Geochemical characteristics and geotectonic setting of early Ordovician basalt lavas in the Ballantrae Complex ophiolite, SW Scotland

ABSTRACTThe consensus of several geochemical studies is a polygenetic origin for the basic volcanic sequence within the Ballantrae Complex ophiolite. This overall agreement masks differences of opinion regarding local geochemical interpretation, the possible correlation of structurally isolated lava tracts, and the degree of structural imbrication responsible for the juxtaposition of the various lava types. Newly acquired data (XRF, INAA, ICP-MS) provide the best evidence yet obtained for the presence of a MORB component and establish a wider distribution for primitive tholeiitic basalts with plate-margin characteristics than had been previously reported. The two principal within-plate sequences (well established from extensive coastal outcrop) are geochemically indistinguishable, with one considered to be the deeper water equivalent of the other. Lithofacies and geochemical similarities encourage correlation of some inland and sparsely exposed examples of within-plate basalt with the well-exposed coastal sequences, and all of this lava type may have originated from a single, ocean island volcano. The diversity of outcrops formed in within-plate and plate-margin geotectonic settings is combined within a dynamic reconstruction of a Tremadoc to Arenig arc-trench system with an active back-arc spreading region. The new reconstruction reconciles for the first time all of the known geochemical and published isotopic age evidence.

Lava flow growth inferred from morphometric parameters: a case study of Citlaltépetl volcano, Mexico

Recent (Quaternary) lava fields, such as those of Citlaltépetl (Pico de Orizaba) volcano in Mexico, are excellent places to obtain precise measurements of flow-field dimensions that can be used to estimate volume, eruption duration and effusion rates. The relationship between these parameters and the influence of some other interrelated features such as lava composition, superficial structures and lava type are important tools that can help to infer conditions when the lavas were active and thus improve understanding of how flow fields grow. The Holocene lavas of Citlaltépetl volcano are homogeneous in composition (dacites) and are generally blocky with well-developed levees. The eruption duration obtained for the Citlaltépetl lavas by a method proposed by C. Kilburn and R. Lopes presents a good correlation with the different lava types morphologically classified here. Results from that method compare favourably with the inferred effusion rates estimated by an empirical cooling method (Graetz). The lavas show different behaviour, mainly controlled by fluctuations in the effusion rate that promote changes from single- to multiple-flow style. The maximum distance achieved by a flow is directly proportional to the effusion rate in Citlaltépetl lavas, but it is always lower for multiple flows, independent of the volume of erupted lava. Observations of Citlaltépetl lavas can be used to understand how lava flow growth occurs on other volcanoes.

Clinopyroxene solid solutions and water in magmas: results in the system phonolitic tephrite-H2O

Clinopyroxene solid solutions crystallized at Ptot = 2 kbar in the natural system phonolitic tephrite-H2O (lava type from Etna eruption 1971) indicate a linear increase of ln Xcpxcats with H2Oliq %. After coupling this with the In Xcpxcats pressure dependence found through experiments, a simple relation has been set up for clinopyroxene- and plagioclase-bearing magmas. This relation allows us to calculate the amount of water dissolved in the melt, at any known pressure in the crustal range.