Discovery of the Early Paleozoic post-collisional granites in northern margin of the Erguna massif and its geological significance

摘要

The Luoguhe intrusion, located in Mohe County, Heilongjiang Province, is mainly composed of moazogranite, quartz diorite and granodiorite, with minor diorite, tonalite, quartz monzodiorite, quartz nionzonite, syenogranite and alkali-feldspar granite. The intrusion can be divided into two lithological units, i.e. quartz diorite and irtonzogranite units, with affinities to high-K calc-alkaline series. The quartz diorite unit (SiO_2: 54.79 percent -58.39 percent, Na_2O/CaO: 0.79-1.53 and Shand index: 0.77-0.82) belongs to metaluminous rocks. And the monzogranite unit (SiO_2: 65.29 percent-66.45 percent, Na_2O/CaO: 1.73-3.43 and Shand index <1.05) can be considered as weakly peraluminous rocks. The intrusion is characterized by high REE abundance (SIGMA REE = 180.2-344.3 mu g/g), medium-strong negative En anomalies (delta Eu = 0.33-0.82), weak REE fractionation [(La/Yb)_N = 4.12-10.45], enrichments in Rb, Th, U, K, La, Ce, Nd, Hf, Zr and Sm, but strong depletions of Ba, Nb, Ta, Sr, P and Ti. These characteristics of major, REE and trace elements indicate that the intrusion was formed in a transitional tectonic setting from compressional to extensional regime, which can be classified as post-collisional granitoids. SHRIMP U-Pb zircon analyses yield ages of 517+-9 and 504+-8 Ma for the quartz diorite and monzogranite units, respectively. The discovery of Early Paleozoic post-collisional granites in the northern margin of the Erguna massif indicates that the northern branch of Paleo-Asian Ocean between Siberian plate and Erguna massif was closed in the Early Paleozoic and the Salair orogeny ended ca. 500 Ma ago.