The onset of the North Atlantic Igneous Province in a rifting perspective

The processes that led to the onset and evolution of the North Atlantic Igneous Province IN A I P) have been a theme of debate ill the past decades. A popular theory has been that the impingement on the lower lithosphere of a hot mantle plume (the 'Ancestral Iceland' plume) initiated the first voluminous outbursts of lava and initiated rifling in the North Atlantic area in Early Palaeogene times. Here we review previous studies in order to set the NAIP magmatism in a time-space context. We suggest that global plate reorganizations and lithospheric extension across old orogenic fronts and/or suture zones, aided by other processes in the mantle (e.g. local or regional scale upwellings prior to and during the final Early Eocene rifting), played a role in the generation of the igneous products recorded ill the NAIP for this period. These events gave rise to the extensive Paleocene and Eocene igneous rocks in W Greenland, NW Britain and at the conjugate E Greenland-NW European margins. Many of the relatively large magmatic centres of the NAIP were associated with transient and geographically confined doming in Early Paleocene times prior to the final break-up of the North Atlantic area

Volcanic constraints on the unzipping of Africa from South America: Insights from new geochronological controls along the Angola margin

The breakup of Africa from South America is associated with the emplacement of the Paraná-Etendeka flood basalt province from around 134 Ma and the Tristan da Cunha plume. Yet many additional volcanic events occur that are younger than the main pulse of the Paraná-Etendeka and straddle the rift to drift phases of the main breakup. This contribution reports on new geochronological constraints from the Angolan part of the African Margin. Three coastal and one inland section have been sampled stretching across some 400 Km, with 39Ar/40Ar, U-Pb and Palaeontology used to provide age constraints. Ages from the new data range from ~100 to 81 Ma, with three main events (cr. 100, 91 and 82-81 Ma). Volcanic events are occurring within the Early to Late Cretaceous, along this part of the margin with a general younging towards Namibia. With the constraints of additional age information both onshore and offshore Angola, a clear younging trend at the early stages of rift to drift is recorded in the volcanic events that unzip from North to South. Similar age volcanic events are reported from the Brazilian side of the conjugate margin, and highlight the need to fully incorporate these relatively low volume volcanic pulses into the plate tectonic breakup models of the South Atlantic Margin

The anatomy of Continental Flood Basalt Provinces: Geological constraints on the processes and products of flood volcanism

The internal architecture of the immense volumes of eruptive products in Continental Flood Basalt Provinces (CFBPs) provides vital clues, through the constraint of a chrono-stratigraphic framework, to the origins of major intraplate melting events. This work presents close examination of the internal facies architecture and structure, duration of volcanism, epeirogenetic uplift associated with CFBPs, and the potential environmental impacts of three intensely studied CFBPs (the Parana-Etendeka, Deccan Traps and North Atlantic Igneous Province). Such a combination of key volcanological, stratigraphic and chronologic observations can reveal how a CFBP is constructed spatially and temporally to provide crucial geological constraints regarding their development. Using this approach, a typical model can be generated, on the basis of the three selected CFBPs, that describes three main phases of flood basalt volcanism. These phases are recognized in Phanerozoic CFBPs globally. At the inception of CFBP volcanism, relatively low-volume transitional-alkaline eruptions are forcibly erupted into exposed cratonic basement lithologies, sediments, and in some cases, water. Distribution of initial volcanism is strongly controlled by the arrangement of pre-existing topography, the presence of water bodies and local sedimentary systems, but is primarily controlled by existing lithospheric and crustal weaknesses and concurrent regional stress patterns. The main phase of volcanism is typically characterised by a culmination of repeated episodes of large volume tholeiitic flows that predominantly generate large tabular flows and flow fields from a number of spatially restricted eruption sites and fissures. These tabular flows build a thick lava flow stratigraphy in a relatively short period of time (c. 1–5 Ma). With the overall duration of flood volcanism lasting 5–10 Ma (the main phase accounting for less than half the overall eruptive time in each specific case). This main phase or ‘acme’ of volcanism accounts for much of the CFBP eruptive volume, indicating that eruption rates are extremely variable over the whole duration of the CFBP. During the waning phase of flood volcanism, the volume of eruptions rapidly decrease and more widely distributed localised centres of eruption begin to develop. These late-stage eruptions are commonly associated with increasing silica content and highly explosive eruptive products. Posteruptive modification is characterised by continued episodes of regional uplift, associated erosion, and often the persistence of a lower-volume mantle melting anomaly in the offshore parts of those CFBPs at volcanic rifted margins

Crystal size distributions (CSD) in three dimensions: insights from the 3D reconstruction of a highly porphyritic rhyolite

Growth histories and residence times of crystals in magmatic systems can be revealed by studying crystal sizes, size distributions and shapes. In this contribution, serial sectioning has been employed on a sample of porphyritic rhyolite from a Permo-Carboniferous laccolith from the Halle Volcanic Complex, Germany, to reconstruct the distribution of felsic phenocrysts in three dimensions in order to determine their true shapes, sizes and three-dimensional size distributions. A model of all three phenocryst phases (quartz, plagioclase, K-feldspar) with 217 crystals, and a larger model containing 1599 K-feldspar crystals was reconstructed in three dimensions. The first model revealed a non-touching framework of crystals in three dimensions, suggesting that individual crystals grew freely in the melt prior to quenching of the texture. However, crystal shapes are complex and show large variation on a Zingg diagram (intermediate over long axis plotted against short over intermediate axis). They often do not resemble the crystallographic shapes expected for phenocrysts growing unhindered from a melt, indicating complex growth histories. In contrast, the three-dimensional size distribution is a simple straight line with a negative slope. Stereologically corrected size distributions from individual sections compare well with stereologically corrected size distributions obtained previously from the same sample. However, crystal size distribution (CSD) data from individual sections scatter considerably. It is shown that CSDs can be robustly reproduced with a sampling size of greater than 200 crystals. The kind of shape assumed in stereological correction of CSDs, however, has a large influence on the calculation and estimation of crystal residence times

Lava-sediment interaction in desert settings; are all peperite-like textures the result of magma-water interaction?

This study reports on lava–sediment interaction in dry depositional settings focusing on the Early Cretaceous volcano-sedimentary sequence in the Huab Basin, NW Namibia, as a detailed example. Here an active aeolian sand sea (erg) system was progressively engulfed by lavas of the Etendeka Flood Basalt Province 133 Ma BP. This volcanic flooding resulted in: (1) the unprecedented preservation of large parts of the active dune system; (2) the development of a variety of sediment interlayers; and (3) a record of excellent examples of the dynamic interaction between lava and aeolian sand. The lava–sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava–sediment contacts at small scales. ‘Peperite’ like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands. Such interaction could involve lava cascading down the front of dune faces, or active ‘aa’ style flow fronts ‘bulldozing’ into and oversteepening dune faces causing rapid influx of sand into the active flow front. Resulting breccia horizons have been found up to 5 m thick. Locally, the aeolian sand is baked to a quartzite by the hot lava. In contrast, on other surfaces, the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction. In places, wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows. Peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with. However, examples from arid environments demonstrate that peperite-forming processes are diverse and may not always require water. Therefore, it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two. This leaves the presence, or lack, of water as an issue for interpreting the processes by which the peperite formed

Quantifying the building blocks of igneous rocks: Are clustered crystal frameworks the foundation?

Most phenocryst populations in volcanic rocks, and those preserved in shallow-level igneous intrusions, are clustered (variously referred to as clots, clumps or glomerocrysts). These clusters of crystals are the building blocks that accumulate to form the high-porosity, touching crystal frameworks from which igneous cumulates form. Examination of touching crystal frameworks in olivine- (komatiite cumulates and experimental charges) and plagioclase-dominant crystal populations (Holyoke flood basalt, Connecticut, USA) reveal complex, high-porosity, clustered crystal arrangements. Olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days. Plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10-10 mm/s to less than 3 years for a growth rate of 5 x 10-10 mm/s based on crystal size distributions. The origin of crystal clusters is likely to involve either (or a combination of) heterogeneous nucleation, remobilization of cumulate mushes or crystals sticking together during settling and/or flow. The spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non-touching crystal populations, and further improve our understanding of crystal packing arrangements and cluster size distributions

The petrogenesis of magmatic systems: using igneous textures to understand magmatic processes

An understanding of the petrogenesis of magmatic systems can be achieved by studying their constituent parts (e.g. crystals) and their chemical trends. Magmas are often travelling from source to emplacement levels together with a growing population of crystals. As they grow and respond to their environment through time, crystals can record the processes of contamination, magma mingling, crystal exchange and magma recharge occurring en route. Detailed quantification of the micro-scale textural parameters of igneous rocks such as crystal shape, size and spatial pattern provides important geometrical information about the crystal population, which can be linked with macroscale magmatic processes. The complex zoning often observed in crystals can also be quantified and linked back to the textures to show how magma chemistry changed during crystal growth. Finally, it is described how these techniques are developing toward 3D chemical mapping of igneous rocks as well as to record 4D experiments

The petrophysical andpetrographical properties ofhyaloclastite deposits: Implicationsfor petroleum exploration

Offshore sequences of volcaniclastic rocks (such as hyaloclastite deposits) are poorly understood in terms of their rock properties and their response to compaction and burial. As petroleum exploration targets offshore volcanic rifted margins worldwide, understanding of volcanic rock properties becomes important both in terms of drilling and how the rocks may behave as seals, reservoirs, or permeability pathways. The Hawaiian Scientific Drilling Project phase II in 2001 obtained a 3 km-(2-mi)-long core of volcanic and volcaniclastic rocks that records the emergence of the largest of the Hawaiian islands. Core recovery of 2945 m (9662 ft) resulted in an unparalleled data set of volcanic and volcaniclastic rocks. Detailed logging, optical petrology, and major element analysis of two sections at depths 1831–1870 and 2530–2597 m (6007–6135 and 8300–8520 ft) are compared to recovered petrophysical logs (gamma ray, resistivity, and P-wave velocity). This study concludes deviation in petrophysical properties does not seem to correlate to changes in grain size or clast sorting, but instead correlates with alteration type (zeolite component) and bulk mineralogy (total olivine phenocryst percentage component). These data sets are important in helping to calibrate well-log responses through hyaloclastite intervals in areas of active petroleum exploration such as the North Atlantic (e.g., Faroe-Shetland Basin, United Kingdom, and Faroe Islands, the Norwegian margin and South Atlantic margins bordering Brazil and Angola)