TY - JOUR
T1 - Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland
AU - Klausen, Martin B.
AU - Szilas, Kristoffer
AU - Kokfelt, Thomas F.
AU - Keulen, Nynke
AU - Schumacher, John C.
AU - Berger, Alfons
N1 - Publisher Copyright:
© 2017 Elsevier B.V.
PY - 2017/11
Y1 - 2017/11
N2 - We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO
2 and Al
2O
3, coupled with a decrease in MgO, total iron (FeO
T) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeO
T, low Al
2O
3/TiO
2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeO
T, high Al
2O
3/TiO
2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb
∗) and LREE-enrichment (La/Sm)
N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.
AB - We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO
2 and Al
2O
3, coupled with a decrease in MgO, total iron (FeO
T) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeO
T, low Al
2O
3/TiO
2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeO
T, high Al
2O
3/TiO
2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb
∗) and LREE-enrichment (La/Sm)
N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.
KW - Andesite
KW - Archean
KW - Basalt
KW - Dacite
KW - Island arc
KW - Subduction
UR - http://www.scopus.com/inward/record.url?scp=85032467192&partnerID=8YFLogxK
U2 - 10.1016/j.precamres.2017.09.014
DO - 10.1016/j.precamres.2017.09.014
M3 - Article
SN - 0301-9268
VL - 302
SP - 50
EP - 73
JO - Precambrian Research
JF - Precambrian Research
ER -