Petrogenetic significance of spinel-group minerals from the ultramafic rocks of the Iti and Kallidromon ophiolites (Central Greece)

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• 作者：Sofia Karipi ; Basilios Tsikouras ; Konstantin Hatzipanagiotou ; Tassos A. Grammatikopoulos
• 关键词：Spinel-group minerals ; Melt-peridotite interaction ; Boninitic melt ; Hydrothermal alteration ; Iti ; Kallidromon
• 刊名：Lithos
• 出版年：2007
• 出版时间：November 2007
• 年：2007
• 卷：99
• 期：1-2
• 页码：136-149
• 全文大小：1334 K

文摘

The peridotites occurring at Iti and Kallidromon ophiolites comprise lherzolite with lensoid dunitic bodies (only at Kallidromon) and harzburgite. The lherzolite contains spinel of aluminous composition forming subhedral to anhedral grains with lobate boundaries. The dunite and harzburgite contain chromite and magnesiochromite, respectively, forming subhedral to euhedral grains. The dunitic chromite displays commonly richer-in-Cr core compositions rimmed by poorer-in-Cr ones. Infrequently, the magnesiochromitic rims show irregular distribution of small areas of poorer-in-Cr composition. On the basis of their Cr#, the lherzolitic spinels and the harburgitic magnesiochromites are analogous to those from abyssal peridotites and oceanic (including back-arc basins) ophiolites, whereas the chromites in the dunite resemble those from arc-related ophiolitic sequences. The lherzolitic spinels show a linear covariation of Cr# with Mg#, compatibly with their being products of restricted partial melting. The richer-in-Cr chromites cluster at high Cr#, as a result of their crystallization from a boninitic melt while the poorer-in-Cr chromites and the magnesiochromites show a linear covariation, with the latter being the Cr-poor end-member of that trend, suggesting crystallization during the evolution of the boninitic melt that interacted with the mantle peridotites. The dunite formation is assigned to the interaction of that melt with the lherzolite after consuming pyroxenes and crystallizing olivine and chromite. This melt subsequently migrated upwards in the harzburgite and evolved, thus magnesiochromite, lower in Cr#, was crystallized. Textural evidence and mineral chemistry data from the harzburgitic magnesiochromite are in agreement with a melt-peridotite interaction after a partial melting event. Later hydrothermal alteration imprinted in the samples by forming ferrian chromite and magnetite at the expense of the spinel-group minerals and developing the silicate assemblage antigorite + talc + tremolite, occurred at conditions analogous to the boundary of greenschist-low amphibolite up to low amphibolite facies. The crystallization of spinel-group minerals induced from a boninitic melt that interacted with mantle peridotites, along with chemical affinities of the magnesiochromite and chromite and plots of pairs of either chromite or magnesiochromite and olivine from the dunitic and harzburgitic samples, are consistent with the origin of the Iti and Kallidromon peridotites in a supra-subduction zone (SSZ) environment.