GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 253-11
Presentation Time: 9:00 AM-6:30 PM

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


CHEN, Julia, Department of Geological Sciences and IIRMES, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840-3902, HOLK, Gregory J., Department of Geological Sciences and IIRMES, California State Univ Long Beach, 1250 Bellflower Blvd, Long Beach, CA 90840 and CLAUSEN, Benjamin L., Geoscience Research Institute, Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350, juliaannchen@gmail.com

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/86Sri values (Beckinsale et al., 1985). The degree of alteration decreases with younger ages.