The Song Ma-Song Du region of northern Vietnam contains the Song Ma Anticlinorium, a polydeformed polymetamorphosed, early Palaeozoic island arc/forearc terrane accreted to the South China plate in Siluro-Devonian times. The Song Ma Anticlinorium is not an Indosinian subduction zone and nor is the post-Triassic, post-Cretaceous Song Ma Fault. The Song Ma Fault is one of many north est-trending, post-Cretaceous, high-angle reverse oblique-slip faults and thrusts responsible for shortening and strike-slip transposition of northern Vietnam. Folds penecontemporaneous with this faulting include the Song Ma Anticlinorium, which is formed probably of thrust-ramp folds. The faulting and penecontemporaneous folding developed during Oligocene sinistral strike-slip on the Song Hong Fault and was reactivated during Pliocene-Quaternary times. We conclude that the present plate boundary is a broad deformation zone between Da Nang and the Song Hong and that parts of the South China plate extend well to the south of the Song Hong Fault.
The Sri Lankan Gondwana fragment is a high-grade gneiss terrain consisting of at least three crustal units, namely the Wanni Complex (WC), the Highland Complex (HC) and the Vijayan Complex (VC), which were juxtaposed during the Pan-African collision. At least six phases of ductile deformation were recognized in the smallest Gondwana fragment, Sri Lanka. There is microscopic and mesoscopic evidence for a first deformation (D-1), which occurred during prograde metamorphism but before the main granulite facies metamorphism. This first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts, microfolds in some metapelites and quartzo-feldspathic gneisses and by a S-1 foliation preserved in crenulation folds and some rootless intrafolial folds. D-2, has two main stages, of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation (L-2). The first stage of D-2, may have been responsible for the formation of a crenulation cleavage of S-1, as seen in garnet porphyroblasts in some metapelites. The compositional layering (S-2) in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage. F-2, folds are mesoscopic, isoclinal, rootless and occur as intrafolial folds, and their limbs are highly attenuated. Major part of the deformation during the latter stage of D-2, was non-coaxial. Many minor and large scale recumbent isoclinal folds (F-3) were produced during D-3. Both D-2 and D-3 were coeval with the peak conditions of granulite facies metamorphism. D, produced very large, gentle, nearly E-W upright folds probably during the beginning of uplift of the entire high-grade assemblage fi from deeper crustal levels, still under granulite facies conditions. D-5 is the second strongest deformation and was responsible for the large-scale upright folds (F-5), which control the present map pattern of Sri Lanka. The superimposition of D-5 folds on D-4 folds gave rise to a large-scale type I fold interference pattern of Ramsay (1967). D-6 deformation caused local refolding of the F-5 folds and was accompanied by amphibolite facies conditions during uplift. The defomation represented by the majority of vein-type structures, such as patchy charnockite veins, cordierite-bearing veins and graphite veins clearly post-date D-5. It is argued that the WC, the HC and the VC in Sri Lanka may be three distinctly different Proterozoic crustal units, brought into contact by tectonism in late Proterozoic at two different stages.
The Himalayan province, which represents the northern platform of the Peninsular India belonging to East Gondwanaland, was strongly affected by Pan-African diastrophism 500+/-25 Ma ago. This brought to an end the protrocted Purana cycle of sedimentation throughout the Peninsular India and the Lesser Himalaya and interrupted basin-filling in its northern Tethyan domain. But the sea returned in the Early Permian along a narrow depression formed due to rifting of the Himalayan crust in what is today the southern Lesser Himalaya. In the rift valley was deposited tillites by glaciers of the Gondwana continent under grip of refrigeration. Along with the glacigene conglomerates were emplaced diamictites generated by submarine slides triggered, presumably, by earthquakes originating from faults delimiting the rift. The diamictities are admired with lava, agglomerate and tuff, indicating widespread volcanism in the rift valley. The rifting culminated in the breaking away of the Tibetan part of the Himalaya in the Late Permian and formation of Neotethys between the Gondwanaland and the Cimmerian microcontinent embodying Tibet, Iran and Turkey. Rivers of the northern Peninsular India flowed in the northerly directions since the Middle Proterozoic through Early Eocene. In the Late Eocene there was a drastic drainage reversal to south and southeast when the Himalaya emerged in the northern front of the Gondwanaland.
The Kondapalli Layered Complex (KLC) consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites, clinopyroxenites, dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole). The KLC is a stratiform intrusion possibly similar to Bushveld, and its various components occur as sheets, bands or lenses in the enclosing sea of charnockites; small scale folded structures are not uncommon. Chilled margins, contact metamorphic zones, xenoliths and late differentiates have not been found for the KLC, which is cut by rare 851+/-28 Ma (whole rock K-Ar age) metadolerite dykes containing intensely clouded plagioclase and a trace of garner. Most rocks of the KLC display layered characteristics, and they essentially comprise plagioclase, orthopyroxene and clinopyroxene in different combinations, with variable proportions and diverse textural relationships. The rocks exhibit textul es that formed both during the magmatic phase of crystallization and during high-temperature post-cumulus to subsolidus deformation and re-equilibration. Mineral analyses for 4 olivines (Fo(95.87)), 19 orthopyroxenes (En(94.51)), 13 clinopyroxenes (Ca39.51Mg54.36Fe4.20), 11 amphiboles (Ca37.31Mg67.38Fe5.34), 13 plagioclares (An(100.76;) An(54)) and 13 chromites [100Mg/(Mg+Fe2+) = 66-26; 100Cr/Sigma R3+ = 73-35], together with 45 whole rock analyses from the KLC are utilized in the present study. The remarkably wide variations in the host rock chemistries reflect the significant changes in modalmineralogies. Two parental magmas (a magnesian liquid and an alumina-rich tholeiitic liquid) are tentatively proposed for the KLC. Some quartz-bearing (anorthositic, enderbiric and other felsic) rocks with conspicuous de formational textures are interpreted as contaminated or mixed rocks occurring at the tectonized junction zones between the KLC and charnockites. The KLC exhibits several distinctive mineralogical features: pure anorthite (An(100)) in zoned spinel-amphibole-orthopyroxenites; exsolution rods or blebs of nearly purr K-feldspar (Or(>90)) in the high calcic plagioclase (An(90)); twinned plagioclase (An(42)) exsolution lamellae in the orthopyroxene (En(45)) of deformed "unusual" enderbites; coronal garnet in rare high Fe-gabbros; inter- and intra-granular compositional variation and different zoning patterns in chromite of the chromitites and ultramafics; and finally, dense networks of fine exsolution lamellae of Al- and Mg-rich chromite in Fe3+- and Fe2+-rich host chromite. The coexisting pyroxenes from the ultrabasic, gabbroic and anorthositic rocks of the KLC, as well as those from the enclosing charnokites, yield similar K-D, values and P (6-8 kbar) - T (830-950 degrees C) estimates;. it is suggested that the KLC has intruded dry country rocks at great depths (lower to middle crustal levels), probably in the period immediately following or coincident with the highest temperature metamorphism of the country rock charnockites. Extensive subsolidus re-equilibration of the KLC]las taken place, with all the rocks (KLC and regional charnockites) retaining signatures of nearly identical physical conditions of formation characteristic of the granulite facies metamorphism.
Late Archaean (2.7 Ga) mantle fertility and the processes of oceanic crust and greenstone formation have been inferred through a derailed geochemical study of the metavolcanics of the Sandur Belt. This belt is made up of two distinct lithotectonic assemblages. ( 1) The autochthonous sequence consists of Yeshwanthanagar Volcanic Block (YVB), Deogiri Block (DB), Western Volcanic Block (WVB), Central Volcanic Block (CVB) and Eastern Volcanic Block (EVB). (2) The allochthonous assemblages are divided into North Central Acid Volcanic Block (NCAVB), Sultanpura Volcanic Block (SVB) and Eastern Acid Volcanic block (EAVB). Autochthonous assemblage was formed as a pericontinental insitu sequence on a shelf to which the allochthonous blocks have been successively accreted along layer parallel faults. Lithological, structural, metamorphic and geochemical discontinuities are found across the different blocks. Volcanic components of these blocks comprise of ultramafic komatiites and/or cumulates, basaltic komatiites, high Mg-basalts, high Fe-tholeiites, tholeiitic dacites, andesites and rhyolites, metamorphosed upto amphibolite facies, but sometime preserving relictigneous mineralogy. REE abundances of most of the metabasalts are 10-40 times of those of chondrite and have generally unfractionated patterns with (La/Yb)(N) similar to 1-5. Fractionated REE patterns are found in the felsic amphibolites of CVB (Hospet zone) and the intermediate-acid volcanic members present only in NCAVB and EAVB. Abundances and ratios of several incompatible diagnostic elements of comparable partioning coefficients such as Nb/Th, Hf/Sm, Zr/Hf, Zr/Yb, Sc/Yb, Nb/Yb, Nb/Th, Th/Ta, Th/Tb and others show large scale variation within and between different blocks. These ratios for the high Mg-komatiitic basalts are nearer to those of primitive mantle. The marked differences in geochemistry between the metabasalts of different blocks indicate that the entire volcanic sequence may not have been derived from the same mantle. Their compositions in general are not comparable to any of the basalts from modern tectonic settings. However a mix of 80 and 20% enriched and normal MORE is very close to their overall composition, which is defined as Archaean Oceanic Ridge Basalt (AORB). Geochemical data suggest that Sandur basalts were generated from a fertile and heterogenous mantle. This fertile mantle either was tapped at the Archaean ridges directly or enriched magma was fed to the ridges from nearby hotspots. Partial subduction of the AORB possibly gave rise to the mafic-felsic volcanics of the NCAVB and EAVB; the two subduction complexes having interbedded turbidites and other sediments derived from within The basin. FeO/MgO, Ce/Nb, Nb/ U, Nb/Th and Nb/La ratios of samples having MgO > 11% indicate that hot spot tectonics might also have played a role for the evolution of this belt, but the present level of information is equivocal. However, derivation of the Sandur metabasalts from a relatively fertile mantle in the form of oceanic ridge basalts seems to be a strong possibility as around 2.7-2.8 Ga ago only small fraction of the continental crust was extracted from the mantle. Subsequent compressional processes appears to have telescoped the two continental volcanic margins.
Granulite facies metapelites at Koliacode in the Kerala Khondalite Belt show compositional bands of cordierite, spinel, sillimanite and biotite alternating with quartz-feldspar rich bands. Reaction textures involving cordierite and garnet show evidence for decompression. Mineral phase equilibria computations provide temperatures of ca. 700-750 degrees C and pressures of 4.5-5.0 kbar. Spinel in these rocks is remarkably enriched in Zn (ZnO up to 9.4 wt. %). The metamorphic P-T conditions and mineral reactions indicate that the terrain underwent rapid and near isothermal decompression.
In the Precambrian basement of the Schirmacher Hills, ductile shear zones have developed at several stages over a wide time-span of tectonothermal history. The earliest shear zones were formed under granulite facies conditions producing well-foliated gneiss. The majority of the ductile shear zones developed under amphibolite facies conditions. The sheared rock is a mylonite characterized by drastic grain refinement of the constituent minerals. The shear zones under amphibolite facies conditions are of four broad categories: an early set subparallel to the gneissic foliation, an early and a late cross-cutting types, sheer zones which developed within massive rocks and lastly, those which developed at the contacts of strong rheological contracts. The last phase of deformation produced discrete shear fractures. All the different phases of ductile shearing were broadly synchronous with emplacement of quartzofeldspathic materials. Pegmatite was emplaced even along some of the discrete sheer fractures, The magnitude of shear strain could be measured from the orientations of newly formed foliations and also from the rotation of early foliations.
Cheralite is reported for the first time from a folded sillimanite schist layer in the garnetiferous quartzo-feldspathic gneisses from the Visakhapatnam area in the Eastern Chats granulite belt. Cheralite of scarce occurrence was recorded from charnockites of Araku area in the catchment of Gosthani river basin, which was incidentally related to the concentration in the beach placers as cheralite component in Visakhapatnam - Bhimunipatnam Coastal region. The monazite data in the Eastern Ghats from quartzo-feldspathic gneiss, sapphirine/cordierite granulites generally shown ThO2 range between 6-10% while hypersthene granulites have 9-19% with an exception of 2% in allanite bearing hypersthene granulites. Cheralite from spinel bearing sillimanite schist layer contains 14-22% ThO2 and shows occasional enrichment towards the grain margins. Plagioclase rims noticed around ilmenite in the spinel bearing sillimanite schist are indicative of partial melt in the granulites. The pegmatite formation along the sheared contacts of garnetiferous quartzo-feldspathic gneisses and spinel bearing sillimanite schists/gneisses is related to metamorphic segregation phenomena in the quartzo-feldspathic gneisses. The enrichment of Th, U and Si is due to the element mobilities during high grade metamorphism. Based on the occurrence of cheralite in the major source rocks, garnetiferous quartzo-feldspathic gneisses, garnetiferous sillimanite gneisses and hypersthene granulites, it is believed that these rocks have substantially contributed to the placer concentrations in the coastal sediments.
Two types of mafic rocks are exposed around the Katekalyan region viz. amphibolites and dolerites and/or meta-dolerites. Amphibolites are further classified as volcanic amphibolites (i.e. meta-volcanics) and plutonic amphibolites (mainly dykes). Dolerites occur as small dykes. Middle Proterozoic or older rock types of the area have been intruded by mafic rocks. Volcanic amphibolites show schistose texture whereas plutonic amphibolites exhibit either schistose or granoblastic textures. Porphyroblastic texture is also observed in some thin sections of both types. Dolerites and/or meta-dolerites show ophitic or sub-ophitic texture. Geochemically all types of mafic rocks may be classified as sub-alkaline high Fe-tholeiites and are enriched in Rb, K, Nb, Sr, Zr,Ti and Y than the primitive mantle. Incompatible elements concentration suggest genetic links between all types of mafic rocks and are indicated to be emplaced in the "within-plate" environment. On the basis of petrological and geochemical data presented, it is suggested that ferro-tholeiitic magma has been emplaced at least twice, one has been metamorphosed (i.e. older emplacements - amphibolitic types) and another is fresh (i.e. younger emplacements - doleritic types). It is also suggested that probably amphibolite dykes act as feeder dykes for volcanic amphibolites (i.e. meta-volcanics). Almost similar mafic rocks, particularly dykes, have been observed in the Antarctic shield which supports the juxtaposition of India and Antarctica and corroborates the views on reassembly of East Gondwanaland.
The Quartz - Pebble Conglomerates (QPC) represent the angular unconformity prevailed between the older group of Sargur rocks and younger Dharwar volcano-sedimentary association in Chikmagalur area of Karnataka state. The older group of rocks consist of amphibolites, Peninsular gneisses, granites, quartzites and quartz-sericite schists whereas the younger group has amphibolites, quartzites, phyllites, chlorite schists, differentiated volcanic rocks and banded iron formations. The QPCs have rutile, ilmenite, pyrite, pyrrhotite, chalcopyrite and rarely uraninite as the major opaque phases. Gamma ray spectrometry, autoradiography and ore petrographic studies have shown that the concentration of uranium in the QPCs is due to solution redeposition of uranium by adsorption or absorption on rutile generated by the decomposition of ilmenite with hydrous iron as another product. The genesis of pyrite is described through a polyplylitic process. The oxidising environments prevailed in Pre-cambrian might have promoted the transportation of U+6, S+6 and Cr+6 and were concentrated in reducing pockets of Bababudan as U+4, S+2 and Cr+3. Cr+3 is represented by fuchsite mica abundant in pyrite rich zones. The scarcity of uraninite is due to the destruction of these grains by oxidation during transportation.
Paleobiogeography, plate evolution, and minor amounts of subduction require a rapidly expanding earth during post-Triassic time, not plate tectonics. An asteroid impact at the P/Tr boundary in the Congo Basin ruptured the lithosphere and, together with another impact in the Carnian of Arizona, caused Earth's volume to subsequently expand. The increase in volume was likely due to inner core and lower mantle transfomation growth at the expense of the fluid outer core. A modified Pacific reconstruction is proposed that closes up the Pacific Basin in the Triassic and allows continents to cover a 55% radius Earth. Some serious weaknesses in the plate tectonics theory are noted, and tectonics by high-energy impacts is discussed.