Geochemical characteristics and tectonic setting of the Northern Patagonian Batholith (40-42 S)

RAPELA, C.W.

San Francisco

Congreso; American Geophysical Union-Fall Meeting; 1987

American Geophysical Union

In the northernmost sector (40-42S) of the Patagonian batholith (PB), Cretaceous to Early Tertiary plutons crosscut an oblique belt of Jurassic age rocks. K-Ar ages decrease westward at 41ºS where five co-magmatic units are recognized: the Lago Moreno quartz-diorite (LHQD, 122-131 Ka), the Los Machis granodiorite (LMCD, 75-100 Ma); the Pireco granite (PC, 30-43 Ma) and the Tristeza leucogranite (TL, 8-10 Ma) in Argentina and the Reloncavi granitoids (REG, 10-16 Ka) in Chile. Most of these units are calc-alkaline. The PG is transitional to peralkaline. The Miocene REG granitoids in Argentina are enriched in CaO and depleted in K20 (low-K suite) at a given Si02 content relative to the Miocene granitoids in Chile. Rb/Sr. Ba/Kb and Y/Sr ratios suggest that fractional crystallization was the main process controlling the observed chemical variation a within a pluton but not between plutons. Changes in the chemistry and distribution of the Tertiary granitoids appear to be related to changes in the geometry of the subduction zone as defined by Cande and Lesli (JGR, 1987). From 49-27 Ma., magmatic activity (including the peralkaline PC granites) is associated with a highly oblique subduction zone. ln contrast, a nearly perpendicular subduction zone existed from 25-O la during the eruption of the calc-alkaline TL and REC granites. During the transition from oblique to perpendicular subduction, compression appears to have decreased and extensional stresses prevailed in both the arc and backarc regions. The time of transition coincides with the peak of eruption of back-arc lavas and alkalic complexes and little or arc granitoid activity in the arc region.