Emplacement of Proterozoic massif-type anorthosite during regional shortening: evidence from the Bolangir anorthosite complex (Eastern Ghats Province, India)

Abstract

A study of the 933±32-Ma-old Bolangir massiftype anorthosite complex (Eastern Ghats Province, India) yielded strong evidence for anorthosite emplacement during regional shortening, and thereby new insights in massif-type anorthosite formation. Several lines of evidence strongly suggest synchronism of plutonism and regional deformation. First, structures in the country rocks, which imply N-S-directed shortening accompanied by E-W extension, are mirrored by a E-W trending post-magmatic foliation and N-S trending shear zones in the anorthosite complex. Near the intrusion, the foliation in the country rocks becomes parallel to the contact and an internal marginal foliation, and foliation triple points occur in the country rocks. Second, synshortening dikes inside and outside the anorthosite complex are filled with pluton-related melts. Third, ferrodiorites, which are considered late-stage differentiates of the anorthositic pluton, concentrate in tectonic voids at the pluton margin. Some of these occurrences have been affected by the last increments of the regional deformation, but others transect the same structures. Ascent mechanism and significance of the adjacent terrane boundary of the Eastern Ghats Belt for ascent and emplacement of the Bolangir anorthosite complex are discussed. The results of this study imply that emplacement of Proterozoic massif-type anorthosite is not restricted to extensional settings.

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

“…Massive and disseminated Fe-Ti concentrations occur in gabbros and anorthosites, and the layered Selvåg gabbroic intrusion (Priesemann and Krause, 1985) represents a minor potential for Fe-Ti ore. Massif-type anorthosites also outcrop in southwest Madagascar (Ashwal et al, 1998) and India, which were both part of East Gondwana. The Eastern Ghats Belt of India hosts four massif-type anorthosite complexes (Bhattacharya et al, 1998;Krause et al, 2001;Dobmeier and Simmat, 2002;Dobmeier, 2006;Chatterjee et al, 2008). Other massifs are also described in south India (Janardhan and Wiebe, 1985), but no ore bodies have been discovered.…”

The Eastern Ghats Belt … A polycyclic granulite terrain

“…Folds/broad warps with N-S trending axial traces and steep southerly plunging axes are later than this shearing. A thrust-shear dominated deformational regime with N-S directed shortening is recognized in Bolangir anorthosite and its country rocks (Dobmeier, 2006).…”

Deep mantle storage of the Earth’s missing niobium in late-stage residual melts from a magma ocean

“…The latter rock series comprises ferromonzodiorites (FMD) and ferrodiorites (FD) that are characterised by extremely high iron contents (10-37 wt.% FeO*) and enrichments of incompatible elements, including HFSE and REE. One suite in the outer marginal zone represented by FMD exhibits moderate to strong deformation structures (e.g., Dobmeier, 2006), whereas the more central FD suite mainly preserved its pristine state, escaping deformation (Bhattacharya et al, 1998). Ferromonzodiorites are exclusively occurring in the outer zone, and showing metamorphic textures.…”

“…Star marks Proterozoic anorthosite complexes within Eastern Ghats Province, T = Turkel, J = Jugasaypatna, C = Chilka Lake. Modified after Dobmeier (2006). $1 mm in diameter in variable modal amounts, embedded in a matrix of fine-grained mafic minerals.…”