Multiple Origins of Hornblende-Rich Cumulates within a Deep Magma Reservoir from the Late Jurassic Gangdese Arc, South Tibet: Implications for Arc Crustal Evolution

Abstract Early fractionation of amphibole is an essential process in the evolution of hydrous sub-arc mantle-derived magmas. However, the petrogenesis of hornblende-rich cumulates in the deep crust and their significant role in arc crustal evolution remain unclear. In this contribution, we focus on Late Jurassic mafic–ultramafic intrusive rocks from the Zedong terrane (central–eastern Gangdese area, South Tibet) and use detailed petrographic, mineralogical and geochemical data to determine the origin of hornblende-rich cumulates with textural and chemical heterogeneities. The clinopyroxene (Cpx) hornblendite with adcumulate texture contains three distinct types of amphibole (cumulus, poikilitic, and minor interstitial). These amphiboles display homogeneous major and trace element compositions, consistent with crystallization from a liquid of constant composition. Rock fabric analysis further suggests that they formed by in situ nucleation and overgrowth at the interface between the mush layer and the convecting parental magma within the magma reservoir. Type I hornblende (Hb) gabbros have an orthocumulate texture with the lowest whole-rock MgO and Mg# values. Intercumulus melt evolution revealed by mineral assemblage and compositional variations indicate that Type I gabbros formed through a peritectic reaction between clinopyroxene precursors and evolved interstitial melts, followed by melt migration and fluid exsolution in a semi-open system. Type II gabbros have the highest whole-rock MgO and Mg# values and are characterized by extensive disequilibrium features. Geochemical modeling indicates that their formation entails a melt-flushing process involving the upward percolation of deep-recharge melts through the crystal mush. This study provides petrological evidence for the multiple origins of hornblende-rich cumulates within a common middle-lower crustal magma reservoir. Their textural and chemical heterogeneities may reflect spatial variations in crystallization regimes across distinct reservoir domains. The Zedong hornblende-rich cumulates have more depleted whole-rock Sr–Nd isotopic compositions and zircon εHf(t) values than those cumulates from Early Jurassic and Cretaceous Gangdese arc. This indicates that their parental magma was derived from a more juvenile mantle wedge and records significant growth in the middle-lower crust. The Cpx hornblendite has the lowest concentrations of SiO2 but the highest Fe2O3T and (Dy/Yb)N ratios. The differentiation trends between the cumulate rocks and coeval non-cumulate rocks within the Zedong terrane indicate that the evolution of the Late Jurassic arc crust was mainly controlled by amphibole fractionation. Modeling shows that melts in equilibrium with the cumulus and brown amphiboles are compositionally analogous to coeval intermediate-acid rocks, and it implies the existence of a trans-crustal magma plumbing system in this arc.