Abstract

The Luoguhe intrusion, located in Mohe County, Heilongjiang Province, is mainly composed of monzogranite, quartz diorite and granodiorite, with minor diorite, tonalite, quartz monzodiorite, quartz monzonite, syenogranite and alkali-feldspar granite. The intrusion can be divided into two lithological units, i.e. quartz diorite and monzogranite units, with affinities to high-K calc-alkaline series. The quartz diorite unit (SiO2: 54.79%–58.30%, Na2O/CaO: 0.79–1.53 and Shand index: 0.77–0.82) belongs to metaluminous rocks. And the monzogranite unit (SiO2: 65.29%–66.45%, Na2O/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 (ΣREE = 180.2-344.3 µg/g), medium-strong negative Eu anomalies (δEu = 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.

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