THE COASTAL BATHOLITH OF PERU: INSIGHTS FROM STABLE ISOTOPES

The Cretaceous-Paleocene Coastal Batholith of Peru formed during the subduction of the Farallon plate under South America as part of the ongoing Andean Orogeny. This >2000 km-long batholith was emplaced at crustal depths <5 km, making it highly susceptible to alteration by hydrothermal systems involving surface-derived water, including seawater and meteoric-hydrothermal water. We present stable isotope data from numerous gabbro, K-rich monzonite, granodiorite, tonalite, and granite plutons of this 184 to 58 Ma batholith that intruded into Precambrian gneiss at the Arequipa segment in the south and into a Jurassic-Cretaceous volcano-sedimentary basin at the Lima segment in the north. Mineral δ¹⁸O values from >60 samples indicate variable magma sources and document hydrothermal alteration. Equilibrium quartz, plagioclase, hornblende, and biotite δ¹⁸O values at T>550°C indicate the preservation of primary magmatic values for most rocks. Most plutons produced quartz and plagioclase δ¹⁸O values (<9‰) that indicate a mantle magma source. Elevated δ¹⁸O values (>9‰) that suggest significant crustal contamination are observed at only a few plutons within the Arequipa segment, consistent with published ⁸⁷Sr/⁸⁶Sr_initial values for such rocks. Perthitic and “sieve” textures in K-feldspar and zoned plagioclase indicate shallow emplacement and fractional crystallization. Alteration minerals include sericite, epidote, chlorite, and actinolite. Non-equilibrium hornblende and biotite δ¹⁸O values from the older plutons indicate infiltration of seawater in the north and meteoric-hydrothermal water in the south. Hydrogen isotope results are inconclusive for the southern Arequipa segment due to the overlap between mineral δD values (–80 to –50‰) formed due to exchange with low-latitude meteoric-hydrothermal waters and magmatic waters, but high δD values (>–50‰) from epidote alteration in the Lima and northern Arequipa segments indicate a seawater-dominated hydrothermal system for this part of the batholith. Neither temporal nor spatial trends are observed in primary magmatic δ¹⁸O values. This indicates a stable continental arc system involving dominantly mantle-derived magma over a span of >120 MY.