Oxygen-Partitioning in Felsic Rocks and Cumulate Minerals in Potassic Magmatic Series (Bodrum Igneous Complex,
SW Anatolia)

Ursula Robert Département de Pétrologie, Université Pierre et Marie Curie, case 110, 4, pl.Jussieu,

75252 Paris Cedex 05, France


Pierre Agrinier Lab. des Isotopes Stables, Institut de Physique du Globe Paris, case 89, 4 pl. Jussieu,

75252 Paris Cedex 05, France


The Bodrum Igneous Complex shares with potassic volcanoes from different regions the presence of at least two well-defined chemical trends differing by their alkali-content. Commonly, different parental magmas have been evoked to explain those lineages. Here, one evolves towards silica oversaturated latites, trachydacites and rhyolites; the second, towards slightly undersaturated trachyandesites and alkali trachytes. They are associated in a large composite volcanic structure of Upper Miocene age, where intermediate facies are, however, the most voluminous and appear in several cycles, between 12 and 7.5 Ma whereas silica-rich felsic rocks seem to be restricted to an interval from 12 to 11 Ma and more alkali-rich felsic rocks from 10 to 9 Ma.

Major and Trace Element, Sr and Nd Isotopic Evidence

The products of the Bodrum Complex are potassic, subalkaline to alkaline, and span a wide range in compositions including a variety of basaltic rocks. Mineralogical evolutions are very similar throughout the major part of the two trends and indicate a relationship; their major and trace element variations suggest that they were generated by low-pressure crystal fractionation from mafic parental magmas, coupled with more complex mixing processes and some crustal contribution in the oversaturated series, but Sr and particularly Nd isotopes fail to distinguish clearly between both series, all values being within the range of those for mafic rocks (basalts, gabbros and cumulates) which include high-Mg, LIL-rich ultrapotassic (UK) varieties with characteristics of near primary magmas derived from an enriched lithospheric mantle.

Sampling the Magma Chamber Evolution

Currently, as in many subalkaline and calcoalkaline volcanoes, intermediate lavas and pyroclastics contain mafic blobs which are very fine-grained glass-bearing crystal-cumulates with compositions extending from basalts to basaltic trachyandesites; exceptionnally, coarse-grained cumulate pyroxenites, melagabbros and gabbros can be found in felsic rock-types. Both types of inclusions show clinopyroxen, a large amount of hydrated phases: phlogopite/biotite and/or amphibole, (but no olivine), plagioclase, rare K-feldspar and accessory apatite or titanite. Minerals evolve from very high mg values to more evolved compositions throughout these xenoliths. Chemically zoned pumice-bearing pyroclastic units may represent the liquid counterparts to the inclusions. Oxygen isotopes have been determined on the latter and on separated minerals from the two types of inclusions.

Oxygen Isotope Evidence for Magma Chamber Processes

The unaltered whole rock d18O values for this complex range from 6.6 to 9.3, excluding some glassy felsic samples (SiO2 > 65) which display values of 12 probably due to hydration. Mafic rocks show high values (6.6 to 8.9) when compared to normal mantle-derived magmas (d18O 5.0-6.5), with the highest values being those of the high-Mg UK varieties thus confirming the unusual characteristics of these rocks. In the Si- undersaturated felsic rocks, d18O shows little variation and is consistent with closed-system fractionation of a system which might have been slighty enriched or contaminated prior to the emplacement in the magma chamber. Despite their generally higher oxygen isotopic values, the main evolution of the oversaturated series may be accounted for by multistep closed-system fractionation which can be followed in detail with the minerals of the cumulates but 18O enriched samples belonging to an early pyroclastic episode and displaying low eNd suggest that assimilation may, however, have contributed in the earlier stage of the differentiating magma chamber.