TY - JOUR

T1 - Eoarchean ultramafic rocks represent crustal cumulates

T2 - A case study of the Narssaq ultramafic body, southern West Greenland

AU - Zhang, Lingyu

AU - Szilas, Kristoffer

N1 - Publisher Copyright: © 2023 The Author(s)

PY - 2024

Y1 - 2024

N2 - Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.

AB - Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.

KW - Archean

KW - Cumulate

KW - Greenland

KW - Itsaq Gneiss Complex

KW - Narssaq

KW - Ultramafic rocks

U2 - 10.1016/j.epsl.2023.118508

DO - 10.1016/j.epsl.2023.118508

M3 - Journal article

AN - SCOPUS:85182171040

VL - 625

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

M1 - 118508

ER -