The Effect Of Algovite Intruding On The Surrounding Sedimentary Rocks Of Zhangxin Area In Jixi Basin

Jixi basin is one of the most important coal basins of Mesozoic Era in the east of Heilongjiang.The basin lies in the Jiamusi-Uplift of Mesozoic Era and to the north of the Dunhua-Mishan Fault.The basin was formed in the early phase of Early-Cretaceous and sealed off in the early stage of Late-Cretaceous. Four stratum units are recognized in Zhangxin area and they are named as Didao Formation(K1d), Chengzihe Formation(K1c), Muling Formation(K1m) and Dongshan Formation(K1ds).Didao Formation is composed of river and lake facies～volcanic clastic sedimentary rocks. Chengzihe Formation is composed of a series of brink lake～swamp facies of sedimentary rocks. Muling Formation is a series of brink lake～delta～swamp facies of sedimentary rocks. Dongshan Formation is a series of land facies composed of volcanic～sedimentary clastic rocks.The sampling points are located in Zhangxin mining area which is 8 kilometres long in east-west direction and 4 kilometres wide in north-south direction with an area of 32km2. The algovite invades the surrounding rocks between sedimentary layers composed of Chengzihe formation.The algovite is an ellipse in plane view with an area of 40km2 and has a thickness of 80~230m.The sampling space in the surrounding rocks in Xifeng tunnel is 4~6m and samples are also taken from the different lithologic layers.On the hanging wall of the algovite intrusion, samples are taken at a space of 20cm in a 10m long section.The sampling depth is from 175.81m to -168.25m in Yellow sea elevation and 133 samples in total are taken.The main types of sedimentary rocks in Chengzihe formation at Xifeng well are mainly lithic arkosic sandstones with some arkosic lithic sandstones. The main types of diagenesis are mechanical indentation, pressolution, cementation, metasomatosis, accreting, crack-infilling and alterationin.Through systematic sampling, slice identifying, physical property analysis, clay mineral analysis, vitrinite reflectance testing and chilkinite crystalization testing, autogenetic mineral analysis,clay mineral assemblage analysis, S% in I/S, the author thinks that diagenetic stages of Chengzihe formation in Zhangxin area are middle and late diagenetic stages.Although the sandstones of Chengzihe formation in Zhangxin area are nearly the same as that in Jixi basin, the sandstones of Chengzihe formation in Zhangxin area contain less quartz grains and more felspar and lithologic fragments.40Ar/39Ar dating shows that the algovite intrusion was formed during 96-101Ma B.P. The Chengzihe formation in Zhangxin area in Jixi basin is intruded by the algovite sills, which shows that the studied area undergone an extending stage during the late stage of Early-Cretaceous and the early stage of Late-Cretaceous.The laumontites are found in the intrusion composed of gabbroporphyrite intruding Chengzihe formation in Jixi Basin. The laumontites are arranged in radial aggregates and concentrates to band. With magma uprising,the temperature and pressure of the magma become smaller and smaller and the solubility of volatilizing components in the magma also becomes smaller and smaller. At the same time,volatilizing components are separated from the magma and concentrated in the boundary belts of the intrusion.Since the magma cools and condenses quickly, the bubbles in the upper part of the algovite intrusion form holes which provide spaces for the forming of laumontites.The cracks in the algovite and its boundary formed by magma cooling provide passageways for the escaping of CO2 and the transporting of Ca2+ and Al3+. Ca2+ and Al3+ react in the liquid magma forming laumontites and calcites.Little by little, laumontites and calcites form mineral assemblages.The data of electronic probe analysis shows that the augites and feldspars in the intrusion supply Ca2+ and Al3+ to form the laumontites. And at the same time, Fe3+ is concentrated in the intrusion. Ca2+ and Al3+ in alteration envelope are more than that in augites.So a light color alteration envelope is formed in the boundary belt of the intrusion.Laumontites are formed by chemical reaction and augites in the intrusion provide Ca2+ for the forming of Laumontites and calcites.The algovites intruding the Chengzihe formation effected the surrounding rocks. According to vitrinite reflectance testing and X-ray analysis on clay minerals,we divided the surrounding sedimentary rocks at Xifeng well into 6 respond belts to the algovite- intruding.From the top to the bottom of the well, they are:beltⅠ(no respond belt),beltⅡand beltⅥ(weak respond belts), beltⅣ(algovite intrusion) and beltⅢand beltⅤ(strong respond belts).According to calculation, the highest temperature of the buried Chengzihe formation at Xifeng well is 259.24oC. The temperature of algovite intrusion is 379.76oC while it intrudes the surrounding rocks and the intruding lasts for 117.7 years.The temperature of the algovite intrusion and the lasting time of intruding make the surrounding rocks metamorphosed weakly.Because of the intruding of algovite, a lot of didrimits are formed in the surrounding rocks and quartz grains are secondary enlarged.The vitrinite reflectance of the surrounding rocks on hanging wall decreases with the increasing of distance between the algovite intrusion and the surrounding rocks changing from 2.31% to 0.79%. The intruding of algovite effects the organic mass on hanging wall and the largest distance effected by the intruding is 129.06m.The average content of chilkinite in the surrounding rocks on hanging wall decreases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 85.7% to 11.5%.The average contents of illite-smectite mixed-layer, kaolinite and chloritoid increase with the increasing of the distance between the algovite intrusion and the surrounding rocks changing separately from 4.3% to 64.8%, 0.0% to 15.0% and 10% to 12.1%. The average contents of chilkinite, kaolinite and chloritoid in the surrounding rocks on bottom wall increase with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 12.3% to 16.7%, 2.5% to 37.7% and 3.5% to 5.8%.But the average contents of illite-smectite mixed-layer decreases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 82.0% to 39.8%.Three kinds of clay mineral assemblages are recognized at Xifeng well.They are:① illite-smectite mixed-layer+chilkinite+chloritoid±kaolinite±chloritoid-smectite mixed- layer assemblage,②chilkinite+chloritoid±chloritoid-smectite mixed-layer assemblage and③chloritoid-smectite mixed-layer+kaolinite + chilkinite + chloritoid assemblage.The porosity of the surrounding rocks on hanging wall decreases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 12.5% to 11.3%.But the porosity of the surrounding rocks on bottom wall increases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 7.0% to 8.8%. The percolation rate of the surrounding rocks on hanging wall increases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 0.16md to 0.66md.And the percolation rate of the surrounding rocks on bottom wall increases with the increasing of the distance between the algovite intrusion and the surrounding rocks changing from 0.21md to 0.41md. The average porosity of the surrounding rocks is 10.4% and the average percolation rate of the surrounding rocks is 0.434md.The porosity of the surrounding rocks at Xifeng well is larger than that of Chengzihe formation in Jixi area.But the percolation rates of both are nearly the same.The intruding of algovite causes two respond belts on the hanging wall:one is a strong respond belt which is 0-7.77m from the algovite intrusion;and the other is a weak respond belt which is 7.77-129.06m from the algovite intrusion.The largest distance effected by the algovite intruding is 129.06m which is 1.2-fold of the thickness of the algovite intrusion.The effect of intruding on hanging wall is larger than that of intruding on bottom wall.