Rolf L. Romer

Composition and density model of the continental crust at an active continental margin—the Central Andes between 21° and 27°S

This paper derives an estimate of the average chemical composition of the Paleozoic Andean basement, based on the geological evolution and from compilations of geochemical and isotopic analyses of Early to Late Paleozoic age metamorphic and magmatic rocks between 21° and 27°S. Geochemical and isotopic data indicate that recycling of the Early Paleozoic metamorphic basement was the predominant process in the formation of felsic magmas in the Central Andes from the Odovician to late Paleozoic. The metamorphic basement, in turn, is derived in part from older intrusions and sedimentary rocks of Eocambrian age. Compositional characteristics of Ordovician and late Paleozoic sediments reflect the erosion of this crust. Mafic metamorphic and igneous rocks do not contribute significantly to the Paleozoic crust. The Paleozoic crust is mainly felsic in composition, which is supported by evidence from lower crustal xenoliths and limited exposures of lower crust. The crust consolidated in the Paleozoic remained largely intact until the formation of the overthickened Andean crust in the Cenozoic. The dominance of the Paleozoic crust in the crust of the present high plateaus of the central Andes is seen in the geochemical and isotopic signatures of the Cenozoic ignimbrite and andesite. Thermodynamic calculation of the stable mineral assemblages from the average bulk composition derived from our compilation indicate that the volumetrically most important minerals are quartz and feldspars under all reasonable P–T conditions expected for the thickened Andean crust. The resulting density–depth model is in accordance with geophysical observations of the velocity–density distribution at 21°–24°S. Radiogenic heat production is estimated from the average U–Th–K concentrations in the Paleozoic metamorphic and granitic rocks that are very similar to average values of the upper crust.

Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and ‘dating’ the increase of oxygen in the Precambrian atmosphere

Abstract The Mooidraai Dolomite Formation is a unit of marine sedimentary carbonates in the upper Transvaal Supergroup which recorded significant changes in the composition of the Earth’s atmosphere and ocean. Previously available data had suggested that this dolomite was about 2.2 Ga old and showed δ13Ccarb values around 0.8‰ PDB, and hence was an exception to the positive excursion of δ13Ccarb values observed worldwide in marine carbonates deposited between 2.25 and 2.05 Ga ago. We studied the Pb–O–C isotope systematics of drill core samples from a highly silicified and aluminosilicate-free sub-unit of the Mooidraai Dolomite Formation, that yields well-preserved micritic dolomite grains. Selective leaching of the silicified dolomite revealed a significant Pb isotopic contrast between the carbonate and the quartz fraction. The former shows only a narrow range of 208Pb/204Pb (35.35–35.58) which does not correlate with 206Pb/204Pb, indicating the absence of Th-derived detrital Pb. The carbonate-bound Pb defines an isochron that corresponds to an age of 2394±26 Ma (2σ; MSWD=1.8, n=22) which we interpret as the diagenetic age of the Mooidraai dolomite. The δ13Ccarb values range from 0.51 to 0.64‰ PDB and confirm the previous data. The δ18Ocarb values range from −2.08 to +0.18‰ PDB and are amongst the heaviest reported yet from Early Precambrian marine sedimentary carbonates. The quartz fraction is considerably lighter than the carbonate fraction, and shows δ18Oqtz values between −9.22 and −6.73‰ PDB (+21.40 and +23.97‰ SMOW). Together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155°C, this indicates that the intense silicification was post-depositional. Comparison of δ18Ocarb values of silicified and non-silicified Mooidraai samples suggests that the post-depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite. The heavy δ18Ocarb values suggest that 2.4 Ga ago the oxygen isotopic composition of seawater differed only little from that of today. The new Pb-Pb carbonate age for the Mooidraai dolomite indicates that the upper Transvaal Supergroup is about 200 my older than previously thought. This explains the ‘normal’ carbon isotopic composition of the Mooidraai dolomite, and suggests that the pronounced increase of the oxygen content in the Precambrian Earth’s atmosphere that is seen in strata underneath the Mooidraai dolomite occurred before 2.4 Ga ago. On this background, it appears unlikely that the positive excursion of the carbon isotopic ratios of 2.25–2.05 Ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen.

The Cadomian Orogeny in Saxo-Thuringia, Germany: geochemical and Nd-Sr-Pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance

Abstract The Cadomian basement and the Cambro-Ordovician overstep sequence in Saxo-Thuringia is characterized by clastic sedimentation from the Late Neoproterozoic to the Ordovician. Magmatism in the Avalonian–Cadomian Arc preserved in Saxo-Thuringia occurred between ca. 570 and 540 Ma. Peri-Gondwanan basin remnants with Cadomian to Early Palaeozoic rocks are exposed as very low-grade metamorphosed rocks in six areas (Schwarzburg Anticline, Berga Anticline, Doberlug Syncline, North Saxon Anticline, Lausitz Anticline, and Elbe Zone). A hiatus in sedimentation between 540 and 530 Ma (Cadomian unconformity) is related to the Cadomian Orogeny. A second gap in sedimentation occurred during the Upper Cambrian (500 to 490 Ma) and is documented by a disconformity between Lower to Middle Cambrian rocks and overlying Tremadocian sediments. Major and trace-element signatures of the Cadomian sediments reflect an active margin (“continental arc”), those of the Ordovician sediments a passive margin. The Cambrian sediments have inherited the arc signature through the input of relatively unaltered Cadomian detritus. Initial Nd and Pb isotope data from the six Saxo-Thuringian areas demonstrate that there is no change in source area with time for each location, but that there are minor contrasts among the locations. (1) Cadomian sediments from the Lausitz Anticline, the Doberlug Syncline and the Elbe Zone have lower 207Pb/204Pb than all other areas. (2) The core of the Schwarzburg Anticline, which is overprinted by greenschist facies conditions and detached, is isotopically heterogeneous. One part of its metasedimentary units has less radiogenic Nd than sediments from other low-grade units of similar age in the same area. (3) Cadomian sediments from the Schwarzburg Anticline show an input of younger felsic crust. (4) The Rothstein Group shows distinct input of young volcanic material. Also, (5) Cadomian sediments from the Lausitz Anticline, the Elbe Zone and parts of the North Saxon Anticline are characterized by input from an old mafic crust. Nd isotope data of the remaining areas yield average crustal residence ages of the sediment source of 1.5–1.9 Ga, which suggests derivation from an old craton as found for other parts of the Iberian–Armorican Terrane Collage. Similarly, the Pb isotope data of all areas indicate sediment provenance from an old craton. The rapid change of lithologies from greywacke to quartzite from the Late Neoproterozoic (Cadomian basement) to the Ordovician does not reflect changes in sediment provenance, but is essentially due to increased reworking of older sediments and old weathering crusts that formed during various hiatus of sedimentation. This change in sediment maturity takes its chemical expression in lower overall trace-element contents in the quartzite (dilution effect by quartz) and relative enrichment of some trace-elements (Zr, MREE, HREE due to detrital zircon and garnet). The Rb–Sr systematics of the quartzites and one Ordovician tuffite was disturbed (most likely during the Variscan Orogeny), which suggests a lithology-controlled mobility of alkali and calc-alkali elements. By comparison with available data, it seems unlikely that only Nd TDM model ages are useful to distinguish between West African and Amazonian provenance. Nd TDM model ages of 1.5 to 1.9 Ga in combination with paleobiogeographic aspects, age data from detrital zircon, and palaeogeographic constraints, especially through tillites of the Saharan glaciation in the Hirnantian, strongly indicate a provenance of Saxo-Thuringia from the West African Craton.

Implications of UPb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accretion of 1.90–1.85 Ga magmatic arcs to the Baltic Shield

Abstract The Palaeoproterozoic growth of the Baltic Shield involved the accretion of several ∼1.90–1.85 Ga old magmatic arcs to the southwest of the Archaean craton and the deformation and migmatization of sedimentary basins located between the arcs. During crustal thickening after the collision of the arcs, the sedimentary basin fill became intruded by peraluminous two-mica granites. Locally, columbite-bearing pegmatites are genetically associated with these granites. Columbites from lithium-cesium-tantalum-type (LCT-type) pegmatites from the Stockholm area (Sormland gneisses) yield UPb ages at 1815–1820 Ma, while niobium-yttrium-fluorine-type (NYF-type) pegmatites from the same area are younger (1795±2 Ma, 2σ). Farther to the north, LCT-type pegmatites from the central Bothnian Basin, that correspond geochemically and mineralogically to those of the Stockholm area, yield UPb columbite ages at 1795–1800 Ma, while LCT-type pegmatites in the sedimentary basin between Skelleftea and Lulea yield less well constrained UPb columbite ages at 1765–1775 Ma. LCT-type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust. They represent markers for the time when the Palaeoproterozoic Baltic Shield suffered sufficient thickening to yield voluminous anatectic melts. The UPb columbite ages from the LCT-type pegmatites indicate that comparable phases of post-collisional crustal thickening of the Svecofennian area of the Baltic Shield occurred later to the north.

U-Pb dating of columbites: A geochronologic tool to date magmatism and ore deposits

We have developed techniques for the U-Pb analysis of the mineral columbite [(Fe,Mn)(Ta,Nb)2O6], and report U-Pb ages obtained for three early Proterozoic columbites from the Baltic Shield of northern Sweden. The U-Pb ages of these columbites agree with other available geochronological data. U-Pb dating of columbite is therefore a potentially powerful tool in establishing reliable ages of pegmatites, alkaline and carbonatitic intrusions, and ore deposits of Sn, W, and REEs, all of which often contain columbite. Furthermore, columbite can be used to date peraluminous granites as it often occurs within Li-P-REE pegmatites associated with such granites.

U-Pb columbite chronology of post-kinematic Palaeoproterozoic pegmatites in Sweden

Rare-element pegmatites of the lithium-cesium-tantalum (LCT) family originate from crustal melts derived from orogenically-thickened continental crust and yield a minimum estimate for the age of orogenic thickening. Pegmatites of this type from the Palaeo

Lead mobilization during tectonic reactivation of the western Baltic Shield

Abstract Lead isotope data from sulfide deposits of the western part of the Baltic Shield define mixing lines in the 206 Pb 204 Pb - 207 Pb 204 Pb diagram. Lead from two types of sulfide deposits have been investigated: 1. (1) Exhalative and volcanogenic deposits that are syngenetic with their host rocks 2. (2) vein deposits. The syngenetic deposits locally show a very wide range of lead isotopic compositions that reflect a variable addition of highly radiogenic lead, while the vein deposits, although they have radiogenic lead isotopic compositions, exhibit only limited isotopic variations. In different provinces of the shield, both types of deposits fall on the same lead mixing array. The slope of the lead mixing lines varies as a function of the age of basement rocks and the age of the tectonic event which produced the lead mobilization and therefore relates the source rock age with the age of lead mobilization. Calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continental rifting. The orogenic events are the ca 360–430 Ma Caledonian, ca 900–1100 Ma Sveconorwegian, and the ca 1800–1900 Ma Svecofennian orogenic cycles. The rifting events correspond to the formation of the ca 280 Ma Oslo rift and the Ordovician (ca 450 Ma) graben system in the area of the present Gulf of Bothnia. Each mixing age indicates that lead was mobilized, probably as a consequence of mild thermal disturbances, and that the crust was permeable to lead migration. The data show that the geographic distribution of sulfide deposits with highly radiogenic lead isotopic compositions coincides with old graben systems, orogenic belts, and orogenic forelands on the Baltic Shield. The ages of vein deposits and their geographic distribution demonstrate multiple tectonic reactivation of the interior of the Baltic Shield in response to orogenic events at its margin.

Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon Granulite Massif, Germany: P-T-t path and geotectonic implications

The Saxon granulites, the type granulite locality, were deeply buried, extremely heated and then rapidly exhumed during the Variscan Orogeny; thus their evolution differs from many granulites elsewhere. The peak-metamorphic assemblages of layered felsic-mafic granulites from a 500 m deep borehole consist of garnet, kyanite, rutile, ternary feldspar and quartz in felsic granulite, and garnet, omphacite, titanite, ternary feldspar and quartz in mafic granulite. A minimum temperature of 1000-1020degreesC, calculated from reintegrated hypersolvus feldspar in felsic and mafic granulites, is consistent with the highest temperature estimates from garnet-clinopyroxene equilibria. Various equilibria in felsic and mafic granulites record a peak pressure of about 23 kbar. Diffusion zoning and local homogenisation of minerals reflect near-isothermal decompression that preceded cooling and partial hydration at medium- to low-pressure. U-Pb dating of titanite yields an age of peak metamorphism at 340.7+/-0.8 Ma (2sigma). However, chemical inheritance from precursor rutile and post-peak Pb loss are also evident, suggesting a protolith age of 499+/-2 Ma (2sigma) and partial resetting down to an age of 333+/-2 Ma (2sigma). Rb-Sr mica ages of 333.2+/-3.3 Ma (2sigma) are interpreted as dating cooling through about 620degreesC. Hence the Saxon granulites were exhumed to the upper crust during the short period of 6-11 Ma, which corresponds to average exhumation and cooling rates of 10 mm/year and 50degreesC/Ma, respectively. Such rapid exhumation is inconsistent with recent numerical models that assume foreland- directed transport of the Saxon granulites in the lower crust followed by extensional unroofing. Instead, high-pressure rocks of the Saxon Granulite Massif and the nearby Erzgebirge experienced a buoyant rise to the middle crust and subsequent juxtaposition with structurally higher units along a series of medium- to low-pressure detachment faults

Crustal Evolution at the Central Andean Continental Margin: a Geochemical Record of Crustal Growth, Recycling and Destruction

Active continental margins are considered as the principal site for growth of the continental crust. However, they are also sites of recycling and destruction of continental crust. The Andean continental margin has been periodically active at least since the early Paleozoic and allows the evaluation of the long-term relevance of these processes. The early Paleozoic orogeny at ca. 0.5 Ga recycled and homogenized the ∼2 Ga old early Proterozoic crust of the Brazilian Shield, which was previously orogenized at ca. 1 Ga, consistent with global models of prominent crustal growth at 2 Ga and a near constant mass of continental crust in the Phanerozoic. The metamorphic and magmatic evolution and the isotopic signatures of the early Paleozoic rocks do not indicate significant crustal growth, either by accretion of exotic terranes and island arcs or by juvenile additions from a mantle source. The dominant inferred mode of crustal evolution in the Paleozoic was recycling of older crust. Destruction of continental crust by subduction erosion is prominent in sections of the present active margin and is also likely to have occurred in the past orogens. Voluminous juvenile magmatism is only observed in the Jurassic — lower Cretaceous extensional magmatic arc. Compositions of mantle-derived magmas from the early Paleozoic to the Cainozoic, as well as late Cretaceous mantle xenoliths, indicate that depleted mantle was already present beneath the early Paleozoic orogen. The old subcontinental, enriched mantle related to the Brazilian shield and bordering Proterozoic mobile belts was modified by asthenospheric mantle in the younger subduction systems. In summary, this transect of the Andes is not a site of major continental growth, but a site where long-term processes of growth, recycling and destruction balance out.

UPb systematics of stilbite-bearing low-temperature mineral assemblages from the Malmberget iron ore, northern Sweden

Minerals with a low thermal stability strongly constrain the history of cooling and later tectonic reworking of an area, provided these minerals can be dated. The possible use of stilbite, a CaAl-silicate of the zeolite group, for geochronologic studies was investigated. Open fractures in the Palaeoproterozoic Malmberget iron ore, northern Sweden, contain low-temperature mineral assemblages with various combinations of apatite, stilbite, calcite, biotite, and less commonly titanite and monazite. Two generations of fractures, that are characterized by calcite and stilbite with distinctly radiogenic initial 87Sr/86Sr at ca. 0.720 and ca. 0.708, are dated at ca. 1740 Ma (monazite) and 1620-1613 Ma (titanite), respectively. Apatite samples, even those intimately intergrown with ca. 1740 Ma old monazite, yield U-Pb ages at 1620-1600 Ma, which indicates that apatite apparently recrystallized and reset its UPb system. Older stilbite yields a secondary lead isochron at 1730 ± 6.4 Ma (2σ), which unequivocally demonstrates that the ambient temperature in the Malmberget area from then on remained below the thermal stability of stilbite (ca. 150°C). Stilbite is a natural ion-exchanger and its U-Pb systematics indicates recent mobility of uranium and lead. However, the 1730 ± 6.4 Ma (2σ) age demonstrates that some of the older stilbite was not disturbed during younger fracturing. Hydrothermally altered and secondary stilbite samples yield scattered lead arrays that correspond to secondary isochrons at ca. 1650-1600 D4a, which agrees with the U-Pb titanite and apatite ages. Thus, in combination with other geochronometers, the generally imprecise stilbite ages provide information on the cooling history of an area.