A Geochemical Review of Amphibolite, Granulite, and Eclogite Facies Lithologies: Perspectives on the Deep Continental Crust

Debate abounds regarding the composition of the deep (middle + lower) continental crust. Exhumed medium- and high-grade metamorphic rocks, which range in composition from mafic to felsic, provide information about the bulk composition of the deep crust. This study presents a global compilation of geochemical data on amphibolite (n = 6,500), granulite (n = 4,000), and eclogite (n = 200) facies lithologies and quantifies trends, uncertainties, and sources of bias in the deep crust sampling. The continental crust's Daly Gap is well documented in amphibolite and most granulite facies lithologies. Igneous differentiation processes likely control the compositional layering in the crust. Al2O3, Lu, and Yb vary little from top to bottom of the crust. In contrast, SiO2, light rare earth elements, Th, and U show a wider range of abundances throughout. Because of oversampling of mafic lithologies, our predictions are a lower bound on middle crustal composition. Additionally, the distinction between granulite facies terrains (intermediate SiO2, high heat production, high incompatibles) or granulite facies xenoliths (low SiO2, low heat production, low incompatibles) as being the best analogs of the deep crust remains disputable. We have incorporated both rock types, along with amphibolite facies lithologies, to define a deep crustal composition that approaches 57.6 wt.% SiO2. This number, however, represents a compositional middle ground; the shallower parts of the deep crust (middle crust) resemble quartz monzonite while the deepest portions (lower crust) more resemble a Ca-rich monzonite. Future studies should analyze more closely the depth dependent trends in deep crustal composition to develop composition models that are not limited to a three-layer crust. Plain Language Summary The composition and origins of the bottom 2/3 of the continental crust has been a topic of geologic debate for many years. Because of the inaccessible depths of these middle and lower sections of the continents, we cannot sample them directly. We must rely on rocks brought to the surface through mountain building and magma entrainment processes. Deep crustal rocks delivered via these processes come from a wide variety of depths and encompass many different chemical compositions. This study seeks to understand and better characterize the average composition of the deep crust (typically from 15 to 40 km beneath the surface) and identify the processes that produced the crust's present-day, chemically layered structure.

Automated summary

Through the study of global geochemical data on metamorphic rocks, the composition of the deep continental crust was explored, finding that Al2O3, Lu, and Yb show little variation from top to bottom, while SiO2, light rare earth elements, Th, and U have a wider abundance range. Oversampling of mafic lithologies led to predictions being a lower bound on middle crustal composition.