70 m.y. of synorogenic plutonism in eastern Borborema Province (NE Brazil): temporal and kinematic constraints on the Brasiliano Orogeny
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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.Keywords:
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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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The generation of pegmatite dikes during the cooling and crystallization of granitic plutons has been calculated using new models for the prediction of granitic melt viscosities and the propagation of dikes. These new models suggest that early in the cooling history of a modeled l0 x l0 x l0 km pluton, dikes cannot propagate, or will be short (on the order of I km), because dl surreunding country-rocks have not yet been significantly heated. However, dikes formed tens to hundreds of thousands of years after intrusion can propagate up to approximately 10 km. Because the far-propagating dikes form late in the magmatic history of the pluton, they will be composed of chemically more evolved magmas than the bulk of the pluton and will crystallize as pegmatites. The model predicts that pegmatites should only rarely be found more than ca. l0 km from their host pluton, that more-evolved pegmatites should be found at greater distances from their host pluton than less-evolved ones, and that pegmatites should not be associated with small plutons. All of these model results are consistent with field observations, and support the petrogenetic relationship between granitic plutons and the evolved pegmatites surrounding them.
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A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the "western Klamath plutonic suite" was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common...
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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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ABSTRACT The Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.
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