Stages of Late Proterozoic magmatism and periods of Au mineralization in the Yenisei Ridge
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Plutonism
Orogeny
Terrigenous sediment
Covers crustal evolution and terrestrial plate tectonics operation from the early Archean to the present day, development of ideas pertaining to accumulations of tectonic models, accounts of structural development in rock of the early Proterozoic age in the northwestern peninsular Indian shield, a n
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The Animikie basin in Minnesota developed during early Proterozoic time over and approximately parallel to the Great Lakes tectonic zone—an Archean suture between an ancient gneiss terrane to the south and a younger greenstone-granite terrane to the north. The evolution of the basin can be divided into an extensional stage with attendant deposition of stratified rocks, and a subsequent compressional stage assigned to the Penokean orogeny.
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Two major supracrustal sequences, the Huronian Supergroup in Ontario and the Marquette Range Supergroup and Animikie Group of Michigan and Minnesota, overlie an Archean basement. These sequences are about 2200—2300 Ma and 1900-2000 Ma old respectively. The major Early Proterozoic tectonic event is the ‘Penokean Orogeny’, which occurred about 1850-1900 Ma ago and included deformation, high-grade regional metamorphism, and extrusive and intrusive igneous activity. This was followed by formation of rhyolitic, ignimbritic volcanic rocks and emplacement of associated granites about 1790 Ma ago. The entire region was subsequently subjected to low-grade regional metamorphism 1650-1700 Ma ago, followed by emplacement of anorogenic quartz-monzonite, in part rapakivi, plutons 1500 Ma ago. Late Proterozoic Grenville and Keweenawan events represent the youngest major Precambrian activity in the region. The rocks involved in the Penokean Orogeny lie along the southern margin of the Archean craton of the Superior Province and are interpreted as representing Early Proterozoic cratonic-margin orogenic activity. The distribution of rocks types and structures associated with the Penokean Orogeny and with similar orogenic belts along the margin of the Archean craton of North America suggest that these orogenic belts may have formed as a result of processes similar to modern plate tectonics, although the data are far from conclusive at present.
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Seismic reflection data from the northeast corner of Banks Island in arctic Canada show that Proterozoic layers were subjected to at least two deformational events prior to the Cambrian. The oldest of these events involved folding and faulting(?) from west-northwest to east-southeast. The ages of the layers that were subjected to this deformation are unknown; however, younger Proterozoic layers, probably correlative with the Shaler Group (ca. 1.1–0.8 Ga), unconformably overlie the folded and faulted layers, and were themselves regionally arched prior to the Cambrian. The relative position of the oldest deformed strata is thus the same as that of the strata observed to the south beneath the Anderson Plains, where thrust faulting and folding deformed pre-Mackenzie Mountains Supergroup (Shaler Group equivalent) layers during at the time of the Racklan orogeny (ca. 1.1–1.2 Ga). Distant effects of the Racklan orogeny may therefore extend to at least the northeast side of Banks Island.
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Summary A general survey is first made of certain aspects of time in plutonism. These include the duration of plutonic times, their limits and subdivisions, the different dates concerned in them, and the unity or otherwise of plutonic processes. More specific inquiries begin with the consideration of time and crystallization. The criteria for the determination of time-sequences in the crystallization of the plutonic rocks are reviewed and examples of their application given. The more complex question of the time-relations of crystallization and deformation is next taken up, chief attention being given to the evidence for the rotation of porphyroblasts. The diagnostic requirements for pre-crystalline, para-crystalline and post-crystalline deformations are examined. The inquiry next moves to time-relationships of a different order, those relating plutonism and orogeny. The implications of folding of a metamorphic succession and the validity of inversion in the crystalline schists are discussed. The importance of metamorphic history, facies and convergence in geological interpretation is illustrated by examples from Scotland, central Europe and elsewhere. The problem of the subdivision of the metamorphic history of a rock and the evidence for monometamorphism or polymetamorphism are considered and examples from the Scottish Highlands are discussed. As the whole operation of plutonism depends upon the activity of migma-magma, it is necessary to deal with the time-relations of crystallization, deformation, granitization, migmatization, metamorphism, intrusion and orogeny. The general principles of the relation between magmatism and orogeny are explored. The micro tectonic and deep-tectonic studies of Demay are summarized and discussed— the discussion leading on to the consideration of the characters of synkinematic intrusion and of synkinematic permeation. Finally, the Granite Series is displayed and discussed. This attempts to relate the plutonic phenomena at the various levels of exposure, and to give a unity to the processes of granitization, migmatization and metamorphism at depth and at successively higher positions and later times. The series begins with the autochthonous granites associated with niigmatites and metamorphites produced in situ in the depths. The movement of the granitized material provides the successive terms of the series. The parting of envelope and core, the structural control of movement, straining off, selective mobilization, overtaking and chemical variation in the series are discussed. The Granite Series ends with the late high-level plutons emplaced as viscous masses. An example of the Granite Series from the Hercynian belt of north-western Europe is presented. The plutonic developments in the Massif Central, Armorica and South-West England are compared. In this belt, the,later members of the Granite Series appear at higher levels as we pass from south to north, and there is a progressive change in the style of contact in the same direction. The higher and younger members to the north have left their deeper and earlier roots in the Massif Central. At the three or four sample levels of the Granite Series provided by this example, fundamental differences of pattern reveal the relationships between place and time in the making of the patterns.
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Field, petrology, and age data on southern Alaskan plutonic rocks now enable the delineation of eight calc-alkaline plutonic belts. These belts of plutons or batholithic complexes are curvilinear to linear and trend parallel or subparallel to the continental margin. The belts represent the principal loci of emplacement for plutons of specific ages, and although there is spatial or temporal overlap in some cases, they are, more commonly, spatially and temporally distinct. Intermediate lighologies, such as quartz diorite, tonalite, and granodiorite, dominate in most of the belts, but granodiorite and granite characterize one. The belts are of Mesozoic or Cenozoic age,...
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Recent structural, petrological and geochronological work on plutons in eastern Borborema Province (NE Brazil) has helped to constrain its tectonic evolution during the Brasiliano/Pan-African Orogeny. 645-630 Ma-old plutons have been intensely affected by solid-state deformation. They show an early, shallowly to moderately-dipping foliation conformable with the regional fabric of country rocks, which is overprinted by a steeply-dipping foliation related to strike-slip shearing. Two compositional groups are recognized: (i) medium-K, epidote-bearing granodiorites, and (ii) coarse-grained granites and associated diorites. 590-580 Ma-plutons were emplaced before or at the very early stage of movement along large transcurrent shear zones, and include high-K calc-alkaline plutons and metaluminous syenites. The 590-580 Ma-old plutons underpin the change from a low-angle tectonic event to transpressive deformation. Once emplaced, these plutons in turn favored the localization of strain needed to promote nucleation of the spatially associated shear zones. Finally, two-mica leucogranites and dike swarms of widely varying compositions were emplaced around 570 Ma at the advanced stages of motion along transcurrent shear zones.
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