TY - JOUR

T1 - Selective metasomatism of ultramafic cumulates within Archean supracrustal sequences

AU - Zhang, Lingyu

AU - Basak, Sampriti

AU - Zakharov, David

AU - Szilas, Kristoffer

N1 - Funding Information: We thank Ole Christiansen, former CEO for NunaMinerals A/S, for providing us with access to the Stor\u00F8 drill cores. Additionally, we appreciate Torsten Vennemann at the University of Lausanne for providing access to the laser fluorination facility. This study forms part of the PhD-thesis of Lingyu Zhang. The authors acknowledge funding from VILLUM FONDEN Grant VKR18978 to Kristoffer Szilas, and China Scholarship Council (No. 202006170037) to Lingyu Zhang. We thank Vincent van Hinsberg for insightful discussions, and Tod Waight for help with EPMA analyses at the University of Copenhagen. We thank two anonymous reviewers for their constructive comments that greatly helped to improve the manuscript, and Associate Editor Vinod Samuel for professional editorial handling. Publisher Copyright: © 2024 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing).

PY - 2024

Y1 - 2024

N2 - The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record.

AB - The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record.

KW - Archean

KW - Greenland

KW - Metasomatism

KW - Storø

KW - Tourmaline

KW - Ultramafic rocks

U2 - 10.1016/j.gsf.2024.101851

DO - 10.1016/j.gsf.2024.101851

M3 - Journal article

AN - SCOPUS:85194001335

VL - 15

JO - Geoscience Frontiers

JF - Geoscience Frontiers

SN - 1674-9871

IS - 5

M1 - 101851

ER -