| Picrite of Emeishan large igneous province with the original magma characteristics, is considered to be the oldest rock in Emeishan mantle plume, which is regarded as the typical samples of Emeishan mantle plume formation mechanism and chemical composition of the mantle source region, the depth and temperature of mantle dynamics background. But researches on the composition of the fluid is relatively small. In this paper, the author selects Binchuan picrite and its symbiotic basalt of Emeishan large igneous province as the object of the study. By analyzing the main elements and trace elements, fluid chemistry and carbon isotopic composition about picrites and basalt samples, this paper shows a detailed discussion of the magmatic origin, magmatic evolution and dynamics background of Emeishan large igneous province as well as its formation. The following sections are the major conclusions:1. Binchuan picrites are all sub-alkaline rocks, the relationship diagram of MgO and the other major elements displays that MgO content of this rock has a positive correlation with TFe2O3but a negative correlation with P2O5, CaO and Na2O+K2O; There are also a positive correlation trends between Ni, Cr and Mg#content.2. According to chondrite-normalized REE patterns and primitive mantle-normalized incompatible trace elements diagrams, the overall change of trace elements diagrams is a bit small, Binchuan picrites samples displayed a similar distribution patterns characteristic with the OIB, relative loss LILE (Rb, Sr, and P) and HFSE (Nb, Th and Hf).3. The occurrence of the fluid matrix components in Binchuan picrites and basalts have a phased release characteristics. At200℃～400℃, the main content of picrite is H2(7.90mm3/g), but that of basalt is CO2(6.51mm3/g) and H2(4.41mm3/g); at400℃～1000℃, the major content of picrite is H2(476.19mm3/g), but that of basalt is H2(47.53mm3/g) and CO2(45.60mm3/g).4. Carbon isotope of CO2and CH4released by Binchuan picrite and basalt roughly have the same isotopic compositional scope between200-400℃and400-800℃; the content of813Cco2values ranges from-26.97‰～-11.27%o, with an average of-19.12%o, the content of813CCH4ranged from-30.38‰～-14.79%o, with an average of-22.58%o, and they all have the characteristic of crust components, the carbon isotopes released by fluid component of CO2and CH4become heavier as the temperature rises. In addition,δ13C value of homologues of CH4exhibit reverse distribution of non-biogenic which is similar with deep mantle.5. Trace element characteristics of Binchuan picrite samples, showing its origins in the OIB source region; compared with symbiotic flood basalts Binchuan picrites has a very high content of H2(485.65mm3/g), and similar with Siberia large igneous province (H2,224.02mm3/g),δ13C value of homologues of CH4in the fluid component exhibit reverse distribution of non-biological, indicating that the source comes from the deep mantle magma composition or core-mantle boundary; combined with incompatible elements Th/Hf-Ta/Hf discrimination diagrams think Binchuan picrites is formed by plume magmatism directly.6. Between MgO content and the rest major elements of Binchuan picrites samples, between the trace elements Ni and Cr content and Mg#, between CaO content and CaO/Al2O3they all have good correlation trends, indicate Binchuan picrite samples derived from the same source, and has experienced continuous magmatic evolution, and there have been clinopyroxene (Cpx) and olivine (Olv) fractional crystallization; carbon isotope at200℃～400～with the characteristic of crust components, some samples Nb/U ratio is about13.34, getting close to the Nb/U ratio of crust (Nb/U of crust is about10), indicating that there is a certain crustal material contamination in form process of picrites; magmatic eruption process releasing a large number of CO2, H2and H2S and other gas components, suggesting the formation of the Emeishan large igneous province has a certain impact on the prevailing environmental change and extinction. |
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