Yindongliang Adakites In The Eastern Section Of North Qilian And Their Geological Significance

The Yindongliang calc-alkaline granodiorite rock body intruded into the paleo-continent margin of Alax, north of the east section of northern Qilian orogenic belt. It is in the Quwushan region, 45 Km to the east of Jingyuan county, Gansu Province. The outcrop area of the rock body is about 85 Km².The results show that the Yindongliang calc-alkaline granodiorite, with enrichment in SiO₂ (⁶7.64%), Al₂O₃ (¹5.29%), Na (Na₂O/K₂O ².11), high Sr and low Y contents (Sr/Y ⁸8.33), enrichment in LREE, but depletion in HREE (La/Yb ³3.02), and very slight negative Eu anomalies ( δ Eu ⁰.96), is very similar to the geochemical characteristics of adakite. Compared with the high-K calc-alkaline adakitic rocks of Yanshanian period in eastern China, it has higher Na₂O contents (⁴.74%), and it is similar to the typical subducted slab-melting generated adakites in MgO (².11 %), Cr (³8.23 μg/g), Ni (³4.66 μg/g) contents and Mg^# (⁵9), indicating obvious interaction of the primary adakitic melt with mantle peridotite, so the Yindongliang granodiorite belongs to typical adakite generated from the subducted slab-melting.The existence of the Yindongliang adakites show that, in Early-Ordovician, the young ocean crust of Laohushan arc-back basin started to northwards subduct beneath the Alax Block, which caused the subducted slab-melting and the primary adakitic melt generated from the subducted slab-melting rising fast and generated obvious interaction with the mantle peridotite. The subducted slab-melting may be an important form of accreation in Quwushan region during Early-Ordovician period.The adakitic melt generated from the subducted slab-melting has the inherent nature with high temperature, sufficient fluids, high oxygen fugacity, which are preferred for Cu, Au and other deeply-sourced metals to be extracted and enriched, and rising fast to the surface of the Earth. Combined with many beneficial factors, Cu, Au and other deeply-sourced metals may be finally economically mineralized. Thus, porphyry copper and gold deposits related to the adakitic rocks in the eastern section of North Qilian may be a new ore-prospecting strategy.