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Overseas Directorate, British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, U.K.
The Velasco alkaline province consists of an early volcanic suite, a series of 14 interfering circular and elliptical alkaline plutons, numerous dykes and a silicified carbonatitic complex. The alkaline rocks, excluding the dykes, are restricted to a narrow NE-trending belt that has been traced for nearly 80 km.
The plutons can be divided into two groups defined by the silica-oversaturated or silica-undersaturated character of the rock units within each pluton. It has been possible to deduce from textural relationships and whole-rock and mineral chemistry that the two groups can be derived from a single pulaskitic magma type, in turn producible by minor fractionation of a mantle-derived trachytic parent. Sr isotopic data indicate that the trend from pulaskite through quartz syenite to granite was developed by progressive crustal contamination due to slow rates of emplacement. In contrast, the undersaturated trend towards foyaite shows no evidence of crustal influence, the plutons probably being emplaced at a greater rate. These plutons exhibit marked sub-vertical crystal layering parallel to the intrusion walls suggesting that crystal-fluid processes were responsible for the fractionation trend.
A separate strongly undersaturated magma batch is represented in the Velasco province and occurs as a melasyenitic dyke-swarm. Some dykes are strongly enriched in rare-earth elements (REE), concentrated in the mineral britholite, and indicate interaction with a hydrous phase enriched in REE.
* British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, U.K.
