Katayama et al. (2003) dated zircon in some ultramafic rocks dominated by garnet and titanian clinohumite (hereafter Ti-clinohumite rocks) from the Kokchetav ultrahigh-pressure (UHP) metamorphic terrane, and interpreted that the zircon cores formed in the mantle in the Proterozoic and the zircon overgrowths formed as a result of UHP metamorphism and mantle metasomatism by slab-induced fluids/melt during the early Paleozoic. However, the data presented in this and their foregoing papers (Muko et al., 2002; Okamoto et al., 2000) cast considerable doubt about their interpretation and conclusions.
Metasomatism of mantle peridotite by crustal materials adds Si (and conceivably Ti, Al, Fe, Ca, Na, K, and large ion lithophile elements), and removes Mg (and conceivably Cr, Ni, and Co). The mantle metasomatism proposed by Katayama et al. (2003) does not explain why the rocks are even more depleted in Si, Na, and K than garnet peridotites (Muko et al., 2002) (Fig. 1), or how the enormous amount of Al, an element known to be relatively immobile in fluid and melt, could be added into a peridotite to produce abundant garnet. Garnet rims in the Kokchetav Ti-clinohumite rocks are significantly higher in Mg relative to the cores (Muko et al., 2002), suggesting that the rocks were enriched in Mg rather than Fe before or during UHP metamorphism.
Katayama et al. (2003) suggested that large amounts of Ti and other high field strength elements (HFSE) were added into a peridotite protolith by metasomatism, based on the experiments by Iizuka and Nakamura (1995) at 80 kbar. The pressure estimates for the Kokchetav UHP rocks are mostly ≤60 kbar, although Okamoto et al. (2000) argued for ≥60 kbar. At these conditions, fluid derived from crustal materials is not enriched in Ti but in Si, causing olivine to transform into pyroxene (Iizuka and Nakamura, 1995). The high HFSE and H2O contents in Ti-clinohumite rocks do require metasomatism, but not necessarily of garnet peridotite in the subduction zone. In contrast, Scambelluri and Rampone (1999) demonstrated that the Ti-clinohumite–bearing rocks from western Liguria (Italy) were derived from Fe-Ti gabbros by Mg metasomatism in an oceanic crust and subsequent high-pressure metamorphism. A Ti-clinohumite rock similar to the Kokchetav ones occurs in the Chinese Su-Lu UHP metamorphic terrane (Yang, 2003). In terms of MgO, CaO, Fe2O3, and Al2O3 compositions, the Ti-clinohumite rocks are intermediate between ultramafites and unaltered Fe-Ti gabbros (Fig. 1). Mg metasomatism of a Fe-Ti gabbroic protolith by fluids from a contacting mantle-derived ultramafic rock in the crust may explain the enriched feature of Ti (and other HFSE), Fe, Al, Mg, Cr, Co, Ni, and volatiles and the depleted feature of Si, Ca, and Na in the Su-Lu Ti-clinohumite rock (Yang, 2003). Subsequent UHP metamorphism at a mantle depth transformed the altered gabbro into assemblages characterized by garnet + Ti-clinohumite + pyroxene + zircon + apatite. It is likely that similar chemical and tectonic processes took place in the Kokchetav Ti-clinohumite rocks, because these also better explain their chemical features.
The Proterozoic zircon cores are interpreted by Katayama et al. (2003) to have formed at depths where plagioclase or spinel was stable. The zircon cores contain high U and some also contain high Th (Table 1 in Katayama et al., 2003), suggesting that they may have formed in crustal materials. The zircon overgrowths contain much lower U and Th, indicating involvement of mantle components. This is consistent with the Mg metasomatism–UHP metamorphism scenario proposed above, but not with the corner convection hypothesis (Katayama et al., 2003). Moreover, pristine mantle peridotites are known to be depleted in Zr and can hardly contain zircon (Bea et al., 2001, and references therein). On the other hand, zircon could be abundant in gabbroic rocks, and it could continue to grow while the rocks transform into Ti-clinohumite rocks during subduction.
Financial support was provided by the Natural Science Foundation of China (No. 40173020).