Rock Association And Geochronology Of The;"Neoproterozoic" Fengshuigouhe Group In The Northwestern Lesser Xing’an Rang, NE China And Its Tectonic Implications

The Fengshuigouhe Group is distributed in the northwestern section of theLesser Xing’an Rang. Tectonically, it is located in the eastern segment of theMongolia-Hinggan Orogen Belt and is adjacent to the Hegenshan-Heihe suture zonebetween the Xing’an and Songnen-Zhangguangcai Range massifs. The formation timeof the Fengshuigouhe Group has been a controversial issue. It is believed that theFengshuigouhe Group formed in the Neo-proterozoic on the basis of stratigraphiccorrelation and field geology. Because the study area experienced the multiplemodification of later tectonic activities, the issues about the association,geochronology and the petrogenesis of the Fengshuigouhe Group have been unsolved.In the present study, detailed studies of petrology, geochronology and geochemistryare carried out for the Fengshuigouhe Group in order to reveal their formation timeand tectonic setting.1. Rock association and geochronology of the Fengshuigouhe GroupField observation and petrographic study indicate that the previously believedFengshuigouhe Group consists of a suit of metamorphic rocks （leptynite） and gneissicgranitoids intruding the leptynite. Detrital zircons from biotite leptynite in theFengshuigouhe Group yield age populations at255Ma,290Ma,320Ma,469Ma,520Ma, implying that the maximum timing of deposition of the Fengshuigouhe Groupis255Ma （Late Permian）. The dating results indicate that the gneissic granites formedin the Early Jurassic （183¹85Ma）. Taken together, we conclude that the protoliths ofleptynites from the Fngshuigouhe Group formed between Late Paleozoic and Early Jurassic, rather than the Neoproterozoic as previously believed.2. Petrogenesis of the Early Jurassic gneissic biotite syenograniteThe Early Jurassic gneissic biotite syenogranites from the Fengshuigouhe Grouphave SiO₂=67.32%-75.61%, Al₂O₃=12.76%-16.71%, Na₂O+K₂O=8.07%-10.40%,Na₂O/K₂O=0.94-1.67, A/CNK=0.91-1.06, A/NK=1.09-1.27, and belongchemically to high-potassium calc-alkaline I-type ones. Moreover, these granitoids areenriched in light rare earth elements （LREEs） and large ion lithophile elements（LILEs）, and depleted in heavy rare earth elements （HREEs） and high field strengthelements （HFSEs）. The initial176Hf/177Hf ratios for primary zircons with the age of183¹85Ma in the granitoids vary from0.282846to0.282971. Their εHf （t） valuesand TDM2ages range from+6.3to+10.8and from754to1092Ma, respectively. TheEarly Jurassic granitoids have high SiO₂,（Na₂O+K₂O） and Al₂O₃, and low MgO,TFe2O3and CaO, implying a possible derivation from partial melting of the lowercontinental crust. The εHf （t） values （+0.63to+10.8） and TDM2ages （754to1092Ma）of zircons imply that their primary magmas were derived from partial melting of thejuvenile lower crust.3. Tectonic setting of the Early Jurassic granitoids and regional tectonicevolutionThe Early Jurassic granitoids, together with coeval mafic-ultramafic intrusiverocks in the Lesser Xing’an-Zhangguangcai Range, constitute a typical bimodaligneous rock association, suggesting an extensional environment. Combined with theEarly Jurassic Calc-alkaline igneous rocks in the Erguna Massif and the easternHeilongjian–Jilin provinces, it is suggested that the Early Jurassic granitoids from theLesser Xing’an Range formed under an extensional environment similar to back-arcbasin related to double-subductions of the Paleo-Pacific plate beneath the Eurasiancontinent and the Mongol-Okhotsk oceanic plate beneath the Erguna Massif.