Hecai Niu

Geochemistry of late Paleozoic mafic igneous rocks from the Kuerti area, Xinjiang, northwest China: implications for backarc mantle evolution

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high eNd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian.

Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay, North Xinjiang

Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

Adakites related to subduction in the northern margin of Junggar arc for the Late Paleozoic: Products of slab melting

Volcanic rocks with adakitic compositional signature have been recognized in the northern margin of ancient Junggar island arc for the Late Paleozoic. These adakites for the early Devonian from the Tuoranggekudouke Group (D1t) are characteristic of high Sr, Sr/Y and (La/Yb)N but low Y, Yb and HREE. Their compositional characteristics are much similar to those of the typical adakites in the world but distinct from those of the normal arc volcanic rocks from the same Group. We conclude that these adakitic volcanic rocks were produced by slab melting during the early period of Paleoasia-ocean lithosphere subduction. This infers that the Paleoasia Ocean in the north Junggar area began a new subduction process in the early Devonian.

Discovery of Mg-righ volcanic rock series in western Altay area, Xinjiang and its geologic significance

A series of Mg-rich volcanic rocks, including rare Mg-rich dacites in the Asherle copper mine, western Altay Area, has been found. This suite of Mg-rich volcanic rocks is characterized by high SiO2, MgO and low Ti contents. Compared with the island arc tholeiite and MORB, these rocks are depleted in HFSE and are therefore similar to boninite. The dacites show V-shaped distribution REE patterns whereas the basalts display flat or LREE-depletton patterns. Based on their geochemical features and occurrence, it is suggested that they were formed in oceanic island arc setting. Its petrogenesis is related to the slab movement. At the early stage, the peridotite of the ocean upper mantle diapirically uprised to the shallow level and started to melt owing to subduction of the ocean slab, forming the island arc tholeiite with pillow structure. At the late stage, the residual refractory peridotites melted again, giving rise to the formation of the parent magma of Mg-rich basalt.

Isotopic evidence for continental ice sheet in mid-latitude region in the supergreenhouse Early Cretaceous

Cretaceous represents one of the hottest greenhouse periods in the Earths history, but some recent studies suggest that small ice caps might be present in non-polar regions during certain periods in the Early Cretaceous. Here we report extremely negative δ18O values of −18.12‰ to −13.19‰ for early Aptian hydrothermal zircon from an A-type granite at Baerzhe in northeastern China. Given that A-type granite is anhydrous and that magmatic zircon of the Baerzhe granite has δ18O value close to mantle values, the extremely negative δ18O values for hydrothermal zircon are attributed to addition of meteoric water with extremely low δ18O, mostly likely transported by glaciers. Considering the paleoaltitude of the region, continental glaciation is suggested to occur in the early Aptian, indicating much larger temperature fluctuations than previously thought during the supergreenhouse Cretaceous. This may have impact on the evolution of major organism in the Jehol Group during this period.

Adakite-type sodium-rich rocks in Awulale Mountain of west Tianshan: Significance for the vertical growth of continental crust

The sodium-rich dacites and albite porphyries of Permian in the Awulale Mountain of west Tianshan have unique chemical and trace element signatures identical to adakite. These intermediate-acidic igneous rocks are characterized by high Na2O, Al2O3 and Sr contents and high Sr/Y and La/Y ratios (> 40 and > 20, respectively), and low Y and Yb contents, and strong depletion in HREE, and positive Eu anomaly. The (143Nd/144Nd)i is in the range from 0.51236 to 0.51248 and the εNd(t) is positive value (+0.79+3.11); the (87Sr/86Sr)i is in the range from 0.7052 to 0.7054. These Nd and Sr isotopic composition features indicate that the source rocks of these adakite-type rocks are from a weakly depleted mantle, or a depleted mantle, but was contaminated by the crustal materials. These adakite-type rocks were most likely derived from the partial melting of new underplated basaltic rocks under the conditions of amphibolite to eclogite transition in the postcollisional environment of North Xinjiang during the Permian Period. They are not only the Phanerozoic juvenile crust materials, but also are probably animportant symbol of the underplating of mantlederived basaltic magmas and the vertical growth of continental crust in the west Tianshan area during the postcollision of Late Paleozoic.

Age, petrogenesis and tectonic significance of the ferrobasalts in the Chagangnuoer iron deposit, western Tianshan

The formation of large iron deposits associated with subduction and its genetic relationships with ferrobasalts are not yet well understood. Here we report a geochemical and geochronological investigation on the newly discovered ferrobasalts associated with the Chagangnuoer iron deposit, western Tianshan. The Chagangnuoer ferrobasalts are characterized by high Fe2O3T (14.55–22.68 wt.%) and MnO (0.36–0.93 wt.%) but low TiO2 (0.70–1.26 wt.%) contents. Analyses of 10 zircon grains yield a weighted zircon U–Pb age of 314 ± 8 Ma. Based on our new petrological and geochemical data, we conclude that the Chagangnuoer ferrobasalts probably have been originated from the partial melting of a spinel peridotite mantle source that has been modified by subduction related fluids. The ferrobasalts have nearly linear positive correlation between MnO and (87Sr/86Sr)i, implying the involvement of subducted Fe–Mn nodules. The mid-ocean ridge basalt (MORB)- and ocean island basalt (OIB)-like geochemical features, as well as moderate Ti/V values (18–36), indicate that the ferrobasalts may have been formed in an extensional back-arc basin setting. Combined with previous studies on the Chagangnuoer iron deposit, we propose a hypothesis that the overlying iron orebodies were likely derived from the ferrobasaltic magma.

Chemical lattice expansion of natural zircon during the magmatic-hydrothermal evolution of A-type granite

Abstract Although thermal lattice expansion is a well-documented nature of crystals, including zircon and zircon-type minerals, chemical lattice expansion of natural mineral is rarely reported. Here we present a comprehensive investigation on three types of natural zircon that records the evolution of the granitic system in Xiangshan, North China, and shows expanding crystallographic parameters induced by chemical incorporation instead of thermal expansion. Prismatic and oscillatory-zoned zircon grains (Type-1A), crystallized early in the granitic magma at high temperatures in a volatile-undersaturated environment, have the smallest lattice parameters (a = 6.603 Å, c = 5.971 Å). Prismatic and altered zircon grains (Type-1B), formed under volatile-saturated conditions and in the presence of F-rich fluid with numerous thorite and xenotime inclusions, have intermediate lattice parameters (a = 6.649 Å, c = 6.020 Å). Pyramidal zircon grains (Type-2), formed in a subsolvus granitic system at relatively low temperatures and coexisted with fluid inclusions, have the biggest lattice parameters (a = 6.677 Å, c = 6.010 Å). Trace elements, including Hf, Th, Ti, Y, and REE, and volatiles content, increase in the structure of zircon from the early to late magmatic origin, which is consistent with the expansion of the lattice parameters. The occurrence of the three zircon types in the Xiangshan arfvedsonite granites is interpreted to reflect the progressive fractionation of granitic melt from hypersolvus to subsolvus conditions. Therefore, we conclude that the lattice expansion of zircon in this study results from chemical incorporation of trace element and volatile components during the magmatic to hydrothermal evolution of granitic magma. Besides, the textural and compositional evolution of zircon can be used as efficient indices for the fractionation and evolution of A-type granitic system.

Paleoproterozoic Copper System in the Zhongtiaoshan Region, Southern Margin of the North China Craton: Ore Geology, Fluid Inclusion, and Isotopic Investigation

The Zhongtiaoshan region is located in the south segment of the North China Craton and hosts a number of significant Paleoproterozoic copper deposits with a total metal endowment of approximately 400 Mt of metal Cu. Among these Cu deposits, the Tongkuangyu and Hujiayu Cu deposits constitute approximately 80 % of the total reserves. The Tongkuangyu Cu deposit, the largest copper deposit in the Zhongtiaoshan region, is hosted in quartz-monzonite porphyry (~2.1 Ga) and its wall rocks of the meta-quartz crystal tuffs of the Jiangxian Group. In contrast, the Hujiayu Cu deposit is hosted within reduced marine sedimentary sequence of the mid-Paleoproterozoic Zhongtiao Group. For decades, as these ancient deposits underwent subsequent metamorphism, the metallogenesis models of the two deposits are highly controversial. More importantly, the nature and origins of the ore-forming fluids and the mechanism of multistage fluid mineralization are still unclear. Summarizing previous researches, we systematically investigate the ore geology, fluid inclusions, and stable isotopes of the typical Tongkuangyu and Hujiayu Cu deposits. The main conclusions are as follows: (1) The oxygen fugacity of hydrothermal system of the Tongkuangyu Cu deposit fluctuates near the Magnetite–Hematite (MH) buffer line, corresponding to the redox state of typical oxidized porphyry copper deposits. The main ore-forming fluids of this deposit consist of magmatic–hydrothermal fluids with middle-high temperature and high salinity. Orebody spatial structure, mineralization types, and alteration features support a porphyry copper deposit model. The formation age of the quartz-monzonite porphyry is consistent with Cu mineralization. We propose that the Tongkuangyu Cu deposit is a porphyry copper deposit formed in an arc-related extension environment. (2) Fluid inclusion studies on the Hujiayu Cu deposit show that the ore-forming fluids of the early mineralization stage are mainly characterized by high salinity and moderate temperature basinal brines. The ore-forming fluids of the late mineralization stage are characterized by CO2-rich and high salinity and high temperature metamorphic hydrothermal solutions, which obviously experience phase separation. Early stage mineralization of the Hujiayu Cu deposit may occur via interaction of oxidized Cu-bearing brines from the underlying red beds [formed after the Great Oxidation Event (GOE)] with the upper reducing carbonaceous shales. In contrast, late stage mineralization at the Hujiayu deposit is likely related to CO2 escaping from metamorphic hydrothermal solutions. The Hujiayu copper deposit is a typical sediment-hosted stratiform copper deposit. The “Hu-Bi” type copper deposits can be comparable with Central Africa Copper Belt.