Zhongmei Wang

Palaeozoic porphyry Cu–Au and ultramafic Cu–Ni deposits in the eastern Tianshan orogenic belt: temporal constraints from U–Pb geochronology

During late Palaeozoic time, extensive magmatism and associated ore deposits were developed in the eastern Tianshan orogenic belt (ETOB), Northwest China, which is part of the Central Asian Orogenic Belt. To understand the petrogenesis of the intrusions in this area, we performed in situ zircon U–Pb and Hf isotopic analyses on the Tuwu–Yandong (TW–YD) stocks and the Xianshan, Hulu, Luodong, and Poshi batholiths. Two major suites of intrusive rocks have been recognized in the ETOB: (1) 338–339 Ma plagiogranite porphyries and 265–300 Ma ultramafic and mafic rocks, of which the former are associated with 323 Ma porphyry Cu–Mo deposits and have enriched radiogenic Hf isotopic compositions (ϵHf(t) = +11.5 to +15.6), which were derived from a depleted mantle source, whereas the latter are associated with 265–300 Ma magmatic Ni–Cu deposits and have variable Hf isotopic compositions (ϵHf(t) = −10.3 to +14.3), indicating an origin via the hybridization of depleted mantle magma and variable amounts of ancient lower-crustal components. The proposed magma sources, combined with the geochemical differences between these two suites of intrusive rocks, indicate that in the lower to middle Carboniferous, a N-dipping subduction zone beneath the Dananhu arc triggered the emplacement of granitic porphyries in the Tousuquan and Dananhu island arc belt in the east Tianshan, leading to the formation of the TW and YD porphyry Cu–Mo deposits. In the Upper Carboniferous to Lower Permian, large mafic–ultramafic complexes were emplaced during the closure of the ancient Tianshan Ocean, resulting in the formation of several magmatic Cu–Ni sulphide deposits.

Paleozoic subduction-related magmatism in the Duobagou area, Dunhuang block: Constrained by zircon U-Pb geochronology and Lu-Hf isotopes and whole-rock geochemistry of metaigneous rocks

The Dunhuang block, situated at the junction of the Tethyan domain, Tarim craton, North China craton and the Central Asian Orogenic Belt (CAOB), consists mainly of Archean–early Mesoproterozoic basement rocks and Paleozoic metamorphic and magmatic rocks. In this study we performed systematic analyses on the Duobagou metaigneous rocks, including zircon U-Pb ages and Hf isotopes and whole-rock geochemistry, to discuss the evolutionary history of the Dunhuang block and the surrounding continental fragments during the Paleozoic. The metaigneous rocks in the Duobagou area have been identified to be amphibole plagiogneisses, amphibolites, and biotite plagiogneisses; the first two are classified as metamafic rocks, and the latter are metafelsic rocks. The whole-rock major and trace element and Sm-Nd isotopic compositions suggest that the amphibole plagiogneisses were probably produced by the fractionation of the amphibolites, and their protoliths were formed from a mantle wedge, which was associated with the emplacement of subduction-related arc basaltic magmas. The biotite plagiogneisses have geochemical affinities to typical Phanerozoic arc volcanic rocks, and in combination with the Nd-Hf isotopic compositions, these indicate that they were derived from island-arc magmas with varying degrees of crustal contamination. The Duobagou amphibole plagiogneisses and amphibolites have metamorphic ages of 425 ± 31 Ma and 442 ± 2 Ma, respectively, that are in agreement with the crystallization age (436 ± 5 Ma) of the biotite plagiogneisses. The geochemical and geochronological features of these metafelsic rocks provide substantial evidence of a major subduction-related Paleozoic magmatic event in the Duobagou area of the Dunhuang block. The available data illustrate that the accretionary orogenesis in the Dunhuang block may have continued to the Carboniferous, and those in the Beishan orogenic belt most likely lasted until the Permian. However, those recorded in the North Altun–Qilian orogenic belt probably ended in the Ordovician. The temporal-spatial distributions of the orogeny-related metamorphic-magmatic rocks in the Dunhuang block seemingly match well with those in the CAOB, supporting the interpretation that the Dunhuang block was most likely involved in the orogenic activities of the CAOB during the Paleozoic. LITHOSPHERE; v. 9; no. 6; p. 1012–1032 | Published online 2 November 2017 https:// doi .org /10 .1130 /L688 .1