Emplacement of the early Miocene Pinto Peak intrusion, Southwest Utah, USA

Abstract

[1] In this contribution, we report rock magnetic, petrographic, and anisotropy of magnetic susceptibility (AMS) data from the Pinto Peak intrusion, all of which bear on volcanic construction. Rock magnetic data indicate that the dominant magnetic mineral phase is low-Ti titanomagnetite of multidomain grain size, the composition of which varies spatially across the intrusion. The intrusion is a porphyritic andesite dominated by Ca-rich plagioclase (>60%) as well as biotite, amphibole, olivine, and opaque minerals. Reflected light petrography reveals mostly euhedral-subhedral (titano)magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals, without exhibiting a consistent mineral alignment fabric. Moderate-to-shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping/plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion. The vent facies rocks yield moderate-to-steep oblate susceptibility ellipsoids. We interpret the flow pattern in the north to reflect subhorizontal flow of magma, filling a tabular sheet-like body associated with propagation of the intrusion to the north. We argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system. The oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks. On reaching shallow crustal levels, the magma deformed and uplifted the roof rocks leading to gravitational instability. As the slide mass released from the roof, an explosive eruption ensued resulting in the emplacement of the Rocks of Paradise tuff and associated effusive lava flows. Following eruption, magma pressure decreased and the magma drained northward forming the northern intrusive phase.

Structural controls on the emplacement and evacuation of magma from a sub-volcanic laccolith; Reyðarártindur Laccolith, SE Iceland

“…Anisotropy of magnetic susceptibility (AMS) allows the rapid and precise measurement of magnetic fabrics from large sample sets (Tarling and Hrouda 1993). It is well established that magnetic lineations and foliations, particularly those reflecting the shape, orientation, and distribution of ferromagnetic minerals (e.g., magnetite), can record primary magma flow patterns in igneous bodies (e.g., Knight and Walker 1988;Archanjo et al 1995;de Saint-Blanquat et al 2006;Stevenson et al 2007a, b;Petronis and O'Driscoll 2013;McCarthy et al 2015a, b) and smaller sheet intrusions (Cañón-tapia and Chávez-Álvarez 2004;Féménias et al 2004;Philpotts and Philpotts 2007;Magee et al 2016). For example, discrete magma lobes have been interpreted within some plutons following recognition of narrow zones comprising parallel fabrics that diverge outwards along-strike and then curve inwards to define bulbous terminations (e.g., Cruden et al 1999;Stevenson et al 2007a;Magee et al 2012b).…”

Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation

“…However, contrasting interpretations of plutonic mineral fabrics as the recorders of magmatic flow or tectonic strain exist, even for similar intrusions and tectonic settings (e.g. Petronis and O'Driscoll, 2013;Tomek et al, 2016).…”