Petrology,geochemistry, and geochronology of boninitic dikes from the Kangqiong ophiolite: implications for the Early Cretaceous evolution of Bangong–Nujiang Neo-Tethys Ocean in Tibet

The Kangqiong ophiolite is exposed in the central–western part of the Bangong–Nujiang suture zone (BNSZ) of central Tibet. This study reports new data for boninitic dikes with the aim of reconstructing the geodynamic and petrogenetic evolution of the Kangqiong ophiolite. Ten samples of boninitic dikes that cross-cut the mafic cumulates have very low TiO₂ (0.34–0.42%) contents and high MgO (6.65–8.25%) contents. LA-ICP-MS U–Pb analyses of zircon from the boninitic dikes yield an age of 115 Ma. They are characterized by positive εHf(t) values varying from +13.1 to +15.0. Taking into account the geochemical characteristics of the mantle section, the Kangqiong ophiolite should be generated in a fore-arc spreading setting resulting from intra-oceanic subduction. Based on our data and previous studies, we propose that the BNSZ represents the major suture and records the Early Cretaceous intra-oceanic subduction of the Bangong–Nujiang Neo-Tethys Ocean, and the Shiquan River–Yongzhu–Jiali ophiolitic mélange belt represents a back-arc basin. These two belts, together with the northern Lhasa subterrane should, represent an Early Cretaceous intra-oceanic subduction system and back-arc basin in central Tibet that is similar to present-day active intra-oceanic subduction systems in the western Pacific Ocean. The final closure of the Bangong–Nujiang Neo-Tethys Ocean might have taken place later than the Early Cretaceous.     

Petrology,geochemistry, and geological significance of the Nadong ocean island,Banggongco–Nujiang suture,Tibetan plateau

We studied oceanic mafic igneous rocks of the Mesozoic Banggongco–Nujang suture zone in western Tibet to constrain the tectonic evolution of these rocks and the region as a whole. Two transects were accomplished. Seven basalt samples from the base of the Nadongshan transect (N1 basalts) have flat chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element variation diagrams that are similar to MORB. Two basalt samples from the base of the Nadongshan transect (N1 basalts), ten gabbro samples from the middle of Nadongshan transect (N2 gabbros), four basalt samples from the bottom of Tanjiuxiama transect (T1 basalts), and four basalt samples from the top of the Tanjiuxiama transect (T2 basalts) are alkali basalts and have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and have primitive-mantle-normalized trace element variation diagrams that are enriched in highly incompatible elements, similar to OIB. LREE concentrations increase from N1 basalts to the T1 and T2 basalts, which have (La/Yb)_N up to 16 and have even higher (Ce/Sm)_N. These data indicate that the Nadong ocean island is an Azores-type ocean island that formed during the mature stage of development of the Banggongco–Nujiang Ocean. The conformable nature of the Nadong ocean island with the Mugagangri Group flysch indicates that the Banggongco–Nujiang Ocean was never a large ocean.     

Early Cretaceous continental arc-related volcanic rocks in the Duobuzha area,northern Tibet: implications for evolution history of the Bangong–Nujiang Ocean

New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon ε_Hf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area.     

Age and nature of the late Early Cretaceous Zhaga Formation,northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.     

Petrology,geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite,LongmuCo–Shuanghu–Lancang suture zone,northern Tibet: evidence for an intra-oceanic arc–basin system

Abstract

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.     

Petrology,geochemistry and U–Pb zircon geochronology of lower crust pyroxenites from northern Apennine (Italy): insights into the post-collisional Variscan evolution

Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary mélanges of the Northern Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature (~850°C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene, Ti-pargasite and spinel. U–Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean concordia U–Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature of the pyroxenites (initial ε_Nd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle. In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush. A thermal event in Late Permian–Middle Triassic caused the partial resetting of zircon U–Pb system.     

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex,China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M₁ assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M₂ stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program Tweequ at 10–12 kbar and 600–650°C. The M₃ assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P–T conditions of 6.0–7.0 kbar and 600–650°C. M₄ is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M₁ and M₄ and the quantitative P–T estimates from M₂ and M₃ define a clockwise P–T path for the Wutai amphibolites.The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M₁+M₂), subsequent isothermal exhumation (M₃), and finally cooling and retrogression (M₄). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.     

Petrology,geochronology and Sr–Nd isotopic geochemistry of the Konso pluton,south-western Ethiopia: implications for transition from convergence to extension in the Mozambique Belt

Granites were shown to be excellent geochronological, structural and geodynamic markers. Among several generations of granites described in the Neoproterozoic of Ethiopia, we studied the post-tectonic Konso pluton to characterise the post-Pan-African evolution of the Mozambique Belt (MB) of southern Ethiopia. The Konso pluton is a composite intrusion of slightly peraluminous and ferro-potassic, bt (biotite)–leucogranites, bt–hbl (hornblende)–granites and subordinate coeval metaluminous monzodiorites, intruded into high-grade gneiss–migmatite associations of the MB. The whole suite displays chemical features of A-type granites. It is LIL- and HFS-elements enriched with Y/Nb and Yb/Ta1.2. The granites and leucogranites show non-fractionated to fractionated REE patterns [(La/Yb)_N=0.3–9.4] with strong negative Eu anomalies. The monzodiorites show fractionated REE patterns [(La/Yb)_N=5.5–7.4] with negligible negative Eu anomaly. The low initial (⁸⁷Sr/⁸⁶Sr)₄₅₀ ratios (0.70113–0.70441) and positive Nd₍₄₅₀₎ values (+1.8 to +3.3) suggest an isotopically primitive source. The Konso granites are likely to be derived from a basaltic parent, with minor contamination by crustal material with high Y/Nb and low Sr initial isotopic ratios. Age of pluton emplacement is constrained by a Rb–Sr isochron and zircon U–Pb data at 449±2 Ma. The Konso pluton is, therefore, the witness of an Ordovician A-type magmatic event, which marks a change from convergence, related to the Pan-African collision, to extension in the Mozambique Belt of southern Ethiopia.     

Remnants of late Permian–Middle Triassic ocean islands in northern Tibet: Implications for the late-stage evolution of the Paleo-Tethys Ocean

In this paper we present new data for the Tianquan (TQ) and Dabure (DB) ocean islands in the western segment of the Longmuco–Shuanghu–Lancangjiang suture zone, northern Tibet, including the results of major and trace element analyses, zircon U–Pb dating, and Hf isotope analyses. Our aim was to assess the genesis of these ocean islands and to consider the implications for the tectonic evolution of the region as a whole. Both TQ and DB retain an ocean-island-type double-layered structure comprising a volcanic basement (basalt and andesite) and an oceanic sedimentary cover sequence (conglomerate, limestone, and chert). The basalts and andesites in the TQ and DB are enriched in light rare earth elements and high field strength elements (Nb, Ta, Zr, Hf, and Ti), yielding chondrite-normalized REE patterns and primitive-mantle-normalized trace element patterns that are similar to those of ocean island basalts. Given the small and generally positive ε_Hf(t) values of the TQ andesites (+ 4.25 to + 6.22) and DB andesites (− 0.59 to + 1.97, mostly > 0), we conclude that the basalts were derived from the partial melting of garnet peridotite in the mantle and that the andesites were formed by fractional crystallization of the mafic parent magma derived from the garnet peridotite mantle. The ascending magmas underwent varying degrees of fractional crystallization but were not contaminated by crustal material. These features indicate that both TQ and DB are typical ocean islands that formed in an ocean basin. Geochemical analyses of cherts from TQ and DB show that they contain terrigenous material, indicating the proximity of a continental margin. The andesites of TQ contain zircons that yield two U–Pb ages of 251 Ma. Given that ages of 246, 247, and 254 Ma had been reported previously, we conclude that TQ formed during the late Permian–Early Triassic. The andesites of DB contain zircons that yield U–Pb ages of 242 and 246 Ma. Taking into account the youngest age of 244 Ma from the DB basalt, we conclude that DB formed during the Middle Triassic. These data, combined with the geological history of the region, indicate that the development of the Longmuco–Shuanghu–Lancangjiang Paleo-Tethys Ocean continued after the early Permian and that the closure of this ocean was diachronous from east to west. The eastern segment of the ocean closed during the Early Triassic; however, the western segment remained at least partially open until the Middle Triassic, although the ocean was relatively small at this time. The ocean finally closed in the Late Triassic.     

Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province,NW China: implications for the tectonic evolution of the Junggar terrane

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).     

40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet,China: petrogenesis and tectonic implications

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende ⁴⁰Ar–³⁹Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO₂ = 46.55–55.75%, MgO = 2.80–7.38%, Mg^# = 36–61, and (Na₂O + K₂O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their I_Sr and ?_Nd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.     

Petrology, geochemistry, and geochronology of the Chah-Bazargan gabbroic intrusions in the south Sanandaj–Sirjan zone, Neyriz, Iran

The Chah-Bazargan gabbroic intrusions are located in the south of Sanandaj–Sirjan zone. Precise U–Pb zircon SHRIMP ages of the intrusions show magmatic ages of 170.5 ± 1.9 Ma. These intrusions consist primarily of gabbros, interspersed with lenticular bodies of anorthosite, troctolite, clinopyroxenite, and wehrlite. The lenticular bodies show gradational or sharp boundaries with the gabbros. In the gradational boundaries, gabbros are mineralogically transformed into anorthosites, wehrlites, and/or clinopyroxenites. On the other hand, where the boundaries are sharp, the mineral assemblages change abruptly. There is no obvious deformation in the intrusions. Hence, the changes in mineral compositions are interpreted as the result of crystallization processes, such as fractionation in the magma chamber. Rock types with sharp boundaries show abrupt chemical changes, but the changes exhibit the same patterns of increasing and decreasing elements, especially of rare earth elements, as the gradational boundaries. Therefore, it is possible that all parts of the intrusions were formed from the same parental magma. Parts showing signs of nonequilibrium crystallization, such as cumulate features and sub-solidification, underwent fracturing and were interspersed throughout the magma chamber by late injection pulses or mechanical movements under mush conditions. The geological and age data show that the intrusions were formed from an Al-, Sr-, Fe-enriched and K-, Nb-depleted tholeiitic magma. The magma resulted from the partial melting of a metasomatized spinel demonstrated by negative Nb, P, Hf, and Ti, and positive Ba, Sr, and U anomalies typical of subduction-related magmas.     

The Middle Triassic evolution of the Bangong–Nujiang Tethyan Ocean: evidence from analyses of OIB-type basalts and OIB-derived phonolites in northern Tibet

International Journal of Earth Sciences - In this paper, we present new major and trace element chemical data for the basalts and phonolites of the Nare ocean island fragment (NaOI), as well as...     

Petrology,geochemistry and metamorphic evolution of Lancang Group in the Changning-Menglian complex belt and its implications on the tectonic evolution of the Paleo-TethysEI北大核心CSCD

Lancang Group within the Changning-Menglian complex belt in the Sanjiang area, Yunnan Province involves many kinds of meta-sediments, including staurolite-kyanite-bearing garnet-mica schist, garnet-mica schist, chloritoid-white mica schist and chlorite-glaucophane-albite schist. Detailed petrographic observation, mineral chemistry analysis and phase equilibrium modelling have shown that these meta-sediments preserve distinctly metamorphic evolutions. The staurolite-kyanite-bearing garnet-mica schist records the decompression and cooling histories related to retrograded metamorphic processes from middle-temperature eclogite facies to amphibolite facies with a peak mineral assemblage of garnet + kyanite + phengite + jadeite formed at the P-T condition of about 19 similar to 30kbar and 600 similar to 750 degrees C. For the garnet-mica schist, the peak metamorphic mineral association constrained by X-Prp and X-Grs in garnet, and Si content in phengite includes garnet + phengite + omphacite + lawsonite + paragonite and the related P-T condition is around 17 similar to 19. 5kbar and 430 similar to 475 degrees C . The chloritoid-white mica schist is characterized by the mineral assemblage of chloritoid + phengite + paragonite + chlorite whereas the peak mineral assemblage includes phengite + paragonite + carpholite. The peak P-T condition defined by Si content in phengite is limited in the range of 17 similar to 19kbar and 300 similar to 330 degrees C. Both garnet-mica schist and chloritoid-white mica schist consistently record heating and decompression processes from lawsonite-blueschist facies to epidote-blueschist facies. Metamorphic reactions and mineralogy of chlorite-glaucophane-albite schist roughly give the P-T condition of 9 similar to 11kbar and 430 similar to 520 degrees C. Studies on the geochemistry of Lancang Group reveal that these meta-sediments show the geochemistry affinity to the continental arc, active continental margin and upper crust sediments. The protoliths are mainly mud rock and sandstone with low maturity and a little of mafic-intermediate volcanic rock. The sediment sources are predominantly intermediate-acid magmatic rocks with old sedimentary contamination to different degree. Tectonic discrimination diagrams show that meta-sediments in the Lancang Group are mainly derived from the continental island arc or active continental margin tectonic setting. Combined with the metamorphism and geochemistry characteristics of these rocks in the Changning-Menglian complex belt, it is therefore inferred that the meta-sediments of Lancang Group display various metamorphic evolutions. Lancang Group are considered to have experienced multi-phase/stage and complex tectonic evolution histories.     

Depositional environment and provenance of the upper Permian–Lower Triassic Tianquanshan Formation,northern Tibet: implications for the Palaeozoic evolution of the Southern Qiangtang,Lhasa, and Himalayan terranes in the Tibetan Plateau

Abstract

This article reports the depositional environment and provenance for the Tianquanshan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone, and uses these to better understand the tectonic evolution of this region. Zircons in the andesite of the Tianquanshan Formation yielded concordia ages of 246, 247, and 254 Ma, indicating that the Tianquanshan Formation formed during the late Permian–Early Triassic. The Tianquanshan Formation consists of flysch and ocean island rock assemblages, indicating that the Longmuco–Shuanghu–Lancangjiang Palaeo-Tethys Ocean continued to exist as a mature ocean in the late Permian–Early Triassic. The detrital zircons in the greywackes of the Tianquanshan Formation yielded peak ages of 470–620, 710–830, 910–1080, 1450–1660, and 2400–2650 Ma, indicating the provenance of the Tianquanshan Formation was either Indian Gondwana or terranes that have an affinity with Indian Gondwana in the Tibetan Plateau (i.e. the Southern Qiangtang, Lhasa, and Himalayan terranes). The Ordovician quartzites, Carboniferous sandstones, Carboniferous–Permian diamictites, and the Upper Permian–Lower Triassic greywackes in the Southern Qiangtang, Lhasa, and Himalayan terranes all contain detrital zircons with youngest ages of ca. 470 Ma, indicating their source areas have been in a stable tectonic environment since the Ordovician, and this inference is supported by the continuous deposition in a littoral–neritic passive margin in these regions from the Ordovician to the lower Permian. Combining the present results with regional geological data, we infer that the Southern Qiangtang, Lhasa, and Himalayan terranes were all in a stable passive continental margin along the northern part of Indian Gondwana during the long period from the Ordovician to the early Permian. At early Permian, because of the opening of the Neo-Tethys Ocean, the tectonic framework of this region underwent a marked change to a rifting and active environment.     

Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange,central Tibet,and tectonic implications

An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28 kbar and 600–650 °C with a low geothermal gradient of ca. 8 °C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole.     

Petrology,zircon U–Pb geochronology and tectonic implications of the A1-type intrusions: Keban region,eastern Turkey

The Southeast Anatolian Orogenic Belt (SAOB), the most extensive segment of the Alpine-Himalayan Orogenic Belt, resulted from the northward subduction of the southern branch of the Neotethys oceanic crust beneath the Anatolian micro-plate. We present new whole-rock geochemistry, zircon U–Pb ages, and Lu–Hf isotope data from the stocks and dykes with a length of up to tens of meters belonging to the Keban magmatic rocks, eastern Turkey. These rocks are represented by syenite and quartz monzonite intruded into the Keban metamorphic complex. The geochemistry data indicates that the samples bear mostly metaluminous, variably high alkalines (K₂O + Na₂O), Ga/Al ratios and zircon saturation temperature, and typically the A-type granite characters. According to the Y/Nb vs Yb/Ta diagram, the Keban magmatic rocks show A₁-type geochemical signatures modified by crustal melts. Syenite and quartz monzonite samples from Keban magmatic rocks give zircon U–Pb ages of 77.4 ± 0.34 Ma, 76.3 ± 0.3 Ma and 76.36 ± 0.34 Ma, respectively. On the primitive mantle-normalised trace element patterns, the Keban magmatic rocks show enrichment in large-ion lithophile elements (LILEs) relative to high field strength elements (HFSEs). They are coupled with slightly negative Nb–Ta anomalies. Chondrite-normalised rare earth-element patterns show strong enrichment in LREEs relative to HREEs, a typical A-type granites feature. The zircons have negative εHf_(t) values that vary from ?2.68 to ?0.41, and Hf model ages (T_DM2) range from 1171.54 to 1329.26 Ma, indicating the enriched lithospheric mantle sources and crustal contribution. The sources and evolution of the alkaline magmas might be related to the post-collisional tectonic setting.     

Zircon U–Pb geochronology and geochemistry of the Maoliger quartz monzodiorites,Inner Mongolia,China: implications for Carboniferous arc magmatism and closure of the Palaeo-Asian Ocean

Recent research has identified an early to late Carboniferous magmatic arc that extends from Suzuo Qi to Xiwu Qi in Inner Mongolia, China, but the eastern extension of this arc is unknown. Understanding the relationship between this arc and the Hegenshan ophiolite belt and Xilamulun Solonker suture zone is important to our understanding of the tectonic evolution of the late Palaeozoic Palaeo-Asian Ocean. Here, we present new zircon laser ablation–inductively coupled plasma mass spectrometry U–Pb and geochemical data for the Maoliger quartz monzodiorites within the Jalaid Qi area. The Maoliger quartz monzodiorites formed at 329 ± 2 Ma, are low-K and tholeiitic, and have geochemical signatures indicative of formation within a magmatic arc. These rocks are large-ion lithophile element (e.g. Rb, Ba, and Sr)-enriched and high-field-strength element (e.g. Nb and Ta)-depleted. Combined with previously published researches, it is suggested that the quartz monzodiorites within the Jalaid Qi area formed contemporaneously with and are geochemically similar to quartz diorites of the Xiwu Qi area and the Baolidao pluton in the Suzuo Qi area. This indicates that the early to late Carboniferous magmatic arc in this region extends eastward to the Jalaid Qi area. This arc is located in an area parallel to a southerly early Permian magmatic arc, suggesting that the Palaeo-Asian Ocean subduction zone migrated south between the early Carboniferous and early Permian. The new data show that the Palaeo-Asian Ocean closed after the late Carboniferous.     

Fluid inclusion geochemistry and Ar–Ar geochronology of the Cenozoic Bangbu orogenic gold deposit,southern Tibet,China

Located along the southern part of the Yarlung Zangbo suture zone in southern Tibet, Bangbu is one of the largest gold deposits in Tibet. Auriferous sulfide-bearing quartz veins are controlled by second- or third-order brittle fractures associated with the regional Qusong–Cuogu–Zhemulang brittle-ductile shear zone. Fluid inclusion studies show that the auriferous quartz contains aqueous inclusions, two-phase and three-phase CO₂-bearing inclusions, and pure gaseous hydrocarbon inclusions. The CO₂-bearing inclusions have salinities of 2.2–9.5% NaCl_eq, and homogenization temperatures (Th) of 167–336 °C. The δD, δ¹⁸O, and δ¹³C compositions of the Bangbu ore-forming fluids are − 105.5 to − 44.4‰, 4.7 to 9.0‰ and − 5.1 to − 2.2‰, respectively, indicating that the ore-forming fluid is mainly of metamorphic origin, with also a mantle-derived contribution. The ³He/⁴He ratio of the ore-forming fluids is 0.174 to 1.010 R_a, and ⁴⁰Ar/³⁶Ar ranges from 311.9 to 1724.9. Calculations indicate that the percentage of mantle-derived He in fluid inclusions from Bangbu is 2.7–16.7%. These geochemical features are similar to those of most orogenic gold deposits. Dating by ⁴⁰Ar/³⁹Ar of hydrothermal sericite collected from auriferous quartz veins at Bangbu yielded a plateau age of 44.8 ± 1.0 Ma, with normal and inverse isochronal ages of 43.6 ± 3.2 Ma and 44 ± 3 Ma, respectively. This indicates that the gold mineralization was contemporaneous with the main collisional stage between India and Eurasia along the Yarlung Zangbo suture, which resulted in the development of near-vertical lithospheric shear zones. A deep metamorphic fluid was channeled upward along the shear zone, mixing with a mantle fluid. The mixed fluids migrated into the brittle structures along the shear zone and precipitated gold, sulfides, and quartz because of declining temperature and pressure or fluid immiscibility. The Bangbu is a large-scale Cenozoic syn-collisional orogenic gold deposit     

Tectonic and sedimentary basin evolution of the eastern Bangong–Nujiang zone (Tibet): a Reading cycle

The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. Their stratigraphic development indicates that both blocks once formed a continuous continental platform until the Late Triassic. Following Late Paleozoic-Triassic rifting, ocean crust formed between both blocks during the Late Triassic creating the Dongqiao-Naqu basin (DNB) among other basins (Yu et al. 1991). The analysis of the rift flank sequences reveals that rifting was dominated by transtension. The basin was shortened by post-Mid-Cretaceous transpression. Thus, the overall basin evolution represents a Reading cycle despite some active margin processes which gave this cycle a special imprint. Major basin parts were preserved despite transpressional shortening suggesting that the eastern BNZ represents a remnant basin. Our understanding of the DNB solves the prior problem of viewing the BNZ as a Mid-Late Jurassic collisional suture although typical collision-related deformation, thickening, mountain building, as well as related molasse formation are lacking. Our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting.