Common Pb isotopic compositions of the Lima, Arequipa and Toquepala segments in the Coastal batholith, Peru: Implications for magmagenesis

Common-Pb isotopic compositions for 65 feldspar samples from the 188-37-Ma old gabbroic to monzogranitic plutons of the Peruvian Coastal batholith show changes along strike that can be related to variable contamination of mantle-derived magmas by the local Precambrian basement. Results have the following isotopic ranges: 206 Pb204Pb = 17.580-20.803; 207 Pb204Pb = 15.555-15.709; and 208 Pb204Pb = 38.104-41.177. With averages of 206Pb204Pb = 18.630, 207Pb204Pb = 15.610 and 208Pb204Pb = 38.500, the gabbroic, dioritic and tonalitic plutons north and east of Lima (the Lima segment) have ratios similar to the isotopically homogeneous reservoir identified by Tilton (1979), Tilton and Barreiro (1980), and Barreiro and Stern (1982) for rocks in central and southern Chile. The homogeneous reservoir has been suggested to be "enriched" subcontinental mantle. More siliceous rocks (granodiorites and monzogranites) in the Lima segment have considerably higher ratios (206Pb204Pb = 18.680-20.803, 207Pb204Pb = 15.610-15.709 and 208Pb204Pb = 38.500-41.177). Nine of ten monzogranites from the ring dike complexes in the Lima segment define a linear array with a slope of 0.1143 (correlation coefficient = 0.9398) and a Pb isochron age of 1.9 Ga. This isochron age has no simple chronological meaning, although it may represent a component of sedimentary debris, derived from 1.9 Ga crustal materials. Feldspars from plutons in the Arequipa and Toquepala segments south of Lima tend to have low Pb-isotopic ratios (206Pb204Pb = 17.580-18.603, 207Pb204Pb = 15.555-15.633 and 208Pb204Pb = 38.104-38.749). The ratios plot between the isotopic signatures of local Precambrian granulites and gneisses and the Chile-type "enriched" subcontinental magmatic reservoir, indicating that these end members were the principal magma sources. The oldest plutonic rocks in the segments (early- to mid-Jurassic) are closest to the Pb-isotopic ratios of the Precambrian basement. Contamination of plutons in the Arequipa and Toquepala segments by unradiogenic Pb from the Precambrian basement, and lack of it in the Lima segment, supports crustal models by Couch et al. (1981) and Jones (1981) which show a thick Precambrian basement layer throughout southern Peru, and an extremely thin layer beneath the Lima segment. © 1986.