PETROLOGY OF GABBROIC XENOLITHS IN 1960 KILAUEA BASALT - CRYSTALLINE REMNANTS OF PRIOR (1955) MAGMATISM

The 1960 Kapoho lavas of Kilauea's east rift zone contain 1-10 cm xeno liths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Tex tures are poikilitic (ol + sp + cpx in pl) and intergranular (cpx +/- pl +/- ol +/- opx). Poikilitic xenoliths, which we interpret as cumula tes, have the most primitive mineral compositions, Fo82.5, Cpx Mg 86. 5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, a nd overall 'open' textures to suggest that they represent 'mush''and ' crust' of a magma crystallization environment. Their phase composition s are more evolved (Fo80-70, CPX Mg 82-75, and An73-63) than those o f the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7-5.8 wt%). The gabbroic xenolith mineral compositi ons fit existing fractional crystallization models that relate the ori gins of various Kilauea lavas to one another. FeO/MgO crystal-liquid p artitioning is consistent with the poikilitic ol-gabbro assemblage for ming as a crystallization product from Kilauea summit magma with appro ximately 8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to e arly 1955 rift-zone lavas (approximately 5 wt% MgO) through approximat ely 28-34% crystallization of the ol + sp + cpx + pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivin e gabbro-norite assemblages crystallized from evolved liquids, such as represented by. the early 1955 and late 1955 lavas (approximately 6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, t hen, scoured the rift-zone reservoir system to entrain portions of cum ulate and solidification zones that had coated reservoir margins durin g crystallization of prior east rift-zone magmas.