Crustal evolution and Phanerozoic crustal growth in northern Xinjiang: Nd isotopic evidence. Part I. Isotopic characterization of basement rocks

Abstract

The Central Asian Orogenic Belt (CAOB) is known as the most important site of juvenile crustal growth during the Phanerozoic. In order to examine the processes of such crustal generation and the role of Precambrian crust in the magma genesis, we conducted geochemical and Nd isotopic studies on both the basement rocks and Phanerozoic granites from the major tectonic terranes in northern Xinjiang: Altai, Junggar, Tianshan and North Tarim. In this paper only the results on the basement rocks are reported. The North Tarim Terrane is composed of Archean bimodal suite (TTG gneisses and amphibolites) and Proterozoic granitic gneisses. This terrane is a fragment of ancient continental crust and is tectonically dissociated from the CAOB. The other terranes in northern Xinjiang (Altai, Junggar, and Tianshan) belong to the CAOB. The Altai and Tianshan are composite terranes probably formed by accretion of Phanerozoic subduction complexes with entrained Proterozoic basement rocks as microcontinental blocks. Geochemical characteristics of amphibolites from Altai and Tianshan suggest their formation in island arc settings. Sm–Nd model ages of the Tianshan basement rocks vary from 1.2 to 2.2 Ga, but mainly concentrated in 1.7–2.1 Ga. Similarly, amphibolites and gneisses of the Altai terrane have T DM in two apparently discrete groups at 0.9–1.5 Ga and 2.4–2.6 Ga. The initial Nd isotope ratios or ε Nd( T) values indicate that a large proportion of basement gneisses from Tianshan were derived from remelting of Paleo- to Mesoproterozoic protoliths (1.7–2.1 Ga). However, this is not the case for the Altai basement gneisses. Granitic gneisses and metasedimentary rocks (schists and phyllites) from the East and West Junggar terranes have much younger T DM ages of 0.7–1.4 Ga. Presence of a minor Precambrian crustal component is possible as inferred from the model ages, but no data have shown Precambrian ages for the Junggar basement rocks. The model age and ϵ Nd( T) data support the idea that the Junggar terrane is composed of young island arc assemblages, and the Junggar Basin itself could be a trapped Paleozoic oceanic crust. Together with some reliable radiometric ages for basement rocks, the present isotopic data and T DM ages indicate that ancient microcontinental blocks constitute a significant proportion of the continental crust in northern Xinjiang, and probably also in the entire CAOB.