Paper No. 8
Presentation Time: 10:20 AM


GIRARDI, James D. and BARTON, Mark D., Dept. of Geosciences, University of Arizona, Tucson, AZ 85721,

Mesozoic magma fluxes in the Coastal Batholith of the central Andes reveal a history of episodic magmatism characterized by broad changes in the bulk composition of the arc and changes in plutonic fluxes by as much as a factor of 5. Estimates of magmatic flux were determined through the analysis of more than 400 published radiometric ages and 56 new U-Pb ages, detailed mapping along a transect at ~27oS and spatial analysis of igneous rocks between 26oS - 28oS, and comparison with published data between 18oS - 30oS. Plutonic fluxes were calculated at 10, 5, and 3 My average intervals to establish different temporal patterns and possible magnitudes of plutonic flux. Four episodes of magmatism, 240 - 230 Ma, 200 - 190 Ma, 160 - 100 Ma, and 70 - 60 Ma reached peak plutonic fluxes of 10 - 60 km3per km of strike length per My, with intervening lulls of relative quiescence. Triassic through Early Jurassic plutons were mainly granites and granodiorites, while Early Cretaceous plutons were dominantly quartz diorites and diorites, and latest Cretaceous plutons were mainly granodioritic. Changes in pluton fluxes and compositions correlate with tectonism including back-arc extension, basin inversion, and trans-tensional episodes.

Comparison of the Coastal Batholith with Mesozoic batholiths of the southwestern United States and Canadian Cordillera reveals a similar pattern of episodic magmatism, although the tectonic setting, timing, fluxes, and compositions vary. During orogenic periods, analogous to the modern Andes, the North American batholiths display roughly double the observed plutonic fluxes and compositions become more felsic (tonalitic to granitic) in composition. In contrast, the Chilean Coastal Batholith evolved toward more mafic and juvenile magmas as flux increased into the Early Cretaceous. This distinctive evolutionary pattern also characterizes early phases of Cordilleran magmatism elsewhere, for example in the Jurassic arc of the southwestern United States.