Clinopyroxene Crystallization from a Basic Magma of Alkaline Affinity

TWO main types of clinopyroxene crystallization trends have been recognized in basic rocks depending on whether the magma is of tholeiitic or alkaline affinity. The augite-ferro-augite trends1 typical of tholeiitic magmas (the Skaergaard and Bushveld intrusions2, for example) are well documented and extend from near the diopside corner of the pyroxene quadrilateral (Fig. 1) almost to the ferrosilite-hedenbergite join. Calcic augite trends typical of alkaline basic magmas such as the Black Jack3 and Shiant Isles4 sills are less well defined, tend to be relatively short (Fig. 1) and lack the marked inflexion present in tholeiitic trends. Also, unlike those from tholeiitic magmas, the clinopyroxenes from alkaline basic intrusions do not coexist with calcium-poor pyroxenes. (Trends apparently transitional between the two have been recorded (for example, from the Kap Edvard Holm intrusion5,6) but need not be considered here.) The tholeiitic examples are from large differentiated intrusions in which the clinopyroxene crystals are relatively homogeneous whereas the alkaline examples are from much smaller sills in which the pyroxenes are strongly zoned. Consequently, the differences in the lengths of the trends, which are defined on the basis of bulk chemical analyses, are probably apparent rather than real.