Origin Of Volcanic Rocks From The Upper Section Of The Baoligaomiao Formation In Sonidzuoqi, Inner Mongolia

The tectonic location of Sonid Left Banner in Inner Mongolia belongs to northorogenic belt of Xing-meng Orogenic Belt, the researched area is located in HegenMountain ophiolite-volcanic arc-accretionary complex。Late Paleozoic volcanics andgranite intrusion are exposed in Hegen Mountain complex, Baoligaomiao Fm volcanics oflate Paleozoic era is the main exposed stratum in the researched area.Baoligaomiao Fm inthe researched area consists of lower clastic rocks and upper volcanics. Through detailedstudy, we know that Baoligaomiao Fm volcanics can easily divided into two parts: thelower part is mainly made up of calc-alkali andesite-dacite-rhyolite and their correspondingvolcaniclastic rock; the upper part is mainly comendite and volcaniclastic rock. Through theresearch of petrology, chronology and geochemistry of Baoligaomiao Fm upper volcanicsin Talabai Farm, Sonid Left Banner, the paper has discussed the property of volcanicmagmatic source and geodynamics background of the magma forming, thus in some wayproviding basis for tectonic evolution of orogenic belt.On the basis of field investigation and research, we have known the feature of theupper part and lower part of Baoligaomiao Fm volcanics: successive volcanic sequence;conformable contact; no fossil erosion surface and obvious volcanic eruption gap. The rocktypes of the upper Baoligaomiao Fm volcanics are rhyolite, pantellerite and Volcanic clasticrock. By zircon U-Pb isotopic dating, we work out the age of the upper Baoligaomiao Fmvolcanics is303.8±3.6Ma,of late Carboniferous epoch, thus the era of volcanic rocks inresearched area is again determined. However, predecessors considered that the volcanicrocks in researched area was of Baiyingaolao Fm, Jurassic.The experimental data of major and trace elements and Rare Earth Element of theupper Baoligaomiao Fm volcanics are as follows: low MgO (0.31～1.48wt%)、TFe2O3(1.96～3.02wt%)，special low CaO(0.082～0.392wt%)，on the diagram ofZr/TiO2-SiO2,the ratio of Zr/TiO2×0.0001is high(0.18～1.05), the content of SiO2is high,thus we can conclude the volcanics is alkaline.In roach, the content of K2O is high(4.41～6.13wt%), on the diagram of SiO2-K2O indicate volcanics is shoshonite-high-K calaalkalineseries.On the diagram of aluminum saturation index, we can deduce the volcanics isweekly peraluminous(A/CNK=1～1.05，A/NK=1.01～1.09). The characteristics of Rare Earth Element of the upper Baoligaomiao Fm volcanicsis as follows:total content of Rare Earth Element is high(ΣREE=263.74～433.22ppm),thefractionation of LREE and HREE is not obvious((La/Yb)n=1.67-5.80), the REE pattern isof week right tilted. Eu negative anomaly is obvious, stated the dissociation has took place,and it mainly happens to plagioclase.In volcanics,the ratio of (Na2O+K2O)/CaO(21.3～99.0), TFe2O3/MgO (9.56～32.4) and (1.29～2.71) are high,the content ofZr+Nb+Ce+Y(633.4～1760ppm) and Zr(414～1428ppm) are also high, all of which hasshown that volcanics is of A granite. In volcanics,the ratio of Y/Nb=2.59～4.23，Ce/Nb=2.98～5.93，the content of Y and Ga(59.1～148ppm,14～30.6ppm)is high,whichshown the volcanics is of A2granite.In roach, Yb+Nb=26.31～52.4ppm，Rb=148～288ppm,the diagram of Yb+Nb-Rb shown that the volcanics is the production ofcontinental marginal arc.