甘肃西秦岭新生代钾霞橄黄长岩火山作用及其构造含义

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

钾霞橄黄长岩是一种产于大洋和 /或大陆环境 ,尤以大陆裂谷最为多见的超钾质火山岩 ,也是世界上出露最少的火成岩之一 ,目前世界只有少数几个国家和地区有这类岩石的报道。甘肃西秦岭新生代火山岩是我国目前唯一有报道的钾霞橄黄长岩产区。本文详细介绍了西秦岭新生代钾霞橄黄长岩及深源包体的岩石学特征 ,提供了该区 13个岩体 30个具代表性的全岩化学和主要矿物的化学成分分析结果 ,6个具代表性的Sr,Nd,Pb同位素分析结果 ,以及 2个火山岩全岩 Ar/ Ar同位素定年结果。研究表明西秦岭新生代钾霞橄黄长岩主要是中新世火山作用的产物。火山岩的化学成分贫 Si O2 和 Al2 O3,富 Mg O,Ca O,Ti O2 和 (K2 O+Na2 O) ,以及具有高的 [Mg O]值 (平均大于 6 8)和 Ni O含量 (平均在 0 .3%以上 )。火山岩中含有丰富的深源包体 ,岩石的结晶程度很差。所有这些表明西秦岭存在原生钾霞橄黄长岩浆。而且本区钾霞橄黄长岩与碳酸岩密切共生 ,显示了典型的大陆区钾霞橄黄长岩的特征。火山岩的Σ REE在 36 5× 10 - 6 ～ 6 4 9× 10 - 6 之间 ,其中轻稀土总量 (Σ L REE)达 319× 10 - 6 ～ 5 6 9× 10 - 6 ,尤以强烈富集大离子轻石元素 (K、 Rb、 Sr、 Ba)和相对亏损高场强元素 (Nb、 Ta等 )为特征。火山岩具有相对高的 1

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 SiO2 and Al2O3, but rich in MgO, CaO, TiO2, and (K2O+Na2O). It is higher in[Mg](>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 Sigma REE of 365x10(-6)similar to 649x10(-6), the Sigma LREE of 319x10(-6)similar to 569x10(-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 Nd-143/Nd-144 (0.512768 similar to0.512911) 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 potassic rocks from Tibet Plateau and adjacent areas. The epsilon (Nd) is between +1.8209 similar to +2.1002, whereas Pb-207/Pb-204 and Pb-208/Pb-204 are 15.476 similar to 15.551 and 38.618 similar to 38.792, respectively. Isotopes Sr, Nd and Pb of the West Qinling karnafugite 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 QinIing 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.