THE GEOCHEMISTRY OF LAVAS FROM THE COMORES ARCHIPELAGO, WESTERN INDIAN-OCEAN - PETROGENESIS AND MANTLE SOURCE REGION CHARACTERISTICS

New mineral and bulk-rock analyses, as well as Nd, Sr and Pb isotope c ompositions are presented for lavas from Grande Comore, Moheli and May otte, three of the four main islands of the Comores Archipelago in the western Indian Ocean, and these data are used to evaluate the petroge nesis, evolution and mantle source region characteristics of Comorean lavas. The typically silica-undersaturated, alkaline lavas from all th ree islands can be grouped into two distinct types: La Grille-type (LG T) 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 ma gmas, all Comorean lavas analysed have experienced compositional modif ications after they segregated from their source regions. Much of this variation can be explained quantitatively by fractional crystallizati on processes dominated by the fractionation of olivine and clinopyroxe ne. Semi-quantitative modelling shows that the consistent and fundamen tal difference in composition between K-depleted LGT lavas and normal KT lavas can be attributed to partial melting processes, provided amph ibole is a residual mantle phase after extraction of LGT magmas at low degrees of melting. Low absolute abundances of the heavy rare earth e lements in LGT magmas are interpreted to reflect partial melting withi n the garnet stability field. In contrast, KT magmas, which do not sho w relative K depletions, are considered to be the products of somewhat larger degrees of partial melting of an amphibole-free source at comp aratively shallower depths. Whereas the Nd and Sr isotopic composition s of Comorean lavas (which show a significant range: Sr-87/Sr-86 = 0.7 0319-0.70393; Nd-143/Nd-144 = 0.51263-0.51288) bear evidence for a tim e-averaged depletion in incompatible elements, the high incompatible e lement 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 th e form of modal metasomatism with the introduction of amphibole (givin g rise to the source of LGT magmas), whereas cryptic metasomatism took place at shallower levels (giving rise to the source of KT lavas). Th e Nd, Sr and Pb isotope signature of the majority of Comorean lavas (b oth LGT and KT) is proposed to be the result of predominant contributi ons from a somewhat heterogeneous source representative of the ambient sub-Comorean mantle, comprising a mixture between a HIMU component an d a component on the depleted portion of the mantle array (possibly th e source of Indian Ocean MORB), with only limited contributions from a n EM I plume component. The lavas erupted by Karthala volcano (the you ngest Comorean lavas), however, have significantly different isotopic compositions from all other Comorean lavas (lower Nd-143/Nd-144 and hi gher Sr-87/Sr-86), suggesting increased contributions from the EM I co mponent.