PETROGRAPHIC ANALYSIS OF THE FAYALITE SYENITE OF GRAPEVINE HILLS, BIG BEND NATIONAL PARK

The Grapevine Hills laccolith is a 3.5 km wide by 200 m thick intrusive complex that solidified 31.9 Ma ago. This rock body is of interest because of the presence of fayalite in a syenite matrix. The bulk chemistry of this rock has been published, but detailed petrographic information is limited.

Five samples were collected and thin sections were prepared in order to investigate the petrography and petrogenesis of this fayalite syenite. Petrographically, the fayalite appears as phenocrysts, most of which have been altered to hematite, magnetite, and epidote. Other magmatic minerals present include alkali feldspar phenocrysts, smaller laths of plagioclase, magnetite, and quartz, predominately as groundmass. Other early-formed minerals that have been extensively altered include pyroxene and hornblende.

The dominant igneous texture in the Grapevine Hills fayalite syenite is a granophyric intergrowth of alkali feldspar and quartz. This texture is micrographic and appears to have nucleated from the larger alkali feldspar crystals. This texture is indicative of rapid undercooling, allowing for simultaneous crystallization of feldspar and quartz. Other textures observed include Carlsbad twinning and compositional zoning within the alkali feldspars. In some of the zoned feldspars there has been selective alteration of certain zones to hematite and epidote. There is also an interesting intergrowth of magnetite and a ferromagnesian mineral, presumed to have originally been hornblende but now almost completely altered to epidote. This texture is interesting because it indicates that the magnetite must have been magmatic in origin and crystallized at the same time as the hornblende.