Geochemical evidence for the orogenic process of West Qinling in Gansu since Neoproterozoic and its metallogenic background

Abstract：Based on geochemical data of stratigraphic units of different levels and granitoid intrusives acquired in West Qinling from the 1980s to 1990s, such as concentrations and ratios of immobile elements and trace element indicators of A-type granites in combination with previous interpretation of geophysical data, the authors investigated the orogenic process of West Qinling since Neoproterozoic, and probed into the relationship between the orogenic process and the metallic mineralization. In comparison with previous studies, the formation-level stratigraphic unit was reasonably adopted on account of its relative homogeneity, the SiO2-P relation was introduced to differentiate I-type granites from other types, and the results have validated previous knowledge with the obtaining of some new understanding. The authors consider that the evolution of West Qinling since Neoproterozoic may be divided into four processes, i.e., the convergence-split-convergence process from late Mesoproterozoic to early Sinian, the extensional process from late Sinian to Ordovician, the westward contraction process from Silurian to Jurassic, and the westward intraplate extension process, which even extended to the whole West Qinling region from Devonian to Mesozoic-Cenozoic. 　The north China block and the Yangtze block was close to each other in certain periods of Mesoproterozoic, Neoproterozoic and Sinian; after Sinian, they were gradually away from each other and developed a multi-island sea basin between them. There were intra-oceanic subduction, arc continent collision and ocean continent subduction before or in Silurian. The collision of the North China craton and the Yangtze craton might have started in Devonian and ended in Jurassic, whereas the S-type granites which were enriched in aluminum-rich minerals and related to syn-collision were formed in Triassic. The collision of the two plates migrated westward from Lueyang to Diebu, so that the Sinian to Triassic strata accreted successively to the North China craton. Subsequent intraplate extension and granitoid intrusion related to subduction and delamination of Yangtze plate also migrated westward from eastern Dangchuan till the development of the extension in whole West Qinling region in Meso-Cenozoic. No matter what the mechanism of arc continent collision was in early Paleozoic, the Diebu-Lueyang fault must have played a leading role in the evolution of West Qinling since at least Silurian. 　The peaks of sedimentary mineralization were Silurian and Devonian, corresponding to the transitional period from ocean splitting to ocean closing. The peaks of hydrothermal mineralization were Triassic and Jurassic during which the Indosinian-Yanshanian magmatism prevailed, corresponding to the transitional period from collision to intracontinental extension.