ISOTOPE CHARACTERISTICS OF A LARGE SILICIC PROVINCE: CRUSTAL VERSUS MANTLE PROCESSES IN THE GENERATION OF THE JURASSIC CHON AIKE PROVINCE (ARGENTINA)

The 230,000 km³ volume of silicic eruptions exposed throughout Southern Patagonia and the Antarctic Peninsula are formally grouped within the Jurassic Chon Aike Silicic Province (CASP). Since the 1980s, it has been advocated that the CASP is largely derived by widespread crustal anatexis during the breakup of Gondwana, yet many aspects of this hypothesis are open for discussion, largely relating to the scarcity of isotopic data within the province.

To test this hypothesis, we investigated two major silicic formations in the CASP of Southern Patagonia. We sampled primarily ignimbrites and minor rhyolitic flow from the El Quemado Complex (EQC), following a 230 km long N-S transect that extends along the eastern margin of the Andes, and the Chon Aike Formation (CA) across the western Deseado Massif (DM).

The combination of zircon in-situ techniques including oxygen isotope analysis (SIMS), and U-Pb geochronology and initial Hafnium isotope compositions (LA-ICP-MS) reveal major differences between the two formations:

(1) Zircon of the EQC is characterized by a high δ¹⁸O signature (range 7 to 9 ‰) which is complemented by negative (crustal-like) initial εHf values (-7.5); though the total range of εHf_i values range from -7.5 to -2.0. Zircon crystallization ages are consistently within error at ca. 150 Ma, thus no north – south trend can be determined. Abundant xenocrystic cores are also preserved and yield ages ranging from ~180 Ma to 2.4 Ga.

(2) Zircon of the CA is characterized by elevated δ¹⁸O values (6-8 ‰), compared to the average mantle-derived zircon value of ~5.3 ‰, and initial εHf values of -2.0 to +1.5. U-Pb ages from all units sampled in the western DM yield ~160 Ma, except for a single isolated 180 Ma rhyolitic dome. Zircon xenocrysts are relatively minor in the CA units.

Isotopic values in the youngest erupted EQC indicate a significant crustal role in the genesis of the silicic magmas. In comparison, the isotopic signature of the CA could either imply variable mixing of crustal with mantle sources and/or a crustal source which is isotopically different to that of the EQC. This variability in both oxygen and hafnium isotopes in the EQC versus the CA suggests a transition in petrogenetic processes that occurred over a ~10 My period which promoted an increase of crustal involvement during the youngest stages of the CASP magmatism.