Late Triassic Intraoceanic Arc Aystem within Neotethys: Evidence from Cumulate Hornblende Gabbro in Gangdese Belt, South Tibet

The Neotethys plays an important role in shaping the Gangdese magmatic belt,southern Tibet.However,the initial time of spreading and subduction of the Neotethys remains contentious.In this study,a suite of late Triassic cumulate hornblende gabbro was identified in the southern margin of the Gangdese magmatic belt.The gabbro exhibits cumulate structure,with hornblende and plagioclase as the primary mineral phases.Isotopic data indicate a hydrous magma source derived from a depleted mantle wedge that has been modified by slab dehydration.Geochemical discriminations suggest that the gabbro was formed in an intraoceanic arc setting,with crystallization ages of ca.220-213 Ma.Hornblende,hornblendelagioclase and ilmenite thermometers reveal that the crystallization temperature of 900-750°C for the gabbro.Hornblende and hornblende-plagioclase geobarometers yield an emplacement depth at ca.14.5-19.5 km.This gabbro constitutes a line of evidence for an intraoceanic arc magmatism that is coeval with the counterparts in the southern Turkey,revealing an intraoceanic subduction system within the Neotethys from west to east in the Late Triassic and that the oceanization of the Neotethys was much earlier than previous expectation.

Petrogenesis of Early Cretaceous hornblende gabbro in Khanka Massif: evidence from geochronology and geochemistry

The authors report zircon U-Pb geochronological, whole-rock geochemical and zircon Lu-Hf isotope data for the hornblende gabbro within the Khanka Massif, with the aim of constraining its formation time and petrogenesis. The zircon U-Pb dating shows that 206 pb^238 Pb ages of zircons from the hornblende gabbro range from 120 to 129 Ma, yielding a weighted mean age of 123±2 Ma, i.e., the Early Cretaceous. The hornblende gabbro has SiO2 of 44.77%-46.58% and belongs to the tholeiitie series on FeOt/MgO-SiO2 diagram. It dis-plays a right-inclined REE pattern with （La/Yb）N ratios of 3.44 to 4.42. The trace element spidergram shows that they are enriched in large ion lithophile elements （LILE） such as Rb, Th, U, K and Pb, and depleted in high field strength elements （HFSE） such as Nb, Ta, Ti and P, indicating an affinity to arc igneous rocks. The ettf（t） values of zircons vary from -2.6 to ＋ 3.9 and Hf model ages （TDM1 ） range from 622 to 883 Ma. These geochemical characteristics indicate that primary magma of the hornblende gabbro could be derived from partial melting of young mantle material acereted during the Neoproterozoie. Combined with the Early Creta-ceous igneous rock assemblages in NE Asia. It is concluded that the hornblende gabbro formed in an active con-tinental margin related to the westward subduction of the Paleo-Paeific Plate beneath the Khanka Massif.

Origin and chronology for hornblende gabbroic intrusions in Urad Zhongqi of western Inner Mongolia and their tectonic implications

The Urad Zhongqi mafic intrusions crop out in western Inner Mongolia and are dominated by hornblende gabbro.Zircon LA-ICP-MS U-Pb dating indicates that the mafic pluton was emplaced in the Early Permian(272 ± 3Ma).The hornblende gabbro rocks have low concentrations of Si O2,high total Fe2O3,Mg O,Ni,Co,Sc and Cr.The rocks are characterized by selective enrichment of large ion lithophile elements(LILE) and light rare earth elements(LREE),and depletion in high field strength elements(HFSE)(e.g.Nb,Ta,Zr,Hf,P and Ti).It suggests that they formed from a primary magma generated by the partial melting of lithospheric mantle material had been previously modified by subduction-related fluids.According to the geochemical data and the geology of the region,the authors found that the Early Permian hornblende gabbroic in western Inner Mongolia formed in a post-collisional extensional setting.