Iain Neill

Sublithospheric small-scale convection—A mechanism for collision zone magmatism

We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau.

Origin of the Aves Ridge and Dutch–Venezuelan Antilles : interaction of the Cretaceous ‘Great Arc' and Caribbean–Colombian Oceanic Plateau?

Abstract: In this paper we reassess the geochronology and geochemistry of three dredge hauls from the SE corner of the Aves Ridge (Caribbean Sea) originally sampled in 1968 by Duke Universitys R.V. Eastward. Two hauls consist of light rare earth element-enriched granitoids with a U–Pb zircon emplacement age of 75.9 ± 0.7 Ma. A further haul contains mostly calc-alkaline island arc basaltic andesites of uncertain age. Petrological, trace element and isotopic constraints indicate that the granitoids have an oceanic crustal source and were formed by melting of the lower arc, oceanic or oceanic plateau crust. The mafic rocks formed by partial melting of an incompatible trace element-enriched mantle wedge, which was probably composed of mantle plume material. Both the dredged rocks and data from the Dutch–Venezuelan Antilles indicate a period of west-dipping underthrusting and subduction beneath, or close to, the Caribbean–Colombian Oceanic Plateau between c. 88 and c. 59 Ma, concurrent with collision of part of the plateau with northwestern South America. Constraints from the geochemistry and geochronology of offshore southern Caribbean arc and plateau rocks suggest that in the southern Caribbean there was no pre-existing west-dipping subduction system during formation of the Caribbean–Colombian Oceanic Plateau, whereas long-lived SW-dipping subduction in the northern Greater Antilles is more probable. Supplementary material: Sample details, major and trace element data (file 1), cathodoluminescence images of analysed zircons (file 2) and whole-rock standards (file 3) are available at http://www.geolsoc.org.uk/SUP18438.

Age and Petrogenesis of the Lower Cretaceous North Coast Schist of Tobago, a Fragment of the Proto–Greater Antilles Inter-American Arc System

The North Coast Schist of Tobago is part of the leading edge of the Caribbean Plate, which has been in oblique collision with northern South America for much of the Cenozoic. The North Coast Schist is dominated by two volcanic “formations” metamorphosed under greenschist-facies conditions during later deformation. The Parlatuvier Formation mostly consists of mafic to intermediate tuffs and tuff breccias with a U-Pb zircon ID-TIMS age of Ma. Trace element data and radiogenic isotopes reveal that the Parlatuvier Formation is derived from a heterogeneous subduction-modified, locally incompatible trace element–enriched, mantle source with some rocks containing the highest 176Hf/177Hf ratios found in the offshore Caribbean. The Mount Dillon Formation comprises silicified tuffs and tuff breccias that are derived from a more isotopically enriched mantle source with a significant slab fluid-related component. A thin belt of amphibolite-facies dynamothermally metamorphosed metavolcanic rocks lies in contact with a younger island arc pluton. Some of these amphibolites have an isotopically similar source to the Parlatuvier Formation but lack a clear subduction-related component. The age, geochemical heterogeneity, and proximal nature of eruption confirm that the North Coast Schist lay within an east-dipping proto–Greater Antilles arc. We propose that the arc system at the time of North Coast Schist magmatism was actively rifting, possibly during development of a back-arc basin. This arc system shut down during the Cretaceous, making way for southwest-dipping Greater Antilles subduction and relative eastward motion of the Caribbean Plate.

Gabbroic-dioritic dykes from the Sanandaj-Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, Western Iran

The Sanandaj–Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic–intermediate dyke swarms with Middle Jurassic (Group 1: hornblende gabbro and diorite) and Late Eocene (Group 2: hornblende–pyroxene gabbro) ages are recognized in the Malayer–Boroujerd Plutonic Complex of the northern SaSiZ. Group 1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group 1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group 2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous–Eocene Neo-Tethyan flat-slab scenario or a Latest Cretaceous–Paleogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small-volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia–Eurasia collision. Supplementary material: Field photographs, photomicrographs, additional geochemical plots, descriptions of analytical methods and tables of geochemical modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.4126196

The Cluanie granodiorite, NW Highlands of Scotland: a late Caledonian pluton of trondhjemitic affinity

Synopsis The Cluanie Pluton is a late Caledonian granitoid emplaced into the Glenfinnan Division of the Moine Supergroup in the NW Scottish Highlands. A field investigation of the pluton and its internal facies is presented along with new major- and trace-element whole-rock XRF analyses, and geobarometric and geothermometric studies. Cluanie is predominantly composed of hornblende granodiorite characterized by varying concentrations of distinctive alkali feldspar megacrysts, with minor amounts of biotite granodiorite and rare mingled porphyritic microgranodiorite. The alkali feldspar megacrysts appear to be magmatic in origin. Rare spectacular pegmatitic concentrations most likely represent physical accumulation of the megacrysts. The pluton is geochemically a high Na/K trondhjemite, the only such pluton known among the Newer Granites of Scotland. On the basis of geochemical evidence and a comparison with partial melting experiments, we propose that the magmas were derived by fluid-rich melting of an amphibolitic source leading to relatively low temperature magmas which were significantly contaminated by Moine metasediments. The pluton was emplaced in the mid crust at about 4.3 kbar during an episode of dextral shear on the Glen Glass Fault related to regional strike-slip faulting on the Great Glen Fault system at c. 425 Ma.