STRUCTURE AND PETROLOGY OF ENCHANTED ROCK BATHOLITH, LLANO AND GILLESPIE COUNTIES, TEXAS

Abstract

Enchanted Rock batholith is one of several moderate-sized plutons that intruded a framework of tightly folded Precambrian metamorphic rocks in Middle Precambrian time. Tectonic features within the mass and structural attitude of wall rocks indicate the northern third is phacolithic. The remainder is both discordant and concordant batholithic, although the schists show a tendency to box the compass around the batholith. The phacolithic part occupies a synclinal trough plunging 35°–40° SE. The batholith consists of four concentric zones: (1) outer zone, medium to coarse leucogranite and granite; (2) intermediate zone, medium to coarse granite and quartz monzonite; (3) intermediate central zone, coarse quartz monzonite and alkalic granodiorite; and (4) the core, fine to medium leuco-quartz monzonite and leucogranite. Rapakivi are most abundant in zone 3. Seriate porphyritic texture prevails in all zones except the core where it is increasingly hiatal. Chilled border rocks are 10–20 feet wide and consist of leuco-quartz monzonite and quartz monzonite. Apophyses of the batholith in metamorphic wall rocks are leuco-quartz monzonite and leucogranite. Hiatal porphyritic texture prevails in chilled border and apophyses rocks. Internal structures of the batholith and its relation to country-rock structures indicate intrusion during the late, most severe stages of regional compression. Tensile stresses along axial planes of folds exerted structural control and guided the rise of the magma, as indicated by the pear-shaped outline of the batholith. The most probable mechanism of magma generation is selective fusion (melting) of random rock material. This occurred at depth in down-buckled zones below the present position of the batholith. Concentration of the proportions of normative constituents of individual rock zones is strikingly grouped about a composition having a 1:1:1 ratio of albite: orthoclase: quartz. Mineralogically there are systematic gradations in proportions of plagioclase, microcline, and quartz between rock zones suggesting that unmixing of sodic plagioclase from microcline was a dominant process. Outline shape of rock zones, textural relations between plagioclase and microcline crystals, and maxima-minima concentrations of unmixed sodic plagioclase suggest migration of part of the unmixed material within the batholith during the later stages of its structural and intrusive history. Field and chemical evidence indicate that probably not more than 5 per cent of the granites of the batholith are of replacement origin. Failure of roof rocks with partial escape of volatiles, cessation of regional tectonism, and depletion of the magma reservoir in the late stages of intrusion probably are responsible for finer grain of the core. Age determinations by the “Larsen Method” give an average age for the batholith of 815 million years.