Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.492b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 492
Physics
Breccia, Clasts, Fracture, Granite, Intrusive Rock, Pseudotachylyte, Veins
Scientific paper
Enchanted Rock batholith in central Texas is a granitic pluton of ~250 km^2 area [1]. The batholith intrudes Precambrian metamorphic rocks and is dated by Rb-Sr at 1,048+/-34 m.y. [2]. Plutonic rocks are exposed in a series of exfoliation domes located along the southeastern edge of the batholith. Numerous subvertical fractures occur along the outer margins of the intrusion and, in a few cases, between exfoliation domes. A number of these fractures contain veins described as pseudotachylytes by Barnes [3]. In hand specimen, the veins resemble pseudotachylytes, forming a branching network and containing angular fragments of local country rock in a fine-grained matrix that locally exhibits an apparent flow texture. Some smaller veins appear to have been injected into host rock. The veins range in width up to ~30 cm with boundaries which are sharp and smooth with undulating surfaces. Thin (~4-5 cm) aplite dikes cut the granite and generally show small amounts of displacement where intersected by the fracture veins (maximum measured apparent displacement ~75 cm). Matrix material in veins includes fragments of quartz, feldspar, biotite, and opaques. Matrix biotite is generally associated with opaques and occurs as numerous, tiny, oriented flakes between fragments of feldspar and quartz in areas which are darker than surrounding matrix. The flow-like textures of matrix materials are defined by these darker areas which are elongated subparallel to fracture direction. The finer-grained matrix biotite apparently derives from comminution of larger grains from host rock. Larger clasts include rock fragments and fragments of alkali feldspar and quartz, with a tendency for the larger fragments to concentrate in the centers of veins. Clasts are irregular in shape and are commonly angular and rarely contorted. Rock fragments are derived from host rock and also include "breccia within breccia" fragments. Single-mineral fragments display evidence of strain; quartz commonly shows undulatory extinction, mosaic texture, and multiple fractures. Feldspar twins commonly are deformed. None of the samples show PDF's in quartz or any evidence for the former presence of glass or quenched melt which might have formed by frictional melting. Fracture vein materials from Enchanted Rock pluton are not pseudotachylytes sensu stricto, in that there is no evidence of frictional melting. The presence of small, comminuted flakes of apparently unaltered biotite in these veins suggests that temperatures did not exceed ~400 degrees-800 degrees C for appreciable periods of time [4, 5, 6] and limits the amount of frictional heating which could have affected these breccias. However, some characteristic features of these rocks are similar to those of pseudotachylytes, including narrow vein-like form, flow-like texture, very fine-grained brecciated nature, extensive deformation of constituent minerals, and localized derivation. These veins represent narrow zones of moderately intense deformation related to the latter stages of emplacement of the pluton or to later tectonic activity. The deformation is not related to impact processes or to fault zones involving any significant displacement. References: [1] Hutchison R. M. (1956) Bull. GSA, 67, 763-806. [2] Garrison J. R. Jr. et al. (1979) Contrib. Mineral. Petrol., 69, 361-374. [3] Barnes V. E. (1940) Texas Univ. Publ. 3945, 645-656. [4] Vedder W. and Wilkins R. W. T. (1969) Am. Mineral., 54, 482-509. [5] Sanz J. et al. (1983) Phys. Chem. Mineral., 9, 14-18. [6] Spray J. G. (1992) Tectonophys., 204, 205-221.
Buchanan Paul C.
Degenhardt J. Jr. J.
Reid Michael A.
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