The Banpur-Balugaon and Bolangir Anorthosite Diapirs of the Eastern Ghats, India: Implications for the Massif Anorthosite Problem

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Abstract
Two anorthosite massifs in the Eastern Ghats share similar structural constitutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution. The (1) circular to near-circular structural patterns both inside and close-to-border outside of the plutons; (2) the merging of these structures with the straight to gently flowing, essentially unidirectional structural trend of the granulite basement within short distances from the border of the plutons; (3) the increase of strain intensity near the border of the plutons; (4) the small but recognizable differences in the dip of the anorthosite flow layers; and (5) the foliation of the granulites, all are strong indications that these anorthosites were emplaced as syntectonic diapirs. In both massifs, anorthosite is by far the dominant lithology, noritic varieties being subordinate and generally formed as narrow dike-like bodies and pods, pockets, and irregular patches near the border. The striking 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 leucoanorthosites to the noritic rocks and wide Mg-Fe variation in the pyroxenes, in contrast to a relatively uniform plagioclase composition. These variations 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 sorted and cryptically enriched layers at different stages of anorthosite diapirism. Diapiraureole structural relationships, petrology, and thermobarometry 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°C. Such considerations, supplemented by a conductive cooling model for anorthosite emplacement and its subsequent evolution, lead to a branched path as the essential topology of the P-T-time history of the anorthosite-granulite associations of the Eastern Ghats. An important corollary of this inference is that a cycle of prograde and retrograde metamorphism of the aureole rocks—before and after the anorthosite invasion, respectively—is an essential consequence of the anorthosite emplacement.