Shi-Yong Liao

Origin of the Dexing Cu-bearing porphyries, SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (∼171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (∼171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044−0.7047, ϵNd(T) values of –1.5 to +0.6, and ϵHf(T) (in situ zircon) values of +2.6 to +4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.

Mantle origin of the Dexing porphyry copper deposit, SE China

The Dexing ore deposit, Jiangxi Province, is the largest porphyry copper deposit in China. Controversies exist regarding the ore-forming source of this deposit. We have conducted Pb isotope analyses of pyrites from the Tongchang and Fujiawu mines. Our results document consistent Pb isotopes from these two orebodies, with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.954–18.320, 15.407–15.517, and 37.888–38.153, respectively. These Pb isotope ratios are consistent with those of ore-bearing adakitic porphyries but distinctly different from those of the Neoproterozoic metamorphic wall rocks, which indicates that the metals were derived from the porphyries. Based on previous S and Os isotopic data and comparisons with more than 20 Mo-bearing deposits worldwide, we further attribute the narrow range of δ34S values of sulphide minerals and high Re–187Os concentrations of associated molybdenites to a mantle origin. This large-scale copper deposit was evidently emplaced in a continental arc setting attending westward subduction of the palaeo-Pacific plate. Partial melting of the downgoing oceanic slab generated the adakitic magmas. The associated metals were extracted from the lithospheric mantle by these magmas during ascent through the mantle wedge.

Petrogenesis and tectonic implications of ultrapotassic microgranitoid enclaves in Late Triassic arc granitoids, Qinling orogen, central China

The origin of microgranitoid enclaves in granitic plutons has long been debated (hybrid magma blobs vs. refractory restites or cognate fragments). This article presents detailed petrography, SHRIMP zircon U–Pb chronology, bulk-rock major and trace element analyses, and Sr–Nd isotope and in situ zircon Hf isotopic geochemistry for microgranitoid enclaves within two Late Triassic granitic plutons in the Qinling orogen. Zircon U–Pb dating shows that the enclaves formed during the Carnian (222.5 ± 2.1 to 220.7 ± 1.9 Ma) coeval with their host granitoids (220.0 ± 2.0 to 218.7 ± 2.4 Ma). Field and petrological observations (e.g. double enclaves, xenocrysts, acicular apatite, and poikilitic K-feldspar or quartz) suggest that the enclaves are globules of a mantle-derived more mafic magma that was injected into and mingled with the host magma. The enclaves are mainly ultrapotassic, distinct from the host granitoids that have high-K calc-alkaline bulk-rock compositions. Although the enclaves have closely similar bulk-rock Sr–Nd isotope [initial 87Sr/86Sr = 0.7046–0.7056, ϵNd (T) = –0.3 to –5.0] and in situ zircon Hf isotope [ϵHf (T) = –1.5 to +2.9] ratios as the granitoids [initial 87Sr/86Sr = 0.7042–0.7059, ϵNd (T) = –0.6 to –6.3, ϵHf (T) = –2.2 to +1.6], chemical relationships including very different bulk-rock compositions at a given SiO2 content lead us to interpret the isotopic similarities as reflecting similar but separate isotopic source rocks. Detailed elemental and isotopic data suggest that the enclaves and the host granitoids were emplaced in a continental arc environment coupled with northward subduction of the Palaeo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with melts interacting with the overlying mantle wedge, formed high-K calc-alkaline granitic magmas, whereas partial melting of diapiric phlogopite-pyroxenites, solidified products of the same subducting sediment-derived melts, generated ultrapotassic magmas of the microgranitoid enclaves. Our new data further confirm that in the Late Triassic time the Qinling terrane was an active continental margin rather than a post-collisional regime, giving new insights into the tectonic evolution of this orogen.

Origin of Early Cretaceous high-K calc-alkaline granitoids, western Tibet: implications for the evolution of the Tethys in NW China

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia attending the transition of the Palaeo- to Neo-Tethys. We present the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Cretaceous plutons (Kalaqigu, Aranbaotai, and Hongqilapu) from this area. SHRIMP zircon U–Pb dating shows that the three plutons were emplaced in the Early Cretaceous. The 114 Ma Kalaqigu pluton is composed of strongly peraluminous high-K calc-alkaline monzogranite and granodiorite. These rocks consist of plagioclase, K-feldspar, quartz, and biotite, and show variable Sr–Nd and Hf (in situ zircon) isotopic compositions [87Sr/86Sr (T) = 0.7098–0.7157, ϵNd (T) = –8.1 to –15.6, and ϵHf (T) = –10.2 to –19.8 (mean –14.8)]. Elemental and isotopic data suggest that the granitoids were generated by partial melting of metasedimentary basement in the normal to thickened lower-crust triggered by underplating or injection of hot mantle-derived magmas in a continental arc setting. The 110 Ma Aranbaotai pluton consists of strongly peraluminous high-K calc-alkaline two-mica monzogranites. These rocks are composed of K-feldspar, plagioclase, quartz, biotite, and muscovite, and also show variable Sr–Nd and Hf (in situ zircon) isotopic compositions [87Sr/86Sr (T) = 0.7075–0.7144, ϵNd (T) = –9.4 to –13.3, and ϵHf (T) = –5.4 to –16.4 (mean –10.0)]. Elemental and isotopic data suggest that the granites were formed by partial melting of subducted sediments at a depths of <40 km to ∼40–50 km in the collision (overthrust) zone during continental collision. The 102 Ma Hongqilapu pluton consists of metaluminous high-K calc-alkaline granodiorites. These rocks are composed of plagioclase + K-feldspar + quartz + amphibole + biotite ± clinopyroxene, and show uniform Sr–Nd and Hf (in situ zircon) isotopic compositions [87Sr/86Sr (T) = 0.7078, ϵNd (T) = –7.6 to –8.0, and ϵHf (T) = –7.2 to –11.2 (mean –9.4)]. Elemental and isotopic data suggest that the granitoids were generated by crystal fractionation of basaltic magmas in a continental arc setting. Our new data suggest that at 114 Ma, the Karakorum terrane was still an active continental arc fuelled by the northword slab subduction of the Mesozoic Tethys; collision of the Kohistan terrane with the Karakorum terrane most likely occurred at 110 Ma, resulting in the closure of the Mesozoic Tethys; since 102 Ma, the southern Karakorum terrane hosted to a new continental arc coupled with the northward slab subduction of the Neo-Tethys.