Geochemistry And Petrogenesis Of Mesozoic Volcanic Rocks From Southeastern Mohe Basin

Mohe Basin, located at the northest of China, makes a whole with Wusumeng Basin inRussia facing Mohe across Heilongjiang. The basin lies in the Ergun block, where it isbordered by Mongolia–Okhotsk suture zone to the north, Tarim River-Xiguitu suture zone tothe east. Since late Paleozoic, the Paleo-Asian Ocean, West Pacific Ocean andMongolia-Okhotsk Ocean has strongly stacked and compounded in the area. The basin aremostly made up of Mesozoic sedimentary stratas, only some Mesozoic volcanics exposed inthe southest. In this Passage I mainly studied the geology, petrography, chronology andgeochemical characteristics of the volcanics in the southest of the basin, the followingconclusions can be made:1．Chronology and stratigraphy of Mesozoic volcanic rocksMesozoic volcanics exposed in the southest of Mohe Basin is early cretaceous,consisting of Guanghua formation and Ganhe formation. The Guanghua formation is a set ofacidic pyroclastic rocks and lavas, mainly are rhyolites. The Ganhe formation is a set ofintermediate-basic rocks and lavas, mainly are basalts and andesites. The result of zirconU-Pb isotopic dating indicates that the ages of the Guanghua formation are128～125Ma, theages of the Ganhe formation are126～118Ma.2．Geochemistry of Mesozoic volcanic rocksPetrogeochemistry characteristics of the Guanghua formation rhyolite is rich silicon andalcali and poor calcium, iron and magnesium with71.02%～77.57%SiO2,0.13%～0.27%MgO,0.91%～4.25%TFe2O3,0.90%～1.28%CaO; The total alkali (K2O+Na2O) accountsfor6.72%～8.29%. The total amount of REE is131.18×10-6～163.01×10-6; Curves of REEdistribution is right dip type, rich light rare earth and depleted heavy ones,(La/Yb)N=8.25～ 17.99; The REE have moderate negative Eu anomalies, δEu=0.25～0.66. Rhyolites areenriched in LILE such as Rb, K, U, Th and LREE, depleted in HFSE such as Nb, Ta, P, Ti, Sr.Zircon Hf analyses conducted by LA-MC-ICPMS show that the Guanghua formation rhyolitehave εHf(t) values of0.1~3.0with Hf modal ages of989Ma～1130Ma, indicating that theGuanghua formation rhyolite derive from partial melting of crust materials.The geochemistry characteristics of the Ganhe formation trachyandesite indicate that thetrachyandesite belong to high potassium Ca-alkaline series-shoshonite series with54.88%～60.46%SiO2,1.76%～3.86%K2O,0.13%～0.27%MgO. The total alkali (K2O+Na2O)accounts for5.27%～7.85%. The total amount of REE is145.27×10-6～218.61×10-6Curves ofREE distribution is right dip type, rich light rare earth and depleted heavy ones,(La/Yb)N=12.67～23.21; The REE have little negative Eu anomalies, δEu=0.77～0.94.Rhyolites are enriched in LILE such as Rb, Ba, K, Th, U and LREE, depleted in HFSE suchas Nb, Ta, P, Ti. Zircon Hf analyses conducted by LA-MC-ICPMS show that the Ganheformation trachyandesite have εHf(t) values of-1.2～4.1.3．Tectonic setting of Mesozoic volcanic rocksVolcanics in Mohe Basin were controlled by Mongolia–Okhotsk tectonic system, nocorrelation with Paleo-Asian Ocean and Paleo-Pacific Ocean tectonic system. Data shows,Mohe Basin occurred as the Mongolia–Okhotsk Ocean closed, its formation process meanedthe east ocean closed, then mountained. Mongolia–Okhotsk Ocean subducted towards thenorth and the south, closed from the east to the west. In the Late Jurassic, Sediments began todeposite, the cover occurred during the continental collision orogeny. From the late LateJurassic to the early Early Cretaceous, the basin was into extension from extrution anddeveloped with the thrust nappe structure and strong tectonic deformation, which meanedorogeny came into the middle. In the middle-late period of Early Cretaceous, the basin was inthe extension, a set of volcanic covers occurred, which meaned orogeny came into the late.Mesozoic volcanics in Mohe Basin occurred in the post-orogeny extension setting. Theywere controlled by Mongolia–Okhotsk tectonic system. Their occurrence was a sign thatMongolia–Okhotsk tectonic evolution would end.