Flux Modulated Magmatic Architecture in Continental Magmatic Arcs

Charles Lewis , Shan de Silva

Petrochronology of the magmatic reservoir that underpinned the Chaxas Volcanic Complex for the duration of its 8.59 – 1.24 Ma lifecycle reveals that magmatic architecture can be regulated by magmatic flux. Modulation of the magmatic reservoir was controlled via two distinct flux regimes during the Pre-Chaxas and Chaxas Complex stages of the Chaxas Volcanic Complex, emphasizing the dynamic nature of magmatic systems that characterize continental magmatic systems. Although Pre-Chaxas eruptions began at 5.44 Ma, intrusions were emplaced as early as 8.59 Ma during the waxing stage of the APVC flare-up. The Pre-Chaxas stage climaxed with the APVC flare-up, resulting in catastrophic eruption of the Puripicar Ignimbrite (PPI) at 4.18 Ma. The overdispersed zircon age spectrum in the PPI spans several Myr, recording the long episodic history of the composite Pre-Chaxas system. Thermal models predict that the preceding 500 kyr episode to the PPI occurred under a magmatic flux of >5 × 10-3 km3/yr with a total intrusive volume of ~ 5300 km3, leaving approximately 4800 km3 of remnant mush. High Zr/Hf zircons in evolved Pre-Chaxas eruptions record assimilation of crustal material during this stage. Starting at 3.54 Ma, smaller predominantly effusive eruptions built the Chaxas Complex. Age spectra from the Chaxas Complex reveal shorter time domains within discrete melt batches injected into Pre-Chaxas cumulates. Thermal modeling suggests maximum fluxes of <7 x 10-4 km3/yr and ~ 1500 km3 of intrusions. The petrochronologic record from the Chaxas Volcanic Complex demonstrates that the construction of composite plutons over millions of years is the result of dynamic magmatic systems that respond to changes in magmatic flux.