Late Precambrian Time Research Articles

Glacially- and tidally-influenced shallow marine sedimentation of the late Precambrian Port Askaig Formation, Scotland

The late Precambrian Port Askaig Formation forms part of the thick succession of metasediments and volcanics of the Dalradian Supergroup of northwest Britain. The glacianegnic Port Askaig Formation is well exposed on the Garvellach Islands in western Scotland and consists of a series of thick and regionally extensive diamictites interbedded with sandstones, siltstones, dolomites and minor conglomerates. Diamictite facies are similar to glaciomarine diamicts described from the Late Cenozoic Yakataga Formation in the Gulf of Alaska, formed by settling of fine-grained suspended sediment and ice-rafting in relatively shallow water (< 250 m) . Two particularly distinctive diamictite units, the Great Breccia and the Disrupted Beds are interpreted as the product of downslope resedimentation events; it is significant that the Port Askaig Formation was deposited during a period of increasing tectonic activity in the Dalradian Basin. Thick and extensive sand sequences interbedded with the diamictite facies are interpreted as tidally influenced shallow marine shelf deposits; sands may have been derived from deltaic sources on the basin margin and transported into the basin during storms. The entire Port Askaig Formation appears to represent an overall shallowing upwards sequence punctuated by smaller scale transgressive (diamictite) and regressive (sandstone) events. Interpretation of the Port Askaig Formation as a succession of glacially and tidally-influenced shelf sediments has significant implications for the reconstruction of palaeoclimatic histories and regional palaeogeographies during late Precambrian time.

Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.

Late Precambrian crustal accretion rates in northeast Africa and Arabia

Apparent late Precambrian growth rates for continental crust in the Arabian-Nubian craton are implausibly high for simple arcaccretion models. If correct, these rates imply that some special, non uniformitarian process occurred to rapidly produce continental crust. Assuming that the dominant mechanism for continental growth is by arc magmatism, roughly 100% of global island-arc production at current rates would have had to accrete in this region from 900 to 600 Ma. One solution to this paradox is that large amounts of preexisting sialic basement have so far gone undetected and should not be included in the estimate for crustal growth during late Precambrian time. Published radiometric data indicate that older continental material makes up the eastern and western margins of the exposed shield. We interpret these and other data to suggest that the larger part of the Arabian-Nubian craton, including the platform obscured by Phanerozoic cover east and west of the exposed shield, is older continental crust. In this case, a growth rate approximately 20% of the modern global rate is required, and nonuniformitarian crustal growth is not necessary to explain late Precambrian accretion of the Arabian-Nubian craton.

Metasomatised Xenoliths from Foster Crater, Antarctica: Implications for Lithospheric Structure and Processes beneath the Transantarctic Mountain Front

Xenoliths in basanite from Foster Crater in the foothills of the Transantarctic Mountains have been used to construct a model of the subcontinental lithosphere and lithospheric processes in this segment of Antarctica. The mafic and ultramafic xenoliths define two mineralogically, texturally and chemically distinct populations. One group is identical to Group II xenoliths, as defined by Frey & Prinz (1978). These have igneous cumulate textures and include wehrlites, rare websterites, dunite and amphibolite. In these xenoliths, the amphibole is always kaersutite and the (very rare) mica a Ti-rich phlogopite (TiO2 > 5%). The other xenoliths have complex metamorphic textures ranging from protogranular to porphyroclastic and are dominated by clinopyroxenites (cpx: Mg/(Mg + Fe) = 0·85–0·95; Ca/(Ca + Mg)>0·53) which show varying degrees of replacement by metasomatic phlogopite. Spinel and anorthite are also present. Collectively, these xenoliths are called the High Calcium Pyroxene Suite (HCPS) in deference to their unusual pyroxene compositions. Moreover, their spinel assemblages indicate equilibrium under variable fO2 conditions contrasting with the Group II spinels which equilibrated at relatively constant fO2. The mineralogy and chemistry of the HCPS xenoliths are unlike meta-igneous rocks and more closely resemble calc-silicate granulites. Thermobarometric calculations on Group II websterite assemblages and comparison with experimental equilibria on granulites and phlogopite stability suggest equilibration temperatures between 850 and 950° at 5 kb pressure, consistent with equilibration at mid to lower crustal depths. A number of processes have acted to modify Group II and HCPS xenoliths including K-metasomatism, dynamic recrystallization, melt generation, melt infiltration and oxidation. The first two are restricted to the HCPS xenoliths whereas the latter affect both groups. The interaction of these processes highlights the differences between Group II and HCPS protolith assemblages and is consistent with derivation from a complex section of lithosphere which has experienced major changes in tectonomagmatic activity since late PreCambrian times.

Contrasting metamorphic terranes near Chéticamp, Cape Breton Highlands, Nova Scotia

Two contrasting metamorphic terranes can be recognized in northwestern Cape Breton Island. One terrane (Pleasant Bay complex) consists of biotite gneiss and quartzite with minor calc-silicate lenses that were metamorphosed in Late Precambrian time (about 550 Ma) and were subsequently intruded by Silurian salic and mafic plutons that were, in turn, deformed and intruded by granite in Devonian time. The other terrane (Jumping Brook complex) consists of volcanogenic and sedimentary schists of probable Silurian age that were metamorphosed in Devonian time. P–T estimates indicate that the older parts of the Pleasant Bay complex were metamorphosed at about 790 °C and 7 kbar (1 kbar = 100 MPa) at low to moderate water fugacities during a major intrusive episode. The Jumping Brook complex exhibits a single progressive metamorphic sequence now disrupted by faulting. P–T conditions during this Devonian (370–390 Ma) metamorphism varied from greenschist (300 °C at &lt;3 kbar) to amphibolite (650 °C at 4 kbar) facies. Metamorphism probably occurred in a thermal dome. The data suggest a moderately deformed basement–cover relation between the Pleasant Bay and Jumping Brook complexes.

The Pancarli nickel-copper sulfide mineralization, eastern Turkey

The Pancarli Ni-Cu mineralization is located in the metamorphic sequence of the Bitlis massif consisting of biotite gneiss, quartz-feldspar gneiss, amphibolite, and metagranitic rocks. The rocks are probably Precambrian in age and have been affected by regional amphibolite-facies metamorphism and by a later cataclasis. There is also evidence of an earlier eclogite-facies metamorphism. The Ni-Cu mineralization occurs as massive sulfide lenses aligned parallel to the penetrative foliation along various levels. The lenses are up to 3 m in length and 2 m in width. The host rock in the immediate vicinity of the orebodies is generally quartz-feldspar gneiss or more rarely quartz-feldspar gneiss and amphibolite. The mineral paragenesis of the ore consisting of pyrrhotite, pentlandite, and chalcopyrite and the low As and Co contents of the ore indicate an orhomagmatic origin of the mineraliza tion. The textures and chemistry of the spinel minerals in the Pancarli deposit together with Cu/(Cu+Ni) ratio of 0.29 of the ore suggest that the mineralization is genetically related to a basic magma. The only mafic rocks in the area are amphibolites that are characterized by a tholeiitic basaltic chemical composition. However, no imprignation and network ores are present in any of the amphibolite bodies. A model based on the segregation of a sulfide melt fraction from a basaltic magma in a magma chamber has been proposed. The subsequent and independent intrusions of the two fractions into the country rocks account for the absence of the impregnation and network ores in the amphibolites, and explain the similarities in geologic setting of the ore and amphibolites as well as their observed field relationships. Structural and textural features of the ores indicate that they have been subjected to regional metamorphism and the later cataclastic deformation that affected the country rocks. The author believes that the ores probably formed during the Pan-African orogenic development of the Bitlis massif in Late Precambrian time. In their present state, the Pancarli ores exhibit varying degrees of weathering with supergene assemblages.

Sedimentary environments of the Simla Group (Upper Precambrian), Lesser Himalaya, and their palaeotectonic significance

The Simla Group of the western Lesser Himalaya comprises a prograding muddy delta sequence that was built by the collapse of a carbonate shelf during Late Precambrian time. The Simla sequence consists of a thick, about 1500 m, coarsening-upwards clastic succession which is divisible into the Basantpur, Chhaosa and Sanjauli Formations. Carbonate appears only in the basal formation (Basantpur Formation). Three subfacies for the basal Simla have been discerned: a carbonate platform to the northeast (Basantpur subfacies), shallow water with more clastic influence to the south (Kunihar subfacies) and relatively deep water adjacent to a fault scarp to the northwest (Kashlog subfacies). The first signs of deltaic sediments appear in the middle of the Basantpur Formation and are well developed in the overlying Chhaosa Formation. The overall characteristics of the Chhaosa facies are those of a delta-front and distributary mouth bar where floods from distributaries and shelf storms lead to very rapid deposition, mostly above wave base. The uppermost Sanjauli Formation is relatively thick bedded, coarse grained and more frequently cross-bedded representing delta platform and distributary channel deposits. A significant change in environment of deposition is observed in the overlying beds, previously known as the Upper Sanjauli Member, and now being separated as the Jagas Formation. The Jagas facies suggests major uplifts and a climatic change to semi-arid conditions. The detritus of the Simla delta came from the Indian Shield situated to the south of the delta. Most constituents indicate mature sediments with occasional lithoclasts of sedimentary and intraformational characters. Faulting presumably continued during the deposition of the Simla Group as is characteristic for rifted continental margins.

Tectonic evolution of the Bala Lineament in the Welsh Basin

The Bala Lineament is one of several NE–SW major linear fault zones traversing the Lower Palaeozoic Welsh Basin. Recent published and unpublished information relating to the lineament is synthesized and an evolutionary model is proposed. Geophysical data suggest it was initiated as a strike-slip structure, probably in late Precambrian times. Unlike the other lineaments, it had no separate Lower Palaeozoic identity; during that period, sedimentation was strongly influenced by contemporary faulting but there is no reason to suspect that deformation was confined to a long, narrow, NE–SW belt or that neighbouring regions were not similarly affected. Moreover, the lineament is not distinguishable from adjoining regions by anomalous end-Caledonian strains. In the Phanerozoic, the lineament became a recognizably discrete tectonic zone during or after the end-Caledonian regional deformation. Then, cleavage and folds along part of the lineament were rotated, and overprinted by a local cleavage, in a broad NE–SW dextral shear zone. This ductile event perhaps denotes the onset of simple shear deformation which affected Carboniferous sedimentation in northeast Wales and culminated in major strike-slip faulting. These faults, which dominate the geological maps of this part of Wales and include the Bala Fault itself, are essentially Variscan structures; their sense and amount of displacement remains uncertain. Previously underemphasized N–S Lower Palaeozoic faults are shown to be as important as the more familiar NE–SW structures in influencing sedimentation patterns along the lineament. It is suggested that the Lower Palaeozoic combination of N–S and NE–SW structures is widespread in the Welsh Basin and is due to distributed regional sinistral shear. Both sets of structures were reactivated repeatedly. In Cardigan Bay, Mesozoic–Tertiary sedimentary basins subsided on these structures, whilst the 1934 Lleyn earthquake and other historical seismicity denote neotectonic activity on the same structures.

Tectonic development of the New Madrid rift complex, Mississippi embayment, North America

Geological and geophysical studies of the New Madrid Seismic Zone have revealed a buried late Precambrian rift beneath the upper Mississippi Embayment area. The rift has influenced the tectonics and geologic history of the area since late Precambrian time and is presently associated with the contemporary earthquake activity of the New Madrid Seismic Zone. The rift formed during late Precambrian to earliest Cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions. The configuration of the buried rift is interpreted from gravity, magnetic, seismic refraction, seismic reflection and stratigraphic studies. The increased mass of the crust in the rift zone, which is reflected by regional positive gravity anomalies over the upper Mississippi Embayment area, has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex. The correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures. The slippage is due to reactivation of the structure by the contemporary, nearly E-W regional compressive stress which is the result of plate motions.

The role of the Cruachan Lineament during Dalradian evolution

Synopsis Geological and geophysical evidence is reviewed for the existence of a trans-Caledonoid (NW–SE) lineament, the Cruachan Lineament, in the Scottish Caledonides, separating a high-density SW Highland block dominated by abundant late Precambrian Dalradian mafic rocks in which the Moine is absent from a low density Moine plus Dalradian Central Highland block poor in mafic rocks. Contrasted metamorphic P-T-time histories are predicted on opposite sides of the lineament. An alternative model of early Dalradian evolution is proposed in which basaltic volcanism and sedimentation in the SW Scottish Highlands occurred in a quasi-oceanic trans-Caledonoid pull-apart basin bounded along its NE side by a major fault now identified as the Cruachan Lineament. The Dalradian basin(s) may have experienced right-lateral translation relative to the crustal block to the SE along a dextral transcurrent fault zone possibly in the vicinity of the Highland Boundary Fault during late Precambrian time. In this model, the Dalradian terrain may provide no evidence regarding the early history of the NW side of the lapetus Ocean.

Structural-metamorphic evolution of late proterozoic ophiolites and precambrian basement in the central asian foldbelt of Mongolia

Abstract The late Proterozoic and Palaeozoic Central Asian foldbelt is regarded as one of the best examples which demonstrates that processes of lateral crustal growth and obduction—accretion tectonics took place since at least late Precambrian times. In Mongolia the Precambrian and early Cambrian rocks were formed in two different environments: continental and oceanic. The Archaean to early Proterozoic complexes are autochthonous and constitute a metamorphic continental basement. The late Proterozoic to early Cambrian ophiolite complexes represent remnants of obducted oceanic crust and occur as intensely deformed nappes and tectonic melanges overthrust on the older basement during the Caledonian orogeny. Distinctly differentiated tholeiitic, calc-alkaline and boninite rocks series together with the lithological characteristics of associated volcanogenic-sedimentary formations indicate that the Mongolian ophiolite complexes were generated in a forearc-marginal basin setting. The culmination of ophiolite formation occurred in late Precambrian to early Cambrian time. Five phases of progressive regional metamorphism have been identified in the basement complex and overlying ophiolite association. Each of these was accompanied by folding and granitoid plutonism and is characterised by a distinct composition of fluid inclusions.

Late Precambrian scyphozoan medusaMawsonites randellensissp. nov. and its significance in the Ediacara metazoan assemblage, South Australia

Ediacara metazoan assemblage, Ediacara Range, South Australia. The size and morphological complexity of this species are apparently of the scyphozoan grade. The characteristic three major zones and deeply cleft marginal lappets of this fossil form are similar to those of the living Coronatae but pedalia are not developed. This new species sheds significant light on the previously enigmatic Mawsonites Glaessner & Wade, and provides the basis for the erection of the Family Mawsonitidae under ?Order Coronatae. The present study confirms the fairly advanced evolutionary grade reached by medusae during Late Precambrian time (the Ediacaran or Ediacarian Period).

The geological and geomorphological setting of the Estuary and Firth of Clyde

SynopsisIn late-Precambrian and early Palaeozoic times, the Clyde Sea Area lay at the junction of a major land mass and the Iapetus Ocean. Closure of the Ocean on its northwestern margin corresponded broadly with initiation of the Highland Boundary and Southern Uplands Faults and formation of southwestnortheast oriented basins of continental sedimentation. Northwest-southeast fractures of the Clyde Belt, along which the Northeast Arran and Southwest Arran troughs of the firth are emplaced, developed across the southwest-northeast Caledonoid grain in Carboniferous-Permian times. The firth and estuary began to assume their distinctive forms in Tertiary times. The area experienced several major glaciations during the Quaternary Period. Late Devensian effects are most distinct: overdeepening of the firth to depths exceeding — 160mOD; deposition of till and meltwater sediments up to 80 m in thickness. Clyde Beds, fossiliferous clays and silty sands up to 15 m in thickness, accumulated c. 13,150–10,000 BP in sub-arctic marine waters in the firth, estuary and Paisley embayment, and in the Lomond basin prior to the Loch Lomond Stadial. The Holocene marine transgression, which produced further modifications in shoreline position, e.g. in Ayrshire, the Paisley embayment and the Lomond basin, was followed by marine regression due to isostatic land rebound.

Rift Structure of the Northern Mississippi Embayment from the analysis of gravity and magnetic data

Gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern Mississippi Embayment which is compatible with geologic and seismic refraction data. The results indicate that this region is a site of a late Precambrian‐early Paleozoic rift system. An anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting. The thickest part of anomalous crust occurs beneath the region of greatest seismic activity, which lies within the geographic limits of the graben. It is suggested from this correlation that strain accumulates within the anomalous crust layer, which acts as an inhomogeneity in a relatively homogeneous lithosphere. Based on the results of this study and previous geophysical studies, the complex tectonic evolution of the upper Mississippi Embayment since late Precambrian time is rift formation possibly along a pre‐existing shear zone, graben development in Cambrian time, reactivations in Permian and Cretaceous time, and subsequent formation of the present Mississippi Embayment and fault zones with large influences from rift structures.

Phycological expertise in geological applications

Our knowledge about fossil algae has been biased for a long time towards groups with calcified tissues such as dasyclads and crustose rhodophytes. This picture has recently begun to change owing to the growing number of discoveries of silicified microbial fossils and fossil microbial endoliths. There is good evidence that both groups contain a large proportion of photosynthetic organisms, particularly algae. Silicified microbial fossils are best known from the Precambrian strata, while microbial endoliths, which apparently evolved during middle to late Precambrian times are common throughout the Phanerozoic including the present. Because of the small size of these fossils, it is possible to study and morphometrically evaluate entire populations rather than individual specimens or body fragments. Thus, we are able to approach fossil taxa using largely biological rather than strictly paleontological criteria and to apply interpretations and reconstructions of biological species. Phycologists entering this field can contribute to the interpretation of such fossils by applying comparisons with modern algae, their life cycles, cellular organization, and cell division patterns. However, they have to learn to deal with post-mortem degradation and diagenetic changes, and to recognize algal remains from mineral features that surround and often obliterate the fossils. This work is often similar to that of a detective or coroner where the use of intuition and imagination is just as dangerous as it is necessary. Detection of microfossils between and within single carbonate grains in otherwise ‘unfossiliferous’ rocks can provide valuable information for applied geology including petroleum exploration.

Radiometric dating of Late Precambrian times

Abstract Between about 1979 and 1981 new radiometric data relevant to the numerical age of the Precambrian-Cambrian boundary became available. Based on this it was suggested in a recent review (Odin et al. 1983) that the numerical age for that boundary was probably much younger than usually accepted, lying probably in the interval of time 520-540 Ma rather than at about 580-600 Ma. This new estimate has provoked discussions which essentially concern the stratigraphical location appropriate to these modern dates. In addition, it has been argued that this new view of the location of the Precambrian-Cambrian boundary did not take into account all the available geochronological results. We consider this last remark as unfounded at the time when our review was submitted for publication (at the beginning of 1982). However, one result of our review seems to have been to rejuvenate interest in the subject so that there is now more data to be discussed. Although the scarcity of fully published results still permits one at present only to propose tentative conclusions, some facts seem well enough established; conversely, in our opinion, other data are certainly not as soundly based as some authors aver. The present comment tries to give complementary information to enable others to judge both our own arguments as well as those of our colleagues. A fundamental new contribution to this problem is in progress, i.e. a formal definition of the Precambrian-Cambrian boundary within the Meishucun stage of China (apparently now strongly advocated by the IUGS-IGCP Working Group

Proterozoic links between Northeastern Brazil and West Africa: A plate tectonic model based on gravity data

Using the Bullard predrift reconstruction for the southern Atlantic, analysis of the major gravimetric trends of the West African craton (2000 Ma) and the surrounding mobile belts allows us to delineate the upper Proterozoic shield boundary and to put forward an evolutionary model for late Precambrian time. Over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults. Gravity anomalies and LANDSAT imagery show a NE-SW lineament in West Africa which can be correlated with a variety of geologic features, the largest being the late Precambrian Gourma aulacogen. At the margins of the craton the deformation patterns and gravity signatures associated with the 600 Ma orogeny were largely controlled by the geometry of the stable craton itself. On the eastern edge (Pan-African belt and Brazilian belt east of the São Luis and Amazonian cratons), the distribution of the anomalies and deduced structures can be explained by a continental collision process. On the western and southern edge (Brazilian belt between the São Luis and Amazonian cratons, Rokelide belt, Mauritanide belt) the gravity signature is different, suggesting a large amount of intensely tectonized basic rocks. A limited spreading process followed by an intense compressional episode is suggested.

The Vendian succession of northeastern Spitsbergen: Petrogenesis of a dolomite-tillite association

The sedimentary record of late Precambrian time is magnificently displayed in the highland snowfields of northeastern Spitsbergen (Svalbard). Vendian strata are represented essentially by the Polarisbreen Group which consists mostly of dolostone and includes two dolomitic glacial units. The oldest sediments in the Polarisbreen Group compose the Elbobreen Formation (c. 400 m), which is divided into four laterally-persistent members. The Lower Carbonate Member (E1, 125 m) contains a distinctive basal dark-grey limestone (with microspar-filled synaeresis cracks) suggested to be of lagoonal origin and associated with minor dolostone, shale and chert. Higher parts of the member are dominantly dolostone, partly stromatolitic, with some shale and sandstone; shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite. Penecontemporaneous dolomite is partially overprinted by microsparry dolomite, thought to be of groundwater origin. The redefined Petrovbreen Member (E2) consists of diamictite and other detrital dolostone. Pronounced thickness variations (2–40 m) are thought to be original depositional features. The member represents the deposits of a short glacial period in which the following depositional processes are inferred: lodgement (massive diamictite), subaqueous meltout (massive and bedded diamictite), ice-rafting (lithologies bearing dropstones, and possibly also diamictite), redeposition by sediment gravity flows (some diamictite and conglomerate; rhythmite and shale), current winnowing (thin tabular conglomerate), subaerial or subaqueous meltwater action (channelled conglomerate and sandstone), periglacial shrinkage (diamictite wedge-fillings). The MacDonaldryggen Member (E3, 230 m) is a monotonous succession of shaly dolostone of lagoonal origin. It grades up into the Slangen Member (E4, 25 m) which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment. The Wilsonbreen Formation (160 m) represents a return to glacial deposition, but this time longer-lasting and with substantial extra-basinal material represented. The Gropbreen Member (W1, 28–107 m) and the Ormen Member (W3, 44–139 m) consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a Middle Carbonate Member (W2, 3–30 m) which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone. The same depositional processes can be recognised as in the Petrovbreen Member, but the Wilsonbreen Formation is overall of somewhat more continental aspect (lower proportion of rhythmite and dropstone structures). In addition there are: basal breccia and crack-fillings formed by frost-shattering of the underlying cemented dolostone, tabular sandstone thought to be formed by wave reworking of outwash, a striated (terrestrial) cobble pavement, supraglacially-derived breccia horizons, periglacial wedges filled by sand and the W2 assemblage of possible lacustrine origin. The Dracoisen Formation (525 m) represents an abrupt return to non-glacial conditions. An upward-deepening wave-dominated succession of pure dolostone (D1, 20 m) and impure dolostone (D2, 105 m) is succeeded by offshore black shale (D3, 150 m) and then by a very-shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics (D5, 10 m) separating dolomitic sandstone and shale (D4, 80 m and D6, 150 m). The contact with the transgressive Cambrian sandstones above is poorly exposed. Deposition of the succession dominantly under marine conditions is inferred, but it is difficult to rule out a lacustrine environment at any particular horizon. This dolomite—tillite association can be explained by penecontemporaneous (and minor secondary) dolomite formation in marginal environments (with warm climatic indicators at some levels) being sharply interrupted, because of rapid climatic changes, by glacial sediments containing abundant detrital dolomite. Since the latter sediments make up only 17% of the 1080m-thick succession, glacial conditions only occupied a small proportion of Vendian time.

Abor Volcanics: further evidence for the birth of the Tethys Ocean in the Himalayan segment

The Himalayan region has a record of several phases of igneous activity which indicate a continued N—S orientated tensional regime since at least late Precambrian time. Though the earlier volcanic eruptions were restricted both in time and space, the Late Carboniferous-Triassic activity was widespread in terms of both the time interval and area covered. The Abor Volcanic activity in the Eastern Syntaxial Bend of the Himalayan fold belt is coeval with the initiation of Late Carboniferous—Triassic Panjal Trap volcanism of the Western Syntaxial Bend and eruption of the Sikkim lavas. Chemical characters of the Abor Volcanics indicate (1) a within-plate rift tectonic setting of the lava eruptions, (2) a transitional nature between tholeiitic and alkalic compositions, (3) relative enrichment in incompatible elements, and (4) high Fe-Ti values similar to many such basalts erupted in the regions of active lithospheric rifting. The olivine-free nature, a linear trend for MgO and low Co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry. However, large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes. Instead, these characters together with the low Zr/Nb ratio and within-plate rift tectonic setting suggests the modification of an upper mantle source region, possibly by fluid phase CO 2 immediately before the partial melting and lithospheric rifting. Combining geology of the region with the geochemistry and inferred tectonic setting of these rocks, the lava eruptions are suggested to have been associated with the 'rift valley stage' of the Tethys Ocean formation in the present Himalayan region.

The geochemistry of the Strathconon amphibolites, Northern Scotland

Synopsis The Strathconon ortho-amphibolites were intruded into Glenfinnan and Loch Eil Division metasediments of the Moine Assemblage before the area was deformed in late Precambrian time. A chemical study has indicated that they consist of a single suite of variously fractionated tholeiitic sills and dykes which can be distinguished chemically both from the Y-poor amphibolites of the Morar Nappe and from more Y-rich amphibolites within the Central Ross Nappe spatially associated with the Quoich Line in Inverness-shire. The chemical distinction between amphibolites from the two nappes requires that major transport of the overlying Central Ross Nappe occurred after the intrusion of the basic sills.