H.M. Rajesh

Role of Pan-African events in the Circum-East Antarctic Orogen of East Gondwana: a critical overview

Abstract Recent studies of Pan-African events in East Gondwana are critically reviewed, particularly recent models of amalgamation of East Gondwana during the Pan-African period. It is pointed out that critical data are insufficient to constrain the newly proposed models and so the classical model of the Grenvillian Circum-East Antarctic Orogen cannot yet be replaced. Grenvillian tectonothermal events with a peak between 1.0 and 1.2Ga assembled different crustal blocks of the East Antarctic Shield with different geohistories. Pan-African tectonothermal reworking took place over wide but selected areas of the orogen. Careful geochronological studies, including SHRIMP dating associated with structural and petrological investigations to correlate ages with those events, are shown to be important, since fluid-rich and/or deformational conditions are equally effective as temperature conditions for mineral recrystallization and resetting of isotopic systematics. Pan-African suture zones, one extending from the Mozambique Belt to the Shackleton Range and another connecting the Mozambique Belt to the Zambezi Belt, are equally possible, although the width of the southern Mozambique Ocean is poorly understood. The extent of Pan-African sutures in the Prydz Bay area is enigmatic, although they represent definite orogens. Palaeomagnetic studies may provide critical constraints in evaluating the sutures, provided that the age of magnetization is well established.

Dextral Pan‐African Shear Along the Southwestern Edge of the Achankovil Shear Belt, South India: Constraints on Gondwana Reconstructions: A Discussion

Achankovil shear zone (ACSZ) (figure 1a), intering charnockites, garnet-biotite gneiss, biotite gneiss, calc-gneiss and granite, as revealed in the preted as a major NW-trending strike-slip shear zone of probable Proterozoic age (Drury et al. 1984), lithological mapping carried out as part of this study (figure 2). A preliminary structural assesshas prominently figured in Gondwana correlative studies (e.g., Windley et al. 1994). Although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across ACSZ (figure 1b), detailed studies indicating the posures indicating a consistent sinistral sense of shear. Gneissosity, forming the secondary compoprobable sense of shear are lacking across this 10– 20 km wide and .100 km long shear zone. Resitional banding, strikes WNW-ESE to NW-SE dipping moderately toward SW (with relatively rare cently, Sacks et al. (1997) reported field observations bearing on the probable sense of shear of the NE dip) and is conformable in all the rock types of the area, in conformity with the general NW-SE ACSZ. After combining field criteria and kinematic analysis of asymmetric features along the SW trend of the ACSZ. Structurally, the terrain has undergone four phases of deformation (summarized in edge of the ACSZ (known as the Tenmala shear zone), they concluded that the ACSZ indicated priRajesh and Santosh 1997). Open to isoclinal intrafolial folds encountered in marily dextral shear, with a minor component of reverse shear. The authors illustrate their discusthe study area are invariably asymmetric. Fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators, including shear bands, asymmetric folds, stretched and tain indicators of a probable sinistral sense of shear and well-developed mineral elongation lineation asymmetric garnet, feldspar and quartz porphyroclasts, hook folds, and lineations. Thus they appardefined by quartz ribbons, trails of xenoblastic garnet grains, and the alignment of the long axes of Kently question the correlation of the dextral Achankovil shear zone (rather Tenmala shear zone) with feldspar augen. Pinch-and-swell structures and boudins commonly show rotation with respect to the sinistral Bongolava-Ranotsara shear zone (BRSZ) in the India-Madagascar Gondwana enfoliation. Tight ‘‘S’’ asymmetric F2 folds and synkinematic pegmatite veins are seen in the western semble. We focus the present discussion on the probable part of the study area. The development of a refold structure by superposed buckling is illustrated in sense of shear of ACSZ in the light of new data. Our study area falls around Punalur and its adjoinfigure 3a. There are smaller asymmetric Sand Zfolds congruent with the larger F1, with one limb ing areas within the Quilon district, southern Kerala, South India and is located between longitudes striking E-W while the other strikes N50°E, showing negative extension. During the development of 76°55′ and 76°59′ E and latitudes 9°1′ and 9°3′ N (figure 2), represented in the Survey of India topo F2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet No. 58C/16. Outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb. Asymmetrically folded veins (figure 3b), the bulk asymmetry of which represents a sinistral sense of shear, have been found 1 Manuscript received May 14, 1997. in a few locations. Elongate quartz lenticles and 2 Centre for Earth Science Studies, PB 7250, Trivandrum 695 031, India. garnet and biotite aggregates define penetrative lin-

Chemical Composition and Tectonic Setting of Chromian Spinels from the Ediacaran–Early Paleozoic Nama Group, Namibia

Petrographic and chemical characterization of chromian spinels from the lower and upper Schwarzrand Subgroup and the upper Fish River Subgroup of the latest Ediacaran to early Paleozoic Nama foreland basin in Namibia is used to infer the tectonic setting of source rocks and evaluate the geotectonic evolution of the Damara Orogen. Electron microprobe analysis of detrital chromian spinels from the Nama Group show three groups of chromian spinels: (i) a group with a Cr# ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A geochemical perspective on charnockite magmatism in Peninsular India

\mathrm{Cr}\,/ ( \mathrm{Cr}\,+\mathrm{Al}\,

The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India

\end{document} ) atomic ratio) between 0.62 and 0.75 and about 0.9% TiO2, which suggest derivation from a volcanic island arc (Niederhagen and Niep members); (ii) a group with a Cr# between 0.75 and 0.88 and TiO2 well below 0.4%, which could be associated with boninitic magmas (Niederhagen, Niep, and Haribes/Rosenhof members); and (iii) a group that displays characteristics of a mid‐ocean ridge basalt source, with a Cr# around 0.3 and TiO2 below 0.5% (Haribes and Rosenhof members). Paleocurrent analyses of the chromian spinel–bearing sandstones of the Nama Basin point to a source area located in the Damara Belt. The chromian spinel petrography and chemistry do not match those of a source from the Paleo‐Mesoproterozoic basement. Although a western provenance for the lower Nama Group is not supported by paleocurrents, the Gariep Belt probably acted as a source by reworking at the final stage of the “foreland deposits” of the Nama Group. Thus, the existing data favor a provenance from the Damara Orogen and indirectly support the tectonic models that argue for an active continental margin on the northern and western borders of the Kalahari Craton.