GSA Connects 2022 meeting in Denver, Colorado

Paper No. 265-16
Presentation Time: 2:00 PM-6:00 PM

MAGMATIC DIVERSITY OF THE PERUVIAN COASTAL BATHOLITH NEAR LIMA, PERU: INSIGHTS FROM OXYGEN AND STRONTIUM ISOTOPES


CONWAY, James, Department of Geological Sciences, California State University, Long Beach, 1250 Bellflower Blvd, Long Beach, CA 90840, HOLK, Gregory J., Department of Geological Sciences and IIRMES, California State Univ, Long Beach, CA 90840, MARTINEZ ARDILA, Ana Maria, Department of Earth Sciences and Biological Sciences, Loma Linda University, 11065Campus St., Griggs Hall Rm 116, Loma Linda, CA 92354, CLAUSEN, Benjamin L., Dept of Earth and Biological Sciences, Loma Linda Univ, Geoscience Research Inst, Loma Linda, CA 92350 and PATERSON, Scott R., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Los Angeles, CA 90089

The Peruvian Coastal Batholith (PCB) of South America runs parallel to the Peru-Chile trench for >1600 km (Atherton, 1984). Onset of plutonism is interpreted to be at the transition from an extensional to weak compressional margin that started ~110 Ma (Atherton 1990; Haederle and Atherton, 2002) with emplacement in the Cretaceous between 105–62 Ma in multiple pulses of magmatism (Pitcher et.al, 1985; Mukasa, 1986; Hildebrand and Whalen, 2014) with variable degrees of crustal assimilation of 20-30% continental material into a mantle-derived primary magma (Kirsch et al., 2016; Paterson and Ducea, 2015, Martinez Ardila et al., 2019). We address the degree of assimilation at the transition between the Lima and Arequipa segments of the PCB using oxygen and strontium isotope data. Quartz and plagioclase from collected samples (n=19) all have d18O values <9‰. Quartz, plagioclase, hornblende and biotite from unaltered rocks have d18O values in apparent equilibrium at T > 550oC, suggesting preservation of magmatic values. Simple mass balance calculations using quartz d18O values representative of the mantle (7‰), miogeoclinal sediments (12‰), and accretionary wedge sediment (15‰) as end members suggest low levels of assimilated material that has experienced surficial weathering (<25%) into a mantle-derived magma. This is consistent with results from the Ica-Pisco area of the northern Arequipa Segment 200 km to the south (Martinez Ardila et al., 2019). Low, homogeneous 87Sr/86Srinitial values (ranging from 0.7038 to 0.7045) suggest the assimilant is from non-continental sediments slightly older than the PCB plutons, perhaps from rapid recycling of the plutons themselves, consistent with Atherton (1990). No significant temporal or spatial trends are observed in magmatic d18O values within these samples. This contrasts with the variable and radiogenic Sri values in the southern Arequipa segment (south of Nazca), suggesting the assimilation of Precambrian basement into these magmas. These results indicate a continental arc system involving dominantly mantle-derived magma over a span of ~40 Ma. Rocks with non-equilibrium mineral d18O values are hydrothermally altered, an expected result for plutons emplaced into shallow crust.