Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

摘要

Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U-Pb dating, geochemistry and Sr-Nd-Hf isotopes was carried out for three types of intrusive rocks from the Hannan region, including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the Tiechuanshan and Huangguan A-type granites. LA-ICP-MS zircon U-Pb dating results show that the studied intrusions have crystallization ages ranging from ca. 880 to 770 Ma. The ca. 880 Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low high field strength elements (HFSE) concentrations that indicate a cumulate origin. These gabbros display subduction-related geochemical signatures with whole-rock initial Sr-87/Sr-86 (I-sr) = 0.704-0.705, epsilon(Nd)(t) = -0.4 to +0.6 and zircon epsilon(Hr)(t) = +6.9 to +8.4, suggesting that they were derived from the partial melting of an enriched lithospheric mantle that had been modified by slab-derived materials. The ca. 860 Ma Tianpinghe I-type granites have high I-sr ( 0.704) and negative epsilon(Nd)(t) (-5.6 to -3.5) values and zircon epsilon(Hf)(t) = -1.9 to +1.6, indicating their sources were mixtures of Neoproterozoic juvenile mafic crust and minor Paleoproterozoic crustal components, whereas the ca. 770 Ma Taojiaba I-type granites have relatively low I-sr (0.704) and positive epsilon(Nd)(t) (+1.5 to +2.4) with zircon epsilon(Hf)(t) +6.2 to +9.5, implying that they were dominantly sourced from the Neoproterozoic juvenile crustal materials. The geochemical and Sr-Nd-Hf isotopic data imply that the ca. 780 Ma Tiechuanshan A-type granites were crystallized from a reduced and peralkaline magma, which was generated by partial melting of a heterogeneous source involving Neoproterozoic juvenile and ancient mafic crust under dry and reducing conditions. In contrast, the parental mag