Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt. Granites are widespread in South China and a majority of them are peraluminous. Particularly, over 91% of the Indosinian granites exposed in the region are peraluminous in composition. It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole, a good mineral barometer commonly identified in metaluminous granites. Muscovite is a common mineral in peraluminous granites, certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites. In this paper, results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported. Based on petrographic studies, the primary muscovite can be discriminated from the secondary muscovites. Muscovites from the Indosinian two-mica granites are enriched in Ti, Al, Mg, and Na, and depleted in Fe and Mn. Geochemically, these muscovites were considered as primary, whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams. Barometer estimations show that pressures calculated for primary muscovites are accurate, but those calculated for secondary muscovites are overestimated. The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar, corresponding to ～19 km in depth, suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.