SEGREGATION OF MONZOSYENITE LIQUID FOLLOWING DEVOLATILIZATION IN THE MONZOGABBROIC BONE SPRINGS SILL, BIG BEND, TEXAS

Bone Springs Sill (BSS) in northern Big Bend National Park differentiated at low pressure from hydrous tephritic magma to a bimodal monzogabbro-monzosyenite intrusion. Fractional crystallization models indicate that monzosyenite liquid was generated by ~50% crystallization of subequal clinopyroxene + plagioclase, lesser Ti-magnetite + olivine, and minor biotite + apatite + nepheline (later converted to analcime). Most of this monzosyenite liquid crystallized as the interstices of the monzogabbro, but after rupturing of the crystalline framework, probably coincident with vapor exsolution, some liquid segregated into discrete monzosyenite bodies in the upper 1/3^rd of the sill. The most felsic monzosyenites in BSS were generated by an additional ~8% fractionation of alkali feldspar + amphibole + nepheline (or analcime) and minor apatite + titano-hematite.

Fluorine-rich apatite began crystallizing in BSS magma either before or during intrusion and biotite began crystallizing shortly after intrusion. Mineral chemistries record Cl loss concurrent with the separation and segregation of the differentiated monzosyenite liquid. This Cl loss is related to a boiling event during which volume expansion ruptured the crystalline framework and allowed migration of the differentiated interstitial liquid. Devolatilization caused an attendant loss of Cl from the melt by its partitioning into the vapor phase. The highest mineral F/Cl is found in the upper 1/3^rd of BSS as a result of F increase with differentiation and Cl loss to vapor exsolution. An apatite-biotite mineral pair included in a Ti-magnetite grain from this section records the highest calculated temperature in the sill: 1093° ± 103° C. This apatite and biotite have high F and low Cl, suggesting that their inclusion postdates devolatilization and segregation of the residual liquid.