Petrogenesis of AlnÖitic Rocks from Malaita, Solomon Islands, Melanesia

Peter H. Nixon, Roger H. Mitchell and Nicholas W. Rogers
Geology Department, P.O. Box 4820, University, Papua New Guinea
Geology Department, Lakehead University, Thunder Bay, Ontario, Canada
University of London Reactor Centre, Sunninghill, Ascot, Berks, U.K.

Summary: Aphanitic and fragmental alnöitic rocks from Malaita contain ultrabasic xenoliths and discrete nodules (megacrysts) of pyroxenes and garnets. Primary minerals in the alnöites are olivine (Fo85), clinopyroxene (diopside-sahlite), natro-melilite (1/3 melilite-2/3 åkermanite), phlogopite (1–9% TiO2), perovskite, spinel (ulvö spinel-magnetite series) and accessory nepheline, melanite, and apatite. Alnöite olivines and clinopyroxenes are compositionally different from those phases in the xenoliths and megacrysts. Rare earth element distribution patterns are linear and indicate strong enrichment in the light rare earths (La/Yb = 42−49). The alnöites are possible primary melts of a pyrolite-type mantle formed by approximately 4% partial melting at depths greater than 120 km, under high carbon dioxide pressures. Despite containing a mantle xenolith assemblage similar to that found in kimberlites, the host Malaita rocks are minera-logically and geochemically different from kimberlite.

Mineralogical Magazine; March 1980 v. 43; no. 329; p. 587-596; DOI: 10.1180/minmag.1980.043.329.05
© 1980, The Mineralogical Society
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