Songjianghe biotite monzonitic granite zircon U-Pb geochronology and geochemical significance

The authors studied geochronology and geochemical data of the Songjianghe biotite monzogranite in the southern Zhangguangcai Range in order to determine its formation age,magma source,and tectonic environment. The results indicate that the Songjianghe biotite monzogranite was formed in the Middle Jurassic with an age of 168. 2 ± 2. 0 Ma( MSWD = 0. 93). The monzogranite was characterized by high alkali and low Ca O and Mg O,belonging to high-potassium calc-alkaline,metaluminous I-type granite. The rock is enriched in large ion lithophile elements such as Rb,Ba,and K and strongly depleted high field strength elements such as P,Ti,Nb,and Ta. It is concluded that the Songjianghe biotite monzogranite was derived from partial melting of amphibolite facies metamorphism mafic lower-crust and its formation was controlled by the Pacific Plate subduction.

Geochemical characteristics and petrogenesis of volcanic rocks from Baiyingaolao Formation in northeastern Hailar Basin

The volcanic rocks from Baiyingaolao Formation in the northeastern Hailar Basin are mainly composed of rhyolite and trachydacite. U-Pb dating of zircon shows these volcanic rocks formed in Early Cretaceous( 128--124 Ma). Geochemical data indicate that they are sub-alkaline series in composition and rich in alkali and potassium. All samples have similar rare earth element patterns characterized by high total rare earth elements contents( ∑REE = 113. 96 × 10- 6-204. 33 × 10- 6),significant fractionation of heavy and light rare earth elements( ∑LREE / ∑HREE = 3. 10-4. 52) with middle negative Eu anomalies( δEu = 0. 46-0. 76).The trace elements are characterized by enrichment in large-ion lithophile elements such as K,Rb,LREE and depletion in high field strength elements e. g. Nb,Ta,HREE,P and Ti,while enriched in Th and U. Rhyolite and trachydacite contain low initial87 Sr /86 Sr ratios( 0. 704 9--0. 7 053) and positive εNd( t) values( ca. 4.15). These data suggest that the magma of rhyolite and trachydacite were derived from mafic lower crust newly accreted from mantle,with the evolutional trend of comagmatic fractional crystallization.