Triggers for the generation of post–collisional porphyry Cu systems in the Kerman magmatic copper belt, Iran: New constraints from elemental and isotopic (Sr–Nd–Hf–O) data

Abstract

Cenozoic tectono–magmatic activities and metallogenic processes in the Kerman magmatic copper belt (KMCB) led to the formation of two contrasting porphyry copper systems in a post–collisional setting: (1) early to late Oligocene (i.e., 33.9–23.03 Ma) Jebal Barez–type intrusive rocks comprising small porphyry copper deposits (PCDs) of normal calc–alkaline and locally tholeiitic affinity, and (2) early Miocene to early Pliocene (i.e., 17–3.6 Ma) Kuh Panj–type granitoids hosting moderate to giant PCDs, such as the world–class Sarcheshmeh deposit with potassic and adakite–like calc–alkaline features. Here I present new Sr–Nd–Hf–O isotopic data from the both economic and sub–economic porphyries. The Kuh Panj ore–bearing granitoids exhibit low ISr (<0.706), high εNd(t) (>−4.6), positive zircon εHf(t) (>+6.1), low δ18O (<6.76‰), and TDMC (Hf model ages) values of 425 to 705 Ma (mean TDMC = 571 Ma). These isotopic data imply that the KMCB economic porphyry systems originated mainly from dehydration melting of a composite juvenile underplated thick lower crust derived from a depleted mantle source.In contrast, the sub–economic porphyries at Jebal Barez show comparatively older Hf model ages (mean TDMC = 1274 Ma), low εNd(t) (<−4.1), and higher ISr (>0.706) values suggesting a source dominated by lower crust. A wide range of εHf(t) values (+2.8 to −10.6) and high zircon δ18O values (>7.31‰) for the Jebal Barez granitoids, are higher than pristine mantle values indicating involvement of the supracrustal recycling materials during zircon growth. These isotopic data, coupled with geochemical characteristics of the Kuh Panj granitoids, such as high SiO2 (>69.5%), K2O (>2.8%) contents, low MgO contents (<1.4), high Sr/Y (>60.9), and La/Yb (>20) ratios, relatively low Mg# (<42.6), and compatible element contents (e.g., Cr ≤ 21.6 ppm) reported in this study, indicate a magma mixing model for the generation of high–K adakite–like porphyries within the garnet amphibolite stability field in the sub–continental lithospheric mantle (SCLM). In new magma mixing model, ore–forming high Sr/Y magmas first involved partial melting of highly metasomatized SCLM that was triggered by post–collisional lithospheric thinning and asthenospheric mantle counterflow, which produced potassic magmas. Underplating of these mantle–derived magmas beneath the lower part of the thickened juvenile lower crust (~45–55 km) would have led to dehydration melting of the lower crust with excess water (>10 wt%).