Formation of Pristine Asthenospheric Partial Melts Within Subduction-Dominated Volcanic Successions: Comparison Between Recent and Paleoproterozoic Sequences of Argentina and Canada

VAN WAGONER NA; HALLER MJ; LEYBOURNE M

Foz do Iguassu

Congreso; 2010 AGU Meeting of the Americas; 2010

American Geophysical Union

A number of hypotheses have been proposed to explain the occurrence of volcanic rocks that represent the partial melts of relatively pristine asthenosphere within volcanic successions dominated by rocks formed proximal to destructive plate margins and characterized subduction zone (SSZ) geochemical signatures. We examine lithogeochemical data from two such examples separated spatially and temporally as a way of considering magmatic processes; the young (< 15 Ma) Patagonia region of southern Argentina and a Paleoproterozoic sequence (~1.9 Ga Amisk Lake) of Canada that is part of the Flin Flon belt of the Trans-Hudson Orogen. In the Patagonia region, young volcanism is characteristically mafic and monogenetic. With distance from the active arc, volcanic rocks become increasingly ocean island basalt (OIB)-like, and progress from rocks with calc-alkalic and strong SSZ influences (e.g., negative Nb, Ta, Ti anomalies, La/Nb > 1, elevated LILE/HFSE), to alkaline rocks with slight Nb-Ta enrichment compared to primitive mantle, La/Nb < 1, and low LILE/HFSE. The preferred interpretation for the alkalic rocks in the back-arc region of Patagonia is that progressive subduction of the Chile Ridge has produced slab-windows through which sub-slab asthenosphere can rise, thereby undergoing decompression partial melting. In contrast to the primarily surficial exposures of the younger Patagonia volcanics, the deformation of the 1.9 Ga Amisk succession has exposed a 10 km-thick volcanic sequence dominated by tholeiitic basalts to basaltic andesites, boninitic basalts, and calc-alkaline to shoshonitic basalts and basaltic andesites. Geochemically, these rocks are characteristic of those found in a modern island arc to back-arc setting. Within the middle of the sequence are atypical rocks characterized by slight LREE enrichment, and primitive mantle normalized signatures with enrichment at Nb-Ta and Ti. These Nb-enriched rocks have low La/Nb (< 0.35) and only slightly LREE-enriched patterns ([La/Yb]n = 1.2 to 2.5). Similar to many of the alkalic rocks of southern Patagonia, Nd isotopic values of the Amisk Nb-enriched, non-SSZ volcanic rocks suggest derivation from depleted mantle, and values overlap the SSZ rocks (e.g., åNd = +4 ± 1.3). However, in contrast to the Patagonia alkalic back-arc rocks, the Nb-enriched Amisk rocks are tholeiitic and garnet was not retained in the source of these melts. There is therefore no compelling reason to invoke a slab-window to explain the Nb-enriched rocks; instead these rocks are more compatible with a model in which the source was rutile-rich, most consistent with partial melting of the subducted slab.