Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric

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.     

辽北法库城子山一带硅灰石矿床地质特征

法库城子山一带硅灰石矿床分布在吉黑地槽南缘调兵山背斜,赋存于早古生代富拉堡子岩组与岩体接触带附近,属接触变质型矿床。区域上构成了东、西两个硅灰石矿带。矿石类型有： 致密块状硅灰石－ 方解石－ 石英型, 硅灰石－ 石英型, 巨晶硅灰石－ 方解石型等三种类型。其主要围岩蚀变为硅灰石化、硅化、碳酸盐化、绿泥石化及角岩化,局部见矽卡岩化。富硅灰岩－ 三叠世侵入岩－ 构造裂隙系统是主要控矿因素。     

Anisotropy of magnetic susceptibility of eclogites: mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

Abstract

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10^?5 S.I. and 28 to 75·10^?5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K_max) and the pole to the magnetic foliation (K_min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris     

Scales of deformation partitioning during exhumation in a continental subduction channel: A petrofabric study of eclogites and gneisses from NW Spain (Malpica‐Tui Allochthonous Complex)

Deformation partitioning is identified as the fingerprint of late Palaeozoic continental subduction that affected various lithologies whose field relationship, thermobarometric and petrofabric features are closely related. Different modes of deformation partitioning can be identified within medium temperature, high‐P eclogite lenses, between them and the host gneisses, and within the latter. Development of foliations and lineations with a coherent attitude in all these rocks and their related structural petrology demonstrate that eclogite enclosures and their country rocks underwent a common, pervasive deformational event. The published P–T stability fields of the eclogite phases that define the microscopic fabric are used to define the metamorphic conditions prevailing during the deformation event and relate it to the subduction process. The mineral equilibria of the gneisses (ortho‐ and paragneisses) fail to record the full range of those P–T conditions, but the field relationships show that eclogites were originally basic dykes emplaced in acid igneous rocks and demonstrate that the eclogites and gneisses shared a common tectonometamorphic evolution. Deformation partitioning within the latter occurred at variable scales and involved (1) meso macroscale preservation of virtually undeformed metagranite bodies, surrounded by (2) pervasively foliated and lineated gneisses, and (3) the simultaneous microscale operation in the latter of ductile and brittle–ductile mechanisms at conditions above 500°C and below 1.5 GPa. A subduction channel tectonic setting is proposed to explain the subduction of upper to mid‐crustal igneous rocks and exhumation subsequent to high‐P metamorphism. Its currently accessible dimensions, and its organization into several lithotectonic units mapped as nappes support tectonic amalgamation of units several km³ in volume. Maximum burial in the subduction channel likely reached depths shallower than the lithostatic pressure implied by geobarometric calculations, possibly conditioned by a sudden pressure drop during the initial retrogression stages accompanying exhumation.     

河北省衡水地区第四纪中—晚期标准地层剖面的建立及其地质含义

依据河北省衡水市西北郊区纪家庄、赵家屯井孔岩心,采用岩性描述、岩相划分、地层测年、古气候孢粉样品分析及钻孔间地层对比等方法,结合前人的研究成果,获取如下认识:(1)纪家庄井孔(ZK1)、赵家屯井孔(ZK2)岩心揭示出衡水地区全新统厚度为8~9 m,上更新统厚度为31~36 m,中更新统厚度为72~75 m;(2)依据各井孔及岩心所揭示的同一岩组底界海拔高程,推测研究区位于向东方向缓倾的古斜坡带上;(3)以岩石地层学、年代地层学为主导,结合以气候地层(孢粉地层)、宏观特征为主并与微观标志相结合的综合划分原则,多井孔岩心揭示出研究区同一地质年代岩组厚度相当,且中更新世以来处于较为稳定的整体沉降环境中,不存在明显的差异升降运动。     

鲁西地区韧性剪切带显微构造研究及岩组分析

通过对鲁西地区韧性剪切带显微构造及岩组特征等的研究,认为该区韧性剪切带具有多期次、多方向、多性质的变形特点,韧性变形主体具有右行剪切性质,最大主压应力方位为ＮＥＮＥＥ。从岩组分析结果可知,区内岩石韧性变形较强,并且复杂,变形以压剪性变形为主,石英变形机制主要为柱在滑移,韧性变形时温度较高。石英自由位位错密度法应力估算结果说明,区内最大差应力为７５．４ＭＰａ,平均值６８．４５ＭＰａ,金矿体的赋存与构     

利用岩矿记忆信息恢复基岩潜山裂缝储层形成的机制

基岩储层具有当今油气储层研究的两大难题,即特殊且致密的基质岩性和储集空间。变质岩作为基岩岩性之一,其储集空间－裂缝的形成与动力作用密切相关。构造动力是致密基岩产生裂缝的主要因素,岩石矿物将记忆其在地质历史过程中所经历的动力特征。在准确采取定向岩心的基础上,利用声发射技术（ＡＥ）和岩组学方法可以恢复基岩储层在地质历史过程中所受应力期次和应力方向,从而了解储层的动力机制。应用上述原理恢复辽河大民屯凹陷     

宁杭高速公路东芦山超高边坡岩体特征分析

对东芦山的岩石成因、结构构造,以及物理力学性质等特征进行研究,指出该类岩石在变形特征上可视为均质弹 性各向同性体。依据边坡区岩体构造的基本特征、空间产状、原生结构面性质及分布规律,对东芦山边坡岩体工程地质岩 组和岩体结构类型进行划分。并在此基础上建立东芦山边坡岩体的地质模型。