扬子地块奥陶系碳酸盐岩重磁化机制探讨

碳酸盐岩是记录古地磁场信息的重要载体,然而,广泛存在的重磁化现象制约了碳酸盐岩在古地磁研究中的应用,其重磁化机制亟待解决.本文对采自贵州羊蹬地区的319块奥陶系碳酸盐岩定向样品作了详细的古地磁学和岩石磁学研究,其结果表明,94%样品(A类)记录了单一剩磁分量A,其解阻温度低于450℃;在地理坐标系下的平均方向为Dg/Ig=3.1°/48.1°(α₉₅=2.9°),对应的古地磁极(87.0°N,2.8°E,A₉₅=3.0°)与扬子地块古近纪-第四纪的古地磁极重合.6%样品(B类)记录了两个磁化分量,其高温分量(450℃~585℃)与A分量显著不同,但明显远离扬子块体早古生代古地磁极;低温分量(< 450℃)与A分量类似.说明羊蹬剖面奥陶系碳酸盐岩记录了两期重磁化.A分量和B低温分量的主要载磁矿物为磁黄铁矿(胶黄铁矿),B高温分量的主要载磁矿物为磁铁矿.这些磁性矿物都是成岩后的次生矿物.其中,解阻温度高于450℃的磁铁矿可能受晚燕山期造山运动影响生成;磁黄铁矿(胶黄铁矿)等矿物可能与印度板块与欧亚大陆碰撞引起的喜马拉雅造山运动所产生的流体作用有关,以后一期重磁化为主.新生代早期青藏高原隆升产生的流体在流经东南缘的碳酸盐岩等沉积岩层时,与原岩发生相互作用,使磁黄铁矿、胶黄铁矿、磁铁矿等磁性矿物生长并获得化学剩磁,造成了广泛重磁化.     

Magnetic reorientation induced by pressure solution: A potential mechanism for orogenic-scale remagnetizations

New paleomagnetic analyses of Triassic, Cretaceous and Eocene strata in the south-central Pyrenees show evidence for a widespread remagnetization, located along the southern border of the Axial Zone, the Internal Sierras, and the northern part of the Jaca-Pamplona basin. This remagnetization, always reversed in polarity, was acquired after an extensive period of Late Eocene–Early Oligocene folding and tilting in the area, and affects limestones, sandstones, marls and red beds. In addition, a characteristic prefolding component was identified in 30% of Upper Cretaceous and Triassic red beds. These results, together with a revaluation of previously published paleomagnetic data from the central Pyrenees, indicate that the spatial distribution of the postfolding remagnetization coincides with that of a domain of pressure solution cleavage. A relationship between the intensity of the remagnetization and the characteristic (prefolding component) with respect to the density of cleavage surfaces, leads us to propose a mechanism for the remagnetization related to the development of pressure solution cleavage that is framed within the tectonic evolution of the central Pyrenees. Partial dissolution of rock under tectonic compression leads to the liberation and subsequent accumulation of insoluble minerals in cleavage planes. Magnetic grains are part of the relatively insoluble residue, and they reorient in the presence of the ambient field after they are freed during dissolution of the rock matrix. Chemical reequilibrium (dissolution and/or neoformation of magnetic carriers) during this process cannot be excluded. The remagnetization mechanism we propose can help to explain widespread remagnetizations in low to moderately deformed rocks without the need of large-scale migration of orogenic fluids.     

Thermal remagnetization and the paleointensity record of metamorphic rocks

We present a quantitative relationship between blocking temperature and time that, in principle, provides a calibration of thermal remagnetization in nature. For a given metamorphic temperature-time regime, one can decide whether a given laboratory blocking temperature (or for paleointensity work, a range of blocking temperatures) is consistent with primary natural remanence (NRM) or with a metamorphic overprint. Independent of the domain structure or the chemical composition of the magnetic minerals, two general types of behaviour are predicted. If the primary NRM possesses laboratory (or primary cooling) blocking temperatures within 100°C or so of the Curie temperature, thermal remagnetization at lower temperatures, even over times as long as 10⁶ years, is improbable. If the blocking temperatures are lower, viscous remagnetization is pronounced at temperatures well below those indicated by laboratory thermal demagnetization. An approximate scale of the “survival potential” of primary NRM in rocks of different metamorphic grades indicates that primary paleointensities are unlikely to be recovered from rocks metamorphosed above high-greenschist facies if the predominant magnetic mineral is nearly pure magnetite, or above middle-amphibolite facies if nearly pure hematite is predominant. Evidence from laboratory experiments and paleomagnetic field studies in metamorphic regions suggests, however, that these survival estimates are unduly optimistic. Chemical remagnetization through the destruction of primary magnetic minerals, and not thermal remagnetization, probably sets an effective upper temperature for the survival of primary NRM.     

Characteristics of magnetic carriers responsible for Late Paleozoic remagnetization in carbonate strata of the mid-continent, U.S.A.

Magnetic analysis of carbonate strata of the mid-continent region of the United States indicates that the Late Paleozoic remagnetization already recognized in the Appalachians also affects almost the entire mid-continent. Magnetic intensity is regionally variable, with more intense magnetizations occurring on intracratonic arches. Magnetization is carried predominately by magnetite, which often occurs in tiny hollow spheroids composed of well formed octahedral microcrysts. Spheroid chemistry and morphology suggests that the magnetite is authigenic, composed of iron extracted from pre-existing pyrite or clay. The timing of the remagnetization, the character of the magnetite, and the regional variation in magnetic intensity support the idea that the remagnetization is a diagenetic consequence of tectonically-driven brine migration.     

Determination of paleointensity from sedimentary and igneous rocks by the method of alternating-field remagnetization

The remanent magnetization of igneous and sedimentary rocks, if not changed by heating or by alteration of magnetic minerals, keep the information about the intensity of that magnetic field in which initial magnetization took place.It has been determined that the dependence of anhysteretic remanent magnetization of such a rock on d.c. magnetic field permits us to find the paleointensity. A method of investigation of rock specimens by means of such remagnetization is suggested and applied to determining the paleointensity for a series of Permo-Triassic rocks.     

The Carboniferous of the Moscow syneclise: Paleomagnetic data

New data for the Early and Late Carboniferous sections of the Russian platform (Moscow syneclise and Donbass) are presented. Magneto-mineralogical studies are carried out to identify the magnetic minerals—carriers of natural remanent magnetization. Extensive Late Paleozoic remagnetization of Carboniferous rocks is revealed. The obtained paleomagnetic data allowed us to determine the average paleomagnetic poles for the Gzhelian, Serpukhovian, and Visean stages of Carboniferous deposits of the Moscow syneclise.     

Pervasive late Paleozoic-Triassic remagnetization of miogeoclinal carbonate rocks in the Basin and Range and vicinity, SW USA: regional results and possible tectonic implications

Relatively unmetamorphosed Paleozoic miogeoclinal carbonate rocks in the Basin and Range of E Nevada, SW Nevada and adjacent California, and W Utah yield low-inclination magnetizations that reflect pervasive, regional remagnetization around the close of the Paleozoic. The rocks range in age from mid-Cambrian through Pennsylvanian and lie generally in a broad belt between the mid-Paleozoic Roberts Mountain Thrust and the late Cretaceous Sevier thrusts. Most of the magnetizations reside in magnetite, but at one site the magnetization is evidently carried by pyrrhotite. Preliminary rock-magnetic data suggest samples with magnetite-borne remanences have wasp-waisted hysteresis curves typical of remagnetized carbonates. The origin of the remagnetization is problematic and probably polygenetic: both the Permo-Triassic Sonoma orogeny and deformation associated with the Ancestral Rockies seem too spatially limited, but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late Paleozoic remagnetization unless combined with thermal resetting of the lowermost units. A number of sites also appear to have undergone some vertical-axis rotation, and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence. However, the probably large age range of the low-inclination components complicates their use for resolving tectonic rotations. Younger, intermediate-stability components of magnetization, probably of Cretaceous or Cenozoic age, are also found in many sites and also probably have multiple origins. At sites farther W, the late Paleozoic component is not found, which probably reflects its destruction by later Mesozoic or Cenozoic heating. At sites farther E, on and near the Colorado Plateau, gray carbonates yield only Cenozoic magnetizations. Some reddish, oxidized carbonates there locally contain a hematite-borne magnetization of late Paleozoic age. However, it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process(es) in the miogeocline.     

Magnetic properties of the Lower Old Red Sandstone lavas in the Midland Valley,Scotland; palaeomagnetic and tectonic considerations

Thermal demagnetization studies of lavas in the Strathmore area of the Midland Valley, Scotland, support overall palaeomagnetic data found in previous studies of these rocks. Reduced directional scatter as compared to some earlier studies, is attributed to more effective demagnetization, resolving some of the directional complexity of previous studies. Combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism. The existence of almost antiparallel directional groups and field tests give supporting evidence for a “primary” (deuteric) origin of the main magnetization of these rocks. Additionally, a second remanence component having shallow reverse directions of magnetization, is attributed to later remagnetization in Old Red Sandstone time. The Midland Valley results are seen in conjunction with other Palaeozoic palaeomagnetic results and possible geodynamic implications are discussed.     

Remagnetization of the Early Paleozoic rocks from southern part of the Huabei basin ( I )

Although paleomagnetic study of the Early Paleozoic for the North China Block (NCB) has witnessed rapid progress since the 1980s, significant difference in the results can be found from the widespread areas in North China. Besides the paleomagnetic techniques used in the laboratories, the difference of these Paleozoic poles could also be due to the early and late Mesozoic remagnetization in the eastern part of China. It is therefore necessary to carry out systematic paleomagnetic and rock magnetic studies for the Early Paleozoic rocks in the NCB. The remagnetizarion re-sults from the northwestern part of Henan Province are reported, and related geological implications are discussed.     

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province,China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.     

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province,China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.

     

Paleomagnetism of early paleozoic geological complexes of Mongolia

Most of the studied Early Phanerozoic rocks of West Mongolia have undergone repeated remagnetization. Secondary magnetization components with normal and reversed polarity are isolated. The magnetization components with normal polarity are associated with the Mesozoic remagnetization of the rocks. The components with reversed polarity were probably formed during the Carboniferous–Permian superchron of reversed polarity. The analysis of the distribution of the reversed-polarity magnetization component in the structure of Mongolia permits some zonation. Within Mongolia, the regions with insignificant post- Permian deformations and complicated post-Permian deformations are identified; also the area of rotations of large geological blocks about the horizontal axis (Khan-Khukhei Ridge) is distinguished. It is hypothesized that in the Ordovician rocks of West Mongolia, the magnetization component that is close to primary was identified. If this is the case, the paleolatitude calculated from this magnetization direction corresponds to the interval 14°–17°–20° (minimal–mean–maximal) of probably northern latitude     

Apparent polar wander relative to Australia during the Precambrian

Palaeomagnetic data for Australian Precambrian rocks allow the preparation of a tentative apparent polar wander curve. The proposed curve helps to “date” the magnetic remanence of many rocks whose age of magnetization or remagnetization is imprecise. Using this curve the CRMs of hematite ore deposits can be shown to have been acquired up to hundreds of millions of years after the deposition of the host rocks.Comparison of the Australian Precambrian apparent polar wander curve with proposed curves for Europe, Siberia, North America and Africa provides further evidence for the suggestion of relative continental movement during the Precambrian. If relative continental drift has not (or could not have) taken place, then “true” polar wander apparently varies relative to the continent for which it is determined.     

Serpentinites and serpentinites: Variety of origins and emplacement mechanisms of serpentinite bodies in the California Cordillera

Most serpentinitized peridotite in orogenic belts is derived from oceanic lithosphere, but the emplacement mechanisms of these rocks vary greatly, as illustrated by the nature of these rock bodies and their contacts. The diverse emplacement mechanisms have important implications for connecting ophiolitic rock occurrences to large‐scale orogenic processes. In the California Cordillera, the largest bodies of ultramafic rocks are parts of ophiolite sheets, such as the Coast Range ophiolite (CRO), that were part of the upper plate of an oceanic subduction system. Such units differ from smaller bodies within subduction complexes such as the Franciscan Complex that were transferred from the subducting plate to the subduction complex during accretion. Some intra‐subduction complex ultramafic rocks occur as nearly block‐free sheets within the Franciscan Complex, and as a part of mafic–ultramafic imbricates or broken formations within the Shoo Fly Complex of the northern Sierra Nevada. Franciscan Complex serpentinite also occurs as sedimentary serpentinite mélange that was partly subducted after deposition in the trench via submarine sliding. Such mélanges include blocks that record older and higher grade metamorphism than the matrix. Sedimentary serpentinite mélange that includes high‐pressure metamorphic blocks is also found in the basal Great Valley Group forearc basin deposits depositionally overlie the CRO. Distinguishing the different serpentinite origins is difficult in the California Cordillera even though a terminal continental collision did not affect this orogenic belt. In more typical orogenic belts with greater post‐subduction disruption, distinction between the types of serpentinite occurrences presents a greater challenge.     

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (??_in) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in ??_in signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the ??_in pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al₂O₃, TiO₂) and ??_in for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations ??_in may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.     

泰国Khorat盆地二叠纪石灰岩的重磁化研究

泰国Ｋｈｏｒａｔ盆地西部的晚二叠世石灰岩的古地磁研究表明磁铁矿为稳定剩磁的主要载体多组分磁分量分离技术揭示了高温磁组分（或高矫顽力）具有呈对分布的正、反极性．但是，应用逐渐展平岩层法可以发现各采样点的平均特征磁化方向在岩层展平至３０％时．磁化方向最为集中．这一发现表明二叠纪石灰岩中所揭示出的磁化方向很可能形成于褶皱（期）过程中．野外观察表明，二叠纪石灰岩在印支期发生强烈褶皱并被晚三叠世湖相石灰岩角度不整合覆盖．所以二叠纪石灰岩的重磁化很可能发生在中、晚三叠世的印支期．这些石灰岩样品切片后经显微镜、扫描电镜和电子探针分析，次生磁铁矿多数与方解石微晶和铁质碳酸钙粒共生，且多分布在方解石脉附近．重磁化很可能是由于印支期造山运动时，铁质碳酸钙受碳水化合物流体的蚀变作用所引起的。     

碳酸盐岩裂缝性储层测井识别及评价技术综述与展望

由于碳酸盐岩裂缝发育机制的复杂性,开展碳酸盐岩裂缝性储层的识别与评价一直是测井分析的热点和难点.本文基于大量文献调研,梳理了碳酸盐岩裂缝的定义、成因和分类,系统归纳了碳酸盐岩裂缝性储层的测井响应特征及评价方法,并进行了实例分析.研究表明,常规测井方法对碳酸盐岩裂缝均有不同程度的揭示,基于不同原理的常规测井方法对碳酸岩盐裂缝的响应程度差异显著,利用常规测井方法对碳酸岩盐储层裂缝识别应遵循综合识别原则;碳酸盐岩裂缝成像测井识别方法中,电阻率成像测井、偶极声波测井和核磁共振测井反映裂缝较为直观,但成本高;基于常规测井资料的灰色关联识别方法、神经网络识别方法、小波多尺度识别方法等非线性数学方法,弥补了常规测井和成像测井识别碳酸盐岩裂缝的不足,并且取得较好的应用效果.     

The significance of tectonic-related tertiary remagnetization along the margins of the Valencia trough

The paleomagnetic data from the margins of the Valencia Trough are derived from Mesozoic and Tertiary rocks from the Balearic Islands, Catalan Coastal Ranges and Eastern Iberian Chain. These rocks are affected by a complex structural evolution consisting of an initial compressive stage followed by one of extension. Cenozoic paleomagnetic data indicate that rotations occurred during the Paleogene compression and before the extension started (Lower Miocene) in the Catalan Coastal Ranges. In contrast, in the Balearic Islands the rotations are synchronous to both compressional (Late Oligocene-Middle Miocene) and extensional tectonics (post-Middle Miocene). In both areas the Mesozoic limestones are remagnetized. In the Catalan Coastal Ranges they display the same direction as the Paleogene syn-compressive deposits whereas in the Balearic Islands they conform with Lower Miocene (Burdigalian) syncompressive rocks. It is concluded that the processes of remagnetization that affected eastern Iberia are related to a compressive rather than an extensional tectonics regime     

Origin of the magnetization of Permo-Carboniferous sediments of Spitsbergen, Svalbard Archipelago

Rock magnetic investigations of Permo-Carboniferous carbonate sediments from two areas on Spitsbergen are described, conducted to identify the carriers of the NRM in these rocks. Since microscopic and magnetic separation techniques could not profitably be applied, the nature of magnetic minerals was investigated by thermal demagnetization of the NRM and decay of saturation isothermal remanence (I_rs) during heating to 600°C, as well as by the distribution of the median destructive fields of the NRM and observation of magnetic susceptibility after subsequent heatings. The results show that the NRM of these limestones resides mainly in magnetite, but creation of magnetic pyrrhotite and of fresh magnetite is observed during heating to 600°C. Presence of sulphides indicates that magnetite is an oxidation product of pyrite or of non-magnetic pyrrhotite. Examination of rock magnetic properties of limestones leads to the conclusion that most of the magnetite in the rocks of the Bellsund area is of detrital origin, whereas the rocks at Festningen contain magnetite derived from pyrite probably during an early stage of the diagenetic process.     

Gas formation mechanism of marine carbonate source rocks in China

It has been proven in exploration practice that thecarbonates in China not only can generate hydrocar-bons, but also form commercial reservoirs. The car-bonates are different from clastic rocks in view of theirdeposition environment as well as their sedimentaryand diagenetic processes. Therefore, the evaluationcriteria and hydrocarbon generation mechanism forcarbonates can not be the same as that for clasticrocks, and it is important to establish a special hydro-carbon generation mechanism and…