Geochronometric and lead isotope data on samples from the Wallace 1 degree by 2 degrees Quadrangle, Montana and Idaho
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Abstract:
Most of the bedrock in the Wallace quadrangle belongs to the Belt Supergroup, a thick (about 18,000 m) sequence of generally fine-grained clastic and carbonate rocks of Middle Proterozoic age. Regional metamorphism prior to Cambrian time prograded the Belt rocks to greenschist facies, and some metal-bearing veins were emplaced in fractures. The Belt rocks were intruded in Late Proterozoic time by basic dikes and sills.Keywords:
Quadrangle
Greenschist
Sill
Dike
Dike
Sill
Large igneous province
Magma chamber
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The Middle Precambrian diabase dikes of the Highland Range in southwestern Montana are moderate- to high-TiO 2 continental tholeiites and are related along a differentiation trend involving strong iron enrichment. Postmagmatic metamorphism and K, Rb, and Sr metasomatism have altered the chemical composition of the igneous rocks of some of the samples. The metamorphic assemblage in the diabase dikes belongs to the low-pressure calcic plagioclase – actinolite hornfels facies, and we suggest that thermal effects associated with the intrusion of the Boulder Batholith are responsible for the metamorphic overprints in these rocks.Combined chemical data from the diabase dikes in the Highland Range, the Ruby Range, and the Tobacco Root Mountains produce smooth differentiation trends for most major oxides and trace elements, and we conclude that one magma was responsible for the dikes in the three ranges. Discrepancies in Rb–Sr age dates obtained for the dikes in the Tobacco Root Mountains can be explained if a Rb, Sr, and K metasomatic event like the one observed in the Highland Range had occurred in the Tobacco Root Mountains as well.Structurally, the diabase dikes in the Highland Range intruded into both east–west- and northwest-trending fractures at the same time. All dikes dip steeply to the north or northeast and are believed to have intruded into tensionally opened fractures related to the opening of the Belt Basin.
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The stratigraphic sequence of the Precambrian complex of the western Sangilen Highlands, Tannu Tuva, include schists, schistose sandstones, fine-gravel conglomerates, micaceous and ferruginous quartzites, graphitic marbles, and gneisses of the Teskhem, Mugur, Balyktyghkem, Chartyss, and Naryn formations, which grade upward into one another without break. Deposits containing Lower Cambrian fossils are also known in the area. The ferruginous quartzites, associated with the upper Mugur, mark the principal boundary in Precambrian sedimentary accumulation and indicate the change from terrigenous sediments to calcareous marine sedimentation. The degree of metamorphism varies considerably, both horizontally and vertically. Terrigenous deposits correspond to the green-schist facies. Local variations in the metamorphism of calcareous deposits apparently result from the variability of original components. Also important is the superimposed injection metamorphism associated with Proterozoic intrusions. --Editor.
Terrigenous sediment
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塔里木大火成岩省是我国境内发现的两个重要的二叠纪大火成省之一,不仅有大规模的溢流相玄武岩喷发,还有复杂多样的侵入岩及其组合。以往的研究主要集中在二叠系等显生宙地层发育区,对于前寒武系等古老层系中是否存在二叠纪岩浆侵入未引起重视。本文通过野外地质调查,在阿克苏地区前寒武系中识别出数条侵入南华系的基性岩墙和较大规模侵入震旦系的基性岩床。岩墙、岩床与围岩之间均显示出典型的侵入接触关系,包括岩体内部结晶粗、边部结晶细的淬冷边结构、岩床顶底面显著的切层现象以及包裹围岩团块等;显微镜下观察表明,辉绿岩具有典型的辉绿结构和嵌晶含长结构。锆石U-Pb年代学结果表明,侵入下震旦统的辉绿岩岩床和侵入南华系的辉绿岩岩墙的侵位时代在误差范围内基本一致,约为290Ma。岩石地球化学特征表明,辉绿岩岩床具有与洋岛玄武岩(OIB)相似的稀土及微量元素配分模式,结合Sr-Nb-Pb同位素测试结果,认为它们来源于碳酸盐化的富集型岩石圈地幔部分熔融,而辉绿岩岩墙具有Nb、Ta、Zr、Hf等负异常的地球化学特征,可能来自曾被俯冲相关流体/熔体交代的岩石圈地幔源区。本文的早二叠世辉绿岩岩床和岩墙的形成时代、岩浆源区和岩浆演化特征、形成构造背景与塔里木大火成岩省镁铁质岩石十分相似,在空间发育上毗邻。因此认为这些侵入的基性岩应属于塔里木二叠纪大火成岩省早期基性岩浆活动的重要组成部分,即塔里木大火成岩省早期不仅有大规模玄武岩喷发,而且在前寒武系等古老层系还存在显著的基性岩浆侵入。本文的研究丰富了塔里木大火成岩省的岩石类型和空间分布认识,并增进了对二叠纪岩石圈深部壳幔作用过程的理解。;The Tarim Large Igeneous Province is one of the two important Permian Large Igeneous Provinces discovered in China, which not only has large-scale overflow basalt eruptions, but also has complex and diverse intrusive rocks and their assemblages. Previous studies mainly focused on the Permian and other Phanerozoic strata, but did not pay attention to the existence of Permian magmatic intrusions in the Precambrian strata. Based on geological survey of field sections, we identified two types of basic rocks in the Precambrian sedimentary strata in the Aksu area, i.e., the basic dikes intruding into the Nanhua System and the basic sills intruding into the Middle and Lower Sinian System. These basic dikes and sills show typical intrusive contact relationship with wall rocks, which is supported by the quenched edge structure showing coarse and fine crystals respectively in the interior and edge of the sills, truncation of the bottom and top of the sills against the sandstone layers, and these basic intrusive rocks wrapping the wall rocks mass, and so on. The microscopic study shows that the basic intrusive rocks are mainly characterized by typical diabasic structure and poikilitic structure. Zircon U-Pb geochronological results show that the intrusive ages of the Lower Sinian diabase sills and the Nanhua diabase dikes are generally consistent within the test error range, about 290Ma. The geochemical characteristics of the diabase sills show that the rare and trace elements distribution pattern of the diabase bed is similar to that of the Ocean Island basalt (OIB). Combined with the Sr-Nb-Pb isotope test results, it is believed that the diabase sills are derived from the partial melting of the enriched lithosphere mantle due to carbonation. The diabase dikes have the negative anomaly characteristics of Nb, Ta, Zr and Hf, and they were derived from lithospheric mantle sources that had been metasomatized by subduction-related fluids/melts. In addition, these Early Permian diabase dikes and sills are very similar to the mafic rocks of the Tarim Large Igneous Province including their formation age, magmatic source, magmatic evolutionary characteristics and tectonic setting, and their adjacent spatial development. Therefore, we believe that these intrusive basic rocks belong to the important component of the early basic magma of the Tarim Large Igneous Province. This means that there are not only large-scale basaltic eruptions, but also a large number of basic intrusions in Precambrian strata during the early stage of the Tarim Large Igneous Province. This study enriched the rock types and spatial distribution of the Tarim Large Igneous Province, and promoted our understanding on the Early Permian crust-mantle interacting processes in the deep lithosphere.
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The Highland Mountains of southwest Montana lie north of the Ruby Range and west of the Tobacco Root Mountains in the Wyoming province of North America. The Precambrian metamorphic basement rocks of the Highland Mountains have been folded into a doubly plunging antiform that trends northeast to east and forms the Highland Mountains dome (O’Neill et al., 1988). O’Neill et al. (1988) classify the Precambrian rocks into five map units: three principal gneisses, a suite of igneous rocks, and well-layered amphibolite. The igneous suite is composed of metabasite dikes and sills (MMDS), aplite and granitic pegmatite sills and minor dikes, and diabase dikes. According to O’Neill et al. (1988), “... a preliminary U-Pb zircon geochronologic study of the crystalline basement rocks of the Highland Mountains document a major 1.8 Ga penetrative metamorphic event.” Robertson (2000) concludes that the basement rocks reached the first sillimanite isograd of amphibolite grade metamorphism. This study looks at the metamorphosed mafic dikes and sills (O’Neill et al.’s metabasites) of the Highland Mountains.
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Dike
Pegmatite
Basement
Batholith
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The Epworth and Goulburn Groups comprise the lowermost sequences of the Proterozoic strata exposed near the northern margins of the Slave (structural) Province of the Canadian Shield. Each group is at least 4600 m thick and underlies a distinct and separate area of not less than 13 000 sq. km.The many features common to the two groups indicate that they are correlative: (1) each lies unconformably on an Archean basement; (2) each is overlain unconformably and successively by kaolinitic sandstone, by dolomite, and by Coppermine River Group basalt and sediments; (3) each has argillite and quartzite near the base, interbedded argillite and limestone in the upper parts, and sandstone at the top; (4) each exhibits features characteristic of deposition in shallow water; (5) both occupy similar structural basins and show the same style of folding and faulting; (6) both are traversed by gabbro dikes and sills of similar age.Field relationships among the Proterozoic strata and the relation these strata bear to intrusive granite, dikes, and sills of known radiogenic age, define the Epworth and Goulburn Groups as of Aphebian age.
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Basement
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Country rock
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The exceptional 3-d exposures of the mid-Tertiary intrusive sheet network on the southern margin of Mount Hillers, Henry Mountains, southern Utah, have undergone no syn- or post-emplacement deformation. The sills and dikes, which formed above the underlying Mount Hillers laccolith, therefore provide an ideal opportunity to study purely magmatic processes in a shallow crustal intrusive sheet network. For this study, field work and laboratory analysis were employed to constrain the timing, emplacement mechanisms, and internal flow characteristics of these sills and dikes. Detailed geologic mapping of cross-cutting relationships, in addition to qualitative textural analysis in the field, indicate that younger, relatively fine-grained dikes cross-cut older, relatively coarse-grained sills. Crystal size distribution, thin section petrography, and major and trace element geochemistry all suggest two distinct batches of magma (one coarse- and one fine-grained) were involved in the construction of the sill/dike complex. Field fabrics and anisotropy of magnetic susceptibility fabrics suggest complex internal flow of the intrusive sheets throughout the growth of the central intrusive igneous body. Field observations indicate that intruding magma exploited radial fractures and bedding planes in the sedimentary host rock. In addition, rigidity contrasts in the host rock were likely an important control on the stratigraphic level of sill emplacement and on intrusive sheet thickness. The proposed construction model for the intrusive sheet network consists of an initial phase of dike-fed sill emplacement in subhorizontal strata. During subsequent growth of the underlying main laccolithic body, which included uplift and rotation of the overburden, continued sill emplacement was followed by radial dike intrusion. This work provides insight into the growth and evolution of shallow crustal magmatic systems, such as those that underlie active volcanoes. Â
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Diabase in the southwestern US intrudes Middle Proterozoic stratified rocks as sills and Early and Middle Proterozoic crystalline rocks as subhorizontal sheets and subvertical dikes. It is discontinuous in a broad belt extending from western Texas to southeastern California. The best known intrusions are sills in Middle Proterozoic strata in Death Valley, Grand Canyon, and central Arizona. Sparse to rare dikes in some of these strata trend mostly north but range from north-northeast to west-northwest. Diabase dikes widespread in crystalline rocks in western Arizona and adjacent parts of southeastern California strike from north to west-northwest, but are predominantly northwesterly. Dikes and sheets are also present in crystalline rocks in the southern Pinaleno Mountains, southeastern Arizona, where dikes strike west-northwest. The northwest trend of the diabase province and prevalent northwesterly trend of dikes in crystalline rocks suggest that intrusion was controlled by an approximately horizontal least compressive stress field roughly parallel to the Grenville Front. Radiometric ages of Arizona and California diabase indicate emplacement at [approximately]1,100 Ma. Paleomagnetic poles from diabase sills and enclosing stratified rocks in Arizona correlate with poles reported from middle and early-late Keweenawan rocks of Lake Superior. Emplacement of the diabase coincides with: (1) the middle Keweenawanmore » eruptive and intrusive episode of the Midcontinent Rift System; (2) a major episode of (middle) Grenville thrusting and deformation documented in the Van Horn area; and (3) a time of abrupt reversal in North American apparent polar wander. These interrelated manifestations presumably arose in response to a major episode of plate interaction and collision between North American and a plate that encroached from the southeast.« less
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Migmatite
Basement
Muscovite
Cordierite
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