Isotopic and geochemical evolution of the Cenozoic basalts from Rio Negro, Patagonia, Argentina.

NTAFLOS, TH; GÜNTHER, M; LABUDIA, C; BJERG, E A; KURAT, G; DINGELDEY, CH

Rio de Janeiro

Congreso; 31st International Geological Congress; 2000

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 geochemical systematics of Patagonian volcanic rocks of the Rio Negro Province, Argentina, indicate different mantle source characteristics that have been derived from two-or more- mantle sources. The oldest rocks, (basaltic andesites) so far detected only in the west have the lowest concentrations of incompatible elements (LREE 30xchondritic) and the highest radiogenic isotopic ratios (87Sr/86Sr up to 0.70502; 143Nd/144Nd down to 0.51254, repectively). The low incompatible element contents indicate high degrees of partial melting. Their negative Nb- and Ta-anomalies, normalized to primitive mantle and enrichment in Sr, Ba and K, indicate crustal contamination and/or mixing with lithospheric subducted slab. The geochemistry of the younger rocks (mainly trachybasalts, basalts and tephrites/basanites) show ocean island basalt (OIB) affinity. Only in the west are some of these lavas slightly depleted in HFSE (with negative Nb-anomalies). The younger lavas show characteristics that can be summarized as follows: a. variable concentrations of incompatible elements suggesting different degrees of partial melting of mantle source-rocks. b. very similar patterns of incompatible elements are present in samples from the far west (close to the Andes) and from the easternmost outcrops (some 300 Km east of the Andes), indicating a similar petrogenesis not related to subduction processes. c. in the east of the area lavas with highly enriched LREE (up to 400x chondritic) occur that can not be explained by a single-stage low degree of partial melting in the source. Compositions of these lavas, normalized to primitive mantle, show distinct negative K anomalies.