Tectonic evolution of the middle segment of the southern Central Asian Orogenic Belt : insights from plutonic and sedimentary rocks in the Alxa Terrane and adjacent regions

Abstract

Situated between the Tarim and North China cratons, the Alxa Terrane, the Yagan and Zhusileng-Hangwula arcs (YZHAs) and the Beishan Orogenic Belt (BSOB) provide an ideal window to understand the tectonic evolution of the middle segment of the southern Central Asian Orogenic Belt (CAOB), which is associated with the consumption of the Paleo-Asian Ocean (PAO). Despite much work, many uncertainties remain, especially on the nature of the BSOB and the subduction history and closure time of the PAO. This Ph.D. project attempts to address the above-stated uncertainties by integrating new petrologic, geochronological and geochemical results from these tectonic units. Zircon U-Pb dating results allow the new recognition of 905-871 Ma orthogneisses and <871 Ma paragneisses in the BOSB, as well as ~1014 Ma orthogneisses in the adjacent Central Tianshan Orogenic Belt (CTOB). The protoliths of these orthogneisses are inferred to be subduction-related I-type granitoids. Based on a review of Neoproterozoic events, new results suggest that the BSOB was closely linked with the CTOB and Tarim Craton, and was located in the periphery of the already assembled Rodinia supercontinent during Neoproterozoic time. New geochronological and geochemical data allow the definition of an early to middle Paleozoic arc assemblage in the Alxa Terrane. The arc-related diorites and granitoids are dated at 460-407 Ma, indicating that the southward subduction of the PAO commenced in the early Paleozoic. Isotopic data document a marked shift from an evolving trend at ca. 460-417 Ma to a juvenile trend at ca. 417-407 Ma, corresponding to a tectonic switch from advancing to retreating accretionary orogenesis. The synthesized data infer that the arc assemblage in the Alxa Terrane represents the western extension of the Paleozoic arc on the northern margin of the North China Craton (NCC). A synthesis of our data with previous work indicates a significant tectonic switch from subduction to post-collision orogenesis at ca. 280-265 Ma in the middle segment of the southern CAOB, reflecting the final closure of the PAO, as evidenced by (1) the marked shift and large variation of zircon εHf(t) and whole-rock εNd(t) values at ca. 280-265 Ma in the Alxa Terrane; (2) the granitoids in the YZHAs recording a tectonic transition from an arc (298-277 Ma) to post-collisional (~230 Ma) setting; (3) the notable change of sedimentary provenance between ~276 Ma and ~256 Ma in the YZHAs. By integrating preexisting investigations, a progressively eastward closure of the PAO is inferred, starting in the west along the northern margin of the Tarim Craton at ca. 320-300 Ma, in the Alxa Terrane at ca. 280-265 Ma, and progressing to the east along the northern margin of the NCC at ca. 260-245 Ma.