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

The Luoguhe intrusion, located in MoheCounty, Heilongjiang Province, is mainly composed of mon-zogranite, quartz diorite and granodiorite, with minor dio-rite, tonalite, quartz monzodiorite, quartz monzonite, syeno-granite and alkali-feldspar granite. The intrusion can bedivided into two lithological units, i.e. quartz diorite andmonzogranite units, with affinities to high-K calc-alkalineseries. The quartz diorite unit (SiO2: 54.79%―58.30%Na2O/CaO: 0.79―1.53 and Shand index: 0.77―0.82) belongsto 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. Theintrusion is characterized by high REE abundance (∑REE =180.2―344.3 μg/g), medium-strong negative Eu anomalies(δ Eu = 0.33―0.82), weak REE fractionation [(La/Yb)N =4.12―10.45], enrichments in Rb, Th, U, K, La, Ce, Nd, HfZr and Sm, but strong depletions of Ba, Nb, Ta, Sr, P and TiThese characteristics of major, REE and trace elements in-dicate that the intrusion was formed in a transitional tectonicsetting from compressional to extensional regime, which canbe classified as post-collisional granitoids. SHRIMP U-Pbzircon analyses yield ages of 517±9 and 504±8 Ma for thequartz diorite and monzogranite units, respectively. The dis-covery of Early Paleozoic post-collisional granites in thenorthern margin of the Erguna massif indicates that thenorthern branch of Paleo-Asian Ocean between Siberianplate and Erguna massif was closed in the Early Paleozoicand the Salair orogeny ended ca. 500 Ma ago.

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 iithological units, i.e. quartz diorite and monzogranite units, with affinities to high-K caic-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 （∑REE = 180.2-344.3μg/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 compressionai 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-collisionai 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.