P– T conditions of crystallization and origin of plagioclase-mantled alkali feldspar megacrysts in the Mesozoic granitoids in the Qinling orogen (China)

Abstract

The Qinling orogen between the North China and South China cratons was intruded at 211–217 Ma by calc-alkaline quartz monzonitic to monzogranitic plutons characterized by I- to A-type geochemistry and in many places contain plagioclase-mantled alkali feldspar megacrysts (rapakivi texture sensu lato). The felsic rocks contain mafic to intermediate magmatic enclaves suggestive of mingling and mixing of mafic and felsic magmas. The P– T conditions of crystallization have been determined for early mineral assemblages (inner parts of alkali feldspar megacrysts and their plagioclase, quartz, amphibole and biotite inclusions) and late assemblages (matrix minerals) of the rapakivi-textured granitoids. Al contents in amphibole from the early and late mineral assemblages yield pressures of 1.2–3.0 and 0.7–3.0 kbar, respectively, and indicate only minor pressure change between the crystallization of the early and late assemblages. Amphibole–plagioclase thermometry gives temperatures mainly of the order of 900 to 1000 °C for both the early and late assemblages indicating nearly isothermal conditions. Feldspar thermometers yield lower temperatures. Relative abundances of minerals and their chemical compositions indicate that the late mineral assemblages tend to be richer in MgO, Na 2O and CaO than the early assemblages. Rapakivi texture is interpreted in this case mainly as a result of compositional changes related to the hybridization between granitic and more mafic magmas. Small release of pressure during crystallization of the magmas may have contributed to the origin of the mantled alkali feldspar megacrysts.