26-6 Tracing Early Paleozoic Arc Processes in Northwestern Gondwana: Nd–Hf Isotopic and Geochemical Evidence from Amphibolites and Orthogneisses, Mérida Andes, Venezuela

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

Presenting Author:

Authors:

Abstract:

High-grade amphibolites and orthogneisses of the Iglesias Complex, Mérida Andes (Venezuela), preserve key records of Early Paleozoic crust–mantle interaction along the northwestern margin of Gondwana. This study integrates whole-rock major and trace element geochemistry with Sm–Nd and Lu–Hf isotopic systematics to constrain protoliths, magmatic sources, and tectonic setting.

Amphibolites occur both as lenses within paragneisses and as dikes intruding orthogneisses. Those hosted in paragneisses display tholeiitic compositions, flat to slightly fractionated REE patterns, positive εNd₅₀₀Ma (+4.2 to +7.3) and εHf₅₀₀Ma (+4.5 to +11.4) values, and Mesoproterozoic to Neoproterozoic model ages, indicating derivation from juvenile mantle sources with minimal crustal contamination. These features suggest emplacement during early extensional or back-arc stages, possibly linked to initial phases of the Famatinian arc.

By contrast, amphibolites intruding orthogneisses and hornblende-bearing orthogneisses exhibit more evolved geochemical signatures, including LREE and LILE enrichment, variable Nb–Ta anomalies, lower εNd and εHf values, and Mesoproterozoic model ages (~1.3–1.0 Ga), reflecting enhanced crustal input and magma differentiation in a continental arc setting. Hornblende orthogneisses record heterogeneous isotopic compositions indicative of mixed mantle–crust sources and variable assimilation, whereas biotite-bearing orthogneisses are peraluminous and isotopically evolved (εNd and εHf ≈ −6 to −8), consistent with partial melting or reworking of ancient felsic crust.

Strongly migmatized greenish lenses previously interpreted as amphibolites are reclassified as calc-silicate gneisses derived from impure carbonate-rich sediments. Their mineralogy, geochemistry, and negative εNd–εHf values support a sedimentary protolith modified by high-grade metamorphism and partial melting.

Overall, the Iglesias Complex records a composite arc-related magmatic system involving juvenile mafic inputs, crustal recycling, and Nd–Hf isotopic decoupling controlled by accessory mineral phases during high-grade metamorphism, highlighting the complexity of Early Paleozoic arc development and crustal evolution along the northwestern Gondwanan margin.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.