| Abstract: | Three distinct types of pillows and pillow lava sequences with different modes of origin have been recognized in the extrusive sequences comprising the upper parts of ophiolite complexes that represent the mafic portion of the floor of an Early Cretaceous back-arc basin in southern Chile. One type of pillow formed by non-explosive submarine effusion. A second type formed by magmatic intrusion into pre-existing aquagene tuff formed by explosive eruption. The third type of pillow occurs within dikes, forming pillowed dikes, possibly as a result of vapor streaming within a cooling dike. Where studied in southern Chile, aquagene tuffs and intrusive pillows decrease and water-lain pillows increase in relative abundance from north to south. This variation corresponds with a north-to-south decrease in both the relative volume of extrusives to extensional dikes and the range and volume of differentiated rocks, suggesting a southward increase in rate of extension relative to rate of magma supply within the spreading ridges at which the ophiolites formed. In the northern part of the original basin where the rate of extension was small relative to the rate of magma supply, magma remained in magma chambers longer, resulting in a greater range and volume of differentiated rocks. The larger volume of more differentiated, cooler and more viscous magmas, in conjunction with the likelihood of more violent eruption of volatile-rich differentiates, may have been responsible for the large volume of aquagene tuff in the northern part of the original basin. These observations in southern Chile suggest that ophiolites which contain a great abundance of aquagene tuffs and intrusive pillow lavas formed in tectonic environments in which the rate of extension was small relative to the rate of magma supply (island arcs, embryonic marginal basins). Ophiolites with predominantly water-lain pillowed and massive lavas formed in tectonic environments in which the rate of extension was large relative to the rate of magma supply (mid-ocean ridges, mature back-arc basins). Thus geologic field data may supplement geochemical data as a tool in distinguishing the original igneous-tectonic environments in which ophiolites originate. |
