Discussion of ‘the Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions by Qiu Hua-Ning and Wijbrans J.R.’

Abstract

The in vacuo crushing procedure employed by Qiu and Wijbrans (2008) is identical to that of Qiu and Wijbrans (2006) and has produced a similar result. The authors provide no description of the sample material analysed and the display of data is highly selective (see also Kendrick, 2007). The new step-heating data produced for crushed garnets are readily explained in terms of degassing from mineral inclusions (Fig. 1) and the argon isotope systematics preclude the ~ 450 Ma 'plateau' or 'isochron' ages being related to primary metamorphic fluid inclusions. The range of apparent ages determined for garnets, phengite and biotite by Qiu and Wijbrans (2006, 2008) extend from ~ 150 Ma to over 1000 Ma and only a subset of garnets yield in vacuo crushing apparent ages of ~ 450 Ma (see Fig. 2). The range of apparent Ar/Ar ages reported by Qiu and Wijbrans (2006, 2008) is similar to that determined for Dabie Shan phengite previously (Giorgis et al., 2000). Such variability is typical of UHP metamorphic rocks where it is well understood that argon has limited mobility and that the prevalence of extraneous Ar makes application of the Ar/Ar technique challenging (Arnaud and Kelley, 1995; Scaillet, 1996; Boundy et al., 1997; Giorgis et al., 2000). In summary, the in vacuo crushing data presented by Qiu and Wijbrans (2006, 2008) appear to add little to our understanding of Dabieshan geochronology, the prevalence of extraneous Ar in UHP terranes, or the in vacuo crushing technique. The data are poorly constrained and remain compatible with the majority of published data, that constrain Dabieshan UHP metamorphism to a Triassic age of 210-230 Ma (see Ames et al., 1993; Chavagnac and Jahn, 1996; Zheng et al., 2003; Chen et al., 2006; Zhao et al., 2007).