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Characteristics and Petrogenesis of Late Mesozoic Granitic Volcanic-Intrusive Complexes in Southeastern China

WANG De-zi, ZHOU Jin-cheng, QIU Jian-sheng, FAN Hong-hai   

  1. Department of Earth Sciences, Nanjing University, Nanjing 210093, China
  • Received:2000-12-20 Revised:2000-12-20 Online:2000-12-20 Published:2000-12-20

Abstract: During the late Mesozoic, magmatic activities in southe astern China were very intensive. In the products of magmatism, the gra nites-rhyolites are predominant, and the andesitic-basaltic rocks are less imp ortant. The statistica l data on the isochron ages for over 200 granite and volcanics samples show that the earl y Cretaceous ages make up about 60% of age population; and the late Jurassic, 20 %; the late Cretaceous, 10%; the middle Jurassic, 10%. The ages of Mesozoic volcanic s and related granites in southeastern China have a tendency of getting younger from the inland toward the coast. In some cases, granites and associated acid ic and interm ediate volcanic rocks are comagmatic. Under the condition of uniformity in time, space and source materials, the granities can be considered as the “roots” of t he central type volcanic apparatuses composed of rhyolites and dacites, forming so -called granitic volcanic-intrusive complexes. The granitic volcanic-intrus i ve complexes in southeastern China may be divided into three types, i.e., syntex is type, continental crust remelting type and A-type. Their petrogenesis is cont rolled by different tectonic settings, rock-forming mechanisms and source materials. The syntexis t ype is dominantly distributed in the coastal area, w hereas the continental crust remelting type is mainly located in the inner land. Th is spatial pettern is constrained by two factors: the composition of b asement rocks and the intensity of crust-mantle interactions. Underplating of basaltic magma took place more intensively in the coastal area of Zhejiang and Fujian provinces during the late Mesozoic. Therefore, the crust-mantle interaction was also more intensive in the coastal area than i n the inner l and. In the west of Wuyishan Mts., the continental crust usually possesses highe r maturity. The meta-igneous materials, such as amphibolites, are rare or absent in the baseme nt rocks of the area. The combining constraints of these two factors led t o a regular spatial distribution of above-mentioned two types of granitic volcani c-intrusive c omplexes in southeastern China. The distribution of A-type granitic volcanic-i ntru sive complexes is mainly controlled by Changle-Nanao fault zone in the coastal a rea of Zhejiang and Fujian provinces. The thickness of both lithosphere and cont inental cr ust is obviously thinned and the asthenosphere is upwelled in this r egion. It is evident that A-type granitic volcanic-intrusive complexes were fo rmed in a extensional tectonic setting and display the post-orogenic features. Since then, the Yanshanian magmatic activities were toward the end. In this pap e r, the authors show the uniformity in emplacement time, space and source mate rials for granitic volcanic-intrusive complexes, taking both Tonglu and Xiangsh an complexes as examples. Some regularities summarized may be very important for un de rstanding the genesis of granitic volcanic-intrusive complexes in southeastern China.