Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric
Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric
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阿曼蛇绿岩辉长岩的岩组研究：AMS 与岩组的比较
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
C. MacLeod
中科院分区：
文献类型：
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作者：
G. Yaouancq;C. MacLeod
从阿曼蛇绿岩中采集了42个辉长岩样品，对其磁化率各向异性进行了测量。通过野外研究和斜长石晶体组构测量，将岩石组构与磁性组构进行了比较。对比研究表明，从古莫霍面到面理辉长岩顶端，73%的岩石具有良好的磁性和岩石组构取向对应关系。在这些岩石中，AMS受位于橄榄石裂隙网络中的次生磁铁矿控制，也可能(但程度较小)受位于单斜辉石出溶片层中的次生磁铁矿控制。AMS椭球体取向与岩石组构取向之间的高度相关性可以用以下事实来解释：磁性面理本质上受制于橄榄石断裂面的取向，而橄榄石断裂面的取向又受制于整个岩浆岩石组构的取向。与原生矿物相相反，这些辉长岩中的磁铁矿晶体的取向并不是由于它们在流动的岩浆中排列，所以它们的择优取向虽然通常模仿岩石组构，但并不具有相同的来源。此外，由于各向异性次生磁铁矿的择优取向远不如斜长石的择优取向完美，AMS和斜长石组构的形状和大小之间没有相关性。在层序的最上层，磁性和岩石组构方向之间没有对应关系。这些岩石的磁性主要由原生磁铁矿和钛铁矿颗粒携带。这些矿物以细小且分散的间隙颗粒赋存，与先前存在的岩石组构既不对齐也不平行。因此，AMS椭球体的各向异性、形状和取向与岩石组构椭球体无关。尽管在Wadi Al Abyad辉长岩中，就像在其他含磁铁矿的岩石中一样(Rochette等人，1992；Archanjo等人，1995)，AMS不能用于评估有限应变椭球体的形状和强度，但当AMS受次生磁铁矿控制时，它可以可靠地用于获得岩石组构椭球体的取向。
The anisotropy of magnetic susceptibility was measured on 42 gabbros sampled across a complete plutonic sequence from the Oman ophiolite. The rock fabrics, investigated in the field and through plagioclase crystallographic fabric measurements, were compared to the magnetic fabrics. This comparative study reveals that from the paleo-Moho to the top of the foliated gabbros level, 73% of the rocks display a good correspondence in orientation, between the magnetic and rock fabric orientation. In these rocks, the AMS is controlled by secondary magnetites located in the fracture network of the olivines, and probably, but to a lesser extent, by secondary magnetites located in the exsolution lamellae of the clinopyroxenes. The high correlation between the AMS ellipsoid orientation and the rock fabric orientation is explained by the fact that the magnetic foliation is essentially constrained by the orientation of the olivine fracture planes, which is in turn constrained by the orientation of the overall magmatic rock fabric. In contrast to the primary mineral phases, the orientation of magnetite crystals in these gabbros is not due to their alignment in a flowing magma, so their preferred orientation, although usually mimicking that of the rock fabric, does have not the same origin. Furthermore, given that the preferred orientation of the anisometric secondary magnetites is much less perfect than the preferred orientation of the plagioclases, no correlation between the shape and magnitude of the AMS and plagioclase fabrics can be established. In the uppermost levels of the sequence there is no correspondence between the magnetic and rock fabric orientation. The magnetism of these rocks is mainly carried by primary magnetite and ilmenite grains. These minerals occur as small and scattered interstitial grains that exhibit neither alignment nor parallelism with the pre-existing rock fabric. Hence, the anisotropy, shape and orientation of the AMS ellipsoid are independent of the rock fabric ellipsoid. Although in the Wadi Al Abyad gabbros, just like in other magnetite bearing rocks (Rochette et al., 1992; Archanjo et al., 1995), the AMS cannot be used to evaluate the shape and strength of the finite strain ellipsoid, it can be reliably used to get the orientation of the rock fabric ellipsoid when the AMS is controlled by secondary magnetites.