Abstract

The compositions of hornblendes from Gibson Peak pluton, a composite intrusion ranging from hypersthene gabbro to trondhjemitic tonalite, appear related as much to position within crystallization sequence as to bulk rock composition. Early-formed hornblende in trondhjemitic tonalite has the composition (Na, K)0.3Ca1.8(Mg, Fe2+, Mn)4+7Fe3+0.6Si7.0Al0.8Ti0.1O22(OH,F,Cl)2.0. Late poikilitic hornblende from hypersthene gabbro has the composition (Na, K)0.5Ca1.8(Mg, Fe2+, Mn)4.6(Fe3+, Ti)0.7Si6.5Al1.3Ti0.2O22(OH, F, Cl)1.9. Hornblende from a pegmatitic residual segregation in diorite has the composition (Na · K0.7Ca1.9(Mg, Fe2+, Mn)4.5(Fe3+, Ti)0.8Si6.1 Al1.7Ti0.2O22(OH, F, Cl)1.9.

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