Mixing of subduction zone and intracontinental magmas at Dexing porphyry Cu (AuMo) deposit, China: Evidence from Ti isotopic compositions

Abstract

At Dexing porphyry Cu (AuMo) deposit, China, the tectonic setting in which granodioritic porphyry magma was generated has remained contentious: petrogenetic models involve either subduction zone magma in a continental arc setting, or magma genesis during Gan Hang intracontinental rifting. Interestingly, Ti stable isotopic variations (δ^49/47Ti_OL-Ti) show the potential to help distinguish subduction zone-, mid-ocean ridge-, and intraplate-related rocks, when compared against SiO₂ concentrations. Here, although the samples represent differentiated magmas that were significantly hydrothermally altered, the δ^49/47Ti_OL-Ti values and SiO₂ concentrations are shown to be insignificantly affected by hydrothermal alteration. A SiO₂-δ^49/47Ti_OL-Ti tectonomagmatic discrimination diagram shows Dexing granodioritic porphyry with a subduction zone signature, and related mafic microgranular enclaves (MME) representing intraplate magma, in agreement with some literature. The simplest way to explain such relations is that already proposed for Dexing and consists in subduction zone-related calc-alkaline (adakitic) magmatism, followed by slab delamination, extension, asthenospheric up flow, and intraplate alkaline magmatism.

Two holes drilled vertically and sampled semi-continuously, over 300 m through the center of the porphyry and 400 m through its margin, show compositional heterogeneity at the meter scale, and repeating cycles of high and low δ^49/47Ti_OL-Ti values, SiO₂, and immobile high field strength element concentrations. These cycles are best explained by a polyphase emplacement of the porphyry through multiple injections of magmas, which must have evolved through fractional crystallization, in a system consisting of interconnected pathways established over ranges of depths and pressures, also showing variations in temperature and composition. The data presented here support field relations, petrographic observations, and suggest the injection and mixing of intraplate mafic magma with subduction zone porphyry magma, and evolution through fractional crystallization and partial assimilation of surrounding metapelites.