TY - JOUR
T1 - Diversification inside a lamprophyric dyke and cone sheet complex converging onto a hidden carbonatite centre (Frederikshåbs Isblink, SW Greenland)
AU - Klausen, Martin Bromann
AU - Frazenburg, Madelaine R.
N1 - Publisher Copyright:
© The Author(s), 2024. Published by Cambridge University Press.
PY - 2023/12
Y1 - 2023/12
N2 - Both radiating dykes and proximal cone sheets converge onto a positive aeromagnetic anomaly of an inferred carbonatitic centre, hidden beneath a retreating edge of the Frederikshåbs Isblink glacier. This convergence, together with sub-parallel incompatible element patterns by all intrusions, suggests a cogenetic relationship that warrants investigation into potential diversification processes. More primitive high- and low-Mg damtjernites, which for three dykes conform to more porphyritic dyke cores and aphyric margins, respectively, can be explained by high-Mg trends being controlled by the fractionation/accumulation of mainly augite and olivine (or other mafic phases), while discordant low-Mg trends require additional decoupled magnetite fractionation. It is proposed that each dyke intrusion tapped the differentiated top of a central magma chamber, occasionally followed by an unconsolidated mafic cumulate mush, excluding denser magnetites, with in situ flow segregation playing a subordinate additional role. Beyond the most differentiated damtjernite, more evolved phonolitic nephelinites to carbonaceous alnöites split into bulk rock geochemical T-trends that can only relate to late-stage segregations into magmas with varying proportions of interstitial igneous (not secondary) analcime and carbonate - collectively increasing in volume with differentiation. While the analcime component also appears to segregate more readily into veins and ocelli than carbonatite, it is speculated if such low viscosity, density and liquidus rest melts, inside igneous centres, more efficiently aggregated into voluminous, buoyant analcime caps above slightly denser carbonatites. Similar converging plumbing systems and diversification processes are proposed for other complexes, where kimberlitic parents were simply extracted from deeper mantle sources.
AB - Both radiating dykes and proximal cone sheets converge onto a positive aeromagnetic anomaly of an inferred carbonatitic centre, hidden beneath a retreating edge of the Frederikshåbs Isblink glacier. This convergence, together with sub-parallel incompatible element patterns by all intrusions, suggests a cogenetic relationship that warrants investigation into potential diversification processes. More primitive high- and low-Mg damtjernites, which for three dykes conform to more porphyritic dyke cores and aphyric margins, respectively, can be explained by high-Mg trends being controlled by the fractionation/accumulation of mainly augite and olivine (or other mafic phases), while discordant low-Mg trends require additional decoupled magnetite fractionation. It is proposed that each dyke intrusion tapped the differentiated top of a central magma chamber, occasionally followed by an unconsolidated mafic cumulate mush, excluding denser magnetites, with in situ flow segregation playing a subordinate additional role. Beyond the most differentiated damtjernite, more evolved phonolitic nephelinites to carbonaceous alnöites split into bulk rock geochemical T-trends that can only relate to late-stage segregations into magmas with varying proportions of interstitial igneous (not secondary) analcime and carbonate - collectively increasing in volume with differentiation. While the analcime component also appears to segregate more readily into veins and ocelli than carbonatite, it is speculated if such low viscosity, density and liquidus rest melts, inside igneous centres, more efficiently aggregated into voluminous, buoyant analcime caps above slightly denser carbonatites. Similar converging plumbing systems and diversification processes are proposed for other complexes, where kimberlitic parents were simply extracted from deeper mantle sources.
KW - alnöite
KW - analcime
KW - calcite
KW - Damtjernite
KW - immiscibility
KW - nephelinite
UR - http://www.scopus.com/inward/record.url?scp=85204973523&partnerID=8YFLogxK
U2 - 10.1017/S0016756824000165
DO - 10.1017/S0016756824000165
M3 - Article
AN - SCOPUS:85204973523
SN - 0016-7568
VL - 160
SP - 2147
EP - 2165
JO - Geological Magazine
JF - Geological Magazine
IS - 12
ER -