285-5 Modeling Mushes: Linking Compaction and Crystallization to Simulate Modal Variation in a Zoned Granitic Pluton

Abstract:

The Whitney pluton of the eastern Sierra Nevada is dominated by two major phases: a relatively coarse-grained interior phase and a finer-grained marginal phase that contains lower abundances of ferromagnesian minerals and quartz but a higher abundance of alkali-feldspar. Because many granitic rocks like these are interpreted as cumulates (Barnes et al., 2019) their compositions are best modeled as mixtures of early-formed crystals with later phases precipitated from trapped interstitial melts. To simulate how the modal differences between the interior and marginal phases of the Whitney pluton might have arisen, models for compaction and fractional crystallization were combined in a series of discrete steps that made it possible to track the compositions of crystal assemblages, melts, and mushes (magmas with ≥70 vol. % crystals) as an initially homogeneous intrusion solidified.

The simulation begins when a 5 km-thick model intrusion cools to 50 vol. % crystallinity and ceases to mix convectively. Crystals in the bottom 1 km of this intrusion then compact to 70% crystallinity and trap 30% interstitial melt. The remaining melt from this compacted layer is expelled upward where it lowers crystallinity in the uncompacted part of the intrusion and re-initiates convective mixing. When magma in the upper part again reaches 50% crystallinity, the cycle repeats. At the end of the simulation the pluton is made up of six compacted layers capped by a 714 m-thick layer that crystallizes and convects until it reaches 50% crystallinity, after which it solidifies without compaction.

 The crystal assemblages and mushes in the first six layers define overlapping tracks on a Quartz-Alkali-feldspar-Plagioclase diagram that trend away from plagioclase towards a melt of nearly constant composition. This composition is initially buffered by the large volume of melt in the upper part of the intrusion, but as the convecting region shrinks melts expressed from the underlying compacted layers drive the remaining melt away from quartz and towards alkali-feldspar. The uppermost layer of the intrusion, which is a 50:50 mixture of this alkali-feldspar rich-melt and a relatively plagioclase-poor crystal assemblage, has a modal composition that is offset from earlier mushes in the same sense that the marginal phase of the Whitney pluton is offset from its interior phase.

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