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Petrogenesis of alnöitic rocks from Malaita, Solomon Islands, Melanesia

Published online by Cambridge University Press:  05 July 2018

Peter H. Nixon ,
Roger H. Mitchell  and
Nicholas W. Rogers
Peter H. Nixon
Affiliation:
Geology Department, P.O. Box 4820, University, Papua New Guinea
Roger H. Mitchell
Affiliation:
Geology Department, Lakehead University, Thunder BayOntarioCanada
Nicholas W. Rogers
Affiliation:
University of London Reactor Centre, SunninghillAscotBerksU.K.
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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.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 329 , March 1980 , pp. 587 - 596
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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