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The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik plutonic complex, Oslo rift valley, Norway; Part 1, Magmatic and secondary zircon; implications for petrogenesis from trace-element geochemistry

P. C. Piilonen, A. M. McDonald, G. Poirier, R. Rowe and A. O. Larsen
The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik plutonic complex, Oslo rift valley, Norway; Part 1, Magmatic and secondary zircon; implications for petrogenesis from trace-element geochemistry
Mineralogical Magazine (June 2012) 76 (3): 649-672

Abstract

A detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stalaker-type pegmatites has a mean Sigma REE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44-67) and negative Eu anomaly (Eu/Eu* = 0.15-0.18). Secondary Tvedalen-type zircon is REE-enriched (Sigma REE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (Sigma REE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (Sigma REE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/U (sub N) ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Stalaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U-Th-Pb isotope analyses indicate that the secondary and altered zircon are depleted in (super 238) U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 76
Serial Issue: 3
Title: The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik plutonic complex, Oslo rift valley, Norway; Part 1, Magmatic and secondary zircon; implications for petrogenesis from trace-element geochemistry
Affiliation: Canadian Museum of Nature, Research Division, Ottawa, ON, Canada
Pages: 649-672
Published: 201206
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 52
Accession Number: 2012-092175
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
N58°58'60" - N59°45'00", E09°45'00" - E10°31'60"
Secondary Affiliation: Laurentian University, CAN, CanadaStatoil, NOR, Norway
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201248
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