滇东南建水火山岩LA-ICP-MS锆石U-Pb测年、地球化学特征及地质意义

滇东南建水地区位于师宗-弥勒构造带的南段,区内出露一套玄武岩–安山岩–英安岩–流纹岩组合。研究这套火山岩的形成时间和形成环境对认识滇东南构造格局有着重要意义。本文首次对建水火山岩进行LA-ICP-MS锆石U-Pb测年,获得两个样品的年龄分别为261.9±2.2 Ma(MSWD=0.80)、264.8±1.7 Ma(MSWD=1.12),属中二叠世晚期,代表建水火山岩的形成时间。建水玄武岩与安山岩具有低TiO_2含量(0.50%~0.88%)、高Mg~#(52.0~64.5)、弱富集LREE((La/Yb)_N=1.42~3.44)、富集LILE(Rb,Th,U,Pb)、轻微亏损HFSE(Nb,Ta,Ti)的特点,具典型岛弧玄武岩的特征;英安岩和流纹岩高ΣREE含量(139.5×10~(–6)~313.6×10~(–6))、富集LREE((La/Yb)N=4.16~9.78)和LILE(Rb,Ba,Th,U)、亏损HFSE(Nb,Ta)、强亏损Sr、Ti、Y等元素的特点与典型的岛弧流纹岩相似,但高钾(K_2O含量平均7.73%)、钙碱性(δ=0.93~2.94)、强过铝质(A/CNK=1.13~2.10)的特点使之区别于一般岛弧酸性火山岩,而具有一些上地壳部分熔融形成的S型花岗岩的特点。综合建水火山岩岩石组合、地球化学和区域地质背景,认为建水火山岩形成于活动大陆边缘的弧后盆地伸展环境,是地幔部分熔融形成的玄武质岩浆结晶分异与上地壳混染作用的共同结果,与滇桂交界处岛弧火山岩、两广交界处岛弧玄武岩等同为哀牢山洋向北俯冲的产物。

LA-ICP-MS Zircon U-Pb Dating,Geochemical Characteristics of Volcanic Rocks in Jianshui,Southeast Yunnan and Their Geological Implications

The age and tectonic setting of a set of basalt-andesite-dacite-rhyolite in Jianshui, Southeast Yunnan, south of the Shizong-Mile structural belt is studied in this paper. LA-ICP-MS zircon U-Pb dating of two samples yield 261.9±2.2 Ma (MSWD=0.80) and 264.8±1.7 Ma (MSWD=1.12) respectively. The basalts and andesites are characterized by low TiO2 (0.50%–0.88%), high Mg# (52.0–64.5), slightly enriched LREE ((La/Yb)N=1.42–3.44), enriched LILE (Rb, Th, U, Pb) and slightly depleted HFSE (Nb, Ta, Ti), which are typical of island arc basalt (IAB). The dacites and rhyolites are characterized by high ΣREE (139.5×10–6–313.6×10–6), enriched LREE ((La/Yb)N=4.16–9.78) and LILE (Rb, Ba, Th, U), depleted HFSE (Nb, Ta), and strongly depleted Sr, Ti, Y, which are similar to those of island arc rhyolite. However, unlike common acid island-arc volcanic rocks, the dacites and rhyolites are characterized by high K2O contents (average 7.73%), calc-alkaline (δ=0.93–2.94) and strongly peraluminous features (A/CNK=1.13–2.10), which are similar to those of the S-type granites. Considering the regional geologic background, rock association, and geochemical features, we argue that this set of volcanic rocks was formed in an extensional back-arc basin of active continental margin. They were derived from partial melting of the mantle and experienced crystallization differentiation and subsequent contamination of the upper crust. Similar to the island arc volcanic rocks at the Yunnan-Guangxi border and the island arc basalts along the Guangdong-Guangxi border, this set of volcanic rocks was most likely formed during the northward subduction of the Ailaoshan Ocean.