| Abstrakt: |
The mineral assemblages within ductile shear zones, which act as open systems, may be strongly influenced by the occurrence of mass transfer processes induced by the infiltrated composite H2O-rich (+ ±boron ±Al) fluid fluxes and mobilising major elements. Metasomatism is the re-equilibration of a rock involving a change in the chemical composition through the interaction of parent rock with an external fluid phase. In the Sakoli Fold Belt of central India, sillimanite occurrences are associated with two major ductile shear zones, viz., one near the eastern margin, trending N–S; and other, near the northwestern margin trending NE–SW formed contemporaneous with D1 and D3 deformations, respectively. The shear zone rocks include basement slices of granodiorite gneiss, quartzite, quartz vein and intrusive granite. Aluminosilicates of andalusite and sillimanite occur in two mutually exclusive domains. While andalusite is ubiquitous in the host metapelites of Sakoli Group, sillimanite is restricted within the shear zones, preferably in mylonitized granite protoliths. Other minerals associated with sillimanite include abundant fibrolite, tourmaline, dumortierite, rutile, sphene, pyrophyllite, quartz, rare biotite and garnet, together with and secondary muscovite (after feldspar) and kaolinite, etc. It is indicative of interaction between infiltrated high-temperature hydrothermal fluid derived possibly from intrusive granite and the host rocks in the shear zone. No evidence of polymorphic inversion between andalusite and sillimanite nor breakdown reaction from muscovite + quartz → sillimanite+ K-feldspar could be established. Textural evidences suggest the growth of sillimanite and associated tourmaline at two pulses, each coinciding with or outlasting the peak of shear zone deformations and metamorphism in Sakoli rocks. Ductile shear zones in the area may thus represent high alumina hydrothermal alteration zones in granitic rocks through the influx of high-temperature fluids from some deeper source. The infiltrating hydrothermal fluids rich in Al, B, and H2O were channelised along the ductile shear zones. It caused extensive chemical alteration of the host rock giving rise to sillimanite, tourmaline, kaolinite, mica, etc., as metasomatic products. The infiltrating fluid was sufficiently high in temperature (>550°C) to produce sillimanite in the later stages of prograde metamorphism of the Sakoli Group of rocks, restricted to ductile shear zones in two separate tectonic and deformation settings. The anomalous growth of sillimanite in the restricted domains was undoubtedly facilitated by high strain environment of shear zones. Highlights: Two sets of shear zones, N–S and NE–SW related to two generations of deformations, respectively, hosts sillimanite mineralisation. Textural characteristics indicate solid-state growth of sillimanite during the syn- to late-tectonic stages of mylonitization in the shear zones. Infiltrating hydrothermal fluids rich in Al, B, H2O along the ductile shear zones, caused extensive chemical alteration of host rocks. The temperature of the infiltrating fluid was high enough to form sillimanite instead of andalucite as the metastatic product. High strain within shear zones facilitated the growth. Minerals like sillimanite, tourmaline, kaolinite, mica, etc., are metasomatic products. [ABSTRACT FROM AUTHOR] |
