T. J. Dempster

Timing of deposition, orogenesis and glaciation within the Dalradian rocks of Scotland: constraints from U–Pb zircon ages

Abstract: The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U–Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian ‘Newer Gabbros’ (470 ± 9 Ma) of Insch and Morven–Cabrach in Aberdeenshire. These age data, together with the existing 590 ± 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.

A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup

High precision/high accuracy U-Pb analyses of zircons from the Ben Vuirich granite in the Scottish Dalradian succession were performed on carefully picked, high integrity, abraded fractions of specific morphologies. Zircon needles, which grew during crystallization, give an age of 590 ± 2 Ma. Stubby prisms show inheritance with an upper concordia intercept of 1448 ± 7 Ma. The age of intrusion at 590 Ma contrasts strongly with a previous U-Pb zircon age of 514+6−7 Ma, determined on bulk-fraction unabraded grains, which, it is argued, must have suffered significant Pb loss. As the Ben Vuirich granite was emplaced between the regional D2 and D3 deformation events in the Dalradian block, the new data imply that all Dalradian sedimentation was Precambrian. Supposed Lower Cambrian fauna1 links between the Dalradian succession and Laurentia are thus discounted, and the possibility that the Dalradian Supergroup has non-Laurentian affinities must be considered. The early D1/D2 nappe-forming Grampian structures are also Precambrian, and may be associated with terrane docking along a tectonically active ocean margin. Later D3 structures and associated peak Barrovian metamorphism suggest a subsequent thickening event, possibly c. 520 Ma ago.

Formation and composition of the lower continental crust: Evidence from Scottish xenolith suites

Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo-Carboniferous. The extremely variable Al2O3 (up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite- and mantle-normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high-grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra-site variations that are consistent with crystal-liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNd and 206Pb/ 204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm-Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm-Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian-derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the average age of the crust.

Zircon growth in slate

Clastic sedimentary and low-grade metasedimentary rocks preserve populations of detrital zircons because of the unreactive nature of this mineral. However, evidence of new zircon growth has been found within highly heterogeneous populations of zircon from several greenschist facies slates from the Scottish Highlands. Small (<10 μm), anhedral, unzoned zircons and discrete overgrowths on rounded detrital grains are very common. These new fine-grained zircons have crystallized at temperatures below 350 °C and have been observed only in polished thin sections; they are absent from conventional mineral separates. Typical separation techniques create severe biases in the heavy-mineral populations of metasedimentary rocks, and recognition of the growth of zircon in such conditions may allow isotopic dating of low-temperature events.

Crystallization of the Shap Granite: evidence from zoned K-feldspar megacrysts

Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H2O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.

Metamorphism and cooling of the NE Dalradian

The Buchan area of Dalradian rocks represents a distinct tectonothermal domain within the Scottish Highlands, and the Portsoy-Duchray Hill lineament on the western margin of this block is one of the best examples of a long-lived domain boundary. Rb-Sr mica cooling ages, from metamorphic rocks from across the trace of this lineament, document a prolonged Ordovician cooling history. Delayed, but then relatively rapid, cooling to the west of the Portsoy-Duchray Hill lineament is recognized whereas to the east the age data point to a high heat flow regime lasting more than 50 Ma. The isotopic ages suggest that c. 460 Ma was a time of active tectonism, resulting in final cooling of the low-grade rocks and differential uplift and rapid cooling of deeper structural levels to the west. The radiometric ages do not support previous tectonic models for the region that have invoked thrust thickening along the line of the Portsoy-Duchray Hill lineament and a new model for the metamorphism is proposed. The overprinting of andalusite-bearing assemblages by kyanite to the west of the Portsoy-Duchray Hill lineament is linked to pressure increases associated with the intrusion of the Newer Gabbros and represents a magmatic loading related to the same tectonothermal event that generated the low-pressure metamorphism, rather than a separate tectonic episode.

Allochthonous metamorphic blocks on the Hebridean passive margin, Scotland

Radiometric age determinations on syn-orogenic intrusive rocks coupled with evidence from field relationships, indicate that metamorphism and ductile deformation was taking place hi the Scottish metamorphic blocks coeval with extension on the Hebridean foreland. Consequently metamorphic rocks now found east and south of the Moine Thrust are regarded as substantially allochthonous, being located within the region where there was formerly a Cambrian-Early Ordovician passive margin sequence and its attendant oceanic crust. We regard these blocks as displaced terranes which acquired their metamorphic and structural signatures outside of the present Hebridean foreland regime. This may have occurred initially at the tune of megacontinent splitting and later on the destructive margin which was initiated hi Laurentia.

Exotic metamorphic terranes in the Caledonides: Tectonic history of the Dalradian block, Scotland

The Dalradian block, a part of the Precambrian metamorphic basement of Scotland, is thought to be exotic to Laurentia, having a provenance in Gondwana. In contrast to the rest of the Laurentian margin, which from ca. 700 to 530 Ma was undergoing extension, the Dalradian block was undergoing severe compression ca. 590 Ma. At the time of splitting of the Late Proterozoic supercontinent, both Gondwana and Laurentia shared a common history of extension; however, at ca. 670 Ma Gondwana converted to a destructive margin and underwent compression, whereas Laurentia remained in extension and passive to 500-510 Ma. The Dalradian block had two major phases of metamorphism and deformation, one in Gondwana, and the other in Laurentia. The second phase of classical Barrovian metamorphism may have been produced by thickening due to emplacement of ophiolitic and other nappes, rather than the earlier internal nappe structures. Late-stage ductile folding in the Moine basement to the north may be related to the final emplacement of the Dalradian block onto Laurentia.

Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.

Low-temperature thermochronology: Resolving geotherm shapes or denudation histories?

Thermal histories derived from apatite fission-track data are remarkably consistent irrespective of tectonic setting and overall rate of cooling. Rapid cooling through the partial annealing zone is typically followed by slower cooling, and such histories can only be explained by systematic errors in the experimentally derived annealing rates used to determine the thermal histories, or a consistent geotherm shape characterized by a relatively low geothermal gradient in the uppermost crust. Differences between the cooling paths characterizing individual tectonic settings indicate that geotherm shape influences the cooling histories. This suggests that crustal geotherms, especially those in orogenic belts, are characterized by a shallow zone of high permeability, allowing the rapid transfer of heat by fluid advection, perhaps together with a near-surface zone affected by enhanced heat loss due to topographic effects. The influence of such controls on cooling histories must be considered prior to using thermochronology data to constrain denudation histories.