Petrogenesis of the Paleoproterozoic Näränkävaara layered intrusion, northern Finland, Part I: The northern peridotites and their relationships with the layered series and recharge events
The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion includes a 1.5–2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic layered series, the latter with tw...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Geological Society of Finland
2022-06-01
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Series: | Bulletin of the Geological Society of Finland |
Subjects: | |
Online Access: | https://www.geologinenseura.fi/sites/geologinenseura.fi/files/bulletin_94_1_2022_jarvinen_et_al_part_i_pp_23-52.pdf |
Summary: | The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface
area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion
includes a 1.5–2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic
layered series, the latter with two peridotitic reversals related to magma recharge. In
addition, a series of poorly known elongate poikilitic harzburgitic intrusions (the northern
peridotites) are found along the NE contact between the intrusion and the granite-gneiss
basement complex. We investigate new mineral and whole-rock geochemical data from
the northern peridotites, with the aim of clarifying their petrogenetic relationship to the
main layered body of the intrusion. The northern peridotites form a 200–400 m thick
cumulate series grading from olivine orthocumulates (OC) at the northern basement
complex contact to olivine-orthopyroxene heteradcumulates (HAC) towards the main
intrusion body in the south. The OC show whole-rock and mineral chemical trends
consistent with origin as rapidly cooled olivine-melt mixtures. The HAC have crystallized
in situ from a relatively Cr- and SiO2-rich magma. Based on lithological and stratigraphical
correlations, the northern peridotites are linked to the emplacement of the magma
that caused the first reversal in the layered series: marginal orthocumulates were
formed at the initial emplacement of a new pulse of LREE-enriched siliceous high-MgO
basaltic (SHMB) magma into the Näränkävaara chamber, followed by heteradcumulate
formation from a fractionating magma with added external SiO2 and fluid. Ubiquitous
granite-gneiss xenoliths and felsic veins in drill core suggest assimilation may have
been a local process. The northern peridotite parental magma shows undepleted metal
ratios suggesting no sulfide saturation occurred prior to emplacement. |
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ISSN: | 0367-5211 1799-4632 |