Middle Archaean Research Articles

Correlations between the North China Craton and the Indian Shield: Constraints from regional metallogeny

The correlation between the North China Craton (NCC) and the Indian Shield (IND) has been a hot topic in recent years. On the basis of ore deposit databases, the NCC and IND have shown broad similarity in metallogenesis from the middle Archaean to the Mesoproterozoic. The two blocks both have three major metallogenic systems: (1) the Archaean BIF metallogenic system; (2) the Paleoproterozoic Cu-Pb-Zn metallogenic system; and (3) the Mesoproterozoic Fe-Pb-Zn system. In the north margin of the NCC and the west margin of the IND, the Archaean BIF-Au-Cu-Pb-Zn deposits had the same petrogenesis and host rocks, the Paleoproterozoic Cu-Pb-Zn deposits were controlled by active belts, and the Mesoproterozoic Fe-Pb-Zn deposits were mainly related to multi-stage rifting. Matching regional mineralization patterns and geological features has established the continental assembly referred to as “NCWI”, an acronym for the north margin of the NCC (NC) and the west margin of the IND (WI) during the middle Archaean to the Mesoproterozoic. In this assembly, the available geological and metallogenic data from the Eastern Block and active belts of NC fit those from the Dharwar craton and the Aravalli–Delhi–Vindhyan belt of WI, respectively. Moreover, the depositional model and environment of Paleoproterozoic metasedimentary manganese deposits in NCWI implied that the assembly may be located at low latitudes, where the conditions were favorable for dissolving ice and precipitating manganese deposits.

Timing of deposition and deformation of the Moodies Group (Barberton Greenstone Belt, South Africa): Very-high-resolution of Archaean surface processes

Thin and largely discontinuous volcanic tuff beds in the alluvial, fluvial, and marginal-marine siliciclastic strata of the Middle Archaean Moodies Group, Barberton Greenstone Belt (BGB), contain rare to abundant zircon. Dating of magmatic zircon grains from five tuff beds and one cross-cutting dike yield ages which span the entire spectrum of BGB history from ca. 3550Ma to 3215Ma and show variable degrees of post-depositional alteration until ca. 2500Ma. The youngest pristine zircon grains in all tuff samples analyzed by CA-ID-TIMS (Chemical-Abrasion Ion-Dissolution Thermal Ion Mass Spectrometry) yield upper intercept and Concordia ages between 3227.6±0.3 and 3216±0.3Ma, and those grains analyzed by LA-SF-ICP-MS (Laser Ablation-Sector Field-Inductively Coupled Plasma-Mass Spectrometry) and SHRIMP (Sensitive High-Resolution Ion Microprobe) yield ages between 3228±9 and 3214±9Ma. Most of these ages are either identical (within error) or significantly older than a CA-ID-TIMS age of 3223.4±0.3Ma, obtained from zircon grains of a tuff horizon of the Schoongezicht Formation at the top of the underlying Fig Tree Group. We interpret this age to mark the time of volcanic eruption, whereas the significantly older and overlapping ages of the stratigraphically higher Moodies tuff zircon grains point either to a high degree of magmatic inheritance and/or a detrital contribution from surrounding sedimentary rocks during or after ash-tuff deposition. The minimum age for deposition of Moodies Group sediments and deformation is provided by a LA-ICP-MS zircon U-Pb age of 3219±9Ma obtained from an undeformed felsic dike which crosscuts folded Moodies strata. This age overlaps with a precise CA-ID-TIMS U-Pb zircon age of 3216.0±0.3Ma obtained from a tuff horizon of the upper Moodies Group. Overall, the new age data collectively imply that the up to 3.7km-thick, sandstone-dominated Moodies Group was deposited and deformed within <1–14Ma, and that Moodies strata thus hold the potential to offer a temporal resolution comparable to active- and passive-margin Quaternary strata (∼0.1–1mm/a).

Kaapvaal craton: Changing first- and second-order controls on sea level from c. 3.0 Ga to 2.0 Ga

Well preserved supracrustal basin-fills including the oldest large depository (Witwatersrand) known makes the Kaapvaal craton an ideal example for studying controls on first- and second-order stratigraphic cyclicity. The duration of the three basins studied (Witwatersrand, Ventersdorp and Transvaal), from c. 3.0 to c. 2.0 Ga is almost twice that of the entire Phanerozoic period, from which the paradigms of classical sequence stratigraphy have been derived. Amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through Earth history. This does not appear to be borne out by the Precambrian record studied here; instead, a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first- and second-orders is preferred. First-order cycles within the c. 3.0–2.0 Ga period on Kaapvaal are synonymous with the entire evolution of a supracrustal basin-fill defined by a specific basin-type; second-order cycles define basin-fill packages of unique character, which reflect large scale shifts in the accommodation–sedimentation balance. Early to Middle Archaean evolution of the Kaapvaal craton encompassed amalgamation of a mosaic of subdomains, to produce an early south–southeastern nucleus by c. 3.1 Ga. Accretion of composite terranes from both north and west followed from c. 3.0–2.7 Ga; this was contemporaneous with evolution of the c. 3.0–2.8 Ga Witwatersrand basin. The high-grade Limpopo terrane was amalgamated with the Kaapvaal craton between 2.95 and 2.69 Ga, although a younger age for this orogensis is also possible. The Witwatersrand first-order depositional cycle was controlled largely by subduction-related tectonic loading and had a duration of c. 190 My. The c. 5 My first-order cycle of the c. 2.7 Ga Ventersdorp basin was essentially controlled by thermal uplift-induced extensional subsidence, and that of the c. 650 My Transvaal basin by extensional and subsequent thermal subsidence. It is noteworthy in the three basins examined that first-order cycles controlled by plate tectonics (Witwatersrand and Transvaal basins) were about two orders of magnitude longer relative to the Ventersdorp “plume tectonic” cycle; second-order cycles also varied greatly, from c. 100 My to c. 1 My. Within the Witwatersrand and Transvaal basins, subsidence allowed extensive seaways to develop, within which classical sequence stratigraphic systems tracts can be recognized, reflecting different stages of shoreline shifts. Within the short-lived Ventersdorp continental basin system, sedimentological criteria can be used to discriminate high and low accommodation systems tracts. The Transvaal first-order cycle was terminated at c. 2.0 Ga by the Bushveld Complex plume event.

Archaean crust in the Rayner Complex of east Antarctica: Oygarden Group of islands, Kemp Land

ABSTRACTArchaean zircon grains from the Oygarden Group of islands in Kemp Land, east Antarctica, record evidence for multiple episodes of recrystallisation, dissolution and growth from the Early to Middle Archaean to the Neoproterozoic. Zircon grains in layered felsic orthogneiss have an age of ∼3650 Ma, a minimum protolith age for this rock. These zircon grains were subsequently affected by a ∼3470 Ma Pb-loss event. Homogeneous felsic orthogneiss that cuts S1, but contains an intense S2 foliation, has disturbed ∼2780 Ma metamorphic zircon cores and rims that suggest a minimum age of ∼2780 Ma for the protolith to the orthogneiss. All zircon U-Pb data display considerable disturbance with further Pb-loss at ∼2400 Ma and ∼1600 Ma, and a major episode of isotopic resetting at ∼930 Ma. The highly disturbed data are related to complexly zoned zircon grains that developed through growth and modification during successive metamorphic events. Zircon cores with relic growth zoning patterns are inferred to have resulted from partial annealing or recrystallisation of older magmatic zircon. Highly luminescent zircon rims that embay cores along curved boundaries are interpreted to have formed through recrystallisation of zircon cores, and not new growth. The ages reported here confirm that central Kemp Land is composed of Archaean crust reworked during the Neoproterozoic Rayner Structural Episode.

Trace element geochemistry and genesis of Precambrian sub-alkaline mafic dikes from the central Indian craton: evidence for mantle metasomatism

Precambrian mafic dikes transect the Archaean granite gneisses of the southern Bastar craton, Central India. On the basis of high-field strength elements, the Bastar mafic dike swarms were sub-divided into an earlier BD1 and a later BD2 system. The BD1 set comprises amphibolites, whereas the BD2 set consists of dolerites/metadolerites. On the basis of field relationships and available age data for enveloping granite rocks, the BD1 dikes are regarded as middle Archaean in age and BD2 dikes are Paleoproterozoic (∼1.8 Ga). The silica and total-alkalis contents of dikes of both swarms classified them mostly as sub-alkaline basalts. The BD1 dikes are low Ti+Fe and high-Mg olivine to quartz normative rocks, whereas the BD2 dikes are predominantly quartz normative with relatively high Ti+Fe and low-Mg contents. The high-field strength element concentrations are relatively higher in the BD2 dikes than in the BD1 dikes for comparable values of MgO and/or Cr. Judging from the geochemical data, these dikes probably were not contaminated by crustal components. The geochemistry and field observations clearly suggest a continental rift environment for the emplacement of both BD1 and BD2 dikes. Rare-earth element patterns and compatible–incompatible trace element modelling suggest that the two swarms were derived from distinctly different sub-alkaline basaltic magmas. The middle Archaean BD1 dikes were derived from ∼15–20% batch melting of a depleted lherzolite mantle source; in contrast, the Paleoproterozoic BD2 dikes were derived from ∼7–10% batch melting of a relatively enriched mantle source. A post Archaean increase in the thickness of metasomatized mantle lithosphere is the probable cause of mantle enrichment.

GRANITE FORMING EVENTS AND THEIR ROLE IN CRUST FORMATION OF THE INDIAN SHIELD

Continental crustal evolution in the Indian shield appears to have started around the middle Archaean and completed by the early Proterozoic. In this time span of around 700 - 800 Ma, the primordial gneissic crust has evolved, recycled and stabilized. The earliest phase of the granitoid activity, from the middle-to late Archaean consists predominantly of a tonalite-trondhjemite-granodiorite (TTG) suite, while the second phase of the Archaean - Proterozoic transition (APT) period consists predominantly of granodiorite-adamellite suite. Minor granite forming events have occurred in the middle-to late Proterozoic. The compositional changes in the granites from the Archaean to the Proterozoic appear to have been mainly controlled by the tectonics and the source compositions at each stage and contribution from the mantle appears to be negligible. Increasing of some large ion lithophile (LIL) elements and decreasing of compatible trace elements in the APT granites compared to the earlier gneissic phase and the overall compositions of the latter (the gneisses) suggest that the middle- to late Archaean polyphase gneisses are the partial melts of the mafic rocks from greenstones and the various phases of the tonalite-trondhjemite-granodiorite (TTG), viz., the hornblende-biotite- and feldspathic gneisses (younging in the same order) are the successive fractionate of similar or same source. The tectonic setting at the time of formation of the TTG gneisses is more of a compressive regime and crustal thickening appears to have played a role in the initiation of partial melting of the source. The appearance of extensive K-rich batholithic granites throughout the shield in a limited time span of 200-300 Ma during the early Proterozoic, increase of LIL elements and high Th, Rb and Sr variations in these batholithic granites suggest that these granites are anatectic in nature and have formed due to the release of compressional forces and initiation of an extensional regime in the shield area. Though sporadic important alkalic granite bodies have formed in the middle to late Proterozoic, they are mostly restricted to the orogenic belts like the Aravalli, the Delhi, the Satpura and the Eastern Ghats. Many of these post-tectonic granites (mostly A-type) give evidence for crust-mantle interaction in their formation and their emplacement in a stable continental crust setting.

GRANITE FORMING EVENTS AND THEIR ROLE IN CRUST FORMATION OF THE INDIAN SHIELD

Continental crustal evolution in the Indian shield appears to have started around the middle Archaean and completed by the early Proterozoic. In this time span of around 700 - 800 Ma, the primordial gneissic crust has evolved, recycled and stabilized. The earliest phase of the granitoid activity, from the middle-to late Archaean consists predominantly of a tonalite-trondhjemite-granodiorite (TTG) suite, while the second phase of the Archaean - Proterozoic transition (APT) period consists predominantly of granodiorite-adamellite suite. Minor granite forming events have occurred in the middle-to late Proterozoic. The compositional changes in the granites from the Archaean to the Proterozoic appear to have been mainly controlled by the tectonics and the source compositions at each stage and contribution from the mantle appears to be negligible. Increasing of some large ion lithophile (LIL) elements and decreasing of compatible trace elements in the APT granites compared to the earlier gneissic phase and the overall compositions of the latter (the gneisses) suggest that the middle- to late Archaean polyphase gneisses are the partial melts of the mafic rocks from greenstones and the various phases of the tonalite-trondhjemite-granodiorite (TTG), viz., the hornblende-biotite- and feldspathic gneisses (younging in the same order) are the successive fractionate of similar or same source. The tectonic setting at the time of formation of the TTG gneisses is more of a compressive regime and crustal thickening appears to have played a role in the initiation of partial melting of the source. The appearance of extensive K-rich batholithic granites throughout the shield in a limited time span of 200-300 Ma during the early Proterozoic, increase of LIL elements and high Th, Rb and Sr variations in these batholithic granites suggest that these granites are anatectic in nature and have formed due to the release of compressional forces and initiation of an extensional regime in the shield area. Though sporadic important alkalic granite bodies have formed in the middle to late Proterozoic, they are mostly restricted to the orogenic belts like the Aravalli, the Delhi, the Satpura and the Eastern Ghats. Many of these post-tectonic granites (mostly A-type) give evidence for crust-mantle interaction in their formation and their emplacement in a stable continental crust setting.

Geological setting, mineralogy, geochemistry and genesis of the Middle Archaean Kalyadi copper deposit, western Dharwar craton, southern India

The Kalyadi polymetallic copper deposit occurs within the Middle Archaean (≥3.0 Ga), medium-grade Kalyadi schist belt which consists predominantly of ultramafic-mafic schists interbedded with chemogenic chert, detrital high Al-Mg schists and siliceous schists. This sedimentary exhalative type (SEDEX type) ore-body is the only copper deposit hosted in cherts in the western Dharwar craton. The Kalyadi supracrustal rocks are intruded by tonalite-trondhjemitic gneisses (ca. 3.0 Ga) and granite (ca. 2.6 Ga). The Kalyadi copper deposit is polygenetic in nature. The primary ores represented by disseminations of pyrite ± linneite and chalcopyrite ± magnetite essentially along the bedding lamination of the metachert are referred to as the metamorphosed chert-sulphide rhythmites of a primary stratiform type. The ore is of low-grade and records imprints of at least two events of deformation. Pyrite is characterised by high-Co values (262–4524 ppm) and high–Co/Ni ratios (3.0–19.7). Rare earth element patterns of the primary ores and the host metacherts are identical, characterised by La enrichment, absence of Eu anomalies and flat to depleted HREE patterns with δ34 S = −0.8‰. The secondary (remobilised) ores are structurally controlled occurring as veins and stringers discordant to the bedding lamination or schistosity. The constituent ores are chalcopyrite-pyrite-pyrrhotite with minor pentlandite. These sulphides with low-Co/Ni ratios (0.87–1.80), have either a strong positive or negative Eu anomaly and show slight HREE enrichment. The δ34 S value ranges from +2.64 to −4.29‰. It is interpreted that the primary stratiform ores and the cherts were derived from volcanogenic hydrothermal fluids as syngenetic/chemical deposits in a deep sea environment. The secondary epigenetic mineralisation is related to subsequent migmatisation, deformational events and granitic activity.

An Archaean tectonic model of the Dharwar craton, southern India: the origin of the Holenarasipur greenstone belt (Hussan district, Karnataka) and reinterpretation of the Sargur-Dharwar relationship

The occurrence of the Holenarasipur greenstone belt is explained by accretionary process at a trench. A sequence of amphibolite (MORB), chert, and banded iron formation associated with komatiitic amphibolite (oceanic island material) is an Archaean analogue to the modern oceanic crust grown through the migration from an active ridge to a trench. At a trench, slices of such a sequence mixed with or became covered by turbidite from land mass (quartzite and conglomerate) to form an accretionary complex. Available SHRIMP data suggest the synchronous formation of nearby tonalite-trondhjemite-granodiorite (TTG) with the greenstone belt by melting of subducting oceanic plate. The Dharwar craton has been built up stepwise: (1) Sargur stage—ocean-ocean collision to form oceanic island arc with TTG and an accretionary complex (older greenstone belt), followed by the collision of such arcs to form mini-continents at the early to middle Archaean (unit 3.0 Ga); (2) Dharwar stage—the amalgamation of mini-continents with newer accretionary complexes (younger greenstone belt) under the same subduction polarity caused by the change from two layered to whole mantle convection with the progressive cooling of the earth (3.0−2.5 Ga). The cessation of TTG activity was also due to the cooling of the earth. Tectospheric peridotite keel formed with TTG has acted as a thermal insulator to stabilize the Dharwar craton after the Archaean.

Major-, trace-, and rare-earth-element geochemistry of the Archaean Maggo gneisses, southern Nain Province, Labrador

The early middle Archaean Maggo gneisses of the southern Nain Province, Labrador, form the southwest portion of the once contiguous North Atlantic (Nutak) Craton (NAC). The gneisses and their late middle Archaean metamorphosed and migmatized equivalents are typical of grey gneiss terranes exposed worldwide. Geochemically the gneisses exhibit a continuous range of composition from 53.7 to 78.4 wt.% SiO2 and straddle the boundary between low- and high-Al trondhjemites. Major-element distributions are comparable to those of other Archaean-aged NAC gneisses (Amîtsoq, Uivak, and Nûk gneisses), however, the Na2O and K2O contents are scattered. The gneisses are depleted in K, Rb, and Ba, are enriched in Sr, and have high-field-strength-element distributions similar to those for NAC gneisses.Rare-earth-element (REE) patterns of Maggo gneisses can be subdivided, on the basis of the nature of the Eu anomaly, into two groups of samples: (i) with negative Eu anomalies and higher Σ REE contents and (ii) with positive to normal Eu anomalies and lower Σ REE contents. The subdivision reflects differentiation processes in the parent magma of the Maggo gneisses. REE patterns are similar to those reported for NAC grey gneiss complexes. On the basis of the (La/Yb)N and (Yb)N values, the Maggo gneisses parent magma is interpreted as being derived by partial-melting processes from preexisting, high-grade (granulite to amphibolite facies) sialic continental crust equivalent to the early Archaean lithologies preserved elsewhere in the NAC.

A comparison of Sr‐Nd‐Pb isotopes in young and old continental lithospheric mantle: Patagonia and Eastern China

It is commonly accepted that beneath the continental crust lies a keel of lithospheric mantle, which extends 50–200 kilometres downward to a transition zone into the asthenosphere. The chemical and physical properties of this reservoir are best known through studies of the basalts and xenoliths that provide samples of the subcrustal mantle. Although sharing many characteristics with oceanic island basalts, some continental basalts become increasingly distinct isotopically as crustal age increases, strongly supporting a permanent association between crust and mantle. Consequently, the distinctive trace element and isotope composition of the lithospheric mantle is able to give important clues to its origin and evolution. The mantle under newly‐created crust is typified by a radiogenic isotope variability that emphasizes the materials from which the continental lithosphere is assembled. Old lithospheric mantle, on the other hand, exhibits more evolved isotopic patterns that attest to the existence of long‐lived, chemically complex systems. A comparison of the Pb, Sr and Nd isotopes in alkalic to sub‐alkalic basalt derived from Phanerozoic (Patagonia) and Middle Archaean to Early Proterozoic (eastern China) subcrustal mantle is useful for identifying ‘end‐member’ components of the lithosphere. One component, having an isotopic composition close to PREMA, either continues to evolve virtually unchanged after incorporation into the lithosphere or is, itself, a relatively new addition even to old lithosphere. Another component, beginning with the isotopic composition of BSE, undergoes significant reduction in U/Pb and Sm/Nd (but not Rb/Sr) upon incorporation into the lithosphere and, with time, shows an increasingly retarded evolution of 206Pb/204Pb and negative εNd‐values approaching the isotopic composition of EMI. Five models are discussed that relate the isotopic composition of the continental lithospheric mantle to that of other parts of the terrestrial system, which may be involved in its origin and evolution. The potential locations of the contributing components and the mechanisms and timing of their assembly into lithosphere are considered. Current knowledge, however, does not allow us to distinguish unequivocally among the various scenarios for the creation and evolution of this reservoir.

Rb–Sr geochronology of the Archaean Maggo gneiss from the Hopedale block, Nain Province, Labrador

The Maggo gneiss, a typical grey gneiss from the Hopedale block of coastal Labrador (southern Nain Province), preserves structures of two major Archaean tectonothermal events, the Hopedalian and the younger Fiordian. Rb–Sr whole-rock analyses of four suites representing the Hopedalian event (3140 ± 95 Ma (1σ), 3125 ± 43 Ma, 3028 ± 199 Ma, and 3025 ± 163 Ma) fall within the early middle Archaean and represent a period of cratonization of the Hopedale block.Two suites with a younger Fiordian structural fabric (2899 ± 116 Ma and 2804 ± 100 Ma) fall within the late middle Archaean and are interpreted as dating a period of reworking that locally reset the Rb–Sr isotopic systematics of the Maggo gneiss. Fiordian ages were also determined from four suites (2927 ± 50 Ma, 2884 ± 86 Ma, 2854 ± 83 Ma, and 2764 ± 89 Ma) that do not display the Fiordian structural fabric, indicating that chemical and mechanical components of the Fiordian event did not occur simultaneously. Age determinations of the Fiordian do not date specific geologic events but seemingly reflect varying degrees of protracted isotopic homogenization during chemical and mineralogical reequilibration.One suite of analyses yields an age of 3305 ± 75 Ma and is interpreted as dating a pre-Hopedalian event. This older age supports field evidence as to the existence of old crust in the Hopedale block, typical also of rocks of the North Atlantic Craton.

The Early Precambrian Rocks, Their Ages, Division and Correlation of the Taihang‐Wutai Region, the Yanshan Region and the Eastern Sector of the Yinshan Mountains

Abstract According to differences of the protolith formations, the early Precambrian strata in the northern part of the North China platform may be divided into the stable stratigraphic region in the west and the mobile stratigraphic region in the east. Based on unconformities, either stratiragphic or tectonic, as well as significant metamorphic thermal events, the two regions may be stratigraphically defined as follows: 1) the middle Archaean Fuping Supergroup composed of the Chenzhuang and Wanzi Groups (stable areas), and the middle Archaean Qianxi Group (mobile area), whose upper limits are all dated at 2800 Ma; and 2) the upper Archaean Wutai Supergroup composed of the Longquanguan, Shizui and Taihuai Groups (stable areas), and the upper Archaean Zunhua, Dantazi and Zhuzhangzi Groups (mobile areas), whose upper limits are all dated at 2500 Ma. A correlation of the above‐mentioned units is also made. The lower Proterozoic Hutuo Group of the stable region is adjusted to comprise the Gaofan, Doucun, Dongye and Guojiazhai Groups. The upper limit of the Gaofan Group is placed at 2350 Ma, Dongye 1850 Ma and Guojiazhai (the lower limit of the Changcheng System) 1700 Ma.

Sulfur isotope studies in early Archaean sediments from Isua, West Greenland: Implications for the antiquity of bacterial sulfate reduction

The sulfur contents and sulfur isotope ratios ( δ 34 S) have been measured for samples collected from the Isua area of West Greenland in an effort to place narrower limits on the time of the rise of sulfate respirers during the Precambrian. The δ 34S values of the Isua sediments (3.7 × 10 9 yr old) including the various facies of the banded iron-formations have their mean values close to zero %. (CDT) (±0.5%.) with a standard deviation of less than 1%.. This comes extremely close to the respective means yielded by the presumed tuffaceous amphibolites (+ 0.3 ± 0.9%.) and by the somewhat younger, between 3.1 and 3.7 ± 10 9 yr, basaltic Ameralik dykes of the region (+ 0.6 ± 1.1%.). In view of the regional distribution of the Isua banded iron-formation sediments, the variety of environmental conditions under which the various facies were deposited and the complete absence of isotopic evidence for sulfate reducers, in contrast to the banded iron-formations of the middle Archaean ( δ 34 S varies from −20 to +20%.), it seems most unlikely that evidence for 'sulfate reducers' existed or will be found in other sediments of Isua age. The very small spread in δ 34S values for the Isua sediments is interpreted as due to minor fractionation during the passage of endogenic sulfur phases to their present sites of emplacement within the sedimentary succession.

The primitive crust: Evidence from the Indian shield

Abstract Four distinct geological settings appear to have existed in the Indian Peninsula during the Precambrian Era. These are represented by (1) Early and Middle Archaean 1 ‘true greenstone belts’ of Holenarasipur, Nuggihalli, Kolar and Sargur, (2) Late Archaean—Early Proterozoic ‘greenstone-like geosynclinal piles’ of Shimoga-Chitradurga, (3) high-grade derivatives of greenstones and granulites of Tamil Nadu-Kerala and (4) Proterozoic platforms of Cuddapah-Vindhyan basins. The ‘greenstone belts’ of Holenarasipur type are elongated, sometimes symmetrical, belts of rhythmic volcanism and fine-grained detrital and chemical sedimentation. The base of these greenstone belts is made up of basic and ultrabasic flows of komatiitic affinities and these are intruded by tonalites and trondhjemites. The main Dharwar greenstone-like geosynclinal piles start with orthoquartzite-carbonate facies showing ripple marks and current-bedding and these develop into thick, graded-bedded eugeosynclinal piles and do not show the characteristic rhythmic volcanism, sedimentation and symmetry of ‘true greenstone belts’. Their volcanism is of a calc-alkaline type. Recent data on these geological settings from the South Indian Shield suggest a thin simatic protocrust with a highly undifferentiated protomantle existing during the Early and Middle Archaean. In the cracks developed due to degassing of the protomantle, ‘true greenstone belts‘ were laid down and the products of partial melting of the basic crust at comparatively shallow depth, i.e. tonalite-trondhjemites were emplaced. This process formed the continental nuclei of mixed, but predominantly basic, composition. Between these continental nuclei, geosynclines were formed in which Dharwar-type greenstone piles were laid down. Partial melting of the lower crust of the upper mantle, at a deeper level than the depth of genesis of trondhjemite, gave rise to potassic granites and resulted in more than two (?) episodes of widespread potash reactivation marking the completion of cratonisation, following which emplacement of dike swarms took place.