青藏高原新生代两类超钾质岩石的成因--实验岩石学和地球化学约束

Petrogenesis of two kinds of Cenozoic ultrapotassic volcanic rocks in Qinghai-Tibet Plateau:Constraints from experimental petrology and geochemistry

青藏高原分布有羌塘—囊谦—滇西和冈底斯两条新生代钾质-超钾质火山岩带。羌塘—囊谦—滇西超钾质岩浆活动的峰值时间为40～30Ma,主体岩石具有Ⅰ型超钾质岩的高MgO和低CaO、Al2O3含量特征;30～24Ma期间羌塘中、西部出现Ⅲ型钾质-超钾质岩浆活动,主体岩石以贫SiO2、高CaO、Al2O3和低MgO/CaO为特征。冈底斯新生代超钾质火山岩也显示I型超钾质岩的高MgO和低CaO、Al2O3含量特征,其形成时间为25～12Ma。综合超钾质岩石的实验资料,可知区内I型超钾质岩的源区以富硅、富钾流(熔)体交代形成的金云母方辉橄榄岩为主;Ⅲ型钾质-超钾质岩浆源区则以斜辉橄榄岩地幔为主。囊谦—滇西Ⅰ型超钾质岩带空间上严格受红河走滑构造带所控制,40～28Ma出现I型超钾质岩浆活动,16Ma转变为OIB型钾质火山岩。岩浆源区从岩石圈地幔向软流圈演变,暗示大型走滑断裂引起的岩石圈地幔减薄和软流圈上涌是导致交代岩石圈地幔金云母分解熔融产生区内I型超钾质岩浆的主控因素。羌塘中部35～34Ma有软流圈来源为主的钠质碱性玄武岩岩浆的喷发,30～24Ma转变为以岩石圈地幔为主要来源的Ⅲ型钾质-超钾质岩浆活动,岩浆源区从软流圈向岩石圈迁移,指示软流圈上涌伴随的富CO2流(熔)体活动是导致古交代岩石圈地幔升温熔融产生Ⅲ型钾质-超钾质岩浆的主控因素,软流圈上涌可能是俯冲板片断离或岩石圈地幔拆沉作用的结果。

Two kinds of Cenozoic potassie-ultrapotassic volcanic rocks of Qiangtang-Mangkang-Dianxi and Gangdise are distributed in Qinghai-Tibet Plateau. The peak time of Qiangtang-Mangkang-Dianxi ultrapotassic magmatism is 40-30 Ma, and the main rocks have the characteristics of high content of MgO and low content of CaO and Al2O3 as type-Ⅰ ultrapotassic rocks, while the 30--24 Ma magmatism of middle and west Qiangtang area show the characteristics of type-Ⅲ potassie-ultrapotassie voleanics, which have low content of SiO2, high content of CaO and Al2O3, and low MgO/CaO ratio. The Cenozoic ultrapotassic volcanies of Gangdise also possess the characteristics of type-Ⅰ ultrapotassic rocks in high content of MgO and low content of CaO and Al2O3, with forming time between 25-12 Ma. Through analysis of the experimental data about ultrapotassic rocks, type-Ⅰ ultrapotassic rocks came from the source region of phlogopite harzburgite that had undergone metasomatism of fluid （melt） enriched in silica and potassium; while the source region of type-Ⅲ potassie-ultrapotassic volcanics is carbonated peridotite mantle bearing potassic amphibole. The Nangqian-Dianxi type-Ⅰ ultrapotassic volcanic belt is strictly controlled by Honghe strike-slip fault zone in space. type-Ⅰ ultrapotassie magrnatism occurred during 40-28 Ma, and the 16 Ma magmatism is translated into potassic basalt with OIB characteristics. The transition of source region from lithospheric mantle to asthenosphere indicates the thinning of lithospheric mantle and upwelling of asthenosphere caused by large scale strike-slip fault, which is the major controlling factor for decomposition melting of phlogopite in the metasornatized lithospheric mantle to generate the type-Ⅰ ultrapotassic magrnatism. The magmatism of 35-34 Ma in middle Qiangtang area is the eruption of sodic alkaline basalt mainly from asthenosphere, and the 30-24 Ma type-Ⅲ potassie-ultrapotassic magma was originated from lithospheric mantle. The transition of magmatic source indicates the activities of CO2-rich fluid caused by the upwelling of asthenosphere is the major controlling factor in causing ancient metasomatized lithospheric mantle to warm up and melt to generate type-Ⅲ potassie-ultrapotassic volcanics. The upwelling mechanism of asthenosphere may be related to the breaking-off of subdueted slab or delamination of lithospheric mantle.