ISSN:
1432-0967
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Geologie und Paläontologie
Notizen:
Abstract The olivine-melilite-nephelinite at Moiliili, a representative of the Honolulu Volcanic Series, experienced post-eruptive fractional crystallization which resulted in the formation of pegmatoids of restricted volume. The pegmatoids can be equated in composition with melilite-bearing nephelinite (≡melilite-bearing titanaugite melteigite and titanaugite ijolite). Pegmatoid olivine, titaniferous Ca-rich pyroxene and melilite display mild decreases in their 100 Mg/(Mg + Fe) ratios, largely in response to olivine fractionation. Titanomagnetite Fe2TiO4 components increased with fractionation, mainly at the expense of Fe3O4. Nepheline and sodalite show only restricted compositional variation, although sodalite solid solution is more extensive than in plutonic analogues. Compared with the host olivine-melilite-nephelinite, the pegmatoids are highly depleted in Cr and Ni and enriched in Rb, Sr, Y, Zr, Nb and Ba. The Moiliili fractionation sequence confirms the low-pressure trend olivine-melilite-nephelinite→melilite-bearing nephelinite, previously defined on the basis of relationships in the synthetic system diopside-akermanite-nepheline, but it provides no support for the proposal that olivine-melilite-nephelinite ultimately may fractionate to phonolite. Phonolitic differentiates can be generated by the low-pressure fractionation of alkali feldspar-bearing olivine nephelinites but the relative volumes of the salic derivatives are minor. These volumetric and other constraints inherent in low-pressure fractionation models employing nephelinitic parents suggest that at least some phonolites may be the products of deep crustal or upper mantle anatexis.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF00371205
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