The movement of magma through the shallow crust and the impact of subsurface sill complexes on the hydrocarbon systems of prospective sedimentary basins has long been an area of interest and debate. Based on 3D seismic reflection and well data, we present a regional analysis of the emplacement and magmatic plumbing system of the Palaeogene Faroe-Shetland Sill Complex (FSSC), which is intruded into the Mesozoic and Cenozoic sequences of the Faroe-Shetland Basin (FSB). Identification of magma flow directions through detailed seismic interpretation of approximately 100 sills indicates that the main magma input zones into the FSB were controlled primarily by the NE-SW basin structure that compartmentalise the FSB into its constituent sub-basins. An analysis of well data shows that potentially up to 88% of sills in the FSSC are <40 m in thickness, and thus below the vertical resolution limit of seismic data at depths at which most sills occur. This resolution limitation suggests that caution needs to be exercised when interpreting magmatic systems from seismic data alone, as a large amount of intrusive material could potentially be missed. The interaction of the FSSC with the petroleum systems of the FSB is not well understood. Given the close association between the FSSC and potential petroleum migration routes into some of the oil/gas fields (e.g. Tormore), the role the intrusions may have played in compartmentalisation of basin fill needs to be taken fully into account to further unlock the future petroleum potential of the FSB.

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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.

Attempts to assess the significance of subducted sediments in the genesis of island arc magmas have been limited by the lack of trace element data on pelagic oozes. Accordingly, we have analysed a series of pelagic clays and nannofossil oozes from the Nazca Plate for REE and other trace elements. A calculated average––Pacific Authigenic Weighted Mean Sediment (PAWMS)––exhibits light REE-enrichment (La n /Yb n ∼4.5), high contents of Ba and Sr, but low abundances of Rb, Nb and Ta. Most significant, however, is the occurrence of large, negative Ce anomalies (Ce/Ce*~0.2). We have attempted to model the contribution of PAWMS-type material to the source of the magmas of the Mariana Island Arc, an intra-oceanic arc far removed from the effect of continent-derived detritus. Only small amounts of pelagic sediment, between 0.3 and 0.5% of the source, are required to develop the small negative Ce anomalies, high Ba/La ratios, and high LIL/HFS element ratios characteristic of these and other arc lavas. However, a small fluid contribution from the dehydrating subducted oceanic crust is required to produce the high Rb/Ba observed in several Mariana suites. The ternary mixing between sediment, mantle host and dehydrating oceanic crust also produces very low Nb and Ta abundances in the arc lava source. However, the very high abundance of Sr (>1000ppm) and the estimated high 87 Sr/ 86 Sr ratio (~0.7150) of PAWMS, results in a model 87 Sr/ 86 Sr ratio of 0.7070. This is far higher than the measured ratios in the Mariana arc lavas (0.7033–0.7040) and may suggest that the subducted sediment has a lower Sr content (<200 ppm) or a lower 87 Sr/ 86 Sr ratio, or that the carbonate-rich component is not involved in source contamination. Volumetrically it would appear that much of the sediment approaching the Mariana trench (~45 km 3 Ma −1 per km of arc) may be recycled into the deeper mantle. This will have the effect of introducing high Ba, Sr, Th, 87 Sr/ 86 Sr, 208 Pb/ 204 Pb and 207 Pb/ 204 Pb material into the mantle. However, incorporation of such material cannot alone satisfactorily account for the trace element and isotope chemistry of ocean islands; oceanic sediments have LIL/Ta, Nb ratios far too high to produce the trace element characteristics of most intraplate magmas.