Volcano Rock Geochemistry And Its Geological Significance Of Carboniferous-Permian In Changning-Fengqing County, Yunnan Province

Volcano rock geochemistry of Carboniferous-Permian in Changning-Fengqing mainly located in eastern margin of Baoshan-Zhenkang block and the north of Changning-Menglian belt in the eastern of Paleo-Tethyan-?Sanjiang area?.the reaserch The study on the volcano rock formation mechanism helps to solve the long-standing dispute of Changning-Menglian tectonic evolution mechanism,however, there are few studies on the volcanic rocks in this area.The result of this paper shows that the volcano rock of Changning-Fengqing area are mainly the series of tensile and alkali basalt, the volcano rocks in fengqing mengyou are the two types of quartz and high alumina basalt,which are belong to the sodium series and whose samples are saussuritized and chloritization by the seawater; the volcano rocks in changning wandian are mainly potassium series high aluminum basalt,which are with low magnesium and poor calcium,however are with rich aluminum and iron,whose tuff samples are filling with arc triangular quartz crystal chip; the Volcano rocks in changning gengga are the sodium series alkaline basalts,many olivines phanerocryst that are iddingsite and pyroxenes can be found in those samples. Trace elements Sr, Nb, Ce are the negative anomaly,while Nd?Pb are positive anomaly,other trace elements,such as La,U,K,Zr,Yb are obvious fluctuation. Besides the trace element U is weak enrichment,K is deficit. The ratio between Th and U(Th/U) is of a low degree, while the content of Si and Na are of a high degree,which reflect that the volcano rocks here formed in the marine environment;K,Nd,Ta are weak enrichment in the volcano rocks of changning gengga,while Sm,P,U are deficit,which indicates the feature of Continental alkaline basalt; Rare earth element distribution curves of the the volcano rocks are right,light rare earth elements are rich, Eu and Ce are negative abnormal, some characteristics,such as La,Sm,Nb,Ta,Zr,Yb,Sr,Pb, ferromagnesian index,felsic index, solidification index and differentiation index are indicating thar the magma source is enriched mantle,which is the partial melting result of mixed phase of plate interior between spinel lherzolite and garnet lherzolite. Magma passed the fractional crystallization of olivine, Rich calcium feldspar and diopsidic augite and the contamination of a spot of earth's crust. Studies suggest that the volcano rocks of Carboniferous-Permian in changning-fengqing are formed in intraplate,however they are different from Emeishan basalt caused by mantle plume,the volcano rocks in fengqing mengyou formed in in an ocean island, corresponding to the Lower Carboniferous volcano stratum-pingzhang(C1pz) in the region;the volcano rocks in changning gengga formed in an intraplate extensional environment on active continental margins, corresponding to the early permian volcano stratum-Woniusi(P1w) in the region;the volcano rocks in changning wandian are mainly the result that early Permian magma on active continental margins flowed into neritic continental shelf, which is closely related to the early Permian volcano stratum-Woniusi(P1w),corresponding to the middle Permian stratum-bingma(P2bm).The experimental results show that changning-menglian belt spread inheritably from passive margin basin of the east of Sibumasu in early Devonian, a large number of silicolite from deep sea were depositing across the region at the same time; the passive margin basin of the east of Sibumasu spread speedly between later Devonian and early Carboniferous,forming oceanic crust, the volcano rocks of the late Carboniferous volcano stratum-pingzhang(C1pz) accreted with silicolite from deep sea in the early, with terrigenous clastic rock in the mid-term,and with the carbonate rocks in the end.which represents seamount double-layer structure,reflecting that the changning-menglian ocean were in sedimentary sequence of regression by upthrowed Persistently at this time;the main oceanic basin got into Subduction stage during the time from the late Carboniferous to the late Permian and got into the stage of remnant ocean basin during the late Permian to the early Triassic period,the origin of the early Permian volcano stratum-Woniusi(P1w) is that the east of changning-menglian ocean subducted to the lower part of the yangtze block,the baoshan-zhenkang block separated from the Sibumasu in the west of changning-menglian ocean,leading to the mantle magma quickly migrate to the surface along the fracture passage,with the tension of rift on the continental margin,the magma were flowing into the east low sea,forming the middle Permian stratum-bingma(P2bm) that basalts accreted with tuff sandstones which are the sediment of short distance transportation,later the sequence of the late Permian is dominated by the Carbonate rocks(P3s);the changning-menglaing belt got into the stage of collisional orogeny during the middle-late Triassic period.