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Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha, Aravalli–Delhi Fold Belt, NW India

Prakash, Abhishek ; Piazolo, Sandra ; Saha, Lopamudra ; Bhattacharya, Abhijit ; Pal, Durgesh ; Sarkar, Saheli

International Journal of Earth Sciences, 2018, Vol.107(6), pp.2265-2292 [Periódico revisado por pares]

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  • Título:
    Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha, Aravalli–Delhi Fold Belt, NW India
  • Autor: Prakash, Abhishek ; Piazolo, Sandra ; Saha, Lopamudra ; Bhattacharya, Abhijit ; Pal, Durgesh ; Sarkar, Saheli
  • Assuntos: Melt-present deformation ; Anatectic migmatites ; Crystal preferred orientation ; Quartz ; Garnet ; Rheology
  • É parte de: International Journal of Earth Sciences, 2018, Vol.107(6), pp.2265-2292
  • Descrição: In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age shear zone in the Aravalli–Delhi Fold Belt. The migmatite exhibits three main deformation structures and fabrics (S 1 , S 2 , S 3) . Elongated garnet porphyroblasts are aligned parallel to the metatexite S 2 layers and contain crenulation hinges defined by biotite–sillimanite–mullite–quartz (with S 1 axial planar foliation). Microstructural evidence and phase equilibrium relations establish the garnet as a peritectic phase of incongruent melting by breakdown of biotite, sillimanite ± mullite and quartz at peak P – T of ~ 8 kbar, 730 °C along a tight-loop, clockwise P–T path. Monazite dating establishes that the partial melting occurred between ~ 1000 and 870 Ma. The absence of subgrains and systematic crystal lattice distortions in these garnets despite their elongation suggests growth pseudomorphing pre-existing 3-D networks of S 1 biotite aggregates rather than high-temperature crystal plastic deformation which is noted in the S 1 quartz grains that exhibit strong crystallographic preferred orientation (CPO), undulatory extinction and subgrains. Mode-I fractures in these garnet porphyroblasts induced by high melt pressure during late stage of partial melt crystallization are filled by retrograde biotite–sillimanite. Weak CPO and non-systematic crystal lattice distortions in the coarse quartz grains within the S 2 leucosome domains indicate these crystallized during melt solidification without later crystal plastic deformation overprint. In the later stages of deformation (D 3 ), strain was mostly accommodated in the mullite–biotite–sillimanite-rich restite domains forming S 3 which warps around garnet and leucosome domains; consequently, fine-grained S 3 quartz does not exhibit strong CPOs.
  • Idioma: Inglês

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