Garnet geochemistry of tungsten-mineralized Xihuashan granites in South China

Abstract

The Xihuashan complex intrusion in South China, which is emplaced at ca. 154Ma, mainly comprises medium-grained porphyritic biotite granite, medium-grained biotite granite, and fine-grained two-mica granite. Medium-grained biotite and fine-grained two-mica granites are important tungsten-bearing granites and contain an unusual amount of garnet. Garnets from this intrusion are dominated by almandine and spessartine, which constitute 94% to 99% of the total molecular composition of the garnet. These garnets display unusual compositional zoning. The cores of these garnets are rich in heavy rare earth element (HREE), Y, Ca, and contain abundant HREE- and Y-rich mineral inclusions. Their rims are free of mineral inclusions and have low of these elements. Two types of fluorite inclusions exist in garnet: Y fluorite and Y-poor fluorite. Garnet exhibits specific Mn zoning with a relatively Mn-poor core but a relatively Mn-rich rim, thus constituting a specific “spessartine inverse bell-shaped profile” that belongs to typical magmatic garnets. All analyzed garnets have high REE content and exhibit HREE-enriched and LREE-depleted patterns with strong negative Eu anomalies. The incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE3+ following a coupled substitution of the type [Fe2+,Mn2+]−1VIII[REE3+]+1VIII[Si4+]−1IV[Z3+]+1IV. The texture and compositional zoning of garnet suggest that garnet growth is largely controlled by the pressure–temperature–composition condition of magmatic evolution, internal crystal-chemical parameters, and kinetics during mineral growth. The garnet core grows in near equilibrium with magmatic melt under a relatively high pressure–temperature (P–T) condition. By contrast, the garnet rim grows rapidly by the coexisting melt-fluid phase and CO2-rich volatile environment under a relatively low P–T condition, which is virtually unfavorable for the incorporation of REE into the magmatic garnet structure. Garnet fingerprints the magmatic-hydrothermal transition during crystallization of these granites.