| Yangtze block has complex and special geological tectonic evolution history which is located in the composite parts between the Circum-Pacific tectonic domain and the Tethyan tectonic domain, it is an important unit in the global tectonic system. Midu area is located in the southwestern margin of the Yangtze block, Yunan Province, and near the Ailaoshan-Red River fault zone, and developed large scale basic volcanic rocks. Howerve, there are different viewpoints about the rock types and the diagenetic tectonic environment of this basic volcanic rocks in the study area. But it has great importance for deeply understanding the initial convergence and fragmentation patterns of Gondwana-Eurasian plates, as well as the tectonic regime of Tethys during the Late Paleozoic.Based on the field investigation and section measuring, the characteristics of petrography and geochemistry of this basic volcanic rocks in the study area were analysied, and the igneous rock characteristics and the source region are discussed.Lava and pyroclastic rock are the main types of basic volcanic rocks in the study area, rhythm and cycle characteristics are significantly, where the erupted performance of pyroclastic rocks are: breccia?crystal tuff?basaltic tuff?tuffaceous sand-stone combination. And the lava eruptions presented the character of microcrystalline basalt?porphyritic basalt? almond- shaped basalt composition. Study area exposed thick pyroclastic rocks and thin lava. The main structure of lava is porphyritic, the matrix structure are crystallite hidden crystal, and phenocrysts plagioclase-based, locality are pyroxene and olivine, showing the development of low-temperature alteration.Major elements analysis shows the Si O2 content is between 49.03% to 54.76%, 52.85% on average, slightly higher than normal, which may be related to the post-weathering. The conten of Ti O2 ranges from 1.38% to 3.58%, an average of 2.38%, which lower than the standard oceanic island basalts. Mg O content is between 1.01% to 9.55%, with an average of 2.14%. Mg# varies from 18% to 56%, which has a wide variation and less than the original magma range. The content of TFe O is high, and the average value is 11.25%, indicated that the magma source maybe presented at a high pressure environment. The content of P2O5, Al2O3, K2 O, Na2 O, Ca O and Mn O is between 0.18% to 0.45%, 14.54% to 18.42%, 0 to 2.91%, 0.20 to 4.67%, 2.29% to 17.09% and 0.10% to 0.19%. Through the analysis we can know that the basic volcanic rocks in the study area belongs to the series of tholiite and has the transition character of alkaline-subalkaline.Trace elements analysis shows that the Cr, Ni content in basic volcanic rocks of study area are low, the average value of Cr and Ni is 296.90?g/g and 74.32?g/g, which lower than the value of the original magma, indicating that the magma upwelling experienced aprocess of fractional crystallization of clinopyroxene and magma Ti-Fe oxide mineral. Through the mafic igneous rocks primitive mantle normalized scheme of trace elementin the study area, we can know that the distribution character of each sample is consistent in addition to individual samples, which mainly loss of Ba, K, Sr, and relative enrichment of Nb, U, LREE. Rocks incompatible element ratios are quite with EM1 OIB or EM2 OIB. The rare earth elements shows that the ?REE=83.79×10-6 to 181.11×10-6, LREE/HREE=5.04 to 7.97, with an average of 6.45, and the fractionation of REE shows obviously and strongly enriched in LREE.(La/Yb)N= 5.60 to 9.57, average of 7.64. ?Eu content is between 0.78 to 1.06, with a weak negative anomalies.Dating results of zircon LA-ICP-MS U-Th-Pb of olivine basalt is 33.22±0.37 Ma, it shows that the basic volcanic rocks in study area should be formed in the Oligocene of Cenozoic.Petrographic and geochemical characteristics of basic volcanic rocks in the study area shows that the basic volcanic rocks was formed in the plate extension environment, and the causes maybe related to mantle plume.The strong incompatible element contents and ratios indicated that the magmatic origin from enriched mantle region, it's the product of partial melting from pomegranate lherzolite and spinel lherzolite. In the process of magma ascent after formation which experienced the fractional crystallization of olivine-chrome spinel plagioclase, clinopyroxene and apatiteilmenite oxides with a much amount of crustal material contamination. |
PDF Full Text Request