Discovery of an adakite-like pluton near Dongqiyishan (Beishan, NW China) - Its age and tectonic significance

摘要

The Dongqiyishan granitic pluton is exposed in the Eastern Beishan area, in the central part of the Southern Central Asia Orogenic Belt SIMS U-Pb zircon dating indicates that the Dongqiyishan pluton was emplaced at 356 ± 2 Ma. Geochemically the pluton is calc-alkaline and characterized by high SiO2 (> 65%), Al2O3 (mostly > 15%), Na2O/K2O ratios (1.3-3.9) and Sr (generally >400 ppm). Samples display positive to weakly negative Eu anomalies (δ_(Eu) = 0.81-2.32), low HREE, Y and Yb (Y<18 ppm, Yb<1.9 ppm), are enriched in LREE, LILE (Rb, Ba, Sr, U, and K), Zr, and Hf, and depleted in HFSE. They have relatively low εNd_(t) values (-2.9-+0.6) and relatively high (~(87)Sr/~(86)Sr)i ratios (0.70581-0.70749). Together, these geochemical traits suggest an adakite-like composition that reflects melting of subducted oceanic crust and the involvement of older continental material. Assimilation and fractional crystallization (AFC) modeling requires significant crustal assimilation to modify an initial MORB-derived andesitic melt to produce the hybrid granitic melt, e.g. - the Dongqiyishan pluton. A back-arc basin to the south (represented by the Yueyashan-Xichangjing ophiolitic belt) closed before the Early Carboniferous Dongqiyishan adakite-like pluton was emplaced into the Silurian volcanic arc. Closure of the back-arc basin potentially provides the continental component seen in the isotopic signature of the Dongqiyishan pluton and required by the AFC modeling. Post-Ordovician flattening of the subducting slab together with crustal thickening from closure of the back-arc basin, led to the formation of a tightly coupled subducting slab and lower crust which resulted in the formation of Dongqiyishan adakite-like pluton. The Dongqiyishan pluton is consistent with southward subduction (represented by the Xiaohuangshan ophiolitic belt) and indicates that oceanic crust was still being produced in the Paleo-Asian Ocean during the Early Carboniferous. Adakite-like granitic rocks may be more abundant than previously thought in the Beishan region.