Balcones Province (Uvalde County; Balcones Fault Zone)

More than 200 plugs, sills and volcanic remnants extend northeast-southwest over some 300 km from east of Austin to west of Uvalde. Many of the plugs form hills, but numerous occurrences have only been encountered in drilling. Some 90% of the intrusions are concentrated around Uvalde (Fig. 167) and there is a smaller concentration known as the Travis volcanic field (Ewing and Caran, 1982) around Austin, where many do not outcrop at the surface. The province parallels the Balcones fault zone and is co-linear with the Ouachita structural belt to the north, but intensive activity pre-dates the major movements along the Balcones faults, many of the igneous intrusions being cut by them. According to Spencer (1969, p. 274), about 50% of the exposed igneous rocks are melilite-olivine nephelinite, 30% olivine nephelinite, a little over 10% phonolite, less than 10% basalt and less than 5% nepheline basanite. Olivine nephelinite contains titanaugite, olivine, nepheline, opaques and a little apatite, biotite and zeolite. The presence of melilite and perovskite distinguishes the melilite-olivine nephelinites. Basanites comprise clinopyroxene, olivine and nepheline, the last being absent from basalts. Phonolites contain alkali feldspar, aegirine-augite, nepheline and analcime, with accessories including scarce brown amphibole and olivine. A mafic variety of the phonolites contains more abundant brown amphibole and pyroxene in an analcime-rich matrix. The well-exposed Pilot Knob, near Austin, consists of nepheline basanite surrounded by pyroclastic rocks and has been described in detail by Barker and Young (1979). This, and numerous other occurrences, are located and named on maps in Ewing and Caran (1982, Figs 3 and 4). Hornfelsed inclusions are common but not abundant in the province, and a zone of spinel peridotite nodules occurs in the quarry at Knippa, near Uvalde (Spencer, 1969, p. 278; Cameron and Cameron, 1973).