滇西南澜沧江带官房地区三叠纪火山岩地质地球化学特征及其构造环境

Geological and geochemical characteristics and tectonic setting of Triassic volcanic suite in Guanfang area along South Lancangjiang belt

官房地区火山岩属三江构造带南澜沧江构造岩浆带北段，中晚三叠世喷发了一套由中三叠统忙怀组酸性火山岩和上三叠统小定西组基性火山岩组成的火山岩系。忙怀组酸性火山岩以高硅、富钾、低钛为特征，SiO2含量平均为75．49％，K2O含量平均为3．88％，TiO2含量平均为0．136％，A／CNK〉1．1（平均为1．53），属高钾钙碱性流纹质火山岩类；∑REE平均为276．85×10^-6，富集轻稀土元素，具强Eu负异常，微量元素在蛛网图上显示出K、Rb、Ba、Th强烈富集，而Ti、P则明显亏损。小定西组也以富钾为特征（K2O含量平均为2．68％），里特曼指数σ在2．23～9．24之间（平均为6．27），为高钾钙碱性玄武岩-钾玄岩系列，∑REE平均为171．83×10^-6，轻度富集轻稀土元素，稀土元素配分型式为缓右倾斜-平坦型，无Eu或弱的负Eu异常；微量元素配分模式以K、Rb、Ba、Th强烈富集，Ti、Y、Yb、Cr则明显亏损为特征。南澜沧江带三叠纪火山岩系的地质、岩石组合和地球化学特征表明其为活动大陆边缘弧的产物。

The volcanic suite of Guanfang area occurs in the north of South Lancangjiang magmatic tectonic zone within the Sanjiang Three-River tectonic belt. A volcanic suite composed of acidic volcanics of Manghuai Formation and basic volcanics of Xiaodingxi Formation was extruded in the Guanfang area in the middle-Late Triassic period. With high silica（SiO2 = 75.49 % on average）, rich potassium（K2O = 3.88 % on average）, low titanium （TiO2= 0.136% on average）and A/CNK high than 1.1（1.53 on average）, the acidic volcanic rocks of Manghuai Formation belong to potassium-rich calc-alkali rhyolitic volcanic rock series. The rocks have an average ∑REE of 276.85×10^-6 and show evident nagative Ti, P and Eu anomalies as well as positive K, Rb, Ba, Th anomalies in the trace element spider diagrams. Basic volcanic rocks of Xiaodingxi Formation are also enriched in potassium（K2O= 2.68% on average）, with their Rittmann index varies in the range of 65.92%- 75.68 % （σ = 6.27 on average）, thus belonging to potassium-high calc-alkali basalt-shoshonite series. The REE distribution pattern is slightly right oblique with an average ∑REE of 171.83 × 10^-6, and there exist no or very weak nagative Eu anomalies. The rocks show evident K, Rb, Ba, Th positive anomalies and Ti, Y, Yb, Cr nagative anomalies in the trace element distribution pattern. According to the rock assemblage as well as geological and geochemical characteristics, it is suggested that the Triassic volcanic suite in Guanfang area along South Lancangjiang belt of Yunnan Province should be composed of epicontinental arc-volcanic rocks.