EMPLACEMENT OF THE IRON MOUNTAIN LACCOLITH, SW UTAH, REVEALED BY ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND FIELD MAPPING STUDIES

Iron Axis magmatism in SW Utah consisted of intrusion of early Miocene (22-20 Ma) calc-alkaline hypabyssal laccoliths and associated volcanism immediately west of the present Colorado Plateau. Laccolith groups on the Colorado Plateau (e.g., Henry and La Sal Mountains) consist of distinct central intrusions from which numerous smaller intrusions radiate, whereas laccoliths of the Iron Axis are single, dome-like intrusions distributed along a NE-SW linear trend, most of which are spatially associated with a NE-SW striking, NW dipping Sevier thrust fault. We sampled 36 Anisotropy of Magnetic Susceptibility (AMS) sites at the Iron Mountain laccolith, with an average of 13 independent samples per site, near uniformly distributed through the intrusion at a spacing of ~ 0.5 km to 1 km between sites depending on exposure. Our intent was to distinguish the mode of emplacement by evaluating the magnetic fabric pattern as well as structural relations between the igneous and country rock contacts. All contacts examined between the intrusion and country rock are either steeply dipping (> 70°) away from the intrusion or faulted. AMS results reveal an orderly pattern of magnetic susceptibility ellipsoids. The mean susceptibility is high (9.93 x 10-3 SI) consistent with a ferromagnetic (s.l.) mineral as the principal magnetic phase. The average corrected degree of anisotropy is low 1.033 (3%), but not atypical for these materials. The shape parameter ranges from strongly prolate (-0.811) to moderately oblate (0.477). Along the margins, the magnetic foliations (K1-K2) are steeply dipping towards and away from the contact with the country rock with magnetic lineations (K1) plunging steeply within the K1-K2 planes. The magnetic fabrics within the central part of the intrusion yield very shallow plunging K1s and variably dipping K1-K2 planes. The preliminary AMS fabric data reveal a pattern consistent with an overall dome-shaped intrusion. We envision a scenario of rapid ballooning of the intrusion during emplacement. This emplacement mechanism adequately explains the faulted and steeply dipping contacts with the country rocks. A similar dome-shaped geometry is revealed by AMS data from the Three Peaks and Granite Mountain laccoliths 20 Km NE of Iron Mountain.