The Kondapalli Layered Complex, Andhra Pradesh, India: Geology, Mineralogy and Chemistry

Abstract

The Kondapalli Layered Complex (KLC) consists of dominant gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites; clinophyroxenites; dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole, but not with olivine or plagioclase); the complex does not contain wehrlite, lherzolite and troctolite. The KLC occurs as minor bands and lenses within a region dominated by charnockites; it is variably deformed and it is cut by rare metadolerite dykes. The KLC represents a discontinuous stratiformtype complex and its disrupted fragments contain different components in disproportionate amounts (Leelanandam, 1991). Most of the KLC displays layered characters, and it essentially consists of plagioclase, orthopyroxene and clinopyroxene in different combinations, with variable proportions and with diverse textural relationships. The inferred order of crystallization was ol, ol+opx, opx(_+plag), opx+cpx(_+plag), cpx(_ plag), opx + cpx + plag, plag(+ cpx + cpx). In chromitites, chromite is not only enclosed by, but also encloses, pyroxene and amphibole; suggesting overlapping periods of crystallization of different phases. The rocks in general exhibit textures showing much subsolidus re-equilibration and cumulate textures are well preserved with adcumulus growth as the dominant form of postcumulus enlargement. Variable deformational textures are particularly conspicuous in the ultramafic rocks, and also in those rocks occurring along the marginal portions and local shear zones. The quartz-bearing (anorthositic, enderbitic and other felsic) rocks, with conspicuous deformafional and cataclastic textures, are interpreted as contaminated or mixed rocks occurring at the tectonized junction zones between the KLC and enclosing charnockites. Symplectic and coronitic textures are typically absent in the rocks from KLC. The KLC is the only one of its type in the entire Eastern Gha t s mobi le belt o f Pen in su l a r Ind i a (Leelanandam, 1990). The 45 whole rock analyses from the KLC represent a fairly extended spectrum in chemical fractionation. The remarkably wide variations in the chemistry of the ultramafic rocks (Table 1) reflect the dramatic changes in modal mineralogies and cumulate nature of the mineral phases. In the chemical variation diagrams, a great similarity is noticed between the differentiation trends for KLC and other layered complexes. A major chemical hiatus exists between the ultramafic and gabbroic groups, and a major one between the gabbroic and anorthositic groups. A rather regular reciprocal relationship between the (MgO + Fe20~) and (CaO+Al203) contents exists among the ultramafic rocks, and also among the gabbroicanorthositic rocks.