Downloaded from gsabulletin.gsapubs.org on October 31, 2010 Geological Society of America Bulletin
Tectonic evolution of the southern Laurentian Grenville orogenic belt
Sharon Mosher
Geological Society of America Bulletin 1998;110;1357-1375 doi: 10.1130/0016-7606(1998)110<1357:TEOTSL>2.3.CO;2
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Tectonic evolution of the southern Laurentian Grenville orogenic belt
Sharon Mosher* Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712
ABSTRACT bordered on its eastern margin by the north- with northwest-directed transport are observed northeast–trending Grenville orogen, which re- along the belt (Davidson, 1986) with crustal im- The Grenville orogenic belt along the south- cords both intracratonic and collisional orogen- brication resulting in burial to depths in excess ern margin of Laurentia records more than esis (Davidson, 1986; Rivers et al., 1989). In of 45 km (Indares, 1993). This orogenic belt 300 m.y. of orogenic activity culminating in Labrador, crustal shortening telescoped older continues northward into Greenland and Scandi- arc-continent and continent-continent colli- basement rocks in an intracratonic setting as a navia, and scattered inliers in the Appalachians sion ca. 1150–1120 Ma. Exposures in Texas result of continent-continent collision farther suggest a lateral continuity along the length of provide a unique profile across the Grenville outboard (Connelly et al., 1995). In Ontario, a the Appalachian orogen (Fig. 1). Recent plate re- orogen from the orogen core to the cratonal long history of island-arc and allochthonous ter- constructions interpret South America as the col- margin. In the Llano uplift of central Texas, rane accretion culminated in continent-continent liding continent (Dalziel, 1991, 1992, 1997, ca. 1360–1232 Ma upper amphibolite–lower collision (Davidson, 1986; Gower et al., 1990; Hoffman, 1991; Moores, 1991; Unrug, 1996) granulite facies, polydeformed supracrustal Culotta et al., 1990). Deep crustal shear zones adjacent to this eastern margin of Laurentia, and plutonic rocks represent the core of the collisional orogen. This exposure contains a 100˚ 00'W suture between a 1326–1275 Ma exotic island- arc terrane and probable Laurentian crust
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ࣿ thrust belt with post–1123 Ma synorogenic sedimentary rocks, which grade into unde- formed sedimentary rocks northward on the FRANKLIN Laurentian craton. MTNS. LLANO FRONT The Texas basement reveals a consistent P LLANO but evolving tectonic setting for the southern H CBP UPLIFT margin of Laurentia during Mesoproterozoic time. This paper summarizes recent advances in our knowledge of the Texas basement and 30˚ 00'N WEST TEXAS proposes plate models to explain the tectonic UPLIFTS evolution of this margin during Mesoprotero- zoic time. The orogenic history is strikingly Sierra Del Cerro Del Carrizalillo similar to that of the Canadian Grenville oro- Cuervo gen and requires a colliding continent off the southern Laurentian margin during the as- 0 150 mi sembly of Rodinia. 0 200 km INTRODUCTION
Grenville-age rocks have become pivotal in Figure 1. Locations of Mesoproterozoic exposures in Texas and Mexico, wells in Central basin constraining plate reconstructions for Rodinia, platform (CBP), and Llano front (H—Hueco Mountains; P—Pump Station Hills). Directions of an early Neoproterozoic supercontinent (Dalziel, tectonic transport (arrows) are shown for Llano uplift and west Texas exposures. Two possible 1991; Hoffman, 1991; Moores, 1991). Lauren- plate boundaries (dashed) are shown. Boundary in Llano uplift represents a collisional suture, tia, which has a critical position in all models, is whereas that in west Texas represents closure of a small ocean basin. The equivalent collisional suture in west Texas is to the south near Cerro Del Carrizalillo and Sierra Del Cuervo. Inset *E-mail: [email protected]. shows location of Laurentian Grenville orogenic belts.
GSA Bulletin; November 1998; v. 110; no. 11; p. 1357–1375; 10 figures; 2 tables.
1357 Downloaded from gsabulletin.gsapubs.org on October 31, 2010 S. MOSHER constructions interpret South America as the col- 1993; Roback, 1996a, 1996b; Carlson, 1998). all of which are polydeformed, and near the Coal liding continent (Dalziel, 1991, 1992, 1997, Smaller exposures in west Texas (near Van Horn, Creek domain boundary, highly transposed Hoffman, 1991; Moores, 1991; Unrug, 1996) Texas; Fig. 4) contain the northern margin of the (Carter, 1989; Nelis et al., 1989; Reese, 1995). The adjacent to this eastern margin of Laurentia, orogen, which appears to be a continuation of the Packsaddle domain structurally overlies the Valley whereas earlier reconstructions show western Llano front (Fig. 1). There, polydeformed Spring domain to the north along a southwest-dip- Africa instead (e.g., Bird and Dewey, 1970; 1400–1300 Ma metasedimentary and metavol- ping ductile thrust zone (Reese, 1995). The Valley Hatcher, 1987). Most reconstructions also show canic rocks were thrust in a transpressional setting Spring domain is a polydeformed granitic gneiss Baltica and the Greenland portion of Laurentia over ca. 1250 Ma carbonate and volcanic rocks terrane that consists of supracrustal and plutonic forming the northern extension of this belt, al- (Soegaard and Callahan, 1994). Synorogenic con- rocks and contains an older crustal component (ca. though Gower et al. (1990) showed only Baltica glomerates and sandstones (ca. post-1123 Ma; 1360 Ma) that may represent the southern margin interacting with Laurentia. Roths, 1993) involved in the underlying narrow of Laurentia (Reese et al., 1992; Reese, 1995; A Grenvillian orogenic belt also borders the thrust belt are undeformed 7 km north of the Roback, 1996a). The Valley Spring domain is the southern margin of Laurentia and is exposed in boundary on cratonal Laurentia. Farther west least-studied domain, and additional work (in the central Texas Llano uplift and smaller expo- along strike in the Franklin Mountains (near El progress) will probably subdivide it further. Each sures in west Texas (Fig. 1). In Texas, the east- Paso, Texas), correlative carbonate and volcanic of these domains, summarized in the following, trending Grenville orogen records both arc- rocks are undeformed (Pittenger et al., 1994). consists of rocks with a wide range in ages (Figs. 2 continent and continent-continent collisional Thus, considered together, the exposures along and 3; Table 1). I have extrapolated these domains orogenesis (Wilkerson et al., 1988; Mosher, the southern margin of Laurentia provide a unique to the central and western uplift, using previous 1993; Reese, 1995; Roback, 1996a; Carlson, profile across the Grenville orogen from the oro- stratigraphic correlations (e.g., Barnes, 1981), but 1998, and this study). Thus, plate reconstruc- gen core onto the cratonic margin. No penetrative further geochronological work is needed to test tions that show the assembly of Rodinia along ductile deformation or metamorphism younger these relationships. Previous stratigraphic names Grenvillian sutures must include a southern than Mesoproterozoic age has affected the ex- are referenced below where helpful in relating re- continent off the southern margin of Laurentia, posed rocks in Texas. cent work to previous literature. an element lacking in most reconstructions (Dalziel, 1991, 1992, 1997; Hoffman, 1991; LLANO UPLIFT Coal Creek Domain Borg and DePaolo, 1994; Torsvik et al., 1996; Unrug, 1996). The exception is Moores (1991) The Llano uplift consists of ca. 1360 to 1232 ± The Coal Creek domain consists of a long- who tentatively suggested that Baltica might lie 4 Ma metavolcanic, metaplutonic, and metasedi- lived, 1326 ± 2 to 1275 +2/–1 Ma plutonic com- off this southern margin of Laurentia. mentary rocks (Table 1; Fig. 2) that have been plex (Figs. 2 and 3), interpreted as an ensimatic In this paper I summarize the results of recent polydeformed synchronous with a moderate- to arc (Roback, 1996a), that shows an early intru- studies along the southern margin of Laurentia high-pressure, upper amphibolite to lower gran- sive, deformational, and metamorphic history and propose the first comprehensive plate model ulite facies regional metamorphism (Walker, unique to that of the rest of the uplift. A large (6 for the Mesoproterozoic tectonic evolution of this 1992; Mosher, 1993; Reese, 1995; Roback, × 2.3 km) tabular body of serpentinized margin. New absolute age constraints and careful 1996a; Carlson, 1998). These rocks were subse- harzburgite (Coal Creek Serpentinite) has been structural and metamorphic studies allow correla- quently intruded by 1119 +6/–3 to 1070 ± 2 Ma, tectonically emplaced within the plutonic com- tion of tectonic events and settings along the mar- syntechtonic to post-tectonic granites (Table 1; plex and shows a complicated history of serpen- gin. Although further work is needed to test the Fig. 2) synchronous in part with a generally stat- tinization, metamorphism, and deformation models proposed herein, a consistent story for the ic, low-pressure, middle amphibolite facies meta- (Gillis, 1989; Mosher and Gillis, unpublished tectonic evolution of the southern margin of Lau- morphism (Walker, 1992; Reed, 1995; Reed data). The oldest parts of the complex are 1326 rentia during Mesoproterozoic time is emerging. et al., 1995; Carlson, 1998). ± 2 to 1301 +2/–1 Ma, folded, well-foliated, Recent U/Pb geochronology (Walker, 1992; gray, mafic to tonalitic gneisses (previously OVERVIEW Reese, 1995; Roback, 1996a) has shown that the mapped as Big Branch Gneiss), found primarily previously described stratigraphy (see Barnes, south of the serpentinite, that underwent dy- Rocks of presumed Grenville affinity lie south 1981) is unusable in its present form. Rocks namothermal metamorphism ca. 1292 Ma of the northeast-trending Llano front, a magnetic mapped as a single unit vary significantly (>100 (Roback, 1996a). North of the serpentinite, and gravity anomaly similar to the Grenville front m.y.) in age and in many cases cannot be genetic- these gray tonalitic gneisses are cut by a in the Appalachians (Mosher, 1993) (Fig. 1). Drill ally related. In the eastern uplift, where most of the younger, dioritic to tonalitic suite (previously cores into basement north of the front contain recent detailed work has been conducted, three in- mapped as part of the Packsaddle Schist Click granitic and rhyolitic rocks of the southern Gran- dividual lithotectonic domains and intrusive rocks Formation; older Packsaddle Schist of Mosher, ite rhyolite terrane (1400–1300 Ma; Van Schmus that crosscut one or more domains have been de- 1993). These younger (1286 +6/–4 to 1275 et al., 1996), whereas south of the front they con- fined (Mosher, 1996; Roback, 1996a)(Figs. 2 and +2/–1 Ma) cogenetic plutonic rocks show well tain gneisses and metasedimentary rocks. About 3). The southernmost domain, the Coal Creek do- preserved cross-cutting intrusive relationships 300 km south of the front, the core of the southern main, is a tonalitic to dioritic arc terrane (Garrison, and truncate the fabric in the older tonalitic Grenville orogen is exposed in the Llano uplift of 1981b, 1985; Roback, 1996a). The Coal Creek do- gneisses, but are themselves foliated (Roback, central Texas (Figs. 2 and 3). The uplift contains a main structurally overlies the Packsaddle domain 1996a). The Coal Creek domain also contains collisional suture between a distinct arc terrane to the north along the southwest-dipping Sandy gabbros, amphibolites, mafic schists, and minor and continental crust, medium-temperature eclo- Creek ductile thrust zone (Carter, 1989; Roback, talc-rich and serpentinite bodies, all of which in- gitic rocks suggesting burial to depths of ~50 km, 1996a; Whitefield, 1997). The Packsaddle domain crease in abundance southward approaching the and evidence of dynamothermal tectonism ca. consists of metavolcanic and metasedimentary Coal Creek Serpentinite. Mafic rocks are Fe-
1150–1120 Ma (Wilkerson et al., 1988; Mosher, supracrustal rocks intruded by metaplutonic rocks, rich, low- to medium-K2O tholeiites composi-
1358 Geological Society of America Bulletin, November 1998 Downloaded from gsabulletin.gsapubs.org on October 31, 2010 Q ऊ e
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Modified after Barnes (1981) and Reed (1996a). lated to the Ouachita orogeny. Spring–Packsaddle domain boundary for southeastern uplift.) (VSD–PSD—Valley ࣿ ࣾ ࣿ ࣽ ࣿ ऀ y z | } y ं ः ौ ࣿ ऀ | } ࣽ y z ं ः ौ ࣿ ऀ | } ࣽ ࣾࣿ ऀ ँं ः ऄ z {| } ~ ौ ् ªª «« ¥¥ ¦¦ YY ZZ TT UU ऄऄ अअ ࣿࣿ ऀऀ ¹¹ ºº ´´ µµ ¯¯ °° ªª «« ¥¥ ¦¦ hh ii cc dd ^^ __ YY ZZ TT UU ऍऍ ऎऎ ईई उउ ऄऄ अअ ࣿࣿ ऀऀ ¹¹ ºº ´´ µµ hh ii cc dd ऍऍ ऎऎ ईई उउ ࣽ ࣾ ࣿ ऀ R S T U £ ¤ ¥ ¦ ऄ अ ई उ Y Z ^ _ c d ª « ¯ ° ´ µ ࣽ ࣾ R S £ ¤ ं ः ऄ अ आ इ ई उ ऋ ऌ ऍ ऎ W X Y Z \ ] ^ _ a b c d f g h i ¨ © ª « ® ¯ ° ² ³ ´ µ · ¸ ¹ º आ इ ऋ ऌ a b f g ² ³ · ¸ Q ¢ ࣾ S ¤ ऊ e ¶ इ ऌ b g ³ ¸ {{ ࣾࣾ ्् ~~ {{ ऄऄ ँँ ࣾࣾ ्् ऄऄ {{{ || }} ौौौ ्् ॎॎ ॑॑॑ ॒॒ ॓॓ ࣾࣾࣾ ࣿࣿ ऀऀ ःःः ऄऄ अअ इइइ ईई उउ ॖॖ ॗॗ ऌऌऌ ऍऍ ऎऎ {{{ || }} ौौौ ्् ॎॎ ॑॑॑ ॒॒ ॓॓ ࣾࣾࣾ ࣿࣿ ऀऀ ःःः ऄऄ अअ इइइ ईई उउ ॖॖ ॗॗ ऌऌऌ ऍऍ ऎऎ {{{ || }} ौौौ ्् ॎॎ ॑॑॑ ॒॒ ॓॓ ࣾࣾࣾ ࣿࣿ ऀऀ ःःः ऄऄ अअ इइइ ईई उउ ॖॖ ॗॗ ऌऌऌ ऍऍ ऎऎ ࣽ y z उ ऎ ॗ ࣿ ऀ | } ऊ ऋ ः ऄ ौ ् ॑ ॒ ࣽࣽ ࣾ ࣿ ँँ ंं ः ऄ आआ इ ई ऊऊ ऋऋ ऌ ऍ yy zz { | ~~ ौ ् ॏॏ ॐॐ ॑ ॒ ॔॔ ॕॕ ॖ ऍ ऎ ॖ ॗ ࣿ ऀँ ऄ अ | }~ ् ॎॏ ॒ ॓ आ इ ऋ ऌ ॕ ࣽࣽ ࣾ ࣿ ँँ ंं ः ऄ आआ इ ई ऊऊ ऋऋ ऌ ऍ yy zz { | ~~ ौ ् ॏॏ ॐॐ ॑ ॒ ॔॔ ॕॕ ॖ ॑ ࣽ ࣾ ࣿ ऀ ं ः ऄ अ आ इ ई उ z { | } ौ ् ॎ ॐ ॒ ॓ ॕ ॓॔ ऌ ऍ ॖ 1080±6 yy || yy ंं ࣿࣿ || yy ंं ࣿࣿ y ࣾ ࣿ { | ࣾ { 1288±2 ࣾࣿ ँ {| ~ ࣽ ࣾ y z { ࣽ ऀ ँ ः ऄ } ~ ौ ् y z ࣿ ऀ ँ ं ः ऄ | } ~ ौ ् ऄ ् ँ ~ ࣾ { ँ ~ ࣿ ऀ ं ः | } ौ ऄ ् ँ ~ ࣿ ऀ ं ः | } ौ zzz {{ ࣽࣽࣽ ࣾࣾ ौौौ ्् }}} ~~ zzz {{ ःःः ऄऄ ऀऀऀ ँँ ࣽࣽࣽ ࣾࣾ ौौौ ्् }}} ~~ zzz {{ ःःः ऄऄ ऀऀऀ ँँ ࣽࣽࣽ ࣾࣾ ࣽ y z ࣽ ࣾ ࣿ y z { | ࣾࣾࣾ yyy {{{ ࣾࣾࣾ yyy {{{ ࣾࣾࣾ yyy {{{ ࣾࣾࣾ yyy {{{ ࣾࣾࣾ yyy {{{ ࣽ z ࣽࣽ ࣿࣿ zz || ࣿ | ࣿࣿ || T UV Y Z[ ^ _ ¥ ¦§ ª «¬ ¯ ° ࣿ ऀँ ऄ अ ऍ ऎ h i ¹ º ࣿ ऀ ऄ अ T U Y Z ^ _ ¥ ¦ ª « ¯ ° ं ः ऄ W X Y \ ] ^ ¨ © ª ® ¯ Q ¢ आ [ \ a ² ࣽ ࣾ ँ ं ः Q R S V W X ¢ £ ¤ § ¨ © ऊ e ¶ आ इ ऊ ऋ ऌ ` a b e f g ± ² ³ ¶ · ¸ ··· ¶¶¶ ²²² ±±± ¬¬¬ §§§ ¨¨¨ ¢¢¢ £££ fff eee aaa ``` \\\ [[[ VVV WWW QQQ RRR ऋऋऋ ऊऊऊ आआआ ँँँ ंंं ࣽࣽࣽ ··· ¶¶¶ ²²² ±±± ¬¬¬ §§§ ¨¨¨ ¢¢¢ £££ fff eee aaa ``` \\\ [[[ VVV WWW QQQ RRR ऋऋऋ ऊऊऊ आआआ ँँँ ंंं ࣽࣽࣽ ··· ¶¶¶ ²²² ±±± ¬¬¬ §§§ ¨¨¨ fff eee aaa ``` \\\ [[[ VVV WWW ऋऋऋ ऊऊऊ आआआ ँँँ ंंं ऀ } ऎ ॗ ः ौ ॑ ऊ ॔ ऀऀ अअ उउ ऎऎ }} ॎॎ ॓॓ ॗॗ ई उ ॒ ॓ ऀऀ अअ उउ ऎऎ }} ॎॎ ॓॓ ॗॗ इ ई उ ॑ ॒ ॓ || }} yy zz ࣿࣿ ऀऀ ौौ || }} yy zz ंं ःः ࣿࣿ ऀऀ ौौ || }} yy zz ंं ःः ࣿࣿ ऀऀ | } y z ࣿ ऀ ौौ ंं ःः ौ ं ः | } ࣿ ऀ | } y z ࣿ ऀ ौ | } y z ं ः ࣿ ऀ ौ ं ः ࣽ y z ं ः ौ ࣽࣽ ࣽࣽ ࣽ ࣽࣽ ࣽ ࣽ ࣽࣽࣽ ࣿࣿࣿ zzz ||| ࣽࣽࣽ ࣿࣿࣿ zzz ||| ࣽࣽࣽ ࣿࣿࣿ zzz ||| ࣽࣽࣽ ࣿࣿࣿ zzz ||| ࣽࣽࣽ ࣿࣿࣿ zzz ||| 1254+6/-4 ࣾࣾ @@ BB ौौ ॎॎ ÀÀ  QQ SS ¢¢ ¤¤ ࣾࣾ @@ BB ौौ ॎॎ ÀÀ  QQ SS ¢¢ ¤¤ ~~ {{ ँँ ࣾࣾ ्् ~~ {{ ऄऄ ँँ ࣾࣾ ्् ~~ {{ ऄऄ ँँ ~ ࣾࣾ ँ { ࣾ ्् ऄऄ ् ~ ऄ ँ ~~ {{ ँँ ࣾࣾ ्् ~~ {{ ऄऄ ँँ ࣾࣾ ्् ऄऄ ࣾ { ऄ ् Mosher, 1996
in 1252±3 T UV Y Z[ ^ _ ¥ ¦§ ª «¬ ¯ ° उ _` d¬ °± µ ࣿ ऀँ ऄ अ ऍ ऎ h i ¹ º आ ऋ [ \ a f ² · ¹¹¹ ººº ¸¸¸ ´´´ µµµ ³³³ ¯¯¯ °°° ®®® ªªª ««« ©©© ¤¤¤ ¥¥¥ ¦¦¦ hhh iii ggg ccc ddd bbb ^^^ ___ ]]] YYY ZZZ XXX SSS TTT UUU ऍऍऍ ऎऎऎ ऌऌऌ ईईई उउउ इइइ ऄऄऄ अअअ ःःः ࣾࣾࣾ ࣿࣿࣿ ऀऀऀ ¹¹¹ ººº ¸¸¸ ´´´ µµµ ³³³ ¯¯¯ °°° ®®® ªªª ««« ©©© ¤¤¤ ¥¥¥ ¦¦¦ hhh iii ggg ccc ddd bbb ^^^ ___ ]]] YYY ZZZ XXX SSS TTT UUU ऍऍऍ ऎऎऎ ऌऌऌ ईईई उउउ इइइ ऄऄऄ अअअ ःःः ࣾࣾࣾ ࣿࣿࣿ ऀऀऀ ¹¹¹ ººº ¸¸¸ ´´´ µµµ ³³³ ¯¯¯ °°° ®®® ªªª ««« ©©© hhh iii ggg ccc ddd bbb ^^^ ___ ]]] YYY ZZZ XXX ऍऍऍ ऎऎऎ ऌऌऌ ईईई उउउ इइइ ऄऄऄ अअअ ःःः ँँँ ंंं ःःः yyy zzz {{{ ~~~ ौौौ ॏॏॏ ॐॐॐ ॑॑॑ ँँँ ंंं ःःः आआआ इइइ ऊऊऊ ऋऋऋ ऌऌऌ yyy zzz {{{ ~~~ ौौौ ॏॏॏ ॐॐॐ ॑॑॑ ॔॔॔ ॕॕॕ ँँँ ंंं ःःः आआआ इइइ ऊऊऊ ऋऋऋ ऌऌऌ yyy zzz {{{ ~~~ ौौौ ॏॏॏ ॐॐॐ ॑॑॑ ॔॔॔ ॕॕॕ ऊऊऊ ऋऋऋ ऌऌऌ ࣽࣽࣽ ࣾࣾࣾ ࣽࣽࣽ ࣾࣾࣾ ࣽࣽࣽ ࣾࣾࣾ आ ॕ Reese, 1995 Helper et al., 1996 Reed et al., 1995 Whitefield, 1997 Rougvie et al., 1996 Includes data from: Walker, 1992 Roback, 1996 a, 1996b, 1243±2 Dat es are U-Pb zircon crystallization ages or (m) U-Pb metamorphic z = zircon r = rutile t = titanite m = monazite ࣾ @ B ौ ॎ À  Q S ¢ ¤ ࣾ @ B ौ ॎ À  Q S ¢ ¤ @ ौ À Q ¢ ࣾ B ॎ  S ¤ ࣽࣽ ࣾࣾ ࣿࣿ yy zz {{ || ࣽࣽ ࣾࣾ ࣿࣿ yy zz {{ || ࣾࣾ ࣿࣿ {{ || ࣽࣽ ࣿࣿ AA CC ्् ॏॏ ÁÁ Ãà RR TT ££ ¥¥ ࣽࣽ ࣿࣿ AA CC ्् ॏॏ ÁÁ Ãà RR TT ££ ¥¥ 1254+5/-3 1147±4 z (m) 1128±6 z (m) ࣿ ऀ ं ः | } ौ ࣽ ࣾ ऀ ँ z { } ~ ࣽ ऀ y z } ࣽ ࣿ A C ् ॏ Á à R T £ ¥ ࣽ ࣿ A C ् ॏ Á à R T £ ¥ ࣽࣾ AB ्ॎ Á RS £¤ ࣽࣾ ࣿ AB C ्ॎ ॏ Á à RS T £¤ ¥ ࣽ ࣿ A C ् ॏ Á à R T £ ¥ ࣾࣾ @@ BB ौौ ॎॎ ÀÀ  QQ SS ¢¢ ¤¤ ࣾࣾ @@ BB ौौ ॎॎ ÀÀ  QQ SS ¢¢ ¤¤ 1070±2 ࣾ { ࣾࣿ ँ {| ~ ई उऊ ऍ ऎ ^ _` c de h i¬ ¯ °± ´ µ¶ ¹ º ¶ ± ¬ § ¢ e ` [ V Q ऊ ँ ¶ ± ¬ § ¢ e ` [ V Q ऊ ँ ¢¢ ££ ¤¤ ¥¥ QQ RR SS TT ࣽࣽ ࣾࣾ ࣿࣿ ¶ e ऊ ¶¶ ·· ¸¸ ¹¹ ±± ²² ³³ ´´ ¬¬ ®® ¯¯ §§ ¨¨ ©© ªª ¢¢ ££ ¤¤ ¥¥ ee ff gg hh `` aa bb cc [[ \\ ]] ^^ VV WW XX YY QQ RR SS TT ऊऊ ऋऋ ऌऌ ऍऍ आआ इइ ईई ँँ ंं ःः ऄऄ ࣽࣽ ࣾࣾ ࣿࣿ ¶¶ ·· ¸¸ ¹¹ ±± ²² ³³ ´´ ¬¬ ®® ¯¯ ee ff gg hh `` aa bb cc [[ \\ ]] ^^ ऊऊ ऋऋ ऌऌ ऍऍ आआ इइ ईई ࣿࣿࣿࣿ ऀऀऀ ऄऄऄऄ अअअ |||| }}} ्््् ॎॎॎ ॒॒॒॒ ॓॓॓ ࣿࣿࣿࣿ ऀऀऀ ऄऄऄऄ अअअ ईईईई उउउ ऍऍऍऍ ऎऎऎ |||| }}} ्््् ॎॎॎ ॒॒॒॒ ॓॓॓ ॖॖॖॖ ॗॗॗ ࣿࣿࣿࣿ ऀऀऀ ऄऄऄऄ अअअ ईईईई उउउ ऍऍऍऍ ऎऎऎ |||| }}} ्््् ॎॎॎ ॒॒॒॒ ॓॓॓ ॖॖॖॖ ॗॗॗ ऍऍऍऍ ऎऎऎ ॖॖॖॖ ॗॗॗ ࣽࣽࣽ ࣿࣿࣿ AAA CCC ््् ॏॏॏ ÁÁÁ ÃÃà RRR TTT £££ ¥¥¥ ࣽࣽࣽ ࣿࣿࣿ AAA CCC ््् ॏॏॏ ÁÁÁ ÃÃà RRR TTT £££ ¥¥¥ ࣽ y z ࣽ ࣾ ࣿ y z { | ࣽࣽࣽ ࣾࣾࣾ ࣿࣿࣿ yyy zzz {{{ ||| ࣽࣽࣽ ࣾࣾࣾ ࣿࣿࣿ yyy zzz {{{ ||| ࣽࣽࣽ ࣾࣾࣾ ࣿࣿࣿ yyy zzz {{{ ||| ࣾࣾࣾ ࣿࣿࣿ {{{ ||| Explanation ·· ¸¸ ¹¹ ºº ²² ³³ ´´ µµ ®® ¯¯ °° ¨¨ ©© ªª «« ££ ¤¤ ¥¥ ¦¦ ff gg hh ii aa bb cc dd \\ ]] ^^ __ WW XX YY ZZ RR SS TT UU ऌऌ ऍऍ ऎऎ ऋऋ आआ इइ ईई उउ ंं ःः ऄऄ अअ ࣽࣽ ࣾࣾ ࣿࣿ ऀऀ ·· ¸¸ ¹¹ ºº ²² ³³ ´´ µµ ®® ¯¯ °° ¨¨ ©© ªª «« ££ ¤¤ ¥¥ ¦¦ ff gg hh ii aa bb cc dd \\ ]] ^^ __ WW XX YY ZZ RR SS TT UU ऌऌ ऍऍ ऎऎ ऋऋ आआ इइ ईई उउ ंं ःः ऄऄ अअ ࣽࣽ ࣾࣾ ࣿࣿ ऀऀ ¦¦ UU ऀऀ ·· ¸¸ ¹¹ ºº ff gg hh ii ऌऌ ऍऍ ऎऎ ऋऋ ºº µµ °° «« ¦¦ ii dd __ ZZ UU ऎऎ उउ अअ ऀऀ ºº µµ °° ii dd __ ऎऎ उउ ࣽࣽ yy zz ࣽ ࣾ ࣿ ऀ ँ y z { | } ~ ࣽ ࣾ ं ः y z { ौ ࣽࣽ ࣾࣾ ࣿࣿ yy zz {{ || ं ः ऄ ौ ् ऊ ऋ ऌ Coarse-grained granites Fine- to medium-grained granites Rhyolitic dikes Comanche Creek and Red Mountain gneisses Packsaddle Domain Valley Spring Domain Lost Creek Gneiss Valley Spring Gneisses Coal Creek Domain Big Branch Gneiss Coal Creek igneous complex with serpentinite Paleozoic ࣽ ऀ y z } ࣽ @ A ौ ् À Á Q R ¢ £ ࣾ ࣿ ं ः B C F G ॎ ॏ ॒ ॓ Â Ã Æ Ç S T W X ¤ ¥ ¨ © Figure 2. Geologic map of the Llano uplift, central Texas, showing lithotectonic do- 2. Geologic map of the Llano uplift, central Texas, Figure ࣽ ࣾ ࣿ mains and U/Pb dates (see Table 1). The uplift is an erosional window through Phanero- window through 1). The uplift is an erosional mains and U/Pb dates (see Table locally cut by normal and oblique-slip faults re- are The rocks zoic sedimentary cover. ࣽ ࣿ ऀ ँ z | } ~ ऀ ँ ः ऄ } ~ ौ ् ॏ ॐ ई उ ऀँ अ }~ ॎ ऎ ॗ ࣾࣿ ँ {| ~ Geological Society of America Bulletin, November 1998 1359 ࣿ ऀ | } ऍ ऎ ॖ ॗ ^ _ ¯ ° Downloaded from gsabulletin.gsapubs.org on October 31, 2010 S. MOSHER tionally similar to present-day island arc tholei- ites and ocean floor basalts (Garrison, 1981a, 1981b). An enigmatic younger metamorphism has been dated as 1256 +2/–1 Ma (Roback, ࣽࣽࣽࣽzzzz yyy Coal Creek Domain 1301-1326 Ma 1996a). The structural stacking within the Coal 1275- Island arc 1292 Ma 1286 Ma
Creek domain (older tonalitic gneisses overly- ࣽࣽࣽࣽࣽࣾࣾࣾࣾऀऀऀऀऀँँँँःःःःःऄऄऄऄzzzzz{{{{}}}}}~~~~ौौौौौ्््् ࣿࣿࣿࣿंंंंyyyy|||| ࣽࣽࣽࣽࣿࣿࣿࣿzzzz|||| ࣾࣾࣾyyy{{{ ing serpentinite that overlies younger plutonic 1255-1247 Ma 1238-1239 rocks that become less mafic away from the ser- Packsaddle Domain ࣽࣽࣽࣿࣿࣿzzz||| ࣾࣾࣾyyy{{{Ma ࣽࣽࣽࣿࣿࣿzzz||| ࣾࣾyy{{Supracrustal rocks pentinite) suggests that the serpentinite repre- ªªªª©©©©§§§§¦¦¦¦YYYY££££¤¤¤¤XXXXVVVVUUUUऄऄऄऄRRRRSSSSःःःःँँँँऀऀऀऀࣽࣽࣽࣽࣾࣾࣾࣾ¨¨¨¨¥¥¥¥¢¢¢¢WWWWTTTTQQQQंंंंࣿࣿࣿࣿ
ःःःःःऄऄऄऄौौौौौ््््ंंंं1244-1274 Ma and intrusions sents arc basement or associated oceanic crust 1098 Ma that has been tectonically imbricated with the ࣿࣿࣿ|||ࣾࣾࣾ{{{ roots of an ensimatic arc (Roback, 1996a). ࣾࣾࣾँँँ{{{~~~ ࣽࣽࣽࣿࣿࣿऀऀऀyyyzzz|||}}} ªªªª©©©©§§§§¦¦¦¦YYYY££££¤¤¤¤XXXXVVVVUUUUऄऄऄऄRRRRSSSSःःःःँँँँऀऀऀऀࣽࣽࣽࣽࣾࣾࣾࣾ¨¨¨¨¥¥¥¥¢¢¢¢WWWWTTTTQQQQंंंंࣿࣿࣿࣿ
ࣽࣽࣽࣽࣽࣾࣾࣾࣾzzzzz{{{{ yyyy The Coal Creek domain rocks have distinctly 1119-1116 Ma 1091-1070 Ma 1254 Ma ࣽࣽࣽzzz yy 1270 Ma ࣽࣽࣽࣾࣾࣾࣾࣿࣿࣿyyyyzzz{{{{||| Valley Spring Domain different geochemical and Nd and Pb isotopic Supracrustal rocks, Late Syntectonic Granites 1232 Ma Post-tectonic Granites 1360 Ma signatures than those of the Packsaddle and ࣿࣿःः CCGGKKॏॏ॓॓ÃÃÇÇËË ࣽࣾࣿऀँंः@ABCDEFGौ्ॎॏॐ॒॑॓ÀÁÂÃÄÅÆÇ ࣽࣽऀऀँँऄऄअअ @@AADDEEHHIIJौौ््ॐॐ॑॑॔॔ॕॕÀÀÁÁÄÄÅÅÈÈÉÉÊ intrusions and continental
ࣾࣾࣾࣾࣿࣿࣿ{{{{||| ªªªª©©©©§§§§¦¦¦¦YYYYXXXXVVVVUUUUऄऄऄऄःःःःँँँँऀऀऀऀ¨¨¨¨¥¥¥¥WWWWTTTTंंंंࣿࣿࣿࣿ1288 Ma Valley Spring domains to the north, indicating ࣽࣽࣽࣽࣽࣾࣾࣾࣾऀऀऀऀऀँँँँःःःःःऄऄऄऄzzzzz{{{{}}}}}~~~~ौौौौौ्््् ࣿࣿࣿࣿंंंंyyyy||||basement(?) ࣽࣽࣽࣿࣿࣿzzz||| ࣾࣾyy{{ that the arc evolved separately from the rest of the uplift (Roback et al., 1995; Roback, 1996a; Figure 3. Structural column showing the spatial relationships among dated rocks and do-
Whitefield, 1997). Initial Pb isotopic data from ःःःःःऄऄऄऄौौौौौ््््ंंंं mains for the Llano uplift.ऄअ HIJKLMNO॔ॕÈÉÊËÌÍÎÏ Coal Creek Serpentinite is represented by solid black in Coal Creek tectonized granitic rocks within the Packsaddle domain. Intrusive rocks in Packsaddle domain and Valley Spring domain shown by random domain and Valley Spring domain and from parallel line dashes are considered part of the same period of igneous activity and are stitching post-tectonic granites plot along a 1.1 Ga plutons for these two domains. Only the 1098 +3/–2 Ma rhyolite dike (Packsaddle domain) and 207Pb/204Pb–206Pb/204Pb isochron (James and post-tectonic granites are undeformed. Sources for data are given in Table 1. Modified from Walker, 1992; Smith et al., 1997), whereas Pb Mosher (1996). isotopic data for Coal Creek domain rocks plot above this isochron (higher 207Pb/204Pb for a specific 206Pb/204Pb) (Roback et al., 1995; Roback, 1996a; Smith et al., 1997, Fig. 13). Sm- Nd isotopic data yield older model ages (1679 to
ࣽࣽࣾRRS££¤ Q¢ 1403 Ma; eNd +1.2 and +3.9) for the Coal Creek domain than for most of the Packsaddle domain 10 km
(1478 to 1276 Ma; eNd +2.4 to +4.8), the Valley