Zircon U–Pb and Hf isotopic study of gneissic rocks from the Chinese Altai: Progressive accretionary history in the early to middle Palaeozoic

Min Sun,Chao Yuan,Xiaoping Xia,Xiaoping Long,Wenjiao Xiao,Guochun Zhao,Shoufa Lin,Fuyuan Wu,A Kröner

doi:10.1016/j.chemgeo.2007.10.026

Abstract

Gneissic rocks in the Chinese Altai Mountains have been interpreted as either Paleozoic metasedimentary rocks or Precambrian basement. This study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a NE–SW traverse in the northwestern Chinese Altai. Petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from I-type granites formed in a subduction environment. Three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources. Zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 Ma, some give Neoproterozoic ages, and a few yield discordant Paleoproterozoic to Archean ages. Zircon Hf isotopic compositions indicate that both juvenile/mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived. In contrast, zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids, and define single populations for each sample with mean ages between 380 and 453 Ma. The general absence of Precambrian inheritance and positive zircon ɛHf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas. Our data can be interpreted in terms of a progressive accretionary history in early to middle Palaeozoic times, and the Chinese Altai may possibly represent a magmatic arc built on a continental margin dominated by Neoproterozoic rocks.

Highlights

In the end of the 20th century folded structures of central Asia were regarded as formed by accretion and collision of the Paleo- Asian oceanic plate and Siberia continent [Berzin et al, 1994; Dobretsov, 2003; Dobretsov et al, 2004; Didenko et al, 1994; Mossakovsky et al, 1993; Simonov et al, 1994; Zonenshain et al, 1990]
The composite Kazakhstan–Baikal continent has a basement that was formed in the Late Proterozoic – Cambrian during subduction of the Paleo-Asian oceanic plate, comprising a collage of Precambrian Gondwanaderived microcontinents and terranes, beneath the Late Proterozoic Kazakhstan-Tuva-Mongolian island arc along the southeastern margin of the Siberian craton
In the Altai–Sayan segment (Charysh–Terekta, Ulagan, and Western Sayan zones), the Charysh–Terekta–Ulagan– Sayan suture-shear zone (CTUSs) consists of fragments of Late Vendian – Early Ordovician oceanic crust of the Ob’–Zaisan oceanic basin, Ordovician blueschists and CambroOrdovician turbidites, Ordovician–Silurian syncollisional granites and metamorphic rocks in thrust and shear zones

Summary

Introduction

In the end of the 20th century folded structures of central Asia were regarded as formed by accretion and collision of the Paleo- Asian oceanic plate and Siberia continent [Berzin et al, 1994; Dobretsov, 2003; Dobretsov et al, 2004; Didenko et al, 1994; Mossakovsky et al, 1993; Simonov et al, 1994; Zonenshain et al, 1990]. The Late Proterozoic – Paleozoic continental margin complexes on the western margin of the Siberian craton (in present coordinates), which include the Late Proterozoic – Cambrian Kuznetsk–Altai island-arc, Ordovician – Early Devonian passive margin, and Devonian – Early Carboniferous active margin.

Results

Conclusion