Geochemical evidence for Plume activity beneath Patagonian Sub-Continental Mantle

NTAFLOS, TH; BJERG, E A; LABUDIA, C; THÖNI, M

Niza

Congreso; European Geophysical Society-American Geophysical Union-European Union of Geosciences (EGS-AGU-EUG) Joint Assembly; 2003

The hills of La Huerta and Valle Fertil, wich belong to the Western Pampean Ranges in the Province of San Juan, NW Argentina, mainly consist of metagabboic to metadioritic rocks that intruded the Proterozoic basement in the Early Middle Ordovician (Famat <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:21.0cm 842.0pt; margin:70.9pt 3.0cm 70.9pt 3.0cm; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> The origin of the Neogene basaltic lavas in southern Patagonia, Argentina is believed to be the result of decompression melting of asthenospheric mantle that upwells through a slab window and impinged the basis of the continental lithospheric mantle (Gorring & Kay 2001, J. of Petrology 42, 1067–1094). We studied in detail the Pleistocene/Pliocene basalts and their enclosed mantle xenoliths from Rio Negro province, northern Patagonia, Argentina. The basalts are basanites, trachybasalts, and alkali basalts and themantle xenoliths are spinel- and/or garnetlherzolites and harzburgites. Geochemically, the basalts from northern Patagonia are similar to those from southern Patagonia. The lavas are characterized by variable concentrations of incompatible elements suggesting different degrees of melting of mantle-rock source (LaN/LuN ratio ranges from 10 to 40). We have not found any evidence for a subduction signature in the lavas. Neither Nb and Ta depletion nor enrichment of LILE was observed. The 87Sr/86Sr and 143Nd/144Nd isotopic ratios plot within the OIB field and range from 0.70345 to 0.70441 and 0.512724 to 512821, respectively. Using the Dynamic Melt Inversion method for REE, the calculated compositions show that the northern Patagonia lavas were produced by 2% and 9% melting of an already metasomatized asthenospheric mantle.