|
Discussion |
Department of Geology, National Museums & Galleries of Wales, Cardiff CF10 3NP, UK Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK (e-mail: aewright@onetel.net. uk) School for Professional and Continuing Education, University of Birmingham, Birmingham B15 2TT, UK
| The first 250 words of the full text of this article appear below. Images appear only in PDF or full-text views. |
 |
Jana Horák writes: |
|---|
Firstly, the conclusion that the new age data for the Arfon Group enables a redefining of the Sarn Complex terrane affinity is questioned. In particular the statement
it also now seems unlikely that the Sarn Complex should be compared with the Padarn area as the Parwyd Gneiss has yielded a Rb–Sr date of 542 ± 17 Ma (Beckinsale et al. 1984). This is not now regarded as the age of the amphibolite facies metamorphism (Horák et al. 1996) but must indicate a considerable retrogression event in the gneissose basement in Late Precambrian–Cambrian time
does not appear to have any relevance to any of the data presented. The Parwyd Gneiss is represented by a single exposure of retrogressed garnet amphibolite and felsic gneiss, which lies entirely within the Ll
n Shear Zone. Although previously included within the Sarn Complex, recent work has defined this as a separate unit of uncertain affinity (e.g. Gibbons & Horák 1990; Gibbons & McCarroll 1993). In contrast to the gneisses, the Sarn Complex is a poorly exposed heterogeneous plutonic complex, of which the most northwestern margin has been involved in ductile and brittle deformation. The Rb–Sr isotopic data age for the Parwyd gneisses has no bearing on the affinity or age of the Sarn Complex and furthermore
