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

article U- 230 Th isotopes 87 Sr/ 86 Sr lower crust assimilation 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 Volcan 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 87 Sr/ 86 Sr ratios, which suggest an initially compartmentalized system. Relatively high ( 230 Th/ 232 Th) activity ratios of ~0.72 in early-erupted calc-alkaline lavas are consistent with interaction with high-U upper crust. Low ( 230 Th/ 232 Th) 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 b2×10 4 yr. In contrast, lavas with greatest calc-alkalinity are also at or near secular

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