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

ALAN R. WOOLLEY
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

R. GARTH PLATT
Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1

G. NELSON EBY
Department of Earth Sciences, University of Massachusetts at Lowell, Lowell, Massachusetts 01854, U.S.A.

Abstract

The North Nyasa Alkaline Province (NNAP) of central and northern Malawi, eastern central Africa, consists of seven nepheline syenite intrusions which were, to varying degrees, affected by the Mozambique Orogenic event. We have analyzed the constituent clinopyroxene in rocks of five of these intrusions by electron microprobe. Three groups of pyroxenes can be distinguished. (a) Alkali pyroxenes range from diopside through aegirine-augite to aegirine. These have total Al < 0.15 (apfu) and are taken to be of primary magmatic origin. (b) Pyroxenes characterized by Al > 0.15 (apfu), with VIAl predominant, form a rim on group-(a) pyroxenes, cut across them and also form, in one intrusion, tiny acicular crystals within nepheline and, locally, feldspar. They comprise aluminian aegirine-augite, aluminian aegirine and omphacite. These pyroxenes are interpreted as metamorphic in origin. (c) The third group comprises Al-rich pyroxenes in which IVAl is predominant. They are diopside and aluminian diopside exhibiting no trend of alkali enrichment. They are probably of igneous origin, but could also have been affected by metamorphism. Although the production of pure jadeitic pyroxenes probably requires a minimum pressure of about 7 kbar, aluminian aegirine with higher contents of Fe3+ can probably be generated at much lower pressures. The preservation of primary igneous pyroxenes in the NNAP intrusions probably indicates that neither high pressures nor particularly elevated temperatures were reached. The formation of the aluminous pyroxenes was essentially the result of an isochemical event; in general, metamorphism of agpaitic rocks will give rise to aluminian aegirine and a jadeitic pyroxene, whereas miaskitic rocks will contain aluminian aegirine-augite and an omphacitic pyroxene.