Hornblendites as a record of differentiation, metasomatism and magma fertility in arc crust

Abstract

The fractionation of hornblende is a common phenomenon in arc magmas, and gives rise to a number of notable geochemical characteristics. However, it is often cryptic, with limited direct petrographic evidence for the fractionation, or even presence of amphibole in volcanic suites. Newly identified hornblendites in the SE Tibet represent direct evidence for hornblende fractionation in the Gangdese arc. The hornblendites have age ranges from 95.3 ± 0.2 to 92.6 ± 0.2 Ma, recording magmatic activities over a ca. 2 Myr interval. Textural relationships indicate that hornblendites form by peritectic reactions between an evolving melt and earlier-formed clinopyroxenites. Relict clinopyroxenes and primitive pargasite record melt evolution from high Sr/Y basaltic andesite to dacite, during cooling from ∼1050 °C to ∼850 °C in the mid to lower crust. The clinopyroxene precursors interacted with water-rich high SiO2 fluids/melts at ca. 800 °C. Zircons have high δ18O values (up to 7.62‰) and positive εHf(t) values (up to +13.0), but bulk rock data show high 87Sr/86Sr ratios (up to 0.7093) and negative εNd(t) values (ca. −3.2). This decoupled signature implies the primary magmatic source was metasomatized by 5–10% recycled subducted sediments. Magmas have a high oxygen fugacity (ΔNNO can up to +2.40) through a combination of metasomatism during subduction, and subsequent melt-mush interaction that is recorded by zoned hornblendes. Metasomatism, widespread and long-term fractionation of amphibole ± garnet ± clinopyroxene, and high oxygen fugacity, are all positive indicators for the fertility of arc magmas with respect to porphyry copper formation. This study demonstrates that late-stage melt-mush interaction can contribute to peritectic hornblendite formation and facilitate magma fertility.