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The role of hydrous mantle-derived magmas in the generation of Late Cretaceous granitoids in the Gangdese batholith: insights from the Shanba and Zongga plutons in the southern Lhasa subterrane, Tibet
Mineralogy and Petrology ( IF 1.8 ) Pub Date : 2020-11-27 , DOI: 10.1007/s00710-020-00728-6 Rui-Qiang Wang , Jian-Sheng Qiu , Ding-Jun Wen , Hang Xu
Mineralogy and Petrology ( IF 1.8 ) Pub Date : 2020-11-27 , DOI: 10.1007/s00710-020-00728-6 Rui-Qiang Wang , Jian-Sheng Qiu , Ding-Jun Wen , Hang Xu
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 K 2 O 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 [ I Sr = 0.7035–0.7042, ε Nd ( t ) = 5.0–6.4, ε Hf ( t ) = 9.6–13.8], with more radiogenic Nd isotopic compositions than those of the Shanba gabbroic diorites [ I Sr = 0.7037–0.7040, ε Nd ( t ) = 3.1–4.1, ε Hf ( 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.
更新日期：2020-11-27
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