Journal of Petrology
Andreas Spath, Anton P. le Roex, Robert A. Duncan
ABSTRACT
New mineral and bulk-rock analyses, as well as Nd, Sr and Pb isotope compositions
are presented for lavas from Grande Comore, Moheli and Mayotte, three of the
four main islands of the Comores Archipelago in the western Indian Ocean, and
these data are used to evaluate the petrogenesis, evolution and mantle source
region characteristics of Comorean lavas. The typically silica-undersaturated,
alkaline lavas from all three islands can be grouped into two distinct types:
La Grille-type (LGT) lavas, which display strong relative depletions in K, and
Karthala-type (KT) lavas, which do not. With the exception of the lavas erupted
by La Grille volcano on Grande Comore, which exhibit the petrographic and geochemical
characteristics expected of primary mantle-derived magmas, all Comorean lavas
analysed have experienced compositional modifications after they segregated
from their source regions. Much of this variation can be explained quantitatively
by fractional crystallisation processes dominated by the fractionation of olivine
and clinopyroxene. Semi-quantitative modelling shows that the consistent and
fundamental difference in composition between K-depleted LGT lavas and normal
KT lavas can be attributed to partial melting processes, provided amphibole
is a residual mantle phase after extraction of LGT magmas at low degrees of
melting. Low absolute abundances of the heavy rare earth elements in LGT magmas
are interpreted to reflect partial melting within the garnet stability field.
In contrast, KT magmas, which do not show relative K-depletions, are considered
to be the products of somewhat larger degrees of partial melting of an amphibole-free
source at comparatively shallower depths. Whereas the Nd and Sr isotopic compositions
of Comorean lavas (which show a significant range: 87Sr/86Sr=0.70319-0.70393;
143Nd/144Nd=0.51263-0.51288) bear evidence for a time-averaged depletion in
incompatible elements, the high incompatible element abundances of the lavas
are interpreted to reflect the effects of a recent mantle enrichment event.
At depths well within the garnet stability field this mantle enrichment is interpreted
to have taken the form of modal metasomatism with the introduction of amphibole
(giving rise to the source of LGT magmas), whereas cryptic metasomatism took
place at shallower levels (giving rise to the source of KT lavas). The Nd, Sr
and Pb isotope signature of the majority of Comorean lavas (both LGT and KT)
is proposed to be the result of predominant contributions from a somewhat heterogeneous
source representative of the ambient sub-Comorean mantle, comprising a mixture
between a HIMU component and a component on the depleted portion of the mantle
array (possibly the source of Indian Ocean MORB), with only limited contributions
from an EM I plume component. The lavas erupted by Karthala volcano (the youngest
Comorean lavas), however, have significantly different isotopic compositions
from all other Comorean lavas (lower 143Nd/144Nd and higher 87Sr/86Sr), suggesting
increased contributions from the EM I component.
Keywords:
basalt petrogenesis;Comores;mantle geochemistry;Ocean Island Basalts
Return to header
Current Contents
................
J. Petrology Editor's Page
This page maintained by P Stuart and
J Rosenbaum
©1996 last updated 21 March 1996