90-2 Field and Modeling Constraints on the Construction of Arc Crust

Abstract:

Exposed arc crustal sections represent unique locations to study the transport, storage, and differentiation of arc magmas from (near-)Moho depths to the shallow crust, offering ‘top-down’ constraints on arc construction processes that in modern arcs are only accessible through ‘bottom-up’ geophysical methods. Several well-studied paleo-island and -continental arc crustal sections exist, and they all show similar first-order structure: a lower crust dominated by mafic cumulates, and a relatively sharp transition to an intermediate to felsic middle and upper crust dominated by granitoid intrusions. Metasediments and other material representing either the pre-existing crust on which the arc was constructed, or tectonically added material, are present in nearly all crustal sections but constitute at most ~30% of the exposed middle and lower crust. Here I will examine models for the construction of arc crust motivated by these sections, focusing on the Bear Valley Intrusive Suite (BVIS), a continuous crustal section spanning from >9 to <3 kbars in the Southern Sierra Nevada Batholith (California). High-precision zircon geochronology shows that this entire section was constructed in <1.5 Myr, making the BVIS one of the few crustal sections where contemporaneous intrusions can be examined across all crustal levels.

Specifically, I will use the BVIS to examine two frequently discussed topics in arc magmatism: 1) trans-crustal, mush-dominated magmatic systems, and 2) magmatic flare-ups. First, it is increasingly thought that mush-dominated magmatic systems are the norm, but the transport of magma through these systems at crustal scales is not well understood. Construction of the BVIS likely required rapid transport of differentiated magmas from the lower to middle crust. I will examine possible mechanisms for this transport within a mush-dominated crustal section. Second, the transition from felsic plutons to mafic cumulates occurs at a shallower level in the BVIS than in the rest of the Sierra Nevada Batholith. This likely results from elevated magmatic fluxes during a flare-up, which raised lower crustal temperatures and allowed mafic magmas to ascend to shallower levels before stagnating and crystallizing. Together, these ‘top-down’ results demonstrate how field, geochronologic, and geochemical data from exposed arc crustal sections constrain the rates and mechanisms of arc construction and can be directly compared with ‘bottom-up’ geophysical images to build a more complete picture of arc crust assembly.

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