The role of external fluid in the Shanggusi dynamic granitic magma system, East Qinling, China: Quantitative integration of textural and chemical data

Abstract

It is well recognized that various degrees of mantle-derived materials are incorporated in the formation of granite, and mantle-derived mafic melts are generally considered to mix with crustal felsic melt. Here, however, we provide an example of the Shanggusi leucocratic granite where external mantle-derived hydrous fluid, rather than mafic melt, might be incorporated into a nearly pure crustal granitic melt system. Field observations suggest that the Shanggusi granite consists of granite porphyry, granite dyke and granitic pegmatite and they have consistent zircon U–Pb ages and molybdenite Re–Os ages. The marginal pegmatite, interconnected miarolitic cavities, heterogeneous molybdenite mineralization and significant variation of micro-texture of the Shanggusi granite physically indicate that strong fluid activities occurred in the granitic system. Accumulation of quartz and K-feldspar and bulk-rock major element data imply that fractional crystallization played an important role in the evolution of the granitic system which, however, cannot reasonably explain the significant trace elements fractionation, non-CHARAC trace elements behavior and simultaneous concave and convex REE tetrad effect of the Shanggusi granite, but which can be best explained by the influence of external fluorine-rich hydrous fluid. Importantly, the chemical fractionation, including bulk-rock trace elements and isotopes, is closely correlated with quantitative textural parameter L max (the average length of the four largest quartz crystals in each sample), indicating that the vast majority of physical and chemical characteristics of the granitic system were most likely controlled by the wholesale fluid flow. The Shanggusi granite is highly siliceous (SiO 2 = 74.91–79.50 wt.%, except granitic pegmatite with SiO 2 = 67.41 wt.%), extremely poor in mafic minerals, and with relative homogeneous bulk-rock major element chemistry and mineralogy, which approximate experimentally pure crustal melt that generated by dehydration melting of sedimentary rocks. The mantle-derived Nd–Pb–O isotopic features, however, require a great deal of contributions from mantle materials. Therefore, it is proposed that external mantle-derived fluid was infiltrated into the Shanggusi granitic system at least before emplacement, and fluid–melt mixing may be an important but not easily recognized part in granite origin. • Quantitative integration of textural and chemical data in igneous petrology • External mantle-derived fluid was involved in the Shanggusi granitic system. • Fluid-induced chemical and textural diversity