Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic,palaeozoic sediments in southern France (Montagne Noire)

Rb-Sr whole-rock and mineral analyses are reported from Early Paleozoic sediments of the southern rim of the French Central Massif (Montagne Noire). The sediments—mainly pelites, silt- and sandstones—can be separated into three different zones (A, B, and C) according to their varying degrees of deformation and transformation, which took place during the Hercynian (= Variscan) orogeny (Carboniferous). For the least deformed, folded but not schistose, fossiliferous Lower Ordovician sediments (zone A), a scatter of the data points resulted, at least partly due to the varying amount of detrital muscovites, which did not equilibrate their Sr isotopes with those of the clay minerals. Fossiliferous Middle Cambrian sediments (zone B), however, are colinear in an isochron plot. The rocks are also folded, and show a beginning schistosity in the pelitic layers. The age of 333±16 m.y. therefore does not correspond to the time of sedimentation, as one would expect from most of the interpretations given in the literature. It is, however, in close agreement with the stratigraphically known time of folding and anchimetamorphism. The same result, within analytical error, of 326±17 m.y. was obtained for Lower Ordovician sediments (zone C), which show a schistosity not only in the pelites but also in some psammitic layers. As for the sediments of zone B, this result indicates open whole-rock systems for Rb and/or Sr during folding and anchimetamorphism. Minerals extracted from psammites of zones B and C gave the following results: albite coincides with the isochrons; detrital muscovites do not coincide and yield ages of about 650 m.y. (Cadomian orogeny) for both zone B and C sediments; detrital chlorites from zone B sediments seem to be only partly reset by anchimetamorphism and therefore plot closer to the whole-rock isochron. From this it must be concluded that an increasing amount of these detrital micas could significantly shift the position of the total-rock points, as might be the case for zone A sediments. From the whole-rock results of zone B and C sediments and the mineralogical and petrological changes during diagenesis and anchimetamorphism we conclude that many of the isochrons obtained from stratigraphically uncontrollable Precambrian sediments should be reinterpreted in favour of metamorphic and not sedimentary ages, especially if the metamorphism reached greenschist facies conditions. Devonian and Cambrian limestones intercalated between the pelites, silstones and sandstones contain Sr of a variable and slightly radiogenic isotopic composition in contrast to the expected ancient sea water Sr. This fact is interpreted again in favour of open whole-rock systems during the postdepositional history of the rock, rather than primary scatter of ancient, radiogenic sea water Sr.     

Isotopic U-Pb ages of monazites and zircons from the crust-mantle transition and adjacent units of the ivrea and ceneri zones (Southern Alps,Italy)

The Ivrea zone forms a part of the Southern Alps and is composed of basic rocks interfingered with granulite facies acidic rocks. According to geophysical evidence, this zone represents the transition between crust and uplifted and overthrusted mantle. Towards the Ceneri zone the metamorphic grade changes to amphibolite facies. Paragneisses, migmatites and anatectic gneisses dominate, within which postmetamorphic granites occur. Concordant monazite U-Pb ages of 275+2 m.y. were obtained from paragneisses of the Ivrea zone. The apparent zircon ages are discordant indicating a minimum age of 1900 m.y. for the oldest population and an apparent lead loss of 99 to 85 % about 285–300 m.y. ago. The zircons show features such as rounded habitus, low trace element contents and well ordered crystal lattices characteristic for detrital, recrystallised populations. Monazite from the neighbouring Ceneri zone migmatite yielded concordant U-Pb ages at 295±5 m.y. The discordant zircon age pattern indicates a time of formation of 450 m.y., similar to other newly formed zircons in anatectic rocks of the Ceneri zone, and an episodic or continuous lead loss at, or until 300 m.y. ago. The majority of the zircons are euhedral and have elevated trace element contents, features typical for zircons formed in the present-day host rocks. Concordant, 295±5 m.y. old monazite dates the formation of the postmetamorphic Mont' Orfano granite. Again zircon fractions yielded discordant ages, pointing in contrast to the above discordancies to a recent or continuous lead loss. The concordant ages of the monazites demonstrate the usefulness of this mineral for dating purposes in metamorphic and granitic rocks and contrast with the discordant age patterns of all zircon suites. From the general agreement between the monazite ages and the time of lead loss inferred from the zircon age patterns as well as from the geological relationships of the rocks and their metamorphic grade it is concluded that 295±5 m.y. is the minimum age for the regional granulite to upper amphibolite facies metamorphism of the Ivrea zone and that the uplift and overthrust of the upper mantle started prior to 295 m.y. ago, and that the basic rocks of the Ivrea zone are synmetamorphic intrusions. The decrease from 310–320 m.y. to 170–200 m.y. of the K-Ar and Rb-Sr mineral ages from the Ceneri towards the Ivrea zone is accompanied by decreases from 450 m.y. to 295 m.y. and on to 275 m.y. in the U-Pb ages of monazites. The zircon age pattern also shows a decrease from 450 m.y. to approximately 300 m.y. The main lowering of the ages occurs approximately at the petrographic boundary between the two zones and is related to the Hercynian uplift and overthrust of the mantle which may have started as early as 450 m.y. ago. The Insubric line which terminates the Ivrea zone towards the North must therefore be of pre-Alpine age, or a precursor of the Insubric line must have existed at the time of the mantle uplift.     

Origin of garnet in the basic granulites around Saltora,W. Bengal,India

Garnetiferous basic granulites occur, as parts of hornblende-pyroxene- and pyroxene granulites, in a Precambrian terrain around Saltora. The chemistry of the garnetiferous basic granulites is broadly similar to that of the hornblende-pyroxene granulites, their immediate precursors, but in detail they have distinctly higher Fe/Mg ratios. The compositions of the major mafic silicates of the garnetiferous varieties do not reflect higher pressures of formation: the Jd/Ts ratios in calcic pyroxenes are similar to those from the non-garnetiferous varieties, and the pyrope contents of garnets are low. Exchange equilibrium in respect of major elements was established among the mafic silicates in spite of garnets being late overprints. The orthopyroxene — calcic pyroxene pairs from the garnetiferous granulites show lower values of K _D(Mg-Fe) ^opx-cpx than those from the non-garnetiferous granulites, pointing to lower temperature of equilibration. The K _D(Mg-Fe) ^opx-hbl K _D(Mg-Fe) ^cpx-hbl relations show that the more magnesian triads equilibrated at lower temperatures; viewed against experimental data regarding the effect of Mg/Fe ratios on the appearance of garnets in basic rocks, formation of garnets by cooling is strongly indicated. Several intergrowth textures, especially garnet-ilmenite and garnet-quartz (±albite) symplectites, and modal relations argue in favour of composite reactions of the type hornblende+ quartz-→calcic pyroxene+garnet+albite+H₂O, which couple hornblende breakdown reactions with orthopyroxene+anorthite→garnet reactions. The approximate range of pressure and temperature conditions, estimated from experimental data, are 6–8.5 kb and 750–830° C. Since garnets formed by cooling in iron-rich granulites, the garnetiferous granulites do not represent higher pressure subfacies of the granulite facies.     

Synthesis and thermal stability of 2 \frac{1}{2}-octahedral sodium mica,NaMg2.5 (OH)2∫i4O10]

A new 2 \(\frac{1}{2}\) -octahedral sheet silicate, NaMg_2.5 Si₄O₁₀ (OH)₂, has been synthesized from oxide mixtures in the temperature range 500–600°C at pressures between 1 and 5 kb. The lattice parameters are a ₀ = 5.298 Å, b ₀ = 9.047 Å, c ₀ = 9.479 Å and ß=99.55°. X-ray data are given in the text. At temperatures above 605° C/1 kb and 630° C/5 kb, it decomposes to magnesiorichterite plus quartz.     

Late Pleistocene glaciation of Páramo de La Culata,north-central Venezuelan Andes

Late Pleistocene glacial features in the Páramo de La Culata region, north-central Venezuelan Andes, include: 1. depositional features: morainic till and fluvio-glacial deposits (terrace gravels); 2. sculptured features: glaciated valleys, cirques, horns, and arêtes; and 3. erosional features: striation and grooving, polished rock, roches moutonnées and whaleback forms, and erratic boulders. Two main levels of moraines were found, an older one at 2600 m elevation, and a younger one between 3000 and 3500 m. The difference in age is reflected by the higher degree of weathering, erosion, and vegetation cover of the lower level, as compared with the higher level. Radiocarbon dating, and a comparison and correlation of these glacial features with those of adjacent regions, indicates that the lower morainic level (2600 m) is probably the result of the main glacial advance of the Late Wisconsin Glaciation. The main morainic level (3000 to 3500 m) was probably formed by the latest Wisconsin glacial advance. The Late Pleistocene snow-line depression reached approximately 1200 m below the present snow-line (i. e., down to approximately 3500 m).