Neoproterozoic tectonic evolution of the Precambrian Aksu blueschist terrane, northwestern Tarim, China: Insights from LA-ICP-MS zircon U-Pb ages and geochemical data

The Precambrian Aksu blueschist terrane (ABT) located in the northwestern Tarim Craton was formerly regarded as a Mesoproterozoic or an early Neoproterozoic complex. Yet, its tectonic significance remains poorly understood due to the lack of reliable age. We have conducted a detrital zircon U-Pb geochronological study of both the metasedimentary rocks from the ABT and the unmetamorphosed sandstones from the overlying Sinian succession to better constrain the age of the ABT. In addition, geochemical analyses were performed on the metasedimentary rocks to establish the broad tectonic setting of the source region of sediments. Our first U-Pb dating results suggest a maximum deposition age of ca. 730 Ma for the protolith of the metasedimentary rocks in the ABT and a maximum deposition age of 602 Ma for the unmetamorphased Sinian sandstones immediately overlying the ABT. Therefore, the blueschist-facies metamorphism in the ABT must have taken place after ca. 730 Ma, but prior to 602 Ma. This metamorphism may manifest the Pan-African orogeny (ca. 700-500 Ma), which is related to the assemblage of Gondwana, in the northern Tarim. Furthermore, the age range of 1.3-0.9 Ga was not recorded in the detrital zircons from both the metasedimentary rocks and the Sinian sandstones, suggesting that the northern Tarim Craton may not be significantly affected by the Grenville-age orogeny. A major age population at Paleopreoterozoic (ca. 2.0-1.8 Ga) was found in all samples, implying a Paleoproterozoic orogeny in the northern Tarim, which is coincident with the timing of the orogeny associated with the assembly of the Columbia supercontinent. Taking together the geochemical and chronological data, we propose that the northern margin of the Tarim Craton was probably a late Neoproterozoic active continental margin and a major source for the sedimentary rocks of the ABT, which provided a mixture of both old recycled sedimentary material from the basement rocks and juvenile material from the igneous rocks.