Subduction Of Neo-Tethyan Ocean Research Articles

Biotite Geochemistry and Its Implication for the Difference in Mineralization in the Xiongcun Porphyry Cu–Au Ore District, Tibet

The Xiongcun Cu–Au ore district is in the southern middle Gangdese Metallogenic Belt, Tibet, and formed during Neo-Tethyan oceanic subduction. The Xiongcun ore district mainly comprises two deposits, the No. I and No. II deposits, which were formed by two individual mineralization events according to deposit geology and Re–Os isotopic dating of molybdenite. The No. I deposit is similar to a reduced porphyry copper–gold deposit, given the widespread occurrence of primary and/or hydrothermal pyrrhotite and common CH4-rich and rare N2-rich fluid inclusions. The No. II deposit, similar to classic oxidized porphyry copper–gold deposits, contains highly oxidized minerals, including magnetite, anhydrite, and hematite. The halogen chemistry of the ore-forming fluid from the No. I and No. II deposits is still unclear. Biotite geochemistry with halogen contents was used to investigate the differences in ore-forming fluid between the No. I and No. II deposits. Hydrothermal biotite from the No. I deposit, usually intergrown with sphalerite, is Mg-rich and classified as phlogopite and Mg-biotite, and hydrothermal biotite from the No. II deposit is classified as Mg-biotite. Hydrothermal biotite from the No. I deposit has significantly higher SiO2, MnO, MgO, F, Li, Sc, Zn, Rb, Tl, and Pb contents and lower Al2O3, FeOtot, Cl, Ba, Cr, V, Co, Ni, Y, Sr, Zr, Th, and Cu contents than the biotite from the No. II deposit. Hydrothermal biotites from the No. I and No. II deposits yield temperatures ranging from 230 °C to 593 °C and 212 °C to 306 °C, respectively. The calculated oxygen fugacity and fugacity ratios indicate that the hydrothermal fluid of the No. I deposit has a higher F content, oxygen fugacity, and log(fHF/fHCl) value and a lower log(fH2O/fHF) value than the hydrothermal fluid from the No. II deposit. The biotite geochemistry shows that the No. I and No. II deposits formed from different hydrothermal fluids. The hydrothermal fluid of the No. I deposit was mixed with meteoric waters containing organic matter, resulting in a decrease in oxygen fugacity and more efficient precipitation of gold. The No. I and No. II deposits were formed by a Cl-rich hydrothermal system conducive to transporting Cu and Au. The decreasing Cl, oxygen fugacity, and temperature may be the key factors in Cu and Au precipitation. Biotite geochemistry allows a more detailed evaluation of the halogen chemistry of hydrothermal fluids and their evolution within porphyry Cu systems.

Origin and Geodynamic Mechanism of the Tibetan Demingding Porphyry Mo (Cu) Deposit from Oceanic Subduction to Continental Collision

Demingding is a promising porphyry Mo-dominated deposit recently discovered in the eastern Gangdese metallogenic belt in Tibet, China. We present zircon U-Pb-Lu-Hf isotopic studies, as well as geochemical data of the late monzogranites and the prior rhyolites from the Demingding porphyry deposit to uncover their origin and geodynamic mechanism. Zircon U–Pb dating yielded precise crystallization ages of 17.3 ± 0.6 Ma (MSWD = 2.5) and 186.5 ± 3.0 Ma (MSWD = 2.0) for monzogranite and rhyolite, respectively. The monzogranite is characterized by high-K calc-alkaline, adakitic affinities, and positive zircon εHf(t) values (+0.9∼+5.6, avg.+3.1) with TDM2 (0.73–1.04 Ga), while the rhyolite has εHf(t) values of (+2.1∼+7.3, avg.+5.2) and TDM2 of (0.76–1.09 Ga) similar to the monzogranite. Our results suggest that the Demingding porphyry Mo (Cu) deposit is related to magma generated from the Neo-Tethyan oceanic subduction. The subsequent monzogranite porphyry was likely formed by the remelting of previously subduction-modified arc lithosphere, triggered by continental collision crustal thickening in Miocene. The lower positive εHf(t) values of monzogranites suggest minor inputs from the Mo-rich ancient crust, suggesting that Mo favors the silicate melt. Such magmatic events and special metallogenesis typify intracontinental processes and porphyry copper deposits, which are normally confined to oceanic subduction and Cu-dominated style, thereby making the continental setting and Mo-dominated style of Demingding exceptional and possibly unique.

Late Mesozoic-Cenozoic multistage exhumation of the central Bangong-Nujiang Suture, Central Tibet

The Cenozoic rise of the Tibetan Plateau has significantly altered global climate change and the biological redistribution of Asia. However, there remain great controversies about its detailed topographical evolution, especially in the hinterland of the Plateau, such as around the Bangong-Nujiang Suture (BNS). Here, we choose the Gaize Basin, which is located in the central BNS, as our target research area. New low-temperature thermochronology data have been used to quantify the cooling history of the intrusive rocks and sedimentary strata from the N-S edges of the Gaize Basin. Our thermochronology results show that 1) zircon (UTh)/He ages range from 54 ± 3.7 to 113 ± 8.4 Ma, 2) apatite fission track ages mostly cluster around ~55 Ma, and 3) apatite (UTh)/He ages are between 34 ± 3.3 and 63 ± 4.4 Ma. Combined with the results from thermochronology modelling, we suggest that there were three cooling stages recorded around the Gaize Basin: 1) the late Mesozoic (120–100 Ma), which was probably due to the BNS closure and subsequent Qiangtang-Lhasa collision, 2) the Late Cretaceous-early Paleogene (85–55 Ma), which was perhaps related to the far-field effects of the Neo-Tethyan oceanic subduction, and 3) 47–30 Ma, in response to the continuous India-Asian collision. Similar to today, low relief topography has finalized in the central Tibetan Plateau after the last cooling stage and is mainly aided by river incision and erosion on both sides of the BNS.

Tectonic transition from Paleo- to Neo-Tethyan Ocean in Tangjia-Sumdo area, Southern Tibet: Constraints from Early Jurassic magmatism

The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 ± 1.1 Ma), monzogranites (194.3 ± 1.1 Ma and 200.5 ± 1.2 Ma), and granites (184.5 ± 2.1 Ma, 185.5 ± 4.0 Ma and 185.6 ± 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated εHf(t) values of the diorite are − 16.7 to − 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block.

Geochronology, whole-rock geochemistry, Sr[sbnd]Nd isotopes, and biotite chemistry of the Deh-Bala intrusive rocks, Central Urumieh-Dokhtar Magmatic Arc (Iran): Implications for magmatic processes and copper mineralization

The Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran hosts a variety of volcano-plutonic rocks of different composition and age formed by subduction of Neo-Tethyan Ocean beneath Eurasian supercontinent and the subsequent processes of post-subduction magmatism. This study focuses on mineral chemistry of biotite and amphibole, zircon UPb geochronology, whole-rock geochemistry, and SrNd isotopic geochemistry in the Deh-Bala granitoid, in an attempt to investigate the porphyry-epithermal mineralization potential. The Deh-Bala intrusions in the central part of the UDMA consist mainly of granodiorite and minor tonalite, with microgranular enclaves (MEs) of gabbrodiorite, diorite, and monzodiorite composition. In addition, the main ore minerals associated with more altered and mineralized varieties of these intrusions include chalcopyrite, covellite, malachite, and pyrite. The silicic, argillic, phyllic (sericitic), and propylitic facies of alteration occur in parts of the Deh-Bala area; these types of alteration, which are typically considered the most important predictors of porphyry copper‑gold and (or) epithermal precious metal mineralization.LA-ICP-MS UPb dating of zircons reveals that the pluton was emplaced during Middle-Eocene (ca. 39.0 Ma). The Deh-Bala granitoids (DBG) are calc-alkaline in composition (Na2O + K2O: 4.8–7.3 wt%) and display SiO2 contents from ∼53 to ∼67 wt%. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with marked Eu anomalies. They are also characterized by an enrichment in large-ion lithophile elements (LILEs, such as Rb, K, and U) and a depletion in high-field strength elements (HFSE, such Nb, Y, Ti, and Zr), displaying obviously depletions with Ba, P, and Sc. The DBG shows whole-rock initial 87Sr/86Sr ratios (ISr) of 0.7048–0.7061, εNd(t) values of −1.79 to +1.64, and Nd two-stage depleted mantle model ages (TDM2) from 747 to 1004 Ma. These isotopic data, combined with the geochemical signatures, indicate that the DBG originated through mixing of mafic and felsic end-members, the former was derived from melting of the lithospheric mantle, and the latter derived from partial melting of Neoproterozoic crustal materials in a continental-arc.Biotite compositions analysed by electron microprobe are consistent with the Deh-Bala magmas being oxidized and chlorine-rich, similar to worldwide porphyry copper‑gold systems formed in arc-related settings associated with convergent margin subduction zones. Such a chlorine-rich magmatic-hydrothermal system is known to be effective in scavenging copper and (or) gold from melt and its transfer to shallow levels.

Early mesozoic arc–back-arc system in the leading edge of the Tibetan Plateau

The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.

Initial subduction of Neo-Tethyan ocean: Geochemical records in chromite and mineral inclusions in the Pozantı-Karsantı ophiolite, southern Turkey

Chromitites in the Pozantı-Karsantı ophiolite in Turkey mainly occur as podiform chromitites within mantle harzburgite and stratiform-like chromitites in mantle-crust transition zone. Chromites in chromitites have varing Cr# from 62.8 to 80.3 and can be divided into two types, namely; intermediate (Cr#: 62.8 – 69.2) and high-Cr (Cr#: 73.9 – 80.3) types. Major elements of the high-Cr chromitite have an affinity with boninite, whereas the intermediate chromitite shows transitional features between MORB and boninite. The compositional differences in clinopyroxene inclusions between intermediate- and high-Cr chromitite, coupled with the relatively high trace element contents (e.g. V, Ga) in the high-Cr chromitite, indicate distinctive parental magmas. Trace elemental profile analysis of a nodular chromite grain in one nodular chromitite sample PK14-41 demonstrates significant but non-systematic variations from the core to the rim, which also confirmed the compositional heterogeneity of the parental magmas. The presence of primary hydrous mineral inclusions such as amphibole in chromite, together with Ca-rich minerals (e.g. calcite), reflect the water-rich and Ca-rich characteristics of the parental magma. The higher fO2 of high-Cr chromitite evidenced by the lower V/Mn values may be due to more oxidized fluids released from downgoing crustal materials. Thus, we conclude that the parental magmas of the Pozantı-Karsantı chromitite were derived from a proto-forearc mantle and evolved to higher fO2 with the subduction initiation at that time, but were water- and Ca-rich in general.

Apatite and zircon geochemistry of Jurassic porphyries in the Xiongcun district, southern Gangdese porphyry copper belt: Implications for petrogenesis and mineralization

The Xiongcun district, a unique area where oxidized porphyry copper-gold deposits (OPCD) and reduced porphyry copper-gold deposits (RPCD) coexist, is located in the central part of the Gangdese porphyry copper belt (GPCB) in Tibet, China, and hosts the only known mineralization related to the Neo-Tethyan oceanic subduction. Apatite and zircon major and trace elements, in situ oxygen isotopes in zircon from fertile quartz diorite porphyry (QDP) of the No. I deposit, fertile hornblende quartz diorite porphyry (HQDP) of the No. II deposit, and barren hornblende quartz diorite porphyry with coarse quartz eyes (QQDP) in the Xiongcun district were determined by EPMA, LA-ICP-MS, and SHRIMP II ion microprobe to investigate the petrogenesis, magma oxidation state, and mineralization differences between the No. I and No. II deposits, as well as the potential of apatite as a metallogenic indicator. The results from this study show that a combination of REE, Ti, oxygen isotopes, and the Th/U ratios in zircon and REE as well as Sr in apatite can be used to track the magma compositions and crystallization histories in porphyry systems. Multiple indicators, including the apatite SO3 content, apatite Mn oxybarometer, and zircon Ce4+/Ce3+ ratio, indicate that (1) the parental magmas of fertile QDP and HQDP containing Cu and Au mineralization were more reduced than that of barren QQDP; (2) despite barren QQDP having higher oxygen fugacity than QDP and HQDP, the lower Cu and water contents in the magma result in a lower potential for ore formation; and (3) the similar oxidation states of QDP and HQDP show that the mineralization differences between RPCD (No. I deposit) and OPCD (No. II deposit) were not caused by the oxidation states of the magmas but were likely caused by the different processes of hydrothermal fluid evolution. Apatite grains from fertile porphyries (QDP and HQDP) have much higher Cl/F ratios than those from barren porphyry (QQDP), indicating that the Cl/F ratio of apatite can be used as a proxy for porphyry Cu-Au mineralization related to the Jurassic porphyry intrusions in the Gangdese porphyry copper belt.

Petrogenesis of late Eocene high Ba-Sr potassic rocks from western Yangtze Block, SE Tibet: A magmatic response to the Indo-Asian collision

The Indo–Asian collision resulted in extrusion of the Indochina Block along the Ailao Shan–Red River (ASRR) shear zone in the Cenozoic, with the emplacement of widespread potassic magmatic rocks. In this contribution, we investigated five potassic felsic intrusions exposed in the western Yangtze Block adjacent to the ASRR shear zone, including the Xiaoqiaotou, Jianchuan, Yuzhaokuai, Laojunshan and South Taohuacun intrusions. New LA–ICP–MS zircon U–Pb results in combination with previous data indicate that these felsic rocks have identical crystallization ages of ∼36–35Ma. They are characterized by high Ba (mostly >1500ppm) and Sr (mostly >1000ppm) abundances, with high K2O contents and K2O/Na2O ratios. They exhibit similar Sr-Nd isotopic components as the coeval shoshonitic mafic rocks exposed in the studied area. Elemental and isotopic data suggest that the five intrusions were likely derived from fractional crystallization of shoshonitic mafic magmas originating from an enriched lithospheric mantle. On the basis of previously published data and results in this paper, we considered that the lithospheric mantle underneath the western Yangtze might have undergone enrichment events twice at least, including the Neoproterozoic oceanic subduction and the Neo-Tethyan oceanic subduction.

Metallogenesis and the minerogenetic series in the Gangdese polymetallic copper belt

The Gangdese is a newly explored porphyry Cu ore belt in China. In fact, except for the porphyry Cu deposits there are a lot of other deposits, forming a polymetallic copper belt in the southern Gangdese (SG, i.e., southern Lhasa terrane) and Gangdese back–arc fault uplift belt (GBAFUB, also called Lungar–Nyainqêntanglha arc). The Gangdese polymetallic copper belt contains porphyry Cu(–Mo–Au), Cu–Au and Mo(–Cu), skarn Cu(–Mo–Au–W), Pb–Zn(–Ag), Fe(–Cu) and Mo–W, breccia-type and hydrothermal vein-type Pb–Zn(–Ag) deposits. The porphyry Cu–Au mineralization occurred at the Xiongcun area of SG during the Middle Jurassic (175–160Ma) and was associated with island arc magmatism related to the northward subduction of the Neo–Tethyan oceanic slab. The Cretaceous (ca. 112Ma) skarn Fe(–Cu) mineralization appeared at the Nixiong area of GBAFUB and was generated in a back arc extensional setting related to the Neo–Tethyan oceanic subduction beneath the Lhasa terrane. The Paleocene–Eocene (65–51Ma) porphyry Mo(–Cu), skarn Mo–W, and skarn Pb–Zn(–Ag) deposits are located in the eastern part of GBAFUB and related to granitoids originating from the partial melting of the old crust with contributions from the mantle. In contrast, the Paleocene–Eocene (60–45Ma) porphyry Cu, skarn-, breccia-, and hydrothermal-vein-type Pb–Zn(–Ag), and skarn Fe–Cu deposits at the Namling area were associated with granitoids derived from the partial melting of the mantle with some input of the old crust. The Oligocene (30–23Ma) porphyry–skarn Cu–Mo–W–Au mineralization occurred at the Zedong area adjacent to north of the Indus–Yarlung–Zangbo suture zone in a post collisional setting. The intensified Miocene (21–13Ma) porphyry–skarn Cu(–Mo–Au), and skarn–hydrothermal-vein-type Pb–Zn(–Ag) deposits formed in the SG, whereas the porphyry Mo(–Cu) and skarn–hydrothermal-vein-type Pb–Zn(–Ag) deposits formed in the GBAFUB. The porphyry Mo(–Cu) mineralization have more substantial contributions from the old crust, in addition to the mantle, than those with porphyry Cu(–Mo–Au) mineralization. The source region of the high Sr/Y intrusions associated with post collisional porphyry Cu(–Mo–Au) mineralization is probably the arc magmas that were generated at the subduction stage and have remained stalled at the boundary between the crust and mantle since the Neo–Tethyan Ocean closed. The early subduction of the Neo–Tethyan oceanic lithosphere played a crucial role in the generation of ore deposits formed in collisional and post collisional settings. Six minerogenic series (or groups) can be recognized in the Gangdese polymetallic copper belt: (1) Jurassic porphyry Cu–Au minerogenic series related to island arc magmatism, (2) Cretaceous skarn Fe(–Cu) minerogenic series in back arc extensional setting, (3) Paleocene–Eocene granitoids-related polymetallic minerogenic series in collisional setting, (4) Oligocene porphyry–skarn Cu–W–Mo–Au minerogenic series in post collisional setting, (5) Miocene Cu–Mo–Au–Pb–Zn–Ag minerogenic series related to granitoids with substantial contributions from the mantle in post collisional setting, and (6) Miocene Mo–Cu–Pb–Zn minerogenic series related to granitoids with significant contributions from the old crust in post collisional setting. During mineralization, some deposits underwent superimposed mineralization, such as the Jiru porphyry Cu deposit where the Eocene and Miocene porphyry-type Cu mineralization occurred. The minerogenic series can be applied to ore prospecting. For example, when either Pb–Zn(–Ag) veins or porphyry–skarn Mo(–W–Cu) orebodies are known in an area, one of them could provide a useful exploration vector for the other.

Zircon U–Pb ages, geochemistry, and Sr–Nd–Pb–Hf isotopes of the Nuri intrusive rocks in the Gangdese area, southern Tibet: Constraints on timing, petrogenesis, and tectonic transformation

Abstract Abundant magmatic rocks of various ages are exposed in Gangdese, southern Tibet. These rocks play an important role in understanding the tectonic transformation from the subduction of Neo-Tethyan oceanic crust to the collision of the Indian and Asian continents. Based on zircon U–Pb ages, geochemistry, and Sr–Nd–Pb–Hf isotopic data of the Late Cretaceous to early Oligocene (~ 96–30 Ma) intrusive rocks in the Nuri Cu–W–Mo deposit, we discuss the Late Cretaceous to early Oligocene tectonic transformation of the region and the origin of Oligocene Cu–W–Mo mineralization in southern Gangdese. The Nuri intrusive rocks represent three magmatic episodes: 96–91, 56–52, and 33–30 Ma. The 96–91 and 56–52 Ma rocks have relatively low (87Sr/86Sr)i (0.7041 to 0.7060), and high eNd(t) (+ 3.1 to + 3.5) and eHf(t) values (+ 3.7 to + 15); the 33–30 Ma rocks have relatively high (87Sr/86Sr)i (0.7061 to 0.7063) and Pb isotopes, and low eNd(t) (− 3.8 to − 1.8) and eHf(t) values (+ 0.6 to + 10.1). The three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in Gangdese. The 96–91 and 33–30 Ma rocks are adakitic, whereas the 56–52 Ma rocks have characteristics of arc calc-alkaline magmatic rocks. The 96–91 Ma rocks were produced by the partial melting of Neo-Tethyan basaltic oceanic crust and minor sediments, whereas the 56–52 Ma rocks were generated by the partial melting of juvenile crust and the 33–30 Ma rocks were formed by the melting of Indian plate lower crust contaminated with overlying mantle materials. On the basis of the regional tectonic and magmatic characteristics, we suggest that Neo-Tethyan oceanic slab subduction and slab roll-back occurred from ~ 100 to 65 Ma, collision between the Indian and Asian continents occurred at 65 to 40 Ma, Neo-Tethyan oceanic slab break-off took place at ~ 50 Ma, and the Indian continent subducted northwards beneath the Asian continent at ~ 30 Ma. From the Late Cretaceous (96–91 Ma) to Oligocene (~ 30 Ma), the geodynamic setting of Gangdese was changed from Neo-Tethyan oceanic subduction to Indian continental subduction. In contrast to the Miocene metallogenic rocks, the magmatic source of Oligocene metallogenic rocks contains old Indian continental crust materials, which result in the formation of intense, unique Cu–W–Mo mineralization, especially W mineralization, in the southern Gangdese.

The exhumation history of collision-related mineralizing systems in Tibet: Insights from thermal studies of the Sharang and Yaguila deposits, central Lhasa

The large, newly discovered Sharang porphyry Mo deposit and nearby Yaguila skarn Pb–Zn–Ag (–Mo) deposit reside in the central Lhasa terrane, northern Gangdese metallogenic belt, Tibet. Multiple mineral chronometers (zircon U–Pb, sericite 40Ar–39Ar, and zircon and apatite (U–Th)/He) reveal that ore-forming porphyritic intrusions experienced rapid cooling (>100°C/Ma) during a monotonic magmatic–hydrothermal evolution. The magmatic–hydrothermal ore-forming event at Sharang lasted ~6.0Myr (~1.8Myr for cooling from >900 to 350°C and ~4.0Myr for cooling from 350 to 200°C) whereas cooling was more prolonged during ore formation at Yaguila (~1.8Myr from >900 to 500°C and a maximum of ~16Myr from >900 to 350°C). All porphyritic intrusions in the ore district experienced exhumation at a rate of 0.07–0.09mm/yr (apatite He ages between ~37 and 30Ma). Combined with previous studies, this work implies that uplift of the eastern section of the Lhasa terrane expanded from central Lhasa (37–30Ma) to southern Lhasa (15–12Ma) at an increasing exhumation rate. All available geochronologic data reveal that magmatic–hydrothermal–exhumation activities in the Sharang–Yaguila ore district occurred within four periods of magmatism with related mineralization. Significant porphyry-type Mo mineralization was associated with Late Cretaceous–Eocene felsic porphyritic intrusions in the central Lhasa terrane, resulting from Neotethyan oceanic subduction and India–Asia continental collision.

The effects of partial melting, melt–mantle interaction and fractionation on ophiolite generation: Constraints from the late Cretaceous Pozantı-Karsantı ophiolite, southern Turkey

The Pozantı-Karsantı ophiolite consists of a mantle unit and overlying crustal rocks. The mantle peridotite is composed of harzburgitic to dunitic rocks that are depleted in Al2O3 and CaO compared to primitive mantle. The low whole-rock Al and Ca values are consistent with the high Cr# [= 100 × Cr/(Cr + Al)] values of spinel that range from 44 to 70. These spinels generally have low TiO2 (< 0.06 wt.%) concentrations, although spinels with high-Cr compositions in certain samples exhibit enrichments of up to 0.16 wt.% TiO2. The concentrations of chondrite-normalized rare earth elements (REEs) indicate depletion toward heavy to middle REEs. However, the entire mantle peridotite samples exhibit marked enrichment in light REEs and large-ion lithophile elements (LILEs) compared to middle REEs. The heavy REE patterns of some peridotite samples are comparable to the calculated melting curves representing various degrees of melting and are modeled by ~ 24% to 30% melting in the spinel stability field. However, some samples are more depleted in the middle REEs than heavy REEs. The patterns of heavy to middle REEs do not follow the melting lines produced by various degrees of melting in the spinel stability field. The heavy REE composition of these peridotite samples suggests that partial melting began in the garnet stability field and continued into the spinel stability field; they represent ~ 22% to 26% melting under various pressure conditions. High anorthite content of plagioclase as well as Nb-depleted composition of mafic cumulates and isotropic gabbros indicate that the crustal rocks of the Pozantı-Karsantı ophiolite formed from a melt that has been produced by melting of depleted source. This melt is thought to have enriched in light REE and fluid mobile elements that were released from the subducting slab during the subduction of Neotethyan ocean. The whole-rock light REE and LILE enrichments and the higher TiO2 concentrations of the high-Cr# spinels in some mantle peridotite samples cannot be explained by simple melt extraction in a mid-ocean ridge (MOR) environment and require melting and enrichment processes in a supra-subduction zone tectonic setting. However, these features of the peridotites can be explained by the interaction of light REEs and TiO2-rich melts and fluids with peridotites that were depleted during melting events in an MOR environment. This interaction, took place in a supra-subduction zone environment, may have increased the light REE concentrations in the peridotites and caused the re-equilibration of the Ti-depleted spinel to produce Ti-richer spinel and therefore explains the geochemical signatures of the Pozantı-Karsantı peridotites.

Lithospheric structural control on inversion of the southern margin of the Black Sea Basin, Central Pontides, Turkey

To illustrate the structural evolution of the Black Sea Basin in the context of Neotethyan subduction and subsequent continental collisions, we present the first lithosphere-scale, ∼250-km-long, balanced and restored cross section across its southern continental margin, the Central Pontides. Cross-section construction and restoration are based on field, seismic-reflection, geophysical, and apatite fission-track data. The structure of the onshore Pontides belt is predominantly controlled by inverted normal faults, whereas the offshore areas are devoid of large structural inversion. The restored section indicates that Cretaceous crustal thinning occurred synchronously with (probably buoyancy-driven) exhumation of a forearc high-pressure blueschist wedge likely during Neotethyan slab retreat. Apatite fission-track data show that structural inversion of the forearc zone, which formed the Central Pontides fold-and-thrust belt, started at ca. 55 Ma. This Eocene structural inversion followed upon collision of the Kirs¸ehir continental block and the arrest of Neotethyan oceanic subduction below the Central Pontides. Compared to the Central Pontides belt, which underwent significant shortening (∼28 km, i.e., ∼33%), the relatively colder and stronger Black Sea lithosphere prevented the northern offshore areas from undergoing inversion. We propose that the location of Cenozoic contractional deformation is related to the absence of lithospheric mantle below the southern Pontides (forearc) zone as a consequence of the Cretaceous high-pressure wedge exhumation.

Review of the tectonic setting of Cretaceous to Quaternary volcanism in northwestern Iran

There are three parallel magmatic arcs in the northwest of Iran, of Cretaceous and Eocene–Miocene to Quaternary ages, trending in a NW–SE direction between the Main Zagros Thrust (MZT) in the southwest and the Tabriz Fault in the northeast. In this study, these volcanic belts are referred to as the Sanandaj Cretaceous volcanic (SCV), Sonqor–Baneh volcanic (SBV), and Hamedan–Tabriz volcanic (HTV) belts, respectively. The SCV belt consists mainly of mafic to intermediate submarine rocks with calc-alkaline affinity, and the SBV belt is composed of basalt, gabbro to dioritic bodies, with extrusive to subvolcanic magmatic textures and tholeiitic to alkaline affinity. These extend along the MZT between the Zagros ophiolite in the west and the SCV belt in the east. The HTV belt is part of the Urmieh–Dokhtar Magmatic Arc belt that extends across the Hamedan to Tabriz, and was active in the Miocene to Quaternary. The petrology and geochemistry of the northwestern Iranian volcanic zones indicate that they were generated at an active continental margin. In addition to the volcanic belts, there is a dismembered ophiolite along the MZT from Kermanshah to Turkey, in a NW–SE direction. These ophiolites are remnants of Neo-Tethyan oceanic crust which was obducted over the Arabian passive margin in the late Cretaceous. In this study, we propose that a collision between the Arabian and Iranian plates may have occurred in the middle to late Miocene, and that the Neo-Tethyan oceanic subduction beneath northwestern Iran ceased for a while. As a result, a gap in volcanic activity occurred between the Cretaceous and the Middle Miocene–Quaternary volcanism events. This gap in activity is not observed in southwestern Iran.

Geochemistry and geodynamic implications of magmatic rocks from the Trans-Himalayan arc.

Present study aims at understanding the genetic and tectonic relationship between the enclaves and enclosing granitoids, acidic volcanics and mafic dykes of the Ladakh plutonic complex. Similar rocks from Lhasa Block (Tibet) are also studied and compared. In terms of SiO2 abundance, the enclaves vary in composition from basic to acidic but are predominantly andesitic-basalt. Mafic dykes intruding the Ladakh plutonic complex are of predominantly andesitic-basalt composition. Granitoids and acidic volcanics from Ladakh and Lhasa blocks are compositionally granodiorite, quartz monzonite and granite. They are predominantly meta-aluminous with slight peraluminous characters. The acidic volcanics, however, have K2O/Na2O>1. All these rocks show calc-alkaline characteristics with high Al2O3 abundance, their rare earth elements (REE) and multi-element patterns depict enrichment of large ion lithophile elements (LILE)-light REE (LREE) and depletion of high field strength elements (HFSE) including Nb, P and Ti. It is suggested that the enclaves in Ladakh plutonic complex probably represent the initial pulses of magmatism, in response to intra-oceanic northward subduction of Neo-Tethyan ocean beneath an immature arc. Subsequently huge pulses of granitoids were intruded as the arc matured, sutured with southern continental margin of Eurasian plate and the lithosphere thickened. The granitoids in turn were cut by mafic dykes and acidic volcanics probably representing the last significant episode of subduction related magmatism in this region. It is suggested that the youngest, highly siliceous acidic volcanics may represent melts generated by partial melting and/or dehydration of upper part of subducted north Indian continental lithosphere and southern Eurasian active margin wedge, subsequent to the closing of Neo-Tethyan ocean and collision of Indian and Eurasian plates.