Petrogenesis and 40Ar/39Ar geochronology of intraplate volcanism from the Juan Fernández Ridge, Nazca Plate, SE Pacific
The Juan Fernández Ridge (JFR) is an intraplate volcanic chain (~ 800 km) emplaced above the Nazca Plate in the SE Pacific, isolated from the nearby active spreading ridges (East Pacific Rise and Chile Ridge). Through geochemistry (in whole rock and crystals), isotopic data (Sr-Nd-Pb) and geochronol...
| Author: | |
|---|---|
| Format: | doctoral thesis |
| Status: | Published version |
| Publication Date: | 2018 |
| Country: | Chile |
| OAI Identifier: | oai:repositorio.anid.cl:10533/232865 |
| Online Access: | https://hdl.handle.net/10533/232865 |
| Access Level: | Open access |
| Keyword: | Ciencias Naturales Ciencias de la Tierra y del Medio Ambiente Geoquímica y Geofísica Vulcanología |
| Summary: | The Juan Fernández Ridge (JFR) is an intraplate volcanic chain (~ 800 km) emplaced above the Nazca Plate in the SE Pacific, isolated from the nearby active spreading ridges (East Pacific Rise and Chile Ridge). Through geochemistry (in whole rock and crystals), isotopic data (Sr-Nd-Pb) and geochronology (40Ar/39Ar) the aim is to understand the petrogenetic processes involved in the generation of volcanism and magmatic evolution of JFR. The four most volumetric volcanic structures of JFR: O'Higgins (~ 9.26 - 8.41 Ma), Alpha (~ 4.63 - 4.58 Ma), Robinson Crusoe (~ 4.10 - 3.40 Ma) and Alejandro Selkirk (~ 0.94 - 0.83 Ma); satisfy a younger E to W age progression consistent with the theory of mantle plumes. The shield-building stage represents almost the total volume of the studied volcanoes, and is composed mainly of basalts with geochemical (e.g., high TiO2 content, high FC3MS and TITAN anomaly) and isotopic signature (FOZO-A with additional DM contribution) that suggest the presence of pyroxenite (formed from recycled oceanic crust) as heterogeneity in the mantle source (mainly peridotites). Pyroxenites in the source are confirmed by a petrogenetic model for the JFR plume that indicates a low potential temperature (range of 1290 - 1322 ° C for Robinson Crusoe vs. 1312 - 1362 ° C in Alejandro Selkirk), pressure of melting termination (2.34 - 2.54 vs. 2.24 - 2.52 GPa) probably related to the lithosphere-asthenosphere boundary, and a similar participation in the final magma of melts originated from pyroxenites (38.6 - 56.4 vs. 35.8 - 55.6 wt%), despite its low presence (weight fraction) in the mantle source (4 - 8 vs. 6 - 12 wt%). The internal compositional variations are explained in terms of fractional crystallization of olivine + clinopyroxene ± plagioclase, mixing and/or magmatic recharge and accumulation of olivine phenocrysts in a shallow magmatic chamber (~ 1 to 3 kbar) where the temperature of the magmas can decrease to 1156 - 1181 ° C. The chemical differences between volcanoes are explained by subtle temporary variations in the potential temperature and degree of partial melting of the mantle plume. O'Higgins and Robinson Crusoe also show a stage of rejuvenated volcanism formed by basanite lava flows erupted after a maximum period of inactivity of ~ 0.25 Ma in O'Higgins, and ~ 1.73 Ma in Robinson Crusoe. Its marked geochemical enrichment and isotopic signature relatively similar to the shield-building stage confirm that it also originates from a mantle plume, but possibly with subtle variations in the proportion of its constituents (peridotite and pyroxenite), temperature and degree of partial melting (both less than the shield-building stage). These magmas rise directly from mantle (> 1300 ° C), capturing xenocrystals, with polybaric crystallization and few differentiation. Some melts are stored for brief periods in small shallow reservoirs (at temperatures ~ 1256 - 1295 ° C). |
|---|