Intraplate adakite-like rocks formed by differentiation of mantle-derived mafic magmas: A case study of Eocene intermediate-felsic porphyries in the Machangqing porphyry Cu-Au mining district, SE Tibetan Plateau

Abstract Intraplate intermediate-felsic adakite-like rocks are widely considered to be generated via partial melting of the mafic lower crust. However, the genesis of phenocrysts of mafic minerals from some adakite-like rocks suggests that they could originate from mantle-derived magmas through assimilation and fractional crystallization (AFC) processes. In this study, we have investigated the petrogenesis of three phases of Eocene porphyries, namely quartz diorite porphyry (QDP-I and QDP-II), granodiorite porphyry (GDP), and granite porphyry (GP), exposed in the Machangqing area of the western Yangtze Craton in the southeastern Tibetan plateau. These porphyries were formed at 34.2–35.2 Ma in an intracontinental extension setting and present adakitic affinities of high Sr/Y (50–95) and La/Yb (50–113) ratios. The similar characteristics of these Machangqing porphyries in terms of whole-rock trace elements and Sr–Nd isotopes, as well as in the mineralogy of amphibole phenocrysts and clinopyroxene antecrysts, indicate that these intermediate-felsic porphyries were derived from the same source. The important observation for investigating the parental magma of these intermediate-felsic porphyries is that they have close temporal and spatial correlations with mantle-derived mafic volcanic rocks. These intermediate-felsic porphyries have slightly enriched Sr–Nd isotopic compositions [(87Sr/86Sr)i= 0.7068–0.7077, eNd(t)= −5.1 to −6.6], affected by limited crustal assimilation, as compared to those of contemporaneous mafic volcanic rocks. The high-Al amphibole phenocrysts and clinopyroxene antecrysts in the QDPs have similar rare earth element (REE) patterns with clinopyroxene phenocrysts in the mafic volcanic rocks, which suggest that these rocks originated from the same mantle-derived mafic magma. In addition, the major elements of these intermediate-felsic and mafic igneous rocks in the Machangqing area show fractional crystallization trends with an increasing magma silica component. Simulations of the major and trace elements of these igneous rocks show that the sequence of fractional crystallization of olivine, clinopyroxene, garnet, amphiboles, biotite, feldspar, and apatite with time promoted the evolution of the magma series. As revealed by the calculated physicochemical conditions during crystallization of the phenocrysts, the initial mafic magmas were derived from depths of greater than 64 km through the partial melting of the metasomatic lithosphere mantle induced by Eocene intraplate extension in the western Yangtze Craton. These initial mafic magmas underwent three stages of fractional crystallization at three levels of the thickened crust: the first at the base of the lower crust (49–55 km); followed by fractional crystallization at the middle crust (24–32 km); and finally at the upper crust (4–5 km). These fractional crystallization processes caused the initial mafic magmas to evolve through the intermediate (roughly represented by QDP) to the felsic compositions (GDP and GP). Whole-rock and mineral geochemistry evidence demonstrates that fractional crystallization of mantle-derived magmas, with a small amount of crustal assimilation, generated these Eocene intraplate intermediate-felsic porphyries in the Machangqing area.