GEOCHEMICAL VARIATIONS IN CENOZOIC BACKARC BASALTS AT THE BORDER OF LA PAMPA AND MENDOZA PROVINCES, ARGENTINA

BERTOTTO, G.W.; CINGOLANI, C.A.; BJERG, E.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2008 vol. x

0895-9811

                       Journal of South American Earth Sciences                                                   James N. Kellogg, Editor-In-Chief                                                                November 22, 2008 SA-613 “Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina” by G.W. Bertotto, C.A. Cingolani, E.A. Bjerg I am pleased to inform you that your paper “Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina” is accepted for publication in the Journal of South American Earth Sciences, Special Issue Mafic and Ultramafic Complexes of South America and the Caribbean, edited by M.P. Escayola and J.A. Proenza. This is an interesting, important paper presenting valuable new chemical data on a series of backarc volcanic centers in La Pampa and Neuquén provinces with a proposed change in dip for the subduction zone. The revisions improved the paper considerably. Sincerely,                                           James Kellogg, Editor-in-Chief Journal of South American Earth Sciences Andean Geophysical Laboratory , Department of Geological Sciences GEOCHEMICAL VARIATIONS IN CENOZOIC BACK-ARC BASALTS AT THE BORDER OF LA PAMPA AND MENDOZA PROVINCES, ARGENTINA Gustavo W. Bertotto 1*, Carlos A. Cingolani 2, Ernesto A. Bjerg 3 1 CONICET - Universidad Nacional de La Pampa. Uruguay 151 (6300), Santa Rosa, La Pampa, Argentina. gwbertotto@yahoo.com.ar 2 CONICET - Universidad Nacional de La Plata. Calle 1 nº 644 (1900), La Plata, Buenos Aires, Argentina. ccingola@cig.museo.unlp.edu.ar 3 CONICET - Universidad Nacional del Sur. San Juan 670 (8000), Bahía Blanca, Buenos Aires, Argentina. ebjerg@criba.edu.ar * Corresponding author Keywords: Basalts; Back-arc; Geochemistry; Central-western Argentina Abstract Back-arc volcanism was active in central-western Argentina (provinces of Mendoza and La Pampa) from the Miocene through historic times. The rocks of 39 monogenetic volcanoes located in this area were studied in order to define their geochemical characteristics. The dominant rock texture is porphyritic, with intergranular, pilotaxitic and hyalophitic groundmasses. The most frequent phenocrysts are olivine followed by olivine-plagioclase-clinopyroxene. Their SiO2 content varies between 42.3 and 51.2 wt.%, the most abundant rocks are trachybasalts, followed by basalts and basanites, all of them alkaline. The rocks display enrichment of incompatible elements that varies according to the geographic location and age. There is an increase in incompatible element concentrations from the southern and central to the northern zones. Also, in the northern part of the study area, the behavior of incompatible elements varies with time; the incompatible element ratios of the Plio-Pleistocene rocks show arc signature, while the rocks of the Miocene De la Laguna volcano show intraplate affinity. We conclude for this sector that mantle source region was modified after the generation of Miocene magmas by subduction-related fluids. These fluids are related to a late Miocene episode of subhorizontal subduction, i.e. after the generation of the rocks of De la Laguna volcano. GEOCHEMICAL VARIATIONS IN CENOZOIC BACK-ARC BASALTS AT THE BORDER OF LA PAMPA AND MENDOZA PROVINCES, ARGENTINA Gustavo W. Bertotto 1*, Carlos A. Cingolani 2, Ernesto A. Bjerg 3 1 CONICET - Universidad Nacional de La Pampa. Uruguay 151 (6300), Santa Rosa, La Pampa, Argentina. gwbertotto@yahoo.com.ar 2 CONICET - Universidad Nacional de La Plata. Calle 1 nº 644 (1900), La Plata, Buenos Aires, Argentina. ccingola@cig.museo.unlp.edu.ar 3 CONICET - Universidad Nacional del Sur. San Juan 670 (8000), Bahía Blanca, Buenos Aires, Argentina. ebjerg@criba.edu.ar * Corresponding author Keywords: Basalts; Back-arc; Geochemistry; Central-western Argentina Abstract Back-arc volcanism was active in central-western Argentina (provinces of Mendoza and La Pampa) from the Miocene through historic times. The rocks of 39 monogenetic volcanoes located in this area were studied in order to define their geochemical characteristics. The dominant rock texture is porphyritic, with intergranular, pilotaxitic and hyalophitic groundmasses. The most frequent phenocrysts are olivine followed by olivine-plagioclase-clinopyroxene. Their SiO2 content varies between 42.3 and 51.2 wt.%, the most abundant rocks are trachybasalts, followed by basalts and basanites, all of them alkaline. The rocks display enrichment of incompatible elements that varies according to the geographic location and age. There is an increase in incompatible element concentrations from the southern and central to the northern zones. Also, in the northern part of the study area, the behavior of incompatible elements varies with time; the incompatible element ratios of the Plio-Pleistocene rocks show arc signature, while the rocks of the Miocene De la Laguna volcano show intraplate affinity. We conclude for this sector that mantle source region was modified after the generation of Miocene magmas by subduction-related fluids. These fluids are related to a late Miocene episode of subhorizontal subduction, i.e. after the generation of the rocks of De la Laguna volcano.