Constraining the deposition time and tectonic background of the Habahe Group of the Altai

Abstract

Habahe Group mainly consists of clastic sediments,which widely distribute in the Chinese Altai and can provide important constraints on the tectonic evolution of the Central Asian Orogenic Belt(CAOB).However,there are different opinions about its deposition time and tectonic background.Previous studies suggested that the Habahe Group formed in Sinian to Mid-Ordovieian or Mid-to Late Ordovieian in a passive continental margin.On the other hand,recent geological investigations reveal a long lasting subduction-related setting in the Chinese Ahai at least since Cambrian.The current geochronological study for detrital zircons from the Habahe Group reveals that the detrital zircons of individual samples,irrespective of their lithological and metamorphic differences,all show similar age patterns,with the early Paleozoic(~(206)Pb/~(238)U age=463±542Ma)grains predominating.The youngest detrital zircons from different samples display similar ages(ca.470Ma),which may reflect the maximum deposition time of the Habahe Group.Dating of growth rim of detrital zircons from migmatite yielded a Mid-Devonian age( 384±6Ma ),which was coeval with intrusion of Early to Mid-Devonian granitic plutons in the area and clearly indicates the minimum deposition time of the Habahe Group. Therefore,the deposition time of the Habahe Group can be constrained to be Mid-Ordovician to Early Devonian.Zircon U Pb dating results indicate that the detrital zircons of the Habahe Group are dominated by early Paleozoic grains,with small proportion formed in Preeambrian.The early Paleozoic detrital zircons are mostly magmatic in origin,and their less rounded shapes indicate a limited transportation.The age and morphological characteristics of the detrital zircons are consistent with those of detrital zircons in active tectonic settings and indicate that the Chinese Altai was under an active continental margin environment during the Early Paleozoie.