Devonian continental arc magmatism in the southern Central Asian Orogenic Belt: Evidence from the Dunhuang Block, NW China

Abstract

Reconstruction of periods and sources of continental arc magmatism provides critical insights for deciphering the thermal evolution and geodynamics of the continental lithosphere and unveiling subduction-related episodes of accretion and orogeny. The Dunhuang Block, located in the southern Central Asian Orogenic Belt, is built upon a Precambrian metamorphic crystalline basement that is intruded by numerous Paleozoic granitoids. In this paper, we present new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes from the Hanxiakouzi and Shuangta granodiorite plutons outcropped in the central and northern Dunhuang Block, respectively. The U-Pb zircon dating indicate that the Hanxiakouzi and Shuangta granodiorites were emplaced at ca. 366–363 Ma, i.e., in the late Devonian. Geochemically, the Hanxiakouzi granodiorites are medium- to high-K, I-type granites possessing high Sr and Sr/Y, i.e., similar to adakites. The Hanxiakouzi granodiorites show negative εNd(t) values (−8.2 to −6.6), significant positive εHf(t) values (+0.7 to +8.1), and evolved Pb isotope features suggesting their derivation by melting of subducted terrigenous sediments, interaction of melts with mantle peridotite, and assimilation of ancient crust during magma ascent. The Shuangta granodiorites belong to high-K calc-alkaline I-type granites with medium Sr and low Sr/Y. The Shuangta granodiorites yield negative εNd(t) values (−2.9 to −3.1) and negative to positive zircon εHf(t) values (−1.8 to +3.6), suggesting their formation by partial melting of ancient mafic crust with addition of minor mantle-derived melt and subducted terrigenous sediments. In general, Hf-in-zircon isotopes characteristics of the late Paleozoic granitic rocks in the Dunhuang Block indicate that the southern Dunhuang area carries more ancient crust materials than the central and northern Dunhuang areas, indicating a lateral heterogeneity of the middle Paleozoic crust. Conclusively, the middle Paleozoic granitoids of the Dunhuang Block were emplaced at an active continental margin of the southern Paleo-Asian Ocean.