The Geochemical Characteristics And Diagenesis Research Of Cheng’an Composite Pluton,Anhui Province

The Cheng’an complex intrusions situated in Qimen, Anhui province is located at the southeastern boundary of Yangtze plate. A few geologists do some research about the Cheng’an complex intrusions. Researches about the monzonitic granite and syengranite have not been done in the Cheng’an complex intrusions would be done.In this study, petrology, isotope chronology, geochemistry methods of Cheng’an complex intrusions were systematically investigated.Then we obtain the main conclusions as follow:1、Cheng’an complex intrusions are composed of granite diorite, monzonitic granite and syengranite. The lithofacies zonation is more obvious to see in Cheng’an granite.The Jiandaoka monzonitic granite has a pulsating intrusive contaction with Cheng’an granite diorite. The contact-relationship has points of resemblance to Liucun syengranite and Cheng’an granite.2、The ziron U-Pb ages from rocks of Cheng’an granite and Jiandaoka monzonitic granite and Liucun syengranite are 151.0±2.8Ma and 131.2±2Ma and 129.4±1.5Ma,respectively.The isotopic ages of inherited ziron focused on 800Ma, which imply numerous diagenetic materials were from the ancient crustal basement. Combined with previous research results, the Conclusion is that the Cheng’an granite is the olddest plution of late mesozoic in the southern Anhui Province.3、Applications of feldspar-theromometer and biotite barometer show the p-T conditions about the Cheng’an granite and Jiandaoka monzonitic granite and Liucun syengranite:T=560℃, P=0.22-0.30Gpa, and 5-8 km; T=580℃, P=0.6Gpa; T=513-520℃, P=0.23-0.24Gpa. Due to the influence of volatile matter, the evaluation results will be lower than actual date.4、The Cheng’an granite and Jiandaoka monzonitic granite belong to high-K calc-alkaline intrusion-rock series with low A.R.and they have peraluminous characteristic. The Liucun syengranite has a weak light-heavy REE fractionation which is completely opposite of Cheng’an granite. The right light rare earth concentration model is the same as the lower crust. LILE element concentration is high,while the HFSE element concentration is low. Standardized graphic primitive mantle of trace element is the right model.5、The Cheng’an granite shows I-type characteristics obviouslly. The parental magma of the rocks might be derived from the partial melting of the ancient continental crust materials and some mantle materials. The magmatism of the Cheng’an granite in research area may occurred under the geodynamic setting of extrusion-extension translation stage, which is basically in accordance with the geodynamic setting of adjacent area. The Jiandaoka monzonitic granite is A-type granite that the partial melting of old continental crust materials. And the monzonitic is usually believed to be formed under a post-collision extensional tectonic setting.6、The Cheng’an complex intrusions have a toally magmatic evolution with the possible effect of partial melting of old continental crust materials and differentiation crystallization.The complex intrusions have a complicated emplacement mechanism. The emplacement model may like this:the first is the rock wall encroachment on the extension,and the second is magmatic-stoping effection.