PETROGENETIC SIGNIFICANCE OF SPINELS FROM ULTRAMAFIC ROCKS FROM THE ITI AND KALLIDROMON OPHIOLITES (CENTRAL GREECE)

Abstract

Cr- and Al-spinels from ultramafic rocks from the Iti and Kallidromon ophiolites were studied. Subhedral to anhedral Al-spinel, with lobate boundaries, occurs in lherzolite, usually surrounded and veined by a rim of magnetite. Cr-spinel in harzburgite is commonly subhedral to euhedral and shows two compositions, with Cr-rich areas irregularly distributed over Cr-poor ones. Ferritchromit and magnetite are developed along rims and fractures (Fig. 1a). Subhedral to euhedral Cr-spinel contained in dunite, commonly forms a core-to-rim zonation of Cr-rich to Cr-poor areas, with frequent ferritchromit rims (Fig. 1b). The two compositions of the harzburgitic Cr-spinels are slightly poorer in Cr, compared to the dunitic ones. The Cr# in the studied spinels displays a wide variability. The lherzolitic and harzburgitic Cr-poor spinels display low Cr# ( 60, characterize arcrelated ophiolitic sequences (Dick and Bullen 1984). The studied Al-spinels (with Cr# = 13.80 - 32.31) are analogous to those from abyssal peridotites (Fig. 2). The dunitic Crrich analyses (with Cr# = 83.06-87.05) are restricted at high Cr# values, while the dunitic Cr-poor ones (with Cr# = 60.33 - 75.53) show linear covariation with Mg# (Fig. 2). The harzburgitic Cr-poor analyses (with Cr# = 43.82 - 55.80) show linear covariation with Mg#, while the Cr-rich analyses have Cr# = 80.38 - 83.00. The dunitic and harzburgitic Cr-rich spinel analyses and the dunitic Cr-poor ones plot at high Cr#, analogous to those from both Alpine type and arc-related peridotites, but possess slightly lower Mg# values. The harzburgitic Cr-poor analyses are also similar to spinels from Alpine-type peridotites (Fig. 2). The studied spinels resemble those occurring in ultramafic rocks from Othrys, Pindos, Lesvos and Eastern Chalkidiki ophiolites. The lherzolitic spinels reveal a sympathetic increment of Cr# with Mg# indicating cogenetic relationship, while the dunitic and harzburgitic ones deviate and trend towards higher Cr#. The chemistry of the spinels from both Iti and Kallidromon suites, display a tectonomagmatic evolution from MORB-type affinities towards island-arc signatures. The lherzolitic, harzburgitic, and most of the dunitic spinel-olivine pairs plot within the OSMA (olivine-spinel mantle array; Fig. 3). The lherzolitic pairs plot in the abyssal peridotite field (although not diagnostic due to overlap with passive margin, oceanic arc and marginal basin peridotites) and close to the fertile mantle composition. The dunitic and harzburgitic Cr-poor spinel - olivine pairs are consistent with a SSZ origin. The dunitic Cr-rich spinel – olivine pairs lie within or near the OSMA and follow the trend defined by the fractionation line of boninites (Fig. 3). Textural evidence and linear evolution of Cr# and Mg# in the studied spinels reflects various degrees of partial melting, at least in the initial stages of the evolution of the host peridotites. However, the genesis of the Cr-rich dunitic and harzburgitic spinels may involve more complicated processes. According to Zhou et al. (2005), such high Cr spinels are related to crystallization from percolating SSZ boninitic melts, compatibly with the plots of the dunitic olivine – spinel pairs along the trend defined by the fractionation line of boninites (Fig. 3). The observed core-to-rim Cr2O3 depletion in the dunitic spinels is a common feature of Cr-spinel crystallised from melt (Leblanc and Ceuleneer 1992). Thus, differentiation of magma resulted in the formation of the Cr-poor compositional areas in the harzburgitic and dunitic spinels. The development of subhedral to euhedral grains in the harzburgites and dunites, in contrast to the anhedral lherzolitic Al-spinels, is also consistent with such an episode. The above evidence suggests a metasomatic origin for the Cr-spinels at the expense of the lherzolitic Al-spinels via boninitic melt-lherzolite interaction, in a SSZ regime. This is consistent with petrographic features and geochemical data from the peridotites, given elsewhere (Karipi et al., 2005). During this process, assimilation of clinopyroxene and orthopyroxene in the host lherzolite caused increment in the SiO2 content, in the percolating melt leading to the precipitation of olivine and Cr-spinel.