Geochemistry of the Late Jurassic granitoids in the northern end area of Da Hinggan Mountains and their geological and prospecting implications

Abstract

The Late Jurassic Longgouhe and Ershiyizhan intrusions, located in the Upper Heilongjiang Basin of northern Da Hinggan Mountains, are closely related to the porphyry Cu-Au mineralizations.. The intrusions are mainly composed of quartz diorite, quartz monzodiorite and granodiorite. Most of them are the rocks of high-K calc-alkaline series, with minor of shoshonite series. Their SiO2 and Al2O3 contents range 61.37% - 66.59% and 15.35% - 17.06%, respectively. The MgO content ranges 2.02% - 3.47%, with a high Mg# index ranging from 44 to 59. The (La/Yb)N and δEu values range from 16.85 to 81.73 and from 0.68 to 0.93, respectively, showing strongly fractionated REE patterns that are similar to those of adakites. The rocks are enriched in Ba, Sr and LREE, but strongly depleted in Nb and Ta, and slightly depleted in Rb and Ti. The Yb and Y contents span of 0.31 - 1.32 μg/g and 4.32 - 12.07 μg/g, respectively, suggesting that the rocks are depleted in Yb and Y. The Sr /Y ratios are between 67.74 and 220.60, showing a clear feature of low-Y and high-Sr. All these geochemical characteristics are similar to adakites. Considering that the Paleo-Asian Ocean and Mongol-Okhotsk Ocean were closed in the Late Paleozoic and Permian-Middle Jurassic, respectively, the origin of the interested adakites can be related to partial melting of delaminated crust which were thickened by the collisional orogeny between the Siberia and Mongol-Sinokorea continents.