大陆解体与被动陆缘的演化

Continental breakup and evolution of rifted volcanic margins

火山型被动陆缘是大陆解体过程中形成的一类陆缘类型,其演化过程与活动陆缘一样复杂多变。随着近年来对大陆解体过程与被动陆缘演化的深入研究,对其沉积过程、岩浆活动以及变质作用研究都有了很大的进展。陆壳减薄解体的过程有许多不同的模式,不对称的简单剪切模式可能是火山型被动陆缘的成因,其机制是软流圈隆起的最大位置从剖面上看与地壳减薄最大位置不在一条垂线上,造成软流圈上升的岩浆在解体的大陆一侧形成火山型被动陆缘。被动陆缘的沉积建造由两套沉积物组成,一套是大陆解体的裂谷阶段所形成的陆相沉积物和双模式火山岩组合,另一套是稳定陆缘的复理石组合;岩浆作用中基性岩类反应了物质直接源于上地幔的主要特点,并有部分受到地壳混染的特征;变质作用中高温低压环境主要发生在裂谷作用阶段,其特点反映了大陆解体过程中随着时间的增温和减压过程,而拆离伸展阶段则被脆性变形所代替。

Volcanic rifted margins are a kind of continental margin formed during continental breakup. Their geodynamic evolution is as complex as that of active margins. Through the study of continental breakup and evolution of passive margins in recent years, the recognition of these margins has greatly improved knowledge of their sedimentary process, magmatism and metamorphism Several models have been proposed for continental breakups. The asymmetry simple shear model may explain the genesis of volcanic rifted margin. Rising mantle asthenosphere and thinning of crust cause volcanic eruption on one side of the passive margin. The lithologicai successions of volcanic rifted margins consist of two assemblages. One is a himodal volcanic assemblage, which formed during the rifting stage of the continental breakup theother is represented by flysch rocks, formed when the passive margin becomes stable The composition of mafic rocks in the volcanic margin reflect their mantle origin and their crustal contamination. High tempera- ture and low-pressure metamorphism can occur during the rifting stage and reflects a temporal trend of increasing temperature and decreasing pressure during the continental breakup.