5Geosciences, University of lkm at Dallas, Richardson, TX 75083-0688, U.S.A. ... samples for three subsurface terranes in west Wxas and eastern New Mexico.
Isotopic and elemental chemistry of subsurface Precambrian igneous rocks, west mxas and eastern New Mexico M e l a n i e A. Barnes', C. Renee Rohs2, Elizabeth Y. Anthoe, W. Randy Van Schmus4,and Rodger E. Denison5 'Department of Geosciences, lkxas B c h University, Lubbock, TX 79409-1053, U.S.A. 2Mineral Area College, Park Hills, MO 63601, U.S.A. 3Departmentof Geological Sciences, University of Dxas at El Paso, El Paso, TX 79968, U.S.A. 4Department of Geology, University of Kansas, Lawrence, KS 66045, U.S.A. 5Geosciences, University of lkm at Dallas, Richardson, TX 75083-0688, U.S.A.
ABSTRACT We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in west Wxas and eastern New Mexico. The most northerly is the Panhandle volcanic terrane, which represents a large part of the Mesoproterozoic southern granite-rhyoliteprovince. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debaca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debaca terranes: Intermediate- to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) &O/Na,O > 1 and A-type trace element characteristics; and (2) &O/ Na,O < 1 and I-type trace element characteristics. All but a few samples from the Panhandle terrane, both north and south of the Debaca terrane, are A-type. Depleted mantle model ages for granites and rhyolites from the northern Panhandle terrane range from 1.50-1.69 Ga. W o samples from the southern Panhandle terrane have model ages of 1.74 Ga, and a third sample's model age is 1.57 Ga. These model ages are older than the four reported crystallization ages of 1.37 -1.4 Ga, indicating that: (1) A-type rocks of the Panhandle terrane contain a significant crustal component; and (2) Panhandle terrane is underlain by > 1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a T, of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debaca terrane is distinctly younger at 1.26 Ga, and is the same as crystallizationages of felsic tuffs associated with shelf carbonates in the Franklin Mountains and Van Horn area. In the crystalline terrane, both A- and I-type granites are present. Model ages for the I-type granites are 1.40-1.47 Ga. These are distinctly younger than the model age for the Panhandle terrane, and an A-type granite has a T,, of 1.35 Ga. These data indicate that granites in the crystalline terrane are not part of the granite-rhyolite province; rather, they constitute a separate group. KEY WORDS: Proterozoic, granite-rhyolite province, A-type granites, Nd model ages, gabbro, crustal growth, Laurentia, Tkxas, New Mexico.
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M. A. BARNES ET AL.
aulacogen and the central basin platform. The central basin platform is underlain by the Pecos maRecent work on Precambrian geology of the fic intrusive complex, a 360 km-long mafic to midcontinent region has focused on its eastern and ultramafic layered intrusion (Kargi and Barnes, south-central parts (Lidiak, 1996; Van Schmus et al., 1995). The intrusion has been modeled as a 9 km1996). In this paper we shift the focus farther west, thick funnel-like body in the south and a 5 kmand we characterize the basement of west Texas and thick sill-likebody in the north (Adams and Keller, eastern New Mexico (Fig. 1). This area encompasses 1996). the western part of the southern granite-rhyolite Outcrops of Precambrian rocks in Texas occur province and the northwestern portion of the L1- only in the Franklin Mountains and Van Horn arano province, the area between the Llano front and eas of west Tkxas and the Llano uplift in central the Precambrian margin. A northeast-trending fea- Texas. Thus, characterization of Precambrian geture known as the Llano front (Mosher, 1998) marks ology for most of the area relies on regional geothe approximate boundary between undeformed physical data, core samples, and cuttings retrieved rocks to the north and deformed rocks to the south. from drillholes. Earlier compilations of the subThe Abilene gravity minimum, a geophysical fea- surface geology for this area include those by ture, overlaps with, and lies just south of, the Llano Flawn (1956), Wasserburg et al. (1962), Ham et al. front. Adams and Keller (1996) pointed out that the (1964), Muehlberger et al. (1966, 1967), Lidiak et size and density-contrast of the Abilene gravity al. (1966), Goldich et al. (1966), Denison and minimum are appropriate for a granitic batholith Hetherington (1969), Bickford and Lewis (1979), such as the Sierra Nevada. Other geologic features Bickford et al. (1981a, 1981b), Van Schmus and in the study area include the southern Oklahoma Bickford (1981), Denison et al. (1984), and Tho-
INTRODUCTION
F’ignm 1. Precambrian crustal provinces in central North America, modified from Van Schmus et al. (1996). EGR, eastern granite-rhyolite province; SGR, southern granite-rhyolite province; CB, Cheyenne belt; MCR, Midcontinent Rift system; PMIC, Pecos mafic intrusive complex (underlies the central basin platform); FM, Franklin Mountains; VH, Van Horn area; LU, Llano uplift; AGM, Abilene gravity minimum; and SOA, southern Oklahoma aulacogen. Dashed boundary is southern limit of Precambrian cratonic rocks.
246
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CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
mas et al. (1984). In the first major study of the basement for this area, Flawn (1956) differentiated subsurface terranes by studying over 800 rock samples and limited geophysical and structural data. Later workers essentially followed the subsurface terrane boundaries established by Flawn, with only minor modifications. In some instances, however, the terranes have been split and more narrowly defined (Denison et al., 1984; Mosher, 1993; Muehlberger et al., 1967). After careful consideration of the previous work, we have elected to use the following terminology for the subsurface terranes of west Texas and eastern New Mexico (Fig. 2). The Panhandle tewane includes the Panhandle volcanic terrane (Flawn, 1956), Amarillo granite (Denison et al., 1984), Sierra Grande terrane (Muehlberger et al., 1967),and part of the 'Ibrrence metamorphic terrane west of the Debaca terrane (Muehlberger et al., 1967). The Panhandle terrane represents the southern graniterhyolite province of the midcontinent in Texas and New Mexico, and it consists of rhyolite and associated granite with minor amounts of gabbroic rock. Igneous rocks in other areas of the granite-rhyolite province are classified as A-type in current granite terminology by Anderson (1983) and Lidiak (1996). Earlier work includes elemental and geochronological data for only four samples (21, 26, 29, and 30) from the Panhandle terrane (Bickford et al., 1981a; Thomas et al., 1984). All four samples are rhyolites and yield U-Pb zircon ages between 1.37 and 1.4 Ga. In addition, Van Schmus et al. (1996) published a Nd model age of 1.57 Ga for sample 26 from this group. We have grouped the Swisher diabasic terrane (originally known as the Swisher gabbroic terrane) and the Debaca terrane of Muehlberger et al. (1967). We retain use of the name Debaca tewane to represent a n intercalated metasedimentary and metavolcanic pile that was intruded by gabbroic and diabasic rocks. The Debaca terrane divides the Panhandle terrane into northern and southern parts (Fig. 2). The only geochronological data for the Debaca terrane are tentative, and are based on its correlation by Denison et al. (1984) a n d Muehlberger et al. (1967) with the Mundy Breccia and Castner Marble exposed in the Franklin Mountains. An ash bed within the Castner Marble yields a U-Pb zircon age of 1.26 Ga (Pittenger et al., 1994). Thus the Debaca terrane is considered to be younger than the Panhandle terrane. The third terrane is one we devised for ease of presentation and discussion of the data. We call it the crystaZZine tewane, and it includes the Chaves granite terrane of Muehlberger et al. (1967), to-
gether with Flawn's (1956) Red River mobile belt, Fisher metasedimentary terrane, Texas craton, and Llano province. The crystalline terrane is divided by the Llano front into northern and southern parts (Fig. 2). Samples from the crystalline terrane include granite, granodiorite, diorite, and granitic gneiss, plus metarhyolite from an outcrop in the Van Horn area. There are two published U-Pb zircon ages (1.33 Ga from Roths, 1993; 1.38 Ga from Soegaard et al., 1996) and two Nd model ages (1.41 and 1.42 Ga, both from Patchett and Ruiz, 1989) for metarhyolite from the Carrizo Mountains of the Van Horn area. The goal of this paper is to present whole rock major and trace element geochemistry and Nd isotopes for a subset of the basement samples examined by Flawn, Muehlberger, and Denison. In addition to the samples mentioned above, the only other published geochemical and isotopic data for the subsurface Precambrian in west Texas and eastern New Mexico are from the Pecos mafic intrusive complex (Kargi and Barnes, 1995;Keller et al., 1989; C. G. Barnes et al., this issue). Because igneous rocks can be used as probes of underlying crust and mantle, these subsurface samples contribute to understanding the nature of crustal evolution and the tectonic setting at the edge of the North American craton during the Proterozoic.
SAMPLE DESCRIPTIONS Introduction Samples used in this study were provided by R. E. Denison at the University of Texas-Dallas and C. G. Barnes at Texas Tech University. Petrographic descriptions were compiled from Flawn (1956), Muehlberger et al. (1967), and Thomas et al. (1984). M. A. Barnes and R. E. Denison reexamined thin sections for consistency of terminology. The samples are listed in the Appendix 1 in numerical order, along with locality information and rock names. Figure 2 shows distribution of the samples in relation to the subsurface terranes, the Pecos mafic intrusive complex, and the Llano front.
Panhandle !&mane Samples located within boundaries of the Panhandle terrane include diabase, rhyolite, welded tuff, undeformed granite, and metarhyolite. Although only one diabase sample (no. 472) from within the terrane was analyzed, Flawn (1956) showed that diabase rocks are found throughout the Panhandle terrane. In addition, the petrography of these
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M. A. BARNES ET AL.
samples suggests that the mafic rocks were intruded into the volcanic rocks of the Panhandle terrane as dikes and sills. Sample 472 is a medium-grained intergranular diabase with a felted texture of small plagioclase laths surrounded by large altered olivine, clinopyroxene, and oxide crystals, and it comes
from a well that also penetrated granite. One other diabase sample (no. 478), located just east of the northern Panhandle terrane, was analyzed (Fig. 2). This sample is coarse-grained, contains highly altered olivine, and shows evidence of applied stress such as bent biotite and wavy plagioclase twins. The
F’ignm 2. Subsurface terrane and sample map for west Tkxas and eastern new Mexico. Samples analyzed for this study are represented by diamonds (A-type), squares (I-type), and circles (mafic). Underlined numbers denote volcanic samples. Crosses represent unanalyzed samples for which thin sections exist, and stars represent U-Pb age samples from Thomas et al. (1984). Llano front is denoted by dashed line. M represents metarhyolite samples, and D represents samples showing features in thin section that are associated with penetrative deformation. 248
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similarity between samples 472 and 478, as well as the close proximity of sample 478 to the Panhandle terrane, suggests that this sample may be associated with the Panhandle terrane, not the crystalline terrane in which the well is located. The non-welded rhyolitic samples (nos. 485,486, 494, 496, and 507) of the Panhandle terrane typically contain 15-25 percent potassium feldspar and plagioclase phenocrysts, and less than five percent altered ferromagnesian minerals. Only one sample (no. 486) in this group of rhyolite, rhyodacite, and dacite contains quartz phenocrysts. Welded rhyolitic tuff or ignimbrite samples (nos. 482, 483, 484, 489, 490, 491, 497, 498, 500, 505, and 508) also occur throughout the Panhandle terrane. Some samples show well-preserved delicate structures such as autolithicvolcanic fragments, flattenedpumice, and glass shards (Flawn, 1956). The percentage of phenocrysts varies in this group from crystal-rich samples (nos. 482 and 483) with greater than 50 percent phenocrysts to crystal-poor samples (no. 497) with less than 10 percent phenocrysts. The subhedral to euhedral phenocrysts are quartz with wavy extinction, alkali feldspar, plagioclase, bent biotites, and altered ferromagnesian minerals. Associated with the volcanic rocks of the Panhandle terrane is a small group ~ t a more h sedimentary nature, including felsic breccia (no. 506), lithic tuff (no. 493), and volcaniclastic sandstones (nos. 499 and 718). The intrusive samples (nos. 509, 513, 514, 605, 620,900, and 905) of the Panhandle terrane are all undeformed medium- to fine-grained granite. Samples 509 and 513 are leucogranites with minor biotite (altered to chlorite) and opaques. Sample 605 is a microgranite with porphyritic texture and less than five percent mafic minerals, and sample 514 is a granophyre (Flawn, 1956). Non-granitic rocks include a group of samples classified as metarhyolites (Fig. 2). These samples (nos. 487,495, 898, 603, and 727) contain 25-40 percent phenocrysts of alkali feldspar, plagioclase, and altered ferromagnesian minerals with less than one percent quartz phenocrysts. Samples 487 and 495, located i n the northern Panhandle terrane, have a granoblastic groundmass, and those from eastern New Mexico (nos. 898,603, and 727) have what could be interpreted as recrystallization features. Denison et al. (1984) suggested that the samples from New Mexico may be part of an older igneous unit that was metamorphosed during eruption and emplacement of magmas of the southern granite-rhyolite terrane (Panhandle terrane). K-Ar dating (Muehlberger et al., 1967) and recent work by
Karlstrom et al. (1997) in north-centralNew Mexico have documented metamorphic ages of 1.4 Ga, which suggest extensive regional metamorphism at this time.
Debaca 'Ilerrane Three mafic samples (nos. 473, 474, and 477) were available from the Debaca terrane. These samples are medium- to coarse-grained subophitic olivine pyroxene gabbro with altered biotite and orthopyroxene, as indicated by presence of chlorite and talc. Sample 474 is from a well, which also penetrated a sequence of metamorphosedvolcanic and sedimentary rocks. Based on numerous petrographic observations, Flawn (1956) proposed that the mafic rocks found in the Debaca terrane were intruded into, and interbedded within, the metavolcanic and metasedimentary sequence. Flawn also concluded that the Debaca terrane is in a broad synclinal downwarp and, because there is no associated gravity anomaly, probably is a relatively thin unit overlying the volcanic rocks of the Panhandle terrane. The presence of volcaniclastic sandstones near the boundary of the Debaca terrane and the southern Panhandle terrane supports this idea (Fig. 2, samples 493 and 499). CrystallineTkmme
As noted previously, the crystalline terrane is a grouping of convenience and includes a variety of sample compositions. Most of the samples are intermediate- to felsic-intrusiverocks; however, there is a small group of more mafic samples in and around the area of the Pecos mafic intrusive complex south of the Llano front (Fig. 2). These samples (nos. 595, 596, and 1161) are noritic to dioritic in composition, and their similarity to previously published descriptions of the Pecos mafic intrusive complex suggests they derive from that complex (Keller et al., 1989; Kargi and Barnes, 1995; C. G. Barnes et al., this issue). The intermediate to felsic compositions found within the crystalline terrane include quartz diorite (no. 597), biotite alkali feldspar granite (nos. 599 and 907), medium- to coarse-grained biotite granite (nos. 502, 519, 520, and TTU-86), leucogranite (nos. 525 and 594), and coarse-grained granophyre (no. 902). In addition, approximately half of the samples (nos. 504,522,580,586,609,615, 908, and 1045) are granitic to granodioritic gneiss, while another group of samples (nos. 525,597,599, 902, and 907) shows petrographic evidence of deformation such as strained quartz and bent biotite.
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M. A. BARNES ET AL.
dried and re-dissolved in 2.5N HCl before being centrifuged and loaded into the cation exchange columns for REE collection. The REE were collected, dried, and redissolved in 1.8N HCl. The REE columns were then used to separate Sm and Nd from one another, and the samples were collected and dried. The Nd samples were loaded on Re filaments with resin (100-200 mesh EiChron LN-SPEC and HPO,), and the Sm samples were loaded on Th filaments. The samples were then analyzed for the SmNd isotopic ratios using a multi-collector, thermal ionization mass spectrometer (TIMS) at University of Kansas Isotope Geology Lab. Standard KU Nd #1 (Van Schmus et al., 1996) was analyzed for each turret of Nd samples to assure that the ratios were accurate. Results of the standard analyses can be obtained from the first author upon request. Biotite analyses were performed at the University of Texas at El Paso on a Cameca SX50. The conditions were 15keV, 15nA, and 5 pm beam diameter. ANALYTICAL METHODS Accuracy of the analyses was monitored by analyzMajor and trace element analyses were done on ing the minerals on the Canada Astimex (CRD1, a Leeman Labs Inductively Coupled Plasma Spec- ALB, RHOD, RhTl, SAN1, and ALM) and NMNH trometer at Texas Tech University. Samples were (OLVF, GROS, MICR, ANOC, ILME, SCAP, and APAT) fused with lithium metaborate and dissolved in 50 standard mounts. Results of the standard analyses ml of five percent HC1 solution. The undiluted so- and the complete data set can be obtained upon lution was used to measure trace elements and a request from the first author. 2/7’s dilution was used to measure major elements. Loss on ignition (LOI) was done at 1000°C on 4-5 g ANALYTICAL REsum for those samples with totals less than 99 weight percent. Duplicate analysis of several samples dem- Major and mace Element Data onstrated that the results presented in this paper The samples fall into three compositional are within analytical error. The analysis also shows groups: (1) mafic samples (44.0 to 49.5 wt% SO,); that, despite variations in grain size of powdered (2) intermediate samples (60 to 65 wt% SiO,); and samples, bias had not occurred when weighing (3) felsic samples ( > 68 wt% SiO,; Thbles 1, 2, and samples for fusions. The instrument was calibrated 3). In both the Panhandle and crystalline terranes, using USGS standards, and accuracy of analyses was felsic compositions dominate our sampling. Only monitored using in-house standards collected at one mafic rock and three intermediate rocks are USGS standard sample sites. Results of the standard found among a total of 27 Panhandle terrane analyses can be obtained from the first author upon samples, and four mafic rocks and two intermediate rocks are found among a total of 24 crystalline request. Sm-Nd isotope analyses were performed at the terrane samples. The majority of the intermediate University of Kansas. Since the samples were small to felsic samples plot in the alkaline field on a Na,O and already finely ground, they were not powdered. + YO versus SiO, diagram and, using either that Samples of 200-300 mg were weighed, placed in diagram or normative calculations, the felsic microwavable Teflon bombs, and spiked with 200- samples are granites or equivalent rhyolites. On an 300p1 of a 14,Nd tracer (KU Nd 2G for felsic rocks A/CNK diagram the intermediate and felsic samples and KU Nd 2B for mafic rocks). Whole-rock samples straddle the peraluminous/metaluminous boundwere predissolved using HF + HN03 on a hotplate. ary. The Panhandle terrane is part of the graniteAfter drying, the samples were redissolved using HF + HNO,, and heated for 1.5 minutes in a micro- rhyolite province, which is characterized by A-type wave at 40 percent power. After cooling and drying, granites and rhyolites (Anderson, 1983; Lidiak, the samples were converted to 6N HCl and dissolved 1996). A-type rocks have high %O/Na,O and FeO,/ again in the microwave. The samples were then MgO ratios, iron-rich biotite, low abundances of Sr, The deformed samples are typically associated with the Llano front or the Pecos mafic intrusive complex (Fig. 2). A final group within the crystalline terrane is similar to samples found within the Panhandle terrane. This group consists of two metarhyolite samples, no. 588 (from east of the Panhandle terrane) and no. 743 (from an outcrop in the Carrizo Mountains). Sample 588 is more gneissic than other metarhyolite samples, which makes it difficult to determine its association. Sample 473 is part of a sequence of metavolcanic and metasedimentary rocks dated at 1.38to 1.33 Ga (Roths, 1993;Soegaard et al., 1996).This metamorphic sequence was thrust over a younger sequence of carbonate,volcanic, and clastic sedimentary rocks during Grenville-age ( ~ 1 . Ga) 1 deformation (Soegaard and Callahan, 1994).
250
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CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
Eu, Sc, V, Ni, Cr, Co, CaO, MgO, and A1,0,, and high abundances of Rb, Ba, Zr, Y, Nb, LREE, Ga, F, Th, and U (Eby, 1990; Whalen et al., 1987). Compared to I-type granites,which are generally calcalkaline, A-type granites are more evolved and fractionated. For this data set, the follow-
ing criteria are used to define the intermediate to felsic samples as either A- or I-type granitoids. The first criterion for characterizing a sample as A-type is a Rb/Sr ratio equal to or greater than one (Thbles 2 and 3). In addition to meeting the Rb/Sr ratio criterion, a
Thble 1. Whole-rock major and trace element analyses of mafic samples. Temane:
NPT
DT
DT
DT
NXT
SXT
SXT
SXT
Sample:
472
473
474
477
478
595
596
1161
SiO,
49.18
45.56 46.33 44.13
48.89
46.51 45.23 49.50
TiO,
1.85 2.62 4.58 15.86 17.77 14.80 11.85 16.14 14.60 17.66 0.29 0.25 0.20 0 2 6
2.69 17.89 10.08 0.18
3.21 2.09 1.55 15.28 14.01 13.27
A1203
MnO
1.71 16.40
6.18 9.92
8.42 7.55
Na,O
2.71
2.65
KZO PZO,
0.57 0.30
0.68 0.21
MgO G O
LO1 Total
Rb Sr Zr Ba Y
Nb sc V
Cr Ni
99.10 15 182 98 210 35.9 1 47.4 301 310
cu
99 136
Zn
86
K20/Na20 Rb/Sr FeOT/FeO,+MgO A/CNK KN/C
0.21 0.08 0.63 0.71 0.33
6.45 7.01
14.67 15.99 12.49 0.24 0.45 0.22 5.04 7.73
7.71 7.97 11.02 11.90
6.66
2.76
6.88
7.91
3.62
3.31
4.26
3.61
2.45
2.49
0.89 0.44
0.98 0.30
1.44
2.30 0.74
0.63 0.33
0.19 0.01
99.16 99.92 99.56
1.99 1.94 100.03
99.35 99.91 99.59
17
14
14
29
57
6
3
328
512 188
439
804
867
276
249
176
81
263 123 446 481 215 30.0 23.9 22.1 8 3 6 17.4 16.4 27.2 269 142 184 45 129 162 96 151 108 93 111
20 102
0.26 0.05
0.25 0.03
0.63
0.67 0.90 0.64
0.84 0.44
17 129
1083
1646 38.0 38.0 10 30 33.2 16.4
117
43
263 47.0
83 28.3
3 52.3
54.9
1
114
328
365
392
18
3
71 30
172 63
242 62
23 118
25 109
69 378
47 80
0.30 0.03 0.70
0.34 0.04 0.77
0.64
0.26
0.08
0.07 0.72
0.78 0.62
0.78
0.68
0.72
0.77
0.02 0.65 0.57 0.28
0.01 0.59 0.51 0.23
oxides as weight percent, other elements as ppm LO1 =loss on ignition NPT = northern Panhandle terrane DT = Debaca terrane NXT = northern crystalline terrane SXT = southern crystalline terrane A/CNK = molar AI,O,/CaO + Na,O + %O KN/C =weight percent YO + Na,O/CaO
sample must also have a K,O/ Na,O ratio greater than one, and high Nb ( > 15 ppm), low V ( < 25 ppm), and low Cr ( < 100 ppm) abundances. Figure 3A shows that the A-type rocks (unfilled symbols) occur predominantly in the Panhandle terrane, and I-type rocks (filled symbols) are the majority in the crystalline terrane. Figure 2 also shows the predominance of A-type samples in the Panhandle terrane and of Itype samples in the crystalline terrane. As noted previously, elemental data for the four U-Pb dated rhyolitic samples from the Panhandle terrane can be found in Bickford et al. (1981a). However, because only major element data are presented, the qO/Na,O ratio is used to determine rock type instead of the Rb/Sr ratio. Samples 26 and 30 have YO/Na,O ratios of 0.95 and 0.81, respectively, which makes them borderline A-types with respect to this criterion (Whalen et al., 1987); nevertheless, all four samples are classified as A-type. Figure 3B is a graph of FeO,/ (FeO, + MgO) ratio versus SiO, weight percent. A-type rocks typically have high FeO,/ (FeO, + MgO) ratios ( > 0.8; Anderson, 1983); however, in this plot there is a large overlap between the A-type and I-type groups and between samples from the Panhandle and crystalline terranes. The overlap between A- and I-type samples indicates that the felsic to intermediate samples in general are evolved, making the whole rock FeO,/(FeO, + MgO) ratio a poor criterion for discrimination. Because the A-type and I-type samples overlap i n the FeO,/ (FeO, + MgO) diagram, we analyzed biotites from five A- and Itype rocks from the crystalline terrane (Thble 4). Biotite mineral chemistry reflects the oxidation
Rocky Mountain Geology, v. 34, no. 2, p . 245-262, 7figs., 5 tables, 1 appendix, November, 1999
251
162 410 20.3 15 2.7 15 41 39 192 55
Zr Ba
bd = below detection limit abbreviationsas in 'hble 1
SFT = southern Panhandle terrane
* denotes A-type, rest are I-type
KN/C
K,O/Na,O Rb/Sr FeOdFeO,+MgO A/CNK
Ni cu Zn
V Cr
sc
Y Nb
153 39
224 156
99.72
71.52 0.41 13.21 4.86 0.05 0.50 0.41 3.38 554
70.63 0.53 15.10 3.03 0.05 0.71 0.47 3.63 5.45 0.13
210 81
99.95
0.08
486.
NPT
485.
NPT
1.50 1.43 0.79 1.19 19.32
1.64 2.61 0.90 1.07 21.75
100 301 443 87 145 161 1391 1091 37.8 19.3 17.7 73.8 16 18 15 25 2.5 8.3 2A 7.0 8 8 23 9 142 148 15 198 15 5 119 26 20 27 2 97 26 16 45 78
171 37
99.52 99.43
1.32 1.06 1.22 1.32 4.66 3.93 0.95 0.81 0.84 1.05 1.00 1.01 8.80 18.80 27.16
135 102
Sr
Rb
LO1 Total
pzos
Na20 KzO
MgO CaO
A1z.0, FezO3 MnO
99.11
77.64 78.78 0.11 0.13 11.95 11.29 0.89 o .7a 0.03 0.02 0.19 0.13 0.44 0.29 4.01 3.60 4.26 4.40 0.01 0 .oo
69.08 0.23 11.92 9.08 0.10 0.40 0.84 3.19 4.22 0.05
SiO,
Ti02
NPT 484.
Sample:
NPT
483.
NPT
482.
Terrane:
1.10 0.54 0.91 1.09 18.37
NPT 498.
NPT 500.
505
NPT
212 152 136 226 50.1 22 2.5 16 98 95 445 84
138 34
2.76 1.09 1.00 1.39 4.66 4.06 0.80 0.88 0.99 1.03 1.04 1.03 3.04 49.45 24.03
26.3 13 1.7 1 169 36 22 5
ao
117
123 26
1.08 0.50 0.73 1.05 5.28
333 974 43.7 20 9.1 43 161 33 6 92
153 305
70.39 79.20 64.19 68.39 0.47 0.09 0.15 0.64 13.46 11.36 11.79 15.38 2.25 0.56 15.91 3 6 9 0.08 0.00 0.14 0.13 0.50 0.07 0.15 1.25 2.35 0.16 0.33 1.63 1.90 3.78 3.96 4.15 5.24 4.10 3.97 4.48 0.04 0.00 0.02 0.22 3.18 99.86 99.33 100.61 99.96
497.
NPT
443 285 1443 1300 61.9 33.4 21 17 5.4 9.6 24 7 120 99 6 4 5 1 26 64
156 291
99.23
70.78 0.4 1 14.72 3.1 1 0.05 0.2 9 0.5 1 4.43 4.89 0.04
487
NPT
NPT 508.
NPT 898.
SPT 489.
SPT 491.
SPT 494' 496
SPT SPT 727.
SPT 603'
588
NXT
SXT 743'
187 78
158 23
2 46 89
120 195
1 63 58
1 20 92
60 311
132 64
0.62 1.40 0.19 2.05 0.80 0.96 1.00 0.97 3.06 10.21
2 71 123 2 55 3 50 5 73 299 2 07 160 703 660 3 72 1442 948 9 83 8 30 99 50.9 51.2 92.1 32.3 48.1 36.7 38.1 58.9 14 22 19 20 11 31 13 23 4.9 6.9 4.4 3.7 6.2 8.4 2.2 8.0 36 7 4 5 16 56 23 7 i a 100 111 209 85 89 187 2 22 11 15 6 8 36 20 5 0 2 18 6 110 3 8 9 9 11 49 13 38 75 24 62 32
128 61
1.35 1.09 1.74 2.52 0.67 1.66 1.15 2.40 2.10 6.80 2.75 0.61 2.81 1.30 0.87 0.91 0.91 0.88 0.79 0.83 0.77 1.01 1.05 1.01 1.58 1.06 1.07 1.06 6.67 31.13 47.03 21.48 18.26 44.78 14.91
110 417 76 8 47 24.7 82.7 24 25 1.9 14.0 11 13 48 133 bd 37 bd 248 61 32
140 28
1.13 1.42 1.47 5.05 0.97 0.99 0.93 1.01 7.10 23.97
250 710 38.0 21 4.1 25 87 145 335 59
158 108
100.69 100.25 99.87 100.80 100.15 99.90 99.69 99.26 99.42 99.59 99.88
63.89 73.84 73.34 76.14 75.99 68.15 65.13 78.53 74.92 73.13 72.72 0.33 0.17 0.51 0.24 0.15 0.57 0.76 0.13 0.33 0.44 0.29 12.73 9.91 12.67 12.89 12.04 16.52 16.83 11.49 12.80 13.74 13.93 13.40 8.84 3.62 1.64 2.51 5.55 4.54 0.72 1.94 2.53 2.2 1 0.14 0.07 0.08 0.01 0.03 0.08 0.05 0.01 0.04 0.06 0.0 1 0 .42 0.09 0.51 0.14 0.22 0.69 1.08 0.13 0.54 0.57 0.09 1.20 0.29 1.18 0.28 0.19 0.37 0.5 7 0.18 0.55 2.2 1 0.94 4 .OO 2.90 3.34 4.17 3.28 2.24 6.2 3 3.03 3.82 4.1 7 4.00 4.53 4.12 4.50 4.54 5.72 5.64 4.20 5.03 4.4 1 2.60 5.6 1 0.07 0.02 0.11 0.03 0.00 0.10 0.30 0.01 0.07 0.1 4 0.07
507.
NPT
lable 2. Whole-rock -jar and trace element analyses of mlcanic samples.
NPT
4.71
0.01
99.72
4.77
0.03
5.16
0.01
100.30 100.27
K#J
p20,
0.80
8
1.82
6.78
0.75
Zn
K,O/Na,O
Rb/ Sr
0.98
13.59
1.05
33.47
1.70
16
8
13.06
1.00
0.77
10.32
1.26
12
5
40
86
4
3.8
44.7 19
87
26 107
abbreviations a s in lables 1 and 2 * denotes A-type, Test are I-type
A/CNK KN/C
2.73
3
cu
18
3
Ni
8 95
4.4
Cr
2.5
24.2 21
423
22 0
70
3 91
V
sc
20
124
Ba
Nb
109
zr
19.9
27
Sr
Y
186
Rb
Total
267
3.74
2.80
2.85
Na20
190
0.65
0.56
0.24
LO1
0.23
0.22
0.15
0.04
MgO CaO
0.87
1.01
0.02
Fez03 MnO
0.02
12.49
10.58
ll.m
A 124
0.51
0.12
0.25
76.84
80.03
0.10
620.
NPT
80.05
513.
SiO,
509'
Sample
TiO,
NPT
Tarram:
6.64
1.04
0.74
1.42
1.26
42
2
21
57
17
9.6
16
22.0
681
167 141
237
99.98
0.13
5.00
3.97
1.35
0.64
5.79 0.00
4.23
0.10
1.14
0.95
27.38
2.52
23
8
3
3 153
1.7
35
121.1
48
13 208
356
7.37 134.83
1.03
0.69
0.90
1.16
36
3
5
17 69
3.2
22.8 12
553
190 234
170
98.90
0.56
2.30
3.66
99.99
0.06
0.00 0.03
0.61
1.67 0.04
0.07
2.00
1.07
14.53
11.59
12.96
14.96
0.68
0.71
0.14
0.28
6
28
13 59
5.1
26.4 6
324
178
142
151
99.76
0.06
3.77
3.72
1.45
0.49
0.03
1.98
13.31
0.23
74.72
502
NXT
1.07 0.78 1.04 5.18
0.82 0.95 4.87
1.01
0.65
1.25
9 9 4 8
1
70
25 162
9.8
74.8 21
1566
559
221
144
99.81
0.20
5.00
4.01
1.85
0.82
0.09
4.21
68.38
77.82
75.30
0.32
71.54
605
SPT
514.
905
900.
SPT
NPT
NPT
5.09
3.36 4.92
30.3
506
93 202
200
99.22
0.98 10.58
0.91
1.31
1.76
53
5
5
63
6
1.2
8.02
0.99
0.87
2.15
1.47
49
3
13
10 87
4.1
0.01
5.21
3.69
0.73
0.14
0.93 0.99 12.19
1.07 12.38
3.43
0.72
0.65
1.41
25
29 1.54
5
9
6 78
2.2
86.5 25
510
49 241
168
22
13
11 78
2.2
15.6 8
1072
153 101
99
100.28 99.99
0.87
0.03
3.31
1.03
0.05
0.38 0.68
0.30
1.96 0.04
1.09
12.96
13.00
12.65 0.02
0.14
0.19
0.25
0.03
75.12
75.62
74.29
2.35
580.
NXT
522
NXT
520.
NXT
2 8 2 5
25.5
1211
163 279
214
99.37
0.05
6.28
3.56
0.93
0.18
0.03
2.10
14.04
0.23
71.98
519'
NXT
0.98
0.85
0.61
0.11
0.54
74
34
58
128 189
17.9
30.6 13
514
601 224
67
99.62
0.22
1.95
3.62
5.67
3.85
0.11
6.81
15.69
0.73
m.96
597
NXT
4.36
1.12
0.75
0.51
0.65
72
4
20
84
46
12.0
52.9 11
699
202 260
104
99.96
0.12
2.91
4.50
1.70
1.16
0.05
3.82
15.32
0.52
69.86
599
NXT
0.02
6.47
2.62
0.45
0.15
0.01
2
1.05 17.33
0.65 1.01
1.53
2.67
14
10
2
6 45
1.0
39.2 17
514
217
92
141
99.54
0.03
6.32
3.00
1.00
0.79
0.34
0.69
71
2
2
4 8
5.4
34.4 11
825
253 173
86
99.07
0.13
3.12
4.52
2.55
2.36
0.87
0.5 0
0.08
3.71
15.52
0.38
68.19
86
NXT TTU-
0.2 0
0.00
0.4 1
12.21
0.10
77.39
908'
NXT
0.84
2.77
2.47
13
1.58 20.18
0.97
0.76
0.15
0.37
81
6
3
18 3
4 60
1.6
28.8 7
521
69 166
191
98 66
12.2
11
32.9
486
403 170
59
99.77 100.24
0.23
1.68
4.58
3.97
1.79
0.08
0.90
12.m
16.06 6.32
0.14
77.29
615.
NXT
0.87
64.19
609
NXT
lhble 3. Whole-rock major and trace element analyses of intrusive samples.
2.44
1.01
0.75
0.25
0.71
74
8
86
133
56
8.7
26.8 8
726
302 183
77
99.65
0.10
2.77
3.90
2.73
1.24
0.07
4.06
14.51
0.45
69.82
504
SXT
SXT
2.1 2
5.16
2.53
1.00
1.04
22.22
1.66
0.98
0.68 0.81
1.94
16
4
8
67
9
1.6
10.8 4
416
461
0.04
5.27
2.99
0.97
0.61
0.02
1.91
13.96
0.23
74.03
907
SXT
125
5.48
1.00
0.86
0.38
0.80
31
bd
43
79
8.52
1.13
0.74
0.60
1.76
33
3
bd
20 73
2.4
2 .o 1
8
8.0
1415
207 173
15.6 5
568
45
120 65
86 50
1.72
10.98
0.01
2.90
3.62
1.19
0.17
0.02
1.13
11.30
0.09
79.61
902
SXT
99.58 100.04 100.02
0.16
0.03
3.37
1.74
0.23
0.05
0.00
2.56
11.53
0.15
79.75
0.07
0.43
48
39
4
37
39
3.8
6.2 3
714
699 103
47
99.73
SXT 594'
3.68
1.64
20
3
bd
70
5
1.8
28
19.7
325
61 29
224
99.49
0.1 4
4.9 9
3.14 0.00
2.8 1
0.96
0.22 0.76
0.03
2.30
15.65
0.3 6
70.37
586
SXT
0.02
1.05
12.57
0.09
76.48
525.
SXT
6.83
1.10
0.76
0.94
1.38
43
107
10
125
42
7.8
13
51.2
I145
88 372
82
low7
0.0 2
4.39
3.1 9
1.1 1
1.24
0.05
4.3 1
13.26
0.46
72.24
1045
M. A. BARNES ET AL.
state of the magma. Additionally, it may be indicative of whether a sample is A- or I-type, because Atype magmas commonly are reducing and I-type magmas are oxidizing. As a result, biotite in A-type magmas is Fe-rich (Anderson, 1983), whereas biotite in I-type magmas coexists with Fe-Ti oxides
o
l
D V Panhandle terrane Crystalline terrane
0
v
v v v I.
I
5B).
.
0.9
0 w
f 0.8 d a,
rn
% 0.7 +.+
0 a,
L
t 9.. .
0-6t
t 0.5
40
50
.
17
60 70 Si02 wt%
80
Figure 3. A, Rb/Sr ratio versus weight percent SiO, for samples from three terranes. Unfilled symbols denote the A-type samples, and filled symbols represent I-type or mafic samples. B, FeO, / FeO, + MgO versus SO,. Symbols as in panel A. 254
and is more magnesian. This is the case for A- and I-type samples from the crystalline terrane (lhble 4; Fig. 4). The biotites from A-type granites studied here have FeO,/FeO, + MgO ratios between 0.8 and 0.9, whereas those for biotites from the I-type rocks are 0.4-0.6. Figure 5A shows that felsic rocks of the Panhandle terrane are less calcic than felsic rocks of the crystalline terrane, in keeping with the A-type character of the former. Examination of the data (lhbles 2 and 3) reveals that intrusive samples tend to be more calcic than the volcanic rocks. Samples from the Panhandle terrane also have higher KN/C ratios than those from the crystalline terrane (Fig. The tectonic discrimination diagrams for felsic rocks from Pearce et al. (1984) also assist in differentiating between A- and I-type rocks. As with any discrimination diagram, separation of the source signature from the actual tectonic environment is difficult, so extrapolation of these diagrams into Precambrian time must be treated with caution. In Figure 6, the Panhandle rocks have higher Rb, Y, and Nb than the crystalline terrane rocks, suggesting a more evolved character. The A-type rocks plot predominantly in the “within-plate”field, whereas the I-type rocks plot predominantly in the “ volcanic arc” field. In summary, no one geochemical parameter uniquely identifies a particular sample as either Atype or I-type, or as belonging to the Panhandle or crystalline terranes. However, samples from the Panhandle terrane are geochemically relatively homogeneous and are predominantly A-type granitoids. In contrast, the crystalline terrane spans a wider compositional range and includes both Aand I-type rocks. Mafic samples occur over a wide geographic area, occurring in all three terranes (Fig. 2). Olivine gabbro from the Debaca terrane is tholeiitic, with c50 percent SiO,, c 5 percent total alkalies, c 1 percent %O, and values of FeO,/(FeO, + MgO) from 0.6 to 0.7 (Thble 1). More specifically, sample 477 can be classified as an olivine ferrogabbro because of its high TiO, and total iron. Diabase (no. 472) from the Panhandle terrane also is tholeiitic, although it has higher SiO,, CaO, Y, Sc, V, Cr, and Cu, and lower TiO,, FeO,, MgO, %O, Sr, Zr, and Zn than samples from the Debaca terrane. Diabase (no. 478) from the crystalline terrane is similar to diabase from the Panhandle terrane (no. 472) and, as suggested previously, may have petrogenetic affinity with gabbroic rocks of the Panhandle terrane. As noted in the sample descriptions, three of the mafic samples in the crystalline terrane are thought
Rockg Mountain Geolom, v. 34, no. 2, p . 245-262, 7figs., 5 tables, 1 appendix, November, 1999
CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
to be part of the Pecos mafic in- mble 4. Biotite analyses for samples from the crystalline terrane. trusive complex (Kargi a n d Barnes, 1995). For further discus- Sample SiO, TiOz AIZO3 FeO MnO CaO NazO KzO CI less0 Total sion of mafic magmatism at the time of the Pecos mafic intrusive 504 ave 35.38 2.37 15.68 20.48 0.44 0.05 0.07 9.33 0.01 0.00 92.94 complex emplacement, see C. G. 519 ave 34.09 3.94 12.53 31.92 0.46 0.07 0.06 8.59 0.30 -0.03 94.15 520ave 35.05 3.59 13.60 30.01 0.37 0.03 0.06 9.04 0.33 -0.03 95.07 Barnes et al. (this issue). Isotopic Data
599ave
35.04 2.94
16.61 19.51 0.40
0.08 0.10
7.85 0.09
0.00
91.10
609 ave
36.54 1.97
14.99 16.51 0.54
0.05 0.07
9.12 0.01
0.00
92.27
As noted previously, a few Nd Numbers of ions on the basis of 22 oxygens T i Fe*' Mg Ca Na K C1 Total model ages and U-Pb crystalliza- Sample Si Al" Al" tion ages are published for rhy0.51 0.28 2.71 2.15 0.01 0.02 1.88 0.00 olitic samples from t h e 504 ave 5.59 2.41 15.62 519 ave 5.65 2.35 0.10 0.49 4.43 0.59 0.01 0.02 1.82 0.08 Panhandle and crystalline ter15.57 0.28 0.44 4.07 0.77 0.00 0.02 1.87 0.09 ranes. There are four crystalliza- 520 ave 5.68 2.32 15.55 0.68 0.35 2.59 2.03 0.01 0.03 1.59 0.02 tion ages ranging from 1.37 to 1.4 599 ave 5.57 2.43 15.34 609 ave 5.69 2.31 0.44 0.23 2.15 2.89 0.01 0.02 1.81 0.00 15.62 Ga (Thomas et al., 1984) and one Nd model age of 1.57 Ga (Van Schmus et al., 1996) for the Pan- ave = mean value of multiple analyses handle terrane (Fig. 7). In the crystalline terrane, or more specifically,the Carrizo Mountains of Van Horn, there are two crystallization ages of 1.33 Ga (Roths, 1993) and 1.37 Ga (Soegaard et al., 1996), and two Nd model ages of A-I 1.41 and 1.42 Ga (Patchett and Ruiz, 1989). lhble 5 gives Sm and 0.8 Nd data for fourteen samples from this study, as well as sample 26, a rhyolite from the northern Pan- n handle terrane, from Van Schmus et al. (1996). The E ~ ~in( n~ b) l e 5 are calculated using a U-Pb crystallization age of 1.37 Ga. The two mafic samples (nos. 472 and 473) are distinct from each other in terms of their Nd Samples model ages (Thble 5). They are 1.44 Ga for the northern Panhandle terrane sample and 1.26 519 609 504 Ga for the Debaca terrane sample. The model age of 1.44 Ga for the diabase from the northern Pan520 599 handle terrane is similar to the crystallization ages (1.37-1.4 Ga; Thomas et al., 1984) of associated rhyolitic rocks (Fig. 7). Because 2.0 2.4 2.8 3.2 sample 472 is mafic, it is permisAT sible to assume that the model age is similar to the crystallization figure 4. Composition of biotites from crystallineterrane samples in terms age. Given this assumption, the of FeO/ FeO + MgO versus A1 iv (22 oxygens). Each symbol represents an presence of coeval mafic a n d individual analysis. Sample numbers as labelled in the figure.
I
i
Rockg Mountain GeologEl, v. 34, no. 2, p . 245-262, 7figs., 5 tables, 1 appendix, November, 1999
255
M. A. BARNES ET AL.
felsic igneous rocks in the Panhandle terrane indicates a period of bimodal magmatism at about 1.4 Ga. The Nd model age of 1.26 Ga for the gabbroic sample from the Debaca terrane is similar to the U-
100 $
.A.
+
V
v V
V
Pb crystallization age of 1.26 Ga (Pittenger et al., 1994) established for a zircon in an ash bed from the Castner Marble. As discussed above, the Debaca terrane has been correlated by Denison et al. (1984) and Muehlberger et al. (1967) with the Mundy Breccia and Castner Marble exposed in the Franklin Mountains. Again, if the model age is close to the crystallization age, a reasonable assumption for a mafic sample, it supports the correlation of the Debaca terrane with the Mundy Breccia and Castner Marble. Model ages for the intermediate to felsic samples show clear differences between the subsurface terranes (Fig. 7). W o samples from the southern Panhandle terrane display the oldest model ages (1.74 Ga) and, in general, model ages of granitoid samples from the Panhandle terrane are older (1.5-1.74 Ga) than those from crystalline terrane samples (1.351.47 Ga). All model ages for Panhandle terrane samples are older than the U-Pb zircon ages of 1.371.4 Ga (Thomas et al., 1984), which suggests that only the mafic rocks of the terrane are new additions to the continental crust. An A-type granite from the crystalline terrane yielded the youngest model age at 1.35 Ga. Model ages of the three I-type samples in the crystalline terrane cluster at 1.40 to 1.47 Ga.
DISCUSSION AND CONCLUSIONS
-- + --
Debaca terrane Crystalline terrane
rn
V
lo/
f=, 0.1
I
40
I
m
,
I
50
1 I
1 I
60
I I
I
1
I
7b
I
8b
I I
Si02 wt% Fignre 5. A, weight percents CaO versus SiO,. B, KN/C
versus weight percent SO,. Symbols as in Figure 3A.
256
Panhandle 'Ilerrane IItvo principal observations concerning the Panhandle terrane arise from this study. The first is that all but a few of the felsic samples from the Panhandle terrane, both north and south of the Debaca terrane, have compositions similar to A-type granitic magmas. As such, the Panhandle terrane is a distinct subsurface unit that consists of homogeneous, undeformed A-type rhyolitic and granitic rocks. It is compositionally and lithologically similar to subsurface and outcrop samples from the midcontinent granite-rhyoliteprovince (Anderson, 1983; Bickford et al., 198lb; Cullers et al., 1992; Lidiak, 1996; Sides, 1980; Thomas et al., 1984). The second observation is that the range of Nd model ages for granites and rhyolites of the Panhandle terrane (1.50-1.74 Ga) is generally similar to that of similar rocks from the southern granite-rhyolite province of northern Oklahoma and southern Kansas (Van Schmus et al., 1996). The difference between model ages (1.5-1.74 Ga) and crystallization ages (1.37-1.4 Ga) in felsic rocks of the Panhandle terrane indicates that these A-type granites and rhyolites contain a significant
Rockg Mountain Geology, u. 34, no. 2, p . 245-262, 7&s.,
5 tables, 1 appendix, November, 1999
CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
crustal component. The fact that model ages from two samples of the southern terrane are older than those from the northern terrane suggestseither that a larger proportion of crust was assimilated in southern terrane magmas or that older crust was melted. Bimodal magmatism in the Panhandle terrane is indicated by similar model age of diabase (1.44 Ga) and crystallization ages of rhyolite (1.37-1.4 Ga) from the northern Panhandle terrane. A series of wells from the northern Panhandle terrane in which sequences of inter-layered gabbro, granite, and rhyolite were penetrated provides additional support for bimodal magmatism. Characterization of these samples is in progress. Bimodal A-type magmatism is commonly associated with extensional tectonics. The data, therefore, are consistent with a petrogenetic model in which upwelling mantle-derived basaltic melt triggers crustal anatexis. Finally, the model ages imply that crust at least as old as 1.7 Ga underlies areas as far south as the southern Panhandle terrane. This conclusion is supported by a T, of 1.71 Ga for the Proterozoic Lanoria Quartzite in the Franklin Mountains (Patchett and Ruiz, 1989). It is also consistent with the proposal by Bickford et al. (1986) that arc-type terranes with ages of 1.63-1.78 Ga extend into northern Tkxas. In addition, it is supported by the findings of Thomas et al. (1984) that the San Isabel batholith (U-Pb zircon age of 1.36 Ga) of the southern granite-rhyolite province intrudes gneissic rocks older than 1.7 Ga. Thus, we conclude that the southern edge of Laurentia must have been at least as far south as the southern granite-rhyolite province during the eruption and emplacement of these rocks at ~ 1 . Ga. 4 Debaca 'Ilerrane
single U-Pb age of 1.16 Ga. See also C. G. Barnes et al. (this issue) for additional discussion of mafic magmatism during this time.
1000 I:
Y+Nbppm
/
A
100 E
Q Q
2 10
10
E
P Q
The T,, of 1.26 Ga for the mafic sample from the Debaca terrane is the youngest model age of the sample set. As noted above, this model age is similar to a U-Pb zircon age from an ash bed in the Castner Marble of the Franklin Mountains (Pittenger et al., 1994), thereby strengthening the correlation of rocks of the Debaca terrane, Franklin Mountains, and Van Horn area (Muehlberger et al.,
L
I
1967).
The difference in the two model ages reported 1 10 100 1( 10 for mafic rocks in this study implies that gabbros of the Debaca terrane are distinct from gabbros of the Panhandle terrane. The question remains, how- Fignre 6. A, Rb versus Nb + Y. B, Nb versus Y. Syn-COLG, ever, whether magmatism in the Debaca terrane was syn-collisionalgranites;VAG, volcanic-arcgranites;WPG, related to the magmatic episode associated with the within-plategranites;and ORG, ocean-ridgegranite. SymPecos mafic intrusive complex, for which there is a bols as in Figure 3A.
Rockg Mountain Geology, u. 34, no. 2, p . 245-262,7figs., 5 tables, 1 appendix, November, 1999
257
M. A. BARNES ET AL.
Thble 5. Sm and Nd isotope analyses. Sample
Sm ppm
Nd ppm
14'Sd144Nd
""Nd/"'Nd
ENd(0)
ENd(t) TDM ( G a p
Rock
"y-pe
diabase rhyolite granite rhyolite granite rhyolite rhyolite
mafic A-type A-type A-type A-type A-type I-type
A-type I-type altered
Northern Panhandle terrane 472
3.33
12.01
.512605 f 13
4.4
1.44
1.4
1.59 1.69
484
1.06
7.0 1
0.0911
.511761 f 15
-0.7 -17.1
509
1.31
8.45
0.0936
.511705 f 13
-18.2
4.1
497
9.4 1
49.21
0.1157
.511984f
9
-12.8
1.5
1.64
513
3.49
20.70
0.1020
.511909 f 10
-14.2
2.4
1.54
26'
8.03
44.45
0.1092
.511969 f 13
-13 .O
2.3
1.57
487
8.16
44.61
0.1106
.512023f 13
-12 .o
3.1
1.50
0.1678
Southern Panhandle tmane 494
9.58
46.83
0.1238
.512007 f 13
-12.3
0.5
1.74
496
3.9 6
18.69
0.1282
.512061f 13
-11.3
0.8
1.74
490
6.53
34.75
0.1137
.512005f
8
-12.3
2.2
1.57
rhyolite dacite rhyolite
19.28
72.82
0.1601
.512597f 11
-0.8
4.4
1.26
gabbro
mafic
A-type I-type I-type
I-type
Debaca terrane 473
Northern crystalline tmane 519 2.79 597 6.43 599 5.76
12.44
0.1355
.512347f 12
-5.7
5.1
1.35
34.13
0.1138
.512075f 13
-11.o
3.6
1.47
27.96
0.1245
.512191 k 15
-8.7
4.0
1.45
granite diorite granite
12.11
0.0979
.511971f 14
-13.0
4.4
1.40
intrusive
Southern crystalline terrane 586
1.96
*fromVan Schmus et al. (1996) calculated after Nelson and DePaolo (1985) ; procedure found in Van Schmus et al. (1996)
#
Crystalline Wmme The geology of the crystalline terrane is more complex than the Panhandle terrane in that it contains both A- and I-type granitoids, resulting in a more heterogeneousbasement. Further interpretation is severely hampered because no crystallization ages for these rocks are available. However, it is clear that model ages for the I-type granites (1.4 to 1.47 Ga) and for an A-type granite (1.35 Ga) are distinctly younger than the model ages for the Panhandle terrane. This, along with the elemental data, show that the crystalline terrane is not part of the southern granite-rhyoliteprovince,but instead constitutes a separate basement unit. The Llano front serves to divide deformed rocks to the south from undeformed rocks to the north, and it is thought to be associated with continentcontinent collision during the Grenville orogeny (Mosher, 1998). Model age values from the crystalline terrane are similar to those of 1.37-Gametarhyolite from the Van Horn area (TDM of 1.40-1.42 Ga; 258
Patchett and Ruiz, 1989) and those of 1.15 Ga granitic rock from the Llano uplift (TDM of 1.39 Ga; Nelson and DePaolo, 1985). Thus, no isotopic distinction is apparent among samples on either side of the Llano front and, within the crystalline terrane, no geochemical distinction is observed on either side of the front. This lack of geochemical or isotopic distinction suggests that the Llano front does not represent a fundamental terrane boundary with respect to crustal age, but instead is simply the edge of Grenville deformation.
ACKNOWLEDGMENTS Major and trace element ICP data were obtained at Texas lkch University as part of the Geosciences Department graduate student support. We thank graduate students Mi-ae Jeon and Songyun Lee for their careful work and assistance in this laboratory. Data on biotite mineral chemistry were collected on a Cameca SX50 microprobe at University of Tkxas
Rockg Mountain Geology, v. 34, no. 2, p . 245-262, 7figs., 5 tables, 1 appendix, Novembet 1999
CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
Figure 7. Map of west Texas and eastern New Mexico showing model ages and crystallization ages. Symbols as in Figure 2. Underlined and italicized numbers are U-Pb ages; others indicate Nd model ages.
at El Paso by lbm Williams and Libby Anthony, and guide this manuscript, and Kathie Marsaglia for her we thank lbm for his time and expertise in obtain- insights into Precambrian geology of the area. Fiing these results. In particular, we thank Calvin nally, we thank Dr. Ed Lidiak and an anonymous Barnes and Sharon Mosher for their many hours of reviewer for their thoughtful and careful evaluations discussion and probing questions, which helped to of the manuscript.
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259
M. A. BARNES ET AL.
REFERENCES CITED Adams, D. C., and Keller, G. R, 1996, Precambrian basement geology of the Permian Basin region of west Wxas and eastern New Mexico: A geophysical perspective: American Association of Petroleum Geologists Bulletin, v. 80, p. 410-431. Anderson, J. L., 1983, Proterozoic anorogenic granite plutonism of North America: in Medaris, L. G., Jr., Byers, C. W., Milkelson, D. M., Shanks, W. C., eds., Proterozoic Geology: Selected Papers from an International Proterozoic Symposium, Geological Society of America Memoir 161, p. 133154. Bickford, M. E., and Lewis, R D., 1979, U-Pb geochronology of exposed basement rocks in Oklahoma: Geological Society of America Bulletin, v. 90, p. 540-544. Bickford, M. E., Harrower, K. L., Hoppe, W. J., Nelson, B. K., Nusbaum, R L., and Thomas, J. J., 1981a, Rb-Sr and U-Pb geochronology and distribution of rock types in the Precambrian basement of Missouri and Kansas: Geological Society of America Bulletin, v. 92, p. 323-341. Bickford, M. E., Sides, J. R, and Cullers, R. L., 1981b, Chemical evolution of magmas in the Proterozoic terrane of the St. Francois Mountains, southeastern Missouri. 1. Field, petrographic, and major element data: Journal of Geophysical Research, v. 86, p. 10,365-10,386. Bickford, M. E., Van Schmus, W. R, and Zietz, I., 1986, Proterozoic history of the midcontinent region of North America: Geology, v. 14, p. 492-496. Cullers, R L., Griffin, T., Bickford, M. E., and Anderson, J. L., 1992, Origin and chemical evolution of the 1360 Ma San Isabel batholith, Wet Mountains, Colorado: A mid-crustal granite of anorogenic affinities: Geological Society of America Bulletin, v. 104, p. 316-328. Denison, R E., and Hetherington, E. A., Jr., 1969, Basement rocks i n far west Texas and south-central New Mexico, in Kottlowski, E E., and Lemone, D. V., eds., Border stratigraphy symposium: Socorro,New Mexico Bureau of Mines and Mineral Resources Circular, New Mexico Institute of Mining and lkchnology, p. 1-16. Denison, R E., Lidiak, E. G., Bickford, M. E., and Kisvarsanyi, E. B., 1984, Geology and geochronology of Precambrian rocks in the central interior region of the United States: US. Geological Survey Professional Paper 1241C, p. 1-20. Eby, G. N., 1990, The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis: Lithos, v. 26, p. 115-134. Flawn, P. T., 1956, Basement rocks of Texas and southeastern New Mexico: University of Texas Bureau of Economic Geology Publication 5605, 261 p. Goldich, S. S., Lidiak, E. G., Hedge, C. E., and Walthall, F. G., 1966, Geochronology of the midcontinent region, United States. Part 2. Northern area: Journal of Geophysical Research, v. 71, p. 5389-5408. Ham, W. E., Dension, R E., and Merritt, C. A., 1964, Basement rocks and structural evolution of southern Oklahoma: Oklahoma Geological Survey Bulletin 95, 302 p.
Kargi,H., and Barnes, C. G., 1995, A Grenville-agelayered intrusion in the subsurface of west Wxas: Petrology, petrography, and possible tectonic setting: Canadian Journal of Earth Sciences, v. 32, p. 2159-2166.
260
Karlstrom, K. E., Dallmeyer, R D., and Grambling, J. A., 1997, 40Ar/39Ar evidence for 1.4 Ga regional metamorphism in New Mexico: Implications for thermal evolution of lithosphere in the southwestern U S A : Journal of Geology, v. 105, p. 205-223. Keller, G. R, Hills, J. M., Baker, M. R , and Wallin, E. T., 1989, Geophysical and geochronological constraints on the extent and age of mafic intrusions in the basement of west Wxas and eastern New Mexico: Geology, v. 11, p. 1049-1052. Lidiak, E. G., 1996, Geochemistry of subsurface Proterozoic rocks in the eastern midcontinent of the United States: Further evidence for a within-plate tectonic setting, in Van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and basins of eastern North America: Geological Society of America Special Paper 308, p. 45-66. Lidiak, E. G., Marvin, R F., Thomas, H. H., and Bass, M. N., 1966, Geochronology of the midcontinent region of the United States. Part 4. Eastern area: Journal of GeophysicalResearch, V. 71, p. 5427-5438. Mosher, S., 1993, Western extensions of Grenville age rocks, Texas, in Reed, J. C., Jr. and six others, eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. C-2, p. 365-378.
-1998, Tectonic evolution of the southern Laurentian Grenville orogenic belt: Geological Society of America Bulletin, v. 110, p. 1357-1375. Muehlberger, W. R, Hedge, C. E., Denison, R E., and Marvin, R E, 1966, Geochronologyofthe midcontinent regions, United States: Part 3. Southern area: Journal of Geophysical Research, v. 71, p. 5409-5426. Muehlberger, W. R , Denison, R E., and Lidiak, E. G., 1967, Basement rocks in continental interior of United States: American Association of Petroleum Geologists Bulletin, v. 51, p. 2351-2380. Nelson, B. K., and DePaolo, D. J., 1985, Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continenk Geological Society of America Bulletin, v. 96, p. 746-754. Patchett, P. J., and Ruiz, J., 1989, Nd isotopes and the origin of Grenville-age rocks in Texas: Implications for Proterozoic evolution of the United States mid-continent region: Journal of Geology, v. 97, p. 685-695. Pearce, J. A., Hams, N. B. W., and Tindle, A. G., 1984, mace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Pittenger, M. A., Marsaglia, K. M., and Bickford, M. E., 1994, Depositional history of the Middle Proterozoic Castner Marble and basal Mundy Breccia, Franklin Mountains, west Texas: Journal of Sedimentary Research, v. 64, p. 282-297. Roths, P. J., 1993, Geochemical and geochronological studies of the Grenville-age (1250-1000 Ma) Allamoore and Hazel Formations, Hudspeth and Culberson counties, west Texas, in Soegaard, K., Nielsen, K. C., Marsaglia, K. M., and Barnes, C. G., eds., Precambrian geology of Franklin Mountains and Van Horn area, mans-Pecos, Texas : Dallas, Geological Society of America South-Central Section Meeting Guidebook, p. 1-35.
R o c h Mountain Geology, v. 34, no. 2, p . 245-262, 7f?gs., 5 tables, 1 appendix, Novembel; 1999
CHEMISTRY OF SUBSURFACE PRECAMBRIAN IGNEOUS ROCKS
Sides, J. R , 1980, Emplacement ofthe Butler Hill Granite, a shallow pluton within the St. Francois Mountains batholith, southeastern Missouri: Geological Society of America Bulletin, v. 91, p. 535-540.
Soegaard, K., Nielson, K. C., a n d Bickford, M. E., 1996, Lithostratigraphy and chronometry of Middle Proterozoic Carrizo Mountain Group, west lkxas: Geological Society of America Abstracts with Programs, v. 28, no. 1, p. 64.
Van Schmus, W. R., Bickford, M. E., and lbrek, A., 1996, Proterozoicgeology of the eastcentral midcontinent basement, in Van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and basins of eastern North America: Geological Society of America Special Paper 308, p. 7-32. Wasserburg, G. J., Wetherill, G. W., Silver, L. T., and Flawn, I? T., 1962, A study of the ages of the Precambrian of ‘Exas: Journal of Geophysical Research, v. 67, p. 4021-4047. Whalen, J. B., Currie, K. L., and Chappell, B. W., 1987, A-type granites Geochemical characteristics, discrimination and petrogenesis: Contributions to Mineralogy and Petrology, V. 95, p. 407-419.
Thomas, J. J., Shuster, R. D., and Bickford, M. E., 1984, A terrane of 1,350-to 1,400-m.y.-old silicic volcanic and plutonic rocks in the buried Proterozoic of the mid-continent and in the Wet Mountains, Colorado: Geological Society of America Bulletin, v. 95, p. 1150-1157. Van Schmus, W. R., and Bickford, M. E., 1981, Proterozoic chronology and evolution of the midcontinent region, North America, in Kroner, A., ed., Precambrian plate tectonics: New York, Elsevier Scientific Publishing Company, p. 261296.
MANUSCRIPT SUBMITTED AUGUST 23, 1998 REVISED MANUSCRIPT SUBMITTED JANUARY 26, 1999 MANUSCRIPT ACCEITED FEBRUARY 26, 1999
Soegaard, K., and Callahan, D. M., 1994, Late Middle Proterozoic Hazel Formation near Van Horn, ’Ikans-Pecos Texas: Evidence for transpressive deformation in Grenville basement: Geological Society of America Bulletin, v. 106, p. 413-423.
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261
M. A. BARNES ET AL.
Appendix 1. Sample locations and identifications. Denison 21 26
29 30 472 473 474 477 478 482 483 484 485 486 487 489 490 491 493 494 495 496 497 498 499 500 502 504 505 506 507 508 509 513 514 519 520 522 525 580 586 588 594 595 596 597 599 603 605 609 615 620
718 727 743 898 900 902 905 907 908 1045 1161 lTU86
#
County
Company
Well name
Well location
Rock name
Lubbock Moore Hartley Potter Hartley Parma Castro Hale Donaley Hartley Randall Oldham Oldham Oldham Gray Hockley Lamb Cochran Hale Yaokum Sherman Lamb Hartley Armstong Lamb curry Dickens Fisher Dallam Potter Potter Potter Dallam Moore Rmsevelt King King Floyd Prsidio Andrews Mitchell Motley Pecos Pecos Pecos
Humble M obi1 Standard Sinclair Sinclair Gulf Sun Oil Amerada El P a 0 Nat. Gas Sincla ir Frankfort She U She 11 Humble Phillips Humble Honolulu Stanolind Stanolind Continental Phillips Humble Standard cf Texas Standard cf Texas Stanolind U n n n Production Humble Humble Humble Sinclair b Prairie Sincla ir Sinclair Humble Sinclair She U Humble Humble Standard of Texas Welch Phillips Sun General Crude Humble Gulf Pure Sun and Ohio Gulf Riclfield Magnolia Shell Co ntinenta 1 Skelly ApachsOlson Honolulu
# I Ferris nl NunleybMcCombs #l Johnson #13 BivinsEst. #I4 Bivins #l-A Keliehor #l Herring # 1 Kurfees # I McMurtry #12 Bivins #l E.B. White #1 L-S Ranch XC-1 Alamosa Ranch 1 Binford #3 Clay #l Hobgood #1 Halsell # I D.C. Reed #2 Fisher # I Rogers i*1 Kathryn # Jackson #1 Buzzard #l-A Palm #l Hopping "1 Jones # I - G Matador +I Crosley et al #l Helen Be10 a 1 Bush "1 Bivens +lox Bivins Estate t 1 Shelton #21 Mastemon #l Bluitt a43 Bateman #l Ross r*1 Daniel #! Espy X 50 Umversity "2 Elwood 4411 Swenmn #l Wilson It1 Harral "A" #l Tyrell #1 Helms #6E Blackman #l Comanche #I Shaw-Federal #5 State #l Burger B28 #l C.M. Hurley #1 Noble Trust a 1 McCnnkey outcrop #1 Federal
sec blkZ9P ELRRSurv. sec183 blk3 TI'bNN SUN. sec375 blk4 HUTCSIrv. secll blk34 ELE/RRSurv. seclO blk21 SCLSurv. 5 BrownSubd. GreggCSL 46 T4 T.A.Thompson 6 N HbOB 12 29 HbGN sec7 blk2S ELbRR 18 blk8 I X N Lge304 CSL SUN. 16 blkB-5 ELRR 36 HI l T R R sec177 b l k M HbGN Lab10 Lge693 SCLSUN. lab19 lge2 19 Castro CSL 19 Hamson and Brown 5 CL ELRR 106 D J.H.Gibson 8 38 GHbH secll9 blkA Thompson SUN 4s L E WMSUN. 141 B-4 HbGN 2s F T.A.Thompson 18 SN 37E 8 C C.U.Cannellee 42 HUTC 1 blkl I W N 23 6 BWF 30 M-30 GbM SUN. 99 46 HUTC 16 50 H m C 76 018 DbP secl4 8 s 37E 114 A J.B. Rector sec27 S.L. Graves SIN. A.B. Duncan Sum. 1 1 0 4 HUTC s e a 9 blklO Univ. Lds. sec25 blk16 SPRR 43 J. Stqhens SIN. 1 41s TbSTL seclO blk126 sec2 blk115 GCbSF 633 97 HVTC W.A. McKinney SUN. 13 11s 26E 6 13s 3lE 2 21s T E 28 20s 38E 3 28N ME 18 4S27E 10 9s 26E Carrizo Mtn 27 9N 19E CNWSE
rhyolite (5) rhyolite (5) welded rhyolite ( 5 ) rhyolite(5) intergranular diabase (2,3,4) olivine gabbro (1,3,4) olivine gabhro (1.3.4) olivine ferrogabbro (1,3,4) diabase (2) rhyolitic crystal-rich welded tuff (2.34) ignimbrite (2,3,4) welded rhyolite (2,3,4) rhyolitic tine-grained tuff (2,3.4) non-welded rhyolite (2,4) metarhyolte (3) welded rhyolite (1,3,4) slightly welded altered rhyolite (1,q ignimbrite(l,3,4) lithic tuff (4) distal non-welded rhyolite (3,q altered metarhyolite (44) dacite (4) rhyolitic crystal-poor welded tuff (1.34) ignimbrite(l,3,4) volcanoclastic sandstone (4) welded rhyodacite (3) coame grained biotite granite (1,3,4) granitic gneiss (1.34) rhyodacitic slightly weldedtuff (2,3,4) brecciated rhyolite (4) rhyodacitic tuff(2,3,4) welded rhyolite (2,3,4) equigranular, leumratic granite (2.34) equigranular. leumratic granite (2.34) granite (3) coarse grained biotite granite (2,3,4) coarse grained biotite granite (1,3,4) biotite granodiorite gneiss (1,3,4) leucocratic granite (l,3.4) granitic gneiss (1,3) granitic gneiss (1.34) metarhyolite (3) alkali feldspar lellcogranite (1.34) biotite twepymxene diorite (2,3,4) hornblende pymxenegabbro (2,3,4) quartz diorite (1,3) biotite alkali feldspar granite (3.4) metarhyolite (1,3) microgranite (1.3) granodiorite gneiss (1,3) granitic gneiss (1,3) granite (3) metasediment (3) metarhyoiite (3) metarhyoiite (3) metarhyolite (3.4) granite (3) coarse grained granite (1.4) granite (23) biotite alkali feldspar granite (3,4) gran itegneiss (3) granitic gneiss (2,3) d i o d e (2) granite ( l p )
Wilbarger Chaves Chaves Lea Lea Union Chaves Chaves Hudspeth Guadabrpe Quay Pecos Carson Winkler Andrews Ward Pews Lubbock
Hill Phillips Gulf Phillips Tenneco Sincla ir Mobil Magnolia Petr. Cn.
#1D Puckett #1 Dauer #l-H University #l Allen Cowden # I Aleshire +1 Belle Elam #l Johnson
26 101 TCRR sec82 blk4 BGNRR secll blk17 11 A 52 D52 seclOl blk34 HUTC SUN. 109 blklO HbGN 8 8 C DbWRR
Sources of rock identification: 1-Flawn, 1956; 2-Muehlberger et al., 1967; 3-Denison, personal communication; 4-Barnes, personal communication; 5-Bickford et al., 1981
262
Rocky Mountain Geolom, v. 34, no. 2, p . 245-262, 7figs., 5 tables, 1 appendix, November, 1999
