Formation and evolution of snowball quartz phenocrysts in the Dongping porphyritic granite, Hebei Province, China: Insights from fluid inclusions, cathodoluminescence, trace elements, and crystal size distribution study

摘要

Quartz phenocrysts are common in many granites, volcanic rocks, metamorphic rocks and hydrothermal deposits. The Dongping porphyritic granite contains large amounts of rounded to sub-rounded quartz phenocrysts. Minerals in this porphyritic granite show weak but distinct orientation. Dense clusters of fluid inclusions and albite crystals exist in the quartz, showing snowball texture. Two types of snowball quartz are recognized: (1) large snowball quartz phenocrysts containing large albite grains distributed in the core and small albite grains distributed in the mantle, and (2) small snowball quartz crystals containing small albite grains. The granite is characterized by high SiO2 (7.15-73.44 wt%), K2O (4.70-5.10 wt%) and Na2O (4.93-5.21 wt%) contents, low CaO (0.41-0.71 wt%), MgO (0.03-0.05 wt%), Fe2O3 (0.87-0.94 wt%), TiO2 (0.02-0.03 wt%) and Sigma REE (7.10-9.80 ppm) contents. The granite has similar mineral and chemical composition with the aplite in the area, and they were derived from the residual melt of the Shuiquangou magma that suffered assimilation fractional crystallization process. Fluid inclusions in the quartz phenocrysts have homogenization temperatures of 260-380 degrees C and salinities of 1.4-12.4 wt% NaCl equiv. for liquid + vapor type inclusions and 34.1-44.3 wt% NaCl equiv. for brine ones. The quartz phenocrysts exhibit homogeneous cathodoluminescence without growth bands or zoning. The trace element contents in the quartz phenocrysts are extremely low (Ti 3.4-5.1 ppm, Al 13-118 ppm, Li < 0.1 ppm, Ge < 0.5 ppm) compared to published data of igneous quartz, which may result from the crystallizing conditions of quartz and chemical compositions of the magma. Crystal size distributions (CSD) of albite in quartz show that the snowball quartz formed in two stages, whereby an increase in nucleation and growth rates resulted in the core-mantle texture of quartz phenocrysts. The co-existence of low-, high-salinity and melt inclusions indicates that the quartz phenocrysts crystallized rapidly in a fluid-enriched magma. Magmatic flow led to mineral rotation and resulted in the rounded and sub-rounded shapes of the quartz phenocrysts. Fluid exsolved from the magma raised nucleation and growth rates and resulted in the two types of snowball quartz. (C) 2019 Elsevier B.V. All rights reserved.