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The type locality for high-temperature, low-pressure regional metamorphism, the Buchan Block in NE Scotland, exhibits profound differences to the rest of the Grampian Terrane. These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis. Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere, the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated. We report SIMS U–Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast (Inzie Head Gneiss) and northwest (Portsoy) corners of the Buchan Block. Around Inzie Head, upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks, at least locally. U–Pb geochronology indicates a crystallisation age for the diorite of 486 ± 9 Ma. Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493 ± 8 Ma. Ages of ca. 490 Ma for magmatism and high-grade metamorphism, which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis, are significantly older than the established peak of Grampian metamorphism (ca. 470 Ma). We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis. Rollback of a NW-dipping subduction zone at ca. 490 Ma produced a back-arc environment (the Buchan Block) with associated arc magmatism and high d T /d P metamorphism. Arrival of an outboard arc resulted in shortening (the initial phase of the Grampian Orogeny) at ca. 488 Ma. Rollback of a NW-dipping subduction zone to the SE of the ca. 488 Ma suture began at 473 Ma and led to lithospheric-scale extension, decompression melting and advective heating of the middle crust, producing the widespread ca. 470 Ma Grampian (classic Barrovian and Buchan) regional metamorphism. Resumed hinge advance and the final phase of shortening cut off the heat supply at ca. 465 Ma, marking the end of the Grampian Orogeny. [ABSTRACT FROM AUTHOR]

Mobility of Au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic Au deposits. This study investigates the mobility of Au, As, and Sb during metamorphism of the Dalradian metasedimentary rocks of Scotland. The metamorphic processes in the Dalradian of Scotland are extremely well studied, and the terrane is an ideal area to investigate mobility of these metals. Our results show that of the 25 major and trace elements analysed, only Au, As, Sb, S and volatile contents as shown by loss on ignition (LOI) values show systematic variation with the metamorphic grade of the samples. Average Au concentrations decrease from 1.1 ± 0.55 ppb and 0.72 ± 0.34 ppb in chlorite and biotite zone rocks down to 0.4 ± 0.22 ppb and 0.34 ± 0.13 ppb in kyanite and sillimanite zone rocks. Average As concentrations decrease from 4.8 ppm (range 0.5 to 17.8 ppm) and 1.96 ± 1.9 ppm in chlorite and biotite zone rocks down to 0.24 ± 0.15 ppm and 0.2 ± 0.12 ppm in kyanite and sillimanite zone rocks. Average Sb concentrations decrease from 0.18 ± 0.15 ppm and 0.11 ± 0.10 ppm in chlorite and biotite zone rocks down to 0.04 ± 0.02 ppm in both kyanite and sillimanite zone rocks. Sulphur and LOI concentrations also show significant decreases. Mass balance calculations indicate that compared to chlorite and biotite zone samples, sillimanite zone samples have an average mass loss of 62 ± 14%, 94 ± 4% and 74 ± 14% for Au, As, and Sb respectively. Every 1 km 3 of chlorite–biotite zone mixed psammitic–pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1.5 t Au, 8613 t As, 270 t Sb, and 1.02 Mt S. The mobility of these elements is strongly controlled by the paragenesis of sulphide minerals. Pyrite, sphalerite, galena and cobaltite (as well as gersdorffite) decrease in abundance with increasing metamorphic grade in the Dalradian metasedimentary rocks. A critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite. This transition is complete by mid greenschist facies in the Loch Lomond samples but is more gradual at Glen Esk occurring between biotite and sillimanite zones. The Au, As, and Sb content of the sulphide assemblage also decreases with increasing metamorphic grade, and we suggest that this is a controlling factor on the mobility of these metals from the Dalradian metasedimentary rocks during metamorphism. Chlorite may be an important host mineral for As in the greenschist facies rocks. Breakdown of chlorite indirectly drives the mobility of Au, As, and Sb, as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite. We suggest that there is potential for significant undiscovered mineralisation in the Central and SW Highlands of Scotland. However, as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the Otago and Alpine Schist's, New Zealand, very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the Dalradian of Scotland. [ABSTRACT FROM AUTHOR]

Abstract: New Re–Os geochronology for the Ballachulish Slate Formation of the Dalradian Supergroup, Scotland yields a depositional age of 659.6±9.6Ma. This age represents the first successful application of the Re–Os system to rocks that have extremely low Re and Os abundances (<1ppb and<50ppt, respectively). The Re–Os age represents a maximum age for the glaciogenic Port Askaig Formation and refutes previous chemostratigraphic and lithostratigraphic studies which correlated the Port Askaig Formation with a series of middle Cryogenian (ca. 715Ma) glacials. Additionally, the Re–Os age strongly suggests that the Port Askaig Formation may be correlative with the ∼650Ma end-Sturtian glaciations of Australia. As a consequence, the correlation of the Ballachulish Limestone Formation with the ca. 800Ma Bitter Springs anomaly is not tenable. Initial Os isotope data from the Ballachulish Slate Formation coupled with data from Australia reveals a radiogenic 187Os/188Os isotope composition (∼0.8–1.0) for seawater during the Neoproterozoic, which is similar to that of modern seawater (1.06). We also report a young, highly imprecise Re–Os age (310±110Ma) for the Early Cambrian Leny Limestone Formation which is constrained biostratigraphically by a polymerid and miomerid trilobite fauna. We suggest, based on the mineralogy of the Leny Limestone, (kaolinite, muscovite and a serpentine group mineral, berthierine), that the Re–Os systematics have been disturbed by post-depositional fluid flow associated with Palaeozoic igneous intrusions. However, it is evident from the Ballachulish Slate Formation results that anhydrous metamorphism does not disturb the Re–Os geochronometer. [Copyright &y& Elsevier]

Abstract: The Neoproterozoic–Cambrian Dalradian Supergroup of Scotland contains three glaciogenic sedimentary units, the Port Askaig Formation, the Stralinchy-Reelan Formation, and the Loch na Cille Boulder Bed, in ascending order. Although the idea, that relatively cold climates occurred during the latest Proterozoic, is widely accepted, the cause of the onset of glaciation is still controversial; we use new carbonate 87Sr/86Sr data to better constrain this environmental change. Some Dalradian limestones have high strontium contents (>1000ppm), low manganese contents and Mn/Sr ratios (<0.2) and, according to their silica, aluminum and potassium contents and Rb/Sr ratios, they suffered insignificant contamination from siliciclastic minerals, thus they are suitable for estimating the 87Sr/86Sr values of paleoseawater in the Neoproterozoic. The 87Sr/86Sr values of limestones of the Islay Formation (ca. 0.7067), sampled on the island of Islay, are similar to the values of previous studies. However, the 87Sr/86Sr values of limestones just below the Port Askaig Formation (ca. 0.7064), collected on the island of Garbh Eileach, have the lowest values recorded in the Dalradian Supergroup. Comparing these 87Sr/86Sr values with coeval 87Sr/86Sr values of other regions in the world, such low 87Sr/86Sr (<0.7065) has only been reported from successions pre-dating the older Cryogenian (Sturtian) glaciation, which supports the idea that the Port Askaig Formation was deposited during the older Cryogenian glaciation. We have reconstructed the variations in the i87Sr/86Sr ratio from the Tonian through the Cryogenian and Ediacaran to the earliest Cambrian. The results show that the 87Sr/86Sr ratio of seawater decreased just before the older Cryogenian glaciation, that it increased by two steps in the early and middle Ediacaran, and that there were at least two abrupt positive excursions in the middle Ediacaran and at the Precambrian–Cambrian boundary. The decline of the 87Sr/86Sr ratio of seawater just before the older Cryogenian Sturtian glaciation suggests that enhancement of weathering, related to break-up of the Rodinia supercontinent and continental flood basalt volcanism, was a likely driver for generating global cooling for the Snowball Earth conditions (). [Copyright &y& Elsevier]

Abstract: The snowball Earth hypothesis suggests that the Neoproterozoic was characterized by several prolonged and severe global glaciations followed by very rapid climate change to ‘hot house’ conditions. The Neoproterozoic Port Askaig Formation of Scotland consists of a thick succession of diamictite, sandstone, conglomerate and mudstone. Sedimentological and stratigraphic analysis of Port Askaig deposits exposed on the Garvellach Islands was carried out to establish the nature of Neoproterozoic palaeoenvironmental change preserved in this thick succession. Particular emphasis was placed on identifying and distinguishing between climatic and tectonic controls on sedimentation. Port Askaig Formation diamictite units are attributed to deposition by sediment gravity flow processes or ‘rainout’ of fine-grained sediment and ice-rafted debris in a glacially influenced marine setting. Associated facies record various depositional processes ranging from sediment gravity flows (conglomerate, massive sandstone and laminated mudstone) to deposition under other unidirectional currents (cross-bedded and horizontally laminated sandstone). The Port Askaig Formation is also characterized by abundant soft sediment deformation features that occur at discrete intervals and are interpreted to record episodic seismic activity. Stratigraphic analysis of the Port Askaig Formation on the Garvellach Islands reveals three phases of deposition. Phase I was dominated by sediment gravity flow processes and sedimentation was primarily tectonically controlled. Phase II was a transitional phase characterized by continued tectonic-instability, an increased supply of sand to the basin and the preservation of current-generated facies. In the third and final phase of deposition, the interbedded units of sandstone and diamictite are interpreted to reflect development of large sandy bedforms and ice margin fluctuations in a tectonically stable marine setting. Sedimentological and stratigraphic analysis of the Port Askaig Formation demonstrates that tectonic activity had a significant influence on development of the lowermost parts of the succession. Climatic influences on sedimentation are difficult to identify during such phases of tectonic activity but are more easily discerned during episodes of tectonic quiescence (e.g.,, Phase III of the Port Askaig Formation). The thick succession of diamictite interbedded with current-deposited sandstone preserved within the Port Askaig Formation is not consistent with deep freeze conditions proposed by the snowball Earth hypothesis. [Copyright &y& Elsevier]

Abstract: Provenance studies involving U/Pb analysis of detrital zircons have become increasingly popular through the application of microanalytical techniques. Adopted one-dimensional methods for the presentation of detrital zircon data, such as probability density distribution plots, generally require various degrees of filtering of discordant analyses. However, a uniform approach for filtering does not exist, making comparison of data from different samples and sedimentary units, as well as different studies of detrital populations, extremely difficult. The problem is further complicated by the need to switch from 207Pb/ 206Pb based ages to 206Pb/ 238U ages when zircon populations within a sample show a range of ages from Archean to Proterozoic or even Phanerozoic. Ion probe analysis of multiple spots within a single zircon crystal and calculation of an internal isochron can potentially eliminate problems associated with discordance, but this is a time consuming approach. Comparison of internal isochron ages for zircons generated by multiple ion probe intra-grain zircon analyses from a sample of psammite within the Dalradian Supergroup of Scotland with a data set from the same sample based on single grain ion probe analysis using various filtering methods suggests good agreement between the two approaches particularly when filtering is based on the probability of concordance. However, when precise information is required, such as the age of the youngest zircon grains so as to accurately constrain the maximum depositional age of the strata, then multiple analyses of individual zircon grains may be required. [Copyright &y& Elsevier]

The Connemara region of the Irish Caledonides is a classic example of regional-scale metamorphism of low pressure and high temperature. This terrane is considered as part of a fold belt comprising metasedimentary and metavolcanic rocks that are correlated with the Neoproterozoic–Lower Paleozoic Dalradian Supergroup of Scotland. In mid-Ordovician time, the extensive and high-temperature metamorphism was superimposed on the Dalradian rocks resulting in the Connemara zoning. The key feature of the zoning is elevated horizontal thermal gradient of ca. 14 °C/km. Geological data and geochronological evidence point to a causative link between metamorphism and associated magmatic intrusions, and a brief period of development for the metamorphic zoning. Magmatic intrusion into the middle part of continental crust is treated as a most plausible source of heat for metamorphism, and other conjectures as to the origin of the zoning (flow of hot fluid through the permeable rocks, fracture conduit flushed by flowing magma) are believed to be improbable. To examine in sufficient detail the problem of the nature of heat source, a series of appropriate calculations have been performed to reach the best agreement between the observed and simulated spatial distribution of maximum temperatures at different times. The mathematical modelling shows that the temperature–spatial structure of the Connemara zoning is best explained by the model version based on mid-crustal heating above the upper contact of magmatic intrusive body gently curved and tilted at an angle between 20° and 40°, with an initial temperature of the magma appropriate to a basaltic melt. The model estimate of total lifetime of the temperature anomaly in the crust is of the order of 5–6 Ma. In general, this is in rather good agreement with the currently available evidence of geochronological duration of metamorphism and magmatism in Connemara. [Copyright &y& Elsevier]

The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions. [Copyright &y& Elsevier]