T. S. Brewer

Episodic rapakivi magmatism due to distal orogenesis?: Correlation of 1.69–1.50 Ga orogenic and inboard, “anorogenic” events in the Baltic Shield

The temporal overlap of 1.69–1.55 Ga westward growth of the Baltic Shield and voluminous, episodic, 1.65–1.50 Ga rapakivi magmatism in the Svecofennian domain to the east has long been recognized. New U-Pb data from southwest Sweden indicate that this westward growth occurred in at least three distinct stages involving convergent-margin magmatism (Gothian orogenesis) at 1.69–1.65, 1.62–1.58, and 1.56–1.55 Ga. This improved resolution along the margin reveals an interesting temporal correlation between these events and the intracratonic rapakivi magmatism. Each stage of convergent-margin magmatism was echoed by a phase of bimodal rapakivi magmatism at 1.65–1.62, 1.58–1.56, and 1.55–1.50 Ga. In addition to this temporal correlation, rapakivi suites form distinct north-trending arrays that are subparallel to, but 500–1500 km east of, the active margin. These temporal and spatial links suggest that recurring subduction along oceanward-stepping zones provided first-order control(s) on episodic mantle melting and consequent bimodal rapakivi magmatism in distal, inboard settings. At the very least, emerging constraints from Baltica do not support previous, purely anorogenic models for the 1.65–1.50 Ga rapakivi magmatism or models that singularly implicate lingering effects from the preceding Svecofennian orogeny.

Timing of accretion and collisional deformation in the Central Asian Orogenic Belt: implications of granite geochronology in the Bayankhongor Ophiolite Zone

Abstract Growing evidence suggests that the mechanism of Palaeozoic continental growth in Central Asia was by subduction–accretion with punctuated collisions that produced ophiolitic sutures between accreted blocks. The Bayankhongor ophiolite is the largest ophiolite in Mongolia and possibly all of Central Asia, and is interpreted to mark the collisional suture between the Baidrag and Hangai continental blocks. New 207Pb/206Pb zircon evaporation ages for granite plutons and dykes that intrude the ophiolite and its neighbouring lithotectonic units suggest that the ophiolite was obducted at c. 540 Ma at the beginning of a collisional event that lasted until c. 450 Ma. The new data, combined with that of previous studies, indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c. 570 Ma, followed by approximately 30 million years of subduction–accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c. 540–450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggests that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt (CAOB) that defines the southern and western margins of the Hangai continental block.

New zircon ages and regional significance for the evolution of the Pan-African orogen in Madagascar

New 207Pb/206Pb single zircon evaporation ages for granulites, gneisses and granites in southern and central Madagascar record a widespread Pan-African metamorphic and magmatic event in the period c. 650–556 Ma, but also earlier ages in the range 1890–1710 Ma, inherited from protolith material and reflecting heterogeneous crustal sources. South of the Ranotsara shear zone, metasedimentary gneisses and granulites contain an early population of detrital zircons with ages in the range 1890–1740 Ma; a detrital grain with an age of 899 ± 2 Ma suggests that some sedimentary protoliths were deposited later than c. 900 Ma. Metamorphic zircons have a mean age of 564.2 ± 0.9 Ma. North of the Ranotsara shear zone, our data provide information on the age of source material of metamorphic rocks: 788.6 ± 0.7 Ma for the time of emplacement of the granitic precursor of a granulite-facies charnockite and 650.9 ± 0.9 Ma for the protolith age of an amphibolite-facies migmatitic gneiss. A structurally conformable alkali granite sheet with a crystallization age of 568.7 ±1.6 Ma contains xenocrystic zircons, one of which has an age of 1229.6 ± 1.0 Ma, inherited from the source of the anatectically derived material. The post-tectonic, alkalic Carion granite has an emplacement age of 556.0 ± 1.7 Ma and provides a minimum age for granulite- and amphibolite-facies metamorphism. Our field data indicate that extensional shear zones are common in central Madagascar, locally controlling amphibolite-facies retrogression of granulite-facies rocks and the emplacement of crustal melt granites. These events record the widespread extensional collapse of the Pan-African orogen in Madagascar.

Petrogenesis of metaluminous A-type rhyolites from the St Francois Mountains, Missouri and the Mesoproterozoic evolution of the southern Laurentian margin

Abstract Predominantly acid volcanic and intrusive rocks of the approximately 1.48 Ga Eastern Granite–Rhyolite Province are exposed only in the St Francois Mountains inlier, Missouri. Combined Nd isotopic, major and trace element analyses are presented for the predominantly acid volcanic rocks of the St Francois Mountains. They are metaluminous to mildly peraluminous, A-type compositions with positive initial e Nd values of 3.1–5.0. They are interpreted to be the result of crystal fractionation of partially melted calc-alkaline igneous rocks formed at a recently active subduction zone. The likelihood of a juvenile, calc-alkaline, subduction-generated protolith for the St Francois Mountains rhyolites leads to a hypothesis in which dacitic parental magmas resulted from partial melting in a continental ‘back-arc’ setting. Basaltic magmatism due to decompression melting of the mantle resulted from lithospheric extension and thinning and provided the heat source for melting in the middle or lower crust. Penecontemporaneous magmatism elsewhere in Laurentia included subduction related calc-alkaline rocks in the Pinware Terrane of Labrador; some felsic igneous rocks of uncertain plate tectonic setting within the Grenville Province also have 1.5–1.6 Ga T DM ages. These are interpreted as magmatic products of approximately 1.5 Ga northward dipping subduction beneath much of the length of the southern margin of Laurentia. The Eastern Granite Rhyolite Province is consequently seen as an integral inboard expression of this subduction related igneous activity. Petrogenetic models involving a mantle plume or the development of a continental rift are not required to explain this granite–rhyolite terrane and evidence against their involvement is discussed.

Alteration and mineralization of an oceanic forearc and the ophiolite‐ocean crust analogy

Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid-ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu-Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low-temperature (<100°C) seawater alteration, with temperatures increasing downward. The rocks are partly (5–25%) altered to smectite, Fe-oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated δ18O and 87Sr/86Sr. Higher temperatures (∼150°C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite-smectite, corrensite, albite, K-feldspar, and quartz (±chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg-chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced serialization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200°–250°C. High 87Sr/86Sr ratios of chloritized (∼0.7055) and sericitized (∼0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low δ34S of sulfide (−2 to −5.5‰) and sulfate (12.5‰) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite.

Grenvillian age decompression of eclogites in the Glenelg-Attadale Inlier, NW Scotland

The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian Moine nappe of NW Scotland. In the eastern part of the inlier amphibolite-facies retrogression of the eclogites is associated with tectonic fabrics, and P–T estimates indicate significant decompression (c. 20 km). Previous Sm–Nd mineral–whole-rock dates indicated that peak eclogite-facies metamorphism occurred around c. 1.08 Ga, which was correlated with the Grenvillian orogeny. However, the middle REE enrichment of the analysed garnets suggests the influence of apatite inclusions. It is therefore likely that the interpretation of the c. 1.08 Ga age is complex, possibly reflecting re-equilibration at lower temperatures. Sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase, and are part of the retrograde amphibolite-facies assemblages. Titanite extensively replaces rutile, and is clearly associated with the retrograde amphibolite-facies event. A second textural type of titanite forms anhedral grains with plagioclase and pargasite, which is developed where the retrograde amphibolite-facies assemblage overprints the eclogite mineralogy. U–Pb dating has yielded the following ages: zircon age of 995 ± 8 Ma, and variably discordant rutile ages between 416 and 480 Ma. U–Pb and Pb–Pb isochrons on titanite and plagioclase/quartz separates yielded ages of 971 ± 65 Ma and 945 ± 57 Ma, respectively, in agreement with the zircon age. Analysed zircons and titanites are texturally part of the amphibolite-facies assemblage. The new zircon age demonstrates that amphibolite-facies metamorphism during exhumation occurred at 995 ± 8 Ma; the titanites could have closed with respect to Pb at this time or alternatively at some time between c. 1000 and 900 Ma. These data clearly demonstrate that parts of the Scottish basement underwent major thick-skinned tectonics during the Grenvillian orogeny. Rutile is part of the eclogite-facies paragenesis, and yet has young ages; these data are best explained by reheating producing near-total Pb loss related to emplacement of the late- to post-tectonic Ratagain Granite Complex at c. 425 Ma, at the end of the Caledonian orogeny.

DNA damage in earthworms from highly contaminated soils : assessing resistance to arsenic toxicity by use of the Comet assay

Earthworms native to the former mine site of Devon Great Consols (DGC), UK reside in soils highly contaminated with arsenic (As). These earthworms are considered to have developed a resistance to As toxicity. The mechanisms underlying this resistance however, remain unclear. In the present study, non-resistant, commercially sourced Lumbricus terrestris were exposed to a typical DGC soil in laboratory mesocosms. The earthworms bio-accumulated As from the soil and incurred DNA-damage levels significantly above those observed in the control mesocosm (assessed using the Comet assay). A dose response was observed between DNA damage (% tail DNA) and As concentration in soil (control, 98, 183, 236, 324 and 436mgkg(-1)). As-resistant earthworms (Lumbricus rubellus, Dendrodrilus rubidus and L. terrestris) collected from contaminated soils at DGC (203 to 9025mgkg(-1) As) had also bio-accumulated high levels of As from their host soils, yet demonstrated low levels of DNA damage compared with earthworms from uncontaminated sites. The results demonstrate that the As-contaminated soils at DGC are genotoxic to non-native earthworms and much less so to earthworms native to DGC, thus providing further evidence of an acquired resistance to As toxicity in the native earthworms.

Petrophysical Properties of Crystalline Rocks

Boreholes are commonly drilled into crystalline rocks to evaluate their suitability for various applications such as waste disposal (including nuclear waste), geothermal energy, hydrology, sequestration of greenhouse gases and for fault analysis. Crystalline rocks include igneous, metamorphic and even some sedimentary rocks. The quantification and understanding of individual rock masses requires extensive modelling and an analysis of various physical and chemical parameters. This volume covers the following aspects of the petrophysical properties of crystalline rocks: fracturing and deformation, oceanic basement studies, permeability and hydrology, and laboratorybased studies. With the growing demands for sustainable and environmentally effective development of the subsurface, the petrophysics of crystalline rocks is becoming an increasingly important field.

Geochemistry of late Mesoproterozoic volcanism in southwestern Scandinavia: implications for Sveconorwegian/Grenvillian plate tectonic models

Abstract: The last magmatic stage before the Sveconorwegian orogeny in the Baltic Shield is represented by bimodal, c.1.16 Ga volcanics from the Bandak Group in southern Norway and largely coeval basalts from the Dal Group in SW Sweden. In both areas, the volcanic rocks are intercalated with sedimentary units and the onset of basin development is marked by deposition of clastic sediments. In the lower part of the Bandak Group, the Morgedal basalts are relatively evolved and geochemical signatures suggest assimilation of lower crustal components. In contrast, the subsequent Gjuve basalts are more primitive and record assimilation of upper crustal rocks similar to exposed basement rocks. In the Bandak Group, the chemistry of the basalts changes within the stratigraphy, such that the older Morgedal basalts (MgO = 5.2–8.8 wt%, initial ϵNd 2.81–3.96, Zr/Y = 4.0–5.1, Zr/Nb = 34–50, La/Nb = 2.4–3.7) are more fractionated and have more enriched Nd isotope ratios compared to the Gjuve basalts (MgO = 6.4–12.3 wt%, initial ϵNd 4.05–4.97, Zr/Y = 3.4–4.7, Zr/Nb = 20–30, La/Nb = 1.5–2.4). This suggests that there earliest basalts are more contaminated, and it appears that the composition of the crustal component changed with time. A thin felsic volcanic unit, the Dalen Formation, separates the two basalt sequences. It represents melting of upper crustal rocks, triggered by injection of mafic magmas into crustal magma chambers. In the Dal Group, basalts form a relatively thin unit (< 500 m) where flows with high MgO values (> 9%) were least affected by assimilation of crustal components. Whole-rock geochemistry and Nd isotope ratios for the Bandak and Dal basalts indicate shallow melting of sub-continental lithospheric mantle in response to extension in a continental back-arc setting, related to subduction along the western margin of Baltica. These constraints envision a similar tectonic evolution with that of eastern Laurentia, which support models of a pre-Grenvillian supercontinent with a long-lived, active margin that reached western Baltica.

U–Pb geochronology of Lewisian orthogneisses in the Outer Hebrides, Scotland: implications for the tectonic setting and correlation of the South Harris Complex

The South Harris Complex of the Scottish Outer Hebrides forms a distinctive component of the Lewisian Complex and contains three supposedly co-magmatic meta-igneous bodies, a diorite, a ‘norite’ and an anorthosite, believed to be part of a Palaeoproterozoic arc. New U–Pb zircon ages obtained from these bodies of 1888 ± 2 Ma, 1890 +2/−1 Ma, and 2491 +31/−27 Ma, respectively, demonstrate that the anorthosite cannot be related to the younger calc-alkaline diorite and ‘norite’. The diorite and ‘norite’, however, probably are related products of arc magmatism. The spatial association of the diorite and ‘norite’ with the substantially older anorthosite suggests that the younger intrusions were emplaced into continental crust. Any suture associated with the South Harris Complex remains cryptic, and is not marked by an obvious change in protolith ages in the adjacent tonalitic gneisses.