40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.
Integrated apatite fission track analysis and vitrinite reflectance data show that well 204/19-1 in the West of Shetland region, UK Atlantic margin, has experienced only limited additional burial beyond present-day depths. Uplift and cooling to present-day levels probably occurred during late Cenozoic (Eocene to Miocene) basin inversion. Fluid inclusion data indicate that Paleocene–Eocene sandstones have experienced temperatures much higher than can be explained by burial alone. Temperatures up to 200±°C indicate the passage of hot fluid through Cenozoic sandstones, which by-passed the pre-Cenozoic section in this and other wells. The hot fluid event must have been of very brief duration (up to 100 years) to show no record in the fission track and reflectance data, implying that the fluids migrated through fracture systems. Oil inclusions in the Cretaceous of well 204/19-1 have a chemistry that suggests derivation from a Kimmeridgian-aged source rock. They occur in cements that show no evidence for the hot fluid event and it is concluded that the cements pre-date the event. Oil inclusions in Cenozoic sandstones have a heavy, degraded character and were trapped at high temperature, suggesting that degradation was related to the hot fluid event. Present-day oils in the West of Shetland region are mixtures, which could reflect components from the two charges distinguished by the integrated thermal and geochemical histories. The inference of fracture-bound flow is consistent with existing models of overpressure development and hydrofracturing.
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Abstract Fluid inclusion and scanning electron microscope‐cathodoluminescence evidence indicates focused hot, saline, diagenetic fluid flow within the Eastern Flank of the Britannia Field, offshore Scotland, UK. The fluid was sourced from the Andrew Salt Dome, 10 km to the east. The fluids, which promoted quartz cementation of the upper zones within the field, were up to ∼30°C hotter and had salinities up to ∼10 wt% NaCl equivalent higher than fluids from lower in the reservoir section. During diagenesis hot saline fluids migrated westwards as part of a radiating ‘diagenetic front’ from the Andrew Salt Dome. Structural dip associated with the Eastern Flank of the Fladen Ground Spur impeded the westward movement of the diagenetic fluid. The quartz cements from the upper and lower reservoir zones can be distinguished by morphology. In the upper zones the quartz cements have well‐developed macro‐crystalline zoning and heterogeneous luminescence across the grain. In the lower zones, the cements are much less developed, unzoned and very weakly luminescent. The diagenetic fluids were primarily focused into Zone 45 within the upper reservoir. Furthermore, within the Main Platform Area the most prolific producing zone is Zone 45, indicating the importance of this interval as a permeable flow unit during both diagenetic and production timescales. Within the Eastern Flank, the quartz overgrowths have a major impact on reservoir permeability and thus well productivity. The overgrowths are most extensive in the originally clean sandstones with low clay content. Clay in optimum volumes (5–10%) can inhibit nucleation of the damaging quartz overgrowths without having a detrimental effect on pore connectivity. These observations provide a predictive concept for use in the search for relative reservoir sweetspots within the degraded Eastern Flank.
A scanning electron microscope based cathodoluminescence technique utilizing a novel collector system reveals complex internal heterogeneities within granitic quartz grains. The technique overcomes the low intensity and limited variation in cathodoluminescence generated by quartz, which hamper conventional cathodoluminescence analysis. Detailed images of zoning patterns in quartz are comparable to those observed in minerals such as feldspar, and attributed to a combination of progressive growth, boundary layer effects and mineral–melt disequilibria produced during fluctuations in melt composition and temperature during the crystallization interval. We attribute such mineral–melt disequilibria to open system, mixing behaviour in the granite plutons sampled.
The East Greenland Caledonides occupy a crucial position in plate-tectonic reconstructions of the Late Mesoproterozoic to Early Neoproterozoic Grenville–Sveconorwegian belt. We present new field and isotopic data from the northern Stauning Alper which indicate that the 1050–930 Ma history of the area was characterized by deposition of extensive clastic sequences. Sources of detritus were dominated by rocks of Mesoproterozoic age, with only limited contributions from Archaean sources, suggesting deposition at a distance from the present Caledonian foreland. A Neoproterozoic granite (938±13 Ma) provides evidence for thermal perturbation at a time of extensional collapse and uplift recorded in NW Scotland, the Grenville Belt of Canada, Labrador and the Sveconorwegian of SW Sweden and southern Norway. Widespread anatexis in the northern Stauning Alper at c. 430–425 Ma resulted from both collisional melting and decompression melting on uplift contemporaneous with the early part of orogenic collapse of the Caledonian Fold Belt. Caledonian deformation was focused at the zone of most extensive granite emplacement. Isotopic evidence suggests that Caledonian granites, previously thought to be entirely post-kinematic, actually predate late Caledonian extension.
Abstract Sedimentary basins developed along the European margin during the earliest, Permian, stage of proto-Atlantic rifting, during a phase of high heat flow. The proximity of some basins to Caledonian thrusts has implied that rifts locally utilized the basement fabric. New mineralogical and palaeomagnetic data show that thrust planes in the Moine Thrust Zone channelled a pulse of hot fluid in Permian time. The fluids precipitated kaolin in fractures in the thrust zone, and with decreasing intensity away from the zone. The high-temperature polytype dickite is largely confined to major thrust planes. Stable H and O isotope analyses indicate that the parent fluid included meteoric water involved in a hydrothermal system. Coeval hydrothermal hematite has a chemical remanence that dates the fluid pulse as Permian. This is direct evidence for post-orogenic activity in the thrust zone, in which the thrusts vented excess heat during regional crustal extension. The example from the European margin exemplifies the importance of deep-seated structures in the release of heat, and the value of kaolinite polytype mapping as a tool to record anomalous palaeo-heat flow.
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