The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications
The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications
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DOI：
10.1016/j.epsl.2015.02.035
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发表时间：
2015-05
影响因子：
5.3
通讯作者：
A. Zeh;M. Ovtcharova;Allan H Wilson;U. Schaltegger
中科院分区：
文献类型：
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作者：
A. Zeh;M. Ovtcharova;Allan H Wilson;U. Schaltegger
Bushveld杂岩（BC）的Rustenburg层状套（RLS）代表了地球上最古老的大型火成岩省（bbb37万km 3），并含有世界上最大的铂族元素，铬和钒储量。然而，它的形成方式、岩浆就位的确切时间和性质、凝固和亚固态冷却的历史仍然是一个有争议的问题。基于岩石学观测的高精度U单键Pb测年结果显示，整个RLS的锆石结晶时间在1.02±0.63 Ma，温度在940°~ 670°之间，来自高度分异的积云间熔体，淬火边缘区岩石的锆石结晶时间为2055.91±0.26 Ma，而RLS中心积云岩石的锆石结晶时间稍晚于2054.89±0.37 Ma。这一时间与现场观测和热模拟的结果一致，热模拟要求岩浆在不到100 ka的时间内以5公里/年的通量速度快速积累，然后在950 ka内结晶并冷却到700°C以下。在2.056 Ga时，次大陆岩石圈地幔（SCLM）内部应力场的变化引发了岩浆的活跃泵送，导致了短时间的熔体积累和极快的通量速率，导致大量岩浆滞留在上地壳内。这种变化是由于在地幔柱上涌事件中，其最低的富含榴辉岩的部分被消除后，SCLM的反弹造成的。
Abstract The Rustenburg Layered Suite (RLS) of the Bushveld Complex (BC) represents Earth's oldest large igneous province (> 370 000 km 3), and contains the world's largest reserves of platinum-group elements, chromium and vanadium. However, its mode of formation, the exact timing and nature of magma emplacement, solidification and sub-solidus cooling history remain a matter of debate. High precision U single bond Pb dates, backed by petrological observations reveal that zircon throughout the RLS crystallised within 1.02±0.63 Ma from highly fractionated intercumulus melts at temperatures between 940° and 670° C. Zircon in quenched Marginal Zone rocks crystallised at 2055.91±0.26 Ma, and slightly later at 2054.89±0.37 Ma in cumulus rocks in the centre of the RLS. This timing is in agreement with field observations and the results of thermal modelling, which require rapid accumulation of magma at a flux rate of> 5 km 3/yr over less than 100 ka, followed by crystallisation and cooling to below 700° C within 950 ka. This short period of melt accumulation with an extreme flux rate, leading to a large volume of magma stalled within the upper crust, is suggested to result from active magma pumping, triggered by stress field change within the subcontinental lithospheric mantle (SCLM) at 2.056 Ga. This change was caused by rebounding of the SCLM after elimination of its lowest, eclogite-rich part during a mantle plume up-welling event.