Controls on post-Gondwana alkaline volcanism in Southern Africa
Controls on post-Gondwana alkaline volcanism in Southern Africa
复制标题
对南部非洲后冈瓦纳碱性火山活动的控制
DOI：
10.1016/j.epsl.2008.01.007
复制
发表时间：
2008
影响因子：
5.3
通讯作者：
F. Cotterill
中科院分区：
文献类型：
--
作者：
A. Moore;T. Blenkinsop;F. Cotterill
已为冈瓦纳大陆分裂前后在南部非洲喷发的金伯利岩和其他碱性火山管建立了一个高精度（± 5 Ma）放射性年龄综合数据库。这些年龄表明，非洲南部的碱性火山活动自冈瓦纳大陆分裂以来一直是间歇性的，并且与非洲其他地方的碱性火山活动大致相同。在南部非洲，近海沉积模式也是间歇性的，基本上连续的沉积时期被一系列与板块扩张机制有关的重大不整合面所打破。火山事件发生在板块扩张历史和近海沉积记录中相关不整合面的重大变化之后，滞后5 - 13 Ma。在区域范围内，火山活动从南部非洲内陆向边缘全面扩散。一些火山管群定义了一个主要的火成岩线性构造，一般的年龄进展向西南方向延伸。部分碱性线性与主要的裂谷带相吻合，但火山活动早于裂谷的表面表现多达90马。这些关系指向碱性火山活动的构造触发因素，而不是在核幔边界附近发起的羽流。在整个南部非洲，碱性岩浆类型也存在系统的地理变异，富镁的金伯利岩发育于次大陆中部的太古代克拉通，而更发育的成分发育于边缘较年轻的褶皱带。这些化学趋势的解释，减少深度之间的交叉CO2-H2O橄榄岩固相线和地热增加地热梯度从太古宙克拉通年轻的边缘褶皱带。
A comprehensive database of high precision (±5 Ma) radiometric ages has been assembled for kimberlites and other alkaline volcanic pipes erupted in southern Africa during the period immediately preceding and following the disruption of Gondwana. These ages show that alkaline volcanic activity in southern Africa has been episodic since Gondwana break-up, and was broadly coeval with episodes of alkaline volcanism elsewhere in Africa. In southern Africa, offshore sedimentation patterns were also episodic, with periods of essentially continuous sedimentation broken by series of major unconformities, linked to plate spreading regimes. The volcanic episodes followed major changes in plate spreading histories and the associated unconformities in the offshore sedimentary record with a lag of 5–13 Ma. On a regional scale, there is an overall younging of volcanic activity from the interior to the margins of southern Africa. Some of the volcanic pipe clusters define a major igneous lineament, with a general age progression younging to the southwest. Parts of this alkaline lineament coincide with major rift zones, but the volcanism predated the surface expression of rifting by as much as 90 Ma. These relationships point to tectonic triggers for alkaline volcanic activity, rather than plumes initiated near the core–mantle boundary. There is also a systematic geographic variation in alkaline magma type across southern Africa, with Mg-rich, less evolved kimberlites emplaced into the Archean craton of the central portions of the sub-continent, and more evolved compositions emplaced into the marginal younger fold belts. These chemical trends are interpreted in terms of decreasing depth of intersection between the CO2–H2O peridotite solidus and the geotherm with increasing geothermal gradient from the Archean craton to the younger marginal fold belts.