Precambrian tectonic evolution of central tianshan, NW China: Constraints from U-Pb dating and in situ Hf isotopic analysis of detrital zircons

摘要

The Tarim Block is an important geologic unit in the reconstruction of the tectonic evolution of the Central Asian Orogenic Belt and the Precambrian Columbia and Rodinia supercontinents. In order to examine the evolution and crustal generation of the Tarim Block, we performed detrital zircon U-Pb dating and in situ Hf isotopic analysis of Devonian sandstones of the Baluntai area in Central Tianshan, which is part of the Tarim Block. Most analyzed zircon grains show oscillatory zoning and have Th/U ratios >0.4, suggesting that they were mainly derived from igneous rocks. A total of about 400 detrital zircon analyses yielded five age populations, namely, early Paleoproterozoic (peak at 2470Ma), middle Paleoproterozoic (peak at 1858Ma), early Mesoproterozoic (peak at 1541Ma), early Neoproterozoic (peak at 952Ma), and late Neoproterozoic (820-750Ma). These peak ages are remarkably consistent with the polyphase tectonothermal events that occurred in the Tarim Block. The peak at 2470Ma indicates the presence of late Neoarchean to early Paleoproterozoic magmatism in the Tarim Block. The two peaks at 1858Ma and 952Ma coincide with the two periods of assembly of the Columbia and Rodinia supercontinents. This further suggests that the Tarim Block was part of these two supercontinents. In fact, the age peak of 1541Ma correlates with the breakup of Columbia, and the age range of 820-750Ma is interpreted to represent the time of the breakup of Rodinia.The zircon Hf model ages suggest three major stages of crustal evolution at 1.0-1.4Ga, 1.8-3.3Ga, and 3.4-3.8Ga. The zircons exhibit a huge range of ε_(Hf)(t) values from -33 to +51, suggesting that they were derived from highly diverse protoliths. However, since most detrital zircons show negative ε_(Hf)(t) values, the protoliths of the Baluntai sandstones of Central Tianshan probably comprise rocks of Archean to Proterozoic crust. Owing to the similar Precambrian basement, lithology and age spectra between Central Tianshan and Tarim, it is argued that Central Tianshan belonged to the Tarim Block in the Precambrian time. Furthermore, euhedral zircons with high Th/U ratios (>0.4) yielded a prominent peak of 447Ma. This can be correlated with an early Paleozoic arc development in Central Tianshan.