The Extensional Setting Of The Early Cretaceous Magmatism In Eastern China-Example From The Yinmawanshan Pluton In Southern Liaodong Peninsula

In the last years, the important development of Mesozoic geological study in the eastern China showed that the early Cretaceous magmatism were very prosperous, at the same time, accompanying with large-scaled metallization, but nowadays, the dynamics of this phenomena lacks some studies. Majority of scholars thought that the early Cretaceous magmatism was closely related to the lithosperic thinning or the tectonic transition, indicating that it formed in the extensional setting, but there are no enough evidences. According to this problem mentioned above, we studied the Mesozoic Yinmawanshan batholith of the southern part of the Liaonan peninsula. Yinmawanshan batholith is one of the Mesozoic granitic batholiths, and has large area (about 1120km2) and various lithologic characters. Meanwhile, its emplacement is closely related to the Liaonan metamorphic core complex, so this point provides an uncommon opportunity to studying the tectonic mechanism of the early Cretaceous magmatism. This paper takes field surveys to the Yinmawanshan batholith in detail, and takes precise isotopic dating and the geochemical analyses. The achievements are listed as follows. 1,the results of dating We took zircon U-Pb isotopic dating by using LA-ICP-MS of the college of geology in Xibei university, and the analyzing results showed that the zircon U-Pb age is 120(4 ~129(2 Ma, showing that the rocks with different petrofacies formed nearly at the same time, and this viewpoint that the Yinmawanshan batholith formed in Triassic is wrong. 2,the forming mechanism of the Yinmawanshan batholithAccording to the geochemistry, the Yinmawanshan batholith can be divided into the diorite and the monzogranite. The Harker diagram, εNd(t)—MgO diagram and the mafic fine-grained enclaves show that the Yinmawanshan batholith is mixed by at least two kinds of magmas. The monzogranite has high SiO2(71-74%)content, low Mg#, (87Sr86Sr)i>0.705,εNd(t)<0, which all indicate the monzogranite came from the low crust. The SiO2 content of the diorite is 54－66％, Mg# is 47-53, and the large ion lithophile element ( LILE ) and the light rare earth element ( LREE ) are rich, but the high field strength element(HFSE)is deficited, so it can be known that dioritic original magma may comes from the enriching lithospheric mantle. The Gudaoling batholith is located in the north of the Yinmawanshan batholith, and has coincident forming age and the geochemical characteristics with the Yinmawanshan batholith. The geochemical characteristics of the Gudaoling granite and its dioritic enclaves show that the early Cretaceous granite came from three end members, that is, the new low crust, the lithospheric mantle and the old low crust. In the isotopic Sr－Nd diagram, the Yinmawanshan batholith has pretty large-scaled composition of the Sr,Nd isotope ( 87Sr/86Sri=0.7092-0.7111, (Nd(t) =-14.90－-19.32 ), which is evidently located among above three end members.Summing up these results, we think that the Yinmawanshan batholith came from the mixing, the fractional crystallization and the crust contamination of the enriched lithospheric mantle and the low crust magma.3,the time relationship between the Yinmawanshan batholith' formation and the tectonic extension The field observation and petrology have clearly showed that the Yinmawanshan batholith intruded in the forming period of the Liaonan metamorphic core complex, so this provided direct geological evidence to the viewpoint that the early Cretaceous magamatism of the eastern China developed in the extensionally tectonic setting. Besides, through the Ar-Ar dating to the ductile deformed rock in the Jinzhou detachment fault of Liaonan metamorphic core complex, Yang et al (2004) found that the age of the hornblende is 112.6(0.5Ma, the age of the muscovite is 111.9(0.1Ma, the age of the biotite is 114－111Ma, the age of the feldspar is 112－105Ma, above ages are all little younger than the emplacement age of the Yinmawanshan batholith, therefore, it is evident that they are the co...