No abstract

The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 f 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin.The Devonian rocks show intense hydrothermal alteration along the fault zone.The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and MO. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks.The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440°C) magmatic fluids occurred with SD -65960 and S'*O around +8.59~. 634S (pyrite) and initial *' Sr/%r ratios (vein calcite) lie mainly within the ranges +3.4%0 to +8.5% and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. 613C values for caliche and vein calcite imply derivation of carbon from non-organic sources.The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (S1'Ochen +13.1% to +16.5%) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/CI and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 f 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere.

In Central Spain, it is possible to distinguish two main types of peraluminous late-Hercynian granites: the PI types and the PS types. The distinction between both types is made on the basis of differences in peraluminosity (PS types are more peraluminous than PI types) and also on the appearance of some characteristic mafic minerals; PS types have biotite, cordierite and monazite as the most typical accessory phase, whereas PI types have biotite, amphibole (in the less evolved facies) and allanite as the accessory mineral. Both granite types have similar trace element ratios and initial Sr, Nd and Pb isotopic signatures, Initial 87Sr/ 86Sr ratios of both types exhibit a large range from 0,7073 to 0,7193, whereas initial BN d varies in a restricted range from -5,4 to -6,6, The scarce associated basic rocks do not play a significant role in the chemical variability of these peraluminous granites which follow low pressure crystal fractionation trends from granodior ite/monzogranite parental magmas, Mixing and AFC modelling of Sr and Nd isotopic data reveal an unrealistically high mantle contribution, Based on major and trace element and isotopic data, an orthogneissic protolith for both granitic series is proposed, Nevertheless, none of the metamorphic country rocks of Central Spain has the appropriate Sr isotopic composition to satisfy the origin of these granitic rocks, and so, it is argued that progressive isotopic re-equilibration of crustal material during the granulization of the lower Hercynian crust, together with the possibility of isotopic disequilibrium during melting (as has been demonstrated in migmatitic terranes in nearby areas) may explain the isotopic differences between the granites and the high level metamorphic country rocks,

Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D 2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D 2 have been dated using SHRIMP U-Pb geochronology. Zircon ages of 424 AE 8 Ma (Vagastie Bridge granite), 420 AE 6 Ma (Klibreck granite) and 429 AE 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D 2 deformation, during mid-to late Silurian time. Titanite ages of 413 AE 3 Ma (Vagastie Bridge granite) and 416 AE 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550-500 8C. A mid-to late Silurian age for D 2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of forelandpropagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc-continent orogeny. The mid-to late Silurian D 2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.

No abstract