Granites’ Geochemistry Features And Tectonic Significance Of Xiangda-Pingda Area, Longling, West Of Yunnan Province

Xiangda-Pingda area, Longling county, which located in the eastern section of western Yunnan Paleotethyan tectonic, It is combination of the collided department of the Gondwana and southern edge of the Eurasian continental block, which is important to research of Paleotethyan orogeny and to find Non-ferrous minerals.In this paper, the research subject is the Mesozoic-Cenozoic granites which outcrop to the Xiangda-Pingda area, by relatively detailed study to compare their petrological and geochemical characteristics, combining the study of petrological chronology, regional geology, petrological tectonics and other aspects, Discussion on the factors of the petrogenesis and tectonic setting of granites, and which furtherly clarify the ancient tectonic evolution of study area. Obtaining the main conclusions are summarized as follows:(1) Mesozoic - Cenozoic granites which outcropped on the Xiangda-Pingda area mainly have three periods: Jurassic granite where outcropped on Xiangda town. Cretaceous granite where outcropped on the east of Xiangda town and two sides of Pingda. Paleogene granite mainly outcropped on north of Mengdui.(2) Three periods granites have same features of " high Al - Na - Mg - K". which belong to strong peraluminous granites, which has the characteristics of high-K calc-alkaline rocks. Generally belong to crustal granite. Its’ REE relatively high, LREE and HREE Fractionated strongly. and Jurassic granite same as the high Sr low Yb granite that Zhang Qi et al (2006) called as "Adakites", while Cretaceous granite and Paleogene granite fall into the low Sr low Yb to low Sr High Yb granite zone in Sr-Yb diagram.(3) Three periods granites mainly fall into what Pearce (1994) redefined as Post-COLG; Such as The results obtained on Hf-Rb-Ta diagram.(4) Jurassic granite has C-type potassium Adakite features, Probably formed in thickened crust start thinning and crustal environment transition squeezing from extension; it is the product of underplating basaltic’s partial melting under the crust, in eclogite-facies. Paleogene granite and Cretaceous granite have Himalayan-type and Nanling-type granites’ geochemical characteristics respectively with low Sr low Yb and extremely low Sr high Yb. Maybe it is different depth’s Partial melting in the normal crustal thickness, or it is product of partial melting of the thinning crustal in regional tension environment.(5) Analysis combining with geological data completed recent years, west of Nujiang - Changning - Menglian combination widely outcrops the Late Mesozoic granites that have the overall trend of continuous evolution, especially clearly manifested in Sr-Yb diagram: From high Sr low Yb type granite of Bangmiao-Rock, to low Sr low Yb type granite of the eastern edge of Tengchong micro plots’ Middle Jurassic granite, Early Cretaceous granite are mainly low-Sr high Yb common type granite, late Cretaceous granite are mainly low Sr high Yb type granite, a small number of low-Sr high Yb type, to low Sr high Yb type granite of Early Cretaceous granite, to mainly low Sr high Yb type granite, few low Sr high Yb type of Late Cretaceous granite; It may reflect the process of gradually upward migration of magma source and decreasing the pressure environment in tectonic setting evolution of later-collision to later-orogenesis.