琼中麻粒岩的成因及地质意义  

Origin and geological significance of granulites from Qiongzhong complex

作　　者：徐德明[1] 马大铨[1] 张业明[1] 谢才富[1] 

机构地区：[1]宜昌地质矿产研究所,湖北宜昌443003

出　　处：《岩石矿物学杂志》2007年第1期35-41,共7页Acta Petrologica et Mineralogica

基　　金：中国地质调查局国土资源大调查项目(200013000127)

摘　　要：对琼中前寒武纪高级变质杂岩中的麻粒岩进行了较系统的岩石学和矿物学研究,并利用不同的矿物温压计估算了麻粒岩形成的温压条件。结果表明,本区麻粒岩形成于高温低压(低p/T)环境,属于典型的低压麻粒岩,其峰期变质温度为824±15℃,压力小于0.5GPa,晚期岩石发生了退变冷却,终止温度为705℃左右,压力与峰期时的压力相近;麻粒岩相变质作用具有顺时针的p-T轨迹,早期可能经历了一个缓慢降压升温的过程,峰期之后的退变则表现出近等压冷却(IBC)的特点。结合岩石变形作用较弱的特点及区域大地构造背景,认为琼中低压麻粒岩形成于陆缘拉张环境,其可能的机制是,由于地壳拉伸减薄,导致软流圈减压上隆产生岩浆,上覆地壳岩石(包括已经结晶的岩浆)受到下部岩浆热源的影响而发生进变质,随后受拉伸扰动的地热梯度向正常地热梯度恢复,从而形成具有降压升温过程和IBC过程的顺时针p-T轨迹。Petrological and mineralogical characteristics of the granulites from the Qiongzhong complex were studied in this paper, and the temperature and pressure conditions for the formation of the granulites were estimated by a variety of geothermometers and geobarometers. The results show that the granulites belong to typical low-pressure granulites formed in an environment of high-temperature and low-pressure （low p/T）, with peak temperature being 824 ± 15℃ and corresponding pressure lower than 0.5 GPa. The granulites were cooled during late retrograde reaction which terminated at the temperature of 705℃, with the corresponding pressure near the peak. Therefore, the p-T path of the low-pressure granulites from the Qiongzhong complex was clockwise, composed of two phases of heating during early slow decompression and nearly isobaric cooling （IBC） during late retrograde reaction. Combining characteristics of the p-T path of the granulites with the features of relatively weak deformation and regional tectonic setting, it is considered that the low-pressure granulites from the Qiongzhong complex were formed in an environment of crustal extension. The possible mechanism is as follows： because the crust was thinned by extension, the crustal heat flux was raised by the introduction of magmas generated through decompression of the asthenosphere, and then prograde metamorphism togk place in the overlying rocks （including crystallized magma） ; afterwards the geothermal gradient disturbed by extension was restored to normal gradient and, as a result, the clockwise p-T path composed of two phases of heating during decompression and nearly isobaric cooling （IBC） was produced.

关 键 词：低压麻粒岩 PT轨迹 地壳拉伸 琼中 

分 类 号：P588.3[天文地球—岩石学] P581[天文地球—地质学]