Yanshanian Magmatic Activity Scale And Dynamic Background Discussion In South China

Through geophysical exploration and field observation,predecessors have found that there is a certain geometric relationship between the equivalent length and thickness of magma intrusion,which has been verified in practical application.In this paper,the early Yanshanian granite in South China is analyzed by GIS,and the volume of plutons is estimated by the empirical formula after fitting.The statistical results show that the total volume of granitic rocks in the early Yanshanian(200-140Ma)in South China is about26000 km~3,and the average rate of magma addition is nearly 4300 km~3/myr.Early Jurassic(200-180Ma)magmatic activity is relatively weak,so the average rate of magma addition of Middle-Late Jurassic can reach 6500 km~3/myr.According to the three indexes of Fe-number,the modified alkali-calcium index and aluminum saturation index,this paper makes a non-genetic descriptive classification of granitic plutons in the early Yanshanian.The results show that the granites of iron,calc-alkaline,peraluminous series and magnesia,calc-alkaline and peraluminous series are the most abundant.Early Yanshanian granites in South China are dominated by biotite granites and dimica granites,and their source rocks are mainly metamorphic sediments and quartz-mica diorites.By calculating the saturation temperature of zirconium and analyzing its spatial distribution characteristics,it is found that there exists a magmatic high temperature zone with nearly east-west direction in Nanling area.Using Nd isotopes to calculate the young components of mantle-derived rocks,it is found that the early Yanshanian granites are mainly crust-derived type and crust-mantle mixed type,but less mantle-derived type.A chronological database is established by compiling the dating data of Yanshanian granites in South China in recent years.The early Yanshanian granites in South China are mainly distributed to the west of Zhenghe-Dapu fault,while the late Yanshanian granites are mainly distributed in the coastal areas,confining the early Yanshanian granites in a mirror"C"shape.The early Yanshanian plutons shows a NE-trending development as a whole.The major axis of the plutonss in Nanling area is nearly EW-trending,and the plutons development is obviously influenced by the pre-existing fault zones and rifts.The large-scale granites in the early Yanshanian in South China were mainly formed under the subduction of the Paleo-Pacific Plate,in which the magmatic activity of the Early Jurassic was influenced by both post-Indosinian extension and subduction plate fragmentation,and the granites of the Middle-Late Jurassic were mainly produced under the background of continuous extension-thinning of the lithosphere.