan Orogenic Belt, South China

AbstractThe Jiangnan orogenic belt is a key to understanding of the Neoproterozoic tectonic evolution of the South China Block. We investigate the mafic‐ultramafic suites of lherzolite, pyroxenite, gabbro, pillow basalt and gabbroic diorite as well as red jasper interbedded with marine marbles that are mainly exposed as fault‐trapped blocks in the Yuanbaoshan and Longsheng domains of the western Jiangnan belt. The post‐collisional granite plutons that intruded the ultramafic‐mafic rocks are developed well. Zircons in the gabbro yield crystallization ages of 867±10 Ma, 863±8 Ma, 869±9 Ma and 855±5 Ma whereas those from the granites show ages of 823±5 Ma, 831±5 Ma, 824±5 Ma and 833±6 Ma. The Neoproterozoic serpentinited ultramafic samples display minor REE enriched pattern with depletion of Rb, Ba, Nb, Ta and Ti, similar to those of SSZ type ophiolite. The coeval gabbro shows tholeiitic features and is characterized by negative Ba, Nb, Ta, Zr, Hf and Sr anomalies and LREE enriched patterns, with a minor negative Eu anomaly. Some zircon grains from the Longshen gabbro yield Neoarchean‐Paleoproterozoic ages (2859–2262 Ma), suggesting its continental arc setting. Geochemical signature of the mafic‐ultramafic rocks is consistent with subduction related setting. The pyroxene‐bearing diorite exhibits a distinctive arc affinity. The zircons from the gabbro show positive ∊Hf(t) values ranging from 3.9 to 13.8. The granitoids are typical S–type granites with high ACNK values (1.15–1.40) and negative ∊Hf(t) values (−15.1 to −3.2), and are classified as collision–related granites. Combined with the occurrences of mafic‐ultramafic rocks, siliceous marble and red jasper mixed with basalt, our new results suggest the presence of a Tonian (863–869 Ma) SSZ ophiolite system and continental arc‐type magmatism in the western Jiangnan orogen.

Introduction of meta-harzburgites using thermal originated antigorites in the Gishaki Ophiolitic area, as a part of Iranian Alpine type Ophiolites.

In this paper, we investigated two originated metamorphosed harzburgites of the metaperidotite complex from the northern part of the Gishaki area. Harzburgite rocks, as main ultramafic complexes of the studied area, have outcrops in the scattered locations of the area. Harzburgitic units were discriminated using petrography and Raman spectroscopy. There are two meta-harzburgite types based on the metamorphic history of their protholites with different mineralogy types. Normal metamorphosed harzburgites (N-Hz) are metamorphosed via phases of retrograde processes and contact metamorphosed harzburgites (C-Hz) metamorphosed visa some different phases containing contact metamorphism. One of the best clues that are different in these two rock types is Antigorite mineral characteristics. Antigorite polymorphs have structural and chemical sensitive characteristics that can discriminate using Raman spectroscopy. It had certain specific chemical differentiation compared with other serpentine polymorphs especially in Silica and molecular water contents. Reviewed antigorites illustrated general differentiations in themselves so that polymorphs hosted by C-HZ rocks could be considered as a group with typical Contact antigorite by geochemical criteria such as lower molecular water, higher Octahedral content and, higher Silica content. An increase in Si and Fe2+ activities cause incurrences octahedral substitutions as temperature-dependent phenomena in Contact antigorites. Diagrams of Silica versus molecular water contents and octahedral contents clearly distinguished modified antigorites from former antigorite polymorphs. Contact antigorite in the Gishaki serpentinites could be considered as metamorphosed products related to metamorphic aureoles.

Genesis and tectonic setting of high-Cr podiform chromitites of the Rayat ophiolite in the Zagros Suture Zone, northeastern Iraq

Chromitite pods hosted by serpentinized harzburgites of the Rayat ophiolite in the Zagros suture tectonic zone, northeasten Iraq, were studied petrographically and geochemically. These chromitites comprise high-Cr podiform ophiolitic chromite deposit and consist of magesiochromite in a matrix dominated by serpentine with minor carbonate minerals. The chromitites contain 40.2 wt% Cr2O3, 15.8 wt% MgO, 11.4 wt% Al2O3, and 16.7 wt% Fe2O3t, and lesser amounts of the other major oxides and traces of Ba, Co, Cu, Ga, Mn, Ni, Rb, Sc, Sr, Ti, V, and Zn. Magnesiochromites of chromitites contain Cr2O3 (55.6 wt%), MgO (15.2 wt%), FeOt (13.8 wt%), and Al2O3 (14.3 wt%), and minor amounts of Ni, Ti, Mn and V; their average Cr# and Mg#are 0.72 and 0.71, respectively. Accessory chromite in the serpenitinized harzburgite contains 56.9 wt% Cr2O3, 9.9 wt% MgO, 19.6 wt% FeOt, and 12.7 wt% Al2O3; with Cr# and Mg# of 0.75 and 0.49, respectively. The relatively high concentrations of compatible trace elements which have strong affinity to Cr and Mg such as Ni, V, Co, and the low concentrations of incompatible LILE (K, Rb, Sr, and Ba) elements as well as the high concentrations of mantle rock-forming elements (MRFE: Si, Al, Fe, Mg, Ca, Cr, Ni, Zn, Co, and Cu) is a characteristic feature of the Rayat chromitite and indicates mantle origin. The chromitites also contain very low Au (3 ppb) and PGE (53 ppb). The IPGEs (Os, Ir, and Ru) are enriched and are ~ 8 times higher than the PPGEs (Rh, Pt, and Pd), with a positive peak of Ru and a negative peak of Pt in the chondrite-normalized diagram. The chondrite-normalized PGE pattern has generally lower concentrations but similar trend to most high-Cr chromitites of the world. The chromitites belong to Group-I “normal ophiolite type” indicated from the Ptn/Pdn vs. Ptn/Irn diagram. Various tectonic discrimination diagrams including the calculated parental melt compositions of Al2O3, TiO2 and FeOt/MgO for the magnesiochromites and chromites suggest that the Rayat chromitites were derived from boninitic magmas within suprasubduction zone in a fore-arc tectonic setting during the formation of the Taurus (Turkey) - Zagros (Iran/Iraq) Ophiolite Belt.

Mineral Chemistry Perspective of Nain Ophiolite Mélange, Central Iran

The present study documents detailed mineral chemi-stry perspective of Nain ophiolite melange (NOM) of Central Iran with an aim of deciphering the mineral systematics and understanding geothermobarometric equilibration. The NOM covers ~600 km2 and is located at the northwest margin of Central Iranian Microcontinental block. NOM is represented by a sheared, tectonized and serpentinized peridotite in-truded by coarse-grained pegmatitic gabbroic dykes, layered gabbro, sheeted dolerite dykes (with typical rodingite alteration) and pillow basalts. Plagioclase in pillow basalt is albitic and indicates its spilitic affinity, while pyroxene is typically quad pyroxene (augite to diopside). Amphiboles belong to calcic group and range from actinolite to magnesio hornblende. Ilme-nite is the characteristic opaque phase. Clinopyroxene thermometry records a temperature span of 1100–1300C, while amphibole thermometry records 979–1145C. Two-feldspar thermometry also records a similar thermometric range. Amphibole barometry shows higher pressure of equilibration for mantle pegmatite in general and a very low equilibration pressure for sheeted dyke. Pyroxene compositions typically indicate a calc-alkaline basaltic (orogenic) parentage. NOM signifies lherzolite ophiolite type in a chromite-free environment and it is analogous to an idealized ophiolite succession, but has been emplaced in the form of discrete tectonic melange.

Geochemical Characteristics of the Gabbros from the Xinlin Ophiolite in the Great Xing'an Range, NE China

AbstractThe Xinlin ophiolite in NE China is generally considered to mark the suture between the Erguna and Xing'an blocks. Compared with the Maihantewula ophiolite and Jifeng‐Gaxian ophiolite in the southern and central parts of the Xinlin–Xiguitu suture zone, the Xinlin ophiolite in the northern part of the suture has not been as thoroughly investigated. Many studies acknowledge the indicators of the Xinlin ophiolite as a suture, but detailed studies of this unit are scarce. In the present work, we provide the geochemical data to constrain the origin of the gabbros in Xinlin ophiolites. The gabbros from the Xinlin ophiolites are texturally heterogeneous, ranging from fine‐grained aplitic to coarse‐grained pegmatitic. The fine‐grained gabbros have flat to slightly enriched LREE patterns, which are geochemically comparable to transitional (T‐MORB) and enriched mid‐ocean ridge basalt (E‐MORB). The pegmatite gabbros exhibit slightly LREE‐depleted patterns, similar to typical N‐MORB that derived from a depleted mantle source. Generally, gabbros from the Xinlin ophiolites are MORB‐like, but also have some arc characteristics such as high Th and low Ta concentrations. Such features is typical in Supra‐subduction zone (SSZ) type ophiolites. Our data, combined with other regional results, suggest that the geochemical signatures of the Xinlin gabbros that vary between arc‐like and MORB‐like were possibly indicative of their derivation from a subduction‐modified depleted mantle.

Early Cambrian Muli arc–ophiolite complex: a relic of the Proto-Tethys oceanic lithosphere in the Qilian Orogen, NW China

The Qilian Orogen in the northern margin of the Tibetan Plateau is the northernmost of the Tethyan domain. Abundant ophiolites record the closure of an early Tethyan ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. The Muli arc–ophiolite complex in the western segment of the South Qilian belt represents remnants of the Proto-Tethyan oceanic lithosphere. It comprises serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert, which are in tectonic contact with Upper Ordovician turbidites. Basalts have typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite indicate that the Muli arc–ophiolite complex represents a super-subduction zone (SSZ)-type ophiolite. U–Pb zircon data indicate formation associated with southward subduction of the Proto-Tethys Ocean during a short interval between 539 and 522 Ma. Results of petrology, geochemistry, and zircon U–Pb dating demonstrate that granitoids intruded into this complex are Middle to Late Ordovician (470–450 Ma) products of subduction-related arc magmatism. Voluminous Late Ordovician–Early Silurian rocks include deep-water marine siliciclastic and volcaniclastic turbidites and abundant volcanic arc rocks located to the south of the Muli arc–ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian–Middle Ordovician ophiolite–arc systems in the eastern South Qilian belt. This indicates that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.

Geochronology, geochemistry and tectonic implications of a new ophiolitic mélange in the northern West Junggar, NW China

The West Junggar, located in the southernmost part of the Central Asian Orogenic Belt (CAOB), is a key region for understanding the Paleozoic evolution of the CAOB. Issues of the timing of initial subduction and tectonic unit connections in northern West Junggar still remain controversial. In this study, we report a new ophiolitic mélange named the E'min ophiolitic mélange in northern West Junggar. The tectonic blocks in the E'min ophiolitic mélange are mainly composed of serpentinized peridotite, serpentinite, gabbros, pillow basalts, and cherts, with a matrix consisting of highly deformed serpentinites. A gabbro exhibits a zircon SHRIMP U-Pb age of 476 ± 2 Ma, and the zircon grains have δ18O values similar to those of mantle zircons. Those basalt samples display depletions of light rare earth element (REE) relative to heavy REEs. They exhibit weak enrichment of Ba and Th, and moderate depletion of Nb and Ta. The basalts display similar geochemical characteristics to that of fore–arc basalts in the present-day fore–arc setting. The gabbros exhibit high MgO and compatible element contents, but low TiO2, total REE and high field strength element (HFSE) contents. They exhibit light REE depletion, enrichment in large-ion lithophile elements, and depletion of HFSEs. The boninite-like geochemical patterns of the gabbros indicate that they were formed in a subduction-related environment, and were derived from an extremely depleted mantle source infiltrated by subduction-derived fluids and/or melts. The E'min ophiolitic mélange has a geochemical make-up similar to those of suprasubduction-zone (SSZ)-type ophiolites formed in a forearc setting. Hence, we propose that the E'min ophiolitic mélange formed in a forearc setting and may represent the initial subduction in northern West Junggar. Based on geochronological data, we propose that the E'min ophiolite, together with the Kujibai, Hoboksar and Hongguleleng ophiolites, formed during a similar period and comprise a huge E–W trending ophiolitic belt.

Ophiolites of the Central Asian Orogenic Belt: Geochemical and petrological characterization and tectonic settings

We present a compilation of published data (field, petrography, ages and geochemistry) from 73 ophiolitic complexes of the Central Asian Orogenic Belt. The ophiolitic complexes, ranging in age from Neoproterozoic to Triassic, have been geochemically classified as subduction-related and subduction-unrelated categories applying recent, well-established discrimination diagrams. The subduction-unrelated category is further subdivided into Mid-Ocean Ridge type (MOR), a common rift-drift stage and Plume type, and the subduction-related category is subdivided into Backarc (BA), Forearc (FA), Backarc to Forearc (BA-FA) and Volcanic Arc (VA) types. The four subduction-related types define highly different geochemical features, with the BA and FA types defining end members showing subduction influence of 10%–100% and 90%–100% subduction influence, respectively, and the two other types (BA-FA and VA) define values between the two end members. The subduction-related category comprises 79% of the examined ophiolites, of which the BA type ophiolites is by far the dominant group, followed by the BA-FA type, and with FA and VA types as subordinate groups. The Neoproterozoic and Ordovician complexes exhibit the highest, whereas those of Silurian age exhibit the lowest subduction-influence. Of the remaining 21% subduction-unrelated ophiolites, the MOR type dominates. Both the subduction-related and subduction-unrelated types, in particular the latter, are commonly associated with alkaline basalts taken to represent ocean island magmatism. Harzburgite, dunite, gabbro and basalt are the common lithologies in all ophiolite types, whereas the BA-FA, FA and VA types generally contain intermediate to felsic rocks, and in the FA type boninites occur. The subduction-related ophiolites types generally show low metamorphic grade, whereas greenschist, amphibolite and blueschist grades occur in the subduction-unrelated and BA types. The highly different subduction contribution (from 0 to 100% in the MOR and FA, respectively), attest to variable dips of the subducting slab, as well as variable flux of subduction-related elements into the mantle above subducting slabs, from where the ophiolite magmas got their geochemical fingerprints. As most MOR ophiolites get subducted to the deep mantle, the subduction-related ophiolites have become a dominant ophiolitic type on Earth's surface through all times supporting the idea about the early start of Plate Tectonics.

Petrogenesis of plagiogranite and associated diorites and mafic rocks in the Habana–Matanzas ophiolites, northwestern half of central Cuba

Structural field relationships, petrography and geochemical features of plagiogranites and associated intermediate (diorite) to mafic rocks (basalt and gabbro) of the Habana–Matanzas ophiolites in northern Cuba suggest that these felsic and mafic rock units represent a boundary case between forearc and volcanic arc ophiolite types. Subduction-related basalts can be subdivided into back-arc, back-arc to forearc, forearc and volcanic arc types. The majority of the basalts plot in the oceanic arc (as well as joint oceanic arc–continental arc) field of the Th/Yb–Nb/Yb diagram. The samples that classify as volcanic arc basalts plot in the continental arc field of the Th/Yb–Nb/Yb diagram and within the island arc tholeiite and boninite fields in the V–Ti diagram. The Habana–Matanzas plagiogranites and associated basalts, diabases and gabbros display significantly negative Ta–Nb anomalies and are depleted in heavy rare earth elements. 40 Ar/ 39 Ar whole-rock plateau ages of these plagiogranites reveal minimum crystallization ages of 98.11 ± 0.50 Ma, providing the first important time constraint for suprasubduction zone oceanic crust formation on the island of Cuba. Based on our petrogenetic modelling and geochemical characterization, we conclude that magmas of the plagiogranites and associated intermediate rocks of the Habana–Matanzas ophiolites were derived from partial melting of highly depleted, pre-existing crustal rocks (basalt and/or gabbro). Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways