Possible imprints of late Paleoproterozoic orogeny in the Dunhuang terrane, NW China: Constraints from igneous and metapelitic rocks

Abstract

The Dunhuang terrane is a NEE trending tectonic belt confined by the Tarim Craton, Beishan Orogen, Alxa Block and Altyn Tagh Fault. To constrain the late Paleoproterozoic geotectonic background of this terrane, a synthetic research on petrology, geochemistry, zircon and monazite geochronology, and zircon Lu-Hf and whole-rock Nd isotopes of late Paleoproterozoic (ca. 1.9–1.8 Ga) igneous rocks and metapelites was conducted. The Sanweishan-Dongshuigou Na-rich rhyolite and granite formed at ca. 1.84–1.82 Ga exhibit enriched light rare earth elements (LREEs), depleted heavy rare earth elements (HREEs) and insignificant Eu anomalies, with low Y (3.44–6.39 ppm) and Yb (0.29–0.53 ppm) but relatively high Mg# (54.5–58.5) and Sr (368–643 ppm), and variable Cr (5.58–29.7 ppm) and Ni (2.82–15.6 ppm) contents, as well as negative to positive εHf(t) (−6.1 to +6.4) values, indicating subduction-related arc magmatism accompanied by crustal-mantle interaction. The Shuwozi granitic gneiss crystallized at ca. 1.81 Ga displays S-type granite affinity (A/CNK > 1.1) and has low Sr/Y (1.0–1.3) and (Gd/Yb)N (1.50–1.98) ratios, and negative εHf(t) (−20.3 to −5.1) and εNd(t) (−4.3) values, suggesting partial melting of crustal rocks at shallow crustal depth. The Hanxia amphibolite with protolith age of ca. 1.86–1.84 Ga is featured by depleted LREE and flat HREE patterns, insignificant Eu anomalies, low Nb/Ta (11.0–12.0), Ta/Yb (0.02–0.04), Th/Yb (0.01–0.04) and Th/Ta (0.7–1.3) but high Ti/V (22.9–26.7) ratios, and positive εNd(t) (2.7–4.0) values, resembling depleted mantle-derived N-MORB- or BABB (back-arc-basin basalt) geochemical affinities. The metapelite, characterized by mineral assemblage of garnet, biotite, sillimanite, plagioclase, K-feldspar, quartz and rutile, documented ca. 1.81 Ga upper amphibolite-facies metamorphism with peak metamorphic P-T conditions being restricted to 690–810 °C and 5.3–6.4 kbar based on conventional geothermobarometry. These magmatic and metamorphic rocks were variously altered and deformed during Paleozoic (ca. 430–360 Ma) tectonothermal event. Integrated with published data, it can be inferred that the Dunhuang terrane underwent ca. 1.89–1.81 Ga subduction-collisional orogeny related to late Paleoproterozoic (ca. 2.0–1.8 Ga) global assembly of the Columbia supercontinent, similar as those recorded in the northern Tarim Craton, Alxa Block and Quanji massif, and then overprinted Paleozoic (ca. 440–360 Ma) orogenic process, corresponding to the previously proposed composite orogeny in the Dunhuang terrane.