Cenozoic kamafugite volcanism and tectonic meaning in west Qinling area, Gansu province.

Kamafugite is an ultra potassic volcanic rock occurred in oceanic and/or co ntinental settings, especially in continental rift. Kamafugite is also an i gneous rock outcropped very rarely in the world. Cenozoic kamafugite report ed in this paper occurs in West Qinling, Gansu Province, China. Petrologica l characteristics of the kamafugite are described. Thirty chemical composit ions of whole rocks and major minerals and six Sr, Nd, Pb isotopic data fro m thirteen volcanic cones and two isotopic ages of Ar/Ar are given. The res ults show that the kamafugite is mainly a volcanic product of Miocene age. It is poor in SiO2 and Al2O3, but rich in MgO, CaO, TiO2, and (K2O+Na2O). I t is higher in[Mg](>68 on average) and NiO (>0.3% on average). It is very b ad in crystallinity. It contains a lot of deep-seated xenoliths. All of tho se mentioned above demonstrate that there is a primary magma of kamafugite in west Qinling. The kamafugite there has a paragenetic relationship to car bonatite, showing that the West Qinling kamafugite typically occurred in co ntinental sittings. Also, the kamafugite has the Sigma REE of 365x10(-6)sim ilar to 649x10(-6), the Sigma LREE of 319x10(-6)similar to 569x10(-6). Espe cially the LILE (such as K, Rb, Sr, and Ba) is very high, but the HFSE (suc h as Nb, Ta) is relatively depleted. The Nd-143/Nd-144 (0.512768 similar to 0.512911) is higher but both Sr-87/Sr-86(0.70412 similar to0.70525) and Ph- 206/Pb-204(18.418 similar to 18.625) are lower than those of Cenozoic potas sic rocks from Tibet Plateau and adjacent areas. The epsilon (Nd) is betwee n +1.8209 similar to +2.1002, whereas Pb-207/Pb-204 and Pb-208/Pb-204 are 1 5.476 similar to 15.551 and 38.618 similar to 38.792, respectively. Isotope s Sr, Nd and Pb of the West Qinling karnafugite are differ from those of ka mafugite from East African rift, Italy and German, and also differ from tho se of Cenozoic potassi rocks in Tibet plateau and adjacent areas. This pape r also discussed the tectonic sitting where the West QinIing primary kamafu gite magma formed. It is suggested that Cenozoic kamafugite volcanism in We st 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 l ower partial melting of the thermal boundary layer at the base of the litho sphere. The thermal boundary layer had previously been a part of the mantle that had been metasomatized and enriched by fluids/melts from the asthenos phere.