Pre-Devonian deformation and metamorphism of the Precambrian basement rocks in northwestern Chinese Tianshan: tectonic significance for the evolution of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is a huge mountain range experienced a long history of accretion of microcontinent, continental fragment and arc complex from Precambrian to Mesozoic. The Chinese Tianshan, the southernmost part of the CAOB, is considered as a Mid-Late Paleozoic orogenic belt. In this orogen, Precambrian rocks, including Paleoproterozoic crystalline basement, Mesoproterozoic sedimentary rocks and Neoproterozoic magmatic rocks are well exposed in the northern part, i.e. Yili area. Structural investigation suggest that (1) these Precambrian rocks extensively suffered amphibolite and greenschist facies metamorphism and migmatization, and (2) a south-directed ductile thrust and rheologic folding occurred in most of lithological units, showing curved stretch lineation perpendicular to the axis of the folded foliation, corresponding a regional compressive deformation event. Middle Devonian un-deformed and non-metamorphic conglomerate and sandstone overlies unconformably the ductily deformed Precambrian metamorphic rocks. Cambrian to Silurian are absent in this area. 40Ar/39Ar step-heating and laser probe analysis on amphibole from amphibolite and mica from the micaschist and leucosome of migmatite yielded consistent ages at ~460 Ma. Although excessive argon cannot be absolutely precluded for the 40Ar/39Ar dating, these new data combined with geological evidence, such as Devonian unconformity and absence of Cambrian to Silurian rocks, certainly suggest a pre-Devonian tectonic event, which is uncommon for the Mid-Late Paleozoic western Chinese Tianshan belt. Thus, this pre-Devonian deformation and metamorphic event should be correlated with the evolution of Tianshan basement. Regarding the Yili and surrounding areas, we propose that this pre-Devonian tectonic event probably correspond to the amalgamation of the Yili and Kazakhstan blocks in Mid-Ordovician. This study is funded by the National Basic Research Program of China (973 Program) (No. 2007CB411301), National Science Foundation of China (NSFC: 40802043) and National Scientific Council (Taipei) (No. 97-2752-M-002-003-PA).