MINERALOGY, ALTERATION, FABRIC, AND TEXTURE OF SILICA-UNDERSATURATED MAFIC LAPILLI TUFFS FROM THE LATE CRETACEOUS BALCONES IGNEOUS PROVINCE, CENTRAL TEXAS, USA

The Late Cretaceous Balcones Igneous Province of central Texas includes more than 200 discrete volcanic centers mostly composed of silica-undersaturated mafic lapilli tuffs.

These tuffs are highly altered but contain relict igneous textures and fabrics. Commercial quantities of oil have been produced from at least 8 fields in these igneous rocks. The availability of core and seismic data associated with oil and gas exploration provides a unique opportunity to study the architecture and composition of these relatively unknown volcanic centers. In this study, five relatively shallow cores from three separate subsurface tuff bodies provide an opportunity to characterize these rocks (SEM, EDS, XRD, CT) without the effects of modern surficial weathering. XRD analyses have proven useful in identifying alteration and cement minerals, many of which are micrometer-size. SEM, EDS mapping, and CT provide detailed images of mineral textures and zonation.

Lapilli and ash grains are subrounded and poorly sorted. Alteration style and intensity varies between adjacent lapilli. Development and cementation of vesicles in grains also varies. Noncontemporaneous lapilli have been extensively mixed by explosive eruptions and slumping of crater walls.

Relict igneous minerals include intermediate ferrochromite-spinel, perovskite, partially-altered titanaugite, and apatite. Volcanic glass and some phenocrysts have been altered primarily to interlayered vermiculite-smectite. Phenocrysts thought to have been olivine are completely altered to serpentine at the crystal centers and to Al-bearing clays around the edges. Other alteration minerals include gibbsite, kassite, and the garnets schorlomite and katoite. Titanite may have been an original igneous mineral, an alteration product, or both.

Smectitic clays are also common as radial cements (hydrothermal products?) surrounding lapilli, and in vesicles. Pectolite (in Na-rich samples) and xonotlite are also pore-filling cements. Sparry calcite cement is abundant in parts of some cores. Pyrite cement is rare. Apatite appears in three roles: volcanic crystals, cement rimming grains and vesicles, and replacement of volcanic glass.

The tuffs have undergone complicated and prolonged post-depositional alteration processes which have affected chemistry and fabric.