Geochemical distinctions between “crustal” and mantle-derived peridotites/pyroxenites in high/ultrahigh pressure metamorphic complexes

Abstract

Abstract Basic and ultrabasic rocks in high- and ultrahigh-pressure collision belts can provide important petrological information. Mantle-derived and “crustal” peridotites and pyroxenites are recognized among these rocks in Phanerozoic orogenic zones. The former were emplaced as mantle magma intrusions or tectonically transferred solid fragments of mantle material in the deeply subsided lithosphere, while the latter are shallow complexes of dikes and sills, which were altered and metamorphosed during subduction process. Both rock types were later exhumed at the Earth’s surface. For geochemical comparison of these types, four groups of rocks were chosen: two sample sets of mantle-derived rocks and two sets of crustal rocks. The mantle-derived rocks include a set of spinel and garnet peridotites of alpine-type bodies from the Eastern and Western Alps and the Ronda massif in Spain as well as a set of pyroxenites from the Eastern Alps. The “crustal” rocks include a set of garnet and spinel peridotites from the Kokchetav massif, northern Kazakhstan, and garnet peridotites from the Western Gneiss Region, western Norway, together with a set of pyroxenites from the Kokchetav massif. Geochemical investigation has revealed that mantle-derived peridotites are characterized, as a rule, by high contents of MgO (35–46 wt.%), Cr (1750–12,770 ppm), and Ni (900–2500 ppm), low contents of FeO (5–10 wt.%), TiO2 (0.01–0.3 wt.%), Zr (0.002–1.2 ppm), Nb (0.001–0.3 ppm), Sm (0.003–0.5 ppm), La (0.005–1 ppm), and Yb (0.006–0.54 ppm), and the total content of REE equal to 0.06–5.2 ppm. Mantle-derived pyroxenites contain 27–35 wt.% MgO, 2300–3300 ppm Cr, 5.5–9 wt.% FeO, 0.02–0.08 wt.% TiO2, 0.2–1.4 ppm Zr, 0.007–0.06 ppm Nb, 0–0.13 ppm Sm, 0.007–0.23 ppm La, 0.02–0.2 ppm Yb, and 0.05–1.6 ppm total REE. “Crustal” peridotites are characterized by high contents of FeO (12–25 wt.%), TiO2 (0.64–2.6 wt.%), Zr (33–179 ppm), Nb (3.4–13.8 ppm), Sm (0.7–4 ppm), La (1–8 ppm), Yb (0.8–3.3 ppm), and total REE (11.5–48 ppm) as well as by comparatively low contents of MgO (15–26 wt.%), Cr (79–244 ppm), and Ni (450–730 ppm). “Crustal” pyroxenites contain 6–21.5 wt.% MgO, 90–230 ppm Cr, 11–21 wt.% FeO, 0.7–1.3 wt.% TiO2, 45–493 ppm Zr, 1–8 Nb, 1.6–4.3 ppm Sm, 4.7–14 ppm La, 1.3–7.4 ppm Yb, and 27–80 ppm total REE. These data permit us to develop D. Carswell’s idea of distinctions between the mantle-derived and “crustal” peridotites and suggest some promising geochemical criteria. The criteria are based on distinctions between the contents of MgO, FeO, TiO2, Cr, Ni, Zr, Nb, REE, etc. in peridotites and pyroxenites. Binary MgO–Cr, FeO–TiO2, La–Yb, Lu–Nd, Eu–Gd, and Sm–ΣREE diagrams give an opportunity to discriminate the compositions in detail and are the most appropriate for practical use. The obtained information may be helpful in understanding the nature of protoliths when studying mafic/ultramafic granulites in high-grade metamorphic rocks.