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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >Geophysical interpretation of the nature and extent of the xade mafic complex, botswana
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Geophysical interpretation of the nature and extent of the xade mafic complex, botswana

机译：博茨瓦纳Xade铁镁质复合物的性质和范围的地球物理解释

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摘要

The Xade Mafic Complex, situated in central Botswana, was identified during the first regional aeromagnetic survey of the country in 1975-77. It is covered by sedimentary rocks of the Kalahari Group and Karoo Supergroup, including Karoo lavas, with a combined thickness varying from 220 m to 1000 m, extending to greater depths in the north beneath the Meso-proterozoic Passarge Basin. What has generally been viewed as the Xade Complex is evidenced by a high amplitude kidney-shaped zoned magnetic anomaly with two semi-linear anomalies extending to the northwest and northeast in a Y-shaped form. The complex is also marked by a coincident Bouguer gravity anomaly. Historical work has been limited, with only three diamond drill holes having been drilled, separately intersecting gabbroic rocks, weathered basalt grading into dolerite, and amygdaloidal lavas followed by Waterberg Group shales. An U-Pb zircon age of 1109.0 ±1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit. This study has shown, for the first time, that the Xade Complex comprises two lobes of potentially different magmatic character, i.e. a Southern Lobe, which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe. The latter is mostly deeply buried in the north and northwest beneath the Passarge Basin, as evidenced by deep magnetic and gravity sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the Southern Lobe. Forward modelling, of both magnetic and gravity data, indicates that the Southern Lobe is a lopolith with a depth extent of approximately 4 km, whereas the southeastern margin of the Northern Lobe has westerly dips consistent with deepening to the northwest. In addition to the historic boreholes, a recent borehole drilled into the Southern Lobe shows that it comprises a mafic volcanic sequence with subordinate gabbro. A further two boreholes drilled into the Northern Lobe margins indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks. The present interpretation has also identified a dyke system associated with the Northern Lobe, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the complex is located in a craton margin setting. The combined extent of both lobes is approximately one-third the size of the Bushveld Complex, making the Xade Mafic Complex the largest Meso-Proterozoic magmatic system in southern Africa. The craton margin setting, size, Meso-Proterozoic age and variability of plutonic and volcanic rocks hallmark the Xade Mafic Complex as being a very large dynamic magmatic system, with potential for Ni-Cu-PGE mineralization.
机译：位于博茨瓦纳中部的Xade黑手党情结是在1975-77年对该国进行的首次区域航空磁测期间确定的。它被Kalahari组和Karoo超级组的沉积岩（包括Karoo熔岩）覆盖，合并厚度在220 m至1000 m之间变化，并向中元古代Passarge盆地下方的北部扩展。通常被认为是Xade复杂体的证据是高振幅的肾形分区磁异常，其中两个半线性异常以Y形延伸到西北和东北。该复合体的特征还在于同时发生的布格重力异常。历史工作受到限制，仅钻了三个钻石钻孔，分别与辉长岩相交，风化玄武岩定为白云石，杏仁状熔岩，随后是Waterberg Group页岩。辉长岩单元的U-Pb锆石年龄为1109.0±1.3 Ma，与Umkondo火成岩时期相当。这项研究首次表明，Xade复杂体包括两个可能具有不同岩浆特征的裂片，即南部裂片，这是历史上确定的肾形带状磁异常，以及迄今无法识别的大型北部裂片。后者主要被深埋在Passarge盆地的北部和西北部，这由深部磁力和重力源所证明，但后者的南部和东部边缘部分露头低于Karoo沉积物，形成了南叶北部的Y形异常。磁和重力数据的正演模拟表明，南部的波罗夫岩是一个深度约4 km的岩层，而北部的波罗地东南边缘的西风倾角与向西北加深一致。除了历史悠久的钻孔外，最近钻入南部裂片的钻孔还表明，该钻孔还包括一个镁铁质火山岩序列和附属的辉长岩。在北叶边缘再钻两个钻孔，表明该岩层由质地异质和岩浆分异的辉长岩序列，以及次要闪长岩和单长岩构成。本解释还确定了与北叶相关的堤系，该堤系可以代表馈线或出口岩浆导管。解释还表明，该复合物位于克拉通边距设置中。两个瓣的合并范围约为布什维尔德复杂体的三分之一，使Xade镁铁质复杂体成为南部非洲最大的中元古代岩浆系统。 Xade镁铁质复合体是克拉通边缘构造，大小，中元古代年龄以及深成岩和火山岩的变异性，它是一个非常大的动力岩浆系统，具有潜在的Ni-Cu-PGE矿化作用。

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