The Kondapalli Layered Complex, Andhra Pradesh, India : A Synoptic Overview

The Kondapalli Layered Complex (KLC) consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites, clinopyroxenites, dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole). The KLC is a stratiform intrusion possibly similar to Bushveld, and its various components occur as sheets, bands or lenses in the enclosing sea of charnockites; small scale folded structures are not uncommon. Chilled margins, contact metamorphic zones, xenoliths and late differentiates have not been found for the KLC, which is cut by rare 851±28 Ma (whole rock K-Ar age) metadolerite dykes containing intensely clouded plagioclase and a trace of garnet. ocks of the KLC display layered characteristics, and they essentially comprise plagioclase, orthopyroxene and clinopyroxene in different combinations, with variable proportions and diverse textural relationships. The rocks exhibit textures that formed both during the magmatic phase of crystallization and during high-temperature post-cumulus to subsolidus deformation and re-equilibration. l analyses for 4 olivines (Fo95-87), 19 orthopyroxenes (En94-51), 13 clinopyroxenes (Ca39-51Mg54-36Fe4-20), 11 amphiboles (Ca27-31Mg67-38Fe5-34), 13 plagioclases (An100-76; An54) and 13 chromites [100Mg/(Mg+Fe2+) = 66-26; 100Cr/ R3+ = 73-35], together with 45 whole rock analyses from the KLC are utilized in the present study. The remarkably wide variations in the host rock chemistries reflect the significant changes in modal mineralogies. Two parental magmas (a magnesian liquid and an alumina-rich tholeiitic liquid) are tentatively proposed for the KLC. Some quartz-bearing (anorthositic, enderbitic and other felsic) rocks with conspicuous deformational textures are interpreted as contaminated or mixed rocks occurring at the tectonized junction zones between the KLC and charnockites. C exhibits several distinctive mineralogical features: pure anorthite (An100) in zoned spinel-amphibole-orthopyroxenites; exsolution rods or blebs of nearly pure K-feldspar (Or>90) in the high calcic plagioclase (An90); twinned plagioclase (An42) exsolution lamellae in the orthopyroxene (En45) of deformed "unusual" enderbites; coronal garnet in rare high Fe-gabbros; inter- and intra-granular compositional variation and different zoning patterns in chromite of the chromitites and ultramafics; and finally, dense networks of fine exsolution lamellae of Al- and Mg-rich chromite in Fe3+- and Fe2+-rich host chromite. existing pyroxenes from the ultrabasic, gabbroic and anorthositic rocks of the KLC, as well as those from the enclosing charnockites, yield similar KD values and P (6-8 kbar) - T (830-950oC) estimates. It is suggested that the KLC has intruded dry country rocks at great depths (lower to middle crustal levels), probably in the period immediately following or coincident with the highest temperature metamorphism of the country rock charnockites. Extensive subsolidus re-equilibration of the KLC has taken place, with all the rocks (KLC and regional charnockites) retaining signatures of nearly identical physical conditions of formation characteristic of the granulite facies metamorphism.