DID KILOMETER-SCALE FLOW OF INTERSTITIAL MELT ERASE EVIDENCE OF INCREMENTAL EMPLACEMENT OF THE ALTA STOCK?

Tectonic dilation is a widely favored pluton emplacement mechanism, but thermal models indicate that the dilation rate needed to sustain a crustal magma body (>~20 mm/yr) exceeds observed continental extension rates (<5 mm/yr). A pluton emplaced by tectonic dilation thus should be an amalgamation of small short-lived intrusions, yet extensional plutons rarely display field evidence of this. New petrographic results from the Alta stock (Wasatch Mountains, Utah), which was recently interpreted as emplaced by tectonic dilation, support synkinematic intrusion and suggest a process that could have obscured evidence of a composite origin.

The Alta stock is a steep tabular granodiorite body about 2 km wide. Mafic layers near the contacts suggest at least three episodes of diking but ~90% of the stock varies only gradually. Textures and modes of 34 samples were examined and, together with published data from 30 samples, these provide fairly uniform and dense coverage. The stock has a felsic porphyritic center that grades into mafic equigranular margins, although sharp internal contacts are locally present. The margins show evidence of submagmatic deformation that corroborates other evidence for synextensional emplacement (Vogel et al., 2001): e.g., feldspar grains have undeformed rims that overgrow intracrystalline faults cutting oscillatorily zoned cores. Phenocrysts in porphyritic samples are undeformed, but their sizes, shapes, and relative abundances match major minerals of equigranular samples; the porphyritic texture and more felsic composition reflect dispersal of these crystals in an aplitic matrix that becomes more abundant inward in the intrusion. We interpret the spatial pattern of deformation, textures and modes to record expulsion of interstitial aplite melt from the pluton margins as a result of mechanical shearing when the intrusion contained less than the rheologically critical melt fraction. Much of the expelled melt infiltrated inward and impregnated the center of the intrusion. This inference is corroborated by modes of porphyritic samples, which lie on a mixing line between equigranular samples and an aplite dike near the pluton margin. Late-stage pluton-scale migration of interstitial melt may have mediated enough matrix recrystallization to obscure evidence that the stock is composite.