Spatial and temporal variations of the tectonothermal imprints

The nature of thermal and tectonic events identified in different crustal domains of the EGB depicts a time-specific segmented course of evolution. It is realized for some time now that the present configuration of EGB is a collage of crustal domains each having its own characteristic history. A critical look at the events in each domain through the time window may actually help us to visualize the evolutionary history in a holistic way.

Protolith isotopic characters of the Ongole domain suggest that the sediments were supplied to an unknown basement from the adjoining Dharwar Craton. The first major tectonothermal activity here occurred at ~1.76 Ga when the supracrustal rocks underwent UHT metamorphism due to emplacement of voluminous mafic magma in a continental arc type tectonic setting (Leelanandam and Vijaya Kumar, 2007). The sedimentary basin must have developed between ~2.60 Ga (youngest Nd-model age for the sediments) and ~1.76 Ga (metamorphic age), but could not be refined further since no systematic provenance study is carried out in this domain. A second pervasive imprint of metamorphism took place at ~1.60 Ga which coincides with the emplacement of pegmatoidal enderbite. The exact nature of this particular metamorphic overprint is not clear from available data, but it might have caused partial melting as reported recently by Upadhyay et al. (2009). It is still difficult to visualize how an UHT-metamorphosed lower crust underwent partial melting unless it was retrogressed by deep crustal shearing or transported to upper level by exhumation or erosion. It should also be noted that no decompression-related retrogressive metamorphism is discernable in UHT-metamorphosed lower crust of the Ongole Domain. There is no unique answer to this problem and it can only be resolved with sound petrological and structural data. For the time being, we simply assign the ~1.60 Ga event as a thermal imprint. In the mean time, the Ongole Domain witnessed intrusion of enderbitic gneiss at ~1.72-1.70 Ga. Lack of geochemical data on this rock suite prevents one from inferring a suitable tectonic setting, but an Andean-type active continental margin setting is one possibility (Dobmeier and Raith, 2003). This domain did not witness any further high-grade events although weaker event of ~1.10 Ga must have left its impression (Mezger and Cosca, 1999). This is consistent with the idea that the Ongole Domain became cratonized with the adjoining Dharwar Craton at ~1.60 Ga. This cratonization process incorporated thermal activity manifested by granitoid emplacement at ~1.60 Ga in the adjacent Vinjamuru Domain (Dobmeier et al., 2006). During this entire sojourn, the Eastern Ghats Province did not exist in its entirety. However, presence of zircon within the UHT-metamorphosed paragneisses of the Domain 2 of the Eastern Ghats Belt with concordant inherited ages in the range ~1.76-1.70 Ga (Bose et al., 2008) produces an interesting idea that such sediments were sourced from the Ongole Domain or its equivalents. Although the sediments have a Nd-model ages of ~2.5 - 2.1 Ga, the strong presence of ~1.76-1.70 Ga age in the zircon inherited domain may also suggest the age of the basement which has been postulated to be thoroughly reworked by later UHT event (Dobmeier and Raith, 2003).

Alkaline rocks were emplaced along the western margin and prominent shear zones of Krishna and Jeypore Provinces of EGB in the time span of ~1.5 – 1.3 Ga. This signifies onset of rifting in the already cratonized part of EGB (Upadhyay, 2008). No metamorphic signature of this period is seen in any rock of EGB. Emplacement of mafic magma at ~1450 Ma in the Vishakhapatnam Domain (Shaw et al., 1997) is the only thermal activity in Eastern Ghats Province. However, the geochemical affinity of such mafic magma is not known.

A flurry of activity took place in the Eastern Ghats Province during the time segment of ~1250-900 Ma. These include UHT-metamorphic event (~1250-1100 Ma?) in an uncharacterized geodynamic setting. The overall counter clock-wise P-T trajectory of this event can be explained by extensional setting. The major problem with this kind of tectonic modeling is how to account for the anomalous heat supply which is a pre-requisite for UHT metamorphism. Recently, Brown (2006) argued that similar UHT metamorphism under low-P high-T prograde stage can be possible in a back arc setting. To establish such affinity we are severely short of documentary evidences. This UHT metamorphism was succeeded by emplacement of porphyritic granitoids (charnockite to granite in composition) at ~1000-950 Ma. A second granulite-grade metamorphism occurred in the time span of ~950-900 Ma when the deep crustal section was exhumed. This event was accompanied by pervasive deformation including deep-crustal shearing (Das et al., submitted). Imprint of this very event is recorded in rocks of Angul, Phulbani and Chilka Domains as well. Metasediments of the Vishakhapatnam Domain did not undergo any younger tectonothermal event of granulite grade, although weak amphibolite-grade reworking at ~550-500 Ma is noted in localized shear zones (as reviewed in Dobmeier and Raith, 2003).

Chilka Domain in the northern part of EGB is notable in three aspects. Firstly, it has a metamorphic history different from that of the adjacent Vishakhapatnam Domain. Secondly, an important event of contact metamorphism took place in the sediments due to emplacement of anorthosite massif at ~983 Ma. Thirdly, and most importantly, the Neoproterozoic metamorphic history in the time span of ~800-500 Ma is unique for this domain in the EGB. Metasediments of this domain have Nd-model age of ~2.2-1.8 Ga. This domain is bounded by Nagavalli - Vamshadhara Shear Zone (NVSZ) in the south and west and Mahanadi Shear Zone (MSZ) in the north. Restriction of Neoproterozoic age only in this shear-zoned bound block poses a possibility whether this domain represents an exotic block or its younger history is controlled by local factors. Some have argued that the ~800 Ma strong imprint is caused by anorthosite emplacement (Dobmeier and Raith, 2003; Upadhyay et al., 2009). There are two problems with this logic. Firstly, ID-TIMS data of zircon from the anorthosite give an upper intercept age of ~983 Ma for crystallization which, if correct,means that the ~800 Ma age has nothing to do with the anorthosite emplacement. Secondly, this ~800 Ma event is very pervasive and recorded in metasediments occurring quite far from the anorthosite massif (Dobmeier and Simmat, 2002; Bose et al., 2008). It is inconceivable that such enormous heat budget can be provided by the anorthosite magma alone. The preponderance of Pan-African (~550-500 Ma) ages in Chilka Block renders its proximity to the Pan-African front.