Paper Number: 263
Tectonometamorphic history and significance of a ca 1450 Ma event recorded in the Proterozoic succession from a part of the Pakhal Basin adjoining the Eastern Ghat Mobile Belt, South India
Adhikary, D.1, Bhattacharjee, S.2 and Sahoo, S.1
1Geological Survey of India, State Unit: Andhra Pradesh, Southern Region, Hyderabad-500068, India; debapriya.adhikary@gmail.com
2Geological Survey of India, Central Head Quarters, Kolkata-700016, India
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The sedimentary rock sequences in the Proterozoic Pakhal basin in the southeastern part of peninsular India are known to be largely unmetamorphosed and locally deformed. However, the present study of the lithopackage in the southeastern fringe of the basin in the Yellandu area provides evidence of low to moderate scale multi-stage deformation and greenschist to amphibolite facies metamorphism besides a prominent ca 1450 Ma geological event. At least three prominent phases of deformation were inferred from penetrative small scale folds, refolded folds and their intersection besides overprinting of the earlier foliation in the Proterozoic successions of Pakhal Supergroup in the studied area. An attempt was made based on the technique of microstructural study to correlate phases of deformation with corresponding stages of growth of metamorphic minerals. The metamorphism is attributed to one of the major tectono-thermal events of the proximal Ongole Domain of the Eastern Ghats Granulite Belt. The earliest structure identified is represented by very tight to isoclinal folds (F1) on bedding plane (S0), with a pervasive axial planar cleavage (S1). The attitude of the S0 and S1 varies from NE-SW to NW-SE due to folding. They have been affected by co-axial, open to tight, upright F2 folds with axial plane striking NNE-SSW. As a consequence, the F1 folds range from recumbent/reclined through inclined to upright type at different places. Both F1 and F2 are related to buckle origin. The latest structure in the area are upright, conjugate folds (F3) and kink bands with axial plane striking N-S, E-W and chevron folds with NW-SE striking axial plane. The F3 structure resulted due to longitudinal shortening during the last stage of deformation. Microstructural study of thin sections of these rocks shows that porphyroblasts of chloritoid, garnet, staurolite and andalusite have different growth relationship with the quartz–feldspar–mica (biotite–chorite) matrix. Garnet, andalusite and biotite exhibit at least two phases of growth. The first metamorphic (M1) event is pretectonic to D1 deformation, the second metamorphic (M2) event is syn D1 (pre-D2 phase) and the third metamorphic (M3) event is post-tectonic with respect to D2 deformation. Staurolite developed during M1 stage shows close association with garnet1 and andalusite1 of this event. Dominant schistosity (S1) swerves around the pre-D1 porphyroblasts of garnet1 and andalusite1. Chlorite1 and biotite1 define the S1 schistosity which indicates the growth of these minerals during the M2 event which is syn-D1 deformation. Andulusite2 and garnet2 occur as porphyroblasts and show post-kinematic relationship with the S1 schistosity. These porphyroblast are developed across the schistosity (S1). The possible metamorphic reactions in M1 phase as derived from textural relationship could be: Ctd + And1 = St + Chl1 and And1 + Grt1 + Bt1 = Ms + St. Inclusion of staurolite and chlorite1 grains within coarse porphyroblasts of andalusite2 and biotite2 suggest the reaction: St + Chl1 + Ms = And2 + Bt2 + H2O. Finally, replacement of the biotite and andalusite by medium grained randomly oriented muscovite2 and chlorite2 is inferred to be post-D2 or syn-D3 deformation. The possible reaction is Bt + And + H2O = Chl2 + Ms2. From EPMA data it is observed that the garnets are mainly almandine in composition (Alm-0.83, Sp-0.06, py-0.06, Gr-0.05) and the porphyroblasts display enrichment in the almandine component from core to rim. Estimates of the peak temperature of the metamorphic event (M1) in this sector of the Pakhal basin are in the range of 4500C to 5500C. The ca 1450 Ma EPMA date obtained from a monazite grain developed at the margin of garnet1 possibly relates to the earliest phase of metamorphism (M1) of the area.