Magma sources and mixing for the Coastal Batholith in southern Peru: insights from new elemental and isotopic data and from comparison with California batholithic rocks

Seventy granitoid samples recently collected from the Arequipa segment of the Cretaceous Coastal Batholith near Ica, Peru are made up of a combination of diorites, tonalites, monzonites, and granodiorites previously divided into super-units from west to east as: Linga, Pampahuasi, Tiabaya, and Incahuasi. A few associated earlier gabbros are also included. Elemental and isotopic data and petrographic analyses from these new samples along with previous data from the same area provides a data set of about one hundred samples. They yield new insights into across-arc geochemical variation, when compared with data from California batholithic rocks in the Sierra Nevada Mountains and in the Peninsular and Transverse Ranges and Mojave Desert. >>Of the 26 new Rb-Sr isotope data points from Peru: nineteen lie on previous isochrons for the four super-units that yield ages of 80-100 Ma; six lie above the isochrons, two of which match a Pampahuasi data cluster previously interpreted as due to magma mixing; one Linga data point lies below the isochrons as part of a 140 Ma apparent isochron previously interpreted as due to mixing. >>Based on principal component analysis (PCA) of data from California and Peru, the data can be summarized in terms of four factors related to: (1) extent of differentiation and/or mixing using a MgO Harker diagram, (2) relative contributions of mantle and lower crustal input using the Nb/Yb ratio and Nd-Sr and Pb isotope plots, (3) magma source depth using Sr/Y and Gd/Yb ratios, and (4) calc-alkalinity using a K₂O Harker diagram and a Rb-Sr bivariate plot. Two other plots of Al₂O₃ content and TiO₂/Fe₂O₃ ratios assess alumina saturation and ilmenite/magnetite variation. >>Comparison with the two thousand California data points is impractical individually and requires a grouping of the samples, so that only mean and standard deviation of California data groups are plotted. The baseline for comparing the PCA factors [least crustal contamination, shallowest depth, and least alkalinity] is the low-Sr_i (<0.704) granitoid data from near Escondido in the north western Peninsular Ranges batholith. >>To interpret the new Peru data: The range of SiO₂ indicates extent of magma differentiation versus mixing with only low SiO₂ in Pampahuasi, high SiO₂ in Tiabaya, and a range of SiO₂ in the other two super-units. The size of the range aids in determining whether the emplacement was during compression or extension. The crustal contamination increases from west to east, with relative proportions of mantle and lower crustal source dependant on local thickness of the crust; the Coastal Batholith in the Ica area is intermediate between having a primary mantle component as in central Peru where the crust is thin and having a significant crustal component in southern Peru where the crust is thick. The magma source depth ranges from shallow in the west to intermediate in the east with Tiabaya from the deepest source. The calc-alkalinity shows a slight trend of increasing alkalinity toward the east. The alumina content shows these rocks as metaluminous with a slightly higher content in the east than west, suggesting an increased crustal component. The TiO₂/Fe₂O₃ ratios suggest a slight increase in the ilmenite to magnetite composition from west to east.