Martin Gillespie

Two Mesoarchaean terranes in the Reguibat shield of NW Mauritania

Abstract Two domains have previously been recognized in the Archaean Reguibat shield of NW Mauritania, based primarily on their gross lithological differences. New fieldwork has identified a major ductile shear zone (Tâçarât–Inemmaûdene Shear Zone) separating these domains and new geochronological studies show that the two domains record different Mesoarchaean histories. As such, the two domains are redefined as the Choum–Rag el Abiod Terrane and Tasiast–Tijirit Terrane. Previous isotopic studies of metamorphic lithologies of the eastern Choum–Rag el Abiod Terrane indicate a succession of crustal growth from about 3.5–3.45 Ga to between about 3.2 and 2.99 Ga. Isotopic data presented in this contribution from the Tasiast–Tijirit Terrane indicate that emplacement of major calc-alkaline plutons occurred at c. 2.93 Ga after volcanism (preserved as greenstone belts) that included late felsic eruptive centres dated at c. 2965 Ma. This Mesoarchaean intrusive and extrusive magmatism was confined to the Tasiast–Tijirit Terrane, where it was emplaced through migmatitic orthogneisses that are the oldest lithodemic unit of the Tasiast–Tijirit Terrane. Widespread bimodal, post-tectonic magmatism in both terranes included major granitic magmatism dated at c. 2730 Ma. The north–south- to NNE–SSW-trending curvilinear Tâçarât–Inemmaûdene Shear Zone that separates the two terranes records late intense transpressive ductile shearing. It has a flower structure over a horizontal distance of about 70 km across its southern portion with unquantifiable sinistral horizontal offset, and east-directed thrusting on its eastern side where it cuts into the Choum–Rag el Abiod Terrane. A new U–Pb zircon age of 2954±111 Ma is presented for a deformed granite confined within the central part of this shear zone. A minimum age for the shearing is provided by a previously determined c. 2.73 Ga age for a post-tectonic granite that cuts across the easternmost part of the shear zone in the Choum–Rag el Abiod Terrane.

Structural setting and petrogenesis of the Ben Vuirich Granite Pluton of the Grampian Highlands: a pre-orogenic, rift-related intrusion

Synopsis The Ben Vuirich intrusion is a small, elongate body of monzogranite that occurs in the Tummel Steep Belt, Perthshire. It was emplaced into Dalradian rocks (Appin Group) at 590 Ma, prior to the D1 phase of the Grampian Event (Caledonian Orogeny), and was strongly deformed during D2. Locally preserved cordierite- and andalusite-bearing hornfelses were altered to garnet ± kyanite-bearing assemblages during post-D2 regional metamorphism. A new study of the lower-grade hornfelses shows that the protolith was an undeformed, fine-grained, parallel-laminated sedimentary unit, confirming that the pluton is pre-orogenic with respect to the Grampian Event. Whole-rock and trace element analyses of 33 samples of the intrusion, together with rare earth elements, Rb–Sr and O-isotope data, show that it is an A2-group monzogranite. This finding supports the hypothesis that the granite, emplaced at a depth of 7–14 km, formed in the same extensional tectonic setting as the Tayvallich lavas at 600 Ma. Geochemical and isotope parameters point to a largely crustal source. The intrusion belongs to a swarm of rift-related, A-type granitoids that originally stretched from the Appalachians to Scotland, and includes foliated granitoids in the Moine. The granitoids formed in response to the early break-up of Rodinia, c. 50 Ma before the development of the Iapetus Ocean.

Mineralogical and petrological evidence for the hydrogeological characteristics of the Tsukiyoshi Fault, Japan

Generic approaches for determining the spatial and temporal variability of a faults hydrogeological properties are being developed by studying the reversed Tsukiyoshi Fault at Tono, central Japan. This fault is associated with zones of deformation/damage that are sometimes wider than the faults displacement. The hydraulic characteristics probably vary laterally over metres to tens of metres and the hanging wall and foot wall have different hydraulic properties. It is also possible that some of these properties may have changed over time, due to physical processes and/or to water/rock interactions, even at relatively low-temperatures and pressures (to a few tens of degrees centigrade and hundreds of bars). It is suggested that mineralogical and structural data can be used to estimate the most transmissive conditions within a fault in the past. These estimates can then be used to select conservative (‘worst-case’) fault parameters for assessing the future safety of underground waste repositories. Such information can also be used to design a repository to avoid faults with permeable crush zones. The study demonstrates the value of integrating detailed mineralogical and petrological studies into a borehole drilling programme for characterizing the hydraulic properties of faults.

Scottish Landform Examples: The Cairngorms – A Pre-glacial Upland Granite Landscape

The Cairngorm massif in NE Scotland (Figure 1) is an excellent example of a preglacial upland landscape formed in granite. Glacial erosion in the mountains has been largely confined to valleys and corries (Rea, 1998) and so has acted to dissect a pre-existing upland (Figure 2). Intervening areas of the massif experienced negligible glacial erosion due to protective covers of cold-based ice (Sugden, 1968) and preserve a wide range of pre-glacial and non-glacial landforms and regolith. This assemblage is typical for many formerly glaciated upland and mountain areas around the world. The cliffs that sharply demarcate the edges of glacial valleys and corries allow the main pre-glacial landforms to be easily identified. The former shape of pre-glacial valleys and valley heads can then be reconstructed by extrapolation of contours to provide a model of the pre-glacial relief of the Cairngorms (Thomas et al., 2004). This relief model (Figure 3) provides a basis for understanding the development of the landscape over timescales of many millions of years, including the role of geology, weathering, fluvial erosion and, lately, glacial erosion in shaping the relief.

Chemical analysis of palaeogroundwaters: a new frontier for fluid inclusion research

Using a combination of fluid inclusion and mineralogical techniques, chemical data have been obtained that help characterise the hydrogeochemistry of palaeogroundwaters in the onshore section of the Eastern Irish Sea Basin, northwest England. The study area, Sellafield, provided an excellent suite of fracture-controlled carbonate cements deposited from shallow to deeply circulating late Triassic to Recent groundwaters. Methodologies developed for the analysis of hydrothermal fluid inclusions were extended to encompass a wide range of low temperature aqueous inclusions. These included single inclusion chemical analysis by UV laser ablation ICP mass spectrometry (Na, K, Mg, Sr, Li, Mn, Fe) and high precision salinity measurements (±1000 ppm TDS) by microthermometric analysis. Closely integrated with these measurements were calcite morphology and cathodoluminescence (CL) studies that were undertaken to provide a relative chronology ‘stratigraphy’ of carbonate cementation and information on redox conditions and salinity with depth. The results demonstrate a clear distinction between the chemistry of the late Triassic groundwaters and the present day, deep, saline groundwaters. Though both have TDS values >100,000 ppm, the former are Ca–Na–Cl brines with Na/Ca wt ratios of 2:1, whilst the latter are Na–Cl brines with Na/Ca wt. ratios greater than 20. For younger generations of calcite, attributable to deposition from groundwater during the Quaternary, TDS values are <100,000 ppm with salinities typically less than 20,000 ppm. Laser ablation analyses of aqueous inclusions in these younger calcites for Na, Sr and Mg plot exactly within the fields defined by the present day saline to brackish groundwaters, and display similar trends. CL patterns for the Quaternary calcites are primarily a function of trace element impurities (Fe and Mn), but all show a marked contrast at the fresh water–saline water transition. This contrast is also reflected in the morphology of the calcite crystals; from c-axis shortened ‘nailhead’ forms in the fresh groundwater zone to c-axis elongated ‘scalenohedra’ forms in the deeper saline zones. Implications for the evolution of the palaeogroundwaters in response to Tertiary uplift with respect to present day groundwater regimes are briefly discussed.

How hot are the Cairngorms

Heat flow measured over the East Grampians batholith in the 1980s was found to be unexpectedly low and at odds with high radiogenic heat production within the outcropping granites and a very large volume of granite predicted from an interpretation of gravity data. Past climate variations perturb temperature gradients in the shallow subsurface leading to erroneous estimates of heat flow. A reconstruction of the surface temperature history during the last glacial cycle has enabled a rigorous palaeoclimate correction to be applied to the heat flow that shows an increase of 25% over previously reported values; revised to 86 ± 7 mW m−2. An interpretation of recent mapping reveals that the surface exposures of the East Grampians granites are the roof zones of a highly evolved magma system. Rock composition, therefore, is likely to become more mafic with depth and the heat production will decrease with depth. This petrological model can be reconciled with the gravity data if the shape of the batholith is tabular with deep-seated feeder conduits. The increased heat flow value leads to revised predictions of subsurface temperatures of 129°C at 5 km depth and 176°C at 7 km depth, increases of 40% and 49%, respectively, compared to previous estimates. These temperatures are at the lower end of those currently required for power generation with Engineered Geothermal Systems, but could potentially be exploited as a direct heat use resource in the Cairngorm region by targeting permeable fractures with deep boreholes.