Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite-actinolite rocks and jadeite-gneisses of t he blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE). The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in t he magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite-actinolite rocks. They were created by low degrees of partial melting of very primitive magma s, akin to spinel-peridotites and have affinities to boninites, probably th rough melting of the peridotitic mantle wedge. Tholeiitic basalts and andes ites with N-MORB affinity, especially in their REE-patterns, were then prod uced by partial melting, possibly in an embryonic back-arc basin. These roc ks were the protoliths of the blueschists of Siphnos. Their enrichment in s ome LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses s how affinities to MOR-granites and volcanic arc granites, intrusion of thei r protoliths in a back-arc environment is likely. The protoliths of the qua rtz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the gl aucophane-jadeite gneisses were andesites. The proposed geodynamic model, solely based on geochemical data, is consist ent with geochemical data from neighbouring islands, though those rock unit s show much higher chemical variability. Consistent with geotectonic models , which are based on structural and geophysical data, the volcanic protolit hs of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an is land-arc, and the evolution of a back-are basin. (C) 2001 Elsevier Science B.V. All rights reserved.