VARIATION IN AGE AND GEOCHEMISTRY OF THE PERUVIAN COASTAL BATHOLITH FROM SOUTH TO NORTH

The Peruvian Coastal Batholith (PCB) extends nearly 2000 km from SE to NW. The two major parts are the Arequipa segment north of Arequipa and the Lima segment north of Lima. The suggested dividing line is in the Lurin valley, specifically in the Quebrada de Tinajas (Cobbing et al. 1977). Our research is studying variation in age and geochemistry between the segments, as pre-Mesozoic basement occurs mainly under the Arequipa segment with little under the Lima segment.

Granitoid samples were taken along a dozen transects from both segments, with this report emphasizing the segment boundary using the Rio Lurin and Rio Rimac transects, SE and E of Lima, respectively. Our new data display trends from the southern Arequipa to northern Lima segments, especially younger age and increased Sr/Y ratio.

The U-Pb zircon ages for the Rio Lurin valley were older at 90 Ma for the Patap gabbro and 72 Ma for the Tiabaya unit. The Rio Rimac ages were younger at 60 Ma for the Santa Rosa unit in the Santa Eulalia pluton and a surprisingly young 37 Ma for what had been mapped as the Paccho unit. For all of our 34 new ages on the twelve transects, the Arequipa segment average age was over 100 Ma and the Lima about 75 Ma.

The Sr/Y ratio for the Rio Lurin averaged 67 with a very high ratio of 96 and 189 in the Santa Rosa; whereas, in the Rio Rimac it averaged 15. The average Sr/Y ratio for our 50 samples increased between the Arequipa and Lima segments from about 23 to 35. According to Profeta et al. (2015), this suggests a Cretaceous crustal thickness under the PCB of 35 km, but 50 km or more in parts of the Lima segment.

The K₂O wt% composition for the Rio Lurin was 1-3% for the Patap, Jecuan, Santa Rosa, and Tiabaya units; whereas, in the Rio Rimac it was above 4% for the Paccho and Santa Rosa units. The average K₂O for our 50 analyzed samples decreased slightly between the Arequipa and Lima segments from about 2.5% to 2%, due to the prominent monzonitic Linga unit in the Arequipa segment. Average Rb, Cs, Zr, Cu, Pb, and W content also decreased from south to north.

Previous research indicates that initial Sr isotope ratios are lower further north in the Lima segment. Thus, over time magmatism trended from south to north, came from a deeper source, decreased in mobile elements, and was less affected by pre-Mesozoic basement. This has some similarities to the LoSY to HiSY trend of Tulloch and Kimbrough (2003).