Geochronological Information Research Articles

Late Paleoproterozoic magmatism in Delhi Fold Belt, NW India and its implication: evidence from EPMA chemical ages of zircons

Zircons from six granitoid plutons, occurring in the northern part of the Delhi Fold Belt, NW India, have been chemically dated using an electron microprobe. Most of the zircons occurring in the granites are chronologically unzoned and gave ages around 1.78–1.71 Ga, but younger ages of 1.1–0.9 Ga have also been obtained from the rim of some grains. Monazite and zircons from the metasediments of Delhi Supergroup adjacent to the granitoids were also analyzed. The older age of 1.9–1.7 Ga obtained for monazites could possibly indicate their provenance age. Detrital zircon grains (mostly rounded) in the Delhi metasediments, which gave older ages of >1.9 Ga could possibly have been derived from the Archaean basement rocks exposed in this Aravalli Delhi Fold Belt. The geochemical signatures of these granitoids are consistent with an extensional tectonic regime. It is quite likely that contrary to the prevailing assumption of the granitoids being ‘intrusive’, our geochronologic data suggests them to be the basement for sedimentation. The new geochronologic information on these granitoids in the northern part of the Delhi Fold Belt and their comparison with other correlative events establishes a widespread thermal event during late Paleoproterozoic.

Behavior of argon gas release from manganese oxide minerals as revealed by 40Ar/39Ar laser incremental heating analysis

Manganese oxides in association with paleo-weathering may provide significant insights into the multiple factors affecting the formation and evolution of weathering profiles, such as temperature, precipitation, and biodiversity. Laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia, yield well-defined plateaus and consistent isochron ages, confirming the feasibility dating very-fined supergene manganese oxides by 40Ar/39Ar technique. Two distinct structural sites hosting Ar isotopes can be identified in light of their degassing behaviors obtained by incremental heating analyses. The first site, releasing its gas fraction at the laser power 0.2–0.4 W, yields primarily 40Aratm, 38Aratm, and 36Aratm (atmospheric Ar isotopes). The second sites yield predominantly 40Ar* (radiogenic 40Ar), 39ArK, and 38ArK (nucleogenic components), at ∼0.5–1.0 W. There is no significant Ar gas released at the laser power higher than 1.0 W, indicating the breakdown of the tunnel sites hosting the radiogenic and nucleogenic components. The excellent match between the degassing behaviors of 40Ar*, 39ArK, and 38ArK suggests that these isotopes occupy the same crystallographic sites and that 39ArK loss from the tunnel site by recoil during neutron irradiation and/or bake-out procedure preceding isotopic analysis does not occur. Present investigation supports that neither the overwhelming atmospheric 40Ar nor the very-fined nature of the supergene manganese oxides poses problems in extracting meaningful weathering geochronological information by analyzing supergene manganese oxides minerals.

The Palaeoproterozoic Kangerluluk gold-copper mineralization (southeast Greenland): Pb and Nd isotopic constraints on its timing and genesis

The Ketilidian Mobile Belt (KMB) in South Greenland is dominated by the Palaeoproterozoic Julianehab Batholith (1,850–1,770 Ma) and is bordered by supracrustal sequences. The batholith is composed of granite, granodiorite, tonalite, diorite and subordinate gabbro and appinite and formed in a juvenile magmatic arc. The supracrustal rocks consist of psammites, pelites and subordinate, interstratified mafic volcanic rocks. A Rapakivi granite suite (1,755–1,735 Ma) intrudes them. Gold is an important metal in the KMB and occurs in various settings and element associations. Here, emphasis is placed on the Kangerluluk Cu–Au mineralization in the eastern part of the mobile belt. It is hosted within a remnant supracrustal succession dominated by redundant pillowed basalts. Structurally, a syn-volcanic association with gold can be distinguished from one that is clearly related to crosscutting quartz veins. In between, calc-silicate formation in the volcanic rocks occurred contemporaneously with a pervasive sericitization (bleaching) of leucocratic granitoids that are intrusive into the volcanic sequence. Bulk rock and mineral (sulfide) Pb isotope data are comparable in the Pb isotope space. The data define two groups of mineralization where one group is characterized by relatively higher 207Pb/204Pb at given 206Pb/204Pb ratios than the other. Pb stepwise leaching (PbSL) dating experiments performed on garnet and epidote associated with both groups of mineralization confirm the offset in the Pb isotope data. With the geochronological information from PbSL, the results indicate: (1) the initial gold mineralization was genetically related to metalliferous fluids associated with the emplacement of late intrusive stages of the Julianehab Batholith (1,800–1,770 Ma); and (2) local remobilization of the initial mineralization (1,700–1,750) involved a Pb isotopically evolved fluid, which tentatively can be correlated with the intrusion of the Rapakivi granites in the area. Radiogenic Pb was not only found in altered host rocks but also in various sulfides, suggesting that fluids may have percolated through and/or originated from uranium-bearing graphitic schists which are abundant in the sedimentary package of the KMB. Lead isotopic results suggest that the Rapakivi granites may have supplied heat for renewed circulation of fluids within the supracrustal rock pile.

Age Constraints on Granitoids and Metavolcanic Rocks of the São Luís Craton and Gurupi Belt, Northern Brazil: Implications for Lithostratigraphy and Geological Evolution

Single-zircon Pb evaporation dating was undertaken on granitoids and metavolcanic rocks of different lithostratigraphic units of the São Luís craton and the bordering Gurupi belt in northern Brazil, allowing the determination of the crystallization ages of these rocks and a partial reassessment of the regional lithostratigraphy. In the Sao Luis craton, zircons from granitoids of the Tromai Suite (dominantly tonalitic) yielded 207Pb/206Pb ages between 2149 ± 5 Ma and 2165 ± 2 Ma, and a metapyroclastic rock of the metavolcanosedimentary Aurizona Group was dated at 2240 ± 5 Ma. In the Gurupi belt, the Itamoari Tonalite (the deformed correspondent of the Tromai Suite) gave an age of 2148 ± 4 Ma, similar to the younger ages of the Tromai Suite. Two felsic metavolcanics of the metavolcanosedimentary Gurupi Group showed crystallization ages between 2148 ± 1 Ma and 2160 ± 3 Ma, which are similar to those of the granitoids of the cratonic domain. The Th/U ratios of the whole set of analyzed samples, calculated from the 208Pb/206Pb ratio, ranged from 0.23 to 0.51, which is typical of magmatic zircons. The determined ages are in good agreement with those of the correlative Eburnean granitoids and Birimian sequences of the West African craton. A Rb-Sr internal isochron was calculated for a collisional-type granitoid (Maria Suprema Granite) in the Gurupi belt, yielding an age of 1710 ± 32 Ma, interpreted as a partial resetting of the isotopic system. Considered as a minimum age, this dating places the granitoid in the Paleoproterozoic. The age of the rocks and of the establishment of the Gurupi belt have been controversial, owing to the widespread Neoproterozoic (Brasiliano/Pan-African) Rb-Sr and K-Ar signature of its rocks. Our data, combined with other recent geological and geochronological information, suggest that rocks of the present Gurupi belt have likely participated in collisional/accretionary processes occurring in the final stages of the Transamazonian orogenic cycle in the Paleoproterozoic (2.0-2.1 Ga). This was part of the major process of assembly of the Atlantica supercontinent. The belt was tectonically reactivated in the Neoproterozoic, with widespread reworking of older rocks and only localized rock generation, leading to its present configuration.

The fluvial system response to abrupt climate change during the last cold stage: the Upper Pleistocene River Thames fluvial succession at Ashton Keynes, UK

The last interglacial–glacial cycle (125–10 ka BP) is characterised by numerous rapid shifts in global climate on sub-Milankovitch timescales, recorded in the ocean and ice core records. These climatic fluctuations are clearly recorded in those European terrestrial sedimentary sequences that span this time period without interruption. In the UK, only fragmentary Upper Pleistocene sequences exist, mainly within the fluvial archive of the major river systems such as the Thames. The response of the upper River Thames to abrupt fluctuations in climate is documented in the fluvial sediments beneath the Floodplain Terrace (Northmoor Member of the Upper Thames Formation) at Ashton Keynes, Wiltshire. A number of criteria are set out by which significant changes in the fluvial system may be established from the sedimentological, palaeoecological and geochronological information contained within the succession. The sedimentary succession is divisible into four facies associations, on the basis of their sedimentology and bounding surface characteristics. These represent distinct phases of fluvial activity at the site and allow changes in fluvial style to be inferred. Palaeoecological reconstructions from pollen analysis of peats within the sequence provides an indication of the nature and direction of Late Glacial environmental change and optically stimulated luminescence and radiocarbon dating methods provide chronological control on the sequence. These data suggest that major changes in fluvial style are recorded within the succession, which can be related to the climatic fluctuations that took place on the oxygen isotope stage 5a/4 transition (approximately 70 ka BP) and the Devensian Late Glacial climatic warm–cold–warm oscillation (13–11 ka BP). The changes in fluvial style are a result of variations in sediment supply to the river resulting from changes in slope stability, vegetation cover and cold-climate mass movement processes and variations in discharge regime caused by changes in precipitation patterns, snow cover, permafrost distribution and vegetation cover.

Sm–Nd isotope systematics and REE data for leucotroctolites and their amphibolitized equivalents of the Niquelândia Complex upper layered series, central Brazil: further constraints for the timing of magmatism and high-grade metamorphism

The Barro Alto, Niquelândia, and Cana Brava Complexes are major Proterozoic layered intrusions in central Brazil that were affected by high-grade metamorphism with associated ductile deformation during the Neoproterozoic (770–795Ma). Recent studies recognized that the Niquelândia Complex comprises two petrologically distinct and tectonically juxtaposed magmatic systems: a younger Upper Layered Series to the west and an older Lower Layered Series to the east. Previous geochronological studies on Lower Series rocks suggested a Paleoproterozoic (ca 2.0Ga) age for the Lower Series magmatic event. New trace element data matched with Sm–Nd isotope data for Upper Series samples yielded well-constrained and original geochronological information. The 1.35Ga age of the Upper Series magmatism reported in this paper indicates a much younger age of the Upper Series compared with the Lower Series. The tectonic contact between these two distinct magmatic systems is now raised to the category of a major Paleo-Mesoproterozoic crustal discontinuity.

A diatom and radiocarbon perspective of the palaeoenvironmental history and stratigraphy of Holocene deposits between Oostende and Nieuwpoort (Western Coastal Plain, Belgium)

Diatom and radiocarbon data of two cores from the north-eastern part of the western Belgian coastal plain are discussed. The palaeoenvironmental reconstruction of these sites and additional stratigraphic and geochronological information, allow an assessment of the general development of this region throughout the Holocene. Prior to c. 7500 yrs cal. BP a rapidly transgressing coast exists. Basal peat development may have been fairly widespread. Extensive erosion channels form towards the interior once the volume of the developing tidal basin has increased sufficiently. In spite of very high rates of sediment accumulation, high-intertidal environments remain poorly developed due to the high rate of RSL rise and the relatively steep slope of the transgressed terrain. Diatom taphocoenoses of this period consist primarily of coastal tychoplankton (mainly Cymatosira belgica), deposited in association with flood transported sediment, and some epipsammon. Hereafter, the incidence of negative tendencies in marine influence increases, culminating between c. 6300 to 5600 yrs cal. BP. Diatom assemblages are now more varied with an improved representation of tidal flat and salt marsh communities. The marine species Paralia sulcata and Pseudopodosira westii become abundant in high intertidal deposits, which may involve selective transport, sedimentation and preservation mechanisms. In general, the environmental changes reflect the development of a broad intertidal sedimentation zone in which sediment supply remained high, combined with a gradual slowing down of the RSL rise. Coastal progradation finally leads to sufficiently sheltered conditions for long-lasting peat growth in the intracoastal area up to perhaps c. 2100 yrs cal. BP. Presumably a coastal barrier, which remains intersected by some inlets, also develops during this period. At places, peat growth is interrupted at about 4200 yrs cal. BP by a marked but short-lived period of marine sedimentation. Within the general framework of a return to a transgressive setting and a destabilisation of the barrier system due to a decreasing sediment supply, climatic conditions and human interference may have played a role in the final termination of peat growth and the resumption of tidal activity. So far, the exact chronology of the return of tidal sedimentation in the study area remains unclear. No general sedimentation cycles, or so-called Dunkerque transgressions, are identified during the deposition of the upper tidal deposits, which are again characterized by a high representation of Cymatosira, reflecting a more open coastal setting. A tentative chronology of regional tendencies in the marine influence is derived from both bio- and lithostratigraphical indications and compared with the assumed chronology of transgressive and regressive phases in the northern Netherlands. Points of agreement are discussed in terms of possible supra-regional processes.

U‐Pb Ages for Zircon and Titanite from the Ramagiri Area, Southern India: Evidence for Accretionary Origin of the Eastern Dharwar Craton during the Late Archean

The north‐south‐trending Ramagiri Schist Belt is trident shaped, with three prongs separating three granitic terranes. Whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite, the central prong includes felsic metavolcanics and volcanogenic metasediments. The U‐Pb zircon age for the pyroclastics in the central prong of the Ramagiri Schist Belt is $$2707\pm 18$$ Ma, considered as the time of emplacement of the felsic volcanics. To the east of the belt, migmatitic Chenna gneisses yielded a U‐Pb zircon age of $$2650\pm 7$$ Ma, inferred as the minimum age of their magmatic precursors. These gneisses had experienced a thermal event at least 100 m.yr. later, as evidenced from the U‐Pb titanite age of $$2545\pm 1$$ Ma. From the central Ramagiri Complex, a granodiorite has identical U‐Pb zircon and titanite ages of $$2613\pm 6$$ and $$2614\pm 4$$ Ma, respectively, and a quartz diorite has a titanite age of $$2595\pm 1$$ Ma. These intrusives with roof‐pendants of pillowed basalts are inferred to have been emplaced as high‐level plutons that cooled rapidly. From the western Gangam Complex, U‐Pb zircon and titanite ages for a granodiorite were $$2528\pm 1$$ and $$2516\pm 3$$ Ma, respectively, and a monzodiorite has yielded a titanite age of $$2510\pm 2$$ Ma. A postkinematic granitic vein intrusive into the Gangam Complex yielded a titanite age of $$2468\pm 4$$ Ma. The three granitoid terranes separated by a <3‐km‐wide schist belt do not have common magmatic or metamorphic histories, as evidenced by their distinct titanite and zircon ages. Intrusion of voluminous western Gangam Complex at 2528 Ma did not alter the titanite ages of granitoids in either the central Ramagiri Complex or the eastern Chenna Gneiss just a few kilometers away. Therefore, the Ramagiri Schist Belt, consisting of highly sheared rocks, could represent a terrane boundary between three disparate granitoid terranes juxtaposed after 2516 and before 2468 Ma. Available geological and geochronological information from Kolar, Ramagiri, and adjoining areas suggests that growth of Dharwar continental crust could have been accomplished by both accretion of a series of arcs onto and collision of a large, 2530 Ma continent with an older (<3000 Ma) continent at around 2500 Ma.

Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

Lower-crustal and upper-mantle xenoliths entrained in Phanerozoic igneous rocks in the northern Southern Rocky Mountains provide the only direct means of studying the petrologic and geochemical nature of the deep-continental lithosphere in this region. Petrographic and geochemical features of peridotite xenoliths from the State Line kimberlite district, near the Cheyenne belt, suggest that during Devonian kimberlite emplacement, infertile, cryptically metasomatized, Archean mantle dipped south beneath a distinctly more fertile, and veined, lithospheric mantle associated with the Proterozoic continental crust of the Colorado province. Lower-crustal xenoliths recovered from the State Line kimberlites and late Cenozoic ultrapotassic igneous rocks at Leucite Hills (north of the Cheyenne belt) are dominated by mafic granulites. This indicates that both the Archean and Proterozoic lower-continental crust was, and may remain today, mafic in composition. Existing major-element data support a tholeiitic basalt precursor for the mafic granulites associated with the Colorado province. Unfortunately, despite its importance for constraining models of lithospheric structure in the northern Rocky Mountains, detailed information regarding the age, trace element and isotopic compositions, or physical attributes of the mafic xenoliths is presently not available. In order to evaluate the age and composition of continental lithosphere in this region and to assess the possible role of mafic lower-crustal rock types as sources for Laramide magmatism we outline a plan (as part of the Rocky Mountain Continental Dynamics project) to obtain geochemical and geochronological information from xenoliths recovered at the State Line kimberlite district and Leucite Hills.

A double focusing mass spectrometer for geochronology

The performance characteristics of a double focusing mass spectrometer designed specifically for geochronology are described. The sensitive high resolution ion micro probe (SHRIMP II) mass spectrometer is ideally suited for in-situ analysis of uranium and/or thorium-bearing minerals to provide geochronological information on micron-sized domains of the minerals. SHRIMP II simultaneously meets the requirements of high mass resolution and high ion transmission efficiency together with good abundance sensitivity. SHRIMP II’s excellent spatial and mass resolution, coupled with its high sensitivity, has enabled zircon geochronology to be revolutionized.

Neogene to Quaternary U-Th-Pb Geochronology Using Opal, Yucca Mountain, Nevada, USA

Finding suitable sites for nuclear waste disposal, as well as a wide range of other environmental problems, requires accurate geochronological information to constrain Neogene and Quaternary geologic and hydrogeologic processes. Opal-rich materials in the semi-arid environments of southwest Nevada yield isotopic ages by both U-series dating in the range of 0 to 500 ka (thousand years before present) and U-Pb dating of materials from 10 Ma (million years before present) to 50 ka. These two methods are being used for hydrogeologic and climate studies associated with site characterization of the potential high-level radioactive waste repository at Yucca Mountain. Opal (Si02.nH20) is formed in low-temperature (less than 50~ environments by precipitation from aqueous solutions or by coagulation of colloidal silica gels. Oxygenated surface and vadose waters dissolve uranium from the felsic volcanic rocks. The limited solubility of thorium and lead in these aqueous solutions with near-neutral pH results in low common Pb and 232Th abundances in opal. Consequently, these opal-rich materials typically have e levated uranium contents and large 2 3 0 T h / 2 3 2 T h . . 2 3 8 u / Z O 4 p b . . . . . . acUvlty and ratios, making opal an attractive candidate for 23~ and U-Pb dating.

40Ar/39Ar geochronology and exhumation of mylonitized metamorphic complex in Changle-Nanao ductile shear zone

New40Ar/39Ar plateau ages from rocks of Changle-Nanao ductile shear zone are 107.9 Ma(Mus), 108.2 Ma(Bi), 107.1 Ma(Bi), 109.2 Ma(Hb) and 117.9 Ma(Bi) respectively, which are concordant with their isochron ages and record the formation age of the ductile shear zone. The similarity and apparent overlap of the cooling ages with respective closure temperatures of 5 minerals document initial rapid uplift during 107–118 Ma following the collision between the Min-Tai microcontinent and the Min-Zhe Mesozoic volcanic arc. The40Ar/39 Ar plateau ages, K-Ar date of K-feldspar and other geochronologic information suggest that the exhumation rate of the ductile shear zone is about 0.18–1.12 mm/a in the range of 107–70 Ma, which is mainly influenced by tectonic extension.

Crustal evolution of the KTB drill site: From oldest relics to the Late Hercynian granites

The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf‐Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high‐ and medium‐pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW‐SE trending structures, Devonian (&gt;370 Ma) medium‐pressure, amphibolite facies, metamorphism (0.6–0.8 GPa, ∼720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE‐SW trending structures and Carboniferous (315–325 Ma), low‐pressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly &lt;320 Ma). Earlier models to explain this contrast by means of a nappe emplacement of the ZEV, in the narrow time interval between the younger regional metamorphism and granite intrusion, could not be substantiated because, contrary to predictions, the same rock association was present in the whole of the drilled 9.1 km and therefore no nappe boundary was drilled. Instead, it is interpreted that shallow crustal levels reached by the ZEV, subsequent to the Devonian event, shielded most of it from the effects of the Carboniferous metamorphism. A lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original ∼3 km depth section in Mesozoic times. An important result of the geological investigations of the KTB project is the recognition that the present‐day geometry of the ZEV is a result of cold, shallow‐level, thin‐skinned tectonics that, in the absence of the view into the third dimension offered by the drill hole, could not have been accurately predicted from the fossil, high‐grade assemblages and structures seen at the surface.

Isotope systematics of Precambrian marbles from the Lewisian complex of northwest Scotland: implications for PbPb dating of metamorphosed carbonates

We present Pb-isotopic data from the late-Archaean Lewisian marbles of Tiree, Iona, and Glenelg, each locality representing a different tectonic history since the mid-Proterozoic. Data from Tiree (unaffected Precambrian foreland to the Caledonian) yield an age of 1656 ± 23 Ma which is within error of broadly defined Laxfordian reworking events, both locally and throughout the Lewisian. Nd-isotopic data confirm a late-Archaean protolith, although low model μ 1 values require considerably lower pre-Laxfordian U Pb ratios. An age pf 1418 ± 56 Ma is obtained from the Iona marble (? foreland, buried beneath Caledonian nappes), again with a low μ 1 and negative ϵ Nd( t). This age is too young for the Laxfordian and might indicate some disturbance of UPb systematics during Caledonian events. No meaningful age data are obtained from the Glenelg marbles (Lewisian inlier in Caledonian thrust sheet) where Grenvillian (∼ 1.1 Ga) eclogite metamorphism preceded Caledonian events and Pb-isotopic compositions appear not to have been homogenised. Leaching experiments for the Tiree and Iona marbles reveal that radiogenic Pb is hosted in silicate phases, with unradiogenic Pb in the carbonates, a reversal of the situation commonly observed in sedimentary carbonates. If a sedimentary precursor is assumed, redistribution of U and Pb probably occurred during high-grade metamorphism recorded by the age regression, accompanied by significant UPb fractionation to yield higher post-metamorphic U Pb ratios. The significance of non-zero Th U ratios (between ∼ 1 and ∼ 3, post-Laxfordian) remains unclear, but may not have any bearing upon the age or nature of the protolith. Our study provides further evidence that Pb-isotopic studies of marbles can yield reliable geochronological information on metamorphic events in the absence of later tectono-metamorphic overprinting, and is therefore a potentially useful technique for reconnaissance geochronology in metasupracrustal belts. However, caution should be exercised in applying this technique to terrains with complex histories.

Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia

The plains sub-province of the Newer Volcanics of Victoria Australia is a lava plain, ranging in age from 4.5 Ma to < 10 ka and covering an area of 15000 km 2 punctuated by generally younger (< 10–300 ka) cinder cones, lava shields, and maars. Analyses of over four hundred and fifty lavas for major and trace element and strontium isotopic composition show that the plains basalt suite is dominated by tholeiitec and transitional basalts with alkalic rock types and basaltic icelandites being less common. There is a continuous transition from alkalic types through to quartz tholeiites and basaltic icelandites. SiO 2 contents are positively correlated with 87Sr/ 86Sr and negatively correlated with K 2O contents. Abundances of incompatible elements such as K, P, Ti, Sr, and Nb are linearly correlated. Trace element and isotopic patterns indicate that a component with a crustal geochemical signature (high 87Sr/ 86Sr, high K/Nb, and high Pb/Ce) was involved in the generation and/or evolution of some basaltic icelandites. The possibility that the crustal geochemical signature reflects assimilation/crystal fractionation processes cannot be entirely precluded, but it is more likely that isotopic and trace element variation for the more magnesian lavas of the sub-province, including basaltic icelandites with relatively high magnesium numbers, reflects heterogeneity in the mantle sources. Projected onto an east-west profile through the sub-province, the strontium isotopic data reveal a north-south boundary (the Mortlake Discontinuity) separating eastern and western sectors with different mean strontium isotopic ratios. The basalts of the eastern sector show a higher mean 87Sr/ 86Sr ratio (0.7047) and the ratios are more variable (0.7037–0.7058) than is the case in the western sector (mean is 0.7042 and all but one sample in the range 0.7037–0.7046). The Mortlake discontinuity coincides with a major tectonic boundary separating the two principal Palaeozoic mobile belts of southeastern Australia. It is proposed that the tectonic boundary extends into the subcontinental lithosphere and that the isotopic and geochemical differences observed in the basalts of eastern vs. the western sectors arise because of differences in the geochemistry of heterogeneous lithospheric mantle on either side of this boundary. Strontium isotopic analyses in conjunction with other geochemical information, geomorphological and petrographic observations, and available geochronological information have also been used to define isotopic domains in the sub-province ranging in size from a few up to several hundred square kilometres. The isotopic domains define individual flows or groups of flows representing individual magma batches and it is postulated that the domains in part reflect small scale geochemical heterogeneity in the lithospheric mantle.

Rb-Sr dating of sphalerite based on fluid inclusion-host mineral isochrons; a clarification of why it works

Nakai et al. (1990, 1993), Brannon et al. (1992), and Christensen et al. (1995a, b) recently published precise Rb-Sr formation ages of sphalerites from various Mississippi Valleytype deposits. This important advance in direct radiometric dating of ore deposition is based on a crush-leach procedure in which the Rb-Sr isotope composition of the fluid inclusion fraction (termed leachate) is analyzed separately from the residual host crystal (termed residue). Isochron arrays were obtained by combining the fluid-host mineral data pairs from several samples. The resulting ages have yielded decisive chronological constraints on controversial genetic models (as summarized, for example, by Christensen et al., 1995a) where few alternative methods for dating are available. Although the method appears successful, the above authors were unable to explain why the separation of sphalerite samples into fluid inclusion and residual solid fractions should yield an isochron age at all. Without an explanation, the accuracy of the ages remains uncertain. The aims of the present communication are to clarifY the origin of the isochron patterns and to evaluate their age significance in the way they have been interpreted hitherto, that is, in the absence of independent geochronologic information. Our arguments are based first on analyses of synthetic fluid inclusions in quartz, which we use as an experimental analogue of fluid inclusion-bearing sphalerites. In building on the analytical approach of Shepherd and Darbyshire (1981) for fluid inclusion dating, Pettke and Diamond (1995a) synthesized RbSr-bearing, aqueous fluid inclusions in quartz under high temperatures and pressures. Subsequent analysis of the separate fluid inclusion and quartz-host fractions revealed Rb-Sr patterns which are very similar to those found in natural sphalerite by Nakai eta!. (1990). Although recognizing that the analogy between the synthetic and natural samples is limited by important differences in crystal-chemical behavior between sphalerite and quartz, we are confident that the similarities between the isotopic results of these studies justifY inferences being drawn regarding the dating of natural sphalerites. Second, by manipulating the data published by Nakai et a!. (1990, 1993), Brannon et a!. (1992), and Christensen et a!. (1995a, b), we examine the nature of partitioning of Rb and Sr between sphalerite and hydrothermal parent fluid, and thus arrive at conclusions on the validity of dating with fluid inclusion-host sphalerite isotope arrays.

Age and character of basaltic rocks of the Yucca Mountain region, southern Nevada

Volcanism in the Yucca Mountain region of southern Nevada in the last 5 m.y. is restricted to moderate‐to‐small volumes of subalkaline basaltic magmas, produced during at least 6 intervals, and spanning an age range from 4.6 Ma to about 125 ka. Where paleomagnetic evidence is available, the period of volcanism at individual eruptive centers apparently was geologically short‐lived, even where multiple eruptions involved different magma types. K‐Ar studies are consistent with most other geochronologic information, such as the minimum ages of exposure‐dating techniques, and show no evidence of renewed volcanism after a significant quiescence at any of the centers in the Yucca Mountain region. A volcanic recurrence interval of 860 ± 350 kyr is computed from a large K‐Ar data set and an evaluation of their uncertainties. Monte Carlo error propagations demonstrate the validity of uncertainties obtained for weighted‐mean ages when modified using the goodness of fit parameter, MSWD. Elevated 87Sr/86Sr initial ratios (Sri) in the basalts, nearly constant at 0.707, combined with low SiO2 and Rb/Sr ratios indicate a subcontinental, lithospheric mantle source, previously enriched in radiogenic Sr and depleted in Rb. Beginning with eruptions of the most voluminous eruptive center, the newly dated Pliocene Thirsty Mountain volcano, basaltic magmas have decreased in eruptive volume, plagioclase‐phenocryst content, various trace element ratios, and TiO2, while increasing in light rare earth elements, U, Th, P2O5, and light REE/heavy REE ratios. These time‐correlated changes are consistent with either increasing depths of melting or a decreasing thermal gradient in the Yucca Mountain region during the last 5 m.y.

Geochemical and isotopic study of a norite-eclogite transition in the European Variscan belt: Implications for UPb zircon systematics in metabasic rocks

Three different geochronological techniques (UPb conventional multigrain, single zircon stepwise Pb-evaporation, and SmNd on minerals) were applied to rocks from a norite-eclogite transition observed in the West European Variscan belt and combined with major and trace element (including REE) geochemistry. The studied rocks belong to a cogenetic tholeiitic suite which originated in an immature back-arc basin or within-plate magmatic environment. The initial ϵ Nd T=480 Ma values around +1.2 imply that the noritic protoliths were emplaced in a geotectonic setting in close association with the continental crust. UPb mulfgrain and the SmNd dating of a norite sample indicate an age of 481 Ma for the basic magmatism. SmNd dating on two whole-rock-garnet pairs yields an age of 408 Ma for the eclogite-facies metamorphism. These two ages are consistent with the present understanding of the European Variscan belt. Finally, the single zircon Pb-evaporation study of an eclogite sample demonstrates the co-existence of several types of zircons. Each zircon population yields a particular type of geochronological information, related to the contrasting behaviours of the UPb isotopic system during the eclogite-facies tectonometamorphic event. These analytical results suggest that complex and heterogeneous behaviour within the UPb zircon system may also be recorded in numerous other samples. This last point raises the question of the significance of results obtained by UPb conventional multigrain dating in metabasic rocks.

Archaean and early Proterozoic basement provinces in Greenland

Information about the age and plate-tectonic setting of Precambrian basement terrains is of major importance for the evaluation of their mineral potential. The Geological Survey of Greenland (GGU) has therefore over the last two decades collected and published geochronological information on Greenland basement areas, based on whole rock Rb-Sr, Pb-Pb, Sm-Nd and zircon U-Pb isotope data. These isotope data, together with other geochemical information, also yield important clues for the plate-tectonic setting of the investigated terrains. GGU does not have laboratory facilities for isotope work, and most studies were therefore carried out in cooperation with university scientists from Denmark and abroad. This effort has led to the recognition of a number of distinct Precambrian basement provinces. A broad outline of results is given below.

The continental crustal age distribution: Methods of determining mantle separation ages from Sm‐Nd isotopic data and application to the southwestern United States

Samarium‐neodymium isotope systematics provide a means of determining the age of the continental crust, where ”age“ refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm‐Nd model age or the mantle separation age. Systematic analysis of rocks from the continents can be used to determine the age distribution by mass in the continents; this is a unique record of the thermal and chemical evolution of Earth over almost 4 b.y. The establishment of mantle separation ages for crustal rock reservoirs requires precise geochronological information typically provided by U‐Pb isotopic measurements of zircon, plus models for (1) Nd isotopic evolution in the mantle source of continental crust, (2) Nd isotopic evolution of crustal reservoirs subsequent to formation, (3) petrogenesis of certain types of granitic rocks in the crust, (4) the mechanisms of formation of hybrid crust (mixtures of material newly differentiated from the mantle with material derived from older crust), and (5) the evolution of the lower parts of the continental crust during continental margin magmatism and intracontinental extension. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The greatest uncertainties come from a lack of adequate information on lower crustal processes and composition. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass‐age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. Almost all of the Phanerozoic additions occur in areas where there was no preexisting continental basement; Mesozoic magmatic additions account for only about 2% of the crustal mass in the area underlain by Precambrian basement. In the region of post‐Precambrian continental growth, the regions farthest from the Precambrian continental edge are composed of mostly (&gt;85%) of material newly differentiated from the mantle. Adjacent to the Precambrian continental edge the new crust is typically composed about 50% of material newly differentiated from the mantle and 50% of old crustal material. There is no correlation between crustal thickness and the crust formation age in the area studied; the age‐area and age‐mass curves are nearly identical.