The Geochronology And Petrogenesis Of Volcanic Rocks In Baiyingaolao Formation From The Southern Manzhouli, Inner-Mongolia

Southern Manchuria located in the central Mongolia - Ergun massif in the Eastern Xingmeng orogenic belt between the North China craton and the Siberian craton. Tectonic Evolution of this area is complicated, including the Paleo-Asian Ocean tectonic domain during Paleozoic, Mongol-Okhotsk tectonic domain during Mesozoic and Pacific tectonic domain since Mesozoic-Cenozoic. Study of Mesozoic volcanic rocks in this area is lagging behind, further research of these volcanic rocks will provide solid information for deeping the spatial and temporal evolution of the late Mesozoic volcanic magmatic activities, further dissecting the petrogenesis of the volcanic rocks and investigating the tectonic setting of the volcanic rocks, and will administer to deepen the comparative study of the whole Daxinganling volcanic belt.Based on our geochronology study and previous research results, the classification and correlation of the volcanic rocks have been established. The volcanic strata in this area were divided into five formations,including Middle Jurassic Tamulangou Formation, Late Jurassic Manketouebo and Manitu formations, Early Cretaceous Baoyingaolao and Meiletu formations. The main rock type of the volcanic rocks are rhyolite, biotite rhyolite, spherulitic rhyolite, alkali feldspar rhyolite, perlite and Rhyolitic lithic crystal-chip ignimbrite according to the geological investigation and petrographic study. Zircon LA-ICPMS U-Pb dating show the volcanic rocks in Baiyingaolao Formation were mainly formed at the earliest Early Cretaceous from 139Ma to 141Ma, different from the previous thought being formed at Late Jurassic or between 120Ma and 130Ma.The geochemistry of major elements indicate that the volcanic rocks from Baiyingaolao Formation in Southern Manchuria are characteristicd by high silica and alkali, lower Aluminum, Calcium and Magnesium, which close to the average composition of the world A-type granites and alkaline rhyolite. In the TAS classification diagram of volcanic rocks, all the samples fall into the range of the rhyolite; The alkali indexs of volcanic rocks vary from 0.80 to 0.98, average 0.91, greater than the minimum value of alkaline rocks of 0.85, near to the average of World A-type granite and alkaline rhyolite, and the alkalinity rate (AR) values range between 3.94 and 6.51, belong to alkaline rocks. The average FeOT/MgO ratio of volcanic value is 6.09 (varying from 2.77 to 36.02), significantly higher than the I-type(average 2.27), S-type (average 2.38) and M-type (average 2.37) granite.The volcanic rocks from Baiyingaolao formation are characterized by relatively high total REE contents (ΣREE=103.30×10-6~488.07×10-6) and strong negative Eu anoalies (δEu=0.01~0.41); The trace element geochemistry are characterized evidently by enrichment of Rb, Th, U, K, strong depletion of Ba, Sr, P, Ti, mediate depletion of Nb, Ta. The volcanic rocks have high Rb/Sr ratios, ranging between 1.73 and 15.33 (average 7.28), significantly different from I–type(average 0.61), S-type (average 1.81) and M-type (average 0.06) granite, also higher than the A-type granites (average 3.52); The 10000Ga/Al ratio ranges vary from 2.41 to 4.07 (average 3.52) were higher than the lower limit of A-type granite(2.6)except one sample, and significantly higher than the average values of I-type (2.1) and S-type granite (2.28).The geochemical characteristics of the volcanic rocks are similar to the continental rift alkaline rhyolite, A-type granite, and equivalent with the low Ba-Sr rhyolite from Da Hinggan Mountains defined by Zhang et al.(2008). Account to the classification illustration of granite (Whalen et al., 1987 and Eby, 1992), the volcanic rocks of Baiyingaolao Formation show an affinity with the A-type granite.Lu-Hf isotope characteristics of zircon show that the volcanic rocks of Baiyingaolao Formation in Southern Manchuria have positiveεHf(t) values, varying from 3.49 to 9.98, and TDM2 ranging from 556Ma to 951Ma, belong to the Neoproterozoic, quite agree with the Nd and Hf madel age of Phanerozoic granites from Xingmeng orogen, suggest that they may ororigined from the same magma source.The characteristics above indicate that the the magma of these volcanic rocks derived from the basic lower crust accreted from depleted mantle during Neoproterozoic. The Calculation Results of Zircon saturation temperatures suggest that the volcanic rocks were formed at the temperature of 768℃to 874℃, the volcanic rocks have lower Sr and Higher Yb contents indicate they were formed under lower pressure, residual phases of source are composed of plagioclase and amphibolite. Combining with the regional researching results, we suggest that the volcanic rocks in Baiyingaolao Formation from Southern Manchuria were formed in an intra-plate anorogenic extensional setting, which is related to the extensional functiom after the closure of Mongol-Okhotsk Ocean.