Zircon U–Pb geochronology and elemental and Sr–Nd–Hf isotopic geochemistry of the Daocheng granitic pluton from the Yidun Arc, SW China

Abstract

The Garze–Litang suture zone, located in the eastern part of the Tethyan tectonic domain, is notable for widespread Late Triassic granitic plutons, which are genetically associated with the evolution of the Paleo–Tethys Ocean. The Daocheng granitic pluton in the eastern Yidun Arc, SW China, is located in the middle of the Garze–Litang suture zone and has an outcrop area of ∼2800km2. In the present study, we report zircon U–Pb ages and elemental and Sr–Nd–Hf isotopic data for the Daocheng granitic intrusion. Secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry U–Pb analyses on zircons yield consistent ages of ca. 216Ma for three samples from the pluton. All the Daocheng granitic rocks are enriched in Si (SiO2=68.1–76.4%) and large-ion lithophile elements but depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). Mineralogical and geochemical features indicate that these are high-K and calc-alkaline and I-type granites. They are characterized by relatively variable initial 87Sr/86Sr ratios (0.7059–0.7102), negative εNd(t) values (−5.7 to −7.8), wide-ranging εHf(t) values (−9.8 to +3.4), and two-stage Hf model ages of 1.04 to 1.88Ga. These isotopic signatures indicate that the source for the Daocheng granite was probably derived from partial melting of a Late Paleoproterozoic to Early Mesoproterozoic mafic–intermediate lower crust with a variably minor addition (<20%) of depleted mantle-derived magma. The parental magma thereafter underwent extensive fractional crystallization of ferromagnesian minerals, plagioclase, apatite, ilmenite, and rutile during emplacement, under temperatures from 785°C to 839°C. In combination with previous studies on synchronous magmatism in the Yidun Arc, we propose that the Daocheng granite was generated in a syncollisional tectonic setting. The westward subduction and closure of the Garze–Litang paleo-ocean triggered the underplating of large-scale mantle-derived magma and provided heat for the anatexis of the lower crust. Hybrid melts including minor depleted mantle-derived magma and lower crustal magma were then generated; these thereafter were continuously injected into a shallow-level chamber and gave rise to the Daocheng granite.