The Banpur-Balugaon and Bolangir anorthosite diapirs of the Eastern Ghats,India: Implications for the massif anorthosite problem

Two anorthosite massifs in the Eastern Ghats share similar structural const itutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution. The (1) circular to near-circular structural pattern s both inside and close-to-border outside of the plutons; (2) the merging o f these structures with the straight to gently flowing, essentially unidire ctional structural trend of the granulite basement within short distances f rom the border of the plutons; (3) the increase of strain intensity near th e border of the plutons; (4) the small but recognizable differences in the dip of the anorthosite now layers; and (5) the foliation of the granulites, all are strong indications that these anorthosites were emplaced as syntec tonic diapirs. In both massifs, anorthosite is by far the dominant litholog y, noritic varieties being subordinate and generally formed as narrow dike- like bodies and pods, pockets, and irregular patches near the border. The s triking features of the whole-rock and mineral chemistries of these massifs are increasing Fe, Ti, Mg, P, and REE, and decreasing Si and Al from the l eucoanorthosites to the noritic rocks and nide Mg-Fe variation in the pyrox enes, in contrast to a relatively uniform plagioclase composition. These va riations may be the result of initial differentiation in a layered complex serving as a precursor to the anorthosites, through modal sorting, rhythmic layering and cryptic changes, and a subsequent mixing of the modally sorte d and cryptically enriched layers at different stages of anorthosite diapir ism. Diapir-aureole structural relationships, petrology, and thermobarometr y suggest a moderately steep, counterclockwise cooling and exhumation path for both massifs to the P-T range: 5 to 6 kbar and 600 to 700 degrees C. Su ch considerations, supplemented by a conductive cooling model for anorthosi te emplacement and its subsequent evolution, lead to a branched path as the essential topology of the P-T-time history of the anorthosite-granulite as sociations of the Eastern Ghats. An important corollary of this inference i s that a cycle of prograde and retrograde metamorphism of the aureole rocks -before and after the anorthosite invasion, respectively-is an essential co nsequence of the anorthosite emplacement.