RELATIVELY ALUMINOUS ALKALI PYROXENE IN NEPHELINE SYENITES FROM MALAWI - MINERALOGICAL RESPONSE TO METAMORPHISM IN ALKALINE ROCKS

The North Nyasa Alkaline Province (NNAP) of central and northern Malaw i, eastern central Africa, consists of seven nepheline syenite intrusi ons which were, to varying degrees, affected by the Mozambique Orogeni c event. We have analyzed the constituent clinopyroxene in rocks of fi ve of these intrusions by electron microprobe. Three groups of pyroxen es can be distinguished. (a) Alkali pyroxenes range from diopside thro ugh aegirine-augite to aegirine. These have total Al < 0.15 (apfu) and are taken to be of primary magmatic origin. (b) Pyroxenes characteriz ed by Al > 0.15 (apfu), with Al-VI predominant, form a rim on group-(a ) pyroxenes, cut across them and also form, in one intrusion, tiny aci cular crystals within nepheline and, locally, feldspar. They comprise aluminian aegirine-augite, aluminian aegirine and omphacite. These pyr oxenes are interpreted as metamorphic in origin. (c) The third group c omprises Al-rich pyroxenes in which Al-IV is predominant. They are dio pside and aluminian diopside exhibiting no trend of alkali enrichment. They are probably of igneous origin, but could also have been affecte d by metamorphism. Although the production of pure jadeitic pyroxenes probably requires a minimum pressure of about 7 kbar, aluminian aegiri ne with higher contents of Fe3+ can probably be generated at much lowe r pressures. The preservation of primary igneous pyroxenes in the NNAP intrusions probably indicates that neither high pressures nor particu larly elevated temperatures were reached. The formation of the alumino us pyroxenes was essentially the result of an isochemical event; in ge neral, metamorphism of agpaitic rocks will give rise to aluminian aegi rine and a jadeitic pyroxene, whereas miaskitic rocks will contain alu minian aegirine-augite and an omphacitic pyroxene.