OXYGEN ISOTOPE CONSTRAINTS ON CRUSTAL MELTING: SIMS DATA FROM JURASSIC RHYOLITES OF THE CHON AIKE PROVINCE (PATAGONIA, ARGENTINA)

Silica-rich granites and rhyolites can be found in many tectonic settings, and the origin of silica-rich magmas remains an important topic in igneous petrology. We report on a suite of Jurassic rhyolites (El Quemado Complex) in Patagonia. We use the oxygen composition of quartz and zircon - both from SIMS and laser fluorination analysis – to have a closer look at the role of crustal contributions to the production of rhyolites in the Chon Aike Province, one of the largest silicic igneous provinces known.

The peraluminous rhyolites and ignimbrites are characterized by a high oxygen isotope signature for both zircon (7.5‰ to 10.1 ‰) and quartz (10.8‰ to 12.5‰). This requires a significant contribution of supra-crustal (i.e., sedimentary or pelitic) material during rhyolite formation in contrast to previous work¹ who argued for smaller (< 20%) contribution from a mafic lower crust. Another key observation is that zircon and quartz are not in high temperature isotopic equilibrium. It is then often assumed that quartz has been reset due to late stage processes². We carried out a series of SIMS profiles across several quartz phenocrysts with magmatic cathodoluminescence zoning. Profiles are mostly flat and pronounced enrichment along cracks are not evident. The SIMS data illustrate that the ¹⁸O composition of quartz is not influenced by secondary alteration in this case. Large ¹⁸O variation in zircon from single samples (up to 2.6 ‰) indicate either a heterogeneous source, an assembly of isolated magma batches or extraction of zircons together with interstitial melt from a crustal mush.We recognize that widespread crustal anatexis seems difficult to achieve due to the high thermal energy required, yet our data support partial melting of ¹⁸O-rich crust to form the rhyolites of the El Quemado Complex, reflecting the large-scale lower crustal reworking associated with the special tectonic and igneous setting of the Gondwana break-up.

¹Pankurst & Rapela (1995): EPSL 134, 23–36 

²King et al. (1997): Geology 25 (12), 1079–1082