Ž . Lithos 57 2001 67–89 www.elsevier.nlrlocaterlithos Slab window-related magmatism from southernmost South America: the Late Miocene mafic volcanics from the Estancia ž / Glencross Area ; 528S, Argentina–Chile M. D’Orazio a, ) , S. Agostini a , F. Innocenti a , M.J. Haller b , P. Manetti c , F. Mazzarini d a Dipartimento di Scienze della Terra, UniÕersita di Pisa, Õia S. Maria, 53, 56126 Pisa, Italy ` b UniÕersidad Nacional de la Patagonia San Juan Bosco-CONICET, BÕrd. Brown 3700, 9120 Puerto Madryn, Argentina c Dipartimento di Scienze della Terra, UniÕersita di Firenze, Õia G. La Pira, 4, 50121 Firenze, Italy ` d Centro di Studio per la Geologia Strutturale e Dinamica dell’Appennino, CNR, Õia S. Maria, 53, 56126 Pisa, Italy Received 1 December 2000; accepted 30 March 2001 Abstract Ž . The Estancia Glencross Area EGA volcanic rocks form a series of five isolated buttes located at the southern end Ž . ;528S of the discontinuous belt of Cenozoic basaltic lava formations occurring in the extra-Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andesites erupted at 8.0–8.5 Ma in a region closely behind the Andean Cordillera. EGA volcanism predated by about 4–5 my the onset of the volcanism in the nearby Pali Aike Volcanic Field, which produced highly primitive, alkaline lavas. Incompatible trace-element distributions and Sr–Nd isotope compositions of EGA rocks are those typical of within-plate OIB-type basalts and are indicative of minimal interaction of sub-lithospheric magmas with enriched reservoirs. The geochemical characteristics of EGA volcanics, as well as their age and location are consistent with a model of slab window opening beneath this region. The high silica content and the garnet signature of the estimated EGA primary magma are explained by a two-stage process involving the initial production of melts from a garnet lherzolite source followed by the reaction of these melts with harzburgite country rocks during their ascent through the mantle lithosphere. The meltrharzburgite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, are likely related to the dominantly compressive stress regime operating in this area during Late Miocene. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Slab window; Mafic magmas; Patagonia; Petrology; Geodynamics; Miocene 1. Introduction The extra-Andean sector of southern Patagonia is characterized by large exposures of Cenozoic basaltic rocks including both alkaline and subalkaline suites. ) Corresponding author. Fax: q 39-50-500-932. Ž . E-mail address: dorazio@dst.unipi.it M. D’Orazio . These rocks generally occur as widespread mesa-for- ming lava flows or as fields of monogenetic scoria cones and related lavas, and in many instances ex- hibit trace-element and isotopic characteristics like those of within-plate OIB-type basalts. The geody- namic significance of the Cenozoic Patagonian plateau lavas with OIB-type signature has been ex- plained according to two main hypotheses, both in- 0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0024-4937 01 00040-8