Occurrence of eclogite in felsic continental rocks as a natural consequence of burial

The occurrence of eclogite enclosed in felsic continental rocks have been subject of intense discussion over several decades. The eclogite facies mineralogy preserves the only evidence that the rocks were buried to great depth and therefore this has significant consequences for the understanding of geodynamic processes in continental collision zones. However, often the enclosing felsic gneiss lacks evidence of this high-pressure metamorphism.

This apparent conflict in metamorphic pressure is founded on experimental phase equilibria studies in which pyroxene and plagioclase in igneous mafic rocks transform at high pressure to the characteristic garnet and omphacite assemblage of eclogite. In contrast, the felsic rocks preserve a mineral assemblage expected to equilibrate at lower pressure. A common explanation is that the enclosing felsic gneiss was retrogressed on the exhumation path, whereas eclogite has been preserved due to slower reaction kinetics in mafic rocks. The preservation of original igneous structures of various mafic rocks and incomplete reactions in metagabbro may also be attributed to metastability. Subsequently, it has been shown that large areas of the enclosing felsic gneiss have retained their original low-pressure mineral assemblage (e.g. Peterman et al., 2009). Consequently, large areas of felsic gneiss were never metamorphosed in the eclogite facies. It is firmly established that fluids play an important role in metamorphic reactions. In contrast, the role of mechanics and the heterogeneity of rock mechanical properties during metamorphism has received less attention.

Mechanical properties of mafic rocks can be different from the surrounding felsic lithologies. Therefore, a mechanical model was proposed for a mafic inclusion in felsic continental rock in which the entire domain was subjected to burial (Podladchikov & Dabrowski, 2017). The model predicted a much higher pressure in the mafic inclusion than in the felsic matrix. The overpressure in the mafic inclusion resulted from difference in compressibility between the mafic and felsic lithologies. This may explain the contrast in high pressure eclogite compared to the enclosing lower pressure felsic continental rocks.

In this study, we combine the mechanical model with thermodynamic calculations using Thermolab (Vrijmoed & Podladchikov, 2022) and apply it to ultra-high-pressure rocks in western Norway. We retrieve density, compressibility, and thermal expansivity for observed rocks from phase equilibria calculations as input into the mechanical models. With Thermolab, complex fluids including more than 40 aqueous species, and multi-component melt and solid solution models, can be conveniently coupled to reactive transport and mechanical models to quantify the effect of burial overpressure.

References:

Podladchikov, Y. Y., Dabrowski, M., (2017), Overpressure by burial, Geophysical Research Abstracts, Vol. 19, EGU2017-18976, 2017, EGU General Assembly 2017.

Vrijmoed, J.C. and Y.Y. Podladchikov, (2022), Thermolab: A Thermodynamics Laboratory for Nonlinear Transport Processes in Open Systems. Geochemistry Geophysics Geosystems, 2022. 23(4).

Peterman, E. M., Hacker, B.R., Baxter, E. F. (2009), Phase transformations of continental crust during subduction and exhumation:Western Gneiss Region, Norway, Eur. J. Mineral. 2009, 21, pp. 1097-1118