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Identifying the ecological vulnerability of the sensitive and fragile ecosystem of the Ya'an-Qamdo section along the southern route of the Sichuan-Tibet transport corridor is of paramount importance to reduce environmental damage resulting from infrastructure construction. This paper divided the Ya'an-Qamdo transport section into 22 subzones according to their ecological environment characteristics. Based on the vulnerability evaluation model established by the fuzzy matter-element analysis method, the eight main assessment indicators of ecological vulnerability were windstorm, rainstorm, snowstorm, extreme temperature, extreme air pressure, geological hazard, natural conditions, and social resources. The rating and ranking of vulnerability in each subzone were based on the weight of the judgment indicators. Scientific processes were used to verify the rationality of both the indicators themselves and their weights. The results of this study show that subzone 9, located in the subalpine cold and humid forest and scrubland zone, is the most vulnerable, and subzone 1, located in the low- to mid-land warm and humid forest zone, is the least vulnerable. The conclusion of the study suggests that targeted measures of ecological protection should be formulated before development and construction of major transportation infrastructure. Construction should evade the most vulnerable areas, and in-depth research on ecological restoration should be carried out in low- to mid-vulnerability areas so that the ecological environment along the route can be protected effectively for sustainable economic and social development. [ABSTRACT FROM AUTHOR]

We present an integrated study involving detailed field investigation, petrography, whole-rock geochemistry, Sr–Nd isotopic compositions, and zircon U–Pb–Hf isotopic compositions of Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons of the southern Lhasa subterrane, Tibet. We use the obtained data to assess the genetic relationship between the granitoids and gabbroic diorites and to better constrain the origin of Late Cretaceous granitoids in the Gangdese batholith. Both the Shanba and Zongga plutons contain Late Cretaceous gabbroic diorites with zircon U–Pb ages of 91–90 Ma, but only the Zongga pluton includes coeval Late Cretaceous (88–83 Ma) granitoids (tonalites and granodiorites) that coexist with the gabbroic diorites. The Shanba gabbroic diorites consist mainly of clinopyroxene, biotite, and plagioclase, whereas the Zongga gabbroic diorites are composed predominantly of amphibole and plagioclase. The Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons are enriched in large-ion lithophile elements and light rare earth elements (REEs), and are depleted in high-field-strength elements and heavy REEs, showing an arc-type geochemical signature. The Zongga gabbroic diorites have lower K2O and light REE contents and Th/Y ratios, as well as higher Ba/La, Ba/Nb, and Rb/Nb ratios, compared with the Shanba gabbroic diorites. The Zongga gabbroic diorites and granitoids have similar Sr–Nd–Hf isotopic compositions [ISr = 0.7035–0.7042, εNd(t) = 5.0–6.4, εHf(t) = 9.6–13.8], with more radiogenic Nd isotopic compositions than those of the Shanba gabbroic diorites [ISr = 0.7037–0.7040, εNd(t) = 3.1–4.1, εHf(t) = 10.9–14.1]. These geochemical and isotopic data indicate that the Zongga granitoids are genetically related to their spatially and temporally associated Zongga gabbroic diorites, whose mantle source was more hydrous than that of the Shanba gabbroic diorites. On the basis of the petrological, geochemical, and isotopic data, we propose that the Shanba and Zongga gabbroic diorites were generated by partial melting of a depleted mantle wedge that was metasomatized by slab sediment melts and by slab dehydration fluids, respectively. The Zongga granitoids were produced by dehydration melting of juvenile lower crust compositionally similar to the Zongga gabbroic diorites, and the parental magma of these gabbroic diorites was also involved in their generation. We conclude that Late Cretaceous underplating of mantle-derived magmas with differing water contents formed juvenile lower crust in the southern Lhasa subterrane; however, only the more hydrous juvenile lower crust was able to produce voluminous granitoids through dehydration melting. Hydrous mantle-derived magmas, therefore, played a key role in the generation of Late Cretaceous granitoids in the Gangdese batholith and contributed not only to crustal growth but also to crustal reworking in the southern Lhasa subterrane. [ABSTRACT FROM AUTHOR]

Granitoids and related mafic microgranular enclaves (MMEs) in continental collision zones may provide crucial insights into the formation and evolution of continental crust. In this paper, we report an investigation into the mineralogy, zircon U–Pb geochronology, whole-rock major and trace elements, and Sr–Nd–Hf isotopic geochemistry of granodiorites and hosted MMEs, as well as monzogranites, from the Zhaxiding intrusive complex in the Gangdese batholith, southern Tibet. Zircon U–Pb dating suggests that these rocks were emplaced during the early Eocene (49 to 47 Ma). Field investigation and petrological observations indicate that the MMEs represent globules of a mafic magma that was injected into the host felsic magma. On the basis of depleted zircon Hf (ε Hf (t) values of +6.5 to +9.2) and whole-rock Sr–Nd (ε Nd (t) values of +1.6 to +2.6) isotopic compositions, along with their whole-rock compositions, the MMEs are interpreted as having resulted from mixing of mantle-derived magma with juvenile crust-derived melts. The granodiorites and monzogranites are metaluminous and have positive zircon ε Hf (t) (+6.3 to +8.9 for granodiorites and +1.7 to +10.2 for monzogranites) and whole-rock ε Nd (t) (+1.6 to +1.9 for granodiorites and +0.5 to +1.1 for monzogranites) values. The granodiorites further exhibit continental crust-like chemical compositions (e.g., enrichment in Rb, K, and Pb and depletion in Nb and Ta) and high Mg# values, suggesting that they originated from partial melting of juvenile crust with the involvement of mantle-derived melts. However, the monzogranites have lower P, Ti, and total rare earth element contents, similar Rb, Ba, and Sr contents, and uniform Eu anomalies compared with the granodiorites, and were likely generated by fractional crystallization of amphibole and Fe–Ti oxides from juvenile crust-derived magma. We propose that underplating of mantle-derived magma likely triggered coeval crustal magmatism and that mixing of the mantle- and crust-derived magmas and subsequent crystallization differentiation formed the intermediate to felsic Zhaxiding intrusive complex in the Gangdese batholith. The continental crust-like bulk compositions of the intermediate to felsic magmatic rocks represent a net addition of juvenile material to continental crust. Our study thus indicates that magma mixing and differentiation play a significant role in the formation of andesitic to dacitic continental crust in collision zones. • The granitoids in Gangdese batholith were emplaced between 49 and 47 Ma. • The host granite and associated MMEs are products of magma mixing. • Mantle and juvenile crust sources contribute to the Zhaxiding intrusive complex. • The syn -collisional magmatism makes a net contribution to continental crust growth. [ABSTRACT FROM AUTHOR]