Zhiwei Bao

Slab break-off model for the Triassic syn-collisional granites in the Qinling orogenic belt, Central China: Zircon U-Pb age and Hf isotope constraints

Numerous Triassic granitoids in the Qinling orogenic belt related to the Late Triassic collision between the North China Craton (NCC) and the Yangtze Block (YB) are important for determining the crustal composition at depth and the geodynamic processes by which the orogen formed. Most of the Triassic plutons in the Qinling orogen were emplaced between 205 and 225 Ma. The granitoid rocks from the southern margin of the NCC, North Qinling, South Qinling, and the northern margin of the YB that were emplaced during this interval have two-stage Hf model ages of 0.60–2.52 Ga (average 2.19 Ga), 0.90–2.66 Ga (average 1.29 Ga), 0.41–3.04 Ga (average 1.48 Ga), and 1.00–1.84 Ga (average 1.34 Ga), respectively, and mean εHf(t) values of −14.5, −0.32, −1.36, and −3.98, respectively. The Hf isotope compositions of the granitoids in different tectonic units differ significantly, mirroring the diverse history of crustal growth of the four units. The temporal and spatial distribution and Hf isotope compositions of the granitoids suggest that there was a unified geodynamic process that triggered the magmatism. Formation of the Triassic granitoid plutons at 225–205 Ma was a consequence of slab break-off or E–W-striking slab tearing, related to slab rollback in the west part of the Qinling orogen and oblique continental collision in the east. Upwelling of the asthenospheric mantle led to partial melting of the subcontinental lithospheric mantle and the lower crust, and mixing and/or mingling of the resulting magmas resulted in the formation of granitoids with diverse geological and geochemical characteristics.

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.

Comparisons on Alteration and Mineralization Paragenesis between the Qiaoxiahala and the Laoshankou Fe‐Cu‐Au Deposit

belt, which is located in the Sawur Late Paleozoic island arc at the northern margin of the Junggar Terrane and hosted by the Middle Devonian Beitashan Formation volcanic and sedimentary rocks, currently incorporates the Fe-Cu-Au mineralization at Qiaoxiahala (1.44 Mt at 43% ~ 53% Fe, 0.55% ~ 2.21% Cu and 0.13 ~ 2.4 g/t Au) (Li, 2002; Wei, 2002) and at Laoshankou (3.26 Mt at 33.5% ~ 36.42% Fe, 9800 t at 0.19% ~ 0.41% Cu, 1400 t at 0.49 ~ 1.31 g/t Au) (Cheng, 2004; Lv et al., 2012). The Qiaoxiahala deposit is located 30 km to the southeast of Fuyun county, Xinjiang, China, and geologically on the southern margin of the Irtysh Fault. Alteration types mainly include garnetization, amphibolization, epidotization, chloritization, k-feldspathization, silicification and carbonatization. Based on field observations and micro-petrography, the paragenetic sequence can be established as (Fig. 1a): (1) early skarn, (2) late skarn, (3) magnetite stage, (4) magnetite-pyrite stage, (5) Cu (-Au) mineralization stage, and (6) late veins. The Laoshankou deposit is located 41 km to the southwest of Qinghe county and at the intersection between the Irysh Fault and Ertai Fault. Although the wall-rock alteration is similar to Qiaoxiahala, a slightly different paragenetic sequence was identified as (Fig. 1b): (1) early skarn, (2) late skarn, (3) magnetite-epidote stage, (4) pyrite-epidote stage, (5) Cu (-Au) mineralization stage, and (6) late veins. At Qiaoxiahala, the typical skarn alteration minerals, such as garnet and amphibole, are more widely present before magnetite mineralization compared with those in Laoshankou, at which the early skarn alteration is characterized by dominant Hypersthene. Magnetite at Qiaoxiahala is associated with quartz, apatite, sphene and K-feldspar in stage Q-III and with pyrite in stage Q-IV, respectively. However at Laoshankou, magnetite is closely associated with epidote in stage-L-III but not in sulfide mineralization stages. Chalcopyrite, probably associated with Au mineralization, usually replaced early-stage magnetite and pyrite in both Qiaoxiahala and Laoshankou, indicating a possible different hydrothermal fluid system.