Tectonic Evolution of the Precambrian Aksu blueschist Terrain, Northwest China: Geochronological and Geochemical Constraints

The Precambrian Aksu blueschist terrain located in the northwest Tarim craton was formerly regarded as a Mesoproterozoic or an early Neoproterozoic complex. In order to obtain the reliable age data we conduct detrital zircon U-Pb geochronological studies on both the metasedimentary rocks from the Aksu blueschist terrain and the unmetamorphosed sandstones from the overlying Sinian succession. In addition, geochemical analysis was also performed on the metasedimentary rocks to establish the broad tectonic setting of the source region of sediments. Our first U-Pb dating results for the detrital zircons from metasedimentary rocks of the Aksu blueschist terrain evidence a maximum depositional age of ca. 730 Ma, confirming the middle Neoproterozoic stratigraphic age. Detrital zircon U-Pb geochronological study also suggests that the age of subsequent blueschist-facies metamorphism should be younger than ca. 730Ma. This implies that the Aksu blueschist terrain could be an indicator of the Pan-African orogeny (ca.700-500Ma) related to the assemblage of Gondwana. Furthermore, the age range 1.3-0.9 Ga is not recorded in the detrital zircons from both the metasedimentary rocks and the Sinian sandstones, which might suggest that Grenville-age orogeny was not significant in the northern Tarim craton. A major age population at Paleopreoterozoic (ca. 2.0-1.8Ga) was found in all samples, implying a paleoproterozoic orogeny in the northern Tarim, which is coincident with the timing of orogeny associated with the assembly of the Columbia supercontinent. Combined with the geochemical and chronological data, we recognize that the northern margin of the Tarim craton may have been a late Neoproterozoic active continental margin and a major source for the sedimentary rocks of the Aksu blueschist terrain, which probably provided a mixture of older recycled sedimentary material from the basement rocks and juvenile materials from the igneous rocks.