Geochemistry and petrogenesis of Mesoproterozoic (~ 1.1 Ga) magmatic enclaves in granites of the eastern Llano Uplift, central Texas, USA

Lithos 125 (2011) 463–481

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Geochemistry and petrogenesis of Mesoproterozoic (~ 1.1 Ga) magmatic enclaves in granites of the eastern Llano Uplift, central Texas, USA R.K. Smith ⁎, Walt Gray Department of Geological Sciences, The University of Texas at San Antonio, San Antonio, TX 78249-0663, USA

a r t i c l e

i n f o

Article history: Received 8 July 2010 Accepted 3 March 2011 Available online 10 March 2011 Keywords: Mesoproterozoic Grenville Llano Uplift Geochemistry Enclaves Granite

a b s t r a c t Mesoproterozoic (~1.1 Ga) plutons of the eastern Llano Uplift, central Texas, USA contain two types of magmatic enclaves (b 1% by vol.). Although volumetrically insignificant, the enclaves contain important petrogenetic information. Type I enclaves are felsic in composition (70–75 wt.% SiO2), with mineral assemblages and chemical compositions comparable with the host granites, but typically display a finer grained texture. They are interpreted as partly chilled disrupted material from the margins and roof of the plutons. Type II enclaves are intermediate in composition (~56–69 wt.% SiO2), with many elements defining trends continuous with the host granites. Both types of enclaves display sharp borders in contact with the host granite suggesting magma quenching with little or no physical exchange between host granite and enclave magma. Type II enclaves contained within the Marble Falls (MF) and Lone Grove (LG) plutons exhibit enrichments in Y, Nb, and Zr relative to their respective host granites. Enrichments in these incompatible trace elements at low SiO2, renders unlikely the possibility that the MF and LG Type II enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends. Because no coeval mafic to intermediate rocks are exposed in the uplift, characteristics of Type II enclave source magma(s) is uncertain. However, assuming source magmas similar to primitive continental arc basaltic andesite, trace-element trends (i.e., incompatible element enrichment and compatible element depletion) can be adequately replicated by a replenishment fractional crystallization (RFC) model. Chemistry of the MF and LG Type II enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas; the more primitive enclave magmas evolving in near chemical isolation by RFC processes. However, evidence from Type II enclaves in two other plutons in the Llano Uplift (Kingsland and Enchanted Rock) suggest that the isolation was non-ideal; i.e., some limited mixing may have occurred. Rapid quenching likely limited the potential for physical and chemical exchange between Type II enclaves and their host granite magmas. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Llano Uplift (see Fig. 1 in Smith et al., 2010) forms part of a discontinuous band of Grenville-aged rocks (ca. 1370–1070 Ma) extending some 4000 km from the Grenville Province of Canada to the Oaxaca complex of southern Mexico (Garrison and Mohr, 1983). It is believed to have formed along a major collisional reentrant associated with the Grenville orogeny (locally the Llano orogeny), and contains remnants of both island-arc and continental margin blocks. Today, the Llano Uplift is a gentle structural dome exposing ca.1360 ± 3 Ma to 1232 ± 4 Ma metavolcanic, metaplutonic, and metasedimentary rocks that have been polydeformed synchronous with a moderate- to high-pressure, upper amphibolite to lower

⁎ Corresponding author. Fax: + 1 210 458 4469. E-mail address: [email protected] (R.K. Smith). 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.03.005

granulite facies regional metamorphism (Carlson, 1998; Mosher, 1993, 1998; Reese, 1995; Reese et al., 2000; Roback, 1996; Walker, 1992). These rocks represent the core of a collisional orogen along the southern margin of Laurentia (Mosher, 1998; Mosher et al., 2008). Subsequently, the high-pressure metamorphic rocks were intruded by 1119 + 6/−3 Ma to 1070 ± 2 Ma late syn- to post-tectonic granites. These plutons form a mappable unit collectively known as the Town Mountain Granite (TMG), a pink, K2O-rich, very coarse- to coarsegrained, generally porphyritic granite with associated fine- to medium-grained, gray to pink granites. They have been classified as syeno- to monzogranites with A-type affinities. More specifically they are high-K, metaluminous to marginally peraluminous, ferroan, biotite-calcic amphibole granites with large-ion lithophile (LIL) element (e.g., K, Rb, and Ba) enrichment. The central and easternmost of these granite bodies (Fig. 1) consist of the Enchanted Rock (ER), Marble Falls (MF), Kingsland (KL), and Lone Grove (LG) plutons. The plutons are zoned and generally circular to elliptical in areal exposure

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Fig. 1. Location of the eastern Granite plutons of the Llano Uplift, central Texas, USA (modified after Mosher, 1996).

(see Fig. 3 in Smith et al., 2010). Gravity and magnetic surveys indicate that the intrusive contacts are nearly vertical in the upper 2 to 3 km of the preserved crust (Muehlberger et al., 1963). Detailed descriptions and characteristics of these plutons are provided in Muehlberger et al. (1963), Smith et al. (1997), Barker and Reed (2010), and Smith et al. (2010). Prior to this study, knowledge of the mineral chemistry, petrology, geochemistry, and petrogenetic characteristics of magmatic enclaves contained within the MF, KL, and LG plutons was sparse. In this paper, we document the petrologic and geochemical characteristics of these enclaves and compare them to enclaves from the ER pluton and the well-studied Cadillac Mountain intrusive complex of Maine. We then hypothesize as to their petrogenesis and relationship to the tectonic setting of the Llano Uplift. Magmatic enclaves of intermediate to mafic composition are common in granitoid plutons, varying in size, shape, degree of cooling, and chemical composition (Barbarin and Didier, 1991; Didier, 1973; Vernon, 1983). Enclaves tend to have rounded, scalloped, or lenticular shapes with fine-grained igneous microstructures. Mineral assemblages are generally similar to the host granitoid, differing only in their proportions. Typically the mineral compositions indicate that they are igneous in origin and not host-rock xenoliths (Vernon, 1984). Vernon (1984) restricted the term enclave to fine-grained, ellipsoidal types of inclusions. Enclave distribution may be locally uniform and concentrated in swarms, but more commonly irregular. Enclave orientations may be aligned in those areas of the granitoid intrusion showing a prominent flow foliation, especially near the margins of the pluton (Vernon, 1984). Field, geochemical, and experimental evidence suggest that most mafic magmatic enclaves form as globules of high-alumina basaltic or other mafic magma that are quenched in and become dispersed throughout the more felsic host granitoid magma as a result of magma mingling early in the crystallization history of the pluton (Frost and Mahood, 1987; Vernon, 1990). Enclave compositions range from mafic to felsic, suggesting that enclave magmas may have been a product of spatially limited magma-mixing (hybridization) between mafic and host granitoid magmas near the base of plutons, followed by mingling of the hybrid magma as globules (enclaves) into the more felsic host at a later stage within the upper levels of plutons.

Microstructural features suggestive of mineral–melt disequilibrium and magma-mixing (hybridism) include; 1) xenocrysts of quartz (ocelli) rimmed with fine-grained aggregates of early-formed minerals, 2) K-feldspar megacrysts (often rimmed with plagioclase, i.e., rapakivi texture) identical to those occurring in the host granitoid, 3) zoned plagioclase phenocrysts, 4) plagioclase with resorbed or dendritic cores, and 5) Ca “spikes” in plagioclase (Vernon, 1984, 1990). K-feldspar megacrysts are often found partially to completely enclosed in the mafic enclaves, suggesting that they are xenocrysts with an igneous origin. However, the composition of mafic enclaves renders them unlikely to have precipitated K-feldspar. Instead Kfeldspar xenocrysts are most likely the result of a partly crystallized granitoid melt that was incorporated into the more mafic magma during hybridization. The finer grained nature of the more mafic enclaves is likely the result of magma quenching [e.g., acicular apatite, chilled margins, and complexly zoned plagioclase (Barbarin, 1990), with mineral alignment reflecting magmatic strain (Paterson et al., 2003)] resulting from the temperature contrast between mafic and granitoid magmas at the time of mingling; i.e., the hotter more mafic magma is quenched to the temperature of the granitoid magma resulting in an undercooled mafic magma producing finer grained enclaves (Vernon, 1984, 1990). 2. Geologic and tectonic setting Precambrian basement rocks of Texas are bisected by the Llano Front (see Fig. 1 in Smith et al., 2010), separating undeformed rocks to the north from rocks to the south that were deformed and metamorphosed during Grenville time (~1100 to1350 Ma). North of the Front, Precambrian basement rocks are dominated by granitic and rhyolitic rocks of the 1340–1500 Ma Granite–Rhyolite Province (Thomas et al., 1984) located adjacent to older rocks of the Yavapai–Mazatzal Province (Nelson and DePaolo, 1985; Van Schmus et al., 1993). In the Llano Uplift, central Texas, the Grenville Province consists predominantly of polydeformed gneiss, schist, and amphibolite, with minimal migmatite (Barker and Reed, 2010; Morris, 2006), sparse metaserpentinite, and local eclogite occurrences (Mosher, 1993). On the basis of lithology, field relations, geochemistry, and U/Pb ages these metamorphic rocks have been subdivided into three lithotectonic

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domains; 1) the Valley Spring domain (1366–1232 Ma) interpreted as continental arc and terrigenous clastics, 2) the Packsaddle domain (1274–1238 Ma) interpreted as a forearc basin, and 3) the Coal Creek domain (1326–1275 Ma) interpreted as remnants of an island arc and obducted oceanic crust (see Mosher et al., 2008). Mosher (1993) has reported U–Pb zircon ages of orthogneiss in the Llano Uplift that range from 1350 to 1220 Ma. Continental collision or A-type subduction is suggested by the presence of medium T eclogites. Late Mesoproterozoic granitic magmatism in Texas occurred between 1119 and 1070 Ma (Garrison et al., 1979; Walker, 1992) on both sides of the Grenville Front (Smith et al., 1997). In the Llano Uplift late syn- to post-tectonic granites (1119–1070 Ma) intruded the Grenville ca. 1360–1232 Ma metaigneous and metasedimentary rocks (Mosher, 1993, 1995, 1996, 1998). These late-stage, coarsegrained, pink, K2O-rich granites are considered by Anderson (1983) to be part of a single “anorogenic” magmatic pulse associated with

465

extension along the Midcontinent Rift (MCR) that occurred ~1100 Ma ago in North America. More recently, emplacement of the granites has been attributed to subduction and post-collisional extension following the Grenville orogeny (Li et al., 2007; Mosher, 1998). Work by Mosher (1993, 1996) and Reed (1996) has shown that final deformation along the Grenville Front occurred after emplacement of some or all of the ~1100 Ma Llano granites. 3. Field relations and sampling The MF, KL, and LG plutons contain magmatic enclaves, mafic schlieren, and xenoliths of the surrounding metamorphic rocks (Packsaddle domain rocks). “Mafic” magmatic enclaves with diameters ranging from 3 cm to b0.75 m are generally rounded (ovoid) to elliptical to lenticular in shape and display fine-grained to porphyritic textures (Fig. 2), whereas the more silicic enclaves,

Fig. 2. (a) Ovoid- to elliptical-shaped felsic enclaves in sharp contact with the host granite with the larger enclave cut by a late-stage aplite dike (Granite Mtn. quarry, MF pluton). They are hypidiomorphic-granular and very fine- to fine-grained with K-feldspar phenocrysts. Hammer length is 28.58 cm (11.25 in.). (b) Lenticular-shaped [~50 cm (19.7 in.) in length], very fineto fine-grained, felsic enclave with plagioclase overgrowths on K-feldspar (microcline) phenocrysts (0.5–2.0 mm in length; forming the classic rapakivi texture) from the host granite (Granite Mtn. quarry, MF pluton). A mafic mineral-enriched border between the host granite and enclave can be seen on the right half of the photo. (c) Lenticular-shaped, fine-grained, intermediate enclave (dashed outline; KL pluton). Hammer length is 29.21 cm (11.5 in.). (d) “Ovoid”-shaped, hypidiomorphic-granular, fine-grained intermediate enclave with crenulated mafic mineral-enriched border zone (i.e., biotite and amphibole) in sharp contact with the host granite (KL pluton). Hammer length is 29.21 cm (11.5 in.). What appear to be phenocrysts of K-feldspar are clusters of smaller K-feldspar grains forming a glomeroporhyritic texture. (e) Ovoid-shaped, fine-grained, intermediate enclave with K-feldspar phenocrysts, crenulated margin, and a slightly more mafic mineral-enriched border zone (top portion of enclave; LG pluton). Coin diameter is 1.8 cm (0.71 in.). (f) Lenticular-shaped, fine-grained, intermediate enclave with partial to complete inclusion of K-feldspar grains and megacrysts [2.5–3.5 cm (1–1.4 in.) in length] from the host granite (ER pluton).

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commonly referred to as autoliths or cognate enclaves (Clarke, 1992), are larger in diameter (10 cm to N1.5 m) and are more abundant (Fig. 2). Relationships to the host granites and plutons include the following; 1) they have been observed only within the porphyritic coarse- and coarse-grained facies of the MF, KL, and LG plutons (see Fig. 3 in Smith et al., 2010), 2) they are sparse and randomly

Table 1 Whole-rock and trace-element chemical analyses of felsic enclaves from the LG, KL, and MF plutons. Sample

LGE-1a LGE-3 KLE5-1b KLE5-2 MFE-42c MFE-45 MFE-51 MFE-52

Rock type

Felsic

Felsic

Felsic

Felsic

Felsic

Felsic

Felsic

Felsic

Major elements (wt.%) SiO2 73.78 75.04 TiO2 0.17 0.11 Al2O3 13.31 12.68 Fe2OT3 2.22 2.59 MnO 0.03 0.02 MgO 0.24 0.06 CaO 1.15 0.86 Na2O 3.32 3.24 K2O 5.60 5.50 P2O5 0.02 0.01 Cr2O3 0.04 0.04 LOI 0.15 b 0.01 Sum 100.00 100.10 FeOTcalc. 2.00 2.33

72.21 0.21 13.11 4.36 0.05 0.26 0.79 3.55 5.35 0.04 n.d.d n.r.e 99.93 3.92

72.15 0.22 12.89 4.42 0.05 0.27 0.84 3.46 5.30 0.04 n.d.d n.r.e 99.64 3.98

71.50 0.42 13.00 4.23 0.08 0.37 1.37 3.87 4.93 0.11 0.03 0.02 100.10 3.81

71.50 0.38 13.30 3.79 0.06 0.41 1.22 4.36 4.23 0.08 0.03 0.40 99.90 3.41

71.20 0.39 13.30 3.53 0.06 0.40 1.26 4.58 4.28 0.12 0.01 0.80 100.10 3.18

71.70 0.40 13.30 3.97 0.07 0.39 1.20 4.59 4.11 0.10 0.01 0.40 100.40 3.57

Trace-elements (ppm) V 12 6 Ni 16 18 Cu 15 25 Zn 33 25 Mo 5 9 Rb 164 170 Sr 149 71.6 Cs 0.5 0.7 Ba 1040 459 Y 28.1 52.8 Zr 156 199 Nb 10 7 Ga 18 21 La 36.5 38.8 Ce 66.4 74.4 Pr 9.05 10.4 Nd 29.3 36.1 Sm 6.2 9 Eu 1.7 1.73 Gd 5.39 8.89 Tb 0.91 1.74 Dy 5.12 10.5 Ho 1.03 2.23 Er 2.95 6.37 Tm 0.44 0.89 Yb 3.1 5.4 Lu 0.55 0.75 U 2.35 2.15 Th 16.5 7.5 Hf 6 8

23 124 40 32 n.d. 267 82 n.d. 333 54.6 190 n.d. n.d. 42.3 90.0 n.d. 40.4 7.9 0.8 7.1 n.d. 8.0 n.d. 5.1 n.d. 6.2 n.d. n.d. n.d. 8.6

26 35 48 41 n.d. 269 83 n.d. 349 51.5 183 n.d. n.d. 40.5 88.9 n.d. 37.8 7.7 0.7 6.6 n.d. 7.6 n.d. 5.0 n.d. 6.0 n.d. n.d. n.d. 6.1

n.d. n.d. n.d. n.d. n.d. 243 65 n.d. 567 81 624 15 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. 204 73 n.d. 427 54 497 20 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. 191 79 n.d. 479 81 538 27 n.d n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. 209 63 n.d. 346 42 547 37 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Samples of unaltered felsic and intermediate enclaves (Tables 1 and 2) were collected from the MF, KL, and LG plutons utilizing a Pomeroy portable rock core drill. Samples were crushed and passed through a multiple splitter until 10–20 g of material was obtained. All analyses from the MF and LG plutons, were conducted by X-ray Assay Laboratories of Ontario, Canada using X-ray fluorescence techniques. Eight samples from the KL were analyzed for Rb. Detection limits for the major elements was 0.01 wt.% and for the trace elements was 10 ppm and 50 ppm for Ba. All analyses from the KL pluton, other than Rb, were conducted at the University of Houston using ICP techniques. n.d., not determined. a LGE, Lone Grove enclave. b KLE, Kingsland enclave. c MFE, Marble Falls enclave. d n.d., not detected. e n.r., not reported.

distributed (i.e., lacking local uniformity), 3) they typically occur only as single enclaves, never as concentrated swarms, 4) they lack any orientation relative to the host granite (e.g., flow banding or parallel oriented phenocrysts), and 5) they lack any apparent correlation with wall rock contacts; i.e., they can occur from 10 to 100's of meters from exposed wall rock-pluton contacts. Xenoliths are typically angular in shape, in sharp contact with the enclosing granite, and most abundant near contacts with the Packsaddle schist. They have a porphyroblastic texture characterized by idioblasts of biotite and hornblende surrounded by a crystalloblastic matrix of quartz and plagioclase. Schlieren include alternating mafic- and felsic-rich zones, which grade into the surrounding granite. They reach lengths of several meters and thicknesses of a few decimeters and have been interpreted (Barker et al., 1996) to result from mechanical segregation of mafic minerals formed by vertical to subvertical movement of magma. Typically the late post-tectonic MF, KL, and LG plutons show more abundant mafic schlieren and silicic enclaves than the late syn-tectonic plutons, but fewer xenoliths (Barker and Reed, 2010). The areal extent of the three plutons is characterized by restricted land access, low relief, considerable ground cover (resulting in limited areal exposure), and moderate weathering. Thus, granite and enclave sampling was mostly restricted to dam sites, quarry sites, and road cuts. 4. Mineralogy and petrography 4.1. Magmatic enclaves Major- and trace-element data separate the magmatic enclaves in the three plutons into two types (Gallegos and Smith, 1996a,b; Gibbs and Smith, 2008; Gray, 2000; Smith and Smith, 1997). Type I enclaves are felsic in composition (~72 wt.% SiO2), with major- and traceelement abundances comparable to the host granite. Type II enclaves are intermediate in composition (~59–67 wt.% SiO2), with many elements defining more primitive (i.e., lower wt.% SiO2) but continuous trends with those exhibited by the host granite. Even though Type I and II enclaves are finer-grained than the host granite, the term “magmatic enclave” is preferred to “microgranular enclave” or “microgranitoid enclave” because the enclave textures are not necessarily microgranular or microgranitoid (see Barbarin, 2005). The intermediate and felsic enclaves comprise b1% of the exposed area of the plutons and have field, textural, and mineralogical features that support a magmatic origin, rather than being xenoliths or restitic material. Additionally, Göbel (1999) noted the presence of an olivine– pyroxene-bearing enclave (57.81 wt.% SiO2) in the porphyritic coarsegrained textural unit of the LG pluton (see Smith et al., 2010). 4.2. Type I (felsic) magmatic enclaves The light-colored felsic magmatic enclaves (Fig. 2a and b) are typically hypidiomorphic-granular and range from very fine- to finegrained porphyritic and are typically in sharp contact with the host granite (Fig. 2a and b). Mineralogically they are composed of the same minerals as the host granites; i.e., microcline, plagioclase, quartz, and biotite as major phases with amphibole (absent in KL), Fe–Ti oxides, titanite, zircon, apatite, and rare fluorite as accessory phases. K-feldspar (microcline) phenocrysts with plagioclase overgrowths (forming the classic rapakivi texture) are also observed (Fig. 2b). Mafic mineralenriched borders are occasionally observed between the host granite and enclave (Fig. 2b). Modal compositions of the felsic enclaves plot in the monzogranite field with lower modal % of biotite, amphibole, and Fe–Ti oxides relative to the host granites. 4.3. Type II (intermediate) magmatic enclaves The dark-colored intermediate magmatic enclaves are typically hypidiomorphic-granular and range from very fine- to fine-grained

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Table 2 Whole-rock and trace-element analyses of intermediate enclaves from the LG, KL, and MF plutons. Sample

LGE-2

LGE-4A

LGE-4B

LGE-4C

LGE-4D

KL2E-1

KL2E-2

KL8E-1

Rock type

Intermediate

Biotite rich

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Major elements (wt.%) SiO2 63.42 TiO2 1.23 Al2O3 12.76 T Fe2O3 11.49 MnO 0.13 MgO 0.86 CaO 2.86 Na2O 3.92 K2O 2.46 P2O5 0.43 Cr2O3 0.04 LOI b 0.01 Sum 99.04 FeOTcal. 10.34

44.35 3.08 8.38 26.08 0.41 1.99 7.48 1.55 2.10 0.89 0.01 b 0.01 95.43 23.47

63.89 1.26 10.83 12.43 0.18 0.92 3.96 3.41 1.51 0.30 0.04 b0.01 98.55 11.18

56.50 0.94 17.30 9.96 0.14 0.62 4.78 6.40 1.61 0.45 0.02 0.05 98.76 8.96

59.58 0.75 17.34 7.86 0.10 0.48 3.23 5.58 3.77 0.32 0.03 b 0.01 98.87 7.07

68.88 0.71 13.74 6.70 0.09 0.82 1.44 4.02 3.65 0.18 n.d.a n.r.b 100.22 6.03

68.00 0.69 13.86 6.96 0.09 0.84 1.40 3.98 3.62 0.17 n.d.a n.r.b 99.60 6.26

66.05 0.89 14.53 7.91 0.11 1.11 1.90 4.54 2.84 0.21 n.d.a n.r.b 100.08 7.12

Trace-elements (ppm) V 51 Co 9.8 Ni 19 Cu 43 Zn 261 Mo 14 Rb 229 Sr 88.2 Cs 2 Ba 254 Y 187 Zr 777 Nb 41 Ga 29 La 69.3 Ce 175 Pr 30.1 Nd 119 Sm 33.6 Eu 1.9 Gd 32.1 Tb 6.15 Dy 35.9 Ho 7.46 Er 21.3 Tm 3.04 Yb 19.4 Lu 2.8 U 1.36 Th 7.5 Hf 21

85 19 12 57 607 38 171 43.2 1.9 200 N 1000 4170 222 45 571 1320 216 780 204 4.12 194 37.6 229 47.7 136 19.3 118 16.8 12.1 111 129

38 9.6 20 31 273 12 118 81.1 1.5 172 455 1180 83 34 408 989 126 414 91 2.63 79.4 14.7 86.6 17.7 50.1 6.97 43.4 6.1 5.13 78.1 44

27 7 16 31 194 17 103 137 3.2 161 420 2190 65 39 275 677 88 312 76.3 3.24 71.7 13.6 82.4 16.9 48.2 6.87 42.7 6.1 7.39 53.8 70

20 6 17 30 160 22 179 139 1.8 526 279 1990 46 36 299 709 88.1 284 59.7 2.93 51.5 9.37 53.7 10.8 30.9 4.43 27.5 3.94 6.23 60.2 63

50 7 41 39 95 n.d. 232 86 n.d. 217 65.6 510 104 n.d. 61 130.9 n.d. 58.2 11.7 0.9 10.1 n.d. 10.7 n.d. 5.9 n.d. 6 n.d. n.d. n.d. 12

47 9 41 42 98 n.d. 242 80 n.d. 211 73.9 463 224 n.d. 66.5 146.6 n.d. 65.2 13.6 1.0 11 n.d. 12 n.d. 6.9 n.d. 6.5 n.d. n.d. n.d. 26.6

60 11 22 42 115 n.d. 236 107 n.d. 272 63.4 598 b.d.l. n.d. 61.5 132.2 n.d. 58.5 12.1 1.1 10.6 n.d. 11 n.d. 6.1 n.d. 5.2 n.d. n.d. n.d. 17.9

Sample

KL11E1-1

KL11E2-1

KL11E2-2

MFE-50A

MFE-50B

MFE-50C

MFE-53

MFE-55

Rock type

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Major elements (wt.%) SiO2 69.33 TiO2 0.54 Al2O3 13.16 Fe2O3T 5.72 MnO 0.08 MgO 0.62 CaO 1.13 Na2O 3.63 K2O 4.82 P2O5 0.12 Cr2O3 n.d. LOI n.r. Sum 99.14 FeOTcal. 5.15

69.36 0.59 13.64 6.37 0.08 0.68 1.09 3.49 5.27 0.12 n.d. n.r. 100.69 5.73

68.44 0.65 13.20 6.62 0.08 0.71 1.16 3.30 5.54 0.11 n.d. n.r. 99.81 5.96

59.00 0.79 17.10 8.93 0.12 1.56 2.11 5.16 4.75 0.13 0.02 0.45 100.30 8.04

60.20 1.52 12.90 12.00 0.19 1.25 4.02 3.50 3.47 0.62 0.01 0.10 100.30 10.80

67.10 1.02 11.60 10.70 0.15 0.62 3.08 4.05 1.72 0.18 0.01 0.05 100.60 9.63

62.50 1.64 10.00 15.20 0.24 1.13 2.57 2.17 3.85 0.40 0.02 0.15 100.20 13.68

58.10 2.09 9.90 17.40 0.29 1.98 4.31 2.39 2.85 0.52 0.02 0.10 100.20 15.66

Trace-elements (ppm) V 36 Co 5 Ni 32 Cu 32

35 4 13 35

38 6 23 41

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. (continued on next page)

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Table 2 (continued) Sample

KL11E1-1

KL11E2-1

KL11E2-2

MFE-50A

MFE-50B

MFE-50C

MFE-53

MFE-55

Rock type

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

Intermediate

73 n.d. 326 81 n.d. 262 100.8 544 126 n.d. 90.7 188.4 n.d. 80.6 14.8 1.2 12.4 n.d. 14.8 n.d. 9.3 n.d. 11.8 n.d. n.d. n.d. 29.6

93 n.d. 323 89 n.d. 302 97.7 507 b.d.l. n.d. 86.1 186.3 n.d. 74.9 14.2 1.2 12 n.d. 13.9 n.d. 8.9 n.d. 11.5 n.d. n.d. n.d. 28

n.d. n.d. 248 118 n.d. 947 71 176 20 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. 160 80 n.d. 615 245 2520 92 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. 93 91 n.d. 250 168 1500 40 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. 330 26 n.d. 221 314 1880 109 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. 190 34 n.d. 266 258 2380 91 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Trace-elements (ppm) Zn 85 Mo n.d. Rb 298 Sr 86 Cs n.d. Ba 276 Y 83.7 Zr 495 Nb 141 Ga n.d. La 80 Ce 172.2 Pr n.d. Nd 71.6 Sm 13.4 Eu 1.1 Gd 10.8 Tb n.d. Dy 12.3 Ho n.d. Er 7.8 Tm n.d. Yb 9.3 Lu n.d. U n.d. Th n.d. Hf 27 a b

n.d., not detected. n.r., not reported.

and porphyritic. A lenticular-shaped intermediate enclave (dashed outline) is shown in Fig. 2c. Under microscopic examination what appear to be phenocrysts of microcline (Fig. 2d, KL pluton) are clusters of smaller microcline grains forming a glomeroporphyritic texture. Type II enclaves are composed of the same minerals as the host granites, namely euhedral plagioclase surrounded by anhedral microline, quartz, biotite, and amphibole. Accessory minerals include Fe–Ti oxides, titanite, zircon, and acicular apatite suggesting quenching. The enclaves contain considerably more modal biotite, amphibole, and plagioclase relative to the host granites and therefore plot within the granodiorite field of the QAP diagram. They typically display a mafic mineral-rich border zone (i.e., biotite and amphibole) in sharp contact with the host granite (Fig. 2d). A magmatic origin is suggested by crenulated margins (Fig. 2d and e), and partial to complete inclusion of K-feldspar grains and megacrysts (Fig. 2f) from the host granites. A biotite-amphibole-rich magmatic enclave (Didier, 1973; Didier and Barbarin, 1991) within the LG pluton (Gibbs, 2003) showed local magma mingling and was sampled along its length (~2.5 m long by 1 m wide) to evaluate an apparently continuous trend of “mafic” to more felsic composition. The main trends found to occur included; 1) a decrease in modal biotite and amphibole, 2) an increase in modal quartz, microcline, and plagioclase, and 3) an increase in grain size. Microprobe analyses conducted on three microcline and plagioclase grains, from the most biotite-amphibole-rich portion of the enclave, show an average composition of Or92Ab8An0 and An16Ab83Or1, respectively (see Gibbs and Smith, 2008 and Appendices A and B) and are comparable to those for the LG host granites. However, enclave biotite compositions (see Appendix B) have higher Fe/(Fe + Mg) than the host granites as well as intermediate magmatic enclaves from the ER pluton (see Smith et al., 1997; Smith et al., 2010). 4.4. Enchanted Rock pluton The Enchanted Rock (ER) pluton (see Fig. 1 in Hutchinson, 1956), a coarse-grained porphyritic granite, is also host to scattered occurrences of intermediate and felsic magmatic enclaves (Smith, 1996;

Smith and Smith, 1997; Smith and Wark, 1992; Smith et al., 1997). These studies show the enclaves to be; 1) elliptical in shape, ranging in size from ~ 10 to 50 cm in length, and oriented length-parallel to the flow foliation of the host granite, 2) widely distributed within the intermediate and outer zones of the pluton, 3) microgranular with a matrix of K-feldspar, quartz, plagioclase, biotite, and amphibole (accessory apatite occurs as acicular needles), 4) contain varying amounts of K-feldspar, amphibole and quartz xenocrysts (from the host granite) and plagioclase phenocrysts, 5) microscopically, textures suggest a rapid cooling stage, indicated by acicular apatite, followed by a slower cooling stage, indicated by poikilitic quartz, and Table 3 Representative whole-rock and trace-element chemical analyses of magmatic enclaves from the ER pluton. Sample

ER-50A

ER-30

ER-5

Host granite

Rock type

Felsic

Intermediate

Intermediate

Avg. O. Z.

Major elements (wt.%) SiO2 70.20 TiO2 0.37 Al2O3 13.47 T Fe2O3 3.03 MnO 0.06 MgO 0.70 CaO 1.78 Na2O 3.61 K2O 4.11 P2O5 0.09 FeOTcalc. 2.73 Trace-elements (ppm) Rb 203 Sr 145 Ba 584 Y 67 Zr 257 Nb 17

66.30 0.60 14.15 4.90 0.08 0.98 2.34 4.27 3.16 0.27 4.41

234 126 310 36 323 19

67.73 0.58 15.20 5.23 0.10 1.16 2.49 4.88 1.97 0.17 4.71

263 114 270 68 226 9

72.37 0.24 12.99 2.17 0.03 0.27 1.08 3.29 5.69 0.06 1.95

222 83 540 60 146 14

ER, Enchanted Rock pluton; Fe2OT3, Total iron as Fe2O3; Avg. O.Z., Host granite Average, outer zone. For additional trace-element data see Smith et al., 1997.

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Fig. 3. (a) A/NK vs. A/CNK and (b) FeOT/(FeO + MgO) vs. wt.% SiO2 in whole-rock samples of the host granites and their enclaves. The felsic and intermediate magmatic enclaves exhibit geochemical distinctions, suggesting different petrogenesis; i.e., the trends on panels ‘a’ and ‘b’ are distinct for each enclave type.

6) a magmatic origin is implied by their crenulated margins and their partial to complete inclusion of K-feldspar megacrysts from the host granite (Fig. 2f).

5. Major element geochemistry Chemical analyses of Type I and II enclaves for the MF, KL, and LG plutons are given in Tables 1 and 2 (see Appendix B) and representative analyses for the ER pluton in Table 3 (Smith and Smith, 1997; Smith et al., 1997). No truly mafic (basaltic) magmatic enclaves have been recognized in the MF, KL, LG, and ER plutons. In contrast to the host granites, compositions of the KL and ER Type I and II enclaves plot close to and on both sides of the metaluminous– peraluminous boundary (i.e., they are metaluminous to slightly peraluminous; Fig. 3a). Fe/(Fe + Mg) ratios (Fig. 3b) show Type II enclaves in the MF, KL, and LG plutons to be more Fe-rich than the ER enclaves and, with the exception of the KL Type II enclaves, chemically distinct from the host granites. The Fe/(Fe + Mg) ratio in Type II KL enclaves overlaps that of the MF and LG host granites at similar SiO2 contents while Type I MF, KL, and LG enclaves are all similar to the host granites. Relative to the KL Type II enclaves, data for the LG and MF Type II enclaves display considerable scatter (Fig. 3a and b). Harker diagrams (Fig. 4) for Type II enclaves from the MF and LG plutons show that Fe2OT3, TiO2, MnO, MgO, and CaO decrease with increasing SiO2, but show considerable scatter. Additionally, Type I and II magmatic enclaves in each pluton exhibit geochemical distinctions, suggesting different petrogenesis (Figs. 3 and 4). That is, the geochemical trends on Figs. 3a and b, and 4 are distinct for each enclave type (i.e., felsic and intermediate magmatic enclaves). On AFM diagrams Type II MF and LG enclaves plot in the tholeiitic field,

whereas the MF, KL, LG Type I and KL Type II enclaves plot along the calc-alkaline–tholeiitic boundary. Many major oxides in the MF, KL, and LG Type I enclaves overlap with and continue trends exhibited by the host granites (Fig. 4). However, enclaves in the MF (felsic) and ER (felsic and intermediate) plutons exhibit slightly lower K and slightly higher Na contents, and appear to define a distinct trend for Mg, respectively (Fig. 4; Tables 1–3). Subsolidus alteration and loss of sodium is suggested by the slightly higher and lower values of Na and K, respectively, as measured during microprobe analysis of alkali feldspar and plagioclase (Smith et al., 2010).

6. Trace element geochemistry Concentrations of the LIL trace elements Ba, Rb, and Sr in Type I enclaves plot along trends similar to the associated host granite plutons (Fig. 5) and display a comparable range of values (Tables 1–3). However, in Type II enclaves the same trace elements show no clear trends. The HFS elements Zr, Y, and Nb highlight the distinct chemistry of Type II MF and LG enclaves when compared to Type II KL and ER enclaves. For example, Zr is enriched to N2000 ppm in the MF and LG plutons, whereas Zr concentrations in the KL and ER plutons are b600 ppm and 325 ppm, respectively (Tables 2 and 3). The MF and LG Type II enclaves also display considerably more scatter in their LIL and HFS trace elements (vs. SiO2) than the KL and ER Type II enclaves (Fig. 5). In the KL Type II enclaves, concentrations of Zr, Y, Ba, and Rb plot in fields similar to the ER host granites and ER enclaves, but Sr values are lower (Fig. 5). As noted in Fig. 5 Type I MF, KL, LG and ER enclaves display trace-element concentrations similar to the host granites.

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Fig. 4. Felsic and intermediate magmatic enclaves also exhibit different trends on major-oxide vs. SiO2 plots. The felsic MF, KL, and LG enclaves are similar to the host granites, while the intermediate enclaves are chemically distinct except for the KL enclaves which are also similar to the host granites. Except for the alkalis, the Enchanted Rock enclaves are only slightly different from the host granite; symbols as in Fig. 3.

On a binary plot of Y vs. Nb (Fig. 6) the MF, KL, and LG Type II enclaves plot in the within plate granite (WPG) field with enrichments in Y and Nb similar to the MF and LG host granites, suggesting they formed in a tectonic setting comparable to the host granites. Additionally, the KL Type II enclaves show a considerable increase in Nb and decrease in Y relative to the MF and LG Type II enclaves, but are comparable to the host granites (Fig. 6). The MF and KL Type I enclaves also plot in the WPG field and show enrichments in Y and Nb comparable to the host granites. The LG Type I enclaves plot predominantly in the volcanic arc granite and syn-collisional granite (VAG + SYN − COLG) field and have Y and Nb enrichments lower than the host granites. Chondritic-normalized rare earth element (REE) patterns of the LG, KL, and ER Type I and II enclaves are similar (as well as being similar to the host granites; Fig. 7); i.e., a negatively sloping “seagull” pattern with negative Eu anomalies. The REE patterns of Type I and II enclaves of the KL and ER, and Type I enclaves of the LG pluton are within the range of the host granites (Fig. 7), but LG Type II enclave REE patterns show considerable enrichment relative to the MF, KL, LG, and ER host granites

(Fig. 7). No REE data has been published for the MF Type II enclaves. However, it may be reasonably assumed that MF Type II enclaves have similar REE data to LG Type II enclaves, as they share the same majorelement composition (Table 2), A/NK vs. A/CNK ratios (Fig. 3a), Fe/Fe + Mg ratios (Fig. 3b), and plot within the WPG field on the Y vs. Nb diagram (Fig. 6). 7. Enclave petrogenesis In the MF, KL, LG, and ER plutons the petrogenesis of Type I and II magmatic enclaves remains uncertain. Type I enclaves from the MF, KL, LG, and ER plutons have mineral assemblages and major- and trace-element compositions comparable with the host granites, but typically display a finer grained texture. Previous studies have shown that enclaves with compositions similar to the host granites are inclusions of silicic-rich magma globules (N71 wt.% SiO2), that originate from the pluton margins or roof (Bonin, 1991; Didier, 1991). Therefore, Type I enclaves are currently interpreted as representing the chilled equivalents of disrupted margins from the

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host granites; i.e., autoliths or cognate enclaves. However, Type II enclaves, especially those from the MF and LG plutons are distinct from the host granites, and cannot be easily explained as early chilled or partly chilled cumulate equivalents to the host granites. Enclave studies in other plutons have noted that enclave compositions are enriched in incompatible trace elements as the result of chemical exchange between the enclave and the host granite immediately surrounding the enclave (Bedard, 1990; Blundy and Sparks, 1992; Bussy, 1991; Debon, 1991; Holden et al., 1991; Orsini et al., 1991; Seaman and Ramsey, 1992). As the enclave minerals crystallize, such as hornblende or zircon, trace elements compatible in enclave phases (e.g., Y in hornblende) are thought to diffuse from the granitic magma into the small volume of interstitial liquid that remains within the enclave. One problem with Type II enclaves is that incompatible traceelement enrichments seem to require phases in the enclave which are not typically observed (Smith and Smith, 1997); e.g., enclaves enriched in Zr (N1800 ppm) contain no zircon (Table 2, sample MF50B and MF-53). However, those Type II enclaves in the MF and LG plutons for which zircon has been observed have Zr contents as low as 176 ppm and as high as 4170 ppm (Table 2). Additionally, enclaves that occur in the same outcrop exhibit different levels of traceelement enrichments, but are similar in size and occur within granite of fairly homogeneous composition. These enclaves lack mineralogical or textural zoning related to their margins, have fairly sharp and straight boundaries, and exhibit no chilled margins. Therefore, it is possible that the enclaves were nearly solid when they came into contact with the host granite immediately surrounding them. Additionally, the host granites have low concentrations of elements that are enriched in the enclaves, and the enclaves with the greatest enrichments in Zr, Y, and Nb are at the lower end of the

471

Fig. 6. Y and Nb (ppm) in whole-rock samples of the MF, KL, LG, and ER plutons and their enclaves; symbols as in Fig. 3. WPG, within-plate granites; ORG, oceanic ridge granites; VAG, volcanic arc granites; and SYN-COLG, syn-collisional granites (after Pearce et al., 1984).

silica range (Fig. 5). Therefore, the authors do not believe that significant interaction took place between Type II magmatic enclaves and the surrounding host granite. If hybridization between enclave and granitic magma occurred, it probably took place at an earlier stage and at a location elsewhere in the magmatic system (Smith and Smith, 1997). It also seems highly unlikely that Type II MF and LG enclaves are the result of partial melting (anatexis) of mafic crustal rocks. If elements such as Zr, Y, and Nb behave incompatibly during partial melting, the enclaves most enriched in these elements would be expected to be the most silicic, whereas the opposite is observed. Another source of enclave magmas may be from mafic to intermediate magmas in the lower portions of compositionally stratified magma chambers (Fig. 8; Smith and Smith, 1997; Wiebe et al., 1997). Wiebe et al. (1997) noted that fractional crystallization and replenishment of mafic

Fig. 5. Representative trace-elements (ppm) vs. SiO2 wt.% in whole-rock samples of the host granites and their enclaves. Ba, Rb, and Sr show no clear trends for these rocks. However, Zr, Y, and Nb highlight the distinct chemistry of the intermediate MF and LG pluton intermediate enclaves, showing strong enrichments in these elements; symbols as in Fig. 3.

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Fig. 7. Chondrite-normalized (after Anders and Grevesse, 1989) whole-rock rare earth plots of intermediate and felsic enclaves for LG, KL, and ER pluton samples. Shaded areas show overall composition variations for the MF, KL, LG, and ER host granites. REE patterns for the intermediate and felsic enclaves for the ER pluton and their host granites are from Smith et al. (1997; see Table 3).

magma could produce intermediate layers with compositions distinctly different from hybrid compositions generated by the direct mixing of original mafic and silicic magmas. Because of their higher densities, mafic magmas would pond as layers near the base of silicic magma chambers. The mafic layers would form a hybrid zone evolving through time by crystal fractionation, replenishment, and magma mixing processes (Smith and Smith, 1997), as well as diffusion between the mafic layers and the overlying silicic magma (Stage I, Fig. 8). Enclave compositions may therefore, reflect fractionation and hybridization processes within layers rather than by direct exchange between enclave and the host granite magmas (Wiebe et al., 1997). From time to time, the evolved mafic to intermediate layers (hybrid zone) might be disrupted (Stage II, Fig. 8), and dispersed as mafic to intermediate enclaves into the overlying granitic magma (Stage III, Fig. 8). Such hybrid zone compositions would be particularly dependent on the nature of the cumulus mineral assemblage, the rate of basalt magma replenishment, and the relative volumes of basaltic and silicic magmas in the magma chamber (Wiebe et al., 1997). Studies of silicic volcanic systems support the existence of the hybrid zones (inter-

mediate zones), and intermediate enclaves occurring at the base of these magma chambers (Bacon, 1986; Bacon and Metz, 1984; Grunder, 1994; and Stimac et al., 1990). A similar conclusion has also been reached in studies of enclaves in plutonic rocks (Barbarin, 1991; Dodge and Kistler, 1990). The late Silurian–early Devonian (~424–418 Ma) Cadillac Mountain intrusive complex (CMIC) serves as an analog for the late syn- to posttectonic granites of the Mesoproterozoic Llano Uplift. Located on Mt. Desert Island off the coast of Maine, USA (Wiebe et al., 1997), the CMIC granites are well exposed, and host to enclaves that are geochemically similar to Type II MF and LG enclaves. Unlike exposures of the Llano plutons (e.g., MF, KL, LG, and ER), the CMIC is tilted so that the base of the intrusion is exposed displaying a 2–3 km section of rocks dominated by interlayered gabbro, diorite, and granite (see Fig. 1 in Wiebe, et al., 1997). The CM granite overlies the gabbro–diorite sequences and is host to felsic and intermediate magmatic enclaves (see Fig. 1 in Wiebe et al., 1997). In contrast to the Llano plutons, layers of chilled basaltic rocks are found, recording hundreds of injections of basaltic magma that ponded at the base of the magma chamber. Fig. 9 illustrates the compositions for

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Fig. 8. One possible mechanism for the formation of enclave magmas may be the result of mixing silicic and mafic to intermediate magmas, due to density differences, forming a hybrid zone in the lower portions of compositionally stratified magma chambers (Stage I). Over time the evolved hybrid zone is disrupted (Stage II) forming enclaves which are then dispersed into the overlying granitic magma (Stage III).

the basaltic chilled rocks, the CM granite, intermediate enclaves (found in the granite), and the cumulate rocks of the gabbro–diorite sequence (Smith and Smith, 1997). The enclave compositions (e.g., Y and Nb) do not appear to lie on mixing-tie lines between the granite, chilled mafic rocks, and the cumulate rocks of the gabbro–diorite sequence of the CMIC (Smith and Smith, 1997). They exhibit depletions in Mg, Ca, and Sr, and enrichments in incompatible elements such as Y and Nb (Fig. 9). Although these variations are broadly consistent with extensive factional crystallization from a basaltic magma (Smith and Smith, 1997), Wiebe et al. (1997) recognized that the enrichments in incompatible elements are simply too high for closed system fractionation. Having studied the CMIC rocks in detail, Wiebe et al. (1997) proposed a replenishment-fractional crystallization (RFC) model involving fractional crystallization of basaltic magmas coupled with multiple replenishments and mixing of residual liquids with fresh basalt (Fig. 10a). In Fig. 10a the ratio Cev/Co (concentration of a given element in an evolved liquid compared to the original magma composition) is plotted as a function of the number of replenishment cycles, and for different bulk distribution coefficients or D values (for a detailed discussion see Wiebe et al., 1997). This model is similar to that proposed by O'Hara and Mathews (1981) for mid-ocean ridge systems. A very important result of the model is that magmas evolving by RFC processes become progressively enriched in strongly incompatible elements, whereas moderately incompatible element concentrations remain fairly constant (O'Hara and Mathews, 1981). Compatible elements become rapidly depleted, but reach a constant concentration no matter how many subsequent replenishment cycles occur. A schematic diagram (Fig. 10b) shows the rapid depletion of compatible elements whether there are 10 or 150 replenishments, while silica remains relatively constant. In contrast, incompatible elements, such as Y and Nb (see Fig. 10b), continue to increase in concentration with additional replenishments. The evolved liquid layers might be disrupted and dispersed as enclaves (see Fig. 8), or might undergo further mixing with granitic magma prior to dispersal as enclaves, generating a range of Type II enclave trace-element compositions similar to those observed in the MF and LG plutons (Table 2). Another possibility which Wiebe et al. (1997) considered is compaction of crystalline mushs and filter pressing that releases the interstitial liquids. Such liquids will also exhibit enrichments in incompatible trace elements and depletions in compatible elements, and this process

473

would therefore decrease the number of replenishments required by RFC processes alone. Geochemical data from the CM (see Fig. 9) superimposed on similar plots of MF, KL, LG and ER host granites and their intermediate enclaves (Fig. 11) highlights the striking similarities of MF and LG intermediate enclaves to the CM intermediate enclaves; i.e., similar depletions of compatible elements (e.g., Ca, Mg, and Sr compatible during crystal fractionation of basaltic magma) and enrichments in incompatible trace elements such as Y and Nb. Unlike the CM enclaves, the Llano Type II enclaves also show enrichments in Rb and Zr (Fig. 11). Numerical modeling reveals that fractional crystallization, partial melting, and simple mixing alone cannot explain the trace element trends in the Llano Type II enclaves (Smith and Gray, 2009). The observed levels of incompatible trace-element enrichment cannot be reproduced without using unrealistic distribution coefficient (D) values. In addition to erroneous (inaccurate) trace-element trends, mixing models using various enclave and host granite compositions as end members produce mineral assemblages and proportions not observed in Type II enclaves. Instead, Smith and Gray (2009) suggested that, similar to the CM enclaves, Type II MF (Smith and Smith, 1997) KL, and LG enclaves may have evolved by RFC processes. To evaluate this hypothesis, a model was developed using the equations of O'Hara and Mathews (1981). The equations were programmed in Fortran 77 allowing the user to input bulk distribution coefficients (D), ratios involving rates of intrusion, crystallization, and eruption, as well as the trace-element compositions of the initial magma (Co) and replenishing magma (Ci). The difficulty in evaluating the RFC model for the Llano enclaves is a lack of exposed mafic rocks that might be indicative of the primitive starting magma. Therefore, a parental magma similar to a primitive basaltic andesite was assumed in the calculations. An iterative approach was utilized in the RFC model; in the first cycle the basaltic andesite composition was input, followed by fractional crystallization of a specified mineral assemblage. The evolved liquid composition was reentered as the starting composition which was replenished by and mixed with a fresh batch of basaltic andesite. The new liquid crystallizes and the process is repeated for the specified number of cycles. To ensure the magma chamber remains constant in volume, the rate of intrusion to crystallization is set to a high value (0.97), with the rates of eruption to crystallization and assimilation to crystallization set to 0. The model was successful in reproducing the observed trends in incompatible trace-element enrichments and compatible traceelement depletions (Fig. 12). A number of fractionating mineral assemblages were evaluated, but plagioclase + clinopyroxene + magnetite gave the best results (a small amount of magnetite fractionation was required to replicate the transition element compositions). This assemblage is thought reasonable for fractional crystallization in a basaltic andesite (Wiebe et al., 1997). The RFC model proved to be relatively insensitive to the extent of fractional crystallization (20–35%), number of replenishment cycles (50–200), and perturbations in the proportions of the fractionating minerals (Fig. 12). 8. Discussion and conclusions The MF and LG Type II enclaves exhibit 3.5 to 4, 2.5, and 10 times, respectively, enrichments in Y, Nb, and Zr compared to the host granites; the most enriched samples generally have the lowest SiO2 contents. Enrichment in incompatible elements, at low SiO2, renders unlikely the possibility that Type II MF and LG enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Such enrichment would most likely be apparent in the more silicic magmas, and not the intermediate compositions represented by the Type II enclaves. Because of this the authors do not believe that significant interaction took place between the MF and LG Type II enclaves and the surrounding host granites. If hybridization between enclave and

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Fig. 9. Select major-oxides and trace-elements vs. SiO2 illustrating the compositions for the Cadillac Mountain intrusive complex enclaves relative to the basaltic chilled rocks, the Cadillac Mountain granite, and the cumulate rocks of the gabbro–diorite sequences (Smith and Smith, 1997).

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Fig. 10. (a) Concentrations of a given element in an evolved liquid compared to the original magma composition (Cev/Co); i.e., concentrations in enclave magma over initial magma concentration and plotted as a function of the number of replenishment-fractional crystallization (RFC) cycles, and different bulk distribution coefficients or D values. The first cycle in each model involves only fractional crystallization, but all subsequent cycles involve replenishment and mixing with fresh mafic magma, followed by some degree of fractional crystallization (see Wiebe et al., 1997). For the dashed curves, f = 0.60 (40% crystallization) and for the continuous curves, f = 0.80 (20% crystallization). (b) Schematic diagram showing the rapid depletion of compatible elements whether there are 10 or 150 replenishment cycles, while silica remains relatively constant (after Smith and Smith, 1997). In contrast, incompatible elements, e.g., Y and Nb (see Fig. 9), continue to increase in concentration with additional replenishment cycles.

granitic magma occurred, it probably took place at an earlier stage and at a location elsewhere in the magmatic system. The MF and LG Type II enclave magmas could have been derived by partial melting of unknown sources distinct from the host granites or mafic crust, then filter pressed into the host granite. Textural evidence however, suggests that the enclaves were nearly solid when they were emplaced into the host granite. A more likely scenario in their evolution (independent of the host granite) is from a more primitive (basaltic?) magma that underwent processes of replenishment plus fractional crystallization (RFC processes). As has been demonstrated by the application of numerical modeling, RFC can reproduce the observed trends of incompatible trace-element enrichment and compatible trace-element depletion (Fig. 12). Type II enclaves associated with the KL and ER plutons display element compositions that are much closer to their host granites (than is observed for the MF and LG Type II enclaves; see Figs. 4 and 5), suggesting that they may represent 1) chilled samples of slightly lessevolved host granite magma, or 2) mixing between the host granite and the RFC-modified primitive magma. In most cases, the KL and ER enclaves and host granite trace-element compositions do not plot along simple fractionation trends (Fig. 5), making it difficult to envision the petrogenesis of the granite from these enclave magmas. It is somewhat easier to envision ponding of the primitive magma at the base of the granite magma chamber (as was assumed for the RFC model) resulting in localized mixing, especially where the two magmas were in intimate contact. Mosher et al. (2008) proposed that assembly of the Llano Uplift resulted from subduction and collision of an exotic arc along the southern margin of Laurentia during Grenville time. Subduction was followed by slab breakoff, upwelling of the asthenosphere, backarc extension, and emplacement of the ~ 1.1 Ga plutons. Consistent with this hypothesis, primitive magmas (generated during subduction and upwelling) may have ponded at the base of the backarc crust providing the heat necessary for melting and generation of the granitic magmas. Older gabbros, tonalities, and trondhjemites do occur in the Llano Uplift (Roback, 1996) and as noted by Huppert and Sparks (1988) some mantle-derived magma (i.e., upwelling of the

asthenosphere) must have invaded or underplated newly accreted and thickened continental crust, therefore, providing the necessary heat source for the generation of the granite magmas. Melting of preexisting crust formed the granitic magma chambers, but the denser more mafic mantle-derived magma remained confined at the bottom of the magma chambers. Evolution of the mafic magma by RFC processes in isolation from the granitic magma could have produced Type II enclaves (MF and LG plutons) with their distinct trace-element trends. Subsequent tectonic forces dispersed the evolved magma as globules into the growing granitic magma chambers. However, evidence from the KL and ER Type II enclaves suggest that the isolation was not ideal and some limited mixing may have occurred. In conclusion, Type II enclaves likely evolved by replenishmentfractional crystallization (RFC) in complete or partial chemical isolation from the granite magmas. The chemical evolution process was similar to that suggested by Wiebe et al. (1997) for enclaves in the Cadillac Mtn. intrusive complex. In contrast to the Cadillac Mountain intrusive complex, the bases of the Llano plutons are not exposed, nor are there exposures of coeval primitive mafic rocks. Consequently, the characteristics of the primitive magma from which the Type II enclaves evolved cannot be identified with certainty. However, using a typical basaltic andesite as the primitive magma, RFC modeling reproduces the observed trends of incompatible trace-element enrichment and compatible trace-element depletion. Therefore, it is easy to envision host granite generation by crustal melting due to underplating or ponding by the basaltic andesite, which is consistent with Mosher's et al. (2008) proposed model for assembly of lithotectonic domains within the Llano Uplift during the Grenville orogeny. Acknowledgments The authors would like to express their appreciation to the landowners for allowing access to their lands, especially Mr. Bo Lusinger, quarry operations manager for the Texas Granite Corporation, for allowing access into the Marble Falls quarries. RKS acknowledges faculty research funds from The University of Texas at San Antonio for chemical analyses of the MF, KL, and LG enclaves. RKS thanks Dr. C.R. Schwandt for

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Fig. 11. Representative major- and trace-elements (ppm) vs. SiO2 wt.% in whole-rock samples of the MF, KL, LG, and ER host granites and their intermediate enclaves superimposed on the Cadillac Mountain data (see Fig. 9).

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Fig. 12. RFC model predictions compared to normalized trace-element concentrations in the, MF, KL, and LG intermediate enclaves. Ce/Co is the ratio of evolved trace-element concentration to assumed starting composition (primitive basaltic andesite) after 50 cycles of replenishment and fractional crystallization (25% fractionation). The RFC model successfully matched the general trends in enclave trace-element concentrations, especially the incompatible element enrichments and compatible element depletions. For ease of comparison to the predicted Ce/Co ratio, MF, KL, and LG enclave trace-element concentrations have been normalized to the primitive basaltic andesite. The model proved to be relatively insensitive to the amount of fractional crystallization (20–35%), number of replenishment cycles (50–200), and perturbations in the fractionating mineral assemblage.

his assistance and access to microprobe facilities at the NASA, Johnson Space Center, Houston, Texas. WG thanks Dr. A. McGuire, University of Houston for conducting ICP analyses of KL and MF enclaves. The authors

acknowledge and express their appreciation to M.E. Bickford, S. Shaw, and an anonymous reviewer for their constructive and detailed reviews, improving the manuscript considerably.

Appendix A. Analytical techniques A.1. Analysis of enclave whole rock major and trace elements Ten kilograms or greater of the unaltered enclave material (from the MF, KL, and LG plutons) were collected at each location in the field. The rock material was reduced to gravel size in a jaw crusher then powdered in a shatterbox with aluminum oxide liner and puck. Approximately 10 g of each powdered sample were then forwarded to SGS Minerals Services, Ontario Canada, and the University of Houston for analysis. Enclave whole rock major-oxide and trace-element compositions were measured using a combination of XRF, ICP, and ICP-MS techniques. Best estimate of the analytical uncertainty for each oxide and trace element was estimated by comparison of measured and certified values for several standards analyzed during the same period as the powdered enclave rock material (Tables A.1 through A.3).

Table A.1 Analytical results vs. standards for University of Houston ICP analyses. SiO2

TiO2

Al2O3

Fe2OT3

MnO

MgO

CaO

Na2O

K2O

P2O5

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

USGS Standard AGV-1 Certified 60.04 Analysisa 59.80 % Dif b 0.4

1.07 1.05 1.8

17.49 16.89 3.5

6.90 6.72 2.6

0.09 0.09 2.2

1.56 1.51 3.5

5.04 5.00 0.7

4.35 4.40 1.2

2.98 2.96 0.7

0.50 0.49 2.4

USGS Standard W-2 Certified 52.52 Analysisa 52.52 b % Dif 0.0

1.06 1.08 2.2

15.38 15.70 2.1

10.76 10.63 1.2

0.16 0.16 1.8

6.37 6.43 0.9

10.88 10.98 0.9

2.14 2.23 4.2

0.63 0.62 1.6

0.13 0.13 0.0

V

Cr

Co

Ni

Cu

Zn

Sr

Zr

Ba

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

10.1 24.0 137.7

15.3 9.4 38.6

16.0 14.9 6.9

60.0 61.3 2.2

88.0 74.2 15.7

661.9 663.2 0.2

226.9 217.4 4.2

1226.5 1217.9 0.7

USGS Standard AGV-1 Certified 121.0 144.0 Analysisa b % Dif 19.0

(continued on next page)

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Table A.1 (continued)

USGS Standard W-2 Certified Analysisa % Dif b a b

V

Cr

Co

Ni

Cu

Zn

Sr

Zr

Ba

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

262.0 268.5 2.5

93.0 109.0 17.2

44.0 42.9 2.5

70.0 59.3 15.2

103.0 117.6 14.2

77.0 73.4 4.7

194.0 191.9 1.1

94.0 93.6 0.4

182.1 171.3 5.9

Result of one standard run conducted during analysis period. Percent difference between certified value and analytical result is best estimate of the analytical error for each element or oxide.

Table A.2 Analytical results vs. standards for XRAL analyses. Internal Standard XRAL-04 SiO2

Certified Averagea %Dif b

TiO2

Al2O3

Fe2OT3

MnO

MgO

CaO

Na2O

K2O

P2O5

Cr2O3

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

48.60 48.94(9) 0.7

0.38 0.38(0) 0.7

14.88 14.88(2) 0.0

9.23 9.26(2) 0.3

0.16 0.16 (0) 0.0

11.70 11.75(2) 0.4

11.01 11.09(5) 0.7

1.35 1.32(1) 2.3

0.43 0.43 (0) 0.0

0.03 0.03(0) 0.0

0.06 0.06(0) 0.0

CANMET SO-3 Standard Ba

Ce

Co

Cs

Cu

Dy

Er

Eu

Ga

Gd

Hf

Ho

Certified Averagea % Diff b

296 276 (7) 7.0

34 34.5(4) 1.5

8 5(0) 52.9

1.2 1.1(0) 12.5

17 15 (2) 10.9

3 2.8(1) 3.7

1.8 1.7(0) 3.3

0.8 0.77 (3) 3.9

10 6 (0) 50.0

3.0 3.2(2) 5.8

4.7 4.67(3) 0.7

0.5 0.6(1) 16.7

Certifiedc Averagea % Diff b

La 17 17(0) 0.0

Lu 0.2 0.24(2) 15.5

Mo 2 2(0) 0.0

Nb 6 6(0) 0.0

Nd 18 17(0) 5.5

Ni 16 13(1) 20.0

Pr 5 4.5(3) 10.6

Rb 39 36(1) 8.6

Sm 3.5 3.5(1) 1.9

Sn 1.0 1.0(0) 0.0

Sr 217 231(5) 5.9

Ta 0.5 0.5(0) 0.0

Certifiedc Averagea % Diff b

Tb 0.5 0.54(2) 7.4

Th 3.9 3.8(1) 1.7

Tl 0.3 0.5 (0) 40.0

Tm 0.3 0.26(1) 15.4

U 1.1 1.14(6) 3.5

V 38 36(2) 6.5

W 0.6 1.0(0) 40.0

Y 17 15.7(5) 8.5

Yb 1.6 1.7(1) 5.9

Zn 52 46 (1) 12.2

Zr 150 159(5) 5.7

c

a b c

Average of 4 runs conducted during analysis period; one standard error of the mean shown in parenthesis in terms of lowest decimal value. Percent difference between certified value and average analytical result is best estimate of the analytical error for each element or oxide. Certified values from CANMET Certificate of analysis shown in bold. All other values are “for information only” per CALMET Report 79–1.

Table A.3 Measured vs. certified values for XRF standards used by SGS Minerals Services. CANMET SO-3 Standard

Certifieda Averageb % Diff c

Certifieda Averageb % Diff c

a

Certified Averageb % Diff c a b c

Ba

Ce

Co

Cs

Cu

Dy

Er

Eu

Ga

Gd

Hf

Ho

296 276 (7) 7.0

34 34.5(4) 1.5

8 5(0) 52.9

1.2 1.1(0) 12.5

17 15 (2) 10.9

3 2.8(1) 3.7

1.8 1.7(0) 3.3

0.8 0.77 (3) 3.9

10 6 (0) 50.0

3.0 3.2(2) 5.8

4.7 4.67(3) 0.7

0.5 0.6(1) 16.7

La

Lu

Mo

Nb

Nd

Ni

Pr

Rb

Sm

Sn

Sr

Ta

17 17(0) 0.0

0.2 0.24(2) 15.5

2 2(0) 0.0

6 6(0) 0.0

18 17(0) 5.5

16 13(1) 20.0

5 4.5(3) 10.6

39 36(1) 8.6

3.5 3.5(1) 1.9

1.0 1.0(0) 0.0

217 231(5) 5.9

0.5 0.5(0) 0.0

Tb

Th

Tl

Tm

U

V

W

Y

Yb

Zn

Zr

0.5 0.54(2) 7.4

3.9 3.8(1) 1.7

0.3 0.5 (0) 40.0

0.3 0.26(1) 15.4

1.1 1.14(6) 3.5

38 36(2) 6.5

0.6 1.0(0) 40.0

17 15.7(5) 8.5

1.6 1.7(1) 5.9

52 46 (1) 12.2

150 159(5) 5.7

Certified values from CANMET Certificate of Analysis are shown in bold. All other values are “for information only” per CALMET Report 79–1. Average of 4 runs conducted during analysis period; one standard error of the mean shown in parenthesis in terms of lowest decimal value Percent difference between certified value and average analytical result is best estimate of the analytical uncertainty for each element or oxide.

A.2. Mineral chemistry Representative microprobe analyses of enclave biotites, amphiboles, and feldspars were determined by wavelength-dispersive analyses using the CAMECA MBX and CAMECA SX 100 Scanning Electron Microprobes at the NASA Johnson Space Center, Houston, Texas. Only minerals that appeared (visually) to be free of alteration were analyzed. Two (2) to 5 analyses were performed on each grain. Counting times were optimized to ensure lowest possible counting errors while minimizing grain damage. In all cases, Na was counted first with X-ray intensities carefully monitored to ensure no significant loss in Na. Standard operating conditions consisted of a focused beam approximately 1 μm in diameter with an acceleration potential of

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15 kV and a sample current between 25 and 30 nA. Counting times were set for 20 s for major elements, F, and Cl, and 50 s for Fe and Mn. Raw data were corrected for atomic number, absorption, and fluorescence using Cameca (PAP) software (with correction factors of Bence and Albee, 1968). Analytical uncertainty was estimated by comparison of measured to certified values from known standards analyzed during the same period as the enclave samples. Analytical uncertainty in the alkali elements is estimated to be 5–7% for K, and 8–10% for Na. For all other major elements analytical errors are estimated to be less than 1%. Appendix B. Enclave supplementary data

Table B1 Representative microprobe analyses of plagioclase and K-feldspar, Lone Grove pluton, see Table 2. Sample

E-4A (3)

E-4A (2)

E-4A (4)

E-4A (3)

E-4A (2)

E-4A (3)

Texture

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

SiO2 Al2O3 CaO Na2O K2O Total Or Ab An

64.97 23.32 3.40 9.75 0.15 101.59 0.01 0.83 0.16

65.94 23.09 3.07 9.65 0.12 101.87 0.01 0.84 0.15

64.88 23.40 3.47 9.67 0.21 101.63 0.01 0.83 0.16

64.67 19.21 0.00 0.89 16.19 100.96 0.92 0.08 0.00

65.23 19.27 0.00 1.13 15.74 101.37 0.90 0.10 0.00

64.33 19.11 0.00 0.86 16.38 100.68 0.93 0.07 0.00

Or = orthoclase, Ab = albite, An = anorthite, Mg-g = microgranular to granular. E-4A = enclave sample 4A, (3) = number of microprobe analyses per mineral grain.

Table B2 Representative microprobe analyses of enclave biotite and amphibole, Lone Grove pluton, see Table 2. Sample

E-4A biotite (4)

E-4A biotite (4)

E-4A biotite (4)

E-4A biotite (4)

E-4A amphibole (4)

E-4A amphibole (4)

E-4A amphibole (4)

E-4A amphibole (4)

Texture

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

Mg-g

SiO2 TiO2 Al2O3 FeOT MnO MgO CaO Na2O K2O P2O5 F Cl Total

34.67 3.28 12.97 32.07 0.27 3.66 0.01 0.20 9.18 0.00 1.22 0.28 97.80

34.42 3.40 12.83 32.07 0.27 3.67 0.04 0.20 9.18 0.00 1.11 0.28 97.48

34.51 3.39 12.95 31.87 0.27 3.62 0.00 0.19 9.32 0.00 1.19 0.27 97.56

35.29 3.12 13.09 31.53 0.23 4.32 0.06 0.21 9.14 0.00 1.34 0.28 98.59

38.30 1.51 9.45 30.10 0.57 3.30 10.18 1.93 1.50 0.05 0.73 0.28 96.87

39.58 1.30 9.77 30.30 0.53 2.33 10.32 1.87 1.49 0.05 0.63 0.30 98.48

39.59 1.53 9.40 30.29 0.56 2.36 10.28 1.90 1.46 0.05 0.68 0.28 98.37

39.57 1.45 9.73 30.25 0.55 2.25 10.33 1.92 1.55 0.06 0.61 0.28 98.54

Mg-g = microgranular to granular, E-4A = enclave sample 4A, (3) = number of microprobe analyses per mineral grain.

Key to the Marble Falls, Kingsland, and Lone Grove magmatic enclave analyzed sample locations Marble Falls pluton (all numbers preceded by MFE) 42, 45, 51, 52, and 53; Granite Mtn. quarry (Posted) along R.R.1431 NW from Marble Fall Texas, 2.6 km (1.6 miles) from intersection U.S. 281 and R.R. 1431 in Marble Falls to roadside pull off, Marble Falls Quadrangle, 98° 17′ 53″, 30° 35′ 25″–30″. 50A, 50B, and 50C; Quarry (Posted) along NE side of R.R.1431, 3.4 km (2.1 miles) NW from intersection of R.R.1431 and Co. Road 416 (Valley View Ln, Granite Shoals, Texas), Dunman Mtn. Quadrangle, 98° 22′ 53″, 30° 36′ 41″. 55; Quarry (Posted) 1.2 km (0.75 miles) south along Wirtz Dam road and R.R. 1431 intersection, Marble Falls Quadrangle, 98° 19′ 41″, 30° 35′ 13″. Kingsland pluton (all numbers preceded by KL) 2E-1, 2E-2; intersection of ranch road 1431 and F.M. 2342, 3.25 km (2 miles) NE on F.M. 2342, left onto Log Country Cove [0.3 km (0.2 miles)], right for 0.2 km (0.1 miles); Kingsland Quadrangle, 98° 23′ 48″, 30° 40′ 27″. E5-1, E5-2; intersection of R.R.1431 and F.M. 2342, 1 km (0.6 miles) NE along F.M. 2342, private property (Posted), Quarry located 0.9 km (0.6 miles) SE along “wagon” road, Kingsland Quadrangle, 98° 24′ 43″, 30° 39′ 32″. E8-1; intersection of R.R. 1431 and R.R. 3404, 2.25 km (1.4 mile) west on R.R. 3404 and 0.2 km (0.1 miles south of R.R. 3404), Kingsland Quadrangle, 98° 29′ 37″, 30° 41′. E11(1–1); E11(2–1), and E11(2–2); intersection of R.R. 1431 and R.R. 2545, 4.6 km (2.85 miles) NE on R.R. 2545,Quarry (Posted) 0.2 km (0.1 mile) west off R.R. 2545, Kingsland Quadrangle, 98° 25′ 24″, 30° 41′ 18″.

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Lone Grove pluton (all numbers preceded by LGE) 1; intersection of Co. Road 216 and State Highway 29; 0.7 km (0.45 mi) northwest on Co. Road 216 to Petrick quarry, Kingsland Quadrangle, 98° 26′ 47″, 30° 44′ 19″. 2, 3; quarries (Eachus Ranch) located 1.45 km (0.9 miles) west of State Highway 29 and R.R. Highway 1461 intersection and 0.3 km (0.19 miles) south of State Highway 29, Kingsland Quadrangle, 98° 28′ 47″, 30° 43′ 48″. 4A, 4B, 4C, and 4D; quarries (Eachus Ranch) located 1.45 km (0.9 miles) west of State Highway 29 and R.R. Highway 1461 intersection and 0.3 km (0.19 miles) south of State Highway 29, Kingsland Quadrangle, 98° 28′ 46″, 30° 43′ 48″. References Anders, E., Grevesse, N., 1989. Abundances of the elements; meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197–214. Anderson, J.L., 1983. Proterozoic anorogenic granite plutonism of North America. In: Medaris Jr., L.G., Byers, C.W., Mickelson, D.M., Shanks, W.C. 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