Thermodynamic Modeling Of The Origin Of A-type Granites

A-type granite has important research significance mainly because of its special petrogenesis and closely connection with tectonics and geodynamics.We can better understand the generation of the A-type granites if this study is to better constrain what are the effect of the composition of source rocks and experimental conditions(temperature,pressure and oxygen fugacity)in closed system.However,there is a lot of controversy about petrochemical constraints for its formation.We use the achievement of experimental petrology with the research methods of thermodynamic simulation for accuracy correction,by controlling the variables such as the composition of source rock,water content,temperature and pressure,oxygen fugacity.According to the indexes of Fe-number,the modified alkali-calcium index and other indicators supplement the discussion on a lithochemical constraint on the origin of A-type granites petrogenesis in closed system.Previous experimental petrology believed that the generation of A-type granite is related to high temperature,low pressure,low oxygen fugacity,and the composition of source rock.However,it is still unclear how the source rock and formation conditions specifically affect the composition of granitic melt formed in the closed system.This study makes detailed discussions and supplements basing on the thermodynamic simulations about the lithochemical constraints of the generation of A-type granite in a closed system.The composition of source rock exert the decisive factor of the presence of A-type granite,and basic to acid rocks are all appropriate sources for A-type granitic melts.The higher the iron content of source rock is,the easier it is to form A-type granites.The conditions of temperature and pressure have a great influence on the formation of A-type granite for any source rock,Maybe it requires suitable mediumhigh geothermal gradient conditions rather than pure high temperature or pressure conditions.Moreover,oxygen fugacity is also a controlling factor for formation of Atype granite in closed systems.We consistently find that the same source rocks are easier to generate A-type granites under the relatively lower oxygen fugacity under the conditions of the same temperature and pressure.In other words,it form in relatively reduced environments.And the source rock with a higher iron content can also generate a mass of A-type granite in a lightly oxidized environment.But with the increase of oxygen fugacity,the output of A-type granite will decrease,and even A-type granite cannot be formed.In general,the generation of A-type granite is not subject to a single factor,but is a combination of source rocks,temperature-pressure conditions and oxygen fugacity.In general,the higher the iron content of source rock and geothermal gradient and the lower oxygen fugacity that is in favour of forming A-type granite.While the iron content of source rock is very low,it is not conducive to the formation of A-type granite.A-type granites are widely developed in southern China.We can combine with typical A-type granites that developed in southern China to discuss some soure rocks under the same oxygen fugacity and at 700-1100?,0.4-1.2GPa,whether it is conducive to the formation of A-type granite.Thus it provides a certain research method for the analysis of the generation of A-type granite in South China.Thermodynamic simulation makes it possible to quantitatively study the evolution of the deep crust,and provides an efficient and economical new way to supplement the evidence of petrogenesis research.