(1. 中南大學(xué) 有色金屬成礦預(yù)測(cè)教育部重點(diǎn)實(shí)驗(yàn)室，長(zhǎng)沙 410083；
2. 中南大學(xué) 地球科學(xué)與信息物理學(xué)院，長(zhǎng)沙 410083)
摘 要: 對(duì)滇西南瀾滄江帶晚三疊世富鉀火山巖的主量元素、微量元素、稀土元素以及Sr-Nd同位素進(jìn)行測(cè)試。結(jié)果表明：巖石以低硅(SiO2含量為50.47%~52.99%)、富鉀高堿(K2O+Na2O為5.66%~9.39%，K2O的平均值為2.8%)為特征，屬于高鉀鈣堿性玄武巖−鉀玄巖系列。巖石富集大離子親石元素、輕稀土元素、虧損高場(chǎng)強(qiáng)元素，無(wú)Eu或弱的負(fù)Eu異常(δ(Eu)=0.85~1.09)。87Sr/86Sr和143Nd/144Nd比值變化范圍分別為0.707 01~0.710 56和0.512 511~ 0.512 569，所有樣品的ε(Nd)均為負(fù)值，ε(Nd)介于−2.48~−1.27之間(平均值為−1.94)。巖石的微量元素特征及Nd和Sr同位素組成與EMⅡ富集地幔來(lái)源一致，表明源區(qū)具有殼幔混源性質(zhì)。EMⅡ型地幔的形成與古特提斯瀾滄江洋向東俯沖消減相關(guān)，也與該區(qū)火山巖起源于陸緣弧的構(gòu)造背景相一致。利用微量元素地球化學(xué)原理，推斷富鉀火山巖應(yīng)來(lái)源于受古特提期俯沖帶流體交代富集的金云母相尖晶石方輝橄欖巖的部分熔融。
關(guān)鍵字: 南瀾滄江帶；三疊世；富鉀火山巖；Sr-Nd同位素；滇西
volcanic rocks and their origins in southern Lancangjiang belt,
western Yunnan Province, China
（1. Key Laboratory of Metallogenic Prediction of Nonferrous Metals, Ministry of Education,
Central South University, Changsha 410083, China;
2. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China）
Abstract:The late Triassic potash-rich volcanic rocks in the southern Lancangjiang belt are characterized by low-Si (SiO2 50.47%−52.99%), high-alkali (K2O+Na2O 5.66%−9.39%) and high-K (K2O 2.8% on average), and belong to potassium-high calc-alkali basalt-shoshonite series. The rocks are also enriched in large ion lithophile element (LILE), light rare earth elements (LREE), and depleted in high field strength elements (HFES), non or slightly negative Eu anomaly (δ(Eu)=0.85−1.09) in the abundance. The results show that the rocks have relatively low 143Nd/144Nd ratios (0.512 511−0.512 569) and relatively high initial 87Sr/86Sr ratios (0.707 01−0.710 56), as well as negative ε(Nd) values of −2.48−−1.27 (−1.94 on average). Their characteristics of trace elements and Sr-Nd isotopic composition are consistent with EMⅡ, indicating that the source consists of crust-mantle mixed material, which is inevitably related to the subduction eastward of Tethyan Lancangjiang oceanic plate. Based on the geochemistry, it is suggested that the primary magma was likely generated from partial melting of spinel harzburgitic source metasomatized by Paleo-Tethyan subduction-related fluids.
Key words: southern Langcangjiang belt; Triassic; potash-rich volcanic rocks; Sr-Nd isotopes; western Yunnan
