A topaz- and amazonite-bearing leucogranite pluton in eastern Xinjiang, NW China and its zoning

The highly evolved Baishitouquan (BST) beryl-mineralised and topaz-bearing amazonite granite pluton is situated in the eastern Tianshan orogen of northwestern China. This pluton exhibits five well-exposed lithological zones, which, gradational from the lowest level, are leucogranite (zone-a), amazonite-bearing granite (zone-b), amazonite granite (zone-c), topaz-bearing amazonite granite (zone-d) and topaz albite granite (zone-e). The rocks are composed mainly of quartz, albite, and K-feldspar with varying amounts of topaz and amazonite. Quartz and topaz phenocrysts are the earliest phases that crystallised from the melt. Amazonite which replaced albite and K-feldspar was formed at the late magmatic stage or during the magmatic-hydrothermal transition. Geochemically, this pluton is characterised by high F (>2 wt.%) and Rb (499.5–1087.04 ppm), low P2O5 (⩽0.06 wt.%), Na2O > K2O, A/NKC = 1.00–1.11, low ratios of K/Rb, Al/Ga, Y/Ho, Zr/Ha and Nb/Ta, Σ14 REE = 28.6–231.9 ppm with gull wing-shaped distribution patterns (LaCN/LuCN = 0.11–0.68, Eu/Eu* = 0.0005–0.0110) and tetrad effects, and δ18O = 9.75–7.32‰. Melt and fluid-melt inclusions coexist with liquid and vapour inclusions. The rocks were originated from a highly evolved granitic magma.The BST pluton exhibits transition in the following aspects from zone-a to zone-e: (1) As quartz and topaz phenocrysts progressively increase in size and crystal euhedral shape, rock textures change from equigranular to porphyritic. (2) Amazonite begins to appear in zone-b and becomes most concentrated in zone-c, whereas topaz begins to appear in zone-d becoming highly concentrated in zone-e. (3) Li and (Al + Ti) increase in white mica. (4) Petrochemically, there are general trends of increasing F, Al2O3 and Na2O, and decreasing SiO2, (Fe2O3 + FeO + MgO + MnO) and K2O. Plots of normative compositions on the Qz–Ab–Or diagram move gradually towards the Ab apex. (5) Overall, Cr, Ni, Co, V, W, Nb, Zr, U, Th and Y decrease, while F, Li, Rb, Hf, Ta, Sn, Sc, Ga and Zn increase. (6) K/Rb, Al/Ga, Nb/Ta, Zr/Hf and Y/Ho decrease, and K/Cs, Th/U (La/Lu)CN and TE1,3 (quantification factor of REE tetrad effect) increase. (7) There is a general decrease in whole-rock δ18O from 9.25‰ to 9.75‰ in zone-a to 7.32‰ in zone-e. (8) Homogenization temperatures of melt inclusions in quartz decrease from about 860 °C for zone-a to about 660 °C for zone-e. It is interpreted that crystallisation of the magma started from zone-a and proceeded upwards to zone-e, and the vertical zoning is produced by fractional crystallisation accompanied by fluid-melt interaction. Some of the distinctive features of zone-e were caused by influx and reaction of meteoric fluid at the post-magmatic stage. Turbulent structures and co-magmatic deformation textures are well recorded in the rocks, and are ascribed to reduced viscosity and enhanced flow rate of the F- and H2O-rich magma.

Research highlights
► Five zones of a topaz- and amazonite-bearing granite pluton are described.
► Fractional crystallization and fluid-melt interaction caused zoning of a granite.
► Fluid-rich magma body crystallizes from lower parts and upwards to the apical zone.
► Fluids in magma enhance non-CHARAC behaviour of HFSE and tetrad effect of REE.
► Abundant fluids in magma will cause turbulence and co-magmatic mineral deformation.