Computer Science
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.4551h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 22, pp.4551-4562
Computer Science
2
Scientific paper
Ca clinopyroxenes (Cpx) in minette lamprophyres and syenites in the southern Abitibi belt show initial 87 Sr/ 86 Sr and Nd at 2680 Ma ranging from 0.7008 to 0.7016 and from +0.8 to +3.0, respectively, indicating the derivation of these magmas from a mildly depleted mantle. Strontium isotope values lie between the bulk earth and the depleted mantle values at 2.68 Ga, interpolated from present-day values and bulk earth initial values. The Cpx is nevertheless high in large ion lithophile elements (LILEs). The melts calculated from Cpx show fractionated REEs ([La] N /[Yb] N = 21-47 for lamprophyres and 34-80 for syenites) and high LILEs relative to high field strength elements (HFSEs). Niobium and titanium are particularly low in the calculated melts (Ti/ Ti * = 0.05 to 0.21 for lamprophyres and 0.05 to 0.26 for syenites). The REEs in the bulk lamprophyres are similar to those in the calculated melts, but those of the bulk syenites are much lower than those of the calculated melts. This disparity is explained by the differentiation of slowly solidifying melts for the coarse-grained syenites, leaving bulk-rock LILE contents much lower than those in "quenched" lamprophyres. Isotopic and chemical data are consistent with a model of producing partial melts in a depleted mantle (low contents of LREEs and strontium, 87 Sr/ 86 Sr of 0.7008, and high ratios of Rb/ Sr and 143 Nd/ 144 Nd), which was enriched by dehydration of subducting slabs. The slab component contained high contents of LREEs and strontium, high ratios of 87 Sr/ 86 Sr (~0.7016) and Sm/ Nd , low contents HFSEs, and low ratios of Rb/ Sr and 143 Nd/ 144 Nd ( Nd = ~ +1). Low Rb/ Sr ratios of this slab component are attributed to low Rb/ Sr of Archaean sediments and slabs. Isotopic data indicate that the subducting slabs did not contain old cratonic material and the mantle metasomatism took place shortly before the alkaline magmatism, probably during the last volcanic episode of the greenstone belt (~2700 Ma), which produced calc-alkaline andesites.
Hart Stanley R.
Hattori Kéiko
Shimizu Nobumichi
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