Title

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Document Type

Article

Publication Date

1-1-2001

Abstract

Neogene plateau lavas in Patagonia, southern Argentina, east of the volcanic gap between the Southern and Austral Volcanic Zones at 46·5° and 49·5°S are linked with asthenospheric slab window processes associated with the collision of a Chile Ridge segment with the Chile Trench at 12 Ma. The strong ocean-island basalt (OIB)-like geochemical signatures (La/Ta <20;; Ba/La <20; 87Sr/86Sr = 0·7035-0·7046; 143Nd/144Nd = 0·51290-0·51261; 206Pb/204Pb = 18·3-18·8; 207Pb204Pb = 15·57-15·65; 208Pb/204Pb = 38·4-38·7) of these Patagonian slab window lavas contrast with the mid-ocean ridge basalt (MORB)-like, depleted mantle signatures of slab window lavas elsewhere in the cordillera, (e.g. Antarctic Peninsula; Baja California). The Patagonian lavas can be divided into a voluminous ∼ 12-5 Ma, tholeeitic main-plateau sequence (48-55% SiO2; 4-5% Na2O 〈 K2O) and a less voluminous ∼ 7-2 Ma alkaline post-plateau sequence (43-49% SiO2; 5-8% Na2O 〈 K2O). Moderately high FeOT(9-11%), and low heavy rare earth element (HREE), γ, and Sc concentrations in all lavas are consistent with melt generation just below the garnet-spinel transition at a depth of ∼ 70 km. The main-plateau lavas from the western back-arc can be modeled by ∼ 10-15% partial melting of an OIB-like asthenospheric mantle source with additions form slab fluid-melt components coupled with crustal contamination (AFC). A three-stage petrogenetic model is envisaged: (1) decompression melting and source region contamination of an OIB-like subslab asthenospheric source by slab melts of the trailing edge of the subducted Nazca Plate; (2) minor contamination of slab window melts with arc components 'stored' in the supraslab mantle wedge and/or basal continental lithosphere; (3) further modification by addition of crustal components during magma ascent. The main-plateau lavas from the eastern back-arc can be modeled by ∼ 7% partial melting of the same asthenospheric source as the influence of arc components diminishes and the intensity of mantle upwelling into the slab window decreases. All post-plateau lavas can be modeled as 1-4% partial melts of the pristine OIB-like asthenospheric source in the widening slab window.

DOI

10.1093/petrology/42.6.1067

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