Timing of the volcanism of the southern Kivu province: implications for the evolution of the western branch of the East African Rift system
Timing of the volcanism of the southern Kivu province: implications for the evolution of the western branch of the East African Rift system
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DOI：
10.1016/0012-821x(89)90152-0
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发表时间：
1989-09
影响因子：
5.3
通讯作者：
P. Pasteels;M. Villeneuve;P. Paepe;J. Klerkx
中科院分区：
文献类型：
--
作者：
P. Pasteels;M. Villeneuve;P. Paepe;J. Klerkx
报道了从南基伍火山省(扎伊尔、卢旺达、布隆迪)采集的大量岩石样品的最新K-Ar年龄。尚未获得超过10 Ma的年龄。这一结果与旧的假设形成鲜明对比，并严重制约了火山活动和裂谷演化之间的关系。10~7.5 Ma拉斑玄武岩以拉斑玄武岩为主，对应一幕裂隙喷发，7.5~5 Ma碱性玄武岩及其分异主要在局部裂谷中喷发。活动高峰发生在6.0~5.5 Ma之间，更新世碱性火山作用仅局限于局部地区。拉斑玄武岩从拉斑玄武岩向碱性玄武岩火山作用的转变发生在7.5 Ma左右，相当于一个大裂谷阶段，相当于基伍湖盆地形成的起始阶段。裂谷中部拉斑玄武岩的分布，以及沿西部活动边界断裂以碱性岩石为主的分布，强化了前者与张力有关，后者与垂直运动有关，可能还与走滑运动有关。西部裂谷的火山活动仅限于位于两个走滑带之间的张力地带的地区。在南基伍地区，正断层与走滑断层相交，这似乎决定了火山活动的位置。岩浆的形成被认为与剪切加热和上升底辟的绝热减压有关。这意味着岩石圈-软流圈边界由于伸展而升温。拉斑玄武岩或碱性岩浆的产生与地幔底辟的可变上升速度或沿剪切带的可变剪切加热有关。岩浆成分和火山活动强度随时间的变化被认为与裂谷演化的主要阶段有关。
New K—Ar datings of a large rock sampling from the South Kivu volcanic province (Zaire, Rwanda, Burundi) are reported. No ages older than 10 Ma have been obtained. This result contrasts with older assumptions and puts severe constraints on the relations between volcanism and rift evolution. From 10 to 7.5 Ma tholeiitic volcanism predominates corresponding to an episode of fissural eruptions; from 7.5 to 5 Ma alkali basalts and their differentiates are mainly erupted in localized rifts. A culmination of activity occurs between 6.0 and 5.5 Ma ago.Pleistocene alkalic volcanism is restricted to localized areas. The transition from tholeiites to alkali-basaltic volcanism dated around 7.5 Ma would correspond to a major rifting phase which corresponds with the initiation of Lake Kivu Basin formation. The distribution of tholeiitic rocks in the central part of the rift, and predominantly alkalic rocks along the western active border fault, strengthens the idea that the former are associated with tension, the latter with vertical, possibly also strike-slip movements.Volcanism in the Western Rift is restricted to areas where tension occurs in a zone which is located between two zones of strike-slip. In the South Kivu area normal faults intersect strike-slip faults and this seems to have determined the location of volcanic activity.Magma formation is considered to be related with shear heating combined with adiabatic decompression in ascending diapirs. This implies heating at the lithosphere-asthenosphere boundary as a result of extension. Generation of tholeiitic or alkalic magmas is connected with the variable ascent velocity of mantle diapirs or with variable shear heating along the shear zone. Changes in both magma composition and intensity of volcanic activity with time are considered to be related to major phases of rift evolution.