Accessory mineral record of magma-fluid interaction in the Laoshan I- and A-type granitic complex, Eastern China

Abstract

The Laoshan granitic complex (LGC) in East China is composed of many intrusive units which can be divided into two distinct suites, I-type and A-type. There exist different assemblages of accessory minerals between the two units. In the I-type suite, the quartz monzonite contains the accessory mineral assemblage of titanite + magnetite + zircon, the biotite monzogranite is titanite + magnetite + ilmenite + zircon, whereas the accessory mineral assemblage in the quartz syenite is rutile + magnetite + zircon. In the alkali granite of the A-type suite, accessory mineral assemblage consists of Fe-Ti minerals including titanite, magnetite, ilmenite and rutile in addition to frequently-observed zircon and sparse pyrochlore. In the arfvedsonite granite, however, magnetite, titanite and rutile disappear, Nb-rich pyrophanite-dominant ilmenite becomes the only Ti mineral. In addition, large amounts of mineral containing REE and HFSE were crystallized. Zircon is also present in the all two facies of the A-type suite, but with different compositions. Especially, zircon in the arfvedsonite granite is enriched in Hf but poor in U and Th relative to the alkali granite. The change in accessory mineral assemblage in the different facies of both I-type and A-type suites reflects variation in temperature and/or fO₂ during crystallization of every magma chamber. Particularly, presence of the large amount of HFSE- and REE-bearing minerals in the arfvedsonite granite may be attributed to the accumulation of fluids at the end of evolution of A-type magma. Geochemically, the fluid is enriched, on one hand, in REE and HFSE, and on other hand, in volatiles (F, P and CO₂).