Metasomatic Modification of the Mesoarchaean Ulamertoq Ultramafic Body, Southern West Greenland
Research output: Contribution to journal › Journal article › Research › peer-review
Standard
Metasomatic Modification of the Mesoarchaean Ulamertoq Ultramafic Body, Southern West Greenland. / Nishio, Ikuya; Morishita, Tomoaki; Itano, Keita; Guotana, Juan Miguel; Tamura, Akihiro; Szilas, Kristoffer; Harigane, Yumiko; Tani, Kenichiro; Graham Pearson, D.
In: Journal of Petrology, Vol. 63, No. 3, egac004, 2022.Research output: Contribution to journal › Journal article › Research › peer-review
Harvard
APA
Vancouver
Author
Bibtex
}
RIS
TY - JOUR
T1 - Metasomatic Modification of the Mesoarchaean Ulamertoq Ultramafic Body, Southern West Greenland
AU - Nishio, Ikuya
AU - Morishita, Tomoaki
AU - Itano, Keita
AU - Guotana, Juan Miguel
AU - Tamura, Akihiro
AU - Szilas, Kristoffer
AU - Harigane, Yumiko
AU - Tani, Kenichiro
AU - Graham Pearson, D.
N1 - Publisher Copyright: © 2022 The Author(s). Published by Oxford University Press. All rights reserved.
PY - 2022
Y1 - 2022
N2 - Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650°C-850°C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ∼800°C-900°C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.
AB - Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650°C-850°C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ∼800°C-900°C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.
KW - Archaean
KW - Greenland
KW - Metamorphism
KW - Metasomatism
KW - Ultramafic rock
U2 - 10.1093/petrology/egac004
DO - 10.1093/petrology/egac004
M3 - Journal article
AN - SCOPUS:85126739717
VL - 63
JO - Journal of Petrology
JF - Journal of Petrology
SN - 0022-3530
IS - 3
M1 - egac004
ER -
ID: 304324221