Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic craton

TY - CHAP

T1 - Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic craton

AU - Szilas, Kristoffer

AU - Tusch, Jonas

AU - Hoffmann, J.E.

AU - Garde, Adam A.

AU - Münker, Carsten

PY - 2017

Y1 - 2017

N2 - Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu-Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with εHf3071 Ma around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.

AB - Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu-Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with εHf3071 Ma around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.

U2 - 10.1144/SP449.2

DO - 10.1144/SP449.2

M3 - Book chapter

SN - 9781786202802

T3 - The Geological Society, London, Special Publications

SP - 19

EP - 38

BT - Crust-Mantle Interactions and Granitoid Diversification

A2 - Halla, J.

A2 - Whitehouse, M. J.

A2 - Ahmad, T.

A2 - Bagai, Z.

PB - Geological Society of London

CY - London

ER -