Chemistry, subsolidus relations and electron petrography of pyroxenes from the late ferrodiorites of the Skaergaard intrusion, East Greenland

Abstract

The ferroaugites, inverted ferrowollastonites and the brown and green ferrohedenbergites from the Upper Zone (UZb and UZc) of the Skaergaard intrusion (Brown and Vincent, 1963) have been studied with the electron microprobe, and where necessary, with the electron microscope. The cloudy “inclusions” in the inverted ferrowollastonite (Wo_ss) of 4471 are established to be strain fields associated with stacking faults, dislocations and sub-grain boundaries. The green pyroxenes of 1881 have undoubtedly inverted from Wo_ss, as both major and minor element chemistry show. The orientation of the tie-line joining coexisting Ca-rich and Ca-poor pyroxenes has also been established for this part of the quadrilateral, together with the Fe-Mg values at which the 4471 inverted Wo_ss would project on to Brown and Vincent's (1963) trend line for Ca-rich pyroxenes. These Fe-Mg values are the same as those of the 1881 brown ferrohedenbergites (Hed_ss).

The subsolidus cooling history of the inverted Wo_ss has been examined in the light of the present data. It is proposed that a Wo_ss of solidus composition Wo₃₉ may either (a) react to a two-phase assemblage of Hed_ss (composition Wo_42.5) + metastable clinoferrosilite, or (b) invert metastably to a Hed_ss of the same composition. For specimen 4471, these two types of subsolidus behaviour may occur in different crystals within the same large mosaic-patterned grain. The proposed model is consistent with difficulty in nucleation of clinoferrosilitic lamellae, combined with the sluggishness of reactions at low temperatures for these Fe-rich compositions. In both case (a) and (b), inversion to Hed_ss (with or without the formation of mosaic texture) precedes exsolution of clinoferrosilite. The two final subsolidus compositions for the host are ∼Wo₄₆ and ∼Wo₄₂, for types (a) and (b) respectively, and the final subsolidus composition of the lamellae is Wo₀-Wo₂.

The brown and green pyroxenes of 4330 show distinct differences in chemistry, the green being richer in Si, and depleted in Al and Ti relative to the brown. The 4330 green pyroxenes are poorer in Mn, and richer in Na, compared to the green inverted Wo_ss. The green colour in these UZc pyroxenes may be due to the drop in Ti content relative to brown pyroxenes.