Geochemistry and geochronology of Paleozoic intrusions in the Nalati (Narati) area in western Tianshan, Xinjiang, China: Implications for Paleozoic tectonic evolution

Abstract

The Nalati (Narati) area in the Chinese western Tianshan is characterized by abundant Paleozoic intrusions, including granites, diorites and gabbros. They are not only indicators of the interaction between crust and mantle, but also useful clues for tracing the tectonic history of the Tianshan Orogen. Most Early Paleozoic granitoids (biotite monzonitic granites and muscovite granites) of this study are from the Yili Block. The biotite monzonitic granites (mixed-source-derived I-type granites) have a zircon U–Pb age of 497.0±5.9Ma, indicating the time of the subduction of the Terskey Oceanic crust in the Late Cambrian. The 427.2±5.7Ma Zircon U–Pb age of the S-type muscovite granites let us interpret that these granites may have been formed during the crust thickening process after the collision between the Yili Block and the Nalati Block. In western Tianshan the Late Paleozoic biotite granites, muscovite granites, quartz diorites, biotite monzonitic granites, granodiorites and alkali granites have respectively a LA-ICPMS zircon U–Pb age of 371.8±6Ma, 357.2±7.5Ma, 313.9±2.5Ma and 296.9±2.4Ma. The biotite granites display I-type geochemical features and are considered to have been derived from a lower continental crust source. The muscovite granites have a pronounced S-type affinity and are considered to have been formed by the partial melting of thickened continental crust after the collision between the Central Tianshan Belt and the South Tianshan Belt. The quartz-diorites are adakite-like and have an I-type affinity, which are considered to have been formed by partial melting of a delaminated lower crust in a post-orogenic extension setting. The granodiorites also show some typical geochemical features of adakite. Their formation is considered to be related to the lower crustal delamination and the ascending of asthenosphere after the collision event. The alkali granites show an A-type granite affinity. They may have been formed in a within-plate tectonic setting and possibly related to rifting as a consequence of extension.The gabbros can be divided into two groups based on their geochemical characteristics and ages. The older gabbros (415–429Ma) with lower TiO2 contents are formed in an arc-related setting. The younger ones (340–318Ma) with higher TiO2 content are formed in an intracontinental setting and influenced by crustal contamination.Magmatic activities recorded by Paleozoic intrusions can constrain the tectonic history of the Tianshan region from subduction of oceanic crust to intraplate extensional collapse and to rifting, and can also corroborate that the final closure of the South Tianshan Ocean occurred in the Late Devonian–Early Carboniferous time.