The Influence Of Volcanic Activity On The Development Of Microbial Rock

The southeast coast of China underwent massive tectonic movements during the early Cretaceous,during which the Pacific Ocean plate swooped westward into the Eurasia,resulting in massive magmatic-volcanic activity along the southeast coast of China,heaping up huge volcanic debris rocks.In shipu xiangshan Zhejiang,the lower cretaceous(Shipu group)with multiple sets of volcanic clastic rocks and sets of microbial rock as the main characteristics,provides a good condition to discuss the effects of volcanic activities on the microbial rock development..In this study,volcanic petrology,sedimentary petrology and geochemistry were studied with the emphasis on the relationship between volcanic clastic rocks and microbial rocks in the Shipu section.depending on sedimentary beds which as the volcanic interface,it divided shipu section into two periods.According to the rock composition and size of particle,we primary concluded that volcanic activity intensity get weak from the bottom to top in each period.Combination of the characteristics of sand matrix,sandstone skeleton,the main and trace elements,we found several provenances of magma including the mantle magma and crustal magma.And the provenance of the clastic was proximal volcanic,the distant volcanic which transformed by water,and small amounts of sedimentary rocks.In addition,after a detailed study on the characteristics of microbial rocks,many types of microbiolites in Shipu area were identified,contains the thrombolites(stratiform and domical)and stromatolites(stratiform,nodular and columnar branching).Depending on the lithology,sedimentary structure of the 8 sets of microbiolites,we primary established the distribution in different depth:laminated stromatolite,wavy stromatolite and stratiform thrombolites,domical thrombolites and small domical stromatolite,domical stromatolite and nodular stromatolite from the shallow to deep water.Through futher analysing the internal characteristic of the microbiolites and nearby stone and content change of K,P,Cu,we concluded the volcanic activities have two influence on the development of microbiolites:on the one hand,huge,fast accumulation of volcano rocks covered microbiolites,and the rising Cu content poisoned the microorganisms,which had a negative effect on microbiolites.On the other hand,the slow injection of the fine volcanic particles(especially the ash)provided an outstanding environment for microbiolites.And the high content of K,P,Cu provided nutrients for microbiolites.Besides,the relative sea level changes which was influenced by volcanic activity also had two effects on the development of microbiolites:on the one side,the rapid rise of sea level drowned a large number of microorganisms,which cause microbial rock to die.On the other side,the slow sea level rise provides a space for further development of microbial rocks.