Distinguishing mantle and crustal contributions in a continental arc volcano: Tatara-San Pedro, Chilean Andes
Abstract
Identifying mantle contributions to arc magmas is fundamental to understanding continental growth and evolution. A long-standing debate in arc magma genesis is whether mantle and slab-derived contributions can be distinguished from crustal contamination at relatively thick crust continental arc volcanoes1,2,3. The Quaternary Tatara-San Pedro complex (TSPC, 36°S, 70.5°W, ~55 km3) of the Chilean Southern Volcanic Zone (SVZ) is a frontal arc stratovolcanic complex exhibiting a large compositional diversity from basalt to rhyolite. High-density sampling has yielded one of the most complete eruptive chrono-stratigraphies of any arc volcano, consisting of lava sequences spanning over 930 kyr. This provides a unique opportunity to evaluate magma source heterogeneity and the effects of mantle and crustal input at the SVZ subduction factory3,4,5,6. We have begun to augment the extensive XRF geochemical dataset with isotope and ICP trace element analyses. After applying filters for crustal contamination, we distinguish three distinct mantle-derived magma types: (1) one derived from “depleted mantle” fluxed by slab derived hydrous fluids, (2) another from “enriched mantle”, and (3) another that appears prevalent throughout the SVZ mantle wedge with generally intermediate but still distinct chemistry. The “fluid-fluxed depleted mantle” endmember has high Sr/Nd, Zr/Nb, low Nb abundances and the lowest 87Sr/86Sr (~0.7038). The “enriched mantle” endmember has much higher incompatible element abundances, La/Yb, HFSE and 87Sr/86Sr (~0.7042). Magmas from this component may have been generated by relatively shallow decompression melting in the upper limb of the convecting mantle wedge. The “prevalent mantle” endmember magmas broadly resemble parental magmas at other SVZ volcanoes, such as Llaima and Villarica. No lavas show contributions from both the “depleted” and the “enriched” components, but many show contributions from the “prevalent” endmember plus one of the others. Extensive MASH processing is precluded by the absence of significant garnet fractionation in any of the endmembers and by eruption of diverse parental magmas over short time intervals. Although most TSPC lavas are impacted by crustal input, all three endmembers remain distinguishable over a large range of elemental ratios. Evolved lavas (>56% SiO2) form distinct elemental arrays trending away from inferred parental magmas. Thus, by combining extensive field and modern analytical approaches, mantle source signatures and crustal imprints can be distinguished despite the continental setting. References: 1Hilldreth and Moorbath, Contrib. Mineral Petrol. 1988; 2Davidson, GCA 1987; 3Davidson et al., Contrib. Mineral Petrol. 1988; 4Fergueson et al., J. Petrol. 1992; 5Singer et al., GSA Bull.1997; 6Dungan et al., J. Petrol. 2001
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V13G..05J
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes