Mingjie Zhang

The Kalatongke magmatic Ni–Cu deposits in the Central Asian Orogenic Belt, NW China: product of slab window magmatism?

The Permian Kalatongke Ni–Cu deposits in the Central Asian Orogenic Belt are among the most important Ni–Cu deposits in northern Xinjiang, western China. The deposits are hosted by three small mafic intrusions comprising mainly norite and diorite. Its tectonic context, petrogenesis, and ore genesis have been highly contested. In this paper, we present a new model involving slab window magmatism for the Kalatongke intrusions. The origin of the associated sulfide ores is explained in the context of this new model. Minor amounts of olivine in the intrusions have Fo contents varying between 71 and 81.5xa0mol%, which are similar to the predicted values for olivine crystallizing from coeval basalts in the region. Analytic modeling based on major element concentrations suggests that the parental magma of the Kalatongke intrusions and the coeval basalts represent fractionated liquids produced by ∼15% of olivine crystallization from a primary magma, itself produced by 7–8% partial melting of depleted mantle peridotite. Positive εNd values (+4 to +10) and significant negative Nb anomalies for both intrusive and extrusive rocks can be explained by the mixing of magma derived from depleted mantle with 6–18% of a partial melt derived from the lower part of a juvenile arc crust with a composition similar to coeval A-type granites in the region, plus up to 10% contamination with the upper continental crust. Our model suggests that a slab window was created due to slab break-off during a transition from oceanic subduction to arc–arc or arc–continent collision in the region in the Early Permian. Decompression melting in the upwelling oceanic asthenosphere produced the primary magma. When this magma ascended to pond in the lower parts of a juvenile arc crust, it underwent olivine crystallization and at the same time triggered partial melting of the arc crust. Mixing between these two magmas followed by contamination with the upper crust after the magma ascended to higher crustal levels formed the parental magma of the Kalatongke intrusions. The parental magma of the Kalatongke intrusions was saturated with sulfide upon arrival primarily due to olivine fractional crystallization and selective assimilation of crustal sulfur. Sulfide mineralization in the Kalatongke intrusions can be explained by accumulation of immiscible sulfide droplets by flow differentiation, gravitational settling, and downward percolation which operated in different parts of the intrusions. Platinum-group element (PGE) depletion in the bulk sulfide ores of the Kalatongke deposits was due to depletion in the parental magma which in turn was likely due to depletion in the primary magma. PGE depletion in the primary magma can be explained by a relatively low degree of partial melting of the mantle and retention of coexisting sulfide liquid in the mantle.

U-Pb age and Hf isotopes of zircon from basaltic andesite and geochemical fingerprinting of the associated picrites in the Emeishan large igneous province, SW China

The ages of many picrites associated with Permian flood basalts in the Emeishan large igneous province, SW China have been debated for a long time. Some researchers believe that these picrites are co-magmatic with the flood basalts whereas others believe that they are Triassic picritic porphyrites or Cenozoic olivine-rich dikes. We report a U–Pb age of 256.2u2009±u20091.4xa0Ma for the youngest zircon grains from the Binchuan volcanic sequence which contains the disputed picrites. This age is within the range of values reported for the intrusive and extrusive rocks of the Emeishan large igneous province. The εHf values (−0.3 to 6.6) of the youngest zircon crystals from the Binchuan volcanic sequence are similar to those (−5 to 10) of co-magmatic zircon crystals from the intrusive rocks of the Emeishan large igneous province. Our new data confirm that the Binchuan volcanic sequence is an integral part of the Emeishan large igneous province. The disputed and undisputed picrites in Sichuan and Yunnan have similar olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. This, together with their spatially close association with the Emeishan flood basalts, indicates that the disputed picrites also belong to the Emeishan large igneous province. The confusing Cenozoic olivine-rich dikes in the region can be distinguished from the picrites of the Emeishan large igneous province by their different olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. The results from this study vindicate the use of the previously-disputed picrites as evidences for Permian mantle plume activity in SW China.

Carbon Isotope Reversals of Changning-Weiyuan Region Shale Gas, Sichuan Basin

Many high yield shale gas areas in the World are discovered carbon isotope reversals: Barnett, Fayetteville (Zumberge et al., 2012), Marcellus (Tilley et al., 2013), Haynesville (Ferworn et al., 2008), Albany shale gas (Gao et al., 2014), Utica shale gas (Xia et al., 1999; Xia et al., 2012; Xia et al., 2013), the Foothills (Tilley et al., 2011), Horn River (Tilley et al., 2013) and Ordos basin shale gas (Wang et al., 2015; Dai et al., 2005). The occurrence probability of carbon isotope reversals in shale gas is far higher than that of conventional gas plays. And, this isotopic abnormal phenomenon has become a typical characteristic of shale gas. Many data show that carbon isotope reversals can be used to indicate the overpressure in shale gas reservoir and shale gas higher yield (Zumberge et al., 2012; Gao et al., 2014; Tilley et al., 2013; Hao et al., 2013). However, isotope reversal occurred both at high and low production area in Marcellus (Madren, 2012). So, there are still different opinions about carbon isotope reversals indicating high yield of shale gas.

The Ore-forming Conditions of the Zhubu Magmatic Ni-Cu-PGE Deposit, Emeishan Large Igneous Province, SW China: Constraints from Hf-Sr-Nd-C Isotopic Compositions and Volatile Chemical Compositions

[Tang Qingyan] Lanzhou Univ, Gansu Key Lab Mineral Resources Western China, Lanzhou 730000, Peoples R China