Fluid flux in the lithosphere beneath southern Tibet during Neo-Tethyan slab breakoff: Evidence from an appinite–granite suite

Abstract

The mechanism of magma generation during continental collision remains enigmatic. Appinite–granite suites commonly occur within collisional orogenic belts, providing an opportunity to monitor lithospheric evolution and the geodynamic processes involved. This paper presents geochronological, mineralogical, and geochemical data for the Eocene (~50 Ma) Pengcuolin appinite–granite suite, which is the first such suite to have been found in the southern Lhasa Terrane, southern Tibet. Two distinct populations of plagioclase are observed in the appinitic rocks: high-An plagioclases that crystallized as an early phase from H2O-saturated mafic magmas, and low-An plagioclases that formed as a late phase at pressures of ~3 kbar and ~5 kbar, as calculated from equilibrium thermobarometry using low-Al and high-Al amphiboles, respectively. The amphibole-rich appinitic rocks have low SiO2 contents and moderate values of Mg#, and are enriched in large ion lithophile elements and depleted in high field strength elements. They have homogeneous initial 87Sr/86Sr ratios of 0.705174–0.705330, εNd(t) values of 0.85 to 1.47, and zircon εHf(t) values of 3.3 to 7.9. These features suggest the appinites were derived from the melting of mantle peridotites that had been permeated by slab-derived fluids. Whole-rock geochemical data, equilibrium melting calculations, and ABS5 modeling results indicate that the primary melts of the appinitic rocks were generated from garnet-stabilized mantle, and that the melts then ascended with crystallization taking place under H2O-saturated conditions and under variable temperatures and pressures. The associated granites have a high and narrow range of SiO2 contents, and they exhibit a high-K calc-alkaline character, and low A/CNK ratios and Mg# values. These geochemical characteristics, plus their widespread distribution and depleted isotopic compositions (87Sr/86Sri = ~0.705, εNd(t) = 1.12–1.59, and zircon εHf(t) = 6.9–9.2), are comparable to those of Cenozoic granitoids elsewhere in southern Tibet and consistent with the partial melting of juvenile mafic lower-crustal materials. This appinite–granite suite not only attests to a flux of slab fluids through the lithospheric mantle during breakoff of the Neo-Tethyan slab, it also sheds light on the vertical profile of magma evolution from the deep mantle to the shallow crust. The emplacement of the appinite–granite suite was facilitated by regional faults, but the rising hydrous basaltic magmas were often trapped by the thick overlying continental crust, and those underplating magmas provided the heat and water that caused the partial melting of the continental crust, finally leading to the widespread Cenozoic felsic (and rare mafic) magmatic rocks of southern Tibet.