NEW U-PB AND RB-SR AGES FOR THE NORTHERN AREQUIPA SEGMENT OF THE PERUVIAN COASTAL BATHOLITH

Age data presented here for the Ica part of the northern Arequipa segment of the Peruvian Coastal Batholith (PCB) update previous K-Ar ages from Moore (1984), Rb-Sr ages from Beckinsale (1985), and U-Pb ages from Mukasa (1985).

Pre-PCB-related units of interest are Precambrian basement, Quilmana basaltic andesite envelop expected to be middle Cretaceous, and pre-PCB gabbro with an age less than the Aptian/Albian boundary (113 Ma). The PCB “super-units” with previous age estimates are from west to east: Linga monzonitic granitoids at 101-94 Ma, Pampahuasi diorite-tonalite at 96-82 Ma, Tiabaya granodiorite at 85-77 Ma, and Incahuasi granodiorite at 85-78 Ma. In two cases, the old Rb-Sr age data suggested magma mixing effects: some Linga data yielded a 140 Ma pseudo-isochron age and the Pampahuasi data are scatted giving no isochron at all.

New U-Pb zircon ages were measured at the Arizona Laserchron Center. Data from cores and rims of twenty zircon crystals in a single Quilmana volcanic sample gave inherited ages of 2800-2200 Ma. Eight beam spots from a single Linga Humay monzogabbro zircon grain gave inherited ages of 540-400 Ma. The remaining Linga and Pampahuasi ages are similar to previous measurements and are listed here in order of age based on field relations: Linga Auquish (105-103 Ma), Pampahuasi (100-96 Ma), and Linga Rinconada (102-94 Ma). Tiabaya at 94-88 Ma is older than previous estimates, but still younger than Linga and Pampahuasi. Incahuasi at 71-66 Ma is younger than previous estimates.

New Rb-Sr age estimates include pre-PCB units. A questionable Precambrian Rb-Sr isochron age of 2540 Ma is based on three gneiss samples. A questionable Rb-Sr gabbro age of 103 Ma is based on three samples. The new Rb-Sr Linga 101 Ma isochron is based on 17 samples, with two or fewer points fitting on the alternate 140 Ma Humay monzogabbro pseudo-isochron of Beckinsale. The new Pampahuasi data is scattered as before with no linear trend. Including the new Tiabaya and Incahuasi data lowers their age estimates to 80 and 74 Ma, respectively.

Myers 1975 lists three age groups (flare-ups) in the Lima segment at 95-90, 95-70, and 70-50 Ma with our data from the northern Arequipa segment correlating best with the first two groups. More U-Pb age data will soon be available from subsets of the Tiabaya and Incahuasi, as well as the basalts and gabbros.