Genesis Significance Of Clay Minerals In The Carboniferous-permian, In Central-eastern Jilin Province

The central-eastern Jilin province locates in the east of East Asian Orogenic Belt. Itsgeotectonic units include Songnen massif, Jiamusi massif and Xingkai massif typically.The late Paleozoic strata of the east-central part of Jinlin Province are mainly composedby a set of sedimentary layers and partial continental deposit layers. Several NE, NNEand approximately EW deep faults can be found in this region, with rocks of mudstone,silty mudstone and argillaceous siltstoue etc. According to the book “Regional Geologyof Jinlin Province”, the metamorphism of this region belongs to the Hercynianmetamorphism of low greenschist facies. Recent research on the vitrinite reflectanceand illite crystallinity in Songliao basin and its surrounding area indicate that the degreeof metamorphism of this region is equivalent to anchizone. Since the grains of theregional metamorphic minerals are very small, it is difficult to use polarizing microscopeto study such minerals. This study therefore employs the XRD technology to determinatethe illite crystallinity, illite (muscovite) type, illite (muscovite) b values, the relativecontent of clay minerals, and the ratio of claymineral mixed layer. To find out the degreeof metamorphism of this region, we also investigate the genetic significance of its clayminerals.The results of this work indicate that, the clay minerals in the Carboniferous-permian in the middle east of Jilin province are illite, kaolinite, chlorite, illite/smectite mixedlayer and chlorite/smectite mixed layer etc. It is also found that the illite crystallinity ofthe sampled minerals in the study area ranges from0.33to0.43, and the average value ofthe illite crystallinities of the Carboniferous rocks is0.38，which indicates that theCarboniferous rocks were probably formed in a low anchizon. In addition, the averagevalue of the illite crystallinities of the Permian rocks is0.43, which means the rocks areproducts of the late diagenetic zon or a low anchizon. It is observed that when changes from Carboniferous to Permian, the illite crystallinity will increase from0.38to0.43gradually. This is a normal buried sedimentary sequence.The polytype of illite (muscovite) is featured by1Md,1Md+2M1and2M1types. TheCarboniferous polytype of illite (muscovite) is mainly2M1, while the Permian polytypeof illite (muscovite) is mainly1Md+2M1. These results show that the Carboniferousrocks are the anchizone product and the Permian rocks probably belong to the latediagenetic belt or advanced diagenesis and low anchizone nearby.The values of the b dimension of the sampled illites (muscovite) vary from8.9946to9.045with an average value of9.004. This means that the rocks are formedin the low-pressure environment. For those illites whose illite (muscovite) b dimensionare higher than9.000，their crystallinities are usually lower than0.42, and theybelongs to the type of1Md. To the contrast, those illites with illite (muscovite) bdimension are lower than9.000, their illite crystallinities are usually higher than0.42,and the polytype of such illites are1Md+2M1. In other words, there is a relationshipbetween the b dimension of illite (muscovite) and illite crystallinity, and between bdimension of illite (muscovite) and the illite (muscovite) polytype.Regarding to the Carboniferous samples, the relative content of illite decreases withthe increase of illite crystallinity. i.e., when the illite crystallinity ranges from0.25to0.42, the relative content of illite in clay minerals is as high as87.71%. when thecrystallinity is higher than0.42, the relative content of illite in clay minerals is62.14%.Different from that, the relative content of illite/smectite increases with the increase ofillite crystallinity. That is, when the illite crystallinity ranges from0.25to0.42, therelative content of illite/smectite in clay minerals is only2.57%. When the crystallinity ishigher than0.42, the relative content of illite/smectite in clay minerals is increased to be23.57%.For the Permian samples, the relative content of illite/smectite increases with theincrease of illite crystallinity. When the illite crystallinity ranges from0.25to0.42, therelative content of illite/smectite in clay minerals is0.71%. But when the crystallinity ishigher than0.42, the relative content of illite/smectite in clay minerals is2.78%. As forthe mixed-layer mineral, when the illite crystallinity ranges from0.25to0.42, both theratio of smectite in illite/smectite and chlorite/smectite mixed layer are6.43%, or12.86%in total. When the crystallinity is higher than0.42, the ratios of smectite in illite/smectite,and chlorite/smectite mixed layer are28.89%and14.44%respectively, or43.33%in total. These results show that with the decrease of formation temperature and pressure,the relative content of smectite in the mixed-layer clay mineral increase.