DOCUMENTING THE TRANSITION FROM ARC FLARE-UP TO TYPICAL ARC MAGMATISM AT THE PURICO-CHASCON VOLCANIC SYSTEM, NORTHERN CHILE: A CRYSTAL-SCALE PERSPECTIVE

Since the mid-Miocene, volcanic activity in the central Andes has occurred in two distinct evolutionary stages. From 10-1 Ma, high volcanic flux rates associated with a regional magmatic flare-up resulted in the eruption of numerous voluminous ignimbrites from a series of multi-cyclic caldera complexes. From 1 Ma to recent, volcanic output has decreased, and eruptions have become significantly smaller and less explosive, concentrated at stratovolcanoes located along the current arc front. In this study, we discuss the transition from flare-up, caldera-dominated magmatism to typical arc magmatism as recorded in crystals from the Purico-Chascon volcanic complex, a long-lived volcanic center in northern Chile.

Volcanism at the Purico-Chascon volcanic complex initiated 1.0 Ma with the eruption of the 60-80 km³ Purico ignimbrite. Following the ignimbrite eruption, activity became significantly less explosive resulting in a series of intermediate lava domes and flows (10 km³ total), the youngest of which (Cerro Chascon) erupted 200 ka. Plagioclase crystals from the Purico ignimbrite have high, restricted ⁸⁷Sr/⁸⁶Sr isotope ratios (0.7085-0.7089) that are similar to whole-rock isotope ratios from the same unit. Cerro Chascon contains similar high ⁸⁷Sr/⁸⁶Sr plagioclase crystals, but also contains a second type of plagioclase with significantly lower ⁸⁷Sr/⁸⁶Sr ratios (0.7060-0.7072).

Thermobarometric calculations based on phase equilibria indicate that crystals within the Purico ignimbrite grew in a single, relatively low temperature, (800-875 °C) upper crustal (100-200 MPa; 4-8 km) magma reservoir. In contrast, crystals from Cerro Chascon appear to be derived from two distinct magmas. Some crystals grew in upper crustal conditions identical to crystals from the Purico ignimbrite, whereas others appear to be derived from a much deeper (350-550 MPa; 12-20 km deep), higher temperature (950-1050 °C) magma source. In addition, olivine crystals (Fo_81-85) within Cerro Chascon yielded olivine-melt temperatures of ~1200 °C, consistent with an even deeper source, possibly within the lower crust. We believe the observed intra-crystalline isotopic variations and changes in magma generation and storage reflect changes in the thermal structure of the crust in response to the waning of the flare-up.