EVIDENCE IN CERRO-PAMPA VOLCANIC-ROCKS FOR SLAB-MELTING PRIOR TO RIDGE-TRENCH COLLISION IN SOUTHERN SOUTH-AMERICA

Late Miocene (approximate to 12 Ma) hornblende-bearing andesitic to da citic (63 to 68% SiO2, 1.2 to 1.9% K2O) ''adakite'' flows at the small Cerro Pampa center in Patagonia (47 degrees 55'S, 71 degrees 25') hav e some of the clearest slab-melt geochemical signatures yet seen in a Phanerozoic center on continental crust. These magmas formed in respon se to melting of the hot, thin slab that was subducting beneath South America prior to the collision of the Chile rise at approximate to 6 M a or at approximate to 10 Ma. Their N-MORB-like Sr-87/Sr-86 (0.70285-0 .70309) and low Pb-206/Pb-204 (18.44-18.59) ratios show that they coul d have been generated by approximate to 3-5% partial melting of eclogi te facies N-MORB oceanic crust. Low FeO/MgO (0.9-1.3) ratios and high Cr (>85 ppm) and Ni (>43 ppm) concentrations indicate some interaction with mantle peridotite. Low epsilon Nd (+6.9 to +5.5), high Pb-207/Pb -204 (15.57-15.58) ratios, and high Ba, Cs, U, and Th concentrations c ompared to N-MORB modeled melts indicate some upper crustal contaminat ion. In comparison with previously proposed Patagonian slab-melts, Cer ro Pampa magmas require less mantle contamination than those at Cook v olcano (54 degrees S) and less crustal contamination than those in the northern Austral Volcanic Zone (49 degrees S to 52 degrees S). These differences fit a ridge-trench collisional (slab-window) model that ex plains the properties of slab-melts formed before (Cerro Pampa) and af ter (Austral Volcanic Zone) ridge collision.