Search Results (4)

The Gangdese Belt is sandwiched between the Yarlung–Zangbo Suture Zone (YZSZ) and the Bangong–Nujiang Suture Zone (BNSZ), and witnessed large-scale magmatic activity during the Early Cretaceous period. Currently, controversies remain regarding the petrogenetic mechanism and tectonic setting of the Early Cretaceous magmatism in the eastern Gangdese Belt. To clarify these controversies, this study conducted systematic petrogeochemical analysis on the Ranwu pluton in the eastern Gangdese Belt. The results show that the main rock types of the Ranwu pluton are monzogranite and granodiorite. The LA-ICP-MS zircon U-Pb ages of the monzogranite (sample CZTW1105) and granodiorite (sample CZTW2051) are 116.3 ± 0.5 Ma and 114.6 ± 0.6 Ma, respectively, collectively indicating that the Ranwu pluton formed during the Early Cretaceous period. The results of in situ zircon Hf isotope analysis show that the εHf(t) values range from −5.0 to 0.5, with corresponding Hf crustal model ages (T_DMC) of 1139–1494 Ma. The Ranwu pluton belongs to the high-K calc-alkaline series and is classified as I-type granite. Combined with geochemical characteristics and tectonic setting discrimination diagrams, it is determined that the granites of this period have geochemical signatures of post-collisional granites and formed in a tectonic setting during the transition from a compressional to an extensional regime. The occurrence of Early Cretaceous post-collisional granites marks the end of the main orogenic stage in the Bangong–Nujiang Suture Zone, and the Gangdese Belt has since transitioned into a tectonic environment of the post-orogenic extensional stage. Full article

A-type granites have been the subject of considerable interest due to their distinct anorogenic geological background. The A-type and arc-related granites are crucial in deciphering the evolution of the ocean closure and continental collision in the Tibet Plateau. The demise of the Bangong–Nujiang suture zone (BNSZ) and the Yarlung–Tsangpo suture zone was accompanied by the emplacement of volumes of syn-collisional and post-collisional granites. Controversy has persisted regarding the contribution of the collisional granites within the Lhasa Block to the growth of the Tibetan Plateau. This study provides key evidence about the evolution of the Lhasa Block and Bangong–Nujiang Ocean (BNO) by the newly documented 1200 km long, Early Cretaceous A-type acidic magmatic belt. The resolution was achieved through the utilization of petrology, whole-rock geochemistry, zircon U-Pb geochronology, and in situ zircon Hf isotope analysis of the Burshulaling Granites in the eastern segment and previous existing data in the central and western segment of the Lhasa Block. The Burshulaling Granites are characterized as peraluminous, high-K calc-alkaline series, indicating a post-collision setting with high temperature and low pressure. The zircon grains from two granite samples yield ²⁰⁶Pb/²³⁸U ages of 115–113 Ma. In situ zircon Hf analyses with ²⁰⁶Pb/²³⁸U ages give ε_Hf(t) of −6.2–0.6, showing prominent characteristics of crust-mantle interaction. Granites from east to west exhibit whole-rock geochemical and geochronological similarities that fall within the well-constrained Early Cretaceous time frame (117–103 Ma) and track post-collisional A-type acidic magmatic belt along BNSZ. We argue that this magmatism resulted from slab break-off or orogenic root detachment, leading to melting and mixing of the lower crust. Meanwhile, this study indicates the existence of the Bangong–Nujiang Ocean southward subduction or a collapse following an Andean-type orogen. Full article

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet hosts a series of dismembered Jurassic ophiolites that are widely considered as remnants of the vanished Meso-Tethys Ocean. In this study we present new compositional, isotopic, and geochronological data from anorthosites and gabbros of the Dongco and Lanong ophiolites in order to test several hypotheses about the nature of subduction in the Bangong-Nujiang Tethys Ocean (BNTO) during the Mesozoic era. Uranium–Pb dating of magmatic zircons separated from the Dongco anorthosites yielded an (average) age of 169.0 ± 3.7 Ma. Zircons separated from the Lanong anorthosites and gabbros yielded U–Pb ages of 166.8 ± 0.9 Ma and 167.3 ± 1.1 Ma, respectively. Zircons separated from the Dongco and Lanong anorthosites have positive ε_Hf(t) values (5.62–15.94 and 10.37–14.95, respectively). The Dongco anorthosites have moderate initial ⁸⁷Sr/⁸⁶Sr (0.703477–0.704144) and high ε_Nd(t) (+6.50 to +7.91). The Lanong anorthosites have high (⁸⁷Sr/⁸⁶Sr)_i (0.706058–0.712952) and ε_Nd(t) in the range of −1.56 to +2.02. Furthermore, the Lanong gabbros have high (⁸⁷Sr/⁸⁶Sr)_i (0.705826–0.706613) and ε_Nd(t) in the range of −0.79 to +4.20. Most gabbros from Dongco and a few gabbros from Lanong show normal mid-ocean ridge basalt (N-MORB)-like primitive mantle (PM)-normalized multi-element patterns. In contrast, most gabbros from Lanong show U-shaped chondrite-normalized rare earth element (REE) profiles. The investigated gabbros are characterized by wide ranges of δEu {(Eu)_N/[(Sm)_N*(Gd)_N]^1/2} values (0.83–2.53), indicating that some of them are cumulative rocks. The trace element contents of all anorthosite samples imply that their composition was controlled by cumulative processes. The geochemical and isotopic compositions of the non-cumulative gabbros from Dongco (δEu: 0.95–1.04) and Lanong (δEu: 0.83–1.03) indicate that their parental melts were derived from melting of heterogeneously depleted, juvenile mantle reservoirs. These rocks have arc-related affinities, indicating that their mantle sources were influenced by minor inputs of subducted lithospheric components. Our preferred hypothesis for the origin of the non-cumulative gabbros from Dongco is that they were formed in a transient back-arc basin (BAB) setting in the middle-western segment of the BNTO, whereas our preferred scenario about the origin of the non-cumulative gabbros from Lanong is that they were generated in a forearc setting in the middle part of the BNTO. We conclude that both geotectonic settings were developed in response to the northward subduction of the BNTO during the Middle Jurassic. Full article

ASTER (Advanced Spaceborne Thermal Emission and Reflection) satellite imagery is useful in assisting lithologic mapping and, however, its effectiveness is yet to be evaluated for lithologic complex such as tectonic mélange. The Mugagangri Group (MG), the signature unit of the Bangong-Nujiang suture zone (BNSZ), Tibet and consisting of ophiolitic mélanges, was previously mapped as a single unit due to its poorly-described internal structures and an informative map with refined lithologic subdivision is needed for future petrologic and tectonic studies. In this paper, based on a combination of field work and ASTER data analysis, the MG is mapped as five subunits according to our newly-proposed lithologic subdivision scheme. In particular, we apply a data-processing sequence to first analyze the TIR band ratios to reveal approximate distribution of carbonates and silicate-dominated lithologies and then the VNIR/SWIR band ratios and false color images to differentiate the lithologic units and delineate their boundaries. The generalized procedures of ASTER data processing and lithologic mapping are applicable for future studies in not only the BNSZ but also other Tibetan ranges. Moreover, the mapping result is consistent with that the MG represents an accretionary complex accreted to the south Qiangtang margin as a result of northward-subduction of the Bangong-Nujiang oceanic crust. Full article