Transactions of the Royal Society of Edinburgh: Earth Sciences

Research Article

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

The Southern Uplands Terrance, Tectonics and Biostratigraphy within the Caledonian Orogen, Our Dynamic Earth, Edinburgh.

J. L. Smelliea1 and P. Stonea2

a1 J. L. Smellie, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, England, U.K.

a2 P. Stone, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland, U.K.

Abstract

ABSTRACT The 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.

(Received October 01 1999)

(Accepted 04 2000)

KEY WORDS

  • back-arc spreading;
  • boninite;
  • island arc;
  • marginal basin;
  • island arc;
  • MORB;
  • ocean island