Source contamination, crustal assimilation, and magmatic recycling during three flare-up events in the Cretaceous Peruvian Coastal Batholith: An example from the Ica-Pisco plutons

Abstract

Source contamination, crustal assimilation and magmatic recycling of older and coeval magmatic materials can potentially have dramatic effects on magma petrology, emplacement and thermal histories, as well as magma addition rates and crustal growth. To establish sources and magnitudes of assimilation and recycling processes in the Ica-Pisco area of the Peruvian Coastal Batholith (PCB), we use data at different scales collected from 125 field localities, 124 thin sections, 10 new U-Pb zircon ages, and 117 elemental and 49 isotopic whole rock analyses.Our new U-Pb zircon ages for the northern Arequipa segment of the PCB document three magmatic flare-up events over 80 m.y. (ca. 136-56 Ma), younging toward the northeast: pre-PCB gabbros at ~136 Ma; the Quilmana volcanics and the Linga, Pampahuasi, and Tiabaya plutons as part of a flare-up at ~120-80 Ma; and a final Incahuasi flare-up event at ~70-56 Ma. Zircon geochronology of xenocrysts and antecrysts provides a record of recycling from isotopically evolved materials, as well as from earlier magmatic events.Field observations reveal widespread evidence for mass transfer processes of older volcanic and plutonic units into younger plutons over a range of scales: (1) host block subsidence and disaggregation at the kilometer-scale during cauldron subsidence; (2) widespread stoping at the meter-scale; and (3) mechanical and chemical disaggregation of xenoliths at the centimeter-scale. Observations at these scales indicate that generally 1–10% of host rock assimilation and 1–5% of magmatic recycling occurred. The high alkalinity in the Linga, Pampahuasi, and Incahuasi plutons is associated with contamination from high-K materials at the magma source and minor crustal contamination at emplacement levels. The Sr/Y ratio and REE whole rock geochemistry indicate that the youngest inboard plutons have a deeper magma source, suggesting a thicker lower crust with greater possibility for assimilation during magma ascent. Sr, Nd, and Pb isotopes indicate a significant contribution from mantle melting, a moderate contribution from melting of subducted Pacific Ocean sediments, and minor assimilation from lower crustal Precambrian and Paleozoic gneisses during magma ascent and emplacement. Assimilation and fractional crystallization (AFC) calculations with Sri and Sr suggest that the Ica-Pisco plutons are composed of at most 25% recycled Pacific Ocean sediments. In summary, the composition of the Ica-Pisco plutons appears to be derived from about 65 to 70% mantle melts and 30–35% isotopically evolved material that is composed of Pacific Ocean sediments, arc and basement crust. Geochemical variations from older to younger plutons show an initial decrease in the mantle component, a general increase in the isotopically evolved component, and an initial increase and later decrease in the volcanic and plutonic host rock components.