Middle Proterozoic Geologic Evolution of Llano Uplift, Texas: Evidence from U-Pb Zircon Geochronometry

Middle Proterozoic geologic evolution of Llano uplift, Texas: Evidence from U-Pb zircon geochronometry

NICHOLAS WALKER Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78713-7909

ABSTRACT through the Adirondack and Appalachian not strongly overprinted by Paleozoic tectonism, Mountains and, in central Texas, occurs as the and the predominant metamorphic grade is low The Llano uplift is a gentle structural dome Llano uplift. Scattered outcrops of Grenville-age amphibolite (McGehee, 1979). As in the case of that exposes Middle Proterozoic crystalline rocks are also found in west Texas and in north- Grenville-age rocks of eastern and southeastern rocks of Grenville affinity. Zircon U-Pb ern and southern Mexico (Fig. la). The precise Canada (Hoffman, 1989) and the Adirondack geochronometry of polydeformed metaigne- paleotectonic setting of the belt is uncertain, but Mountains of New York (McLelland and oth- ous rocks and post-tectonic plutons in the up- the overall high metamorphic grade, presence of ers, 1988a), the Llano uplift is characterized by a lift defines a more protracted and complex deep-seated igneous and metamorphic rocks, polymetamorphic history (Wilkerson and oth- geologic evolution than previously docu- mylonite-bounded thrust sheets, and orogenic ers, 1988; Schwarze, 1990) and ductile deforma- mented. Results indicate that the uplift har- reworking of pre-1200 Ma crust has led most tion recorded by synmetamorphic foliations, bors crust at least 1303 m.y. old, demonstrate students of the Grenville to conclude that it is a fold vergence, stretching lineations, and rare that igneous protoliths of some metamorphic collisional orogen. mylonite zones (Nelis and others, 1989; Carter, units were emplaced between 1,252 and 1,232 The Llano uplift of central Texas (Fig. lb) 1989; Mosher, 1991). m.y. ago, and constrain major Grenville de- includes the largest exposed tract of Grenville- Early U-Pb geochronologic studies of several formational processes to have taken place be- age rocks in the southern United States. It there- post-tectonic plutons within the uplift demon- tween 1,232 and 1,116 m.y. ago. This fore serves as an important locale for studying strated their Grenville age (Tilton and others, metamorphosed and deformed crust was then Middle Proterozoic orogenic processes operative 1957; Silver, 1963). Subsequent Rb-Sr and perforated by a suite of coarse-grained gra- along the southern margin of the continent. Un- K-Ar investigations concentrated on determina- nitic plutons the emplacement ages of which like exposures of rocks of Grenville affinity in tion of the timing of metamorphism and on em- range from 1116 to 1070 Ma. the Adirondacks and Appalachians, the uplift is placement ages of the post-tectonic plutons When considered in conjunction with the present geometric organization of metamorphic units within the uplift, the U-Pb data suggest that contacts between some of the metamorphic units are tectonic. As in the case of Grenville tracts elsewhere, it appears that the Llano crust is structurally imbricated. The Middle Proterozoic temporal and geologic evolution of the uplift is similar to that described from other Grenville-age terranes in North America, thus verifying the synchroneity of orogenic processes along most of the Middle Proterozoic oceanward margin of Laurentia.

INTRODUCTION

The Grenville belt is the youngest Precam- brian orogen of regional extent in North Amer- ica. Metamorphism, deformation, and plutonism culminated -1.2-1.0 b.y. ago throughout the Figure 1. a. Surface and subsurface distribution of Grenville-age (-1.3-1.0 Ga) rocks in orogen. This belt of crystalline rocks is discon- North America, showing location of the Llano uplift (after Mosher, 1991). tinuously exposed in southeastern Canada and b. Location of Llano uplift in Texas.

Geological Society of America Bulletin, v. 104, p. 494-504,6 figs., 1 table, April 1992.

494

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 EVOLUTION OF LLANO UPLIFT, TEXAS 495

Katemcy Pluton Lone Grove Pluton

EXPLANATION I I Paleozoic & younger rocks 1 Post-tectonic granitoids pvvl Red Mountain / Big Branch gneisses ¡¡EE] Big Branch Gneiss Enchanted Rock I 1 Packsaddle Schist Batholith ^jjSjSj) Lost Creek Gneiss V//A Valley Spring Gneiss Legion Creek • Zircon sample locality Pluton 0 10 km 98°30' I

Figure 2. Simplified geologic map of the Llano uplift, showing distribution of Precambrian metamorphic rocks and post-tectonic plutons and zircon sample localities. Sample numbers are keyed to Table 1. (Map after Muehlberger and others, 1967).

(Zartman, 1964, 1965; DeLong and Long, viously considered to be depositional, are likely phic units across tens of kilometers. These ob- 1976; Garrison and others, 1979). There have tectonic. Last, the results demonstrate that the servations led to the following "stratigraphy" of been, however, no attempts to use ziron U-Pb uplift underwent a geologic evolution broadly metamorphic units from inferred oldest to geochronometry to extract protolith ages from similar to that of other Grenville-age terranes in youngest (Fig. 3): Valley Spring Gneiss, Lost the metaigneous rocks, to delineate the timing of North America. Creek Gneiss, Packsaddle Schist. On the basis of Grenville orogenesis, or to establish the precise interpreted conformable relationships, the meta- chronology of post-tectonic pluton emplace- GEOLOGIC SETTING plutonic Big Branch Gneiss was considered to ment. Lack of such data has hampered accurate intrude the stratigraphically youngest part of the understanding of the geologic evolution of the The Llano uplift is a gentle structural dome Packsaddle Schist. The metaplutonic Big Branch uplift. within which some 9,000 km2 of Middle Prot- Gneiss in turn is intruded, as is the Packsaddle This paper presents results of a U-Pb zircon erozoic crystalline rocks are exposed in an ero- Schist, by the metaplutonic Red Mountain geochronometric study that focused on determi- sional inlier through Paleozoic and Mesozoic Gneiss. The broad open folds used to deduce the nation of ages of metaigneous protoliths of the sedimentary rocks (Muehlberger and others, relative age relationships, however, are now major metamorphic units and on the chronome- 1967; Barnes, 1981). Within the inlier, polyde- known to be the product of the youngest of five try of post-tectonic pluton emplacement. Results formed, multiply metamorphosed metaigneous Middle Proterozoic synmetamorphic deforma- of this study lead to three major conclusions. and metasedimentary rocks host a suite of post- tions (Mosher, 1991). First, the data document the presence of tectonic granitic plutons (Fig. 2). In the absence Following deformation, this metamorphosed 1232-1303 Ma metaigneous rocks in the uplift. of protolith age data and detailed structural in- crust was perforated by numerous post-tectonic Second, on the basis of U-Pb ages of the metaig- vestigations, a simple conformable stratigraphic granitic plutons and temporally related sills and neous rocks and their present geometric organi- stacking of principal metamorphic units was in- dikes. On the basis of color, grain size, and field zation, the results call into question formerly ferred on the basis of their present positions relationships, Stenzel (1932, 1935) proposed a accepted contact relationships between most within broad, open folds (see Barnes, 1988, and classification of the granites in the uplift that is principal metamorphic units; such contacts, pre- references therein) and correlation of metamor- still used. The oldest, pink, coarse-grained gran-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 496 N. WALKER

southeastern part of the uplift and textures and point Rb-Sr isochron age of 1167 ± 15 Ma for geothermobarometry of metamafic rocks in the the Red Mountain Gneiss. Mineral and whole- northwestern and northern uplift, the following rock K-Ar ages from many of the units, includ- metamorphic history has been proposed (Wil- ing metabasalts that cut metaserpentinite spa- kerson and others, 1988; Schwarze, 1990; tially associated with the Packsaddle Schist Walker and others, 1990). In eclogite remnants (Garrison and others, 1979), are in the range of from the northwest uplift, an early, as yet un- 1000-1080 Ma. These younger dates apparently dated, event at pressures as great as 15 kbar is reflect Ar leakage in response to emplacement of postulated, assuming compositions of orthopy- the post-tectonic granites. roxene cores represent equilibrium with coexist- The U-Pb geochronometric results presented ing garnet. These metamorphic conditions were below reveal a more protracted and complex succeeded by a dynamothermal event at 650- geologic evolution of the uplift than that de- 750 °C and 6-11 kbar for which sparse evi- duced from previous field and geochronometric dence is preserved in some metamafic rocks in studies. the northwestern and northern part of the uplift. The predominant amphibolite metamorphic Zircon U-Pb Geochronometry grade throughout the uplift, recognized by mineral parageneses and postkinematic symplectitic Representative samples of the major litho- rims on early garnet, records P-T conditions logic type within each principal metamorphic near 525-650 °C at pressures of ~3.5 kbar unit and of several post-tectonic Town Moun- (McGehee, 1979; Garrison and others, 1979; Figure 3. Former inferred composite strat- tain Granites were collected for U-Pb zircon Schwarze, 1990). In many localities, this late igraphic column of the Llano uplift, based on geochronometric investigation. Sample localities static metamorphism has been ascribed to relative position of the metamorphic units are described in Appendix I and shown in Fig- regional-scale reheating and hydration induced within broad open folds (after McGehee, ure 2. by intrusion of the voluminous postkinematic 1979). Compare to the revised chronostrati- Zircons were separated from 10- to 30-kg granitic magmas (Bebout and Carlson, 1986). graphic column and inferred geologic rela- samples using standard heavy-liquid and mag- tions shown in Figures 6a and 6b. netic techniques. Zircon concentrates from each GEOCHRONOMETRY sample were split by magnetic character using a Frantz Isodynamic Barrier separator and then ites are termed "Town Mountain Granites" and Previous Studies sieved to provide populations of desired size constitute the post-tectonic plutons examined in range. Representative subpopulations were hand- this study. In a pioneering U-Pb study, Tilton and others picked to purity and most were air abraded in a (1957) concluded that the Lone Grove pluton device similar to that described by Krogh STRUCTURAL HISTORY (Fig. 2) had an ~ 1100 Ma U-Pb zircon age and (1982). Zircon dissolution and ion-exchange thus was of Grenville affinity. Results of subse- procedures were comparable to those described Deformed Middle Proterozoic metamorphic quent Rb-Sr and K-Ar studies on some of the by Krogh (1973) and Parrish and others (1987). 208 rocks throughout the uplift are characterized by metamorphic units were interpreted to indicate In the early part of this study, a mixed Pb- 235 transposed metamorphic fabrics. The structural that deposition and emplacement of metamor- U tracer was used for measurements of the evolution of the uplift has been studied in great- phic protoliths and subsequent deformation and concentrations of Pb and U, whereas in the latter 205 233 235 est detail in the better-exposed southeastern part, metamorphism occurred in the interval 1199— stages, a mixed Pb- U- U tracer was em- where five phases of noncoaxial synmetamor- 1129 Ma (Zartman, 1965; Garrison and others, ployed. Pb and U were loaded on Re filaments phic folding, each associated with development 1979). Rb-Sr, K-Ar, and limited U-Pb investiga- and analyzed on the Finnigan MAT 261 multi- of metamorphic foliation, have been docu- tions of the post-tectonic plutons yielded ages of collector mass spectrometer at Austin. Pb was mented (Nelis and others, 1989; Carter, 1989; -1050-1100 Ma (Aldrich and others, 1958; analyzed in static multicollector mode employ- Mosher, 1991). The overall tectonic transport Silver, 1963; Zartman, 1964; DeLong and Long, ing Faraday cup collection of masses 208, 207, direction based on vergence of folds and stretch- 1976; Garrison and others, 1979). 206, and 205 while simultaneously collecting ing lineations is toward the northeast. Previous ages assigned to the metamorphic mass 204 in a secondary electron multiplier. Such a deformational history highlights the units were based on K-Ar and Rb-Sr studies. Uranium was analyzed in static multicollector danger in interpreting the nature of original con- Zartman (1964) presented Rb-Sr whole-rock mode employing Faraday collectors only. Addi- tacts between protoliths to the principal meta- data for an eastern sample of Valley Spring tional analytical details are given in the foot- morphic units, based only on present stacking Gneiss, which when recalculated using decay notes of Table 1. order within the uplift. constants recommended by Steiger and Jäger (1977), yields an isochron age of — 1172 ± 17 U-Pb Data and Observations METAMORPHIC HISTORY Ma (Garrison and others, 1979). For a western sample of Valley Spring Gneiss, Garrison and Analytical results and calculated ages are Although the predominant metamorphic min- others (1979) reported a Rb-Sr isochron age of a given in Table 1. Discordia arrays for each sam- eral assemblages indicate amphibolite-facies whole-rock-muscovite pair of 1129 ± 9 Ma. ple are shown in Figures 4 and 5. All upper metamorphism (McGehee, 1979), work by Geochronometric data have not been reported intercepts and associated uncertainties are stated Carlson and Nelis (1986), Wilkerson and others from the Lost Creek Gneiss or the Packsaddle at the two-sigma level and were calculated by a (1988), and Schwarze (1990) has demonstrated Schist. Garrison and others (1979) reported a modified York-type linear regression (York, a polymetamorphic history for the uplift. On the 6-point whole-rock Rb-Sr isochron age of 1199 1969) similar to that described by Parrish and basis of rare staurolite inclusions in garnet in the ± 23 Ma for the Big Branch Gneiss and an 8- others (1987).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 EVOLUTION OF LLANO UPLIFT, TEXAS 497

TABLE 1. ZIRCON ISOTOPIC DATA, LLANO UPLIFT, TEXAS

Sample, Amount Concentration^ Pb isotopie composition" Age and uncertainty tt fraction properties^ analyzed (ppm) (Ma) (mg) 206pb,/23Su 207pb,/235lI 207Pb»/206Pb*

1. Big Branch Gneiss (type locality) ntn,0°,+l50/im,ab. 1.5 24.76 131.9 0.14838 0.08458 48732 1265 ± 1.8 1278 ± 2.2 1300 ± 3.9 nm, 1°, +150 /im, ab. 2.3 26.52 150.4 0.14743 0.08436 50251 1195 ± 1.9 1233 ± 2.4 1301 ± 4.4 m, 2°, +150 /im, ab. 1.6 28.46 164.5 0.14768 0.08430 48780 1177 ± 2.0 1218 ± 2.3 1293 ± 4.3 m, 2°, -100 +64 nm 2.1 42.30 280.3 0.13451 0.08449 13341 1034 ± 2.3 1116 ± 2.5 1279 ± 3.0 m, 3°, +150 /im, ab. 1.0 29.43 168.9 0.15014 0.08485 20000 1182 ± 2.2 1221 ± 2.6 1289 ± 4.4 m, 5°, +150 /im, ab. 1.3 25.93 154.6 0.15106 0.08513 12563 1142 ± 2.3 1195 ± 2.8 1295 ± 4.3

2. Big Branch Gneiss (near Red Mountain) nm, 1°, -150 +100 fm, ab. 0.6 33.33 194.8 0.10651 0.08413 5528 1162 ± 2.1 1188 ±2.6 1236 ± 4.1 rn, 1°, +150 nm, ab. 1.0 27.39 169.8 0.11531 0.08566 3798 1101 ± 2.3 1151 ± 2.6 1245 ± 4.1 m, 2.5°, -75 +64 /im, ab. 0.9 15.69 94.41 0.11720 0.08499 4277 1132 ± 2.2 1169 ±2.4 1239 ± 4.0 m, 5°, -100 +75 /im, ab. 1.1 39.65 276.5 0.11111 0.08364 7530 985 ± 2.0 1068 ± 2.3 1241 + 3.8

3. Lost Creek Gneiss nm, 4°, +150 /im, ab. 0.7 28.21 156.7 0.16239 0.08297 21834 1218 ± 1.8 1231 ± 2.0 1254 ± 3.2 m, 5°, +100 firn, ab. 0.8 28.40 171.2 0.15933 0.08418 7593 1130 ±2.7 1173 ±3.0 1253 ± 4.4 m,6°,-100+I50|im, ' 1.2 34.20 200.2 0.15852 0.08328 1410t 1161 ±2.2 1193 ± 2.6 1253 ± 4.1 m, T, +150 ¿im, ab. 1.4 34.44 217.9 0.16710 0.08326 16340 1081 ± 2.1 1140 ± 2.6 1254 ± 3.6

4. Packsaddle Schist (northwest uplift) nm, 3°, -150+100 (

5. Packsaddle Schist (southeast uplift) m, 2.5°, +75 /im, ab. 1.2 1609 130.6 0.16634 0.08501 4600 1064 ± 2.1 1225 t 2.8 1245 ± 4.0 m, 2.5°, +75 fim 1.1 15.05 112.5 0.17086 0.08709 2389 926 ± 2.8 1019 ± 3.3 1229 ± 4.8 m, 5°, +75 /i m 1.1 22.06 178.9 0.16301 0.08963 1623 858 ± 3.2 967 ± 3.8 1221 ± 5.3 m, 10°, +75 /im, ab. 0.8 19.17 161.1 0.16385 0.08564 3388 830 ± 2.7 945 ± 2.9 1233 ± 4.3

6. Red Mountain Gneiss nm, 1°, +100 /im, ab. 1.4 14.38 80.88 0.15689 0.08330 8606 1204 ± 1.9 1216 ± 2.1 1237 ± 3.3 nm, 2°, +150 Jim, ab. 1.0 36.33 205.1 0.15486 0.08377 7117 1200 ± 1.9 1215 ± 2.3 1242 ± 3.6 m, 2°,+100-150/im, ab. 1.4 33.96 212.7 0.17691 0.09420 1090 1090 ± 3.4 1137 ± 3.9 1228 ± 5.3 nm, 2°, +100 150 /im, ab. 1.3 37.01 218.4 0.16114 0.08712 2492 1166 ± 2.5 1189 t 2.8 1233 ± 4.6

7. Valley Spring Gneiss nm, 3°, -100 +64 /im, ab. 1.2 56.27 333.6 0.17870 0.08295 10593 1147 ± 1.9 1179 ±2.3 1235 t 3.7 m, 4°, +150 /im, ab. 3.3 69.47 419.9 0.18946 0.08288 12210 1127 ± 1.8 1166 ±2.3 1238 ± 3.8 nm, 4°, -150 +100 /im, ab. 1.6 50.43 312.4 0.20187 0.08371 7423 1102 ±2.1 1150 ± 2.4 1241 ± 4.0 m, 4°, 150+100/im, ab. 1.3 44.20 275.5 0.19783 0.08333 1096 ± 2.0 1145 i 2.5 1239 ± 3.9

8. Melarhyolite dike nm, 1°, +150 /im, ab, 3.1 15.54 104.2 0.15821 0.07874 5252 1086 ± 2.1 1089 ± 2.6 1096 ± 5.0 nm, 1°. -150 +75 /im, ab. 1.4 17.04 109.6 0.19035 0.09308 839 1064 ± 3.2 1076 ± 3.8 1098 ± 9.3 m, 2°,-150 +75/im, ab. 2.2 19.44 124.3 0.15226 0.07936 4320 1071 ± 2.3 1079 ± 2.6 1097 ± 4.8 m, 3°, -150 +75 /im, ab. 1.6 17.75 112.2 0.16918 0.08387 1850 1082 ± 2.9 1088 ± 3.6 1101 ± 6.1

9. Lone Grove pluton nm, 3°, +150 fim, ab. 2.0 26.54 169.1 0.11442 0.07666 19048 1074 ± 2.0 1080 t 2.3 1094 ± 4.1 m, 3°,+100 (im, ab. 2.0 27.99 178.4 0.11152 0.07684 14481 1074 ± 2.1 1080 t 2.7 1093 ± 4.6 nm, 4°, +150 /im, ab. 1.2 18.41 124.1 0.13343 0.07765 9183 1020 ± 2.3 1045 ± 3.1 1098 ± 4.7 m, 4°, +100 /im, ab. 1.4 23.14 153.1 0.12662 0.07645 28926 1038 ± 1.9 1057 ± 2.1 1096 ± 3.9

10. Katemcy piuton nm, 3°,-200+150 um, ab. 1.8 60.21 396.7 0.10754 0.075631 30303 1041 ± 2.0 1052 ± 2.3 1074 ± 4.0 nm, 3°, -150+100 /im, ab. 1.1 30.29 225.8 0,12231 0.075651 62500 929 ± 1.7 975 ± 1.9 1080 ± 3.8 m, 3°, -200 +150 /im, ab. 1.8 28.01 234.0 0.12298 0.076364 19231 838 ± 2.1 909 t 2.6 1085 ± 4.6 m, 4°,-200+150/im, ab. 0.7 27.31 251.8 0.14271 0.076195 58823 760 ± 1.8 852 í 2.1 1094 ± 3.9

11. Legion Creek pluton nm, 0.5°,-250+150/im, ab. 1.6 53.93 336.2 0.10301 0.07797 17094 1013 ± 2.1 1049 ± 2.5 1124 ±4.6 nm, 0.5°,-150 +100 /im 0.7 70.39 492.5 0.10544 0.07775 27548 985 ± 2.0 1030 ± 2.3 1127 ± 4.3 nm, 0.5°,-150 +100 «ra, ab. 1.7 41.49 275.9 0.10424 0.07825 11507 1033 ± 2.3 1062 ± 2.7 1122 ±4.8 nm, 0.5 V 75+64 firn 1.2 55.58 399.2 0.10428 0.07814 16750 961 ± 2.4 1014 i 2.6 1128 ± 4.5

12. Enchanted Rock hatholith (marginal phase) nm, 1°,-200+150 «m, a„. 0.8 39.34 265.1 0.13011 0,076566 18545 1020 ± 2.1 1042 ± 2.4 1090 ± 4.7 m, 2°,-200+150 (im, ab. 1.2 45.51 354.2 0.12088 0.07726 12587 892 ± 2.4 953 ± 2.8 1098 ± 4.8 m, 2.5°,-150 +100 /im ab. 1.4 66.14 545.2 0.12412 0.07736 14189 845 ± 2.3 920 ± 2.7 1104 ±4.7 m, 3°, -100 +75 /im, ab. 1.8 76.19 673.1 0.12022 0.07813 8846 792 ± 2.4 880 ± 2.9 1110 ± 5.0 m, 10°, -100 +75 ¿im, ab. 1.2 54.66 449.8 0.I32I1 0.07732 38461 712 ± 1.9 818 ± 2.2 1120 ± 4.0

W nonmagnetic; m, magnetic. Value in degrees is the side tilt of the Frantz magnetic barrier separator. Frantz separator was operated at a magnet current of 1.8 amps, with a forward tilt of 20°. Values in microns indicate size range of analyzed zircons prior to abrasion. Zircon fractions were air abraded for 2 to 12 hr. § Asterisk denotes radiogenic Pb corrected for common Pb in zircon and laboratory blank Pb. Common Pb compositions estimated from Stacey and Krameis (1975) with a ±0.2 uncertainty assigned to the 2fJ7Pb/2rj4Pb ratio. Total procedural blanks over the course of this study ranged from 43 to 12 pg. ••Measured ratios corrected for mass fractionation of ~0.10%/atomic mass unit, based on replicate analyses of NBS SRM 981 and SRM 982. ff Decay constants: 23% = 1.5513 E"l0/yr; 235U = 9.8485 E-'°/yr. Atom ratio 238U/235U = 137.88. Uncertainty in the calculated ages is stated at the two-sigma level. Uncertainties based on total summed uncertainties in calibrations of mixed Pb-U tracers, measurement of isotopic ratios of Pb and U, common and laboratory blank Pb isotopic ratios, Pb and U mass fractionation corrections, and reproducibility in measurement of NBS Pb and U standards.

For each metaigneous sample, upper concor- pretations are based on published data bearing All zircon fractions are characterized by dia intercepts are interpreted as crystallization on protoliths and field relationships coupled to euhedral, multifaceted crystals the aspect ratios ages of igneous protoliths. Upper intercepts the following observations of external and of which vary from 3:1 to 10:1. There is no defined by data from the post-tectonic plutons internal zircon morphology, and U-Pb isotopic megascopic evidence for a mixed population in are taken as crystallization ages. These inter- systematics. a given sample (that is, heterogeneous color,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 RED MOUNTAIN GNEISS 1240

1200

\ \ -Q -Q 1150 Z Upper Intercept toa a 1239 +5/-3 Ma

1120 /

1.B9 1.97 2.05 2.13 2.21 2.29 2.37 2.45

207 235 207 235 Pb/ U Pb/ U

Figure 4. Concordia plots of zircon data from the principal metamorphic units. Upper intercept uncertainties are stated at the two-sigma level. Isotopie data given in Table 1.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 EVOLUTION OF LLANO UPLIFT, TEXAS 499

1303 +5/-3 Ma, which is taken to be the age of emplacement of the protolith of the gneiss. Near the sample locality, the gneiss exhibits a relict intrusive relationship to mafic schists (Gil- lis, 1989) previously mapped as Packsaddle Schist. Thus, wall rocks to the Big Branch Gneiss are in excess of 1,303 m.y. old. Big Branch Gneiss near Red Mountain (Sample 2). This microcline augen orthogneiss, previously mapped as part of the Big Branch Gneiss (McGehee, 1979), is intruded by the protolith of the Red Mountain Gneiss and where sampled, is in contact with the Packsaddle Schist. This orthogneiss is strongly foliated and lineated, and these fabric elements are parallel to those of the nearby Packsaddle Schist. More detailed discussion of geologic relationships and g structural interpretations for this unit are in Car- Figure 4. (Continued). ter (1989). The orthogneiss is characterized by augen of microcline in a matrix of quartz, plagioclase, hornblende, and biotite. Four variably bimodal size distribution, nature of inclusions, cordant than are those with lower magnetic abraded zircon fractions define a linear array form of crystals). Petrographic and scanning susceptibility and of larger size (use of the total that intercepts concordia at 1238 +8/-6 Ma electron microscope examination of polished U concentration in the zircon fraction to confirm (Fig. 4). Thus, although mapped as part of the and hydrofluoric acid-etched representative zir- this observation is rendered unreliable owing to Big Branch Gneiss, the zircon data demand that cons from each sample revealed no evidence of air abrasion of most fractions). The simple U-Pb this sample is temporally unrelated to the type detrital or xenocrystic cores within the zircons. systematics are attributed to three factors. First, Big Branch described above, which is —65 m.y. Rather, internal zircon morphology is character- the zircons are characterized by uranium con- older than this sample. ized by micron-scale concentric zoning, typically centrations of a few hundred parts per million or Lost Creek Gneiss (Sample 3). The Lost about a euhedral core. Such patterns are charac- less (measured on nonabraded fractions, Table Creek Gneiss is chiefly a microcline augen gneiss teristic of igneous zircons. The possibility that a 1), which resulted in little radiation damage to with local migmatitic domains. This unit of volumetrically minor overgrowth of metamor- the zircon lattice by U decay. Second, as shown probable metarhyolite (Garrison and others, phic zircon is present on zircons from the meta- below, metamorphism took place shortly after 1979) is exposed in the northwestern part of the igneous rocks cannot be excluded but is emplacement of the protoliths. Therefore, the uplift (Barnes, 1988) and thins to the southeast considered unlikely for the following reason. accumulated radiation damage by U decay prior (Fig. 2). Four zircon fractions from a sample Given the overall felsic nature of these rocks to metamorphism was minimal. Last, unless composed of microcline augen, plagioclase, (which yield abundant zircon), most zirconium U-Pb systematics in zircon were wholly reset quartz, biotite, and minor hornblende define a in the rock is undoubtedly housed in zircon. during metamorphism (a phenomenon that is well-correlated discordia trajectory with an Because zircon is not a common reactant in undocumented) for each sample, the colinearity upper intercept of 1252 ± 3 Ma (Fig. 4). The documented metamorphic reactions, zirconium of abraded and nonabraded zircon data and gneiss occupies a position between the Valley from zircon is not readily released during meta- simple U-Pb isotopic systematics are strong evi- Spring Gneiss and the Packsaddle Schist and morphism. The low zirconium concentration dence for the cogenetic nature of the zircon thus was previously inferred to be older than the in the metamorphic fluids is therefore likely to population within a given sample. Packsaddle Schist but younger than the Valley prevent substantial growth of metamorphic Spring Gneiss. This intercept age is analytically zircon. ZIRCON U-Pb RESULTS distinct from and older than that of the Valley When displayed on concordia diagrams (Figs. Spring Gneiss (see below) but is similar to the 5 and 6), data from each sample define highly Metaigneous Rocks zircon U-Pb age of the Packsaddle Schist as de- correlated linear discordia trajectories. Data scribed below. from several samples plot near the upper con- Big Branch Gneiss (Sample 1). The Big Packsaddle Schist. The Packsaddle Schist is cordia intercept. Lower concordia intercepts for Branch Gneiss is a variably foliated and lineated a heterogeneous unit consisting predominantly each sample are <150 Ma, which is interpreted predominantly tonalitic to dioritic orthogneiss of mica schist, amphibolite, marble, and lesser to reflect recent Pb loss. Following variable (Barnes, 1981; Garrison, 1985) exposed in the quartzofeldspathic gneiss. Stratigraphic relation- times of air abrasion, zircon fraction analyses southeastern uplift (Fig. 2). Where sampled at ships are clearest in the southeastern uplift where remain colinear with their nonabraded counter- the type locality, the gneiss is composed of pla- several formations within the schist have been part, demonstrating no detectable inheritance of gioclase, hornblende, and quartz with minor bio- named and described by McGehee (1979). xenocrystic zircon or mantling by metamorphic tite, magnetite, and accessory titanite and zircon. Although Packsaddle Schist contains rocks zircon. In addition, the discordia patterns for Six zircon fractions were analyzed from this for which the protoliths are predominantly sed- each sample are rather simple in that those zir- sample, the data from which are shown in Fig- imentary, some rocks are clearly of igneous cons that possess higher magnetic susceptibility ure 4. The data define a well-correlated linear origin (see McGehee, 1979, and references and are of smaller size typically are more dis- discordia array that defines an upper intercept of therein). Such metaigneous rocks typically have

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 500 N. WALKER

Figure 5. Concordia plots of zircon data from post-tectonic piutons. Upper intercept uncertainties are stated at the two-sigma level. Iso- ENCHANTED ROCK BATHOLITH topie data given in Table 1. 1080 (marginal phase)

conformable contacts with overlying and under- lying metasedimentary rocks, leading to the conclusion that they represent metavolcanic \ Upper Intercept -Q rocks. Two metavolcanic rocks from the Pack- a. 1082 ±6 Ma saddle Schist were collected for geochronometric investigation. There is no physical evidence for a mixed zircon population within either sample, and the simple U-Pb systematics and colinearity of the analyzed zircon fractions suggest that the zircon population within each sample is cogenetic. Thus, the upper intercept ages 20? 235 cited below are taken as the time of emplace- Pb/ U ment of the volcanic precursor to each sample.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 EVOLUTION OF LLANO UPLIFT, TEXAS 501

Northwestern Uplift (Sample 4). Five zircon fractions from a quartz-amphibole-garnet- plagioclase-microcline gneiss from the northwestern part of the uplift define a discordia trajectory that intersects concordia at 1243 ± 2 Ma (Fig. 4). The intercept age is interpreted as the time of crystallization of the magmatic protolith of this component of the Packsaddle Schist. Southeastern Uplift (Sample 5). Foliated Packsaddle Schist adjacent to the Big Branch Gneiss of sample 2 was collected for geochronometry. This sample is characterized by a well- developed foliation and lineation that are concordant with those of the Big Branch Gneiss (Barnes, 1981; Carter, 1989). Quartz, plagioclase, microcline, and minor biotite and hornblende are the major minerals that compose this rock. Four abraded zircon fractions yield an upper concordia intercept of 1247 +8/ 6 Ma (Fig. 4), analytically indistinguishable from the upper intercept obtained from the Packsaddle Schist sample in the northwestern uplift. a b Red Mountain Gneiss (Sample 6). The Red Mountain Gneiss is composed of a number Figure 6. a. Composite chronostratigraphic column of metamorphic and plutonic rocks in of sill-like bodies of metagranite that intrude the uplift, based on new U-Pb zircon ages. Double line separating the Big Branch and Lost both the Packsaddle Schist and a unit previously Creek Gneisses denotes the substantial hiatus between emplacement of the Big Branch Gneiss mapped as Big Branch Gneiss (Clabaugh and protolith and that of the Lost Creek Gneiss. Boyer, 1961). Where these sills intrude the b. Inferred geologic relationships between the principal metamorphic units and post-tectonic Packsaddle Schist near the sample locality, the plutons, based on U-Pb zircon ages and superposition of units in synmetamorphic open folds. sills are folded concordantly with the enclosing Packsaddle Schist (see Nelis and others, 1989). Mineralogically, the gneiss is composed of define a discordia trajectory with an upper con- and others (1979) indicated that emplacement quartz, microcline, and minor plagioclase, bio- cordia intercept of 1232 ± 4 Ma. Lack of evi- took place between -1050 and 1100 Ma. The tite, and hornblende. dence for mixed zircon populations, absence of zircon data presented below refine this reported Four abraded and nonabraded zircon frac- xenocrystic cores, and simple U-Pb systematic» chronometry of pluton emplacement. tions give an upper intercept of 1239 +5/-3 Ma lead to the interpretation that this is the time of Melarhyolite Dike (Sample 8). Numerous (Fig. 4), interpreted as the emplacement age of emplacement of the magmatic protolith of this dikes of rhyolite, Fe-rich rhyolite (melarhyolite), the granite protolith. component of the gneiss. and diabase cut the metamorphic rocks of the Valley Spring Gneiss (Sample 7). The Val- uplift. These dikes cannot in every case be ley Spring Gneiss is a widespread unit (Fig. 2) Post-tectonic Plutons mapped into plutons but it is likely that many of consisting chiefly of microcline-plagioclase- them are genetically linked to the voluminous quartz-biotite gneiss particularly in the eastern Following metamorphism and deformation of granitic plutons. Assuming that many of these uplift but with minor calc-silicate, marble, am- the principal metamorphic units, the Llano crust dikes are genetically related, DeLong and Long phibolite, and quartzite elsewhere (Barnes, was perforated by a suite of coarse-grained, pre- (1976) reported a multipoint whole-rock Rb-Sr 1988). Mineralogic and chemical characteristics dominantly alkali granite plutons and numerous isochron age of 1106 ± 6 Ma, which they took of the eastern metaigneous Valley Spring Gneiss silicic to intermediate dikes and sills (Fig. 2). as the time of emplacement of the dikes. indicate a bulk composition similar to that of The origin of these plutons is uncertain. The A melarhyolite dike exposed in the southeast- granite (Billings, 1962), and field relationships plutons are the only Precambrian relicts of ern uplift was sampled where it cuts the Pack- suggest that much of this unit, particularly that postkinematic geologic processes in the uplift, saddle Schist. Four zircon fractions define a in the eastern uplift, is a metamorphosed rhy- and therefore they cannot be uniquely tied to reasonably well-correlated discordia trajectory olitic volcanic rock (Garrison and others, 1979; specific tectonic settings as might otherwise be for which the upper concordia intercept is 1098 Barnes, 1988). The zircon sample reported here recorded by a temporally equivalent sedimen- +3/-2 Ma (Fig. 5), in good agreement with the was taken from the same eastern locality of Val- tary or extrusive record. age reported by DeLong and Long. ley Spring Gneiss for which Zartman (1965, his U-Pb and Rb-Sr ages of several Town Moun- Lone Grove Pluton (Sample 9). The Lone sample 1) reported a Rb-Sr age of 1172 Ma. tain plutons and dikes reported by Tilton and Grove pluton has been the subject of a thorough Zircon data for the Valley Spring Gneiss are others (1957), Silver (1963), Zartman (1964, K-Ar and Rb-Sr study by Zartman (1964). shown in Figure 4. Five abraded zircon fractions 1965), DeLong and Long (1976), and Garrison Zartman reported ages (recalculated for 1977

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 502 N. WALKER

decay constants) ranging from 1010-1100 Ma in the uplift is primary. Figure 6b shows simpli- (1982), who estimated the closure temperature from this pluton, and Garrison and others fied, interpreted contact relations among the for Nd in garnet at -600 °C. (1979) reported a multipoint whole-rock and principal metamorphic units in the uplift, based The oldest rock dated thus far in the uplift is mineral Rb-Sr isochron age of 1056 ±12 Ma. on the geochronometric data presented earlier. the type Big Branch Gneiss at 1303 Ma, a unit Analyses of four zircon fractions from a sample Given that the relative positions of these units formerly considered to be among the youngest collected at the Petrick quarry site of Zartman within folds represent their stacking order, the metamorphic rocks in the uplift. The Big Branch (1964) define an upper intercept of 1091 +4/-3 zircon data demand that at least some of the intrudes wall rocks of mafic schist, which de- Ma (Fig. 5). contacts between the units are tectonic. The mands that pre-1303 Ma rocks reside within the Ketemcy Pluton (Sample 10). This pluton is 1232 Ma Valley Spring Gneiss is thus postulated uplift. The type Big Branch Gneiss and the schist the northwesternmost exposed pluton in the up- to be tectonically overlain by a lithologic "pack- it intrudes are compositionally more mafic than lift. No previous geochronometric ages have age" composed of the 1252 Ma Lost Creek are the other principal metamorphic units in the been reported from this pluton. Four zircon frac- Gneiss, the 1247-1243 Ma Packsaddle Schist, uplift. Furthermore, the gneiss and wall rocks tions were analyzed, and the results (Fig. 5) de- and the 1239-1238 Ma orthogneisses that in- are in close spatial association with metaserpen- fine a discordia trajectory with an upper trude the schist. The 1303 Ma Big Branch tinite (Barnes, 1981; Garrison, 1981; Gillis, concordia intercept of 1071 ±2 Ma. Gneiss and its enclosing wall rocks of mafic 1989). These observations, coupled with the fact Legion Creek Pluton (Sample 11). The Le- schist are in turn structurally above the Pack- that the gneiss and enclosing wall rocks are at gion Creek pluton is another Town Mountain saddle Schist, Lost Creek Gneiss, and Valley least 50 m.y. older than the other major meta- Granite that cuts the Packsaddle Schist. Three Spring Gneiss. morphic units, raise the possibility that this cou- abraded and one nonabraded zircon fraction In the southeastern uplift, where the Lost plet is allochthonous relative to the other from this sample define a discordia array for Creek Gneiss is absent, a zone of mylonite sepa- metamorphic units. If so, at least part of the which the upper intercept is 1116 +6/-4 Ma rates the Valley Spring Gneiss from the Pack- Llano uplift is constructed of unrelated crustal (Fig. 5). This is the oldest member of the Town saddle Schist (J. Reese, 1991, personal com- blocks. Mountain Granite suite so far identified in the mun.). In most areas, however, neither ductile This study also underscores problems with uplift. shear zones nor brittle faults have yet been iden- unit correlation and nomenclature in the uplift. Enchanted Rock Batholith (Sample 12). tified along these proposed tectonic contacts, Rocks mapped as Packsaddle Schist are in- The Enchanted Rock batholith is a well-studied, perhaps in part because of the lack of detailed truded by the 1303 Ma type Big Branch Gneiss compositionally zoned pluton. Early studies studies. Deformation fabrics, imparted by tec- (sample 1), and yet Packsaddle Schist dated documented the petrologic evolution and em- tonic juxtaposition of some of the principal meta- elsewhere in the uplift yields 1247 and 1242 Ma placement mechanism of the batholith (Hutch- morphic units, however, may have been textur- ages. It is unlikely that the Packsaddle Schist inson, 1956; Ragland, 1962, 1970; Ragland ally annealed and mineralogically overprinted in protolith formed over a time span of -55 m.y. and others, 1968). A whole-rock Rb-Sr isochron response to reheating and hydration due to em- Thus, the mafic schist intruded by the type Big defined by seven whole-rock and two plagio- placement of the post-tectonic Town Mountain Branch Gneiss is here considered to be unrelated clase samples yields an age of 1048 ± 30 (Garri- Granite suite. The sampled granites were em- to the Packsaddle Schist elsewhere in the uplift. son and others, 1979). Five zircon fractions placed over an ~48 m.y. span, commencing at Furthermore, the 1303 Ma type Big Branch from the outermost phase of the batholith yield a 1116 Ma and terminating at 1070 Ma. Gneiss is substantially older than the 1238 Ma well-correlated discordia array with an upper The timing of major deformation and dyna- microcline-rich "Big Branch" gneiss (sample 2), intercept of 1082 ± 6 Ma (Fig. 5). mothermal metamorphism in the uplift is brack- suggesting that these two rocks are genetically eted to have occurred between 1232 and 1116 unrelated. Further geochronometric study cou- DISCUSSION Ma, as deduced from the age of the youngest pled with geologic and structural mapping is nec- deformed metamorphic unit (Valley Spring essary to examine the validity of previous The geochronometric data presented above Gneiss) and that of the oldest post-tectonic correlations of polydeformed metamorphic indicate that the uplift underwent a more com- pluton (Legion Creek Granite). Attempts to date rocks across tens of kilometers in the uplift. plex and protracted Middle Proterozoic evolu- the earliest recognized high-pressure metamor- The U-Pb zircon ages determined in this study tion than that documented by earlier studies. phic event by Sm-Nd methods on garnet- are typically 40-100 m.y. older than reported The long-held view of simple depositional con- clinopyroxene pairs from three eclogitic rocks Rb-Sr whole-rock or mineral-isochron ages of tacts and intrusive contacts (Fig. 3) between the from the northwestern uplift resulted in isochron the metamorphic rocks and the post-tectonic principal metamorphic units is challenged by the ages of —1000 Ma. This isochron age is inter- plutons. For the metamorphic rocks, the cited results of this study. Figure 6a shows the chrono- preted to reflect disturbance of the Sm-Nd sys- Rb-Sr ages probably reflect a combination of stratigraphic relationships of the metamorphic tem as a result of reheating in response to variable times of isotopic homogenization as a units in the uplift, based on U-Pb zircon ages. intrusion of the post-tectonic granites. Given result of differential cooling rates following dy- When compared to Figure 3, Figure 6a empha- that temperatures of metamorphic rocks during namothermal metamorphism and subsequent sizes the disparity between relative stratigraphic the late, static metamorphism were in the range static overprinting related to emplacement of the position of the units and their protolith ages. On of 525-650 °C over a substantial part of the post-tectonic plutons. The Rb-Sr ages of the the basis of the U-Pb geochronometry reported uplift (McGehee, 1979; Wilkerson and others, post-tectonic plutons and those of the metamor- above, it is unlikely that the present geometric 1988; Schwarze, 1990), this interpretation is phic rocks may be influenced by a flux of tecton- organization of the principal metamorphic units supported by the work of Humphries and Cliff ically driven fluids through these rocks, perhaps

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 EVOLUTION OF LLANO UPLIFT, TEXAS 503

during the late Paleozoic Ouachita orogeny in a Grenville-age rocks in west Texas. TDM model (Facilities and Instrumentation), and the Geol- manner analogous to that described by Oliver ages of the Texas Grenville-age rocks, chiefly in ogy Foundation of the University of Texas. (1986). the range of 1.5-1.2 Ga, are equivalent to those of granulites from Virginia and Mexico, suggest- APPENDIX I. SAMPLE LOCALITIES COMPARISON TO OTHER ing that similar geologic processes were operable NORTH AMERICAN ROCKS during Grenville orogenesis from the central 1. Big Branch Gneiss (type locality). Collected at intersection of Althaus-Davis road and Big Branch OF GRENVILLE AGE Appalachians, through Texas, and into southern Creek, 4.6 km north of intersection of Althaus-Davis Mexico (Patchett and Ruiz, 1989). road and ranch road 1323, in Blanco County. Locality The evolution of the Llano uplift outlined The timing of metamorphism, deformation, same as 4 of Barnes (1988). above is strikingly similar to that described for crustal imbrication, and magmatism cited above 2. Big Branch Gneiss (near Red Mountain). Col- lected on west side of Comanche Creek, ~1.0 km rocks of Grenville age elsewhere in North Amer- illustrates the general synchroneity of Grenville north of intersection of Hardin Ranch road and Co- ica for which U-Pb zircon data have been re- orogenesis along the Middle Proterozoic ocean- manche Creek, Llano County. ported (see articles in Moore and others, 1986; ward margin of Laurentia. Field, geochronomet- 3. Lost Creek Gneiss. Sampled in McCulloch review by Hoffman, 1989). ric, and isotopic studies of Grenville rocks from County on east side of state highway 71,3.2 km north of junction of state highway 71 and ranch road 386. Geologic relations and U-Pb geochronometry southeastern Canada to southern Mexico con- 4. Packsaddle Schist (northwest uplift). Taken in the Canadian Grenville province have been firm that the collisional event was truly of con- from roadcut on west side of state highway 71,4.3 km recently summarized by Davidson (1990) and tinental scale. north of intersection of highway 71 and ranch road Hoffman (1989). In the northwestern zone of 386 in McCulloch County. the Canadian Grenville, reworking of 1710- CONCLUSIONS 5. Packsaddle Schist (southeast uplift). Collected 1630 and 1460-1430 Ma plutons during Gren- from east side of Comanche Creek at intersection of Hardin Ranch road and Comanche Creek, Llano ville processes commenced at 1240 Ma but per- The Llano uplift of central Texas consists of County. sisted to 1160-1030 Ma. In the central zone, polydeformed metamorphosed rocks yielding 6. Red Mountain Gneiss. Sampled on east side of structural imbrication is documented by 1160- zircon U-Pb isotopic ages ranging from 1303 to Sandy Creek, 2.8 km east of intersection of Click road 1030 Ma zircon ages. In the southeastern zone, 1232 Ma and nondeformed granitic plutons and Sandy Creek, Llano County. the Grenville Supergroup of metasedimentary 7. Valley Spring Gneiss. Sampled on east side of emplaced 1116-1070 Ma. The Llano crust was Park road in Inks Lake State Park, -1.8 km north of and metaigneous rocks is ca. 1250 Ma and is reworked as a result of metamorphism and intersection of park boundary and Park road, in intruded by plutonic rocks ranging from 1250- deformation during Grenville orogenesis in the Bumet County. Locality same as lgn of Zartman 1080 Ma. Metamorphism and crustal thickening interval 1231-1116 Ma. Whether this meta- (1965). is dated as > 1160-1050 Ma. morphism and deformation was episodic or 8. Melarhyolite dike. Sampled on southeast side of Sandy Creek, 1.7 km east of intersection of Click road McLelland and others (1988b) reported zir- progressive in nature is irresolvable from the and Sandy Creek, Llano County. con ages of numerous metaigneous rocks from present U-Pb data set, but further geochrono- 9. Lone Grove pluton. Collected from the aban- the Adirondacks in New York state that range metric study and on-going field efforts are un- doned "Petrick quarry," at the intersection of Texas from ca. 1321 to 1055 Ma. From these data and derway to improve the temporal resolution of highways 29 and 261 in Burnet County. Locality same geologic events in the uplift. as 3 of Zartman (1964). field relationships, they recognized three meta- 10. Katemcy pluton. Sampled from granite quarry morphic episodes: an early dynamothermal The present stratigraphic organization of meta- on north side of ranch road 1222, ~4.2 km west of event at ca. 1225 Ma, a widespread contact- morphic units in the uplift cannot be entirely junction of road 1222 and ranch road 386 in Mason metamorphic event induced by anorogenic em- primary and probably reflects structural imbri- County. placement of plutons at 1150-1130 Ma, and a cation during Grenville orogenesis, a feature 11. Legion Creek pluton. Taken from west side of state highway 16 in Gillespie County, 1.3 km north of final dynamothermal episode at 1075-1055 Ma. prevalent in other Grenville-age terranes in bridge across Legion Creek. In west Texas, Grenville rocks are thrust North America. 12. Enchanted Rock batholith (marginal phase). upon a foreland of mixed volcanic and sedimen- Collected from northwest side of Sandy Creek, -1.5 tary rocks (King and Flawn, 1953). Rhyolite in ACKNOWLEDGMENTS km from bridge over Sandy Creek in Enchanted Rock State Park, Gillespie County. the allochthon is dated at 1350 Ma (S. A. Bow- ring, personal commun., cited in Hoffman, Reviews of early drafts by colleagues Bill 1989), whereas 1150-1130 Ma granite and Carlson, Sharon Mosher, Bill Muehlberger, rhyolite (Copeland and Bowring, 1988) com- Doug Smith, and Eric James were most helpful, REFERENCES CITED

pose part of the foreland. as were discussions with these individuals. Aldrich, L. T.. Welherill, G. W, Davis, D. L„ and Tillon, G. R„ 1958, Ra- dioactive ages of micas from granitic rocks by the Rb-Sr and K-Ar In the state of Chihuahua in northern Mexico, Comments by GSA Bulletin reviewers Edward methods: Eos (American Geophysical Union Transactions), v. 39, Blount and others (1988) described a metaigne- Lidiak, Andrew Coleman, and Paul Fullagar led p. 1124-1134. Barnes, V. E., 1981, Geologic atlas of Texas—Llano sheet: University of Texas ous complex in which plutons have 1328 and to an improved final product. Steve Clabaugh Bureau of Economic Geology, scale 1:250,000. 1988, The Precambrian of central Texas, in Hayward, O. T., éd., Cen- 1280 Ma zircon ages. The metamorphic rocks enthusiastically provided expertise on regional tennial field guide Volume 4: Boulder, Colorado, Geological Society of are cut by an undeformed 1078 Ma granite geology. Paul Warren and Bob Roback ably as- America, South-Central Section, p. 361-368. Bebout, G. E., and Carlson, W. D., 1986, Fluid evolution and transport during (N. Walker, unpub. data). sisted in the preparation of mineral separates. metamorphism: Evidence from the Llano uplift, Texas: Contributions to Mineralogy and Petrology, v. 92, p. 518-529. Patchett and Ruiz (1989) recently determined This research was supported by National Sci- Billings, G. K., 1962, A geochemical investigation of the Valley Spring Gneiss Sm-Nd model ages of numerous igneous and ence Foundation Grants EAR88-04717 (Crustal and Packsaddle Schist: Texas Journal of Scienoe, v. 14, p. 328-351. Blount, J. G., Walker, N. W., and Carlson, W. D., 1988, Geochemistry and metamorphic rocks from the uplift and from Structure and Tectonics) and EAR86-18996 U-Pb zircon geochronology of mid-Proterozoic meta-igneous rocks

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021 504 N. WALKER

from Chihuahua, Mexico: Geological Society of America Abstracts Geochimica et Cosmochimica Acta, v. 37, p. 485-494. geothermobarometry and compositional zoning in garnet [M.A. thesis]: with Programs, v. 20, p. A205. 1982, Improved accuracy of U-Pb ages by the creation of more concor- Austin, Texas, University of Texas, 275 p. Carlson, W. D., and Nelis, M. K., 1986, An occurrence of staurolite in the dant systems using an air abrasion technique: Geochimica et Silver, L. T., 1963, U-Pb isotope ages from zircons from some igneous rocks Llano uplift, central Texas: American Mineralogist, v. 71, p. 682-685. Cosmochimica Acta, v. 46, p. 560-567. from the Llano uplift, central Texas: Geological Society of America Carter, K. E., 1989, Grenville orogenic affinities in the Red Mountain area, McGehee, R. V„ 1979, Precambrian rocks of the southeastern Llano region, Special paper 73, p. 243-244. Llano uplift, Texas: Canadian Journal of Earth Sciences, v. 26, Texas: University of Texas Bureau of Economic Geology Geological Stacey, J. S., and Kramers, J. D., 1975, Approximation of terrestrial Pb isotope p. 1124-1135. Circular 793, 36 p. evolution by a two-stage model: Earth and Planetary Science Letters, Clabaugh, S. E., and Boyer, R. E., 1961, Origin and structure of the Red McLelland, J., Lochhead, A., and Vyhnal, C., 1988a, Evidence for multiple v. 26, p. 207-221. Mountain Gneiss, Llano County, Texas: Texas Journal of Science, metamorphic events in the Adirondack Mountains, New York: Journal Steiger, R. H., and Jäger, E., 1977, Subcom mission on geochronology: Conven- v. 13, p. 7-16. of Geology, v. 96, p. 279-298. tions on the use of decay constants in geo- and cosmochronology: Earth CopeJand, P., and Bowring, S., 1988, U-Pb zircon and ^Ar/^Ar ages from McLelland, J:, Chiarenzelli, J., Whitney, P., and Isachsen, Y„ 1988b, U-Pb Planetary Science Letters, v. 36, p. 359-362. Proterozoic rocks, west Texas: Geological Society of America Abstracts zircon geochronology of the Adirondack Mountains and implications Stenzel, H. B., 1932, Precambrian of the Uano uplift: Geological Society of with Programs, v. 20, p. 95-96. for their geologic evolution: Geology, v. 16, p. 920-924. America Bulletin, v. 43, p. 143-144. Davidson, A., 1990, Crusta! structure of a transect across the southwestern Moore, J. M., Davidson, A., and Baer, A. J., 1986, The Grenville province: 1935, Precambrian structural conditions in the Llano region, in The Grenville orogen: Geological Association of Canada Program with Ab- Geological Association of Canada Special .Paper 31, 358 p. geology of Texas, Volume 2, Structural and economic geology: Univer- stracts, v. 15, p. A31. Mosher, S., 1991, Western extensions of Grenville-age rocks: Texas, in Reed, sity of Texas Bureau of Economic Geology Bulletin 3401, p. 74-79. DeLong, S. E., and Long, L. E., 1976, Petrology and Rb-Sr age of Precambrian J. C., ed.. The Precambrian: Boulder, Colorado, Geological Society of Tilton, G. R„ Davis, G. L., Wetherill, G. W., and Aldrich, L. T., 1957, Isotopic rhyolitic dikes. Llano County, Texas: Geological Society of America America DNAG Centennial Volume. ages from granites and pegmatites: Eos (American Geophysical Union Bulletin, v. 87, p. 275-280. Muehlberger, W. R., Denison, R. E., and Lidiak, E. G., 1967, Basement rocks Transactions), v. 38, p. 360-371. Garrison, J. R., Jr., 1981, Coal Creek Serpentinite, Llano uplift, Texas: A in the continental interior of the United States: American Association of Walker, N. W., Carlson, W. D., and Mosher, S., 1990, Proterozoic evolution of fragment of a Precambrian ophiolite: Geology, v. 9, p. 225-230. Petroleum Geologists Bulletin, v. 12, p. 2351-2380. the Uano uplift, Texas: Geological Society of America Abstracts with 1985, Petrology, geochemistry, and origin of the Big Branch and Red Nelis, M. K., Mosher, S., and Carlson, W. D., 1989, Grenville-age orogeny in Programs, v. 22, no. 7, p. Al 13. Mountain Gneisses, southeastern Llano uplift, central Texas: American the Uano uplift of central Texas: Deformation and metamorphism of Witkerson, A., Carlson, W. D., and Smith, D., 1988, High-pressure metamor- Mineralogist, v. 70, p. 1151-1163. the Rough Ridge Formation: Geological Society of America Bulletin, phism during the Uano orogeny inferred from Proterozoic eclogitic Garrison, J. R„ Jr., Long, L. E., and Richmann, D. L., 1979, Rb-Sr and K-Ar v. 101, p. 876^883. remnants: Geology, v. 16, p. 391-394. geochronologic and isotopic studies, Uano uplift, central Texas: Contri- Oliver, J., 1986, Fluids expelled tectonically from orogenic belts: Their role in York, D., 1969, Least squares fitting of a straight line with correlated errors: butions to Mineralogy and Petrology, v. 69, p. 361-374. hydrocarbon migration and other geologic phenomena: Geology, v. 14, Earth and Planetary Science Letters, v. 5, p. 320-324. Gillis, G., 1989, Polyphase deformation of the Middle Proterozoic Coal Creek p. 99-102. Zartman, R. E., 1964, A geochronologic study of the Lone Grove pluton from Serpentinite, Uano uplift, Texas [M.A. thesis]: Austin, Texas, University Parrish, R. R., Roddick, J. C., Loveridge, W. D., and Sullivan, R. W., 1987, the Llano uplift, Texas: Journal of Petrology, v. 5, p. 349-408. of Texas, 88 p. Uranium-lead analytical techniques at the geochronology laboratory: 1965, Rubidium-strontium age of some metamorphic rocks from the Hoffman, P. F., 1989, Precambrian geology and tectonic history of North Geological Survey of Canada Paper 87-2, p. 3-7. Llano uplift, Texas: Journal of Petrology, v. 6, p. 28-36. Ameirca, in Bally, A. W., and Palmer, A. R., eds., The geology of North Patchett, P. J., and Ruiz, J., 1989, Nd isotopes and the origin of Grenville-age America—An overview: Boulder, Colorado, Geological Society of rocks in Texas: Implications for Proterozoic evolution of the United America, Geology of North America, v. A, p. 447-512. States mid-oonttnent region: Journal of Geology, v. 97, p. 685-695. Humphries, F. J., and Cliff, R. A., 1982, Sm-Nd dating and the cooling history Ragland, P. C., 1962, Chemical, radiometric, and mineralogic trend surfaces of Scourian granulites, Scotland: Nature, v. 295, p. 515-517. within the Enchanted Rock batholith, Uano and Gillespie Counties, Hutchinson, R. M., 1956, Structure and petrology of the Enchanted Rock Texas [Ph.D. dissert.]: Houston, Texas, Rice University, 100 p. batholith, Llano and Gillespie Counties, Texas: Geological Society of 1970, Composition and structural state of the potassic perthites as re- America Bulletin, v. 67, p. 763-805. lated to the pedogenesis of a granitic pluton: Uthos, v. 3, p. 167-189. King, P. B., and Flawn, P. T., 1953, Geology and mineral deposits of Precam- Ragland, P. C., Billings, G. K., and Adams, J.A.S., 1968, Magmatic differentia- brian rocks of the Van Horn area, Texas: University of Texas Bureau of tion and autometasomatism in a zoned granitic batholith from central Economic Geology Bulletin 5301,218 p. Texas, U.S.A., in Ahrens, L. H., ed., Origin and distribution of the MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 25,1991 Krogh, T. E., 1973, A low contamination method for hydrothermal decomposi- elements: New York, Pergamon Press, p. 795-823. REVISED MANUSCRIPT RECEIVED SEPTEMBER 16, 1991 tion of zircon and extraction of U and Pb for isotopic age determination: Schwarze, E. T., 1990, Polymetamorphism in the Llano uplift: Evidence from MANUSCRIPT ACCEPTED SEPTEMBER 18,1991

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/4/494/3381570/i0016-7606-104-4-494.pdf by guest on 28 September 2021