滇西腾冲中更新世英安质岩浆的爆发机制

The eruption trigger of the dacitic magma in Tengchong during the Middle Pleistocene

本文对腾冲马站钻孔的中酸性火山岩进行了岩相学、锆石SIMS U-Pb年代学及地球化学研究,确定其岩石类型、形成时代及岩浆喷发前的岩浆状态,从而揭示腾冲火山喷发机制。根据矿物组成将腾冲钻孔的中酸性火山岩分为两层:上层灰白色角闪熔岩(矿物组合:单斜辉石+斜方辉石+角闪石+黑云母+斜长石+钾长石+钛磁铁矿+磁铁矿+石英);下层黑色辉石熔岩(矿物组合:单斜辉石+斜方辉石+斜长石+钛磁铁矿+磁铁矿)。腾冲钻孔中酸性火山岩的锆石均呈半自形-自形,振荡环带明显,为岩浆成因。测年结果表明,灰白色角闪熔岩的结晶年龄为0.7Ma,黑色辉石熔岩中最年轻的锆石为0.6Ma;结合上覆中更新世粗面岩,我们推断其喷发时代为中更新世。地球化学显示角闪熔岩和辉石熔岩都为高钾钙碱性英安岩。相似的全岩和斑晶核部的地化特征,指示角闪英安岩和辉石英安岩来自同一个岩浆房。综合斑晶的地化特征及平衡结晶的温压条件,我们认为早期的岩浆房经历一次基性岩浆补给事件,导致火山爆发产生黑色辉石英安岩,后期岩浆房又经历一次酸性岩浆补给事件,导致火山爆发产生灰白色角闪英安岩。两次岩浆补给事件是导致火山爆发的直接原因。

Temporal-spatial and physical-chemical conditions of pre-eruption magma chamber are vital factors to unravel the eruption mechanism. In this study, we carry out high-resolution geochronological dating for volcanic rocks collected from Mazhan Drilling Core, Tengchong, China. Zircons of dacites are euhedral to semi-euhedral with clear oscillatory zoning which may imply a magmatic origin. Dating of zircons gets a result of^0.7Ma, with a Middle Pleistocene overlying layer, which means the eruption happened at the Middle Pleistocene. Geochemistry of dacites shows high potassic calc-alkaline affinity. Dacites in Drilling Core could be divided into an upper part and the lower part based on the existence of amphibole. Amphibole bearing dacites in the upper part with mineral assemblages：Cpx＋Opx＋Pl＋Il＋Mag. Some augite crystals are rimmed by enstatite and pigeonite, and lower An plagioclase are rimmed by higher An value, which are all suggest that mafic magma injected into the magma chamber before the eruption; Pyroxene bearing dacites from lower part with mineral assemblages：Cpx＋Opx＋Pl＋Kf＋Bi＋Il＋Mag＋Q. The increased temperature and pressure of plagioclase with lower An value show that silicic magma injected into the magma chamber before the eruption. Furthermore, the dacites from two layers have the similarity of bulk compositional and mineral core composition indicate these two different dacites initially from one mamga chamber, but experience two separate composition mamga injection event. The mamga injection induced the instability of the magma chamber, which is the direct cause of this eruption.