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Evidence of crustal contamination of mafic rocks associated with rapakivi rocks: an example from the Nordingrå complex, Central Sweden

ANDERS LINDH^*,*, ULF BERTIL ANDERSSON^,, THOMAS LUNDQVIST and STEFAN CLAESSON

^* Geological Department, Sölvegatan 13, SE-223 62 Lund, Sweden
Department of Earth Sciences, Villavägen 16, SE-752 36 Uppsala, Sweden
Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden
Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

^* Author for correspondence: anders.lindh{at}geol.lu.se

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. _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 _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.