Deformation of granitic rocks across the brittle-ductile transition
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Mylonite
Greenschist
Orthoclase
Mylonites in the Dabie orogen are fault and shear zone related rocks formed under metamorphic conditions.They can be divided into greenschist,amphibolite,granulite,and eclogite facies mylonites.The main criterion for identifying the mylonites in the Dabie Mountain is the ductile(plastic)deformation of index minerals of the constituent metamorphic facies.This is clearly different from the former definition and description of mylonite.Formation epoch of the mylonites in the Dabie Mountain was approximately the same as(or a little later than)that of peak metamorphism of their host rocks.From older to younger,the series follows the order eclo-gite facies mylonite→granulite facies mylonite →amphibolite facies mylonite→greenschist facies mylonite.According to lithology and texture,the mylonites can be further divided into second and third subclasses if necessary.Former mylonites of eclogite and granulite facies are always superimposed by the later facies and show the appearance of amphibolite or even greenschist facies mylonites;the earlier formed mylonites are thus preserved as relics in their retrograded products.Mylonites in the low-grade metamorphic rocks in southern and northern Dabie Mountain only experienced the metamorphic history of greenschist and amphibolite facies mylonitization during the Paleozoic-Triassic period.
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Orthoclase
Alkali feldspar
Batholith
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Misho Granitoid pluton is located in NW of urumieh Lake. Main Mineral of pluton are Guartz , Alkaline feldspar in microcline and orthoclase types , Two Type of biotite and plagioclase in Range Albite – oligoclase . plagioclase and alkaline feldspar Have Mirmekitic and pertitic textures respectively. Microcline shows seprated Mineral and Patches shapes . also plagioclase are seprateds Mineral . Mirmekite Show circle Mineral in around and center of plagioclase. New euhedral zircon and apatite are inclusions in biotite, plagioclase and quartz. Geochemical charactrisic of pluton’s samples are ploted in syn clossional environment. Patches of microcline within orthoclase and mirmekitic textures are attributed to syn crystalization and Those textures are not affected by subsulidus changings. Samples of this pluton show Minralogical and Textture syn crystalization. Main factor in generation this condition are quick arising the melt.
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Strain in a narrow mylonite zone has been estimated from deformed garnetiferous porphyroblastic aggregates and from preferentially oriented plagioclase porphyroclasts with high aspect ratios. In the undeformed metamafic dyke hosting the mylonite, the mineral aggregates have spheroidal to slightly oblate shapes, and plagioclase is nearly randomly oriented. In the mylonite, the mineral aggregates are prolate ellipsoids, and plagioclase in the aggregates and matrix is symmetrically oriented about the mylonitic planar fabric. Comparison with the radii of spheres of equal volume shows that the ellipsoidal mineral aggregates underwent triaxial strain, with maximum extension of 50–140% parallel to X and with shortening of up to −30 and −45% parallel to Y and Z, respectively. The maximum strain ratio varies between 1.9 and 4.2 (mean of 10 measurements = 3.1). The orientation and aspect ratios of elongate plagioclase grains measured in the X–Z plane indicate an intermediate value (2.7) for the strain ratio. Plagioclase deformation was apparently accommodated by dislocation glide on (010), recovery processes (subgrain rotation), and microcracking. The effects of mechanical anisotropy in plagioclase, however, were subordinate to the strain regime, strain ratio, and initial aspect ratio of grains in determining the final aspect ratio and rest position of these porphyroclasts.Both the deformed garnetiferous aggregates and the plagioclase porphyroclasts record state of strain in the mylonite. This suggests that the preferred orientation of densely packed feldspars of high aspect ratio potentially may be used to estimate strain in tectonites.
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Authigenic
Anorthite
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Synneusis, the drifting together and systematic attachment of crystals in a melt, is involved in three of the known origins for mantled feldspars: (1) overgrowth, (2) exsolution, and (3) filling of skeletal plagioclase. Quartz latite dikes of the Chocolate Mountains, California, contain plagioclase crystals in parallel and twinned synneusis. Sanidine mantles consisting of segments, each crystallographically parallel to the adjacent plagioclase crystal, enclose synneusis structures. One-feldspar biotite granite of the Golden Horn batholith, Washington, has mantled feldspars with complex, multiple albite mantles formed by synneusis of zoned alkali feldspars followed by subsolidus exsolution. Skeletal plagioclase formed by resorption is filled with orthoclase in two-feldspar biotite granite of the Golden Horn batholith. Synneusis, before and after resorption, produced mantled feldspars with systematic crystal orientations. Wiborgite granite of Finland has anhedral orthoclase enclosed in large numbers of plagioclase crystals. An origin by synneusis is suggested for this texture.
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Batholith
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A new equilibrium diagram for the system Or: Ab: An has been constructed on the basis of the feldspar content of a large number of rocks. It indicates that an increase in the anorthite content of the plagioclase present corresponds to an increase in the possible maximum of orthoclase in the rocks. Thus the eutectic point between ortho-clase and albite lies at about 45 Or: 55 Ab, whereas the boundary between the orthoclase and plagioclase fields in the vicinity of andesine is near 70 Or: 30 ().
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Orthoclase
Dacite
Alkali feldspar
Phenocryst
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Petrology and structure of La Chilca Shear Zone, Catamarca. Field, petrological and structural studies allowed to define and and to characterize a ductile shear zone on the metamorphic basement of the northwestern Sierras Pampeanas. La Chilca Shear Zone has a NNW trend for about 24 km and a width of 1-4 km in the southern border of the Sierra de Aconquija and northern part of the Sierra de Ambato. It is composed by mylonites, protomylonites and blastomylonites that are the product of the ductile deformation that affected the metamorphic basement originally composed by migmatites and gneisses. The metamorphic conditions during the deformation reached greenschist facies to amphibolite facies, with temperatures around 350 to 500 oC. The dominant mylonitic foliation C is generally trending N 330°/42° ENE. On this plane there is a stretching mineral lineation of N68°/42°. Reverse displacements toward the southwest (N 248°) were determined for this shear zone. Internal local features of direct displacements suggest some complexities in the deformation kinematics.
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