| Meosozoic igneous rocks are common at the joint part of Qinling, Qilian and Kunlun orogens, whereas the origin and tectonic setting of them are still controversial. This paper present geochronological and geochemical data for the Triassic magmatic rocks, which consist of andesite, dacite, granitic porphyry and granodiorite, from the Tongren-Zeku region, western Qinling terrane. SHRIMP zircon U-Pb dating yield Middle Triassic ages of 240.1±2.4 Ma,245.0±2.8 Ma and 240.5±3.6 Ma for andesite from Tongren area, andesite and granodiorite from Zeku area, respectively.All of them were enriched in some large iron lithophile elements (eg., Th, U, Pb) and light rare earth elements, but depleted in high field strength elements (eg., Nb, Ta, Ti) and heavy rare earth elements, with showing arc-like distribution patterns of trace elelments. The andesites and granodiorites have high MgO, Cr, Ni contents, Mg# values and relatively low K20/Na2O ratios, with high (87Sr/86Sr)t ratios (0.7083 to 0.7084) but low sNd(t) (-7.52 to -6.94) and zircon εHf(t) values (-5.58 to -1.70), indicating that they were generated by partial meling of an enrich lithosphere mantle with involvement of crustal materials. Comparing with coeval andesites and granodiorites, granitic prophyries exhibit lower MgO, Cr, Ni contents, Mg# values, but elevated K2O/Na2O ratio. Except one grain with a positive εHf(t) value (+4.1), εHf(t) values of the other young grains from the granitic prophyries (-14.2～-9.1) were lower than that of andesite and granodiorite. In addition, the granitic prophyires contain some Meso to Paleo-proterozoic and Late-Archean inherited zircon with negative εHf(t) values varying from -7.4 to -2.5. We inferred that the granitic prophyries were originated by magmas produced by partial melting of lower crust mixing with melts derived from enriched lithosphere mantle. We conclude that the Middle-Triassic magmatic rocks formed during the northern subduction of Palaeo-Tethys. |
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