In:
Geological Magazine, Cambridge University Press (CUP), Vol. 138, No. 4 ( 2001-07), p. 371-386
Abstract:
Gabbro and leucogabbro are volumetrically important rocks in the Nordingrå rapakivi
complex, East Central Sweden. Plagioclase, ortho- and clinopyroxenes, and olivine dominate the gabbro. Perthitic orthoclase and quartz are interstitial in relation to the major minerals. The present work
is based on 232 major-element and a large number of trace element analyses together with 15 whole rock Sm–Nd isotope analyses of the Nordingrå gabbroic rocks. ε Nd(T) values are negative, −1.1 to −3.2;
the most negative values come from the gabbro. Most rocks are enriched in iron, some extremely enriched; none represent primitive mantle melts. The range of Mg-numbers is the same in the gabbro
and the leucogabbro. Plots of the Ni-content vs. the Mg-number are scattered, but there is a positive correlation between these two parameters. The primary mantle-normalized ratios between similar
trace elements are normally strongly different from one. Values larger as well as smaller than one are found for the same ratio in different rocks. The rare earth elements are only weakly fractionated with
small Eu anomalies, negative for the gabbros and positive for the leucogabbros. The primary magma of the Nordingrå gabbro-anorthosite is thought to have been derived from a mildly depleted mantle
source. Variations in the degree of partial melting of a reasonably homogeneous enriched mantle do not explain the observed chemical evolution. Crystal differentiation can account for some geochemical
features, especially the Fe-enrichment. Crustal contamination is required by other characteristics as, for example, the negative ε Nd(T) values and the irregular and sometimes high primary-mantle normalized
incompatible trace-element ratios. Al-rich relic material from the formation of the rapakivi granite melt is another source of assimilation. Most probably contaminants are heterogeneous, including
undepleted crust (represented, for example, by early Svecofennian and Archaean granitoids), depleted crust (restitic after rapakivi magma extraction), and to some degree the associated rapakivi magma
itself. Significant parts of this crust should be Archaean in age.
Type of Medium:
Online Resource
ISSN:
0016-7568
,
1469-5081
DOI:
10.1017/S0016756801005672
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
2001
detail.hit.zdb_id:
956405-6
detail.hit.zdb_id:
1479206-0
SSG:
13
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