滇西腾冲更新世粗面安山岩Ar-Ar年代学、地球化学特征及其构造意义

腾冲芒棒地区广泛发育新近纪以来的火山岩,本文首次对该区粗面安山岩进行的高精度40Ar/39Ar同位素定年,其坪年龄分别为2.1±0.1Ma和2.0±0.1Ma,代表其喷溢时代为早更新世。岩石地球化学特征研究显示,粗面安山岩属高钾钙碱性系列,富集大离子亲石元素、亏损高场强元素,具有弧火山岩与大陆板内火山岩的双重属性,推测其形成于后造山环境。综合研究表明,腾冲地块更新世火山岩的形成与印度-欧亚板块俯冲-碰撞造山隆升后的伸展垮塌、富集地幔部分熔融作用密切相关,而印支陆块向东南逃逸以及腾冲弧形走滑断裂带(龙川江断裂带、大盈江断裂带)的活化促使了该期火山岩的喷发与侵位。

40Ar/39Ar geochronology and geochemistry of Pleistocene trachyandesite in Tengchong, western Yunnan, China, and its tectonic implication

Mangbang area is located in the east of Tengchong with widely distribution of the Late Cenozoic volcanoes. ⁴⁰Ar/³⁹Ar analysis on matrix separate from two trachyandesite samples collected from the Mangbang area yielded a high-precise plateau age of 2.1±0.1Ma and 2.0±0.1Ma, indicating the effusive age as Early Pleistocene. Analyzed samples appear as high-potassium calc-alkaline features, and are enriched in large ion lithophile elements (LILE), but depleted in high field strength elements (HFSE), which show the arc volcanic and intraplate volcanic transition characteristic, speculating that they may form in post-orogenic environment. The generation of Pleistocene volcanic rocks in the Tengchong Block is related to extension collapse after the India-Asia collision result in partial melting of an enriched mantle source. The Indosinian Block escape to the southeast and rejuvenation of Tengchong arcuate strike-slip fault zone (e.g., Longchuanjiang fault belt and Dayingjiang fault belt) induced the volcanic eruption and emplacement.