The Patagonian batholith at 48°S latitude, Chile; Geochemical and isotopie variations

Plutons of the Patagonian batholith at 48°S can be divided into two series, a calc-alkaline tonalitic (CAT) series and a calc-alkaline granodioritic (CAG) series. The CAT series is characterized by low K₂O and other LIL element concentrations and predominately consists of tonalite and leucotonalite with little or no K-feldspar. The CAG series is characterized by higher K₂O and LIL element concentrations and consists of quartz monzodiorite, granodiorite, and granite. Both rock series also contain gabbros. Chemical differences of these two rock series suggest that differentiation of two distinct parental magma types occurred in the formation of the Patagonian batholith. Geochemical studies from other regions in the batholith confirm the existence of these two primary magma groups, and suggest that they were derived from different mantle sources or different depths in the mantle wedge beneath the magmatic arc. The evolved granites and leucotonalites of the batholith are likely partial melts of mafic and intermediate rocks of both magma series induced by a continuing magma flux from the mantle. Sr, Nd, and Pb isotope data suggest that crustal contamination has played an important role in the petrologic and chemical evolution of the batholith at 48°S. These data suggest that the batholith is composed of mixtures of mantle-derived magmas and crustal components, and that there was a progressive decrease in crustal involvement during evolution of the batholith. Initial 87Sr/86Sr ratios range from 0.7036 to 0.7074 (εSr = -14 to 40) and initial ¹⁴³Nd/ ¹⁴⁴Nd range from 0.51279 to 0.51217 (εNd = 6.7 to -6). There is a good correlation between age and isotopic composition regardless of lithology. The oldest plutons have the highest initial ⁸⁷Sr/⁸⁶Sr ratios and the lowest εNd, and progressively younger plutons exhibit progressively lower initial ⁸⁷Sr/⁸⁶Sr ratios and higher εNd. The apparent decrease in contamination through time that is suggested by the isotope data can be explained by a process in which the basement accretionary wedge complex dried out and became more refractory with time. This evolution resulted in decreased potential for chemical interaction between magmas and basement material, and thus younger plutons were less contaminated.