Chromite and PGE geochemistry of the Elekda臒 Ophiolite (Kastamonu, Northern Turkey): Implications for deep magmatic processes in a supra-subduction zone setting

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• 作者：Cahit D枚nmez ; Serdar Keskin ; Kurtulu艧 G眉nay ; Ayd谋n Olcay 脟olako臒lu ; Yahya 脟ift莽i ; 陌brahim Uysal ; Aytekin T眉rkel ; Nail Y谋ld谋r谋m
• 关键词：Chromite ; Deep magma processes ; Platinum group elements (PGE) ; Elekda臒 ophiolite ; Kastamonu ; Northern Turkey
• 刊名：Ore Geology Reviews
• 出版年：March, 2014
• 年：2014
• 卷：57
• 期：Complete
• 页码：216-228
• 全文大小：2654 K

文摘

The geochemistry of chromium spinel and platinum group elements (PGE) provides significant information about conditions in the mantle during the emplacement and genesis of ophiolitic complexes. The Elekda臒 ophiolite is a greenschist to amphibolite facies ophiolite that was emplaced onto the Sakarya Zone, in the central part of the Pontides of northern Turkey. NW-SE trending Elekda臒 ophiolite hosts small scale chromitite occurrences in serpentinized harzburgite and dunite, and have maximum resource of several thousands t Cr₂O₃.

The chromitites the Elekda臒 ophiolite can be divided in to 鈥渉igh-Cr鈥?and 鈥渉igh-Al鈥?types. The calculated melt composition of the high-Cr chromitites is 9.4-13.2 wt.% Al₂O₃, 0.2-0.4 wt.% TiO₂ with FeO/MgO ratios of 0.4-1.9, which are indicative of boninitic compositions. However, the high-Al chromitites have 14.7-17.3 wt.% Al₂O₃ and 0.3-0.8 wt.% TiO₂ with FeO/MgO ratios of between 0.3 and 0.8, which are consistent with MORB-like tholeiitic affinities. The total PGE content of the chromitites range between 82 and 1466 ppb, and the average PGE content of the high-Cr and high-Al chromitites are 267 ppb and 195 ppb, respectively. The chromitites are characterized by low Pd/Ir ratios of 0.02-0.20 that is characteristic of non-fractionated chromitites. The chondrite normalized PGE pattern of the chromitites resemble those of mantle-derived Cr-rich chromitites in ophiolites of the supra-subduction zone (SSZ) elsewhere.

Mineral and bulk rock PGE data for the Elekda臒 chromitites indicate that high-Cr chromitites formed from Cr-rich and, Al- and Ti-poor boninitic melts, most probably in the fore-arc setting in the deeper part of the mantle. This was succeeded by the mineralization of high-Al chromitites from a differentiated boninitic melt at shallow depths close to the Moho Discontinuity.