[en] Microsyenitic and microdioritic dykes from Alconera on the northern side of Burguillos del Cerro Anticlinarium give a K-Ar age of 281+-7 M.A. This age is close to that reported for the nearly Feria syenite which has similar petrographical and geochemical characteristics and whose intrusions fall on a common axis NW-SE. This confirms that these alkaline subvolcanic rocks were emplaced during a period from Stephanian to lower Permian, subsequent to the last phase of varisc deformation

No abstract available

No abstract available

[en] Radiometric data for the Westcarpathian neovolcanites did not include basalts. Using the argon method a basanitoid group of ages of about 7,5 mil. years near B. Stiavnica and N from Lucenec (Maskova, Podrecany), and about 2,5 mil. years old basanites in the surroundings of Filakovo were identified. (author)

[en] Full text: Crustal and upper mantle xenoliths are hosted in basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Qebrada Las Conchas, Province Salta, Argentina. Most of the crustal xenoliths have granitoid composition ( quartz - plagioclase - k-feldspar -garnet) whereas mafic compositions (plagioclase - clinopyroxene -garnet ±hornblende) are exceedingly rare. The xenoliths show a well equilibrated granoblastic structure and the minerals are compositionally unzoned. Metamorphic conditions have been of granulite facies temperature ca. 850 - 900 at lower crustal depth with pressure of ca. 10 kbar from thermo barometric calculations. Sm-Nd mineral isochron ages are 94.9±8.4Ma, 91.6±13.5Ma, 89.0±4.2Ma (granitoid composition), and 110.7±23.2Ma (mafic composition). These ages are within the errors in good agreement with the age of basanitic volcanism. Sm-Nd isochron ages are considered as closure ages and temperature at the respective time has been still above the Sm-Nd systems' closure temperature (>600-700 ). Sm-Nd and Rb-Sr isotopic signatures (¹⁴⁷Sm/¹⁴⁴Nd t0=0.1225 - 0.1608); ¹⁴³Nd/¹⁴⁴Ndt0 0.512000 - 0.512324; ⁸⁷Rb/⁸⁶Sr = 0.099 - 0.172; 87Sr/86Sr = 0.708188 - 0.7143161) and common lead isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 18.43 - 18.48; ²⁰⁷Pb/²⁰⁴Pb = 15.62-15.70; ²⁰⁸Pb/²⁰⁴Pb = 38.22 - 38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina apart from the lower ²⁰⁸Pb/²⁰⁴Pb ratios of the Early Paleozoic basement. Sm-Nd depleted mantle model ages of c. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a Proterozoic protoliths for both with distinct ages of the last thermal overprint. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature gradient in a crust of nearly normal thickness at c. 90 Ma and to a prominent thermal anomaly in the lithosphere. If this anomaly has been caused by uplift of the lithosphere/asthenosphere boundary by extensional tectonics the extension must have been restricted to the mantle lithosphere. Interpretation of the tectonic setting remains ambiguous and both a 'true rift' and a rifting in a possible Cretaceous back arc have to be considered in future investigations. The crustal composition is identical with the compositionally homogeneous Early Paleozoic crust of NW Argentina and N Chile. A thick mafic lower crust seems unlikely considering low frequency of mafic xenoliths and composition of the granitoid xenoliths. (author)

[en] A particular K-Ar technique, the Cassignol technique, has been developed in order to date Upper Pleistocene and Holocene volcanic rocks. We describe here its principles and its technology. The limit of detectability of the radiogenic Ar portion corresponds to an error of less than 10³ a for K-rich minerals and a few 10³ a for basalts. The reliability of the results and the validity of the correction for atmospheric contamination have been checked by analysing historical lavas and by comparison with data obtained from radiocarbon and thermoluminescence dating methods. The results demonstrate that the technique is capable of achieving K-Ar dates as young as 2000 a with a few centuries accuracy. A precision of ± 1.5% is obtained for samples older than 10⁵ a. Moreover, in rocks older than 10⁷a, the technique permits the accurate dating of minute amounts of pure separated mineral phases. A reconstruction of the recent volcano-tectonic evolution of the Naples area has been carried out. It allows us to establish a model for estimating volcanic hazards in the Phlegrean Fields. The dated terrestrial pyroclastic deposits have been recovered from cores collected in the central Tyrrhenian Sea. The land-sea correlations allow us to refine the chronology of the marine oxygen isotope records for the past 60,000 a. 59 refs.; 7 figs.; 5 tabs

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[en] Geochemical and geochronological data from Alumine riolitic welded tuffs are analysed. Minor elements show enrichment in Rb, Th and K and depletion in Nb, Ti, P and Sr. La/Yb ratios are low. The geochemical features are consistent with a volcanic arc genesis. The radiometric data obtained by K/Ar method point out a Paleocene age for these rocks, allowing to correlate them with the Ventana Formation or the equivalent Auca Pan Formation. (author)

[en] The radiometric age data obtained by different dating methods have been interpreted in terms of possible orogenic activities prevailing in the Himalaya. In general, the age data confirm four main events, the Precambrian, the Late Precambrian-Cambrian Assyntian (Caledonian), the Late Palaeozoic-Hercynian and the Late Cretaceous-Tertiary Himalayan orogeny. The mineral dates are particularly significant in delineating different phases of the last i.e. the Himalayan orogeny which indicates main activity of the young Himalayan metamorphism around 70 to 50 Ma and followed by a momentous phase of major uplift during 25 to 10 Ma, which was responsible for the rise of the deeper part of the Himalaya into great folds and thrust slices and the formation of nappe structures. (author)

No abstract available