Cenozoic kamafugite volcanism and tectonic meaning in west Qinling area, Gansu province.
YU XueHui 1, MO XuanXue 1, Martin FLOWER 2, SU ShangGuo 1 and ZHAO Xin 11. China University of Geosciences, Beijing 100083, China 2. Department of Earth and Environment sciences, University of Illinois at Chicago, Chicago, IL 60607, USA.
Kamafugite is an ultra potassic volcanic rock occurred in oceanic and/or continental settings, especially in continental rift. Kamafugite is also an igneous rock outcropped very rarely in the world. Cenozoic kamafugite reported in this paper occurs in West Qinling, Gansu Province, China. Petrological characteristics of the kamafugite are described. Thirty chemical compositions of whole rocks and major minerals and six Sr, Nd, Pb isotopic data from thirteen volcanic cones and two isotopic ages of Ar/Ar are given. The results show that the kamafugite is mainly a volcanic product of Miocene age. It is poor in SiO 2 and Al 2O 3, but rich in MgO, CaO, TiO 2, and (K 2O+Na 2O). It is higher in(68 on average) and NiO( 0.3% on average). It is very bad in crystallinity. It contains a lot of deep-seated xenoliths. All of those mentioned above demonstrate that there is a primary magma of kamafugite in west Qinling. The kamafugite there has a paragenetic relationship to carbonatite, showing that the West Qinling kamafugite typically occurred in continental sittings. Also, the kamafugite has the ΣREE of 365×10 -6 ～649×10 -6 , the ΣLREE of 319×10 -6 ～569×10 -6 . Especially the LILE (such as K, Rb, Sr, and Ba) is very high, but the HFSE(such as Nb, Ta) is relatively depleted. The 143 Nd/ 144 Nd(0.512768～0.512911) is higher but both 87 Sr/ 86 Sr(0.70412～0.70525) and 206 Pb/ 204 Pb(18.418～18.625) are lower than those of Cenozoic potassic rocks from Tibet Plateau and adjacent areas. The ε Nd is between +1.8209～+2.1002, whereas 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are 15.476～15.551 and 38.618～38.792, respectively. Isotopes Sr, Nd and Pb of the West Qinling kamafugite are differ from those of kamafugite from East African rift, Italy and German, and also differ from those of Cenozoic potassi rocks in Tibet plateau and adjacent areas.This paper also discussed the tectonic sitting where the West Qinling primary kamafugite magma formed. It is suggested that Cenozoic kamafugite volcanism in West Qinling was a response to strong upleft of Tibet Plataeu after the India-Europeasia plate collision. The primary kamafugite magma might originate directly from the mantle during the lithospheric extension, and formed by lower partial melting of the thermal boundary layer at the base of the lithosphere. The thermal boundary layer had previously been a part of the mantle that had been metasomatized and enriched by fluids/melts from the asthenosphere.
【Fund】： 国家自然科学基金 (项目编号 :49973 0 14 ) ;; 国家重点基础研究发展规化 (G19980 40 80 0 );; 国土资源部青藏高原专项计划经费的资助
【CateGory Index】： P588.15
【CateGory Index】： P588.15