We welcome the comments by Yang, who proposed that the Kokchetav ultramafic rocks have an Fe-Ti gabbroic protolith based on whole-rock major element chemistry (Muko et al., 2002), and have been subjected to a Mg metasomatism at crustal depths. The Kokchetav Ti-clinohumite–rich rocks described in our paper contain abundant volatile and high field strength element (HFSE)–bearing minerals. Combined with zircon U/Pb dating and trace element characteristics, we interpreted these features as fluid-infiltrated mantle metasomatism at a subduction zone.
The Kokchetav ultramafic rocks have clearly experienced long and complex metamorphic evolution that consequently buries their original histories as well as compositions. They contain abundant serpentine and amphibole minerals along secondary veins that resulted from later stage overprinting during exhumation. The whole-rock analysis would therefore include an effect of such alteration, and to use major chemistries to correlate the nature of protolith for metasomatic rocks, as shown by Yang, could be greatly misleading. However, examination of mineral inclusion and compositional zonation of refractory minerals, such as garnet and zircon, can reveal evidences prior to such overprintings (e.g., Schertl et al., 1991; Zhang et al., 1997; Katayama et al., 2000).
Compositions of garnet from the Ti-clinohumite–bearing ultramafic rocks compared with those from different rock types are shown in Figure 1. The analyzed garnet has similar composition to that of orogenic peridotites in the Dabie Mountains (Zhang et al., 1995), but is significantly different from those of associated eclogites and diamond-bearing pelitic gneisses from the same locality (Shatsky et al., 1995; Zhang et al., 1997; Okamoto et al., 2000). The garnet displays a pronounced zonation with increasing pyrope content from core (51–56 mol%) to rim (58–63 mol%); this is probably associated with the reaction of Chl + En = Fo + Prp + H2O. The pyrope-rich rim contains abundant inclusions of Ti-clinohumite, ilmenite, zircon, and apatite; in contrast, the garnet core includes green spinel, amphibole, and phlogopite. The absence of Ti-bearing phases in the garnet core suggests that Ti enrichment occurred during high-pressure recrystallization. This fact is not consistent with the suggestion of the Fe-Ti gabbroic protolith enriched in Mg during magnesium metasomatism prior to subduction as proposed by Yang. Trace element abundance of stubby-shaped zircon separated from the ultramafic rocks (see Figure 3 in Katayama et al., 2003) also indicates that they recrystallized at high pressures where garnet is stable. The zircon rare earth element abundances display significantly different patterns from those of associated eclogite and country gneisses, but are similar to the characteristics reported in kimberlite. The calculated pressure (>35 kbar) required for the coexistence of low-F Ti-clinohumite, olivine, and ilmenite is consistent with our proposed model.
These lines of evidence preserved in refractory minerals favor that HFSE-rich metasomatism of garnet-bearing peridotites took place at mantle depths rather than Fe-Ti enrichment and Mg metasomatism in gabbroic rocks at crustal depths. The Kokchetav ultramafic rocks also contain rare inherited zircons of Proterozoic ages with pronounced heavy rare earth element enrichment. Large differences for the radiogenic ages and trace element characteristics, including U and Th concentrations, raise a plausible interpretation that the inherited zircons formed during one or more Proterozoic events and subsequently resided in the mantle environment. These zircons served as seeds for zircon growth during the HFSE-rich metasomatic processes.