Role of mantle-derived magma in genesis of early Yanshanian granites in the Nanling Range, South China: in situ zircon Hf-O isotopic constraints

Abstract

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived mag-mas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, eHf(t) =11.60.3 and O=5.40.3. In combination with whole-rock geochemical and Sr-Nd isotopic data, the parental magma of the Qinghu monzonite were likely derived from the partial melting of re-cently-metasomatized, phlogopite-bearing lithospheric mantle without appreciable crustal contamina-tion. The Lisong and Fogang granites and the mafic microgranular enclaves (MME) within the Lisong granites have a wide range of zircon Hf-O isotopic compositions, with Hf and O isotopes being nega-tively correlated within each pluton. The Lisong MMEs were crystallized from a mantle-derived magma, similar to the parental magma of the Qinghu monzonite, with small amount of crustal assimilation. The Lisong and Fogang granites were formed by reworking of meta-sedimentary materials by man-tle-derived magmas and mixing of the mantle- and sediment-derived melts to varying degrees. It is thus concluded that these two Yanshanian granites in the Nanling Range were formed associated with growth and differentiation of continental crust.