High-Mg# Adakitic Rocks Formed by Lower-crustal Magma Differentiation: Mineralogical and Geochemical Evidence from Garnet-bearing Diorite Porphyries in Central Tibet

Modern arc adakites with high Mg# values (molar 100 x Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg# adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg# adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg# (Mg# = 45-56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg# > 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg# adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at similar to 1GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([Sr-87/Sr-86](i) > 0.709, Nd[t] < -3.4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O >5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg# adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge.