Theofilos Toulkeridis

Characterization, provenance, and tectonic setting of Fig Tree greywackes from the Archaean Barberton Greenstone Belt, South Africa

Abstract In this study, mineralogical and geochemical data, as well as Sr and Nd isotopic compositions of early Archaean greywackes from the Fig Tree Group are presented to provide further information about the evolution of the Barberton Greenstone Belt (BGB) of the Kaapvaal Craton, South Africa. The chemical data on the collected greywackes from the BGB suggest limited chemical modification of the source rocks. The Eu/Eu * anomaly is negative with an average value of 0.76 which is slightly higher than that of Eu-depleted crustal rocks (0.65). High Cr and Ni concentrations and a Cr/Ni ratio of about 1.6 indicate derivation from ultramafic sources without fractionation. Although rounded zircons occur, the combination of CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values and Eu/Eu * ratios suggests little transportation until final deposition, whereas the juvenile crustal material comes from local sources. The siliciclastic material seems to be of mixed felsic and mafic-to-ultramafic origin. The tectonic setting of the source areas appears to reflect a continental island arc of Andean type, supporting the existence of an active plate-tectonic geodynamic system in early Archaean time. Sr isotopic data indicate an alteration during a post-depositional event, most probably at 2.6–2.7 Ga, which disturbed the Sr isotopic system. The Sm–Nd isotopic system must also have been altered since deposition of the rocks, as suggested by e Nd values both lower and higher than expected. The calculated depleted-mantle model ages of 3.0 to 3.8 Ga are in the range of BGB sedimentary rocks. The higher model ages appear to reflect the underlying belt, which is represented by the Onverwacht Group and the surrounding intrusives, as being the major source of the greywackes.

Fluctuations of Caspian Sea level: Beyond climatic variations?

The continued rising mode, since about 1978, in the level of the Caspian Sea cannot be ascribed to a change in the hydrologic balance that is governed solely by runoff, precipitation, and evaporation. Such a change fails to explain some of the chemical links (K/Cl, Rb/Cl, Ca/Cl, Sr/Cl, and K/Rb) between Kara Bogaz and Caspian Sea waters. A more realistic model must include a tectono-hydrodynamic factor that allows for subsurface addition of saline water to Caspian Sea and Kara Bogaz waters. The subsurface water component could be akin to the coastal thermal springs in the area, but must dissolve some evaporite deposits in the subsurface before addition to the waters of the inland sea. A budget calculation gives an estimate of 2% to 9% subterranean addition of saline water to the seawater. Such an addition could cause a rise in the level of the Caspian Sea from 63 to as much as 225 cm within the past 20 yr.

Sm-Nd dating of Fig Tree clay minerals of the Barberton greenstone belt, South Africa

Sm-Nd isotopic data from carbonate-derived clay minerals of the 3.22-3.25 Ga Fig Tree Group, Barberton greenstone belt, South Africa, form a linear array corresponding to an age of 3102 +/- 64 Ma, making these minerals the oldest dated clays on Earth. The obtained age is 120-160 m.y. younger than the depositional age determined by zircon geochronology. Nd model ages for the clays range from approximately 3.39 to 3.44 Ga and almost cover the age variation of the Barberton greenstone belt rocks, consistent with independent evidence that the clay minerals are derived from material of the belt. The combined isotopic and mineralogical data provide evidence for a cryptic thermal overprint in the sediments of the belt. However, the highest temperature reached by the samples since the time of clay-mineral formation was <300 degrees C, lower than virtually any known early Archean supracrustal sequence.

Deformed A-type granites in northern Malawi, east-central Africa : pre- or syntectonic?

Metamorphosed and deformed A-type granites ranging in age from 1087 to 1119 Ma occur in strike-slip and reverse-slip shear zones of the Ubendian and Irumide belts of northern Malawi. Peak metamorphic conditions in the granites and their wallrocks reached about 12–13 kbar and 680–740°C. Microstructures reveal that the granites experienced a seemingly progressive deformation from magmatic to solid state. Magmatic deformation is manifested by the coexistence of aligned magmatic potassium feldspar, plagioclase, amphibole, biotite and ilmenite with undeformed quartz and schlieren structures. High-temperature solid-state deformation is characterized by widespread grain-boundary migration, dynamic recrystallization and myrmekitic potassium feldspar replacement. The tectonic foliation is generally subparallel to the magmatic foliation in the granites and also subparallel to the borders of the plutons and to the foliation in the wallrock. Kinematic indicators in both granite and wallrock reveal sinistral movement in the Ubendian-belt shear zones and top-to-the-ESE thrusting in the Irumide-belt shear zones. The overall parallelism of magmatic and tectonic foliations would suggest syntectonic emplacement of the granites with respect to a c. 1100 Ma Irumide orogeny. However, similar deformation patterns of granite-related dykes and some Pan-African pegmatites and geochronologic work suggests that the first deformation of the granites, their related dykes and some of the pegmatites occurred during the Pan-African orogeny. This study implies that a purely structural approach to distinguish syn- and pretectonic granites is rendered difficult when post-emplacement deformation occurred under high-grade metamorphic conditions. Strongly heterogeneous deformation with a pronounced degree of coaxial flattening in the granites allows magmatic structures to be preserved in low-strain zones and to be passively rotated into parallelism with penetrative deformation structures. Our work also implies that there was no Irumide orogen in northern Malawi.

Differentiated hydrothermal and meteoric alterations of the Lueshe carbonatite complex (Democratic Republic of Congo) identified by a REE study combined with a sequential acid-leaching experiment

The behavior of rare-earth elements (REE) during hydrothermal alteration and progressive weathering has been studied by acid-leaching experiments on minerals and whole-rock samples of the Lueshe carbonatite complex (NE of Democratic Republic of Congo). The fresh and hydrothermally altered samples were successively leached with acetic acid (HAc, 1 N) and hydrochloric acid (HCl, 1 N). An additional leaching step with nitric acid (HNO3, 1 N) was applied on meteoric weathered material. After this sequential extraction procedure, the residues were dissolved in a fluorhydric-nitric solution (HF+HNO3). The Lueshe carbonatite complex contains a suite of REE-carriers identified as Ca-bearing minerals including carbonates, apatite and pyrochlore as primary minerals that are accompanied by hydrothermal ancylite-(Ce), parisite-(Ce), synchysite-(Ce) and monazite-(Ce). The hydrothermal alteration induces an enrichment in the light REE, but the authigenic minerals (except monazite) do not, in fact, play an important part in the pre-lateritic REE stock, because of their low amounts. During the early stage of weathering, the residual minerals retain the bulk of the REE, the distribution being only slightly modified. During intense weathering, the leaching experiments indicate that the REE are redistributed among secondary REE-phosphate minerals and bonded to Fe-hydroxides, in the lateritic profile.

Widespread relics of high-pressure metamorphism confirm major terrane accretion in Ecuador: a new example from the Northern Andes

Metamorphic rocks from two sections through the North Andean Cordillera Real in Ecuador contain phengites as relics of high-pressure (HP) metamorphism. Their maximum Si contents per formula unit are (1) 3.25 in Al-rich metapelites, (2) 3.35 in ordinary metapelites and garnet-rich metabasites, and (3) 3.45 in garnet-bearing orthogneisses. Detailed pressure–temperature paths were derived by calculating P–T pseudosections. Garnet-bearing metapelite and metabasite yielded a P–T path starting at 1.4 GPa and 525°C. Peak temperature conditions were 560°C at <1.4 GPa. The retrograde path passed through 0.7–0.8 GPa at 500°C. An adjacent metagranitoid experienced a similar P–T evolution at slightly lower temperatures of about 30–50°C. This is the first reported occurrence of HP metamorphism in the Cordillera Real; it is widespread and evidently was the result of continental collision. We hypothesize that a single microcontinent collided with the South American continental margin in Early Cretaceous time. In contrast to our findings, previous models have simply suggested the amalgamation of several terranes in the late Mesozoic or earlier in the region of Ecuador. Therefore, we propose that a widespread search for HP relics should be undertaken. Such relics provide a general criterion for defining subducted terranes and their respective boundaries.

K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

SummaryWhite micas from metamorphosed and hydrothermally-altered basaltic lavas, conglomerates, quartzites and shales in and around the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The minerals were separated into various grain sizes between < 0.4 and 10μm, and characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in the basaltic lavas suggests an age of 1994 ± 60 Ma, and that of the smaller mica particles (< 2μm) in the quartzites defines a younger age of 1917 ± 66 Ma. This range is considered to reflect the timing of long-lived hydrothermal alteration in the Ventersdorp Contact Reef. The older age is slightly younger than the intrusion of the Bushveld Complex (2060–2054 Ma) and the Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be associated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal activity that extended throughout the Griqualand Basin and even into the Witwatersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the period between 2.0 and 1.9 Ga was significant, as far as alteration, and possibly also gold mobilization, was concerned.ZusammenfassungHellglimmer von metamorph und hydrothermal überprägtem Basalt, Konglomerat, Quarzit und Pelit aus dem Ventersdorp Contact Reef im Witwatersrand Becken, Südafrika, wurden mittels der K-Ar Methode datiert, um das Alter der postsedimentären thermischen Überprägung einzugrenzen. Mineralkörner wurden in einzelne Fraktionen zwischen < 0.4 und 10μm separiert und auf deren Zusammensetzung, Paragenese und Textur mittels Röntgendiffraktometrie und Elektronenmikroskopie untersucht. Das Isochronenalter für alle Hellglimmerproben aus dem Metabasalt liegt bei 1994 ± 60 Ma, jenes für die Glimmer mit einer Korngröße < 2μm im Quarzit bei 1917 ± 66 Ma. Diese Altersspanne wird als Ausdruck einer lang anhaltenden hydrothermalen Veränderung des Ventersdorp Contact Reef interpretiert. Das ältere Alter ist etwas jünger als die Intrusion des Bushveld Komplex (2060–2054 Ma) und die Vredefort Katastrophe (2020 Ma) - zwei gut dokumentierte Ereignisse, die das Witwatersrand Becken erfaßten. Das jüngere Alter könnte mit der eburnischen Orogenese entlang des Westrandes des Kaapvaal Kratons in Zusammenhang stehen, während der es zu Fluid Migration und hydrothermaler Aktivität quer über den Kontinent vom Griqualand Becken bis in das Witwatersrand Becken kam. Die neuen K-Ar Alter bestätigen somit, daß die Zeitspanne von 2.0–1.9 Ga wesentlich für die Alteration und möglicherweise auch für die Mobilisierung von Gold im Witwatersrand Becken war.

Crystallization conditions of fundamental particles from mixed-layer illite-smectite of bentonites based on isotopic data (K-Ar, Rb-Sr and δ18O)

Rb-Sr and oxygen isotope studies, in addition to K-Ar isotopic determinations published previously, are reported on diagenetic and hydrothermal fundamental particles (particle thickness of 0.03 to 0.05 nm and particle ab size of 0.02–0.05 µm) of the East-Slovak Basin. The combined data set allows us to ascertain the crystallization conditions of the illite material from two bentonite units collected at two basinal sites located ~20 km apart, and characterized by prolonged diagenetic conditions induced by progressive burial. A bentonite rock characterized by a short hydrothermal event from the Zempleni mountains to the SW of the East-Slovak basin is also studied.For the two first sites, the δ18O values increase in one case and decrease in the other, when the size of the diagenetic fundamental particles from bentonite samples increases. These variations suggest that temperature increased in one and decreased in the second of the two samples collected in the basin, while the particles were growing. In the case of the hydrothermal bentonite, the δ18O values of the different size-fractions consisting of fundamental particles remain about constant, suggesting constant temperature and fluid chemistry.The Rb-Sr dates of the fundamental particles of the three bentonite rocks were systematically higher than the corresponding K-Ar ages. The 87Sr/86Sr ratios, which are initially involved in the particle nucleation, appeared higher than that of contemporaneous sea-water. In all cases, the initial 87Sr/ 86Sr ratio decreases when particle size increases, which implies supply of external Sr into the bentonite units. This external Sr seems to have had an 87Sr/86Sr ratio close or identical to that of the contemporaneous sea water. This means that Sr, probably of sea-water origin, progressively diffused from host shales into the bentonite units, during burial diagenesis. In turn it favors the suggestion made previously about diffusion of K from shales into the bentonite layers during illitization of the smectite from these units.

Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, delta 18 O) evolution of the clay minerals from faulted, carbonate-rich, passive paleomargin of southeastern Massif Central, France

ABSTRACT Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, d18O) data of < 0.2 µm clay fractions from core samples have been used to enhance our understanding of the evolution of the faulted, carbonate-rich, passive paleomargin in the southeastern French Massif Central. The fault system described at the lower part of the sedimentary sequence was active at 190 ± 20 Ma during a rifting tectono-thermal activity with important migration of hot fluids. It was active again afterwards, probably after maximum burial occurring at 110-120 Ma, as a result of the compressive Alpine activity in the region. This late shearing induced an overall throw of at least 1160 m and subsequent erosion of as much as 2000 m of s diments. However, it probably did not induce major fluid movements, which means in turn that the mineralizing fluids responsible for economic ore deposits in the margin relate to the 190-Ma-old Liassic hydrothermal event. The Rb-Sr, K-Ar, and oxygen isotopic systematics of the clay fractions suggest that late faulting activity occurred under very low water-to-rock conditions, which did not favor widespread fluid migration. The clay material of the fault yields unreasonably widely scattered Rb-Sr and K-Ar dates, from 70 to 183 Ma, because of variable contents of radiogenic 40Ar and some unrealistic initial 87Sr/86Sr ratios, even below 0.700. The 190 Ma illites that crystallized in the fault at 200-210°C also have abnormally low d18O values of about +13.3, leading to a d18O value of the pa eofluids as high as +8.1. The 87Sr/86Sr ratios and REE distributions of acid-leached clay residues delineate two types of fluids that have probably interacted with the clay minerals. One, with an 87Sr/86Sr ratio of 0.7250 and a humped REE distribution pattern, is believed to be of hydrothermal origin, and the other, with an 87Sr/86Sr ratio of 0.7082 and a flat REE pattern, might be of recent continental origin.

Compositions chimiques et isotopiques d'eaux de la mer Caspienne et de tributaires de la région de Makachkala (Russie) : premières données sur le fonctionnement d'un système endoréïque particulier

Abstract The δ18O and 87Sr/86Sr values of thermal waters from the western coast of the Caspian sea, which contribute to its total budget, result from interactions with host rocks; they even suggest a supply by ‘deep’ waters. In fact, the Sr contents and the 87Sr/86Sr values of the river and thermal waters are not sufficient for the calculation of an equilibrated budget, implying that other sources contribute to the budget of the Caspian waters. The 206Pb/207Pb ratio of the waters from the Sulak river also suggests local anthropogenic pollution.