CHROMIAN AMPHIBOLES AND MICAS IN ULTRAMAFIC INCLUSIONS IN QUATERNARY LAVAS OF KAMCHATKA AND THE KURILES

Abstract

Both of the main morphological types of chromiferous 'upper mantle' amphiboles and micas, i.e. the interstitial and poikilitic, previously found in inclusions from alkalic basalts, have now been identified in ultramafic inclusions in lavas of the Kurile-Kamchatka island arc. On the whole, the micas from the ultramafic inclusions from Kamchatka and the Kuriles are low-titanium, high-alumina chromian phlogopites. Their compositions resemble those of the interstitial micas from lherzolite inclusions in alkalic basalts, from which they usually differ only in a somewhat higher mafic index. Finds of amphiboles and micas in ultramafic inclusions from lavas of the Kurile-Kamchatka island arc suggest that mantle matter underwent metasomatic transformation at depth in that arc. The specific properties of the amphiboles from ultramafic nodules in island-arc lavas are a relatively low alkalinity and variable alumina content, both directly related to the composition of the main minerals of the inclusions.