Dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals, yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low. Results from crystal size distribution (CSD) measurements on harrisitic olivines from Rum, Scotland, combined with estimated olivine growth rates, suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long, may have exceptionally short crystal growth times (several hours to several hundreds of days). This, together with very low calculated nucleation densities for harrisitic olivine, supports the interpretation of harrisite being a disequilibrium texture, developed in response to supersaturation of the magma in olivine. We propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the Rum magma chamber floor, beneath cooler resident magma. It is envisaged that the picrite sheets were largely free of suspended olivine crystals. Coupled with the olivine-enriched composition of the melt and the increasing cooling rate, this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei. The enhanced supersaturation caused rapid growth of olivine once nucleation began, with skeletal and dendritic shapes. It is suggested that the observed, interlayered sequences of harrisite and cumulus peridotite found throughout the Rum Layered Suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling.
Igneous sills are common components in rifted sedimentary basins globally. Much work has focused on intrusions emplaced at relatively shallow palaeodepths (0 – 1.5 km). However, owing to constraints of seismic reflection imaging and limited field exposures, intrusions emplaced at deeper palaeodepths (>1.5 km) within sedimentary basins are not as well understood in regard to their emplacement mechanisms and host-rock interactions. Results from a world-class, seismic-scale outcrop of intruded Jurassic sedimentary rocks in East Greenland are presented here. Igneous intrusions and their host rocks have been studied in the field and utilizing a 22 km long ‘virtual outcrop’ acquired using helicopter-mounted lidar. The results suggest that the geometries of the deeply emplaced sills ( c. 3 km) are dominantly controlled by host-rock lithology, sedimentology and cementation state. Sills favour mudstones and even exploit centimetre-scale mudstone-draped dune-foresets in otherwise homogeneous sandstones. Sills in poorly cemented intervals show clear ductile structures, in contrast to sills in cemented units, which show only brittle emplacement structures. The studied host rock is remarkably undeformed despite intrusion. Volumetric expansion caused by the intrusions is almost exclusively accommodated by vertical jack-up of the overburden, on a 1:1 ratio, implying that intrusions may play a significant role in uplift of a basin if emplaced at deep basinal levels. Supplementary materials: Uninterpreted versions of Figures 7, 8 and 11 are available at http://doi.org/10.6084/m9.figshare.c.3281882
Deposition and subsequent preservation of the Jurassic-Cretaceous Etjo Sandstone Formation of Namibia represents a complex interplay between climatic and tectonic factors and related variations in extrabasinal sediment supply. The aeolian and fluvial deposits indicate semi-arid to arid climatic conditions throughout the deposition of four distinct sedimentary units. The succession records either an upward increase in aridity or an upward increase in aeolian sediment supply, represented by a transition from a fluvially dominated basal unit, through a marginal fluvial-aeolian unit to an exclusively aeolian unit. A combination of inherited palaeotopography and syndepositional extensional faulting provided the space necessary for the accumulation of much of the succession. A basinwide unconformity (super surface) divides the succession. This hiatus resulted partly from a lack of available preservation space and partly from a shutdown in aeolian activity related to a regional climatic reorganization. A subsequent shift in the palaeowind direction from northwesterly to southwesterly exploited sand reserves in the Paraná Basin of South America and led to the resumption of aeolian sedimentation across the region. Variations in preserved bedform thickness were directly controlled by differential amounts of tectonic subsidence across the basin. A second major super surface towards the top of the succession resulted from the regional shutdown of large tracts of the aeolian system following the eruption of Etendeka flood basalts across the region.
The emplacement of igneous intrusions into sedimentary basins mechanically deforms the host rocks and causes hydrocarbon maturation. Existing models of host-rock deformation are investigated using high-quality 3D seismic and industry well data in the western Møre Basin offshore mid-Norway. The models include synemplacement (e.g., elastic bending-related active uplift and volume reduction of metamorphic aureoles) and postemplacement (e.g., differential compaction) mechanisms. We use the seismic interpretations of five horizons in the Cretaceous-Paleogene sequence (Springar, Tang, and Tare Formations) to analyze the host rock deformation induced by the emplacement of the underlying saucer-shaped Tulipan sill. The results show that the sill, emplaced between 55.8 and 54.9 Ma, is responsible for the overlying dome structure observed in the seismic data. Isochron maps of the deformed sediments, as well as deformation of the younger postemplacement sediments, document a good match between the spatial distribution of the dome and the periphery of the sill. The thickness [Formula: see text] of the Tulipan is less than 100 m, whereas the amplitude [Formula: see text] of the overlying dome ranges between 30 and 70 m. Spectral decomposition maps highlight the distribution of fractures in the upper part of the dome. These fractures are observed in between hydrothermal vent complexes in the outer parts of the dome structure. The 3D seismic horizon interpretation and volume rendering visualization of the Tulipan sill reveal fingers and an overall saucer-shaped geometry. We conclude that a combination of different mechanisms of overburden deformation, including (1) elastic bending, (2) shear failure, and (3) differential compaction, is responsible for the synemplacement formation and the postemplacement modification of the observed dome structure in the Tulipan area.
<p><span>The connections between the Earth&#8217;s interior and its surface are manifold, and defined by processes of material transfer: from the deep Earth to lithosphere, through the crust and into the interconnected systems of the atmosphere-hydrosphere-biosphere, and back again. One of the most spectacular surface expressions of such a process, with origins extending into the deep mantle, is the emplacement of large igneous provinces (LIPs), which have led to rapid climate changes and mass extinctions, but also to moments of transformation with respect to Earth&#8217;s evolving paleogeography. But equally critical are those process which involve material fluxes going the other way&#8212;as best exemplified by subduction, a key driving force behind plate tectonics, but also a key driver for long-term climate evolution through arc volcanism and degassing of CO<sub><span>2.</span></sub></span></p><p><span>Most </span><span>hotspots, kimberlites, </span><span>LIPs are sourced by plumes that rise from the margins of two large low shear-wave velocity provinces in the lowermost mantle.</span><span> These thermochemical provinces have likely been quasi-stable for hundreds of millions, perhaps billions of years, and </span><span>plume heads rise through the mantle in about 30 Myr or less. LIPs provide a direct link between the deep Earth and the atmosphere but </span><span>environmental consequences depend on both their volumes and the composition of the crustal rocks they are emplaced through. </span><span>LIP activity can alter the plate tectonic setting by creating and modifying plate boundaries and hence changing the paleogeography and its long-term forcing on climate. Extensive blankets of LIP-lava on the Earth&#8217;s surface can also enhance silicate weathering and potentially lead to CO<sub><span>2</span></sub> drawdown (cooling), but we find no clear relationship between LIPs and post-emplacement variation in atmospheric CO<sub><span>2</span></sub> proxies on </span><span>very long (>10 Myrs) time-scales</span><span>. Hotspot and kimberlite volcanoes generally have relatively small climate effects compared with that of LIPs (because of volumetric and flux differences), but the eruption of large kimberlite clusters, notably in the Cretaceous, could be capable of delivering enough CO<sub><span>2</span></sub> to the atmosphere to trigger sudden global warming events.</span></p><p><span>Subduction is a key driving force behind plate tectonics but also a key driver for the long-term climate evolution through arc volcanism and degassing of CO<sub><span>2</span></sub>. Subduction fluxes </span><span>derived from full-plate models</span><span> provide a powerful way of estimating plate tectonic CO<sub><span>2</span></sub> degassing (sourcing). These correlate well with zircon age frequency distributions and zircon age peaks clearly correspond to intervals of high subduction flux associated with greenhouse conditions. Lows in zircon age frequency are more variable with links to both icehouse and greenhouse conditions, and only the Permo-Carboniferous (~330-275 Ma) icehouse is clearly related to the zircon and subduction flux record. </span><span>A key challenge is to develop reliable full-plate models before the Devonian in order to consider the subduction flux </span><span>during the end-Ordovician Hirnantian (~445 Ma) glaciations, but we also expect refinements in subduction fluxes for Mesozoic-Cenozoic times as more advanced ocean-basin models with intra-oceanic subduction are being developed and implemented in full-plate models.</span></p>
Silicic volcanic rocks are associated with most, if not all, continental ×ood basalt provinces and volcanic rifted margins, where they can form substantial parts of the eruptive stratigraphy and have eruptive volumes >10 4 km 3 . Poor preservation of silicic volcanic rocks following kilometer-scale uplift and denudation of the volcanic rifted margins, however, can result in only deeper level structural features being exposed (i.e., dike swarms, major intrusions, and deeply subsided intracaldera µlls; e.g., North Atlantic igneous province). The role of silicic magmatism in the evolution of a large igneous province and rifted margin may therefore be largely overlooked. There are silicic-dominated igneous provinces with eruptive volumes comparable to those of maµc large igneous provinces ( >10 6 km 3 ), but that have low proportions of basalt expressed at the surface. Some silicic large igneous provinces are associated with intraplate magmatism and continental breakup (e.g., Jurassic Chon Aike province of South America, Early Cretaceous eastern Australian margin), whereas others are tectonically and geochemically associated with backarc environments (e.g., Sierra Madre Occidental). Silicic volcanic rocks formed in these two environments are similar in terms of total eruptive volumes, dominant l ithologies, and rhyolite geochemistry, but show fundamental differences in tectonic setting and basalt geochemistry. Large-volume ignimbrites are the dominant silicic volcanic rock type of continental flood basalt and silicic large igneous provinces. Individual silicic eruptive units can have thicknesses, areal extents, and volumes that are comparable to, or exceed, in
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