Geochemistry and geochronology of paleozoic sedimentary and plutonic rocks in the eastern Tianshan : implications for the evolution of the southern Central Asian orogenic belt

Abstract

The Eastern Tianshan Belt (ETB) represents an important component of the southern Central Asian Orogenic Belt (CAOB) between Eastern Europe-Siberia and Tarim-North China, and thus its evolution is of vital importance in understanding the evolution of the southern CAOB. Although extensive investigations were carried out on the ETB in the last decade, controversies still surround the accretionary and collisional processes operative during the development of one microcontinent (Central Tianshan; CTS) and two oceans (Junggar and South Tianshan). This Ph.D. research attempts to resolve the controversial issues by incorporating detrital zircon study with petrological, geochronological and geochemical investigations on Paleozoic sedimentary/magmatic rocks in the ETB.

New detrital zircon studies on late Paleozoic (meta-)sedimentary rocks from the North (NTS) and South Tianshan (STS) indicate that the CTS was once part of the northern Tarim during Precambrian time because of their indistinguishable detrital zircon age profiles and Hf isotope compositions. Particularly, Ordovician-Silurian (475-440 Ma) detrital zircons with low negative εHf(t) values predominatein Paleozoic sediments in and surrounding the CTS (including northern Tarim), suggesting that coeval magmatic rocks constituted the main body of the CTS at that time. In combination with previous studies, the CTS most probably developed as a continental arc along the northern Tarim, facing the subduction of the Junggar oceanic plate in the early Paleozoic, with its crust progressively thickened in a compressional environment during subduction zone advance, which gave rise to the generation of thickened lower curst-derived adakitic rocks in the CTS at ~450 Ma.

Through applying zircon Hf isotope mapping of Paleozoic (500-380 Ma) magmatism from different tectonic units of the ETB, the incipient opening of the STS back-arc basin along the northern Tarim is restricted to have occurred in the Late Ordovician, which is consistent with the discovery of ~450 Ma adakitic rocks in the CTS and is further substantiated by the fact that well-defined ophiolitic rocks in the STS are mostly younger than ~445 Ma.

The opening of the STS back-arc basin, accompanied with the separation of the CTS as a microcontinent, was probably triggered by the concurrent rollback of the Junggar oceanic plate, resulting in a regional tectonic switching from subduction zone advance (before 450 Ma) to retreat (since ~450-440 Ma), as supported by the immediate change in detrital zircon εHf(t) values from dominantly negative to positive, the exhumation of high-pressure mafic granulites at the northern margin of the CTS at ~430 Ma and the emergence of a series of Devonian-Carboniferous island arcs in the Junggar Ocean.

The closure of the eastern segment of the STS Ocean between CTS and Beishan most likely took place in the Late Devonian, as evidenced by the occurrence of ~380 Ma regional metamorphism in the southern CTS, whereas the Junggar Ocean was most probably closed in the latest Carboniferous, as evidenced by recognition of ~300 Ma regional metamorphism in the northern CTS, consistent with 325-310 Ma mafic-felsic magmatism showing subduction-related signatures and ~290 Ma mafic plutons exhibiting within-plate basalt affinities.