Shallow and deep crustal control on differentiation of calc-alkaline and tholeiitic magma

The role of changing crustal interaction and plumbing geometry in modulating calc-alkaline vs. tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath Volcán Parinacota. Changes in petrologic affinity correlate with periods of cone-building, sector collapse, and rebuilding of the volcano over the last 52 ka, and are well explained by changes in magma recharge regime. With increasing recharge and magma output, lavas transition from low-Fe, strongly calc-alkaline, phenocryst-rich silicic compositions to medium-Fe, near-tholeiitic, mafic, and aphanitic characteristics. Strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously; these are characterized by distinctive ⁸⁷Sr/ ⁸⁶Sr ratios, which suggest an initially compartmentalized system. Relatively high ( ²³⁰Th/ ²³²Th) activity ratios of ~ 0.72 in early-erupted calc-alkaline lavas are consistent with interaction with high-U upper crust. Low ( ²³⁰Th/ ²³²Th) activity ratios of ~ 0.55 and up to 33% Th-excess in younger near-tholeiitic lavas correlate with steep REE patterns, indicating lower-crustal interaction. Thorium-excesses at the time of eruption approach the maximum that can be generated via small-degree garnet-residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short, on the order of < 2 × 10 ⁴ yr. In contrast, lavas with greatest calc-alkalinity are also at or near secular equilibrium, suggesting stagnation times > 3 × 10 ⁵ yr in the upper crust. In addition to more traditional explanations tied to magma source, expression of low-Fe 'calc-alkaline' (CA) vs. medium-Fe 'near-tholeiitic' (TH) magma series at the scale of individual volcanoes is likely to be modulated by transitions from compartmentalized, stagnant, assimilation-prone 'dirty' systems (CA) to 'clean' systems (TH) that are characterized by rapid magma throughput and minimal opportunity for upper-crustal contamination.