Early Paleozoic arc–back-arc system in the southeastern margin of the North Qilian Orogen, China: Constraints from geochronology, and whole-rock elemental and Sr-Nd-Pb-Hf isotopic geochemistry of volcanic suites

Abstract

The North Qilian Orogen is considered to represent an oceanic suture zone formed by closure of the North Qilian Ocean, which formed the northern part of the Proto-Tethys Ocean along the northern margin of the eastern Gondwana during the Early Paleozoic. However, the Early Paleozoic subduction system remains controversial, especially along the southeastern margin where it is connected to the Qinling Orogen. Here we present zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Pb-Hf isotopic data on a suite of andesitic–dacitic volcanic rocks from the Chenjiahe Group and basalts–basaltic andesites from the Hongtubao Formation in the southeast margin of the North Qilian Orogen. SHRIMP and LA-ICP-MS zircon U-Pb dating of dacite and basaltic andesite sample yield crystallization ages of ca. 443Ma and 438Ma. The andesitic–dacitic volcanic rocks exhibit calc-alkaline feature, with LREE- and LILE-enrichment, Nb–Ta depletion, moderate whole-rock (87Sr/86Sr)i (0.7055–0.7071), εNd(t) (−4.5 to 0.3) and εHf(t) (−0.3 to 5.6) values, and enriched Pb-isotopic compositions. Our data suggest magma derivation from ancient crustal sources involving juvenile materials by magmatic underplating, followed by MASH processes under a continental arc setting. The basalts–basaltic andesites are characterized by medium- to low-K tholeiitic, LREE- and LILE-enrichment and Nb-Ta depletion. Their whole-rock Sr-Nd-Pb-Hf isotopes show depleted mantle features with lower (87Sr/86Sr)i (0.7044–0.7047), positive εNd(t) (3.0–5.9) and εHf(t) (11.1–11.3) values, and Nd-Pb-Hf isotopic features that exhibit affinity with the Tethyan tectonic domain. These features could reflect partial melting of an asthenospheric mantle that witnessed recent subduction-related metasomatism and experienced fractional crystallization in an initial back-arc setting. We suggest that the Early Paleozoic arc–back-arc system was formed during the southward oceanic subduction beneath the Qilian Block in the southeastern margin of the North Qilian Orogen and that the Early Paleozoic subduction system might be inconsistent in different parts of the North Qilian Orogen.