Depth-dependent and temporal evolution of magma addition rates and geochemical compositions in the plutonic record of continental arc crustal sections

Abstract:

Magma transfer from the mantle to the crust in magmatic arcs is an important step in the global cycling of elements and volatiles as well as heat from Earth's interior to the atmosphere. Plutonic arc crustal sections provide an opportunity to study exposed magma plumbing systems and better understand fundamental questions regarding their temporal and compositional evolution through time. In this contribution, we estimates magma addition rates (MARs) in continental arcs during flare-up periods determined from exposed proportions of igneous arc rocks, and present a detailed investigation into one continental arc section exposing magmatism from ~ 5 to 35 km depth emplaced during regional crustal shortening and thickening in the North Cascades.

We present lateral (forearc to backarc) and depth‐dependent (volcanic rocks to deep crust) arc‐wide magma addition rates calculated from three continental arc crustal sections preserving magma flare‐up periods. Volume addition increases with crustal depth and less magma is added in the forearc (~15%) and backarc (~10% to 30%) compared to the main arc. Crustal‐wide MARs for each section are remarkably similar and around 0.7–0.9 km³/km²/Ma and these flare-up rates are comparable to oceanic arc MARs.

In one of these crustal sections, the North Cascades Crystalline Core, we demonstrate that (1) the volume of magmatism increases and (2) becomes less focused into individual reservoirs with depth. Tonalitic compositions dominated at each exposed level but isotopic data indicates that the percentage of mantle contribution to arc magmas increases with time during the flare-up. MARs of three plutonic bodies emplaced at 2 to 9 kbar are relative constant and independent of depth, however, at the temporal peak of the flare-up, high MARs form a magma chamber in the mid-crust, which overcomes the regional compressional stress field to form a spherical-shaped body. Tectonic shortening and magma additions contribute to crustal thickening resulting in a Moho depth of ~ 78 km, which is interpreted to terminate of arc magmatism due to mantle wedge pinching.