Mesozoic multiple magmatism and porphyry–skarn Cu–polymetallic systems of the Yidun Terrane, Eastern Tethys: Implications for subduction- and transtension-related metallogeny

Abstract

Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with relatively high Sr/Y features. The Mesozoic porphyry–skarn Cu–polymetallic systems of the Yidun Terrane hosted by the Late Triassic and Late Cretaceous intrusions provide chances to investigate the spatial and genetic relationships between subduction- and transtension-related magmatism and mineralization, and determine the petrogenesis and fertility of magmatic rocks. Zircon U–Pb ages indicate that the Late Triassic pre-ore quartz diorite porphyries and syn-ore quartz monzonite porphyries were emplaced at ~225 and ~215Ma, respectively. The Late Cretaceous syn-ore monzogranite porphyries were emplaced at 83–78Ma. Our new data, combined with previously published geochronological data, show the spatially overlapping distribution of the multiple Mesozoic porphyry systems in the Yidun Terrane. Although all the Late Triassic intrusive rocks share similar geochemical characteristics, the pre-ore quartz diorite porphyries have normal arc-related chemical features with low Sr/Y and (La/Yb)N ratios, high Y and Yb abundances, while the syn-ore quartz monzonite porphyries exhibit high Sr/Y and (La/Yb)N ratios, low Y and Yb abundances. All samples show similar Sr–Nd–Hf isotopic compositions [(87Sr/86Sr)i=0.7060–0.7117, εNd(t)=−6.7 to 0.0, zircon εHf(t)=−4.0 to +3.0], suggesting that they were probably derived from partial melting of juvenile lower crust. Trace-element patterns and partial melt modeling indicate that the quartz diorite porphyries were likely formed by partial melting of normal thick lower crust, while the causative quartz monzonite porphyries were probably formed by partial melting of eclogitized, thickened lower crust. We propose that pre-ore quartz diorite porphyries were probably generated earlier via the subduction of Garze–Litang oceanic crust, and syn-ore quartz monzonite porphyries were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. Thus, these high Sr/Y quartz monzonite porphyries host several economically important porphyry Cu deposits, such as Pualng, Xuejiping and Songnuo. However, the Late Cretaceous syn-ore monzogranite porphyries have lower εHf(t) values (−8.4 to −2.9) and εNd(t) values (−7.5 to −3.8) than the Late Triassic porphyries, indicating that the former mainly originated from ancient crustal materials. New dataset and previous studies suggest that the Late Cretaceous post-collisional transtension triggered the asthenospheric upwelling and the underplating of mafic magmas, induced the partial melting of garnet-bearing amphibolite and thus caused the emplacement of monzogranite porphyries and associated porphyry–skarn Cu–Mo deposits.