Geochemical control on the evolutionary history of the Ballantrae Complex, SW Scotland, from comparisons with recent analogues

A combination of geochemical and field investigations of the Ballantrae Complex allows some control on the relative timing of events in its tectono-magmatic history. In particular, the presence of boninitic lava and hyaloclastite confirms an oceanic, supra-subduction zone origin for this early Ordovician ophiolite assemblage. Boninite eruption was preceded by a substantial phase of late Tremadoc island-arc volcanicity and the boninitic units themselves are interbedded with early Arenig primitive island-arc tholeiites. Various supra-subduction zone extensional processes have been invoked to explain boninite occurrences in recently active oceanic island arcs; interarc rifting as a precursor to marginal basin formation is preferred as a model for the Ballantrae Complex. Evidence to support this preference is drawn from the early to mid-Arenig part of the Complex where sandstones derived from mature volcanic arc lavas are conformably overlain by tholeiitic lavas erupted in a back-arc basin; a possible analogue is seen in the active Bransfield Strait marginal basin. Thus, an evolutionary sequence can be envisaged for the Ballantrae Complex which begins with the establishment of an oceanic island arc where supra-subduction zone extension led to the genesis of boninitic lavas. Next, as extension continued, a marginal basin developed wherein sandstones derived from the arc lavas were deposited and tholeiites of ocean island type were erupted. Imbrication of the back arc sequence and accretion of the ophiolite terrane was initiated in the late Arenig by arc-continent collision.

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