Hornblendite in the Lower Crust: A Possible Source for Porphyry Cu Deposits

Abstract Lower crustal sulfide‐bearing Cu‐rich cumulates, mainly occurring as hornblendite, have been proposed as a critical source component for the development of giant porphyry Cu deposits (PCDs); however, their mineralogical and geochemical nature remain elusive. We present integrated geochemical and geochronological investigations on a suite of hornblendite from the Gangdese continental arc in southern Tibet. Zircon U‐Pb geochronology documents a prolonged magmatic cooling and crystallization spanning from the Late Triassic to the Early Jurassic (ca. 207∼200 Ma; ∼6 myr). The hornblendite is characterized by (a) variable SiO 2 , Al 2 O 3 , MgO and Fe 2 O 3 T contents, negative anomalies of Nb, Ta, Zr and Hf, high Sr/Y ratio and Cu content (up to 1,430 ppm); (b) relative oxidized conditions represented by elevated bulk Fe 3+ /(Fe 3+ +Fe 2+ ) and Ce/Ce* ratios; (c) depleted mantle‐like Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr (t) = 0.7038 to 0.7046, ε Nd(t) = +5.1∼+6.4), which are consistent with those in the coeval PCDs. These results imply that these hornblendites were originated from partial melting of mantle wedge metasomatized by subduction‐derived fluids. Considering the temporal and spatial distribution of subduction‐ and collision‐related PCDs, we propose that the deep‐seated hornblendite cumulates in the lower part of the arc crust could serve as an important potential source for PCDs and have played an important role in Cu mineralization in the Gangdese arc.