New geochemical and combined zircon U-Pb and Lu-Hf isotopic data of orthogneisses in the northern Altyn Tagh, northern margin of the Tibetan plateau: Implication for Archean evolution of the Dunhuang Block and crust formation in NW China

摘要

Dunhuang Block is previously considered to be an eastern part of the Tarim Craton, but now it is proposed to be the western extension of the Alxa Block of the NCC as a result of displacement along the Altyn Tagh fault. The oldest basement rocks of the Dunhuang Block, named Aketashitage Complex, were mainly exposed in the northern Altyn Tagh. Migmatites of the Aketashitage Complex are sodic and subalkaline in composition. Zircon U-Pb dating of the tonalitic and monzogranitic migmatites indicates that these rocks were produced by strong deformation of earlier formed tonalites with innumerous granitic veins. The tonalitic melanosome of the migmatites was generated approximately at 2.7-2.8 Ga, whereas the granitic leucosome was likely produced by partial melting of the earlier tonalitic melanosome at 2.51-2.53 Ga. Zircon SHRIMP U-Pb dating of metadiorites exposed in the Aketashitage Complex yielded a crystallization age of 2498 +/- 10 Ma. The metadiorites are characterized by high Sr and Sr/Y and low Y, but their low SiO2 and high Cr, Co, and Ni indicate similar geochemical characters of modern low-Si adakites. Their high MgO, Cr, Co, Ni, and Mg-# and high epsilon(Hf)(t) suggest that the metadiorites were probably produced by partial melting of slab-melt metasomatized mantle peridotites. In combination with previously published age data, it is concluded that the orthogneisses in the Aketashitage Complex were generated during three periods, namely at 2.83, 2.71-2.77 and 2.50-2.57 Ga. Except for the metamorphic rims, zircons from the Aketashitage tonalitic and monzogranitic orthogneisses have T-DM(c) model ages ranging from 3.1 Ga to 3.5 Ga and suggest that their formation involved partly the recycling of Paleoarchean to Mesoarchean crust. The younger T-DM(c) model ages of 2.7-2.9 for the metadioritic rocks suggest another period of formation of juvenile crust in the Neoarchean. The similarity of crustal formation and growth between the Dunhuang Block and the northern Tarim Craton therefore suggest that the Dunhuang Block had become a part of the Tarim Craton in the late Neoarchean. The lack of Eoarchean crust and different patterns of Hf model ages suggest that the Dunhuang Block is not a western extension of the Alxa Block of the NCC. (C) 2014 Elsevier B.V. All rights reserved.