Late Proterozoic Age Research Articles

Geology, Petrography, and Geochemistry of Younger Granites and Related Radioactive Pegmatite from Gabal El Urf Area, North Eastern Desert, Egypt: Implications for the Evolution of the NYF-Type Rare-Elements Granitic Pegmatite

Gabal El Urf region is located in the northern Egyptian Eastern Desert and overlain essentially by older and younger granitoids of Late Proterozoic age. These granitoids contain numerous outcrops of pegmatites, mostly occurring as dikes and veins or huge separated masses. In the contrary, pegmatites form lens-like bodies enclosed within the younger granites along their margins. Previous radiometric surveys showed anomalous concentrations of radioactivity on pegmatite body, located to the south of the study region. The younger granites and the associated radioactive pegmatite are studied in details with the aim to describe this pegmatite, petrogenesis and its relation with the encolsing granite. The younger granites are monzogranite and syenogranite according to mineralogical and chemical classification. Petrographically, the two younger granitic phases are strikingly similar. Both consist of potash feldspar, plagioclase and biotite with some hornblende in the earlier phase. Gabal El Urf younger granites are categorized as highly fractionated I-type granites with metaluminous to mildly peraluminous characters, which emplaced at 600 ± 11 Ma in post-collisional environment. The radioactive pegmatite is located within the syenogranite and enriched with Zr, Nb, Ta, Y, Th, U and REEs; accordingly it was classified as NYF-type rare-elements pegmatite. Metal-bearing generations of minerals within the rare-elements pegmatite include zircon, uranothorite, betafite, aeschynite, fluorite, pyrite and ilmenite.

LA‐ICP‐MS dating of detrital zircon grains from the Cretaceous allochthonous bauxites of Languedoc (south of France): Provenance and geodynamic consequences

AbstractThe Cretaceous of southern France is characterised by a long erosional hiatus, outlined with bauxite deposits, which represent the only remaining sedimentary record of a key period for geodynamic reconstructions. Detrital zircons from allochthonous karst bauxites of Languedoc (Southern France) have been dated using LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), in order to specify the age of deposition and to constrain the provenance of the weathered material. We analysed 671 single detrital zircons grains from three karst bauxitic basins, stretching from close to the Variscan Montagne Noire to the present‐day Mediterranean Sea. Analytical results provide Variscan (300–350 Ma) and Late Proterozoic (550–700 Ma) ages as primary groups. In addition, Middle‐, Late Proterozoic and Early Archean (oldest grain at 3.55 Ga) represent significant groups. Mid‐Cretaceous zircons (118–113 Ma) provide a pooled age of 115.5 ± 3.8 Ma, which constitutes the maximum age for bauxite deposition. Results also suggest a dual source for the Languedoc bauxite: one generalised sedimentary source of regional extent and a localised source in the Variscan basement structural high, that has been progressively unroofed during Albian. Integration of these new findings with previously published thermochronological data support the presence of an Early Cretaceous marly cover on the Variscan basement, which has been weathered and then, removed during the Albian. The Languedoc bauxite provide a spatial and temporal link between the uplift of southern French Massif Central to the north, and the Pyrenean rift and its eastward extension to the south. These new results allow to constrain the timing and distribution of uplift/subsidence during the mid‐Cretaceous events in relation with the motion of the Iberian plate relative to Eurasia.

QUARTZITES OF THE TOVKACHIV SUITE OF THE LATE PROTEROZOIC OF THE OVRUCH AND TOVKACHIV DEPOSITS ARE AN UNIQUE OBJECT FOR INDUSTRY, SCIENTIFIC RESEARCH, EVERYDAY LIFE AND GEOLOGICAL HERITAGE

Information about the geological and tectonic structure of the Ovruch and Tovkachiv quartzite deposits is presented. Attention is focused on the uniqueness of the quartzites of the Tovkachiv Formation of the Ovruch series of the Late Proterozoic age, the deposits of which are developed by the open-pit method in the town. Pershotravneve, for industry, scientific research, everyday life and geological heritage. It is proposed to classify the quartzite section, which was discovered by quarries, after working off the mineral reserves, as objects of geological heritage. Based on the analysis of time-varying satellite images, the change in the area of two quartzite quarries and four water bodies near them was studied. The dynamics of the change in area and its trend during 1989-2020 were clarified.

Мicrobiota of bauxites in the Eva deposit (Guinea-Bissau)

The important role of the biogenic factor in sedimentary ore genesis now, after more than a hundred years after the publication of V. I. Vernadskys and Ya. V. Samoylovs papers, has become an indisputable fact for everyone. Discussions mainly take place as to how productive the activity of organisms was in the formation of specific exploitable minerals. In 2004, a joint monograph of Russian geologists Biomorphic Structures in Bauxite was published. Using the examples of dozens of deposits of a huge stratigraphic range from the Late Proterozoic age to the Quaternary period and wide geographic range (Africa, Eurasia, Australia and America), active participation of various microbial communities in the bauxite formation was shown in the monograph. This article demonstrates new results of electron microscopic study of ores from Eva bauxite deposit, which is little known in Russia, and continues researching this topic, which was initiated by E. L. Shkolnik.

Petrogenesis of the Early-Triassic quartz-monzodiorite dykes from Central Jebilet (Moroccan Meseta): Trace element and Nd-Sr isotope constraints on magma sources, and inferences on their geodynamic context

A dyke swarm composed of hypabyssal rocks of intermediate composition cross-cuts Variscan granites and Palaeozoic metasedimentary country-rocks of the Jebilet range. Based on K-Ar isotope dating on kaersutite phenocrysts, these dykes are Early Triassic in age (ca. 240 ± 10 Ma).Due to more or less pervasive alteration, the primary mineralogical assemblages are often difficult to unravel. Plagioclase and amphibole are ubiquitous, but the extent of biotite, for instance, is subject to discussion. Two distinctive types of rocks are clearly present according to their mineralogical composition: most samples are characterized by a light-brown to greenish hornblende, while some samples are characterized by the presence of kaersutite, suggestive of an alkaline affinity.Incompatible trace element data, restricted to those elements which are considered to be relatively immobile during alteration processes, allow two groups of samples to be distinguished. The largest one displays distinctive negative Ti and Nb anomalies similar to those typical for magmas extracted from mantle sources, which interacted with hydrous fluids and silicate melts, and were thus enriched in Th and LREE relative to the HFSE. However, Nd and Sr data preclude late stage crustal contamination as a suitable explanation of these features in all but the most evolved samples. The LREE enrichment combined with relatively radiogenic Nd isotope signatures are interpreted to reflect the characteristics of source materials, which evolved in a rather primitive, ensimatic setting.The second group comprises only three samples but is of major interest as it does not show any clear Nb negative anomaly and its Th/Nb systematics suggests that crustal components were not involved in their genesis. This group is interpreted as having been produced through partial melting of mantle sources which did not suffer hydrous metasomatism nor recycling of old crustal components, but were enriched in a within-plate setting by low-degree mantle melts.Based on these data, the following tentative geological scenario is proposed. Lithospheric stretching and decompression would have triggered partial melting at relatively deep level of the lithosphere, and produced magmas with within-plate, alkaline-affinities. The ascent of these hot mantle melts would have triggered partial melting of fertile, hydrated domains of Late Proterozoic age present in the overlying mantle and/or lower crust, thereby generating the volumetrically predominant group of samples with « calc-alkaline » geochemical features, namely Nb and Ti negative anomalies. The two types of magmas likely interacted to produce hybrid liquids with variably deep Nb anomalies but broadly constant εNd values, as a result of the weak difference of their isotope signatures.These magmas are much younger (ca. 240 ± 10 Ma) than the last Variscan granites dated at ca. 300 Ma. On the other hand, they largely predate the tholeiitic continental flood basalts of the Central Atlantic Magmatic Province (CAMP) emplaced at ca. 200 Ma, and were thus emplaced amidst the 100 Ma time span between the end of the Variscan orogeny and the lithospheric break-up that led to the formation of the Atlantic Ocean. Therefore, the Jebilet dykes neither represent the ultimate manifestation of the Variscan orogeny nor a premise of the Atlantic opening. Instead, they document a particular event within a protracted intra-continental rifting history spanning the whole Permo-Triassic period. In general terms, this distensive event might be viewed as a local consequence of the dextral mega-shear which separated Europe from North Africa in Late Palaeozoic times. In this scope, the studied dykes could be described as a set of tension gashes, resulting from a local change in stress regime. Their formation was accompanied by a small pulse of intraplate magmatism, possibly because favorable fertile materials occurred locally at depth. At a larger scale, the whole dyke swarm cross-cutting the Variscan basement in Morocco is indicative of an aborted rifting event, largely predating the successful break-up of Pangea and the early opening of the Central Atlantic Ocean.

The magmatic evolution and genesis of the Quaternary basanite-trachyphonolite suite of Itasy (Madagascar) as inferred by geochemistry, Sr-Nd-Pb isotopes and trace element distribution in coexisting phases

The Itasy is a Pleistocene-Holocene volcanic field in central Madagascar, located to the west of the Ankaratra volcanic complex. It comprises scoria cones and lava domes (>120), with associated pyroclastic fall and mafic lava flows, covering an area of ab. 400 km2. The last volcanic episodes probably dated ca. 6000–7100 y BP; warm springs and geysers are active. The juvenile samples comprise a peculiar, almost bimodal, rock suite ranging from potassic leucite-kaersutite-bearing basanites, tephrites and phonotephrites, to benmoreites and titanite-haüyne-bearing trachyphonolites (MgO from 9–10 wt% to 0.1 wt%). These rocks show continuous and overlapping variations in the bulk-rock and phase composition (olivine, clinopyroxene, amphibole, feldspar, leucite, haüyne, nepheline, oxides, apatite, titanite, glass and other accessories). The basanites have homogeneous isotopic composition (87Sr/86Sr = 0.70366–0.70378, 143Nd/144Nd = 0.51274–0.51277, 206Pb/204Pb = 18.7–18.9, 207Pb/204Pb = 15.53–15.56; 208Pb/204Pb = 38.89–39.01), and a marked enrichment in the most incompatible elements (LILE and HFSE reach 100–215 times primitive mantle). These features are consistent with low degrees of partial melting of a volatile-, LILE- and HFSE-rich, amphibole-bearing peridotitic mantle induced by uplift during an E-W-directed extensional regime, as is found in central Madagascar. The marked changes in the geochemical composition, and small variations of the Sr-Nd-Pb isotopes in the trachyphonolites (87Sr/86Sr = 0.70425–0.70446, 143Nd/144Nd = 0.51266–0.51269, 206Pb/204Pb = 18.18–18.39, 207Pb/204Pb = 15.49–15.51; 208Pb/204Pb = 38.38–39.57) with respect to basanites and tephrites point to a limited amount of crustal contamination by the relatively low-206Pb/204Pb, low-143Nd/144Nd, high-87Sr/86Sr Precambrian basement rocks (of Middle Archean to Late Proterozoic age), and highlight the geochemical effects of titanite and anorthoclase removal on the trace element fractionation trends, a feature also shown in the trace element composition of the phenocrysts in trachyphonolites.

Source to sink relation between the Eastern Kunlun Range and the Qaidam Basin, northern Tibetan Plateau, during the Cenozoic

Understanding the source to sink relationship through time between the Eastern Kunlun Range, one of the major mountain belts in the northern Tibetan Plateau, and the actively deforming Qaidam Basin to the north has important implications for unravelling the growth history of the Tibetan Plateau. In this study, U-Pb dating (laser-ablation–inductively coupled plasma–mass spectrometry) of detrital zircons from 22 sandstone samples (Paleocene to Holocene) collected from four sections within the southwestern Qaidam Basin is combined with provenance analysis and new seismic profile interpretation to investigate the mountain building of the Eastern Kunlun Range and its effects on the development of the Qaidam Basin. The U-Pb age distributions of detrital grains from Paleocene strata are characterized by a major component of Paleozoic to late Proterozoic ages. Furthermore, carbonate debris containing foraminifera have been recognized in the Paleocene conglomerate sequences. We thus suggest that the Eastern Kunlun Range was already exhumed prior to the Paleocene. The southward onlaps of Paleocene to Oligocene strata observed on seismic profiles and the appearance of a Mesozoic component in the detrital zircon age spectra of Eocene to Oligocene strata indicate that the Qaidam Basin was widening southward during that early Cenozoic period. Well-developed post-Oligocene growth strata and the increasing proportion of Mesozoic and Paleozoic U-Pb ages in detrital zircon grains from late Neogene strata demonstrate that the relief of the Eastern Kunlun and Altyn Tagh Ranges increased, leading to isolation and narrowing of the Qaidam Basin, from Miocene to the present. The inferred pulsed deformation in the Eastern Kunlun Range highlights the complex growth history of the Tibetan Plateau.

Lead Reservoirs and Metallization in Nigeria: Example of a Contrasting Geological Terrains

Whole rock and mineral extracts from Nigerian goldfields of late Proterozoic age are significantly more radiogenic than those from the much younger Mesozoic lead-zinc mineralization in the Benue Trough, their common formation by hydrothermal processes notwithstanding. Their marked contrasts in lead isotope compositions can be attributed to differences in source reservoirs, structural settings and geological history rather than their age of mineralization. The Mesozoic sulphides have a remarkable uniformly unradiogenic composition for their age, placing the deposits in the lower crustal geotectonic environment that indicate a source of much lower µ. In contrast, mineral extracts from the southwestern and northwestern Nigeria gold deposits show increasingly radiogenic composition and plot in the mixed and upper crustal geotectonic fields respectively, typical of sources enriched in U/Pb and Th/Pb ratios, with clear indication of larger upper crustal reservoir of much higher µ. Therefore, exploration efforts may need to emphasize near-source mineralization prospects of low radiogenic Pb composition for economic deposits.

Geochemical Exploration for Gold and the Association of As-Cu-Pb-Sn-Zn-Li in the Upper Proterozoic Granitoids of the Wadi Rumman Area, Southwest Jordan

The geochemical studies were conducted in an area covering about 15 km2 of the south side of the Wadi Rumman area, SW Jordan. The study area included a part of a basement of complex exposed rocks in southwestern Jordan. The complexes comprised igneous and metamorphic suites, mostly from the late Proterozoic age. A systematic geochemical sampling was conducted using rock and heavy mineral-panned concentrate of geochemical samples. Fifty rock samples were collected from the granitic rocks, simple pegmatite dyke, quartz veins and alteration zone, which covered the area. Next, 45 heavy mineral-panned concentrate samples were collected from the alluvium in the stream sediment within the catchment area, and the dray was sieved to less than 1 mm grain size. The geochemical samples were analyzed for their trace elements and gold by using Ione Conductive Coupled Plasma Emission Spectroscopy (ICP-AES) and the Atomic Absorption Spectrometer (AAS) at Natural Resources Authority (NRA) Labs. The results of the geochemical analysis indicated the presence of gold and heavy minerals in the study area, a result considered abnormal in the rock samples. A strong positive correlation was recorded of Au with As, Cu and W (r = 0.82, 0.7 and 1.0), as with Mo (r = 0.83), Cu with Pb (r = 0.83), Sn with Mo (r = 0.73), Mo with W (r = 0.97), Zn with W (r = 0.71), and Li with Bi (r = 0.7). These correlations revealed gold associated within the hydrothermal alteration, quartz veins and pegmatite dyke. Abnormal metals such as As and Bi were good path-finders to find Au. The HM samples showed low positive linear correlations among the concentrations of Au with As, Zn and Li, and negative linear correlations of Au with Pb, Sn, Bi and W. The combination of both rock and heavy mineral concentrate samples shows four geochemical anomalous areas of gold and heavy minerals. The geochemical signatures of Au and As in the Wadi Rumman appear to be in the hydrothermal alteration, quartz veins and pegmatite dyke, respectively. Heavy mineral concentration sampling delineated the Au geochemical anomaly in area 1 specificities with the rock geochemical anomaly of area 1.

Surface dispersion of uranium in the Al-Hayt area, NE Arabian Shield

Al-Hayt area, although located on the Cenozoic basaltic plateau of Harrat Khaybar, is threatened by uranium contamination emanating mainly from the rhyolite hills of Asfar Thwelil just outside the eastern boundary of the lava field. This outcrop is an exhumed volcanic dome belonging to the Shammar Group of Late Proterozoic age. The rock units in this dome are porphyritic and flow rhyolites with some ignimbrites. The flow rhyolite is especially enriched in uranium and much of this is concentrated in thin vertical veins of uraninite and uranophane. Radiation measurements were carried out in the field using a portable gamma ray differential spectrometer that was capable of counting radiation at 5 spectral intervals representing uranium, thorium, potassium and total radiation (TC1 & TC2). Four traverses containing 67 points were made across the hills of Asfar Thwelil and nearby plains, and some measurements were also carried out in various parts of the Al-Hayt Town itself. Radiation measurements were also carried out in several parts of the eastern and northern shield for the sake of comparison. Soil samples for chemical analysis were also collected along the traverses and from the town and its main quarry. The samples were sieved to 3 sizes (coarse, medium and fine) and analyzed for 34 elements using the Neutron Activation technique. Results of the chemical analyses reveal the concentration of uranium and thorium in the fine soil fraction; although this may indicate adsorption onto clay minerals, a similar trend by immobile elements such as Zr, Hf, Ta, REE and Co rules out this possibility and points to heavy mineral concentration in the fine fraction as the main cause. Radiation was measured in 5 exploration trenches, and the upper layers (alluvium) were found to be the least radioactive, while layers containing salt and calcrete were highly enriched in radioactive elements. Correlation coefficients from soil samples are strongly in favor of the concentration of U & Th with other incompatible elements in heavy minerals such as zircon. Factor analysis of the same samples provides a number of underlying causes that control their chemistry; most important of all is the heavy mineral content followed by the influence of basaltic detritus form the Harrah.

Temporal evolution of the Angavo and related shear zones in Gondwana: Constraints from LA-MC-ICP-MS U–Pb zircon ages of granitoids and gneiss from central Madagascar

Granitoids of Neoproterozoic age are well exposed in the Precambrian basement rocks of Madagascar, in the vicinity of and within the Neoproterozoic dextral Angavo shear zone (ASZ). Pre-kinematic granitoids, a syn-kinematic granite, and a granitic gneiss have been dated using Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). One pre-kinematic granitoid yields a weighted mean 206Pb/ 238U igneous zircon crystallization age of 823.6 ± 6 Ma, and a second a 756.6 ± 9 Ma igneous crystallization age and a 563 ± 11 Ma metamorphic age. A syn-kinematic granite gives a weighted mean 206Pb/ 238U zircon age of 549.8 ± 4 Ma, which points to the time of movement in the ASZ. A granitic gneiss yields a weighted mean 206Pb/ 238U protolith age of 732 ± 13 Ma and records a 558.8 ± 4 Ma metamorphic age. Accordingly, the ASZ was active and associated with the high-temperature metamorphism of the Angavo-Nondiana belt during the Ediacaran. The late Proterozoic ages of igneous intrusions recorded by the granites show that pre-tectonic granites were sheared in the ASZ, and recorded granulite–amphibolite facies metamorphic events. These U–Pb data suggest a link between the ASZ and the Achankovil shear zone in India, which were adjacent in several proposed Gondwana reconstructions.

SHRIMP U-Pb ages of detrital zircons in metasandstones of the Taean Formation, western Gyeonggi massif, Korea: Tectonic implications

Detrital zircons in three metasandstones of the Taean Formation, western Gyeonggi massif, Korea, were analyzed using a sensitive high-resolution ion microprobe (SHRIMP), in order to provide geochronologic constraints for the depositional age as well as the protolith ages. This formation mainly comprises the sandstone-mudstone units which experienced regional metamorphism at the transitional greenschist-amphibolite facies condition. The majority of detrital zircons yielded concordant ages defining four major age components: (1) Late Archean-Early Proterozoic (2.5–2.4 Ga); (2) Middle to Earliest Late Proterozoic (1.0–0.9 Ga); (3) Middle Late Proterozoic (820–750 Ma); and (4) Silurian (∼430 Ma). The youngest group gives a weighted mean 206Pb/238U age of 431.3 ± 4.1 Ma (n = 17), confirming the Upper Paleozoic deposition of the Taean Formation. Together with this Silurian peak, the presence of Middle to Late Proterozoic ages is characteristic for the source rock, suggesting that the western Gyeonggi massif could be correlative with the South China craton. On the other hand, the lack of Paleoproterozoic age of ∼1.87 Ga is in marked contrast with its predominance in the basement rocks of central to eastern Gyeonggi massif. The overall distribution pattern of detrital zircon ages is consistent with that of the Devonian metasedimentary rock in the Imjingang belt, suggesting an extension of this collisional belt towards the south.

Geochemical exploration for gold in the Nile Valley Block (A) area, Wadi Allaqi, South Egypt

The present work aims to estimate Au-anomalous distribution patterns, the optimum grain size fraction and pathfinder elements for gold placer. The obtained data of analyzed elements (Au, Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Ga, Hg, Li, Mn. Mo, Ni, Pb, Rb, Sb, Se, Sr, Te, Ti, V, Y and Zn) in both the fine (−0.25 mm) and coarse (−1 mm+0.25 mm) grain size fractions of 32 dry stream sediment samples, which have been derived from metamorphosed Island Arc volcanic rocks of Late Proterozoic age, revealed that most of the analyzed elements in both grain size fractions are asymmetrically distributed and did not pass the tests of normality. The coarse grain size fraction appears to be the better size fraction for Au detection and can be considered as the optimum grain size fraction for future application in regional stream sediment surveys. The presence of Au anomalies in the upper part of the stream and beside the mine is either sourced by the main Au-mineralization or new potential extension of the mineralization in the study area. Silver, Cd, Se, Zn and Te can be considered as useful pathfinder elements for Au in the coarse grain size fraction, and they may be used for future geochemical exploration for Au in the area. Recommendation of the authors is pointing to perform lithogeochemical survey in the eastern and western parts of the mine.

U-Pb single zircon grain dating of Present fluvial and Cenozoic aeolian sediments from Gabon: consequences on sediment provenance, reworking, and erosion processes on the equatorial West African margin

Abstract U-Pb ages obtained from detrital zircon from terrigenous sediments are used to determine the sources. Present fluvial sand-bars of the Ogooué river yield age spectra of detrital zircons in agreement with Archean and Early Proterozoic sources found in the drainage. The large proportion of Late Proterozoic zircons cannot be derived from primary erosion of the watershed basement rocks, since there is no formation of that age in the area. This later group of zircons is in good agreement with reworking of the aeolian Paleogene Batéké Sands, by regressive erosion in the upper reaches of the Ogooué river, as they contain a majority of Late Proterozoic age zircons. The sources of Late Proterozoic zircons in the Batéké Sand are very distant, and transported and reworked – at least in part – by aeolian processes. Our results, together with the widely distributed Paleogene sediments over continental Africa, suggests that Paleogene was a time of subdued erosion of the cratonic areas and extensive reworking, transport and deposition within continental Africa. In contrast, our results from the Ogooué river indicate active present incision of the cratonic area, erosion of the previous continental sediments, and export of the river bed-load to the continental margin. This temporal evolution of erosion-transport-deposition is correlated with the drastic climate change that occurred during the Cenozoic, leading to a more efficient mechanical erosion, and it correlates with the increase of terrigenous flux to the margin, observed during the Neogene.

Tectonic evolution of the western margin of the Ordos Basin ( Central China)

The western half of the Ordos Basin lies in the north of Central China, on the western margin of the North Chinese Platform. In the southwest and south it borders the Qilian and Qinling folded systems. Their frontal northeastern flank is thrust over the Paleozoic nappe of the Ordos Basin and consists of a series of large parallel linear folds separated by thrusts. The basement of the basin is made up of deformed metamorphic rocks of tentatively Late Proterozoic age. Carbonate and terrigenous intracratonic facies accumulated on the Ordos block from Early Cambrian through Middle Ordovician. From Middle Ordovician through Middle Carboniferous, the western edge of the block was uplifted as a land as a result of Caledonian tectonic movement and underwent denudation for about 150 Myr. In the Late Paleozoic, a series of coal-bearing formations was deposited. In the Triassic, the Ordos Basin became filled with fluviolacustrine delta facies. Thrusts and folds on the southwestern margin of the basin formed in the Mesozoic and were intensified by Himalayan tectonic movements.

Geochronological and Petrological Constraints for Tectonic Evolution of the Central Greater Himalayan Sequence in the Kharta Area, Southern Tibet

Abstract Within the Kharta area, east of Mount Qomolangma (Everest), garnet sillimanite gneisses and granites including mafic lenses that form the Greater Himalayan Sequence (GHS) were displaced beneath the North Col Formation by the lower South Tibetan Fault (LSTF) and above the Lesser Himalayan Sequence by the Main Central Thrust (MCT). Zircons from mafic lenses, probably former dikes, were dated by thermal ionization mass spectrometry to yield discordant U‐Pb results. One sample gave a discordia line with an upper intercept age of 971 Ma, suggesting a late Proterozoic age for emplacement of the dike. Zircon SHRIMP analyses show that the garnet sillimanite gneisses and granites were derived mainly from early Paleozoic rocks produced by high‐temperature metamorphism and/or partial melting of Neoproterozoic (meta)sedimentary rocks. These crystalline rocks were buried beneath southern Tibet and experienced metamorphic conditions of 750°–800°C and 14 kbar at \documentclass{aastex} \usepackage{amsbsy} \usepa...

The oldest rocks of Greece: first evidence for a Precambrian terrane within the Pelagonian Zone

The Pelagonian Zone in Greece represents the westernmost belt of the Hellenide hinterland (Internal Hellenides). Previous geochronological studies of basement rocks from the Pelagonian Zone have systematically yielded Permo-Carboniferous ages. In this study we demonstrate, for the first time, the existence of a Precambrian crustal unit within the crystalline basement of the Pelagonian Zone. The U–Pb single-zircon and SHRIMP ages of these orthogneisses vary from 699 ± 7 Ma to 713 ± 18 Ma, which identify them as the oldest rocks in Greece. These Late Proterozoic rocks, which today occupy an area of c. 20 × 100 km, are significantly different from the neighbouring rocks of the Pelagonian Zone. They are therefore interpreted as delineating a terrane, named here the Florina Terrane. During the Permo-Carboniferous, Florina was incorporated into an active continental margin, where it formed part of the basement for the Pelagonian magmatic arc. The activity of this arc was dated in this study by single-zircon Pb/Pb ages as having taken place at 292 ± 5 Ma and 298 ± 7 Ma. During the Alpine orogeny, Florina, together with the Pelagonian Zone, eventually became a constituent of the Hellenides. Geochemically, the Florina orthogneisses represent granites formed at an active continental margin. Because of the Late Proterozoic ages, this Late Proterozoic active continental margin can be correlated to a Pan-African or Cadomian arc. As the gneisses contain inherited zircons of Late to Middle Proterozoic age, the original location of Florina was probably at the northwestern margin of Gondwana. Similar to other Gondwana-derived terranes, such as East Avalonia, Florina approached the southern margin of Eurasia during Palaeozoic times, where it became part of an active continental margin above the subducting Palaeotethys. These interpretations further indicate that terrane accretion was already playing an important role in the early pre-alpine evolution of the Hellenides.

U-Pb zircon (ID-TIMS and SHRIMP) evidence for the early ordovician intrusion of metagranites in the late Proterozoic Canaveilles Group of the Pyrenees and the Montagne Noire (France)

Abstract Depending on the quality of the zircon grains available for analysis, two methods may be used to date igneous rock emplacement, namely U-Pb TIMS with isotope dilution or in situ U-Pb SIMS (SHRIMP). Both methods have been used to determine, in a precise and accurate manner, the emplacement age of the granitic protolith of the various orthogneisses in the Pyrenean Axial Zone. More specifically, four representative samples of G1, G2 and a “transition gneiss” yielded reliable datings with an average age of 473 ± 4 Ma for each sample. The surrounding sediments of the Canaveilles Group were constrained by zircon grains from interlayered metarhyodacite and dated at 581 ± 10 Ma using the SHRIMP method, clearly giving this group a late Proterozoic (Vendian) age. Finally, the Somail orthogneiss of the Montagne Noire, equivalent to that of the Canigou, yielded an age of 471 ± 4 Ma with the in situ U-Pb method, which is identical to the dating of the Pyrenean samples. In addition, most of the studied orthogneisses recorded a wide range of significant concordant inherited ages spanning from early Archaean (3.5 Ga) to Pan-African/Cadomian (600–800 Ma). Bearing in mind the calc-alkaline affinity of the studied rocks, this work demonstrates the huge contrast between the active Gondwana margin in the north (“South European terrane”) and the remarkably homogeneous continental plate that existed from Arabia to Morocco during the Ordovician.

Laser ablation ICPMS dating of zircons in Erzgebirge orthogneisses: evidence for Early Cambrian and Early Ordovician granitic plutonism in the western Bohemian Massif

U-Pb isotopic data obtained by laser ablation ICPMS analysis of nine zircons with centre to margin oscillatory growth zones from a K-feldspar-rich augen gneiss in the allochthonous Lower Crystalline Nappe of the Erzgebirge domain of the western part of the Bohemian Massif yield a concordia age of 524 ± 10 Ma (2 sigma). This Early Cambrian age represents the time of magmatic crystallization of the protolith of a representative, from near Měděnec, of the allochthonous “Red gneiss” whose igneous nature is shown by the presence of (deformed) xenoliths. Data from TIMS analysis of zircons with variable proportions of unzoned xenocrystic cores surrounded by oscillatory-zoned overgrowths point to magma derivation from upper Proterozoic, or older, rocks. Data obtained for five zircon grains from another “Red gneiss” in the Lower Crystalline Nappe (in the Klinovec anticline) plot below the concordia with the age of the one point that is near concordant being 519 ± 26 Ma (2 sigma). These data, together with internal features of the zircons, are consistent with Early Cambrian granitic plutonism also in this part of the Erzgebirge but with later Pb loss, possibly associated with considerable fluid movement during thrust nappe development. Another sample of a coarse-grained orthogneiss from the autochthonous St Catherine9s dome yielded a significantly younger Early Ordovician age of 480 ± 10 Ma (2 sigma) calculated from eight zircon analyses. However, three zircon grains from the same sample gave a significantly older near-concordant Late Proterozoic age of ca. 620 Ma. Provided that the age difference of ca . 40 Ma between orthogneisses from Měděnec - Klinovec and St Catherine9s dome holds also for other orthogneisses in the Erzgebirge, zircon U-Pb age data could be used to discriminate between allochthonous and autochthonous units in this region. The ca. 25 Ma difference between the Early Cambrian protolith age of the augen gneiss from near Měděnec determined by the laser ablation ICPMS technique and a previously reported older age of 550 ± 9 Ma for a nearby sample determined by the Pb-Pb evaporation technique is accounted for on the basis of the latter not being adequate for dating zircons with a small xenocrystic component. This study demonstrates the importance of high spatial resolution dating techniques, such as SHRIMP or laser ablation ICPMS, in dating zircons with complex growth history that are common in crustally-derived melts.

Volcaniclastic Aggradation in El Tarr Area, Southeastern Sinai, Egypt: Petrological and Geochemical Evidence

The Tarr complex forms the southernmost portion of the Kid Group in the Southeast Sinai Peninsula. This complex consists of diverse interstratified volcanic and volcaniclastic sedimentary strata and cogenetic hypabyssal intrusive rocks of Late Proterozoic age. The occurrence of slightly preserved glass shards and volcanic clasts within volcaniclastic sediments indicate a narrow time span between eruption and final deposition by resedimentation. The volcano-sedimentary sequence of El Tarr area displays a low metamorphic grade and is nearly undeformed. These volcaniclastic rocks are characterized by rapid lateral and vertical facies and petrofacies variation. Modal analysis of these rocks defines a source area comprising mainly volcanic and intrusive rocks, with no indication of continental detritus or detritus from exotic sources. The studied sequence is divided into three stratigraphic intervals on the basis of the predominant lithosomes in each of the intervals. The lowermost stratigraphic interval contains metamorphosed felsic pyroclastic rocks with marble intercalations, continues vertically to mafic and intermediate volcanic flows, pyroclastic fall and flow deposits and local interbeds of clast-supported conglomerates, associated with cross-laminated sandstones with lensoidal layers of tuffaceous mudstones. These rocks were deposited in a low relief, proximal alluvial fan volcanic apron environment. The middle stratigraphic interval consists of thick massive to bedded coarse to medium-grained sandstones and interbeds of tuffaceous mud/siltstones, with lensoidal beds of metamorphosed mafic volcanics. Rocks in the middle interval were deposited in distal alluvial fan volcanic apron environment. The upper stratigraphic interval contains matrix/clast-supported conglomerates of mixed sources that were deposited in poorly confined, shallow proximal braided river channels on a fluvial plain. Tectonics are perceived to be a major control on the positioning and long-term evolution of alluvial systems. By increasing slope gradients through uplift and tilting, or by changing local base-level, incision can be stimulated leading to a switch in the active area of sedimentation. The switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the El Tarr volcaniclastic rocks during Late Proterozoic time. A lack of pedogenic modification throughout most of the channelized sediments suggests high rates of sedimentation which inhibited soil development. The whole studied sequence is similar to rock units in arc-flank volcanic complexes which are interpreted to have accumulated principally on river flood plain and alluvial fan and local aeolian environment.