Formation of continental crust in a temporally linked arc magma system from 5 to 30 km depth: ~ 90 Ma plutonism in the Cascades Crystalline Core composite arc section

Abstract:

Continental crust has an andesitic bulk composition with a mafic lower crust and a granodioritic upper crust. The formation of stratified continental crust in general and the vertical extent of processes active in arc crustal columns leading to the differentiation of primitive, mantle-derived melts entering the lower crust are highly debated. To investigate where in the crustal column magma mixing, fractionation, assimilation and crystal growth occur and to what extent, we study the ~ 90 Ma magmatic flare-up event of the Cascades arc, a magma plumbing system from ~ 5 to 30 km depth. We focus on three intrusive complexes, emplaced at different depths during major regional shortening in an exceptionally thick crust (≥ 55 km¹) but which are temporally related: the upper crustal Black Peak intrusion (1-3 kbar at 3.7 to 11 km; ~ 86.8 to 91.7 Ma²), the mid-crustal Mt. Stuart intrusion (3.5-4.0 kbar at 13 to 15 km; 90.8 and 96.3 Ma³) and the deep crustal Tenpeak intrusion (7 to 10 kbar at 25 to 37 km; 89.7 to 92.3 Ma⁴). These intrusive complexes are well characterized by geochronology showing that they have been constructed incrementally by multiple magma batches over their lifespans and thus allow the monitoring and comparison of geochemical parameters over time at different depths. We use a combination of whole rock major and trace element data and isotopes combined with detailed investigation of amphibole, which has been recognized to be important in the generation of calc-alkaline rocks in arcs to test the following hypotheses: (a) compositional bimodality is produced in the lower crust, whereas upper crustal levels are dominated by mixing to form intermediate compositions, or (b) differentiation occurs throughout the crustal column with different crystallizing phases and their compositions controlling the bulk chemistry.

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2. Shea 2014: PhD thesis, Massachusetts Institute of Technology

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