Abstract
A small micromonzonite pluton intruding Miocene extrusives of the Marysvale volcanic field of southern Utah has undergone a sequence of explosive brecciation, syenite veining, rheomorphism, calcium metasomatism and zeolitization. Skarn-like occurrences of magnetite and Al-spinel-rich feldspathoidal rocks within the outcrop are of two main types: (1) an olivine-bearing variety with occasional REE-rich minerals (zirconolite) occurs as veins and seams in desilicated latite screens and xeno-liths.; (2) an aluminous, corundum- and hibonite-bearing variety with accessory zirconolite, perovskite and armalcolite occurs mainly as fillings to the micromonzonite breccias. The chemically disparate types are interpreted as having initiated as propylitic, and argillic/alunitic, alterations of latite country rock in the early hydrothermal system of the pluton. Subsequent intrusion of the micromonzonite to higher levels has incorporated these products into the igneous complex, in which they have experienced varying degrees of nephelinization, pyrometamorphism, rheomorphism and veining.
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