Sedimentary rocks and modern sediments sample large volumes of the Earth’s crust, and preserve units that vary greatly in age and composition. Determining the provenance of component minerals is complicated by the ability of some minerals to be recycled through multiple sedimentary cycles, so minerals from completely unrelated sources may end up in the same sedimentary basin. To untangle these multi-stage signals, two or more chemical signatures measured in minerals with different stability are required. For instance, labile minerals, such as feldspar, can break down rapidly during sedimentary transport, while refractory minerals, such as zircon, can be much more resilient and survive repeated recycling. One sedimentary succession suitable for testing this hypothesis is the Upper Carboniferous Millstone Grit Group, a fluvio-deltaic, upward-coarsening sequence of mudstones, sandstones and conglomerates deposited in the Pennine Basin of northern England over c. 14 myr. New isotopic data have been measured in detrital K-feldspar and zircon from five of the seven stages, complementing previous work in the area [1,2,3]. Two K-feldspar Pb isotope peaks at 206Pb/204Pb = 12.5–15.5 and c. 18.4 indicate derivation from Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age peaks at c. 2700, 1000–2000 and 430 Ma reflect a mixture of Archaean basement, Proterozoic sediments and Caledonian granites, while Hf model ages form two broad peaks at c. 4500–3000 and 2300–1500 Ma, indicating contributions from both juvenile and reworked crust. Strong similarities between potential sources in this complicated region mean no one mineral or isotopic system can provide a unique provenance determination. Instead, comparing first-cycle and multi-cycle minerals with different hydrodynamic properties is necessary to untangle the full story. Combining these results with published garnet, monazite and muscovite data demonstrates the power of multi-proxy provenance work, indicating a primary source area in the Greenland Caledonides, with minor contributions from Norway and Scot-land. Comparisons between zircon U–Pb distributions in Palaeozoic sediments suggest long-lived sedimentary systems recycled material around the North Atlantic over c. 100 myr, much of it ultimately derived along the Grenvillian margin of Laurentia. This consistency is interrupted only by regular variations in palaeoflow direction, reflecting tectonic evolution in the region.
Recent work on the evolution of the Earth’s continental crust has focussed on the ability of refractory minerals, such as zircon, to preserve information about crustal units long since lost from the rock record. Techniques used in these studies include U-Pb ages of crystallisation, δ18O to track crustal contamination, and Hf model ages of source rock extraction from the mantle. Such analyses, either alone or in combination, can provide insight into questions on a range of scales, from identifying early recycled material in younger crust, to locating similar (possibly related) source rocks now separated by great distances and investigating whether early crustal production were regional or global in extent. Work on detrital zircons from key areas of Hadean and Early Archaean crust - the Jack Hills (eastern Australia), Acasta Gneiss and Slave Province (Canada) and the Limpopo Belt (southern Africa) - has suggested crustal generation was episodic, with distinct peaks every 0.3-0.6 Ga from 4.5-1.2Ga. Younger sediments from other areas of the world could record many of the same events, and/or identify new ones. One such area is north-west Scotland, which comprises various unconformable Proterozoic and Palaeozoic (meta)sedimentary units that have sampled the underlying basement, and potentially basement from further afield within Laurentia and possibly Baltica. While many studies in other localities have presented combined U-Pb and Hf data to identify episodes of crustal production, the addition of δ18O analyses can distinguish between production of juvenile crust and reworking of pre-existing (sedimentary) material. We present the first correlated in situ U-Pb, δ18O and eHf data from detrital zircons sampled throughout the region, including the Meso- to Neoproterozoic Torridonian and Moine Supergroup and Cambro-Ordovician sediments. These data identify significant juvenile extraction events at c.1.6, c.2.2 and c.3.3Ga, without regard to stratigraphic level. In addition, three zircon crystallisation episodes correspond with the onset of supercontinent stabilisation - Superia, Nuna and Rodinia. The extraction events identified in this study fall between those recognised from older sediments in previous work, suggesting possible differences in the records of crust generation preserved in different terrains. Such differences may be of assistance when tracking continental cratons through the formation and destruction of supercontinents such as Rodinia. These samples also contribute to our understanding of Scottish regional geology, and our ability to correlate these detrital repositories with those in other parts of the world. Previous U-Pb provenance work indicates these Scottish rocks correlate with similar sequences in Greenland and Labrador, Canada, all derived from now-absent units on greater Laurentia. The Scottish data represent the first crustal evolution work of this kind to be undertaken on this significant craton.
Detrital zircons from the Lower Carboniferous clastic rocks of the Midland Valley of Scotland have been dated using U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to determine which source areas contributed sediment to the basin during its development, and to investigate whether provenance changed during deposition of these units. Specific provenance detection using U/Pb dating of zircons has never been attempted in these rocks, and there are uncertainties remaining about the regional paleogeographic setting for the Midland Valley. Four samples from the Dinantian Strathclyde Group have been analysed, and the units are locally known as the Fife Ness, Anstruther, Pittenweem, Sandy Craig, and Pathhead formations. The formations are composed of shallow marine, deltaic, fluvial and floodplain deposits and these predominantly siliciclastic sedimentary rocks are interbedded with thin fossiliferous carbonate bands. The samples are quartz arenitic, sub-arkosic and lithic arkosic medium-grained sandstones, predominantly from a fluvial origin. The British Geological Survey developed a lithostratigraphy which is the most used framework for the Strathclyde Group (Browne et al., 1997), but a different biostratigraphical framework based on palynology has been proposed by Owens et al. (2005). In addition to identifying provenance, the zircon age populations for each formation are compared to test which stratigraphic framework is correct. More broadly, the provenance data provides a way to improve the regional palaeogeographic setting for the Midland Valley. Zircon ages in the Strathclyde Group are dominated by Late Mesoproterozoic to Late Palaeoproterozoic (0.9 – 2.0 Ga) and Early Palaeozoic (350 – 450 Ma) ages which reflect Caledonide (Laurentian-Baltica margin including Scotland, Scandinavia, Greenland, Newfoundland), Grampian and internal Midland Valley source areas. Notable peaks occur at 400 Ma, 1.0 —1.1 Ga, 1.3 Ga, 1.6 – 1.7 Ga, and 2.7 Ga, and the Proterozoic age peaks are consistent with a Dalradian source. Although the age spectra for each formation are broadly similar, the proportions of age populations differ and age peaks present in the Anstruther Formation are absent in other formations. For instance, the Anstruther Formation has a significantly larger proportion of Archaean-aged zircons compared to the Pittenweem Formation, and contains a 1.3 Ga peak which is absent in the other formations. This suggests that source areas evolved throughout the deposition of the Strathclyde Group. The dominance of Mesoproterozoic and Palaeoproterozoic ages relative to Palaeozoic ages contrasts to similar ages units in the Pennine Basin and offshore North Sea, where the latter dominate the age spectra (Hallsworth et al., 2000; Morton et al., 2001). This may reflect the proximity of the Dalradian terrane and organisation of river systems draining into the Fife section of the Midland Valley during the Dinantian.
Recent improvements in analytical capabilities allow us to reveal details of magmatic processes at an increasingly finer spatial and temporal scale. In situ analyses of the isotopic and trace element composition of accessory minerals at the sub-grain scale have proven to be effective tools for solving a wide range of geological problems. This study presents new data on accessory minerals including monazite & zircon, examined by in situ LA-ICP-MS and Laser Ablation Split Stream (LASS) techniques, analyzing multiple isotopic systems (U-Pb + Sm-Nd, and U-Pb + Lu-Hf in monazite and zircon, respectively) in order to track geochemical changes over time through a magmatic system. The late Cretaceous granitoids of the Old Woman Mountains in the Mojave Desert, California, provide an excellent opportunity to apply these analytical techniques. The peraluminous granites of the Sweetwater Wash, Painted Rock, and North Piute plutons represent different depths of the magmatic system, and are well understood in terms of field relations and whole-rock geochemistry. A preliminary study on the Sweetwater Wash monazites (Fisher et al., in preparation) has revealed significant inter-grain isotopic heterogeneity in the composition of the source region (~1700 Ma); however, the U-Pb ages show an isotopic resetting during emplacement at ~75 Ma. This decoupling of U-Pb and Sm-Nd isotopic systems is suggested by Fisher et al. to be due to recrystallisation and/or dissolution-reprecipitation of monazite. If grain boundary diffusion of Pb overrides the more kinetically limited volume diffusion, then the U-Pb systematics will be reset while Sm and Nd remain immobile in the monazite structure as essential structural components of the lattice. This new data will allow the further investigation of these preliminary results, providing new insights into the observed isotopic disequilibrium, with the LASS technique accurately linking the multiple isotopic systems. This will provide important insights into monazite isotope systematics, which will have implications for the geochronology community. Systematic sampling through transects of each pluton will also allow the geochemical homogeneity of each pluton to be assessed. Additionally, this study is the first application of the LASS technique to a magmatic system, and thus will provide further insight into the petrogenesis of the Old Woman and North Piute Mountains, and continental arc granites in general.
The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.
Abstract Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T – t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.
Sedimentary rocks and modern sediments sample wide areas of the crust, which preserve units that vary greatly in age and composition. Determining the provenance of component minerals is complicated by the ability of some minerals to be recycled through multiple sedimentary cycles, so minerals from completely unrelated sources may end up in the same sedimentary basin. To untangle these multi-stage signals, two or more chemical signatures measured in minerals with different structural stability are required. For instance, feldspars can break down rapidly during sedimentary transport, while zircons can be much more resilient and survive repeated recycling. One sedimentary succession suitable for testing this hypothesis is the Upper Carboniferous Millstone Grit Group, a fluvio-deltaic sequence of upward-coarsening mudstones, siltstones and sandstones deposited in the Pennine Basin of northern England. New data from throughout this sequence clearly indicate two main feldspar populations, consistent with previous work, but also a minor third group which may correlate with zircons previously thought to be multi-cycle. Since the suggested source region for these rocks is northwest Scotland and the Southern Uplands, which contain material from as far away as Greenland, these data have significant implications for transport distances of both labile and resistant minerals.
Abstract The Archaean North Atlantic Craton underpins much of North America, Greenland and northern Europe, and incorporates the Earth's oldest extant continental crust. This paper reviews the current understanding of the region's crustal evolution, and considers our ability to investigate interrelationships between different fragments of the North Atlantic Craton. Detrital zircons from Mesoproterozoic to Cambrian basal sediments in NW Scotland have been re-examined in light of new data from the Archaean Tarbet supracrustal unit and the Palaeoproterozoic Rubha Ruadh granite. Hf model ages are recorded from 4160 to 1410 Ma, peaking at c. 3350 Ma, and are associated with U–Pb crystallization ages from 3670 to 1070 Ma, peaking at c. 2700 and 1700 Ma. The Rubha Ruadh granite is consistent with partial melting of Northern Region basement without contamination by juvenile magmas or supracrustal material, while the Tarbet Supracrustals record a minimum model age of c. 3200 Ma. Each of these units records Hf model ages that imply remelting of Eoarchaean (4000–3600 Ma) crust. Similar distributions of crystallization and model ages have been identified around the North Atlantic Craton, suggesting that Eoarchaean crust was once extensive in the region and constitutes the foundation of both Scotland and the North Atlantic Craton. Supplementary material: All new zircon U–Pb-Hf-O data from this study are available at www.geolsoc.org.uk/SUP18776 .
The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.