ORIGINAL PAPER Shoji Arai Æ Jiro Uesugi Æ Ahmed H. Ahmed Upper crustal podiform chromitite from the northern Oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for Cr concentration Received: 5 February 2001 / Accepted: 15 December 2003 / Published online: 17 February 2004 Ó Springer-Verlag 2004 Abstract A new type of podiform chromitite was found at Wadi Hilti in the northern Oman ophiolite. It is within a late-intrusive dunite body, possibly derived from olivine-rich crystal mush, between the sheeted dike complex and upper gabbro. This chromitite forms small (<30 cm in thickness) pods with irregular to lenticular shapes. Neither layering nor graded bedding is observed within the pods. The chromitite is in the upper crust, by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the Moho transition zone to the upper mantle. It is dis- tributed along a small felsic to gabbroic melt pool within the dunite body, which was formed by melting of gab- broic blocks captured by the mush. Chromian spinel was precipitated due to mixing of two kinds of melt, a basaltic interstitial melt from the mush and an evolved, possibly felsic, melt formed by the melting of gabbro blocks. The podiform chromitite reported here is strik- ingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions. The for- mer contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions. Chromian spinel of the upper crustal podiform chromitite from Oman has relatively low content of (Cr 2 O 3 + Al 2 O 3 ), the Cr/(Cr + Al) atomic ratio of around 0.6, and the relatively high TiO 2 content ranging from 1 to 3 wt%. We conclude that assimilation of relatively Si-rich materials (crustal rocks or mantle orthopyroxene) by olivine-spinel saturated melts can explain the genesis of any type of chromitite. Introduction The genesis of podiform chromitite has been enigmatic and the relevant deep magmatic process has been controversial (e.g., Thayer 1964, 1969; Dickey 1975; Cassard et al. 1981; Lago et al. 1982; Paktunc 1990; Leblanc and Nicolas 1992). The podiform chromitite displays various occurrences suggesting various origins (e.g., Thayer 1964; Lago et al. 1982; Leblanc and Ce- uleneer 1992; Arai and Yurimoto 1994; Zhou et al. 1994; Ballhaus 1998; Be´dard and He´bert 1998; Matveev and Ballhaus 2002). Lago et al. (1982) demonstrated the dike-like nature of podiform chromitite, and pro- posed that spinel concentration is due to turbulent magmatic current within conduits of the mantle peri- dotite. Arai and Yurimoto (1994) and Zhou et al. (1994) ascribed its genesis to a peridotite/melt reaction and subsequent mixing of relatively Si-rich melts pro- duced by the reaction and a primitive melt within conduits in the upper mantle to the Moho transition zone. Be´dard and He´bert (1998) suggested that the podiform chromitite also forms through assimilation of lower crustal rocks into peridotitic intrusion around the Moho transition zone. The origin of podiform chro- mitite, especially nodular-textured one, was also ex- plained by magma mingling (Ballhaus 1998) and by exsolution of a fluid phase from a water-rich melt (Matveev and Ballhaus 2002). For the stratiform chromitite in layered intrusions, a newly replenished primary melt was mixed with a fractionated melt from an earlier melt (Irvine 1977) or with a granitic melt derived from partial melting of wall crustal rocks by earlier melts (Irvine 1975). More recently Roeder and Reynolds (1991) and Campbell and Murck (1993) also suggested an important role of magma mixing in stratiform chromitite formation. As in the other ophiolites, the podiform chromitites, which have dunite envelope within harzburgite, always occur in the upper mantle to Moho transition zone in the Oman ophiolite (e.g., Auge 1987; Ahmed and Arai Editorial responsibility: V. Trommsdorff S. Arai (&) Æ J. Uesugi Æ A. H. Ahmed Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan E-mail: ultrasa@kenroku.kanazawa-u.ac.jp Tel.: +81-76-2645726 Fax: +81-76-2645746 Contrib Mineral Petrol (2004) 147: 145–154 DOI 10.1007/s00410-004-0552-8