广西那龙地区中三叠世火山岩地球化学特征及构造意义  

Geochemistry and Tectonic Significance of Middle Triassic Volcanic Rocks in Nalong,Guangxi Area

作　　者：胡丽沙[1] 杜远生[1] 杨江海[1] 黄虎[1] 黄宏伟 黄志强 

机构地区：[1]生物地质与环境地质国家重点实验室,中国地质大学,武汉430074 [2]广西地质勘察总院

出　　处：《地质论评》2012年第3期481-494,共14页Geological Review

基　　金：国家自然科学基金资助项目(编号40972078和40921062)的成果

摘　　要：右江盆地西南缘广泛出露早—中三叠世火山岩,目前对这些火山岩没有精确的年代学和地球化学研究。本文选择广西那坡县那龙地区下三叠统罗楼组与中三叠统百逢组之间的一套厚层状分布的火山岩作为研究对象,对其进行了LA-ICP-MS锆石U-Pb同位素定年、锆石主微量元素及全岩的地球化学研究。锆石206Pb/238U同位素年龄为241.2±1.9Ma(MSWD=0.69),代表火山岩的结晶年龄。锆石稀土元素总量较高,富集重稀土元素,具有明显Eu负异常及Ce正异常,含有较低的Th、Th/U、Ce/Sm和δEu值及相对较高的Hf、Yb/Gd和U/Ce值,指示原岩岩浆源区有地壳熔体的组分。火山岩全岩SiO2含量为51.28%～56.17%,并显示低TiO2含量(0.79%～1.28%)和高Al2O3含量(13.52%～15.51%)的特征。稀土元素地球化学特征表现为Eu负异常(δEu=0.60～0.71),富集大离子亲石元素(Rb、Cs、Ba、U、Sr、Pb、Th、K)和轻稀土元素而亏损高场强元素(Nb、Ta、Zr、Hf、P、Ti)及重稀土元素,与岛弧火山岩的特征一致。Ce/Pb及Nb/U值接近大陆地壳,Zr/Y值和Zr含量偏高,表明原始岩浆源区可能为地壳熔体和俯冲洋壳熔体组成的混合物,岩浆在形成上升过程中有深海沉积物或地壳物质的加入,岩石成因与俯冲作用有关。但Ba/Th值(2.92～69.75)小于300,表明原始岩浆在上升过程中受到流体混染的程度比较小。结合前人研究成果可得出,广西那龙中三叠世岛弧火山岩的发现,代表早中生代古特提斯洋东延部分存在一个俯冲消减带。Early to Middle Triassic volcanic rocks are mainly distributed in the southwestern margin of Youjian Basin. There is no accurately ages and geochemistry study of those rocks. Now we choose a layered volcanic rocks between Luolou Formation（ Lower Triassic） and Baifeng Formation （Middle Triassic ） in Nalong, Guangxi area as our subjects. We analyzed zircon LA-ICP-MS U-Pb isoplote age, major elements, trace elements of volcanic rocks in order to limiting the voleaniac rocks erupting time and discussing the source and tectonic significance. Volcanic rocks of Nalong yielded a LA-ICP-MS U-Pb zircon isoplote age of 241.2 ± 1.9 （ MSWD = 0. 69 ） Ma which suggesting that these volcanic rocks erupted in the Middle Triassic. Geochemically of zircons is enriched in REE and have higher enrichment heavy REE. It has a strong negative Eu anomalies and positive Ce anomalies, with low Th, Th/U, Ce/Sm and 8Eu values and relatively higher Hf, Yb/Gd and U/Ce values which could indicate that zircon is a description of the continental crust in a deep melt sources where have been a stable plagioclase crystallization differentiation. The Whole volcanic rocks SiO2 are 51.28% - 56. 17% and have a lower TiO2 （0.79% -1.28% ） and higher A1203 （13.52%- 15.51% ）. These volcanic rocks have enrichment LILE and LREE and lack of HFSE, significantly similar to typical subduction-related arc volcanic rocks. Ce/Pb and Nb/U values is similar to continental crust; Zr/Y and Zr values are higher which indicates that their mantle source is made up of Crust melt, Oceanic crust melt and so on. There have been added in deep-sea sediments or crustal material during plate subduction, but the Ba/Th values （2.92 - 69.75 ） is less than 300, indicating that mantle contamination is smaller. The determined of Island-arc volcanic rocks in Nalong, Guangxi area indicate that there has being a subbuction zone of eastern part of Paleo-Tethys Ocean during Late - Palezoic to Mesozoic.

关 键 词：右江盆地 中三叠世 锆石U-PB年龄 地球化学 岛弧火山岩 古特提斯洋 

分 类 号：P588.14[天文地球—岩石学]