Paul N. Taylor

Early Proterozoic crustal evolution in the birimian of Ghana: constraints from geochronology and isotope geochemistry

Abstract Representative suites of Ghanaian granitoids, metavolcanics and metasediments were selected from Birimian rock units for RbSr, Pb/Pb and SmNd analyses. Isotopic data on the igneous rocks provide evidence for a major Early Proterozoic crust-forming episode which took place over a maximum time interval from ∼ 2.3 to ∼ 2.0 Ga by a differention from a depleted mantle source. There is little evidence for the involvement of significantly older (e.g. Archaean) crust in the genesis of the igneous rock units. The only exception so far found is the Winneba granitoid, which appears to contain a magmatic contribution from presumably underlying Archaean basement. SmNd model ages indicates that Birimian sediments were derived from adjacent penecontemporaneous volcanic belts. The Birimian of Ghana forms part of a major Proterozoic (Eburnean) episode of juvenile crustal accretion which has been recognised in the surrounding areas of Mauretania, Senegal, Ivory Coast, Burkina Faso, Mali and Niger, where it has been dated at 2.1–2.2 ga (abouchami et al.) The Birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents.

Petrography, chemistry and isotopic ages of Peninsular Gneiss, Dharwar acid volcanic rocks and the Chitradurga granite with special reference to the late Archean evolution of the Karnataka craton, southern India

Abstract New petrographic, major and trace element, and isotopic age data are presented for four suites of rocks from the low- to medium-grade part of the South Indian Archaean craton in central Karnataka. Two suites are from the Peninsular Gneiss basement to the late Archaean Dharwar Supergroup, the third is a suite of Dharwar potassic rhyolites and the fourth is from the Chitradurga Granite which has intrusive contacts with the Dharwar Supergroup. The Chikmagalur Granite (granodiorite), which with its host gneisses forms part of the basement (Peninsular Gneiss) to the Dharwar Supergroup in the Bababudan basin, yields RbSr and Pb/Pb whole-rock isochron dates of 3080 ± 110 Ma and 3175 ± 45 Ma, respectively, an initial 87 Sr 86 Sr ratio of 0.7013 ± 0.0009 and a μ1 value ( 238 U 204 Pb ) of 7.98. Host tonalitic gneisses to the Chikmagalur Granite give an RbSr whole-rock isochron date of 3060 ± 160 Ma with an initial 87 Sr 86 Sr ratio of 0.7015 ± 0.0004 and a Pb/Pb whole-rock isochron date of 3190 ± 100 Ma with a model μ1 value of 8.00. A suite of granites and granodioritic-tonalitic gneisses (Chitradurga granitic rocks and gneisses) forming part of the Peninsular Gneiss basement to the Dharwar Supergroup west of the Chitradurga belt yields RbSr and Pb/Pb whole-rock isochron dates of 2970 ± 100 Ma and 3028 ± 28 Ma, respectively, with an initial 87 Sr 86 Sr ratio of 0.7035 ± 0.0013 and a model μ1 value of 7.62. The suite of Dharwar acid volcanic rocks from north of Honnali defines a poorly fitted 207 Pb 206 Pb linear array (MSWD 15.8) which gives an apparent age of 2565 ± 28 Ma and a model μ1 value of 7.46. The poor fit of this ‘isochron’ shows that a closed system was not maintained, a conclusion supported by high K abundances which are attributed to a syn- or post-magmatic ion exchange process. The Chitradurga Granite which intrudes the older rocks of the Dharwar Supergroup in the Chitradurga belt yields a well-fitted Pb/Pb whole-rock isochron date of 2605 ± 18 Ma with a model μ1 value of 7.68. The UPb relations in these suites from Karnataka are in marked contrast with the commonly severe U depletion and, consequently, unradiogenic Pb isotopic compositions observed in deeply eroded, high-grade Archaean gneiss terranes such as those bordering the North Atlantic. This contrast indicates that the cratonic rocks in central Karnataka represent a relatively high level in the original Archaean continental crust.

Dating the oldest terrestrial rocks — fact and fiction

Abstract This paper is a review of well-documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks, mainly using Rb/1bSr, Sm/1bNd and U/1bPb decay schemes. The oldest known stable sialic crust of true continental character, predominantly composed of calc-alkaline orthogneisses, was produced during major mantle-differentiation some 3.7−3.5 Ga ago and is represented on several continents. Enclaves of older supracrustal rocks, as well as a major remnant of a volcano-sedimentary sequence of greenstone belt affinity dated at ∼3.8 Ga, are preserved within the surrounding gneisses in southern West Greenland. Recent ionmicroprobe evidence for ∼4.1-Ga-old detrital zircon grains in a younger early Archaean metasedimentary sequence in Western Australia is also discussed. Several cases are reviewed where claims for early Archaean (> 3.4 Ga) dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data. Some of the geological and geochemical mechanisms which have given rise to claims for spuriously old Sm/1bNd, U/1bPb and Pb/Pb dates are critically discussed. To supplement published data, we present several new sets of whole-rock Sm/1bNd and Pb/Pb isotopic data from West Greenland, Labrador, Zimbabwe and eastern India relevant to the overall topic of this review.

Early Archaean granulite-facies metamorphism south of Ameralik, West Greenland

Relict high-pressure granulite-facies rocks have been found in the Amiˆtsoq gneisses and inclusions of the older Akilia supracrustal association, on islands south of Godthab. Only amphibolite-facies assemblages have been found in Ameralik dykes and younger rocks from this area. The Amiˆtsoq gneisses are depleted in Rb and U relative to those of Ameralik and Isua. Well-fitted Pb/Pb and Rb-Sr isochrons on Amiˆtsoq granulites indicate that this depletion, correlated with the granulite-facies metamorphism, occurred ca. 3600 Ma ago. Textural features suggest that the present cpx + opc + gnt + plag + qtz + hbl assemblages evolved from earlierintermediate-P assemblages (cpx + opx + plag), probably during cooling from the metamorphic peak. Re-equilibrium of olderintermediate-P assemblages in local environments of low ƒH2O, during the ca. 2800-Ma metamorphism of the Malene supracrustals, is feasible but is considered unlikely. Either interpretation requires crustal thickness of at least 20 km and geothermal gradients of⩽30°C/km, ca.3600Ma ago. The higher heat production of early Archaean times was apparently dissipated through oceanic, rather than continental, areas.

Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland

Ph isotopic analyses are reported for 119 samples of late Archaean (ca. 3000-2800 Myr) calc alkaline orthogneisses and associated anorthosites from southern West Greenland. Over most of the area. PbPb whole rock isotope systematics indicate derivation of the magmatic precursors of the gneisses and anorthosites from a source region with a typically mantle-type UPb ratio (μ1 value of 7.5) at. or shortly before, ca. 3000-2800 Myr ago. In contrast, in the Godthaabsfjord region, late Archaean Nuk gneisses and associated anorthosites were emplaced into or through early Archaean (ca. 3700 Myr) Amitsoq gneisses, and crystallised with variable proportions of two isotopically distinct types of Pb which commenced their respective crustal developments at ca. 3000-2800 Myr and at ca. 3700 Myr ago. Isotopic and other geochemical constraints demonstrate that Nuk gneisses and their temporal equivalents were not produced by reworking or melting of Amitsoq gneisses. Mixing of early and late Archaean Pb results from contamination of the magmatic precursors of Nuk gneisses and anorthosites (characterised by mantle-type Pb at time of emplacement) with ancient, unradiogenic Pb derived from ca. 3700 Myr-old Amitsoq-type continental crust invaded by the Nuk magmas. The contaminant is considered to be a trace-element enriched fluid phase released from dehydrating older continental crust during progressive burial and heating by emplacement of calc alkaline magmas in the late Archaean ‘accretion differentiation superevent’. This was followed by mixing of the released fluids with younger Nuk magmas. Pb isotopic compositions of late Archaean gneisses and anorthosites outside the Godthaabsfjord region provide no evidence for the presence of early Archaean Amitsoq-type continental crust in southern West Greenland in areas more than a few tens of km outside the known outcrop of Amitsoq gneisses. We suggest that early Archaean crust does not exist at depth in late Archaean areas with undisturbed Pb-isotope systematics, either in Greenland or elsewhere in the North Atlantic craton. Pb-isotope evidence for crust magma interaction, involving selective extraction of certain trace elements by a fluid phase from wall rock and subsequent mixing between magma and contaminant fluid, provides a powerful tool for detection, sub-surface ‘mapping’, and geochronological and geochemical characterisation of deep, ancient continental crust.

Metalliferous sediments from DSDP Leg 92 : The East Pacific Rise transect

Abstract The carbonate-free metalliferous fraction of thirty-nine sediment samples from four DSDP Leg 92 sites has been analyzed for 12 elements, and a subset of 16 samples analyzed for Pb isotopic composition. The main geochemical features of this component are as follows: i) very high concentrations of Fe and Mn, typically 25–39% and 5–14%, respectively; ii) Al and Ca contents generally less than 2% and 5%, respectively; iii) high Cu (1000–2000 ppm), and Zn and Ni (500–1000 ppm) values; and iv) Co and Pb concentrations of 100–250 ppm. In terms of element partitioning within the metalliferous fraction, amorphous to poorly crystallized oxide-oxyhydroxides removed by the second leach carry virtually all of the Mn, and about 90% of the Ca, Sr and Ni. The well-crystallized goethite-rich material removed by the third leach carries the majority of Fe, Cu, and Pb. These relations hold for sediments as young as ~ 1–2 Ma, indicating early partitioning of hydrothermal Fe and Mn into separate phases. Calculated mass accumulation rates (MAR) for Fe, Mn, Cu, Pb, Zn and Ni in the bulk sediment show the same overall trends at three of the sites, with greatest MAR values near the basement, and a general decrease in MAR values towards the tops of the holes (for sediments deposited above the lysocline). These relations strongly support the concept of a declining hydrothermal contribution of these elements away from a ridge axis. Nevertheless, MAR values for these metals up to ~200 km from the ridge axis are orders of magnitude higher than on abyssal seafloor plains where there is no hydrothermal influence. Mn/Fe ratios throughout the sediment column at two sites indicate that the composition of the hydrothermal precipitates changed during transport through seawater, becoming significantly depleted in Mn beyond 200–300 km from the axis, but maintaining roughly the same proportion of Fe. Most of the Pb isotope data for the Leg 92 metalliferous sediments form approximately linear arrays in the conventional isotopic plots, extending from the middle of the field for mid-ocean ridge basalts toward the field for Mn nodules. The array of data lying between these two end-members is most readily interpreted in terms of simple linear mixing of Pb derived from basaltic and seawater end-member sources. The least radiogenic sediments reflect the average Pb isotope composition of discharging hydrothermal solutions and ocean-ridge basalt at the EPR over the ≈4–8 Ma B.P. interval. Pb in sediments deposited up to 250 km from the axis can be almost entirely of basaltic-hydrothermal origin. Lateral transport of some basaltic Pb by ocean currents appears to extend to distances of at least 1000 km west of the East Pacific Rise.

Pb, Sr and Nd isotope constraints on the Archaean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil

Abstract Three Archaean gneissic complexes (Bonfim, Belo Horizonte and Campo Belo) in the southern part of the Sao Francisco craton were studied by means of UPb (zircon and titanite, monazite) and SmNd, RbSr and PbPb (whole rock) methods. In the Bonfim metamorphic complex the period 2780-2700 Ma is marked by in situ anatexis of gneisses, intrusion of tonalites, mafic dikes and late granites, and deposition of the Rio das Velhas greenstone belt. SmNd T DM model ages on its gneisses, amphibolites and granites range from 2800 to 3000 Ma, indicating involvement of older sialic crust in the 2780-2700 Ma event. In the Belo Horizonte metamorphic complex UPb zircon geochronology has yielded ages of 2860, 2776 and 2712 Ma and RbSr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800-2750 Ma (with large uncertainties) and 87 Sr/ 86 Sr initial ratios of 0.700-0.710. The highest initial ratio (0.710) obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution. Existence of older crust is suggested by UPb zircon ages of ∼ 3030, 2920 and 2880 Ma from inherited grains in metavolcanics and orthogneisses. RbSr ages of 2250-2130 Ma on granitoids and UPb (titanite, monazite) ages from 2320 to 2030 Ma on granitoids, felsic veins and amphibolite enclaves indicate reworking of this complex in the Palaeoproterozoic. In the Campo Belo metamorphic complex granite and greenstone lithologies have yielded ages of ∼ 3380-3000, 2900 and 2650 Ma by UPb zircon and RbSr and PbPb whole rock methods. T DM ages on its granites, gneisses and granulites have yielded ages from 3070 to 2780 Ma indicating involvement of older sialic material during the 2650 Ma event. These complexes indicate major events during the period 2860-2700 Ma, including reworking of established sialic crust with components up to 3380 Ma old. Late granites were injected between 2700-2600 Ma, preceding final assembly and stabilization of the Archaean crust.

Nagssugtoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents—chemical and isotopic evidence

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sri ca. 0.703; model μ1 values ca. 8.0; initial eNd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Stromfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon UPb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada.

Palaeoproterozoic basement province in the Caledonian fold belt of North-East Greenland

Abstract RbSr and SmNd model ages of basement gneisses in the Caledonian fold belt of North-East Greenland show that most of these rocks are related to a Palaeoproterozoic event of crust formation around 2000 Ma ago. UPb within-zircon (SHRIMP) analyses on three samples yield ages of 1974±17, 1764±20, and 1739±11 Ma (2σ values). Archaean rocks (∼ 3000 Ma) are also present, but do not appear to be common. However, SmNd analyses demonstrate that the Proterozoic gneisses may contain substantial proportions of older (probably Archaean) crustal material. The discovery of a major province of mainly juvenile Palaeoproterozoic crust, which extends some 900 km along the coastal outcrops of North-East Greenland, implies that as much as one third of the whole of the Greenland shield may have been generated in the Palaeoproterozoic.

Isotopic and chemical variation in granites across a Proterozoic continental margin—the Ketilidian mobile belt of South Greenland

Abstract Pb-Pb and Rb-Sr isotope systems are compared for Proterozoic granites in the central part of the Ketilidian mobile belt and in the Archaean craton bordering the mobile belt. Very significant differences are found and it is concluded that the granites from the border zone contain large proportions of Archaean crustal material, whereas this is not the case for the granites in the central part of the mobile belt. This shows that most of the Ketilidian mobile belt is not underlain by Archaean crust. The genetic difference between the granites in these two geological settings is also reflected in their chemical compositions. The granites from the central part of the mobile belt have a wider compositional range with, on average, less SiO 2 and more CaO and ferromagnesian constituents than those from the border zone. The latter are leucocratic high-SiO 2 granites, relatively enriched in most incompatible trace elements.