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Strontium isotope systematics for plagioclase of the Skaergaard Intrusion (East Greenland); a window to crustal assimilation, differentiation, and magma dynamics

Graham Hagen-Peter, Christian Tegner and Charles E. Lesher
Strontium isotope systematics for plagioclase of the Skaergaard Intrusion (East Greenland); a window to crustal assimilation, differentiation, and magma dynamics
Geology (Boulder) (February 2019) 47 (4): 313-316

Abstract

We present in situ laser ablation-multicollector-inductively coupled plasma-mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest (super 87) Sr/ (super 86) Sr (sub i) (calculated at 56 Ma) values ( approximately 0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of approximately 0.7044, which is uniform throughout the middle approximately 2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An65-70 at the base and top to approximately An25 approaching the purported "Sandwich Horizon." Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic ( (super 87) Sr/ (super 86) Sr (sub i) up to approximately 0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor in situ contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 47
Serial Issue: 4
Title: Strontium isotope systematics for plagioclase of the Skaergaard Intrusion (East Greenland); a window to crustal assimilation, differentiation, and magma dynamics
Affiliation: Aarhus University, Department of Geoscience, Centre of Earth System Petrology, Aarhus, Denmark
Pages: 313-316
Published: 20190226
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 22
Accession Number: 2019-024197
Categories: Isotope geochemistryIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2019106
Illustration Description: illus.
N68°04'60" - N68°15'00", W31°45'00" - W31°34'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201914
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