T139 Fifty Years of Innovation in Petrology and Orogenic Systems II: A Tribute to Lincoln Hollister

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 205

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

Paper number: T139-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

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

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286476

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