PETROCHEMISTRY AND MAGMA CHAMBER PROCESSES OF THE PERUVIAN COASTAL BATHOLITH GRANITES IN THE NORTHERN PART OF THE AREQUIPA SEGMENT

Petrographical and geochemical data are used to explore magmatic processes in the Cretaceous plutons of the Peruvian Coastal Batholith (PCB) in the Arequipa segment. These plutons have a range of ages from 105 to 66 Ma and intrude Precambrian and Jurassic-Cretaceous sedimentary and volcanic rocks. This area is characterized by a number of intrusions listed from oldest to youngest: monzonitic units (monzogabbro, monzodiorite, monzonite, and monzogranite), tonalite-diorite, and granodiorite. The gabbro-diorites both precede and are contemporaneous with the other granitic units. The monzonitic units are uniquely high in K₂O, which may be due to high fluid flux from the mantle. The granodiorites are in general higher in Al₂O₃, implying incorporation of crustal material. Geochemical trends suggest that the main processes producing compositional variations was fractional crystallization spread out by magma mixing. The data seem to indicate that the magma sources for the monzonitic units and tonalite-diorite are different than that for the granodiorite. This is supported by the Sr/Y and REE ratios of granodiorites which are higher than the other units suggesting a deeper source and/or thicker crust resulting in more assimilation of continental crust. A possible petrogenetic model for the PCB plutons include partial melting, fractional crystallization interrupted by mafic magma pulses, and mixing in two cycles of differentiation. Isotopic compositions of the granitoids are similar to the associated volcanic envelop indicating that they are from the same source. The Sr_i values are 0.704-0.706 showing a slight increase from gabbros to granodiorites and from older to younger indicating that crustal contamination increased with time. The Pb isotopes suggest a parental magma derived from an upper mantle source mixed with crustal materials. The monzonitic and tonalite-diorite plutons represent early magmas emplaced into a juvenile arc setting that experienced minimal contamination from ancient crustal materials. On the other hand, the granodiorites represent magmas emplaced into a more mature arc setting with more involvement of crustal materials. Finally, the timing of the magmatic events in this segment of the PCB can be divided in two distinct magmatic episodes and arc settings, 105-90 Ma and approximately 66Ma.