MIXED MAGMA SOURCES FOR THE SOUTHERN AREQUIPA SEGMENT OF THE PERUVIAN COASTAL BATHOLITH: EVIDENCE FROM STABLE ISOTOPES

The Cretaceous-Paleocene Coastal Batholith of Peru formed during the subduction of the Nazca plate under South America as part of the ongoing Andean Orogeny. This study focuses on the ‘Caldera’ Intrusive Complex, which is divided into numerous 184 to 70 Ma plutons intruded into Precambrian gneiss (Mukasa, 1986) that include gabbro, K-rich diorite, granodiorite, tonalite, and granitic compositions, and hosts the massive Cerro Verde/Santa Rosa porphyry-Cu deposit. Mineral δ¹⁸O values and petrographic analysis from 10 samples indicate variable magma sources and document hydrothermal alteration. Primary minerals in order of abundance include plagioclase, quartz, hornblende, biotite, orthoclase, augite, hypersthene, olivine, and magnetite. Alteration includes plagioclase to sericite and epidote, biotite to chlorite, and augite to green amphibole. K-feldspar displays perthitic texture. “Sieve” textures indicate shallow emplacement. Zoned plagioclase suggests fractional crystallization. Equilibrium quartz (+10.4), plagioclase (+8.2), garnet (+7.1), hornblende (+6.1), and biotite (+6.2) δ¹⁸O values from the Precambrian Charcani Gneiss indicate T ~ 590°C. Altered 100 Ma (?) gabbro near Cerro Verde has hornblende (+5.8) and biotite (+4.5) δ¹⁸O values in equilibrium with magmatic fluids. Quartz (+9.4 ± 0.9; n = 3) and biotite (–1.9 to +3.4; n = 4) δ¹⁸O values from 184 Ma(?) Laderas tonalite indicate meteoric-hydrothermal fluids altered this oldest plutonic unit. The 84 Ma Tiabaya granodiorite experienced a lesser degree of alteration by meteoric fluids, as indicated by equilibrium (~ 550°C) quartz (+8.0 ± 0.4, n = 2), plagioclase (+6.0), and hornblende (+3.6 and +5.9) δ¹⁸O values and non-equilibrium biotite δ¹⁸O values (+1.0 and +2.9). The 70 Ma Linga diorite/granodiorite has quartz (+8.0 ± 0.2, n = 2), orthoclase (+6.9, n = 2), plagioclase (+5.7 ±0.3, n = 2), hornblende (+5.3 ± 0.2, n = 2), and biotite (3.0 ±0.6, n = 2) δ¹⁸O values in equilibrium (540 – 640°C) with magmatic fluids (~ +5.5‰). δ¹⁸O values 1.0 to 1.5‰ lower than those from similar rocks emplaced into eugeosynclinal sediments 700 km to the north (Gonzalez et al., 2017) suggest a lower-¹⁸O Precambrian gneiss assimilant; this is consistent with higher ⁸⁷Sr/⁸⁶Sr_i values (Beckinsale et al., 1985). The degree of alteration decreases with younger ages.