signature=1b8b230a6322310c1629e544ba5dcb63,Silicate-perovskite and majorite signature komatiites fro...

摘要：

Three komatiite-tholeiite sequences from the Archean Abitibi Southern Volcanic Zone (SVZ), which are separated by major terrane boundaries, show systematic differences in composition, rare earth element (REE) patterns and high field strength element (HFSE) /REE fractionation. Komatiites from Tisdale Township are Al-undepleted, with MgO = 12–24%, Al2O3/TiO2 = 13–17, CaO/Al2O3 = 1–1.3, and high Ni (722–1275 ppm) and Cr (1875–2820 ppm) contents. Mg-tholeiites and Fe-tholeiites are spatially associated with komatiites. Both komatiites and Mg-tholeiites have flat REE patterns [(La/Yb)n = 0.6–1.2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1 (calculated using primitive mantle normalized values and adjacent REEs in the incompatibility sequence of elements in mid-ocean ridge basalt; MORB), suggesting that they may have formed by high-degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths (<300 km). Munro Township komatiites are Al-undepleted, with MgO = 22–30%, Al2O3/TiO2 = 17–20, CaO/Al2O3 = 1.0–1.2, and high Ni (831–1590 ppm) and Cr (1250–2760 ppm) contents. They have light rare earth element (LREE) depleted patterns [(La/Yb)n = 0.2–0.4], and Nb/Nb*, Zr/Zr*, and Hf/Hf* > 1, consistent with having been derived from Mg-perovskite enriched sources at depths >700 km below the transition zone, whereas spatially associated tholeiites have (La/Yb)n = 1–2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1, suggesting a shallow undepleted mantle source, distinct from the komatiites. Komatiites and tholeiites from Boston Township are characterized by MgO = 11–29%, Al2O3/TiO2 = 4.5–5.4, CaO/Al2O3 = l.4–2.5, high Ni (203–3420 ppm) and Cr (194–1965 ppm) contents; Al, Sc, Y, and heavy rare earth element (HREE) depletion, and pronounced negative normalized Zr and Hf anomalies (Zr/Zr* and Hf/Hf* < 1), in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300–600 km. The fractionated HFSEs and REEs of Munro - and Boston-area komatiites indicate that these Archean komatiites may have preserved some signatures of early mantle differentiation, which so far has only been inferred based on high-pressure experimental studies and theoretical predictions. Further geochemical studies of Archean komatiites, especially precise multiple trace element analysis, combined with improved measurements of partition coefficients over a wide pressure range, will provide better understanding on early mantle differentiation, stratification, and the transition zone.

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