Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 214

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PÁLL IMSLAND
cores, have most frequently been explained as resulting from mixing of
different magmas where, in a general sense, a relatively evolved magma
containing plagioclase phenocrysts becomes mixed with relatively primitive
phenocryst free magma, prior to or during eruption. This process has been
discussed by a number of authors with reference to samples of varying
petrology and from various tectonic environments (see e.g. Kuno, 1950;
Upton & Wadsworth, 1967; Anderson, 1976; Dungan & Rhodes, 1978). A
variation of this mixing model presumes that a relatively evolved, more or
less crystalline rock, is remobilized by more primitive magma which
resorbes the plagioclases of the rock and crystallizes a more An rich
plagioclase on its margins (see e.g. Bentor, 1951; Upton et al., 1967). The
resorption features resulting from these processes may be either a distinct
zone of resorption cavities (Upton et al., op.cit.) or irregularly distributed
cavities (Dungan & Rhodes, op.cit.). Both these resorption types occur in
the Jan Mayen phenocrysts but only the distinct resorption zone pheno-
crysts are generally zoned on the margins by a more anorthitic plagioclase.
This would indicate that the samples containing these reversely zoned
phenocrysts with a distinct resorption zone represented mixed magmas. The
same may be true of other samples containing resorbed plagioclase pheno-
crysts, if mixing is the sole explanation of these features. No attempt to
identify the compositions of the mixing magmas or the processes leading to
the mixing has been made.
Desai & Anderson (1974) concluded that plagioclase microphenocrysts in
basalts were formed prior to eruption of the magma, probably during ascent
and effervescence. Their size relative to groundmass grains and proper
phenocrysts is in accordance with this conclusion as well as their general
tabular crystal form which indicates relatively slow growth when compared
to the skeletal or more irregular crystal forms of quench crystals (Bryan,
1972) or crystals precipitated from greatly undercooled magmas (Lofgren,
1974b). In the Jan Mayen basalts the plagioclase microphenocrysts fre-
quently form glomerophyric clusters. The movements of ascending magma
are more likely than is a stationary magma to bring individual crystals
together, so that they can by further crystallization grow into coherent
clusters. Under movements the magma is also likely to support more readily
the ions wanted by a growing crystal. A tabular growth (D/G=unity) may
thus be expected to occur with relatively fast crystal growth during ascent.
In the Jan Mayen case the plagioclase microphenocrysts are mostly
normally zoned, showing an even increase in the Ab component from centre
to margin and frequently a thin zone of more highly albitic composition at
the extreme margin. In occasional samples of glassy groundmass this
extreme marginal zone is lacking. As an example of the range of the zoning,
the compositions are given here from one basalt sample (Jan 59): centre=
An74, margin inside extreme marginal zone=An67, groundmass=An62. The
extreme marginal zone is most probably the result of crystallization after