Two types of tourmaline with igneous and metamorphic origin in metamorphic rocks of north Golpayegan, Isfahan province
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Two types of tourmaline with igneous and metamorphic origin in metamorphic rocks of north Golpayegan, Isfahan provinceKeywords:
Tourmaline
Isograd
Inversion of metamorphic zones has been reported from the lower Himalayas by many workers. An example of such an inverted metamorphic zone has been studied in detail around Gangtok. Theoretical analysis of relationship among isograd, isotherm, and isobar leads to the conclusion that inversion of metamorphic zones may be caused under three conditions: (1) the temperature gradient is horizontal, (2) the temperature gradient is upwardly directed, and (3) slope of the isotherm is greater than the slope of the isograd, both being in the same direction. The present example is best explained as the result of an upwardly directed temperature gradient caused by distortion of the isotherms by tectonic deformation.
Isograd
Temperature Gradient
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Introduction. 1. Review of the Microscope and Mineral Optics. 2. Rock-Forming Minerals and Their Optical Properties. 3. Classification of Igneous Rocks. 4. Texture and Structures of Igneous Rocks. 5. Classification and Textures of Metamorphic Rocks. Index.
Texture (cosmology)
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Isograd
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Abstract A sharp line delimitating the distribution of tourmaline (termed as a ‘tourmaline‐out isograd’) is defined in the migmatite zone of the Ryoke metamorphic belt, Japan. The trend of the tourmaline‐out isograd closely matches that of the isograds formed through the regional metamorphism, suggesting that it represents the breakdown front of tourmaline during regional metamorphism. This is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline‐out isograd. The breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline‐bearing high‐temperature metamorphic rocks. Near the tourmaline‐out isograd, large tourmaline crystals occur in the centre of interboudin partitions containing leucosome. In the melanosome of the intervening matrix, reaction textures involving tourmaline are locally observed. These observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism. Boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt. Considering that the tourmaline‐out isograd lies close to the schist–migmatite boundary, these effects might have enhanced melt generation and segregation in the migmatite zone of the Ryoke belt. The evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and/or fluid. This implies that the tourmaline‐out isograd can reflect a significant amount of mass transfer in the anatectic zones.
Tourmaline
Isograd
Migmatite
Solidus
Sillimanite
Cordierite
Andalusite
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