Western Yunnan Nanchang Nanjing - The Menglian Rift With The Geochemistry Of The Volcanic And Tectonic Setting Of The Evidence

The Changning-menglian rift, which is a Late Palaeozoic Volcanic Zone, is an important polymertallic ore belt in Three-River area of south-weatern China. Predecessors did a lot of work, but now there are some argument on its structural property and its evolution too. Volcanism is developed in Late Palaeozoic. there are important to study on volcanic rocks and its lithogeochemical characteristics of volcanic rocks for the causes and rift evolution. After we analysis of the results in previous work and actual survey Laochang volcanic rocks of the middle part, this dissertation summarized the structural setting of the volcanic rock with geochemical, coupled with petrology and isotope geochemistry.By Changning-Menglian rift zone of volcanic rock, The major conlusions are summarized as follow:Rock series:volcanic belong to alkaline basalt series and tholeiite series through the research of the volcanic distribution and rock series. It is the neutral basic----basic alkaline basalt series in prophase. Volcanism is strength, wide range. It's the main types in this area. The volcanic is basic tholeiite series in Advance. Volcanism is weakly, narrow range. Distribution in the region, there are less alkaline basalt series, and tholeiite series more from north to south.Geochemistry:Changning-Menglian rift zone volcanic rocks, generally with SiO2, Al2O3, total alkali (K2O+Na2O) content generally low,TiO2,K2O,total iron content was high, rare earth elements, noneδEu,δCe exception, trace elements, Th high.Different part of geochemistry:In the middle part, basalt high content TiO2, K2O, FeOFe2O3. Rare earth elements are not enriched. REE variation is not obvious. Large-ion lithophile elements are not enriched; By geochemical discrimination, this section of oceanic basalt ridge basalt, formed in the intercontinental rift. Lithologic characteristic of basaltic in the southern part is between the basalt of the middle part and tholeiite of the northern part. That are products of crystallization differentiation of cognate magma in evolution of the different stages in the rift.Rift evolution:Changning-Menglian rift zone is the rift, witch formed in the Carboniferous, extruded in Early Triassic. It related with mantle plume. Accompanied by volcanism, the evolution process can be divided into four phases:intraplate continental rift(D-C1), stability(C2+3), intercontinental rift(C2+3-P1), extrusion(P).The results from the volcanic rift of view.this text thinks:the infant oceanic crust only distribute in the northern part of Tongchang Street. It developed in narrow range. Evolution is in a short time. Therefore, the Changning-Menglian Rift should be continental rift with a part of the infant oceanic crust.