Parageneses of Ordovician sub-greenschist to greenschist facies metabasites from Wales, U.K.

摘要: 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 following phases: epidote, prehnite, pumpellyite, and actinolite. In Wales associated diagenetic anchizone pelitic rocks have assemblage Chl Ab Ttn Qtz ± Ep Prh Pmp Stp Ms. This assemblage, typical prehnite-pumpellyite facies, occurs in across a full range (0.31-0.65) MgO/(MgO FeO) ratios (indicated chlorite). Assemblages epizone pelites comprise Act; pumpellyite is present 0.54; these diagnostic prehnite-actinolite respectively. True metabasites, Act, also found association pelites. The contrasting within local area, or even single intrusion, demonstrate that P-T conditions for directly overlap, distinction controlled whole rock composition. two cases studied control turn seen be function original intrusive history, primary igneous chemical zoning being reflected distribution assemblages. These well documented metabasite show lower upper boundaries cannot equated respectively onset