Abstracts of Other Conference Presentations

Abstract

Many physical (structural and textural) and compositional (chemical and mineralogical) attributes of volcanic rocks are correlated with techtonic setting. However, only a few casual relations are known. Modern subduction-related volcanic rocks are persistently highly porphyritic and dominantly fragmental, especially if subaerial. These features are expected foe magmas rich in H₂O, because ascending magmas become supercooled by effervescence leading to crystallization and explosive eruption. The high H₂O is plausibly related to subduction of cool lithosphere rich in hydrous minerals. Has subducting lithophere always been cool and and hydrous? Deep submarine extrusions of identical magmas would be less porphyritic, less vesicular and not so explosive. Variable vesicularity and associated sulfide deposits could, however, help reveal original depths of extrusion. Magnesian olivineis stabilized by H₂O in siliceous melts; consequently, olivine in blocky and/or fragmental (highly viscous) volcanic rocks in suggestive of a hydrous magma. Nearly all tholeiitic basalts crystallize cpx before opc at low pressures. Anomalous early opx in them seems best explained by siliceous contamination. But how do we account for Mauna Loa? Granitic contamination may deplete basalt in Na₂O with consequent reversal of plagioclase zoning. Little altered oceanic ridge basalts have diagnostic chemical compositions, but most old rocks are strongly altered. Reclosure of the Atlantic would likely preserve parts of Iceland and Azores. How would we interpret them?