GSA Connects 2022 meeting in Denver, Colorado

Paper No. 171-1
Presentation Time: 9:00 AM-1:00 PM


LENHARD, Nathaniel, Springfield, MO 65897

Magma mixing is a common occurrence within volcanic systems and a potential precursor to a volcanic eruption. However, the mechanisms behind the occurrence of magmatic enclaves within a host lava remain debated. A common hypothesis to the origin of these enclaves is from either repeated injection of magma prior to an eruption, or from the disruption of a stratified magma chamber. Typically, magmatic enclaves and their host magmas differ in mineral assemblage and composition, but enclaves and magmas in the Andean Central Volcanic Zone (CVZ) are commonly similar. These similarities lead to difficulty identifying end member compositions and the hybridization of the magmatic system. This is further complicated by eruptive material from intermediate composite volcanoes in the CVZ showing influence by the crust, mid-crustal MASH zones, and subduction-derived magmas. Here, we use enclaves and host lavas from Volcán Ollagüe to decipher the magmatic history. This study utilizes the Chasca Orkho lava series which contain the greatest volume and largest sized enclaves of any eruptive stage in Ollagüe’s history. To evaluate the evolution of the Chasca Orkho series and the relationship between the enclaves and host lava, we determine the major and trace element contents with electron probe microanalyzer (EPMA) and laser ablation ICP-MS analysis on individual plagioclase phenocrysts from both the host rock and enclaves. Plagioclase textures in enclaves show sieving and resorption around the cores and mantles of phenocrysts, while lava samples show similar disequilibrium textures in mantles and rims. Trace element data shows triangular mixing trends while host lava samples have larger ranges of Sr (850-3000ppm) and Ba (130-1500ppm) compared to enclaves (Sr 1000-2000ppm; Ba 160-1000ppm). Additionally, EPMA analysis shows overlapping An content with 28%-52% An for the host lavas and 27%-68% An for the enclaves. The andesite enclaves in lavas represent a snapshot of the magmatic processes in the shallow reservoir. We suggest that a significant degree of mixing occurred between host lava and enclave samples before eruption. Our data provides evidence of a stratified magma chamber that was thrown into disequilibrium by the introduction of a new andesitic composition magma into the shallow reservoir.