Geochronology and Petrogenesis of Triassic High-K Calc-Alkaline Granodiorites in the East Kunlun Orogen,West China:Juvenile Lower Crustal Melting during Post-Collisional Extension

This study reports zircon U-Pb and Hf isotopes and whole-rock elemental data for granodiorites from the East Kunlun orogen. The zircon U-Pb dating defines their crystallization age of 235 Ma. The rocks are characterized by high-K calc-alkaline, magnesian and metaluminous with（K2O＋Na2O）=6.38 wt.%–7.01 wt.%, Mg#=42–50 [Mg#=100×molar Mg/（Mg＋Fe OT）], A/CNK=0.92–0.98, coupled with high εHf（t） values from-0.65 to-1.80. The rocks were derived from partial melting of a juvenile mafic crustal source within normal crust thickness. The juvenile lower crust was generated by mixing lithospheric mantle-derived melt（55%–60%） and supracrustal melt（40%–45%） during the seafloor subduction. Together with available data from the East Kunlun, it is proposed that the studied Middle Triassic granodiorites were formed in post-collisional extension setting, in which melting of the juvenile lower crust in response to the basaltic magma underplating resulted in the production of high-K granodioritic melts.

Geology, Geochronology, and Hf Isotopic Composition of the Pha Lek Fe Deposit, Northern Laos: Implications for Early Permian Subduction-Related Skarn Fe Mineralization in the Truong Son Belt

The Truong Son metallogenic belt in central Laos and Vietnam is an important Fe-Cu-Sn-Au polymetallic ore district. The Pha Lek Fe deposit is closely related to Late Carboniferous– Early Permian I-type granitic magmatism, and contains >50 Mt @ 45% to 50% of Fe ore. Ore minerals occur mainly as magnetite and hematite in the skarn alteration zone between a granitic pluton and metamorphosed Middle–Upper Devonian carbonates. The granitic pluton comprises granodiorite and granite, with zircon U-Pb dating indicating synchronous emplacement at 288.2±1.3 and 284.9±1.2 Ma, respectively. Zircons from these granitoids have ε_(Hf)(t) values of 2.9–11.2 and relatively young TDM2 ages(<1.0 Ga), indicating an origin by partial melting of depleted mafic crust or magma mixing. Previous studies have shown that these granitoids have high Y, Yb, and K_2O contents, and low Sr and Na_2O contents, which are interpreted as the melting of mafic continental crust. Pyrite of the main mineralization stage yields an ^(187)Re/^(188)Os-^(187)Os/^(188)Os isochron age of 287±17 Ma, indicating that mineralization is associated with Pha Lek granitic magmatism. A Late Carboniferous–Early Permian subduction-related skarn-type Fe mineralization model is proposed for the Pha Lek deposit. More evidence is needed to verify a hypothesis of volcanic overprinting during Late Triassic post-collisional extension.

Permian Radiolarians from the A'nyemaqen Mélange Zone in the Huashixia Area of Madoi County,Qinghai Province,Western China,and Their Implications on Regional Tectonism

The Huashixia area in the Madoi County, Qinghai Province, western China is one of the most important areas for the studies of the formation and evolution of the A＇nyemaqen mélange zone due to the well exposed, relatively complete tectonostratigraphic successions. However, the timing and evolutionary process of the A＇nyemaqen paleo-ocean has long been poorly understood. Recently, we obtained abundant radiolarians from the siliceous rocks of the Maerzheng Formation of this mélange zone in Huashixia. Total 19 species in 10 radiolarian genera have been identified. They belong mainly to the Albaillellaria, Spumellaria, Entactinaria, and Latentifistularia. Four radiolarian assemblage zones are established, including： the Pseudoalbaillella rhombothoracata, Pseudoalbaillella scalprata, Ruzhencevispongus uralicus, and Follicucullus bipartitus-Follicucullus sp. cf. F. ventricosus zones. Radiolarian faunal correlations suggest a late Artinskian-early Kungurian age and late Capitanian-early Wuchiapingian age for the first two zones from the lower part and the last two zones from the upper parts of the Maerzheng Formation, respectively. All radiolarians are typical of deep water elements living in deep oceans in low latitudes during the Permian. Previously, the radiolarians of Early Carboniferous to Early Permian age and fusulinids of Early-Middle Permian age have also been reported from the A＇nyemaqen mélange successions. The combination of the newly found radiolarians with the previously reported faunal and microfloral assemblages as well as radiometric ages obtained from the same stratigraphic units indicates that the A＇nyemaqen paleo-ocean may have opened initially during the Early Carboniferous, expanded from the Early-Middle Permian, reached its acme in the early Late Permian, shrunk during the latest Permian to Early Triassic, and eventually was closed in the Middle Triassic.