Showing papers on "Craton published in 1987"

Precambrian geology of India

Abstract: Introduction. Western Dharwar craton. Eastern Dharwar craton. Granulite terrain. Eastern Ghats. Bhandara craton. Singhdhum craton. Meghalaya. Aravalli craton. Rift valleys. Himalayas. Index

Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping

Abstract: Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.

Role of asthenosphere and lithosphere in the genesis of Late Cenozoic basaltic rocks from the Rio Grande Rift and adjacent regions of the southwestern United States

Abstract: Late Cenozoic alkalic and tholeiitic basalts from the Rio Grande rift region display a wide range of Nd and Sr isotopic compositions which indicate the involvement of “enriched” mantle (EM), “depleted” mantle (DM), and crust in basalt petrogenesis. Isotopic compositions of alkali basalts (this study and published data) reflect the characteristics of their mantle source and correlate with tectonic setting: In the southern Basin and Range, a region of pronounced lithospheric extension and thinning, alkali basalts with eNd = +7 to +8 are derived from DM, which corresponds to upwelled, oceanic-type asthenosphere; alkali basalts from the Great Plains, northern Rio Grande rift, and eastern Colorado Plateau, regions which have undergone less lithospheric extension, have distinctly different isotopic compositions (eNd = 0 to + 2) and were derived from EM; alkali basalts from the southeastern Colorado Plateau-Basin and Range transition zone have isotopic compositions that are intermediate between DM and EM values (eNd = + 6.6 to + 3.3) and were derived from the DM/EM boundary. The correlation of isotopic signatures and upper mantle geophysical properties with tectonic setting suggests a physical model for the evolution of basalt sources during lithospheric extension and rifting. Prior to regional extension and rifting, EM corresponds to lithospheric mantle with a history of trace element enrichment and isolation from the underlying asthenosphere. Once rifting begins, lithospheric extension and asthenospheric up-welling lead to thermal thinning and conversion of lithospheric mantle to asthenosphere. EM can take on asthenospheric physical properties, yet remain intact and a source of basalts until, at an advanced stage of rifting, it is physically replaced by convective mixing with the underlying DM/astheriosphere. Tholeiites, because they equilibrate at shallower depths than alkali basalts, generally equilibrate within EM. In contrast to alkali basalts, tholeiites typically are crustally contamined, probably because their lower volatile contents inhibit their ascent through the crust. Throughout much of the region, tholeiites assimilated small amounts ( < 10%) of middle to upper crustal wall rock. In contrast, a variety of volcanic rocks (including tholeiites) from within the rift have assimilated lower crustal wall rock, probably because the background temperatures in the lower crust of the rift are higher and because these rocks are associated with large-volume, long-lived volcanic fields that locally enhance thermal input into the lower crust. Lithospheric mantle of the Rio Grande rift region is younger than lithospheric mantle beneath the Archean craton to the north (1.7 versus 2.7 b.y. old), which is compatible with the higher eNd values of EM-derived volcanic rocks of the rift region. A “plum pudding” model of the mantle, in which the least refractory, more incompatible-element-enriched heterogeneities preferentially determine the isotopic composition of melts, can account for the isotopic compositions of basalts in the rift region.

Crustal genesis on the Oregon Plateau

Abstract: The areal and temporal distribution and the chemical and isotopic characteristics of Cenozoic volcanic rocks from the Oregon Plateau reflect the changing tectonic conditions affecting this area over the last 20 m.y. Following subduction zone related calc-alkaline volcanism, flood basalt eruptions occurred in the time interval 18–14 Ma. This primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the Juan de Fuca ridge that resulted in changes in the geometry of subduction against the Pacific Northwest. The compositions of the mid-Miocene flood basalts show signs of extensive crystal fractionation, in some cases accompanied by crustal contamination. Roughly 11 m.y. ago, basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions. This implies that as extension in the Oregon Plateau proceeded, the crust became less of an impediment to the passage of relatively unfractionated mantle melts. Initial Sr, Nd, and Pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava. Primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area. These observations suggest that an area of some 130,000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern North America over the last 20 m.y. The intensity of basalt volcanism as manifest in the Columbia River Group and in the Oregon Plateau does not appear to reflect the presence of a “hot spot” beneath the area. Both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by “back arc” spreading in a manner analogous to the formation of the marginal basins of the western Pacific. This spreading initiated along the western border of the Archean Wyoming craton with the location of mantle ascent, melt production, and extraction controlled by the changing geometry of subduction, the existence of a zone of lithospheric weakness, and the presence of a thick, low-density, subcontinental mantle “keel” existing beneath the Archean craton in this area.

The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt

Abstract: New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid-Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite-amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn-tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting. Sm-Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis. The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high-grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content. There is ubiquitous evidence for the resetting of Rb–Sr total-rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist-facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.

Distribuição regional e relações tectônicas do magmatismo pós-paleozoico no brasil

Abstract: The distribution of post-Palaeozoic igneous rocks in Brazil and its continental margin is summarized. It is suggested that those rocks are closely related to the structure and age of the basement and also to epeirogenic movements. The reactivated areas comprise many preexisting precambrian zones of structural weakness and some previous palaeozoic covers affected by younger tectonic movement s (such as tectonic arches, geosutures, flexures etc.). The activization occurred since the late Permian in the Amazon Craton and since the Triassic in other reglons. Basalt flows, sills and dykes tend to be concentrated in intracratonic sedimentary basins. The maximum total thicknesses of flows and sills in Parana and Parnaiba basins coincide with areas of maximum subsidence during the Palaeozoic. Dykes occur all over the precambrian shields, specially in areas of late stabilization. Alkaline rocks, including carbonatites and kimberlites, are concent rated in arches and flexures around the Parana Basin, but they also occur locally in the Amazon Craton and in the arched south margin of the Parnaiba Basin. They appear to be related to preexisting zones of weakness reactivated by the mesozoic continental fragmentation. Such zones are placed not far from precambrian cratonic borders. It is not certain that transform oceanic faults are placed in the prolongation of continental orogenic belts, fault zones, aulacogens or other clearly recognized tectonic features. However, the alkaline Mecejana volcanism in Ceara State is located in the coastal region near the end of an oceanic transform zone. Alligned centers of igneous activity ordered according to age, which could have been caused by the displacement of the South American Plate over hot spots, have not yet been observed in Brazil. Some models proposed by recent papers are discussed in order to explain the distribution of the brazilian mesozoic-cenozoic magmatism.

Evolution of a Lower Paleozoic Continental-Margin Carbonate Platform, Northern Canadian Appalachians

Abstract: The western margin of the northern Appalachian orogene and adjacent craton is composed of a lower Paleozoic, low-latitude carbonate platform which originally lay along the northern margin of Iapetus Ocean. Parts of the platform interior are now exposed in Quebec, much lies beneath the Gulf of St. Lawrence, and the outer shelf and deep-water deposits crop out in western Newfoundland. The shelf break and upper slope are nowhere exposed, but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes. Four separate phases recording platform growth and demise can be differentiated, reflecting the interplay between tectonics, eustasy and the evolving lower Paleozoic biota. Phase 1, Preplatform Shelf, reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartzarenites. Phase 2, Narrow, High-Energy Platform, is characterized by mixed siliciclastic-carbonate peritidal sedimentation throughout, manifest as three grand cycles. Contemporaneous deep-water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites. Phase 3, Wide Low-Energy Platform is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged inmore » the form of two unconformity-bounded megacycles. The adjacent deep-water slope was a narrow belt of debris flows and wide carbonate-shale apron, deposited in the lower part of an oxygen minimum zone. Phase 4, Foundered Platform, documents the initial uplift, faulting, subsidence, and fragmentation of the platform in a sequence of peritidal to subtidal to deep-water carbonates, reflecting the initial stages of the Taconic orogeny.« less

Craton-scale distribution of Archean greenstone gold deposits; predictive capacity of the metamorphic model

Abstract: Genetic models should not only attempt to explain features of Archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts. The application of genetic models developed with such a philosophy can have predictive capacity in gold exploration.The metamorphic model, one of a number of current genetic models for Archean gold deposits, involves generation of auriferous H 2 O-CO 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion. Crustal-scale fault-shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites. Declining temperature below the amphibolite-greenschist transition, combined with sulfidation of Fe-rich host rocks, is a particularly efficient Au-depositing mechanism. The model suggests that the major controlling factors (in declining order of importance) are: (1) high temperature-low pressure, generally low-strain, metamorphic regimes, (2) crustal-scale fault systems, (3) unaltered Fe-rich rocks (tholeiites, banded iron-formation) at a suitable stratigraphic-metamorphic level, and (4) abundant Au-enriched komatiites and S-rich sediments in source regions.The low-pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most, if not all, generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in Archean terranes. All four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2.7 + or - 0.1-b.y.-old rift-phase greenstone belts, interpreted to have formed under conditions of high crustal extension and extensive crustal thinning (i.e., with predicted high geothermal gradients, high fault density, limited synvolcanic alteration, and abundant komatiites and sulfidic sediment horizons). More widespread, commonly older, platform-phase greenstone belts, which formed under conditions of lesser crustal extension and thinning, are less mineralized, as predicted by the metamorphic model.The ability of the model to explain the regional distribution of known Archean gold deposits lends credence to its predictive capacity. For example: (1) greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential, (2) the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated, (3) areas with the highest potential can be defined in rift-phase greenstone belts by the coincidence of critical parameters such as specific host-rock sequences and metamorphic facies, (4) prospective structural sites for gold mineralization can be postulated in platform-phase greenstone belts affected by diapiric tectonics, and (5) the possible spacing of large deposits can be estimated. Further, the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny, including alteration patterns, metamorphic P-T paths, and regional-scale structural regimes.

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

Abstract: The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

Proterozoic accretionary tectonics at the southern margin of the Archean Wyoming craton

Abstract: The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones. From north to south and structurally lowest to highest, these blocks include (1) Archean crystalline basement and associated early Proterozoic miogeoclinal rocks; (2) an amphibolite-grade orthogneiss terrane of unknown age containing probable rift-related mafic intrusive rocks; (3) a 1750–1790 Ma, marginal basin(?) succession consisting of upper amphibolite-grade pelitic and volcano-genie schist, associated peraluminous granite, and minor ultramafic rocks; and (4) a 1750–1790 Ma intermediate to mafic plutonic-metamorphic complex interpreted as the deep roots of an island-arc system. The earliest deformation, D1, produced a synmetamorphic, penetrative, horizontal transposition foliation and associated recumbent folds. Tectonic blocks were juxtaposed along major mylonite zones during D2. Macroscopic anil microscopic structures associated with D2 indicate northward thrusting along low-angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the Wyoming craton at about 1750 Ma. Thrusting occurred at minimum temperatures of 475 °C and produced an inverted metamorphic gradient in pelitic rocks north of the belt. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of dextral strike slip (D3) between 1750 and 1400 Ma. Cataclasis (D4) and brecciation associated with the Laramide orogeny (D5) locally overprint earlier structures. Oblique convergence between an island arc and the Archean continental nucleus may explain accretion, crustal shortening, and subsequent strike slip. The Phanerozoic accretionary events of eastern and western North America provide an analog for this model. Presence of similar lithologies and shear zones south of the Cheyenne belt suggest that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.

Crustal heterogeneities and the thermal structure of the continental crust

Abstract: The observed linear relationship between surface heat flow (q0) and the radiogenic heat production of near surface rocks (A0) discovered almost two decades ago has been a primary tool used to infer the distribution of heat production in the continental crust. Recent evidence on the nature of compositional heterogeneities in the continental crust has led us to reexamine the q0-A0 relationship and the likely implications for the thermal structure of the lower continental crust. Numerical simulations of crustal thermal models indicate that some interpretations derived from this relation may be flawed. In particular, the length scale determined from the q0-A0 function, normally thought to describe the depth distribution of crustal heat sources, is significantly affected by the horizontal scale of crustal heterogeneity. As a consequence, most estimates of lower crustal heat flow and lower crustal temperatures obtained using heat flow data with the q0-A0 relationship are overestimates. This overestimation is (proportionally) largest in areas of low mantle heat flux such as cratons, where basal heat flux may be overestimated by more than a factor of two.

Archaean strike-slip faulting and related ensialic basins: evidence from the Pilbara Block, Australia

Abstract: Archaean sedimentary and volcanic successions in the Lalla Rookh Basin and c. 2950 Ma old Whim Creek Belt in the Pilbara Block, Western Australia, were deposited in basins with roughly the same configuration as their present outcrop. Basins were fault-bounded and developed in an ensialic setting, overlying older (3500 to 3300 Ma old); deformed and metamorphosed supracrustal rocks and granitoids. The basin margin faults are now part of a pattern of strike–slip faults which were active during the later stages of regional batholith emplacement. In both cases, structural patterns and style of basin filling are similar to younger basins related to strike–slip faulting. The Lalla Rookh Basin was dominated by coarse clastic sedimentation, comprising alluvial–fan, braided–stream, fan–delta and lacustrine facies. The Whim Creek Belt contains bimodal volcanics and clastic sediments, which comprise alluvial, subaqueous fanglomerate, submarine-fan and basinal facies. Regional strike–slip faulting and the development of the Lalla Rookh Basin and Whim Creek Belt, in response to externally imposed deformation, records an important step in the cratonization of the Pilbara Block. Late Archaean sedimentary basins, dominated by coarse clastic facies and situated adjacent to major strike–slip faults, in other cratons may have a similar origin.

Terrestrial heat flow in Botswana and Namibia

Abstract: We report terrestrial heat flow measurements from 25 new sites in Botswana and Namibia. Twelve sites from the Archean Kaapvaal-Zimbabwe Craton and the Limpopo Belt have a mean of 47 ± 4 mW m−2 (standard error of the mean) but show a trend of increasing heat flow from less than 40 mW m−2 within the cratonic nucleus to about 60 mW m−2 at its edge. Thirteen sites in the Proterozoic and Pan African mobile belts that surround the craton have a mean of 66 ± 3 mW m−2, increasing from about 60 mW m−2 at the cratonic margin to more than 70 mW m−2 several hundred kilometers away. Radiogenic heat production in near-surface basement rocks was measured for four sites on the craton and seven sites in the mobile belts. The limited heat flow-heat production data from the craton are insufficient to establish a correlation. Heat flow and heat production data from the mobile belts, when combined with published data from six sites in Zambia, exhibit a linear trend with an intercept of 50 ± 3 mW m−2 and a slope of 5.5 ± 1.1 km. Some but not all of the observed difference in surface heat flow between the mobile belts and the cratonic nucleus may be due to differences in crustal heat production. That part of the regional variation in surface heat flow which originates at greater depths implies a fundamental difference in thermal structure of the lithosphere between the craton and the mobile belts in southern Africa.

Eastern Asia in the Cretaceous: New paleomagnetic data from South Korea and a new look at Chinese and Japanese data

Abstract: Lower Cretaceous continental sediments and volcanic rocks have been sampled in the Gyeongsang basin (South Korea). A detailed analysis of 160 specimens reveals two stable components of magnetization, a postfolding, low-temperature (LT) component with normal polarity and a stable, high-temperature (HT) component with both normal and reversed polarities. The blocking temperature of the HT component ranges from 540°C up to the Neel temperature of hematite. This component predates the folding of the series and yields a pole at λ = 68°N, Φ = 205°E, A95 = 6°. The prefolding magnetization indicates that South Korea stood at a latitude nearly identical to its present-day latitude in the Lower Cretaceous. Comparison of Korean data with (revised) paleomagnetic data from North and South China and Japan shows that all these blocks of Eastern Asia occupied the same positions in term of latitude and were probably parts of a single craton since at least the Lower Cretaceous. The paleomagnetic directions of the Sino-Korean block show large discrepancies with respect to the expected directions of the “stable” Eurasian (Siberian) block. An interpretation involving significant N-S motion (∼1000 km) between either North and South China or between North China and Siberia appears to contradict geological data. A preferred interpretation is that the Siberian data are in serious error and that the combined China/Korea Lower Cretaceous pole provides a valid estimate for the Eurasian plate as a whole.

Paleomagnetism of Upper Devonian reefal limestones, Canning basin, Western Australia

Abstract: Two conflicting models have been proposed for the middle Paleozoic position of Gondwana with respect to Euramerica. A pole from Morocco (Msissi Norite) that may be of Late Devonian age suggests that a broad ocean (∼3,000 km) separated the northern continents from Gondwana, whereas results from Mauritania (Gneiguira Supergroup) indicate that this ocean had closed by Devonian time. Detailed thermal and alternating-field demagnetization experiments have been carried out on red and gray limestones from seven sites (89 samples) in an essentially undeformed Late Devonian reef complex, northern Canning basin, Western Australia. After minor tilt correction, the site mean characteristic directions average 42.7°/−28.9° (declination/inclination) with k = 61.6 and α95 = 7.8°. These results position northwestern Australia at 15°S latitude in the Late Devonian, a paleolatitude consistent with the presence of a well-developed reefal sequence. An early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post-Devonian directions for Australia resemble the observed declinations and inclinations. If the Smith and Hallam reconstruction for Gondwana is used, the paleomagnetic pole calculated for the Late Devonian of Western Australia agrees with the Msissi Norite result from Morocco and suggests that an ocean still separated Gondwana from Euramerica in the Late Devonian. The pole measured from the Gneiguira Supergroup in Mauritania may be based on remagnetizations of late Paleozoic age. The directions determined from Canning basin limestones are similar in inclination to but differ by 40° in declination from paleomagnetic results from the Upper Devonian Comerong Volcanics, Lachlan fold belt, southeastern Australia. Canning basin samples are from the stable craton, and Comerong Volcanics samples are from the Tasman fold belt. The discrepancy between the two Australian results may be best explained by tectonic rotation of the Comerong Volcanics locality.

The subduction‐ and collision‐related Pan‐African composite batholith of the Adrar des Iforas (Mali): A review

Abstract: A large composite calc-alkaline batholith, in the Iforas region, Mali, occurs close to the Pan-African suture between the 2000 Ma old West African craton and the Trans-Saharan mobile belt. Its location in an embayment of the West African craton is probably responsible for the important production of magma. The Iforas batholith intrudes the western border of an old continental segment affected by early nappe tectonics (D1 event) and is flanked to the west by the Tilemsi palaeo–island arc. The batholith comprises several successive stages. The cordillera (>620 Ma), probably post-dating the D1 event, is essentially composed of volcanosedimentary sequences. The collision (620–580 Ma) is marked by the production of abundant granitoids mostly emplaced by the end of the D2 EW compressional event. The post-collision tectonic stages (D3 and D4; 580–540 Ma) are characterized by strike-slip movements, reversals in the stress field, and a rapid switch from calc-alkaline to alkaline magmatism. Magmas corresponding to each step show distinctive geochemical trends but all share low 87Sr/86Sr initial ratios (0·7035–0·7061). The possible successive sources have been evaluated from different entities in the Inforas region: Eburnean granulites for lower crust, Tilemsi palaeo–island arc for depleted subduction source and the Tadhak undersaturated province for asthenospheric more primitive mantle. A geodynamic model is proposed where all the calc-alkaline groups originated from a classical subduction source (depleted upper mantle modified by hydrous fluids from the subducted oceanic plate) which, some fifty million years after the beginning of the collision, was taken over by an asthenospheric source producing the alkaline province.

Heat flow and heat production in the Namaqua Mobile Belt, South Africa

Abstract: Results of heat flow and heat production surveys at 13 localities in the Namaqua mobile belt, southern Africa, are presented. Twelve stations traverse the 1200–1100 m.y. old gneissic and metamorphic Bushmanland and Gordonia subprovinces. One station is situated in the Richtersveld subprovince, a 2000–1730 m.y. old volcano-sedimentary domain mildly affected by the 1200–1100 m.y. old orogeny. Surface heat flow in the mobile belt ranges from 39 to 81 mW m−2 and averages at 61 ± 11 mW m−2. Surface heat production ranges from 0.8 μW m−3 in mafic rocks to extreme values of 6.9–12.3 μW m−3 in the Springbok district, western Namaqualand. High heat production in the Springbok district is associated with a thin, tabular granitic intrusive and is not representative of the basement. The mean heat production of the remaining stations in the mobile belt is 2.3 ± 1.1 μW m−3. Linear regression analysis of the heat flow-heat production data yields a poor fit (correlation coefficient = 0.62) and imprecise estimates for the intercept (34 ± 6 mW m−2) and slope (11 ± 3 km). The eastern margin of the mobile belt has relatively low heat flow and heat production; geothermal, geological, and gravity data suggest a relative depletion of crustal radiogenic heat production in the region. However, the average Namaqua heat flow is higher than worldwide averages for Proterozoic terrains. There is no geological or geophysical evidence for recent mantle heating below the Namaqua mobile belt, suggesting that an elevated crustal radioactive component is largely responsible for the elevated heat flow. In this respect, the mobile belt is similar to several mid-late Proterozoic heat flow provinces that are characterized by high average surface heat flow and probably have elevated crustal radiogenic heat input. These observations, and seismic data, suggest that the Namaqua crust consists of an upper layer that contributes 40–50%, on average, to the heat flow and a depleted lower layer; the inferred heat flow at midcrustal levels is 50–60% of the average surface heat flow. The mobile belt's average surface heat flow is 40–85% higher than that from the basement of the adjacent Archaean Kaapvaal craton, and its inferred midcrustal heat flow is similar to the lowest surface values in the craton. Heat production data from the Kaapvaal craton indicate that its upper crust is not depleted of heat sources. While enhanced radiogenic heat input from the Namaqua upper crust may account for part of the higher-than-craton heat flow, it cannot account for the full excess. Consequently, a larger equilibrium mantle heat flow, related to a reduced lithospheric thickness below Namaqualand, seems to be required by the new data.

Evidence from mantle xenoliths for an enriched lithospheric keel under the Outer Hebrides

Abstract: Several independent lines of evidence suggest the presence of old regions in the Earth's upper mantle, particularly beneath stable cratons, which are enriched in incompatible elements such as barium and the light rare earth elements (LREE). Such 'lithospheric keels' are well documented beneath the Archaean of South Africa1–4 and North America5,6. Recent studies7 have shown the Lewisian of Scotland to be underlain by heterogeneous mantle, fragments of which are entrained by magmas that later intrude the crustal rocks of north-west Scotland. Here we present chemical and isotopic data from megacryst suites in these magmas, which point to niineralbgical complexity in the lower crust and upper mantle. Moreover, variable trace-element ratios in the xenoliths are now recorded as extreme isotopic variations. The Sr and Fb isotopic compositions are similar to those of lamprbites6 and micaceous kimberlites8 respectively. In contrast, the Nd isotopic ratios are highly variable (eNd = −3.4 to −32.6) and overlap with those observed in garnet inclusions9 in diamonds (South Africa) and Iamproitic6–potassic5 volcanic rocks (USA). We suggest that the Hebridean lithospheric mantle has evolved in a manner totally unlike that of the mantle source regions tapped by Tertiary volcanic rocks erupted in the North Atlantic volcanic province.

Extent and bathymetry of North American Platform Seas in the Early Silurian

Abstract: The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post-Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed. The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene-Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral-stromatoporoid/stromatolite/paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coral-stromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coral-stromatoporoid cycles (East Iowa Basin). With shallow bathymetries of 0–30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0–60 m and 10–90 m the deeper-water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.