Parageneses of Ordovician sub-greenschist to greenschist facies metabasites from Wales, U.K.
Bevins, Richard E.; Robinson, Douglas
European Journal of Mineralogy Volume 5 Number 5 (1993), p. 925 - 936
published: Jan 1, 1992
manuscript accepted: Mar 30, 1993
manuscript received: Nov 16, 1992
ArtNo. ESP147050505008, Price: 29.00 €
Abstract Sub-greenschist to greenschist facies metabasites of Ordovician age from the Lower Palaeozoic Welsh Basin are characterized by mineral assemblages comprising ubiquitous quartz + albite + titanite + chlorite, along with usually at least three of the following phases: epidote, prehnite, pumpellyite, and actinolite. In Wales metabasites associated with diagenetic and anchizone pelitic rocks have the assemblage Chl + Ab + Ttn + Qtz ± Ep ± Prh ± Pmp ± Stp ± Ms. This assemblage, typical of the prehnite-pumpellyite facies, occurs in metabasites across a full range (0.31-0.65) of MgO/(MgO+FeO) ratios (indicated by MgO/(MgO+FeO) ratios in chlorite). Assemblages in metabasites associated with epizone pelites comprise Chl + Ab + Ttn + Qtz ± Ep ± Prh ± Pmp ± Act; pumpellyite is present in metabasites with MgO/(MgÖ+FeO)Chl ratios of 0.54; these assemblages are diagnostic of the prehnite-actinolite facies respectively. True greenschist facies metabasites, characterized by the assemblage Chl + Ab + Ttn + Qtz + Ep + Act, are also found in association with epizone pelites. The contrasting assemblages found within a local area, or even within a single intrusion, demonstrate that in the Welsh Basin P-T conditions for the prehnite-pumpellyite and the prehnite-actinolite facies directly overlap, and that the distinction is controlled by whole rock composition. In two cases studied in Wales the whole rock control is in turn seen to be a function of the original intrusive history, with primary igneous chemical zoning being reflected in the distribution of the prehnite-pumpellyite and prehnite-actinolite facies assemblages. These well documented metabasite assemblages show that in Wales the lower and upper boundaries of the anchizone cannot be equated respectively with the onset of the prehnite-pumpellyite or the greenschist facies.