Mr. Xiaoran Zhang Department of Earth Sciences, HKU
Numerous controversies still surround the accretionary/collisional processes operative during the Paleozoic evolution of the Eastern Tianshan Belt (ETB), which is an important component of the southern Central Asian Orogenic Belt. This Ph.D. study attempts to resolve the controversial issues by incorporating detrital zircon study with petrological, geochronological and geochemical investigations on Paleozoic sedimentary and magmatic rocks in the North (NTS), Central (CTS) and South Tianshan (STS) that are the three subdivisions of the ETB.
New detrital zircon studies on late Paleozoic (meta-)sedimentary rocks from the NTS and STS belts indicate that the CTS block was once part of the northern Tarim during Precambrian time because of their indistinguishable detrital zircon age profiles and Hf isotope compositions. The discovery of ~450 Ma thickened lower crust-derived adakitic rocks in the CTS block, combined with previous studies, suggests that the CTS most probably developed as a continental arc along the northern Tarim, facing the subduction of Junggar oceanic plate in the early Paleozoic. Results from zircon Hf isotope mapping of 500-380 Ma magmatism from different tectonic units of the ETB suggest that the incipient opening of the STS back-arc basin (that subsequently developed into the STS Ocean) may have occurred in the Late Ordovician, when the CTS was separated as a microcontinent. The closure of the eastern segment of the STS Ocean most likely took place in the Late Devonian, as evidenced by the finding of ~380 Ma regional metamorphism in the CTS, whereas the Junggar Ocean was most probably closed in the latest Carboniferous to earliest Permian, as evidenced by recognition of ~300 Ma regional metamorphism and ~290 Ma mafic intrusions exhibiting within-plate basalt affinities.