Re. Bevins et D. Robinson, PARAGENESES OF ORDOVICIAN SUB-GREENSCHIST TO GREENSCHIST FACIES METABASITES FROM WALES, UK, European journal of mineralogy, 5(5), 1993, pp. 925-935
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.