10-10 Constraints on Rates of Sill Emplacement Processes in the Shallow Crust: Buckhorn Ridge Intrusion, Henry Mountains, Utah

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

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Abstract:

The Buckhorn Ridge intrusion in Utah’s Henry Mountains provides an exceptional opportunity to place quantitative constraints on the rates of igneous emplacement processes in the very shallow crust. The intrusion is a tongue-shaped sill emplaced during the Oligocene as part of a sill / dike complex overlying the laccolithic Mount Holmes intrusive center. This dioritic, plagioclase-hornblende porphyry sill intrudes shallowly dipping sedimentary strata and its full 3-d shape is exceptionally well preserved. The intrusion has a maximum thickness of ~60 m near its emergence from dike-like feeder conduits and thins progressively until its distal termination ~1300 m away. The exceptional 3-d exposure of the sill provides a framework to estimate a wide range of emplacement characteristics of this intrusion.

Magma driving pressure calculations suggest emplacement at a depth of less than 1-2 km, consist with constraints from thermochronology, mapping, and reconstructions of Oligocene stratigraphy. Paleomagnetic analysis of the diorite suggests it was emplaced after the underlying, unexposed main laccolith body had already made space for itself by uplifting and rotating the host rock. We estimate a conservative maximum duration of emplacement for the 0.02 km³ sill of ~1 year. This estimate is based on the flux through a dike network, which is controlled by several parameters, most notably magma viscosity (melt viscosity calculated from geochemical data and adjusted to account for phenocryst abundance).  

Weak but consistent fabrics (both field-measured and magnetic) defined by phenocryst alignment are developed throughout the sill. The weak fabric development is likely related to partitioning of emplacement-related strain into phenocryst-poor intrusion margins and host rock. Quantitative estimates suggest phenocryst-poor (~5-10% by volume) intrusion margins had magma viscosity approximately five times lower than the adjacent phenocryst-rich (30-35% by volume) intrusion interior. Partitioning of strain into the low viscosity sill margins would also result in localized shear-related heating of these margins, promoting a positive feedback loop. Emplacement-related strain also produced a marginal breccia zone of magma and sedimentary host rock up to ~1 m thick. Spectacular exposures of this breccia zone exist along much of the base of the intrusion. Estimates suggest the sill tip propagated at least several meters per day and that strain rates in the marginal strain partitioning zones exceeded 10^-5 to 10^-6 per second.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10471

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