Structural evidence supporting a remnant origin of patchy charnockites in the Chilka Lake area, India

Abstract

Dark patches of charnockitic rocks characterized by orthopyroxene occur within garnetiferous granite gneisses (leptynites) in a granulite-migmatite suite around the Chilka Lake, Orissa, within the Eastern Ghats belt in the Indian Precambrian. Analysis of structures of different scales observed in this terrain establishes the presence of three phases of deformation. S, is pervasive in the metapelitic granulites (mainly khondalite), while in the migmatite complex composed of leptynites, charnockites and quartzofeldspathic veins, Sj is present exclusively within the charnockite lenses and bands, and shows different stages of obliteration in the associated leptynites. Thus, the charnockite patches must be earlier than the surrounding migmatitic rocks. The charnockite patches and the surrounding leptynitic gneisses are chemically quite different and the two rock types are not related by any prograde or retrograde transformation. The shapes and disposition of charnockite patches in the mixed exposures are found to be largely controlled by the third phase of folding and locally associated shearing. The kinematics of this late deformation are not favourable for fluid ingress from deeper levels.

Discussion of the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India

“…The structural evidence, such as preferred orientation of orthopyroxene crystals in the patchy charnockites, does not preclude our hypothesis of remnant origin (Bhattacharya, Sen & Acharyya, 1993). Dobmeier & Raith emphasized the difference in signatures of deformation in large enderbite layers and small patchy charnockites.…”

“…In the five sets of modal analyses reported in Bhattacharya, Sen & Acharyya (1993) three are distinctly charnockites in the cores, their relative alkali feldspar:plagioclase proportions being 16:20, 18:20 and 14:18. Among the other two, one with the ratio 16:25 is practically a charno-enderbite, while the other with 32:7 is an alkali feldspar charnockite.…”

“…(c) The most significant aspect of the modal data from cores, margins and detached patches of charnockites as given in table 2 of Bhattacharya, Sen & Acharyya (1993) is the variation of alkali feldspar:plagioclase ratios from cores to margins, and detached patches, as for example, in HM-1, from 14:18 (A) through 17:16 (B) to 21:9 (D).…”

“…5 in Bhattacharya, Sen & Acharyya, 1993). Dobmeier & Raith have completely overlooked this significant field feature.…”

“…1b), and we feel that the sketchy line drawing ( fig. 5) in Bhattacharya, Sen & Acharyya (1993) is hardly suitable as proof for the boudinage of enderbitic layers. Figure 1b clearly shows that arrays of arrested-type charnockite domains are oriented at high angle to the compositional layering, and parallel to the axial planar foliation of late upright folds.…”