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

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 > 550^oC, 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/⁸⁶Sr_initial 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 Sr_i 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.