Petrology and geochemistry of the Karaj Dam basement sill: Implications for geodynamic evolution of the Alborz magmatic belt

The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low I_Sr = 0.70453–0.70535, ɛNd (37.2 Ma) = 1.54–1.9, and T_DM ages ranging from 0.65 to 0.86 Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of ²⁰⁶Pb/²⁰⁴Pb = 18.43–18.67, ²⁰⁷Pb/²⁰⁴Pb = 15.59, and ²⁰⁸Pb/²⁰⁴Pb = 38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma.     

Coupling forward modelling of garnet growth with monazite geochronology: an application to the Rappold Complex (Austroalpine crystalline basement)

Results from forward modelling of garnet growth and U–Th–Pb chemical dating suggest three periods of metamorphism that affected metapelitic rocks of the Rappold Complex (Eastern European Alps). Garnet first grew during Barrovian-type metamorphism, possibly during the Carboniferous Variscan orogeny. The second period of metamorphism produced monazite and resulted in minor garnet growth in some samples. Variable garnet growth was controlled by changes to the effective bulk rock composition resulting from resorption of older garnet porphyroblasts. Monazite crystals have variable morphology, textures and composition, but all yield Permian ages (267 ± 12 to 274 ± 17 Ma). In samples in which there was Permian garnet growth, monazite forms isolated and randomly distributed grains. In other samples, monazite formed pseudomorphous clusters after allanite. This difference is attributed to higher transport rates of monazite-forming elements in samples which underwent dehydration reactions during renewed garnet growth. The third and final period of garnet growth took place during Eo-Alpine (Cretaceous) metamorphism. Garnet of this age displays a wart-like texture. This may reflect transport-limited growth, possibly as a result of repeated dehydration during polyphase metamorphism.     

The Application of Basement Tectonic Research to the Development of Natural Resources—Example Midcontinent North America

An understanding of an area in four dimensions is an important factor in utilizing our natural resources. The additional aspect of change through time, particularly the tectonic processes that have shaped the architecture of an area, can influence the interpretation of the origin and characterization of a resource. An example is provided of the influence that the patterns created during the formation of the continent in central North America demonstrates the continued influence of the original tectonic features and how they have persisted through time. It is this persistence and rejuvenation, that has controlled the occurrence of many of the natural resources on which we depend. Other references are provided to specific examples of the relationships between tectonics, particularly within the crystalline basement rocks, and our natural resource system.     

Neoproterozoic and Cambro-Ordovician magmatism in the Variscan Kłodzko Metamorphic Complex (West Sudetes,Poland): new insights from U/Pb zircon dating

The Kodzko Metamorphic Complex (KMC) in the Central Sudetes consists of meta-sedimentary and meta-igneous rocks metamorphosed under greenschist to amphibolite facies conditions. They are comprised in a number of separate tectonic units interpreted as thrust sheets. In contrast to other Lower Palaeozoic volcano-sedimentary successions in the Sudetes, the two uppermost units (the Orla-Googowy unit and the Kodzko Fortress unit) of the KMC contain meta-igneous rocks with supra-subduction zone affinities. The age of the KMC was previously assumed to be Early Palaeozoic–Devonian, based on biostratigraphic findings in the lowermost tectonic unit. Our geochronological study focused on the magmatic rocks from the two uppermost tectonic units, exposed in the SW part of the KMC. Two orthogneiss samples from the Orla-Googowy unit yielded ages of 500.4±3.1 and 500.2±4.9 Ma, interpreted to indicate the crystallization age of the granitic precursors. A plagioclase gneiss from the same tectonic unit, intimately interlayered with metagabbro, provided an upper intercept age of 590.1±7.2 Ma, which is interpreted as the time of igneous crystallization. From the topmost Kodzko Fortress unit, a metatuffite was studied, which contains a mixture of genetically different zircon grains. The youngest ²⁰⁷Pb/²⁰⁶Pb ages, which cluster at ca. 590-600 Ma, are interpreted to indicate the maximum depositional age for this metasediment. The results of this study are in accord with a model that suggests a nappe structure for the KMC, with a Middle Devonian succession at the base and Upper Proterozoic units at structurally higher levels. It is suggested here that the KMC represents a composite tectonic suture that juxtaposes elements of pre-Variscan basement, intruded by the Lower Ordovician granite, against a Middle Palaeozoic passive margin succession. The new ages, combined with the overall geochemical variation in the KMC, indicate the existence of rock assemblages representing a Gondwana active margin. The recognition of Neoproterozoic subduction-related magmatism provides additional arguments for the hypothesis that equivalents of the Teplá-Barrandian domain are exposed in the Central Sudetes.     

Multi-electrode resistivity survey for groundwater exploration in the Harare greenstone belt, Zimbabwe

A multi-electrode resistivity survey, carried out over metasedimentary strata and metavolcanics in the Harare greenstone belt in northeastern Zimbabwe as part of a groundwater resources investigation, illustrates the ability of this technique to produce high-resolution images of the subsurface, which are useful for groundwater resources assessment. The resistivity results provide a clear view of the thickness of the weathered regolith and of the distribution of the various lithological units. Using a combination of apparent formation resistivity and inferred depth of weathering, it is possible to characterize the various lithologies on the geophysical profiles. These assigned lithologies show excellent correlation with the mapped geology, and the main lithologies, metabasalt, meta-arenite, granodiorite and banded iron formation can be clearly identified. The banded iron formation is characterized by low resistivity values, while a combination of the depth of weathering and resistivity values are used to distinguish between the meta-arenite on one hand and the metabasalt and granodiorite on the other. The multi-electrode method is successful in identifying potentially favourable zones for obtaining groundwater, such as areas with a maximum depth of weathered regolith, zones of fracturing and faulting, and high porosity and permeability zones associated with lithological contacts.

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Resumen Un estudio de resistividad multi-electrodo que se llevó a cabo en estratos meta sedimentarios y metavolcánicos en el cinturón de rocas verdes de Harare, en la parte noreste de Zimbabwe, como parte de una investigación de recursos de agua subterránea, ilustra la capacidad de esta técnica para producir imágenes de alta resolución de la subsuperficie, las cuales son útiles para la evaluación de los recursos da agua subterránea. Los resultados de resistividad suministran una visión clara el espesor del regolito meteorizado y de la distribución de varias unidades litológicas. Mediante el uso de una combinación de resistividad aparente de formación y de una profundidad inferida de meteorización, es posible caracterizar las diferentes litologías en los perfiles geofísicos. Estas litologías asignadas muestran una excelente correlación con la cartografía geológica y se pueden identificar las litologías predominantes como metabasalto, meta-arenita, granodiorita y formación de hierro bandeado. Esta última se caracteriza por valores bajos de resistividad, mientras que se utiliza una combinación de profundidad de meteorización y valores de resistividad, para distinguir entre meta-arenita por un lado y metabasalto y granodiorita por el otro. El método multi-electrodo es exitoso para identificar zonas potencialmente favorables para captar agua subterránea, las cuales pueden ser áreas con una profundidad máxima de regolito meteorizado, o bien zonas de fracturamiento y fallamiento, o también zonas de porosidad y permeabilidad altas asociadas con contactos litológicos.

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Résumé La prospection électrique par la méthode de multiélectrodes, effectuée pour linvestigation des eaux souterraines cantonnés dans des roches métamorphiques dorigine sédimentaire et volcanique de la ceinture de Harare-Zimbabwe a mis en évidence les possibilités de cette technique de réaliser des images à grande résolution du sous-sol qui sont très utiles pour lévaluation de la ressource en eaux souterraines. Daprès la distribution de la résistance il résulte une image très claire de lépaisseur des regolithes altérés ainsi que de la distribution des différentes unités lithologiques. Daprès les profiles géophysiques on peut caractériser les différents lithologies en utilisant les résistivités apparentes de la formation respective et les informations sur la profondeur de la zone altérée. Les lithologies ainsi déterminées montrent une excellente corrélation avec les cartes géologiques et on peut très bien identifier les lithologies principales comme les metabasaltes et les meta-arenites, les granodiorites et les formations de fer rubanées. Cettes dernières sont caractérisées par des faibles valeurs de la résistivité, tandis que en utilisant les résistivité et la profondeur de la zone altérée on peut distinguer les meta-arenites de granodiorites et des metabasaltes. La méthode des multiélectrodes est donc capable didentifier des formations favorables à laccumulation en eau comme les zones de faille et des fractures, ainsi que les zones à grande porosité et perméabilité, associées aux contactes lithologiques.

     

Early Mesozoic unroofing pattern of the Dabie Mountains (China): Constraints from the U-Pb detrital zircon geochronology and Si-in-white mica analysis of synorogenic sediments in the Jianghan Basin

This paper presents the results of an integrated U-Pb detrital zircon geochronology and Si-in-white mica analysis for synorogenic sediments in the Jianghan Basin to the south of the Dabie Orogen. The results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early Mesozoic Dabie Mountain. Si contents of detrital white micas range from 3.09 to 3.34 atoms pfu for the upper Triassic sandstones whereas 3.06 to 3.59 atoms pfu for the lower and middle Jurassic sandstones. The majority of detrital white micas in the lower Jurassic sandstones is phengitic and originated exclusively from the Dabie high- to ultrahigh- pressure rocks. The U-Pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late Triassic to early and middle Jurassic. For the upper Triassic sandstone, the U-Pb age clusters of these zircons are characterized by ~ 420-450 Ma, ~ 750-820 Ma, ~ 1050-1200 Ma and ~ 2500 Ma with minor Luliangian (~ 1700–2000 Ma) components. In contrast, the zircon ages of the Jurassic sandstones are dominated by the Luliangian (~ 1700–2000 Ma) ages with only minor Caledonian (~ 420-450 Ma) and Greenville (~ 1050-1200 Ma) ages. In combination with other available geological data, it can be concluded that the Dabie HP-UHP rocks might initially be exposed to the surface at the beginning of early Jurassic (~ 190 Ma). The Jiangnan terrain (also named “Jiangnan old continental in Chinese) to the south of the Jianghan basin provided the predominant supply of upper Triassic sediments, whereas the Paleoproterozoic Yangtze crustal materials (overlying the present Dabie Complex at the time) were the important provenance of the Jurassic sediments in the Jianghan basin.     

中天山基底与塔里木克拉通的构造亲缘性

中天山基底与塔里木克拉通的构造亲缘性问题涉及中亚大地构造单元的划分,倍受学术界关注.在诸多学者的研究基础上,特别是塔里木北缘物质成分和年代学成果基础上,通过区域地质调查,对托克逊县干沟与和静县阿拉沟两地的前南华纪地层序列及其岩石组合进行了专门研究,并筛选出4件浅变质砂岩样品做碎屑锆石U-Pb测年研究,获得165组年龄数据.结果表明,发育在中天山地区的前南华纪地层序列、岩石组合、地层接触关系、沉积环境与塔里木北缘的基本一致,可比性好,揭示了二者之间密切的构造亲缘性.其Th/U比值密集分布在0.4～4.0,表明岩浆锆石占绝大多数.锆石U-Pb测年产生了4个年龄峰值,分别为950Ma、1 550Ma、1 920Ma和2 480Ma,表明中天山较好地保存了元古代的4次重大构造-岩浆活动信息.这些年龄峰值在塔里木陆块均有对应岩浆体的发育,也与塔里木周缘的前寒武纪年龄谱吻合,进一步佐证了中天山基底与塔里木克拉通曾经是一个统一块体的认识.结合区域构造分析,认为中天山陆块是在南华纪以来,逐渐从塔里木克拉通拉张裂离出来的;伴随早古生代天山洋的俯冲,一个奥陶纪-志留纪火山弧发育在这个裂离的中天山陆块之上.     

基于磁力资料的珠江口盆地构造单元分布特征

珠江口盆地是南海北部陆坡最大的前新生代沉积盆地,油气资源丰富,由于新的地球物理资料未得以充分应用等问题导致盆地内部构造单元分布特征存在诸多不同看法,这些问题制约了盆地的进一步勘探和开发。本文以最新实测1∶10万高精度航磁数据为基础,采用切线法对研究区航磁异常深度进行反演计算,结合钻井、地震及南海北部陆域物性资料研究珠江口盆地磁性基底分布特征。在充分调研珠江口盆地已有构造单元划分的基础上,以盆地磁性基底展布特征为基础,结合断裂、区域构造等对珠江口盆地内部构造单元进行研究。研究表明:珠江口盆地磁性基底深度在0～9 km之间,磁性基底呈"三隆两坳"构造格局,整个坳陷区具有"南北分带,东西分块"的特征; NE向深大断裂为控盆断裂,常为盆地二级构造单元的边界,NW向断裂常控制次一级构造单元并影响其展布形态。     

鄂尔多斯盆地基底结构及早期沉积盖层演化

鄂尔多斯盆地基底主要形成于太古宙—古元古代,岩石组成极为复杂,大多经历了较强的区域变质作用,变质程度一般达到了(高)角闪岩相—麻粒岩相,属变质程度较深的区域变质岩系,主要是各种片岩、片麻岩及变粒岩、石英岩、大理石及花岗片麻岩等;基底结构具有较为明显的分区特征,可划分为北部、西北部及中南部三个大区,走向总体以北东向为主;从演化过程来看,鄂尔多斯盆地基底是华北克拉通形成过程中的太古宙微陆块之一,古元古代末可能是其与华北拼合为一体的关键时期。早期沉积盖层主要经历了长城纪陆内裂陷、蓟县纪边部沉降、青白口纪—南华纪整体隆升和震旦纪边缘拗陷4个演化阶段,沉积环境总体以海相为主,晚期局部具陆相特征,不同时期的沉积物特征有较大差异,可能主要受构造、气候环境的共同制约;早期沉积盖层整体上受基底结构和构造控制明显,越早期影响越大;盆地西南边缘一直是最活跃的构造沉降区,基底构造所处位置特殊可能是其最主要的原因。     

MATHEMATICAL GEOLOGY

正20141850 Chen Dongyue(School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China);Chen Jianping On 3D Ore Prospecting Modeling of Comprehensive Information for Huangshaping Polymetallic Deposit(Journal of Geology,ISSN1674-3636,CN32-1796/P,37(3),2013,p.489-495,12 illus.,12 refs.) Key words:polymetallic ores,data bases,Hunan Province