TY - JOUR
T1 - A Geochemical Review of Amphibolite, Granulite, and Eclogite Facies Lithologies
T2 - Perspectives on the Deep Continental Crust
AU - Sammon, Laura G.
AU - McDonough, William F.
N1 - Funding Information:
L. G. Sammon would like to acknowledge Caerwyn Hartten for her assistance in curating the metamorphic datasets. L. G. Sammon and W. F. McDonough gratefully acknowledge the NSF for support (grants EAR1650365 and EAR2050374). We would also like to thank Derrick Hasterok, Tatsuki Tsujomori, and anonymous other reviewers for the insight, suggestions, and encouragement on this study.
Funding Information:
L. G. Sammon would like to acknowledge Caerwyn Hartten for her assistance in curating the metamorphic datasets. L. G. Sammon and W. F. McDonough gratefully acknowledge the NSF for support (grants EAR1650365 and EAR2050374). We would also like to thank Derrick Hasterok, Tatsuki Tsujomori, and anonymous other reviewers for the insight, suggestions, and encouragement on this study.
Publisher Copyright:
© 2021. American Geophysical Union. All Rights Reserved.
PY - 2021/12
Y1 - 2021/12
N2 - 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.
AB - 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.
KW - Geochemical modeling
KW - amphibolite
KW - deep crust
KW - eclogite
KW - granulite
KW - lower crust
KW - middle crust
UR - http://www.scopus.com/inward/record.url?scp=85121683392&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85121683392&partnerID=8YFLogxK
U2 - 10.1029/2021JB022791
DO - 10.1029/2021JB022791
M3 - Article
AN - SCOPUS:85121683392
VL - 126
JO - Journal of Geophysical Research: Solid Earth
JF - Journal of Geophysical Research: Solid Earth
SN - 2169-9313
IS - 12
M1 - e2021JB022791
ER -