Age and petrogenesis of A-type granites in the middle segment of the Bangonghu–Nujiang suture, Tibetan plateau

Abstract

During geological investigations conducted over the past several years, the authors found a group of A-type granites in the middle segment of the Bangonghu–Nujiang suture on the Tibetan plateau. These granitic plutons occur as stocks with exposed areas of less than 1km2 and lithologically include biotite granite and granodiorite porphyry, which had intruded into Cretaceous sedimentary rocks. Petrogeochemically, the A-type granites contain high SiO2 levels varying from 68.62%–75.36wt.%. Moreover, they contain relatively high Fe2O3T (0.86–5.39wt.%) and low Al2O3 (12.76–15.54wt.%) levels and are weak peraluminous to metaluminous. A trace element N-MORB-normalized spider diagram showed apparent enrichment of the large ion lithophile elements (LILE) Rb, Th, U, K, and Pb with marked depletions of Ba and Sr. The high field strength elements (HFSE) Nb, Ta and Ti were sharply depleted, with relative enrichment of Zr and Hf. These signatures are characteristic of A-type granite. The granite samples had high rare earth element (REE) contents with ∑REE=122.37–291.19ppm. Light REE were rich relative to heavy REE with LREE/HREE=4.89–9.58. And negative Eu anomalies were sharp with δEu varying from 0.14–0.5 and a mean value of 0.34. Their chondrite-normalized rare earth element distribution patterns appear as gently right-dipping V-types.Zircon U–Pb LAICPMS dating for three granitic plutons yielded weighted mean ages of 109.6±1.4Ma (MSWD=3.7), 112.3±0.51Ma (MSWD=0.21) and 113.73±0.5Ma (MSWD=0.21), indicating that they formed in the late stage of the early Cretaceous epoch. The Nd, Sr and Pb isotopic results showed that the granites were rich in radiogenic isotopes. The 87Sr/86Sr and ISr ratios varied from 0.719842–0.786395 and 0.706900–0.710378, respectively. The 143Nd/144Nd ratios were from 0.512123–0.512392, with relatively large negative εNd values (−3.37 to −10.34). The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranged from 18.703–19.070, 15.680–15.732 and 39.121–39.576, respectively, exhibiting isotopic signatures typical of crustal sources. All of the data demonstrated unanimously that these granites were derived from partial melting of arc basaltic rocks underplated in the lower crust during oceanic crust subduction. The high-temperature heat source for melting of anhydrous basaltic rocks of the lower crust was from upwelling asthenosphere through slab windows formed as a result of slab break-off during post-collisional extension of the orogenic zone.