NDLI: Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway

Content Provider	Springer Nature Link
Author	Lapen, Thomas J. Medaris, L. Gordon Johnson, Clark M. Beard, Brian L.
Copyright Year	2005
Abstract	Combined Sm–Nd and Lu–Hf age and isotope data indicate that Mg- and Cr-rich ultramafic rocks at Sandvik, Western Gneiss Region (WGR), Norway, originated from depleted Archean lithospheric mantle that was chemically and physically modified in Middle Proterozoic time. The Sandvik outcrop consists of garnet peridotite and garnet-olivine pyroxenite and thin garnet pyroxenite layers. These contain two principal mineral assemblages: an earlier porphyroclastic assemblage of grt + opx + cpx ± ol (1,200–1,000°C, 40–50 kbar) and a later kelyphitic assemblage of grt + spl + am ± opx ± ol (700–750°C; 12–18 kbar). A CHUR Hf model age indicates a period of melt extraction at ca. 3.3 Ga for garnet peridotite, reflecting extremely high Lu/Hf ratios and very radiogenic present-day 176Hf/177Hf (εHf=+2,165). Lu–Hf garnet-cpx-whole rock ages of two olivine-bearing samples (garnet peridotite and garnet-olivine pyroxenite) from the outcrop are ca. 1,255 Ma, whereas two olivine-free garnet pyroxenites yield Lu–Hf ages of ca. 1,185 Ma. All Sm–Nd garnet-cpx-whole rock ages of these samples are significantly younger (ca. 1,150 Ma for garnet peridotite and ca. 1,120 Ma for garnet pyroxenite). The isotope systematics indicate that the Lu–Hf ages represent cooling from an earlier period of formation/recrystallization for garnet peridotite whereas they probably reflect formation/recrystallization ages of the garnet pyroxenite. The Sm–Nd ages and isotope systematics of the garnet peridotite samples are consistent with an episode of LREE metasomatism, perhaps facilitated by a fluid of carbonatitic composition that strongly decoupled Sm–Nd and Lu–Hf. The Sm–Nd ages of the garnet pyroxenite may represent either LREE metasomatism or cooling, and, like the peridotites, Lu–Hf ages are older than Sm–Nd ages. The age data, as well as the inferred Nd isotope composition of the fluid that affected the olivine-bearing samples, suggest that these rocks were not in contact during the LREE metasomatic event. Moreover, the pyroxenite layers cannot have been emplaced as magmas into the host peridotite. The pyroxenite layers are interpreted to be tectonically juxtaposed with the host olivine-bearing samples sometime after 1,150 Ma but before development of kelyphite.
Starting Page	131
Ending Page	145
Page Count	15
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	150
Issue Number	2
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	2005-08-13
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Mineralogy Mineral Resources Geology
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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