EMPLACEMENT STYLE OF A POST-CALDERA INTRUSION: INSIGHTS FROM STRUCTURE AND MAGNETIC ANISOTROPY OF THE ALAMOSA RIVER PLUTON (SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD, COLORADO)

The multicyclic Platoro caldera complex (~30.1–28.4 Ma) located in the Southern Rocky Mountain volcanic field (Colorado) hosts several post-collapse intrusions. The largest intrusion, the ~9 × 4 km, ~NE–SW trending subvolcanic Alamosa River pluton, was emplaced into lavas and ignimbrites of the Platoro complex. We combine field and mapping, petrography, anisotropy of magnetic susceptibility (AMS), and rock magnetic data to document emplacement mechanisms of a shallow post-caldera intrusion. The Alamosa River pluton has steep, sharp intrusive contacts, along which centimeter to meter thick, finger-like apophyses up to 15 m long intrude the host rock. The pluton contains three textural varieties: fine- to medium-grained equigranular, fine-grained microporphyritic, and coarse-grained porphyritic monzonite. Contacts between the textural varieties vary from sharp to gradational. We collected 54 AMS samples from the pluton and its host rocks. The high bulk susceptibility (1–9 × 10^–2 SI), temperature variations of low-field magnetic susceptibility, isothermal remanent magnetization, and hysteresis analyses conducted on representative specimens indicate pseudo-single domain magnetite as the dominant carrier of the AMS signal. The AMS fabrics are interpreted as recording magmatic strain during pluton emplacement. The degree of anisotropy (P) varies greatly from 0.7 to 6.8 %, and defines areal map domains of low and high P values. In addition, the shape parameter (T) is dominated by oblate AMS ellipsoids suggesting flattening strain. Most of the magnetic foliations are steep, whereas the plunge of the associated magnetic lineations vary from subhorizontal to subvertical. In general, the fabric along the pluton margins is both margin-parallel and perpendicular at different outcrops. Perpendicular fabrics tend to occur near domains where the monzonite intrudes into the host rock in form of finger-like apophyses. Moreover, the fabrics in the host rocks are discordant to those in the pluton along its margin. Our preliminary interpretation is that the Alamosa River pluton was emplaced by multiple, successive magmatic pulses. We propose an emplacement model in which magma was initially emplaced as interconnected, finger-like apophyses. Then, the pluton grew by the arrival of younger magma pulses.