Early Miocene Mafic Arc Volcanism of the Patagonian Andes Revealed with Detrital Multi-Chronometer and Trace Element Geochemistry from the Magallanes Basin 50-52°S
Abstract
Sedimentary basin infill derived from mixed mafic and recycled source areas pose a challenge to resolving unique provenance using conventional methods such as detrital zircon geochronology. Here we present detrital zircon U/Pb LA-ICPMS ages in combination with 40Ar/39Ar geochronology and geochemistry data from mafic clasts collected from the early Miocene infill of the Magallanes foreland basin to understand variations in felsic to mafic igneous sources during the tectonic evolution of the Southern Patagonian Andes. Within the Magallanes Basin, modal analysis of sandstone composition and conglomerate clast lithology document an upsection increase of mafic sediment through the early Miocene fluvial Río Guillermo Formation. Several sources for this mafic sediment have been proposed, including Eocene plateau lavas, the Late Jurassic through Miocene Southern Patagonian Batholith, and the Late Jurassic Sarmiento Ophiolitic Complex, with each source requiring different or variable orogenic and erosional historieszircon U/Pb age distributions allow for all three sources to be viable contributors to the basin. However, 40Ar/39Ar dating of 7 volcanic clasts within the Río Guillermo Fm. indicate the dated clasts are predominantly Miocene in age ( 22-24 Ma) with a single Late Cretaceous age ( 84 Ma). Major element geochemistry data show that the mafic clasts are mostly basaltic andesite and have a calc-alkaline signature. High La/Ta ratios (> 36) indicate arc-derived magmas, whereas Ba/La ratios < 20 indicate a MORB component. We propose that the mafic volcaniclastic basin infill records a Miocene mafic continental arc source area within the upper volcanic levels of the Southern Patagonian Batholith, rather than either the Sarmiento Ophiolitic Complex or Eocene plateau basalts. These findings highlight the development of a more mafic end-member composition of continental arcs, possibly as a result of the thinned lithosphere and ophiolitic terranes within the Patagonian orogen host rock. We suggest that the early Miocene phase of mafic arc magmatism, which postdates Eocene backarc slab-window magmatism, reflects a return to calc-alkaline volcanism along the Southern Patagonian margin prior to middle Miocene subduction of the Chile Ridge spreading center.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T41D0340V
- Keywords:
-
- 1165 Sedimentary geochronology;
- GEOCHRONOLOGYDE: 8108 Continental tectonics: compressional;
- TECTONOPHYSICSDE: 8169 Sedimentary basin processes;
- TECTONOPHYSICSDE: 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS