PARAGENESES OF ORDOVICIAN SUB-GREENSCHIST TO GREENSCHIST FACIES METABASITES FROM WALES, UK

Sub-greenschist to greenschist facies metabasites of Ordovician age fr om the Lower Palaeozoic Welsh Basin are characterized by mineral assem blages comprising ubiquitous quartz + albite + titanite + chlorite, al ong with usually at least three of the following phases: epidote, preh nite, pumpellyite, and actinolite. In Wales metabasites associated wit h diagenetic and anchizone pelitic rocks have the assemblage Chl + Ab + Ttn + Qtz +/- Ep +/- Prh +/- Pmp +/- Stp +/- Ms. This assemblage, ty pical of the prehnite-pumpellyite facies, occurs in metabasites across a full range (0.31-0.65) of MgO/(MgO+FeO) ratios (indicated by MgO/(M gO+FeO) ratios in chlorite). Assemblages in metabasites associated wit h epizone pelites comprise Chl + Ab + Ttn + Qtz +/- Ep +/- Prh +/- Pmp +/- Act; pumpellyite is present in metabasites with MgO/(MgO+FeO)Chl ratios of < 0.54, while actinolite occurs in metabasites with MgO/(MgO +FeO)Chl ratios of > 0.54; these assemblages are diagnostic of the pre hnite-actinolite facies respectively. True greenschist facies metabasi tes, characterized by the assemblage Chl + Ab + Ttn + Qtz + Ep + Act, are also found in association with epizone pelites. The contrasting as semblages 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 t hat the distinction is controlled by whole rock composition. In two ca ses studied in Wales the whole rock control is in tum seen to be a fun ction of the original intrusive history, with primary igneous chemical zoning being reflected in the distribution of the prehnite-pumpellyit e and prehnite-actinolite facies assemblages. These well documented me tabasite 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.