Research On The Global Cenozoic Andesites About Their Temporal-spatial Distribution And Tectonic Settings

A set of discrimination diagrams of the tectonic setting interpretation for the granitoids and basalts had been proposed by a group of scientists headed by Prof.Pearce from 1970 to 1980 in 20th century,which created a method to recognize tectonic environment of igneous rocks.In contrast,previous researches for the andesite tectonic settings and its geochemical characteristics were limited,possibly attributed to the relative complicated source region of the andesite and the changes of their geochemical composition during formation.In this case,it is hard to distinguish their tectonic environment using traditional method.Mass data for andesite have been accumulated in several decades although there are lots of arguments and questions for the andesite tectonic settings.Based on the GEOROC and PetDB databases widely used,this paper mined the Cenozoic data of the global andesite,and the main results have been obtained as following:? 12 different tectonic settings of Cenozoic andesite can be recognized based on the statistics analyses of 58763 Cenozoic andesite from databases.The results demonstrate that 67.71%of global Cenozoic andesite was formed at convergent margin.In addition,andesite can also be found in other tectonic settings such as continental intraplate,continental flood,oceanic island,rift and mid-oceanic ridge.? The Cenozoic andesites from variety tectonic settings were mainly formed in Miocene.Combined with the temporal distribution of basalt and other intermediate-acid volcanic rock,a great global magmatic event had been occurred.The coeval uplifting of Tibetan Plateau,the thickening of Andean's continent,global desertification and other significant events were much likely related to this strong magmatism.Mantle activity could be an important inducement of the Miocene global events.? We found that both commonly used trace elements discrimination diagrams and LILE/HFSE diagrams for the tectonic settings of the basalts were suitable for andesite samples.This result not only expanded the scope of basalts discrimination diagrams application,but also illustrated that the geochemistry characteristics of andesite can also discriminate its different tectonic settings.Oceanic island andesites(OIA)geochemical characteristics are different from Oceanic island basalts(OIB),which is the reason why some OIA samples cannot be limited into the OIB region in those discrimination diagrams.? This thesis used big data mind to explore the incidence relation between oceanic island andesites(OIA)and island arc andesites(IAA).As result,nearly 20,000 OIA-IAA discrimination diagrams came out from 440,000,000 combinations,and finally we picked up 6 of them with good discrimination power,including lg(Cs/Ta)-lg(Cu/Ta),lg(CaO/Nb)-lg(Cs/Zr),lg(Cu/Ta)-lg(Co/Nb),etc.It opens up a new way for studying the genesis and geodynamic setting of andesite.? Scientific development makes us step into the big data time.The preliminary achievement in this thesis reveals that scientific big data could be an important way and method for studying the tectonic setting and geochemical characteristic of igneous rocks.