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Abstract The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using our P-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering.
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This content will become publicly available on May 15, 2026
Reactive thermodynamics of crustal eclogitization and foundering
Regional metamorphism and densification (eclogitization) of the lower crust can affect the lithospheric dynamics of mountain belts, but the coupled effects of reaction rate, temperature, and composition on metamorphism are poorly understood. We present a reactive thermodynamic model of the granulite–eclogite transition to investigate the long-term buoyancy and gravitational stability of the lower crust. First, we characterize the conditions for which orogenic crust attains negative buoyancy by determining its reactive mineral assemblage and density under prescribed pressure–temperature–time paths. Using existing metamorphic rate data, we calibrate a Damkoḧler number (a relative reaction rate) to parameterize the catalytic effect of aqueous fluids. The depth necessary for negative buoyancy is sensitive to temperature and Da, ranging from ∼45 to Image 1 for a basaltic-andesite composition (54 wt.% SiO2). Second, using a Rayleigh–Taylor instability analysis, we suggest that, while cold eclogitic crusts Image 2 could obtain large thicknesses of ∼10 to Image 3 and would founder within Image 4. We hypothesize that such foundering events are a natural consequence of convergent tectonics, where the aqueous fluids and high pressures required for metamorphism are known to exist. The Pampean flat slab in the Central Andes provides geophysical evidence linking slab fluids to eclogitization and densification of the thickened continental crust. Lithospheric foundering coupled to convergent tectonics through eclogitization could explain many observations of orogenic hinterland deformation and magmatism.
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- Award ID(s):
- 2323318
- PAR ID:
- 10647644
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Earth and planetary science letters
- Volume:
- 658
- ISSN:
- 0012-821X
- Subject(s) / Keyword(s):
- Eclogite Granulite Orogeny Geodynamics High pressure metamorphism
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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