In the Main Central thrust zone of the central Nepalese Himalaya, monazites decrease in age downsection between the Main Central thrust and Main Central thrust 1 (MCT-1 in Fig. 1) from ca. 22 to 3.3 Ma (Catlos et al., 2001). This astonishingly systematic result (Kohn et al., 2001) has been criticized, e.g. by Searle et al. (2002), who attributed the monazite growths to fluid infiltrations at low temperatures despite the fact that several of the monazites occur as inclusions in garnets. To explain the downward younging age data Robinson et al. (2003) portray the famous inverted metamorphism as polyphasal, a consequence of successive accretions of thrust slices to the base of the Main Central thrust sheet.
They appear to suggest that monazite/garnet growths resulted from downward heat conduction from overlying thrust sheets. It is likely that burial metamorphism can be ruled out because pre-Cenozoic mica dates show that deepest level in the sections was not significantly heated during the Cenozoic (cf. Copeland et al., 1991). Unfortunately the authors fail to locate the Main Central thrust 1 (see Catlos et al., 2001) or the Ramgarh thrust in the dated sections in central Nepal. Therefore the time lapse between thrusting and the initiation of garnet/monazite growth cannot be estimated. No structural or textural evidence is cited to show that the garnet growths were associated with Himalayan thrusts. Setting aside the significant question of how the monazites formed, their model can be tested using monazite and 40Ar/39Ar mica dates from the Main Central thrust zone in the Marsyangdi and Darondi sections in central Nepal (Catlos et al., 2001).
The Greater Himalayan Sequence near to the sections initially cooled extremely rapidly and then resided at temperature (T) <350 °C by 15–13 Ma (Vannay and Hodges, 1996; Godin et al., 2001). During the middle to late Miocene, a cooling rate of 6–17 °C/m.y. has been estimated for the Greater Himalaya and the Main Central thrust zone (Searle et al., 1997, 1999; Stephenson et al., 2001). The cooling rates suggested by the age data of Catlos et al. (2001) are 15–100 °C/m.y. (Fig. 1). Taking 16 monazite-mica pairs, 8 are consistent with the cooling rates quoted above, 2 are fairly close (25 and 28 °C/m.y.), and 5 are much higher. Although one is negative, it is clear that most are consistent with the “regional” cooling rate.
Since the hanging wall of the Main Central thrust cooled below 350 °C by ca. 10 Ma. (Fig. 1), it was too cold to heat its footwall enough to permit garnet growth at that time and later (England et al., 1993). Also, the section between Main Central thrust and Main Central thrust 1 cooled below 350 °C at ca. 3–5 Ma and therefore, contrary to Kohn et al. (2001), was too cold to permit garnet growth (3.3 Ma and T >500 °C, Catlos et al., 2001) in the underlying rocks. If new garnet did grow at this time in the lower parts of the sections then it occurred at temperatures much less than those estimated by Catlos et al. (2001) and lower also than those generally ascribed to garnet growth in pelites (e.g., Spear, 1993).
Given the questions raised above, it is clear that the tantalizing model of Robinson et al. (2003) requires testing by extensive field and laboratory work not least to establish a sound structural and metamorphic history.