282-4 Evolving Viscosity in Thermotectonic Dynamic Systems: Deformation and Petrological Evolution of Magmatic and Metamorphic Hornblendites in Lower Crustal Xenoliths from Mercaderes, Colombia

Session: Crustal Petrology, Part II

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Abstract:

The decoupling of dense material from the lithospheric column (“lithospheric drip”) is often invoked to explain our observed evolved bulk crustal composition; however, the temporal and spatial scales of drips remain poorly understood. The strength and viscosity of the material involved in drips exert a first-order control on the magnitude and speed of decoupling. The rheology of the continental lithosphere is commonly described using end-member models; however, meso- and microscale heterogeneity, and continued compositional and mineralogical evolution over time, complicate the use of static homogeneous viscosities for crustal layers. 

Observations from the natural rock record are critical for refining evolving viscosities through the lithospheric column, particularly in dynamic arc lithosphere. While the modern lower continental crust is typically inaccessible, lower crustal xenoliths hosted in a 280 ka tuff in Mercaderes, Colombia, sample the lower continental crust under the Northern Volcanic Zone of the Andes. The Mercaderes suite is the only known locality in the world where we access an essentially modern snapshot of continental arc lithosphere, providing an ideal natural laboratory to examine the evolving strength of this material.

The Mercaderes lower crustal xenoliths range from hornblende-rich to clinopyroxene-rich with variable garnet abundance. The hornblende-rich samples comprise two groups: one defined by magmatic textures and another characterized by foliation and crystal plastic deformation accommodated by amphibole ± clinopyroxene. A magmatic hornblendite yields a titanite U-Pb age of 1.6 ± 1.2 Ma; EBSD on individual titanite grains shows negligible intragranular deformation, suggesting this is a crystallization age. A foliated hornblendite yields a U-Pb titanite age of 2.5 ± 2.0 Ma, while another foliated hornblendite yields a U-Pb apatite age of 6.6 ± 13 Ma. EBSD on individual titanite grains shows intragranular deformation, suggesting partial to total resetting of the U-Pb dates by deformation ± metamorphism.

These titanite and apatite data support both the presence of igneous amphibole-rich rocks crystallizing in the lower continental crust and the deformation during metamorphism of amphibole-bearing lithologies that accommodate shear, possibly even drip. In contrast, hornblende poor xenoliths (e.g., garnet pyroxenites) are undeformed. Both igneous and deformed hornblendites contribute to the bulk strength of the lower crust under Colombia, which changes during deformation and petrological evolution, leading to a spatially and temporally heterogeneous viscosity structure.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6519

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