| Popis: |
Supra-subduction zone (SSZ) ophiolites are being recognized more and more frequently in the geologic record. Submarine volcanism in intra-oceanic arcs is one tectonic setting in which such ophiolites might form. The Southern Seamount Province of the Mariana Arc represents the best documented example of an active intra-oceanic submarine arc which can serve as a modern analogue for some SSZ ophiolites. The Southern Seamount Province comprises nine submarine volcanoes, at least seven of which are active or dormant. These edifices have erupted plagioclase-clinopyroxene-olivine-phyric basalts, andesites, dacites and dacitic pumices. These lavas are similar to those of the subaerial edifices in the central Marianas (SiO2 = 49–70%, TiO2 = 0.4−1.3%, MgO = 1.2–5.9%, Ba = 140–450 ppm, 87Sr/86Sr = 0.70346−0.70354), but are somewhat more enriched in the light rare-earth elements. The geochemical characteristics of subaerial Mariana Arc lavas are preserved in the submarine lavas, and the SSP lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices. We are ignorant of the composition of any lavas erupted between the principal edifices, which may be expected to dominate the volcanic section of SSZ ophiolites. Nevertheless, the characteristic high ratio LIL/HFSC, low Ti, Ni, and Cr, and diagnostic mineral compositions (calcic plagioclase, relatively Fe-rich olivine) are likely to be preserved in unaltered submarine arc lavas. The Mariana active arc is built on back-arc, or thinned frontal arc, crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic, hypabyssal, and volcanic rocks of back-arc, frontal arc, and active arc origin. Such complicated intrusive relationships, if preserved in structurally disrupted ophiolites, will make it difficult to reconstruct the geologic history of an SSZ ophiolite. |
