TY - JOUR

T1 - An overview of the lithological and geochemical characteristics of the Mesoarchean (ca. 3075) Ivisaartoq greenstone belt, southern West Greenland

AU - Polat, A.

AU - Frei, Robert

AU - Appel, P.W.U.

AU - Fryer, B.

AU - Dilek, Y.

AU - Ordonez-Calderon, J.C.

PY - 2008

Y1 - 2008

N2 - In this review we summarize the major lithological and geochemical characteristics of the Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt, Nuuk region, southern West Greenland. In addition, the geological characteristics of the Ivisaartoq greenstone belt are compared with those of other Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes variably deformed gabbroic to dioritic dikes, actinolite schists, serpentinites, siliciclastic sediments, and minor cherts. The Ivisaartoq rocks underwent at least two stages of postmagmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. The trace element systematics of the least altered rocks are consistent with a subduction zone geodynamic setting. On the basis of lithological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc/backarc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean suprasubduction zone oceanic crust.The Sm-Nd isotope system appears to have remained relatively undisturbed in picrites, tholeiitic pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (Nd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts (Nd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90%) of Ca-plagioclase and are interpreted as anorthositic cumulates of the lower oceanic crust brought to the surface by upwelling gabbroic magmas. Alternatively, the inclusions may represent the xenoliths from older (>3075 Ma) anorthositic crust onto which the Ivisaartoq magmas were emplaced as an autochthonous sequence. However, no geological evidence has been found for such older anorthositic crust in the region. The anorthositic cumulates have significantly higher initial Nd values (+4.8 to +6.0) than the surrounding gabbroic matrix (+2.3 to +2.8), suggesting two different mantle sources for these rocks.

AB - In this review we summarize the major lithological and geochemical characteristics of the Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt, Nuuk region, southern West Greenland. In addition, the geological characteristics of the Ivisaartoq greenstone belt are compared with those of other Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes variably deformed gabbroic to dioritic dikes, actinolite schists, serpentinites, siliciclastic sediments, and minor cherts. The Ivisaartoq rocks underwent at least two stages of postmagmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. The trace element systematics of the least altered rocks are consistent with a subduction zone geodynamic setting. On the basis of lithological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc/backarc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean suprasubduction zone oceanic crust.The Sm-Nd isotope system appears to have remained relatively undisturbed in picrites, tholeiitic pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (Nd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts (Nd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90%) of Ca-plagioclase and are interpreted as anorthositic cumulates of the lower oceanic crust brought to the surface by upwelling gabbroic magmas. Alternatively, the inclusions may represent the xenoliths from older (>3075 Ma) anorthositic crust onto which the Ivisaartoq magmas were emplaced as an autochthonous sequence. However, no geological evidence has been found for such older anorthositic crust in the region. The anorthositic cumulates have significantly higher initial Nd values (+4.8 to +6.0) than the surrounding gabbroic matrix (+2.3 to +2.8), suggesting two different mantle sources for these rocks.

KW - Faculty of Science

KW - isotop

KW - Archean

KW - isotope

KW - ocelli

KW - anorthosite

KW - pillow basalt

KW - oceanic crust

KW - greenstone belt

U2 - 10.1130/2008.2440(03)

DO - 10.1130/2008.2440(03)

M3 - Journal article

VL - 440

SP - 51

EP - 76

JO - Special Paper of the Geological Society of America

JF - Special Paper of the Geological Society of America

SN - 0072-1077

ER -