Origin of chromite nodules in podiform chromitite from the Kızıldağ ophiolite, southern Turkey
Graphical abstract
Introduction
Podiform chromite deposit is one of the special deposits in mantle peridotite of ophiolites, and fundamentally is lenticular, tabular, pencil-shaped, or irregular in form (Thayer, 1964). The podiform chromite bodies commonly occur in harzburgite and are accompanied by thin segregation of dunites (e.g., Zhou et al., 2001, Huang et al., 2004). These enveloped dunites are proposed to be the result of local melting of the pyroxene in the surrounding harzburgite (Zhou et al., 1996). In most podiform chromite deposits, besides the ordinary massive and disseminated chromite ores, nodular chromitites are also observed, which is a unique characteristic in podiform chromite deposits (e.g., Thayer, 1969, Ahmed, 1982, Prichard et al., 2015). These nodules commonly have ellipsoidal shapes, and range in size from 3 to 50 mm (~10–25 mm in most cases) (Pavlov et al., 1977). Generally, chromite nodules occur in groups and are in contact with each other (e.g., Ahmed, 1982, Paktunc, 1990). Many studies reported that the texture of nodular ores appears to be primary and can provide important clue to the mode of formation of podiform chromite deposit (e.g., Cassard et al., 1981, Ballhaus, 1998, Zhou et al., 2001, Gundewar and Sinha, 2013).
Although the nodular textures of chromitite have been emphasized in metallogenic theories of the economic podiform chromite deposits, the origin of chromite nodules is still controversial. Firstly, Thayer (1969) pointed that the nodular texture could be the product of aggregation of free-formed chromite grains prior to settling. Then, Dickey (1975) proposed the nodular textures may be generated by the snowballing of chromite crystals in a turbulent zone of magma segregation. Cassard et al. (1981) attributed the formation of nodules to pelletization driven by elutriation in parts of magmatic conduits where downward motion of chromite remains in dynamic equilibrium with ascending magma flow. As the research progresses, the chromite nodules were considered as the results of incomplete mixing between the Cr-rich melt and Mg-rich melt in a magma conduit (e.g., Pavlov et al., 1977, Ballhaus, 1998). Zhou et al. (2001) proposed that the nodular chromite grains are produced by primitive melt-mantle harzburgite interaction that followed by mixing of the new-formed melt droplets with the primitive magma in porous dunite conduits. Others raised that nodule is formed by crystallization and growth of the dendritic chromite from a magmatic system that is undercooled and/or supersaturated in Cr (e.g., Greenbaum, 1977, Leblanc, 1980, Prichard et al., 2015). There is no agreement on how these chromite nodules form up to now, which is mainly due to that chromite is isotropic and predominantly opaque, posing difficulties in recognizing the substructures during routine microscopic studies (Ghosh et al., 2017).
Analytical techniques-electron backscattered diffraction (EBSD) has become available for performing complex petrographic analyses, which provides a new approach to evaluate the hypotheses of chromite nodule origin. The EBSD technique is capable of determining the crystallographic orientation of grain lattices in the intergrowths (e.g., Xu et al., 2015) that are crucial to distinguish whether they are components of a single crystal or grew as a cluster of randomly oriented nuclei. Recently, the EBSD technique has been used to study the crystallographic relationships (e.g., Vukmanovic et al., 2013, Prichard et al., 2015, Ghosh et al., 2017, Yudovskaya et al., 2019) and deformation mechanism of chromite grains (e.g., Satsukawa et al., 2015). However, only one study has discussed the details of possible formation processes of chromite nodules (Prichard et al., 2015). On the basis of EBSD and geochemical observations of unusual skeletal chromite nodules from the Troodos ophiolite in Cyprus, Prichard et al. (2015) found that these nodules were emanated from a single skeletal chromite core. However, nodular texture without skeletal core is the more general phenomenon observed from worldwide ophiolite complexes, and the scenario underpinned by the skeletal chromite nodule may not be a representative case.
In this study, we carried out a detailed microtextural and geochemical study on a chromite nodule without skeletal cores from the Kızıldağ chromitite using electron backscattered diffraction (EBSD), electron microprobe analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) techniques, that provide new insight into the contentious question of how the chromite nodules form.
Section snippets
Sample description
The Kızıldağ ophiolite is located in southeastern Turkey, and forms part of a discontinuous ophiolite belt along the northern margin of the Arabian plate (Ricou et al., 1984, Dilek and Thy, 2009). It has been considered as the most well-preserved ophiolite among the Turkish ophiolites (Chen et al., 2015). The Kızıldağ ophiolite constitutes a complete ophiolite assemblage which includes (from bottom to top) mantle tectonites, ultramafic–mafic cumulates, sheeted dikes, plagiogranites, and
Electron backscatter diffraction (EBSD) analysis
One large-size chromite nodule from the nodular chromitite sample KZ15-30 was selected for the electron backscatter diffraction (EBSD) analysis (Fig. 2a). The sample surface was prepared for EBSD via chemical–mechanical polishing using colloidal silica and given a thin carbon coat to prevent charging in the SEM. EBSD measurement was undertaken using a Zeiss SUPRA 55 FESEM with an Oxford NordlysNano EBSD acquisition camera, housed in FESEM Laboratory, China University of Geosciences (Beijing).
Orientation analysis of chromite nodule
Electron backscatter diffraction (EBSD) data were processed to create the inverse pole figure (IPF) map that is color-coded based on the orientations of chromite grains, exhibiting the crystallographic orientations of the analyzed grains. Red, green, and blue colors are designated to grains whose 〈1 0 0〉, 〈1 1 0〉, and 〈1 1 1〉 axes, respectively, are parallel to the projection of IPF (Fig. 2c). All other orientations are color-coded and are assigned blends of these three basic colors based on
Temperature estimation of the magma system
Geothermometric methods based on the Fe-Mg exchange between the coexisting olivine and chromite have been widely applied to the petrogenesis of many ultramafic complexes (e.g., JAN and HOWIE, 1981, Xiao et al., 2016). Based on the Fe-Mg exchange thermometry between olivine and chromite (Ballhaus et al., 1991), here the calculated temperatures for the nodular chromitites most center on ~ 750 ℃, below the common magmatic temperatures, indicating significant Fe-Mg equilibration between olivine and
Conclusions
The new investigations of geochemical features and crystallographic orientations of chromite nodules in the Kızıldağ podiform chromitite give us an opportunity to rethink the previous hypotheses proposed for the formation of nodular ore texture. The following conclusions can be drawn:
- 1.
It is hard to choose a preferred one among these hypotheses that can generate an acceptable interpretation to match observations from different perspectives.
- 2.
Ophiolitic chromite deposits and the associated dynamic
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This study was supported by the National Natural Science Foundation of China (grants Nos. 42002049 and 91962217). We thank Chang-Ming Xing and Dan Wu for the EPMA and LA-ICP-MS analysis, and Ben-Xun Su and Yan Xiao, Wei Lin, Ke-Zhang Qin, Yang Chu for the assistance in the field trips in the Kızıldağ ophiolite.
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