INSIGHTS INTO MAGMA EMPLACEMENT RATES INTO THE SHALLOW CRUST FROM THERMAL MODELING AND THE PALEOSECULAR VARIATION RECORD, HENRY MOUNTAINS, UTAH

The cooling rate of sheet-like intrusions in the shallow crust is a function of magma sheet thickness, temperature of emplacement, country rock temperature, and the thermal diffusivity of the earth materials. A simple, one-dimensional model with a low thermal diffusivity and high emplacement temperatures provide a first-order estimate of the maximum amount of time to cool below a given temperature. These models suggest thin sills (<50 m thick) emplaced into the shallow crust (country rock temperature <250˚C = <10 km depth with a geothermal gradient of 25˚C/km) cool below 585˚C (blocking temperature of magnetite) in <100 years. In contrast, thicker sills (>200 m) at the same conditions would take >1000 years to cool below 585˚C. Thin intrusions in to the shallow crust can therefore potentially capture the position of the virtual geomagnetic pole (VGP) at their time of emplacement, while the cooling time for thicker intrusions is long enough to average out some of the movement of the VGP. Preservation of the paleosecular variation record – i.e. record of movement of the VGP with time – in shallow intrusions means that their paleomagnetic record may preserve information about the length of time elapsed during emplacement. To test this hypothesis, we collected paleomagnetic data from three sheet-like intrusions of varying thickness in the Henry Mountains of Utah: Maiden Creek sill (approx. 20 m), Trachyte Mesa laccolith (approx. 50 m), and Copper Ridge laccolith (approx. 300 m). Thermomagnetic susceptibility and hysteresis analysis indicate pseudosingle domain magnetite is the primary carrier of the paleomagnetic signal. Alternating field (AF) demagnetization indicates these diorites are characterized by one- or two-component signals, where the high coercivity component is interpreted to record the thermoremnant magnetization (TRM). In all three cases, the intrusions record a significant amount of secular variation, indicating that we can reject the hypothesis that they were emplaced as a single pulse of magma. Instead, the data suggest it likely took at least centuries for each of the intrusions to be emplaced. This implies even geometrically simple sheet intrusion can have complex, pulsed emplacement histories.