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Rock Alteration


Sulfide Mineralization


The Askot Crystallines

  The Askote Crystallines, consisting of a variety of quartzo-feldspathic schists, granitic gneisses, migmatites, and hybridized rocks, are hosts to epigenetic sulfide mineralization near the township of Askote.  The area under present investigation is located on the slightly-overturned northern limb of the Askote synform, near its south-eastern closure (see map).  The rock types consist of low- to high-grade schists of pelitic composition near the thrust contact, and grade upwards into rocks of granitic composition and gneissic texture.  They appear to be the metamorphosed equivalents of a layered sequence of argillaceous sediments.  The metamorphic effects observed reflect the phenomenon of inverse metamorphism in which rocks with low metamorphic grades occur at the base, while those with higher grades lie higher up the sequence.  The occurrence of sericite-chlorite schists at the base of this unit is of significance and it appears that mylonitization along the thrust plane has brought down the grade of metamorphism.

Dioritic sills, altered to amphibolites, and having a uniform composition throughout, mark the contact of the Crystallines with the underlying Sedimentaries.  They also occur interbedded with the underlying quartzites.  The contact of the Sedimentaries and Crystallines is thus drawn above the dioritic amphibolites.  Near the contact zone, the Crystallines consist of contorted garnet schists with large, clearly rotated garnets.  Just at the contact, the garnet schists are intercalated with chloritoid phyllites.  Shear effects are evident around these phyllites, thus indicating a profound shearing in this part.  Large impersistent lenses of medium grained augen gneiss containing abundant biotite are frequently seen in the schists near the contact.  These may have constituted a single unit that was broken up due to shearing and granulation accompanied with the folding and thrusting of the Crystallines over the Sedimentaries.  Boudinage or 'Sausage structure' is well exhibited by competent layers of graphitic shaly bands upto 3 cm. thick, within the schists.  The traces of the boudins mark the relicts of sedimentary layering.

The sericite-chlorite schists grade upwards into light coloured medium grained muscovite-biotite schists, and thence to granitic rocks consisting of garnet and feldspar-bearing muscovite-biotite augen gneiss, permeation gneiss and migmatites.  There are no sharp contacts, the change from low to high-grade metamorphic assemblage and pelitic to granitic composition is perfectly gradational, and it appears that the rocks have undergone a progressive granitization with an increase in the metamorphic grade.  On the basis of textural characteristics the granitic rocks are classified into three groups -- augen gneiss, permeation gneiss, and migmatites.  There is a complete concordance in the foliation of the pelitic and granitic rocks in the area under investigation.  Discontinuous patches of the pelitic rocks within granitic ones are occasionally seen (see map), the trend of these being identical with the host, thereby suggesting a coherence of rocks throughout transformation.  Near the contact with the schists, the granitic rocks occasionally contain very thin concordant veins of basic material that shows no reaction with the walls whatsoever.  The veins consist of hornblende (50%), calcic plagioclase (40%), and opaques (10%).

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S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150