Pillow Talk: Volcanic rocks of the Karoo that formed many leagues under the Gondwanan Sea

New field mapping has discovered numerous sections of pillow lavas with hyaloclastites at the lowest levels of Jurassic Karoo volcanic sequences across Lesotho and South Africa. Vesicularity (content and size) and geochemistry of the ca. 1 to 50 m thick pillow lavas sequences presently preserved at 1 670 to 2 150 m asl reveal they originated below 500 m of water. Most of the initial 87Sr/86Sr ratios of the pillow lavas scatter around ca. 0.708, which is also the value of Jurassic seawater. There is an increase in the δ18O values concomitantly with increasing alteration, a well-known feature at low-temperature when marine basalts alter to high-δ18O bearing minerals, such as clays and zeolites, stripping 18O by mass balance from seawater. This feature, combined with element behavior of the most altered samples, as well as the Sr-isotope signatures, suggest that the Karoo pillow lavas and hyaloclastites most likely altered under low-temperature conditions in seawater. Geochemistry of the major oxides and trace elements of the Karoo pillow lavas reveal they define tholeiitic basalt with minor basaltic andesite. The pillow lavas define Th/Yb-ratios well above the mantle array indicating derivation from subduction-related components, and their Ti-V relations are similar to mid-oceanic ridge basalt (MORB). The Nd-Sr isotope data for the pillow lavas indicate that the primary magma assimilated ca. 10 to 15% of continental crust. This is consistent with knowledge from mapping that sills and dykes cover close to 50% of the surface geology surrounding the sub-horizontal pillow lava sequences, and from drill-cores that these dolerites reached a minimum crustal depth of ca. 6 km below Jurassic sea level. The Karoo pillow lavas erupted between 186 and 184 Ma in relatively deep seawater of a possible rift system linked to fragmented continental lithosphere surrounded by ocean spreading domains, such as at the open-end of the East African rift across the Afar. By 183 Ma, Karoo volcanics extruded above sea level and after 182 Ma the pillow lavas were uplifted by at least 1 700 m. Thereafter, during farther up warping across the Karoo Large Igneous Province (KLIP), lavas across Lesotho reached heights over 3 500 masl by 140 Ma, and then eroded to their near present heights around 120 to 100 Ma as determined through a number of thermo-chronology and offshore sedimentation analyses. Thus, unlike the volcanics of the Ethiopian LIP, which erupted across continental crust at 2 000 masl ca. 30 to 40 million years prior to extensional tectonism across the Afar triple junction, KLIP reached its highest topography ca. 30 to 40 Myrs after early initiation across a triple junction system well-below sea level. This difference in timing between crustal extension and epeirogeny has implications for geodynamic and geochemical models that trace dispersal across southern Gondwana between Africa and Antarctica.

How does a monzogranite turn into a trachydacitic extrusion mantled by basinal volcaniclastics and peperites? The case of South-Ouessant, Armorican Variscides (France)

How many volcanic bodies are being confused with plutonic ones worldwide? The purpose of this study is not to provide an answer to this question, but rather to illustrate this issue through an example from the French Armorican Variscides. It concerns a magmatic body cross-cutting highly strained terranes in the Ouessant Island, regarded for decades as a granitoid (monzogranite) on the basis of both its coarse-grained texture and its mineralogy. However, the volcanic origin of the metamorphosed series flanking this foliated body is here recognized by pillow lavas, deposit layers, and fiamme-bearing volcaniclastics, all emplaced onto the soft-substrate floor of a fault-bounded basin. Among other things, similarities between feldspar megacrysts/porphyrocrysts in both the volcaniclastics and the adjoining (formerly) monzogranitic massive body lead us to reinterpret the latter as a trachydacitic extrusion. In our model, the corresponding viscous lava progressively flowed in the basin, recovering the earlier volcanic formations and inducing load effects on the underlying soft sediments, along with compaction of the previously deposited pumices, to produce fiamme. The interpretation of the South-Ouessant area as a Visean transtensional volcano-sedimentary basin provides a new perspective on the distribution of the Variscan pull-apart basins in the Armorican Massif.

A critical revision of the Liguride complex in the Pollino area (Southern Apennines)

<p>The geodynamic reconstructions of the Apennine-Tyrrhenian system strongly rely on chronological and P-T data derived from the study of ophiolite-bearing units accreted in orogenic belts, frequently affected by HP-LT metamorphic overprint. Consequently, we carried out a detailed geological survey, combined with the study of calcareous nannofossils and the analysis of mineralogical and petrographic features of low-grade metamorphic rocks, in order to reconstruct the tectono-stratigraphic relationships among different formations of the Liguride Complex and perform a critical review of the existing literature on the P-T evolution of the Liguride accretionary wedge exposed in the Pollino area of the Southern Apennines.</p><p>A geological-structural survey allowed us to distinguish four major tectonic units, characterized by an overall decrease of metamorphic grade from top to bottom. The tectonic units consist of: i) slices of continental crust rocks consisting of Albitic gneisses, Garnet gneisses and Amphibolites; ii) the Frido Unit Auctorum p.p. with a variable intensity of deformation and HP/LT metamorphic grade. This unit consist of a typical ophiolitic assemblage, including serpentinites with metadoleretites or alterated peridotites, pillow lavas, foliated metabasites, metalimestones and metabreccias, quarzites, and jaspers. The upper part of the succession is made up of calcschists and low-grade metapelites, displaying variable PT conditions from about 7 kbar and 200 °C to 12 kbar and 350 °C. A wide variation of P-T conditions suggests that the Frido Unit consists of different thrust sheets, showing a progressive increase of the metamorphic grade moving from north to south;  iii) the Seluci-Cogliandrino Unit, consisting mainly of metapelites and slices of an upper Jurassic seafloor succession with pillow lavas;  iv) the non-metamorphic Nord Calabrian Unit represented, from the bottom, by ophiolites, shales with pencil cleavage (Crete Nere Formation) and by a prevailing calciclastic unit (Saraceno Formation), topped by thrust top deposits (Albidona Formation).</p><p>Calcareous nannofossil assemblages were studied in samples coming from the main successions of the investigated area, in order to provide age constraints for the deformation of the Liguride accretionary wedge. Results show that Frido Unit did not preserve calcareous nannofossils in all analyzed samples, because of the strong deformation produced during the HP-LT metamorphic overprint. In the upper part of the Seluci-Cogliandrino Unit, Eocene inf. CNE4 biozone has been documented. On the other hand, in the lower part of the Saraceno Fm a late Albian age (CC9 p.p.) was documented, based on the occurrence of Eiffellithus turriseiffelii and Hayesites irregularis. Moreover, the occurrence of Discoaster lodoensis, Reticulofenestra dictyoda and Toweius callosus framed the lower stratigraphic interval of the Albidona Fm to the early Eocene (Ypresian; CNE4 p.p.). Based on the above data, the current views on the Cretaceous-Paleogene geodinamic evolution of the southern Apennine thrust and fold belt should be substantially revised.</p><p> </p>

Fossilized Endolithic Microorganisms in Pillow Lavas from the Troodos Ophiolite, Cyprus

The last decade has revealed the igneous oceanic crust to host a more abundant and diverse biota than previously expected. These underexplored rock-hosted deep ecosystems dominated Earth’s biosphere prior to plants colonized land in the Ordovician, thus the fossil record of deep endoliths holds invaluable clues to early life and the work to decrypt them needs to be intensified. Here, we present fossilized microorganisms found in open and sealed pore spaces in pillow lavas from the Troodos Ophiolite (91 Ma) on Cyprus. A fungal interpretation is inferred upon the microorganisms based on characteristic morphological features. Geochemical conditions are reconstructed using data from mineralogy, fluid inclusions and the fossils themselves. Mineralogy indicates at least three hydrothermal events and a continuous increase of temperature and pH. Precipitation of 1) celadonite and saponite together with the microbial introduction was followed by 2) Na and Ca zeolites resulting in clay adherence on the microorganisms as protection, and finally 3) Ca carbonates resulted in final fossilization and preservation of the organisms in-situ. Deciphering the fossil record of the deep subseafloor biosphere is a challenging task, but when successful, can unlock doors to life’s cryptic past.