Journal of Petrology | Volume 35 | Number 4 | Pages 1095-1125 | 1994
© Oxford University Press 1994
research-article |
Mantle and Crustal Effects on the Geochemistry of Proterozoic Dikes and Sills in Sweden
Max Planck Institut für Chemie Saarstrasse 23, D-6500 Mainz, Germany
Received December 15, 1992; Revised typescript accepted October 6, 1993
ABSTRACT
Chemical and Sr-Nd isotopic data are presented for 14 Proterozoic dolerite intrusions from Sweden. The dolerites are unmetamorphosed and belong to two age groups at 1.26 and 0.95 Ga. The analysed rocks were selected from over 80 samples characterized for initial 87Sr/86Sr ratio, and represent those from each intrusion that have little to no crustal contamination effect on Sr isolopes. In these tholeiitic to mildly alkaline basalts, MgO varies from 4 to 14%, and TiO2 from 1.0 to 4.7%. Incompatible-element abundance patterns show similar shapes at widely varying overall enrichment levels; La varies from eight to 64 times primitive mantle. Many samples have apparent Sr anomalies; positive for the less incompatible-element enriched rocks, negative for those with the highest abundances, Hence Sr varies by a factor of only two, whereas neighboring elements vary by a factor of seven. Small Eu anomalies correlate with the larger Sr variations. All samples have negative Nb anomalies, correlated with 
Nd 1 values that vary from –0.5 to + 3.5 for both dolerite age groups. 87Sr/86Sr1 values are systematically different in the two groups: 1.26-Ga dolerites vary from 0.7026 to 0.7035, and 0.95-Ga dolerites from 0.7041 to 0.7051.
Rather than urging any particular model of subcontinental mantle evolution, we use the dolerite results to deconvolute the effects of crustal contamination, differentiation including cryptic differentiation effects, and mantle-lithosphere interactions on possibly asthenosphere-derived magmas. Problems related to addition of Rb and 87Sr giving false isochrons, as well as raised 87Sr/86Sr from syn- or post-crystallization fluid effects, were eliminated by the sample selection criteria. In the selected dolerites, crustal contamination is not a major control on geochemistry, because all incompatible elements, including those of both high and low crustal abundance, are positively and similarly correlated. Overall abundances have been affected by migrations of residual magma after much olivine and plagioclase crystallization. Only Sr is an approximate indication of the parent magma's enrichment, whereas all other incompatible elements may be depleted or enriched by factors greater than two. On entering the crust, the magmas varied by a factor of only two in overall incompatible-element enrichment. They had Nb anomalies correlated with Nd1, and the 0.95-Ga dolerites possessed higher 87Sr/86Sr than had characterized the 1.26-Ga dolerites. All mantle effects could be interpreted as resulting from a lithospheric magma source that experienced addition of subduction-related components 1.9–1.7 Ga ago, and maybe locally 1.6–1.0 Ga ago. We prefer a model where asthenosphere-derived melts interact with lithospheric wall rocks, acquiring variable Nd down to –0.5 and Nb anomalies. Up to two-thirds of the budget of incompatible elements, but not much Nb, were added to the magmas. Untimately, the contribution of any mantle-source is unconstrained, except that significant lithosphere has to have been involved. The mantle-related geochemical effects, which result in only weak constraints on process and source, are very difficult to isolate in an objective approach to intracontinental basalts, because of superimposed differentiation and contamination effects.
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