INFLUENCE OF CRUSTAL THICKENING ON ARC MAGMATISM - NEVADOS DE PAYACHATA VOLCANIC REGION, NORTHERN CHILE

Neogene through Pleistocene lava flows of the Nevados de Payachata reg ion (lat 18-degrees-S) on the Altiplano of northern Chile fall into tw o discrete chemical groups defined by age and incompatible element con centrations. The Neogene suite (10.5-6.6 Ma) has trace element concent rations comparable to arc magmas erupted on thin crust in central Chil e. Pleistocene lava flows (0.29-0 Ma) are enriched in incompatible ele ments relative to Neogene samples but have similar Sr, Nd, and Pb isot opic ratios. Incompatible element enrichment in mafic rocks reflects d eep-crustal or subcrustal processes. Neogene volcanism in northern Chi le immediately followed a period of intense crustal thickening. Uplift rates accelerated at approximately 15 Ma, indicating that the ''thin- crust''-type Neogene magmas actually traversed a thickened crust. The isotopic and trace element relations are the result of contamination o f mantle-derived basalts in an upwardly growing lower-crustal interact ion zone, defined to be the part of the lower crust at or above its so lidus. Because the lower crust reaches postorogenic thermal equilibriu m slowly, the lower-crustal interaction zone during Neogene magmatism had essentially the same thickness as it did prior to deformation. The rmal relaxation of the lower crust after thickening produced an upward ly growing column of crust near its solidus. Only 15 m.y. after crusta l thickening did ascending, mantle-derived mafic magmas encounter prev iously unmelted, fertile crust near its solidus in the upper fringes o f the interaction zone. Here the magmas were contaminated with incompa tible element-enriched crustal melts, forming the parental magmas for the Pleistocene suite.