Petrogenesis Of The Early Stage Of Late Paleozoic Magmatic Rocks In Lhasa Terrane, And Its Implication For The Opening Of Songdo Tethys

Although the Permian Songdo ocean-type eclogite in the Lhasa Terrane wasidentified as the remnant of the Paleo-Tethyan (this paper named it as the SongdoTethyan Ocean) oceanic lithosphere in recent years, the anser to the question ofwhen and how this Thetyan to form is remains poorly constrained because of therare occurrence of related geological records.This study reports the newly identified Xainza mafic rocks (gabrroic rocksand basalts) and Nang granitoids that trans-Songdo eclogite belt in the LhasaTerrane, and then using them for the analysis of whole-rock major, trace element,zircon U-Pb dating, and zircon Hf isotope. Zircon U-Pb dating for Xainza maficrocks and Nang granitoids has respectively yielded the emplacement age of366Ma, and355～341Ma. Xainza samples are calc-alkaline, mainly beingcharacterized by high TiO2contents (1.3～1.8%) and a wide range of Mg#ratios(50～62). They are only depleted in some particular high field strength elements(HFSEs, e.g., Nb-Ta, P, Ti), enriched in some large ion lith elements (LILEs, e.g.,Th, U). These samples exhibit some obviously characteristics similar to themarginal rift volcanic-like and island arc-like rocks, and with negative zircon εHf(t) values, these suggest they are derived from an enriched source which is likelythat the subcontinental lithospheric mantle. Nang gneissic granitoids are high-Kcalc-alkaline to shoshonitic, consisting mainly of granodiorite and monzogranitethat are characterized by high SiO2(69～76%) and K2O/Na2O ratios. Thepresence of muscovite, high aluminum saturation index values (A/CNK=1.10～1.52), and high corundum (>1%) suggest that the Nang gneissic granitoids are ofS-type granites. All the granitoid samples are enriched in light rare earth elements(LREEs) and LILEs, depleted in Ba, Eu, Sr, and HFSEs. Whole-rock zirconsaturation temperatures vary mainly from780C to800C (up to859C, with anaverage of808C). The occurrence of dioritic enclaves, high zircon saturationtemperatures, low Pb and high Ba abundances, and negative zircon εHf(t) values(-7.2～-1.2, averaging=-4.4), imply that the gneissic granitoids are ofhigh-temperature S-type granites resulting from the anatexis of shallow maturecrustal materials with significant contributions of mantle-derived materials. Considering the Early Carboniferous magmatic arc north of Songdo andDevonian-Carboniferous sedimentary records that are dominated by limestoneand fine-grained detritus interbedded with volcanic rocks preserved in the LhasaTerrane, this paper proposes that the Xainza mafic rocks (including basalts) andNang gneissic granitoids were probably emplaced at a back-arc extensionalbackground. Such a region may form a back-arc basin that ultimately evolvedinto the Songdo Tethyan Ocean.