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Evidence from Peridotite Xenoliths, Knippa, Texas

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Evidence from Peridotite Xenoliths, Knippa, Texas Making the Southern Margin of Laurentia themed issue Composition of the mantle lithosphere beneath south-central Laurentia: Evidence from peridotite xenoliths, Knippa, Texas Urmidola Raye1,*, Elizabeth Y. Anthony2, Robert J. Stern1, Jun-Ichi Kimura3, Minghua Ren2, Chang Qing3, and Kenichiro Tani3 1Department of Geosciences, University of Texas at Dallas, 800 W. Campbell Road, Richardson, Texas 75080, USA 2Department of Geological Sciences, University of Texas at El Paso, 500 W. University Avenue, El Paso, Texas 79968-0555, USA 3Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan ABSTRACT Calculated mean seismic velocities Vs = GEOLOGIC SETTING 4.44 km/sec and Vp =7.87 km/sec show no Mantle xenoliths in ~83 Ma basanites systematic difference between lherzolites The continental crust of Texas—and much from south-central Texas provide a rare and harz burgites, and agree with present of southern Laurentia—was generated as part of opportunity to examine the lithospheric geophysical measure ments of upper mantle the ~1.37 Ga southern granite-rhyolite province mantle beneath southern Laurentia. These velocity beneath Texas. The seismic veloci- (Fig. 1) (Anthony, 2005; Barnes et al., 2002; peridotites represent lithosphere at the ties calculated for these samples will provide Bickford et al., 2000; Reese et al., 2000; Whit- boundary between Mesoproterozoic con- important constraints for interpretation of meyer and Karlstrom, 2007). In the southeast- tinental lithosphere and transitional Gulf EarthScope and other geophysical data sets. ern part of the Llano uplift (Fig. 1), dioritic and of Mexico passive margin. Here we report tonalitic gneiss are inferred to represent a 1.33– petrographic, mineral, and major element INTRODUCTION 1.30 Ga allochthonous magmatic arc (Mosher, data for 29 spinel peridotite xenoliths from 1993; Roback, 1996). This arc is thought to Knippa and use these to characterize the Mantle xenoliths entrained in alkaline mag- have accreted to Laurentia during the Grenville lithospheric mantle beneath south central mas are an important source of information orogeny at ~1.1 Ga due to N-dipping subduction Texas. The xenoliths comprise spinel-bearing about the composition and physical state of beneath Laurentia (Mosher, 1998). Young and lherzolites and harzburgites with coarse, subcontinental mantle. Abundant localities Lee (2009) studied trace-element compositions equigranular textures. Some peridotites con- are found in western North America (Wilshire of Knippa peridotites and found enrichments in tain veins of lizardite . There are no pyrox- et al., 1990). The majority of these samples fluid-mobile trace elements (e.g., La) relative to enites or eclogites . The peridotites contain are located in the Basin and Range Province fluid immobile trace elements (e.g., Nb). They olivine (Fo89-92), orthopyroxene (En89-92), clino- where Mesozoic and Cenozoic tectonic ele- concluded that these trace-element patterns pyroxene (Wo40-45En45-49Fs3-5), and spinel . ments have overprinted Precambrian litho- were caused by subduction-related fluid meta- Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzo- spheric formation. Very few sample localities somatism that modified previously melt-depleted lites (Cr# = 0.14–0.21) and harz burgites are found in tectonic provinces that represent continental lithosphere. They suggested that the (Cr# = 0.25–0.36). Mineral and major ele- the southern edge of Laurentia. The Knippa continental lithospheric mantle represented by ment compositions indicate that the lherzo- locality, located in central Texas, is one such these xenoliths may have been the upper plate lites are residues after <10% melt extraction example. The locality lies within the Bal- during Mesoproterozoic subduction. from primitive upper mantle and the harz- cones igneous province (Fig. 1) at the nexus of Following Mesoproterozoic subduction and burgites formed by <15% melt extraction. Mesoproterozoic and transitional lithosphere the Grenville orogeny, the lithosphere of southern Calculated oxygen fugacities indicate equili- of the Gulf coastal plain. The lithosphere was Laurentia was affected by three major tectonic bration of the harzburgites at –1 to +0.61 affected by the Mesoproterozoic accretion and events during Phanerozoic time—two episodes and lherzolites at 0 to –2.6 log units with subsequent Paleozoic tectonism. Character- of rifting and ocean opening separated by con- respect to fayalite-magnetite-quartz (FMQ) ization of these samples therefore informs us tinental collision (Thomas, 2006). The first rift- buffer, simi lar to lightly metasomatized of a very different set of events and permits ing episode in Early Cambrian time (~530 Ma) spinel perido tites elsewhere. The degree of comparison of subcontinental mantle across a was associated with opening of the Iapetus melt depletion and oxidation of the Knippa broad region. Ocean. During or shortly after this, a continental peridotites are consistent with present data This study reports petrographic descriptions, sliver that ultimately became the Precordillera sets for slightly metasomatized lithospheric major-element whole-rock analyses, and min- of Argentina rifted away (Thomas and Astini, mantle and/or backarc samples rather than eral compositions for these spinel-peridotite 1996). Associated with early Paleozoic ocean forearc settings. Equilibration temperatures xenoliths. We characterize the depletion his- formation, a passive continental margin formed range from 824 to 1058 °C (mean= 916 °C), tory, thermometry, oxidation state, and seismic and persisted throughout most of Paleozoic time. calculated at reference pressure of 2.0 GPa. velocities. These results complement a recent Collision of Laurentia with Gondwana during trace-element study from the same locality by the final stages of Pangea assembly resulted in *[email protected] Young and Lee (2009). the Ouachita orogeny during Pennsylvanian Geosphere; June 2011; v. 7; no. 3; p. 710–723; doi:10.1130/GES00618.1; 13 figures; 3 tables; 1 supplemental table file. 710 For permission to copy, contact [email protected] © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/710/3340191/710.pdf by guest on 28 September 2021 Composition of the mantle lithosphere beneath south-central Laurentia 102°W 100° 98° 96° 94° 92° mafic crust of Jurassic age (Mickus et al., 2009); 36°N and (4) thinned crust of southern Laurentia. The N Oklahoma Arkansas underlying lithospheric mantle may have a simi- S. OK larly complex history. aulac Knippa peridotite xenoliths are hosted by ogen basanites of the Balcones Igneous Province, which is characterized by isolated, monogenetic 34° Orogen APM Mesoproterozoic igneous centers that formed numerous small plugs, laccoliths, sills, tuff rings, and lava lakes craton (1.4 Ga) (Barker et al., 1987; Spencer, 1969). Balcones Igneous Province basanites give 40Ar/39Ar ages of ~81.5–83.5 Ma (Griffin et al., 2010). The 32° Texas xenoliths are spinel peridotites. The absence Ouachita of garnet- and plagioclase-peridotite indicates derivation from a depth range of 40 to 85 km, Llano Grenville Front (approx.) i.e., within the upper part of the subcontinental uplift ~1.1 Ga deformation Louisiana lithospheric mantle. l crust 30° P na SAMPLE DESCRIPTION BI itio ns ra Spinel-peridotite xenoliths were collected Knippa T from the Vulcan quarry, situated in Knippa, Mexico Uvalde County, Texas (29.278°N, 99.657°W). 28° 200 km Host basanites are dense, black, and fresh. Splitting time These lavas are aphyric to sparsely phyric in Gulf of Mexico hand speci men, and in thin section have scat- Jurassic oceanic tered pheno crysts (90% olivine and 10% clino- lithosphere pyroxene). The black groundmass contains finely 0.5 s 1.0 s 1.5 s dispersed clinopyroxene, plagioclase, olivine, 26° nepheline, titaniferous magnetite, melilite, zeo- Figure 1. Location of Knippa mantle xenolith locality in south-central Texas, showing sim- lite, amphibole, phlogopite, and apatite. Mg# plified crustal provinces. Knippa peridotite xenoliths are hosted by ~83 Ma basanites that ( = Mg/Mg + Fe) of the basanites range from erupted along the lithospheric discontinuity separating Mesoproterozoic lithosphere of the 0.67 to 0.75, have low Ni and Cr abundances, Texas craton and the Jurassic transitional lithosphere of the NW Gulf of Mexico passive and show strong, light rare-earth element margin. The Ouachita orogen approximates the boundary between the North American (LREE) enrichment, with chondrite-normalized craton to the north and west and transitional crust to the east and south. Geophysical stud- (La/Yb)N = 19–24 (Griffin et al., 2010). ies show that orientation and magnitude of splits correlate to crustal provinces (Gao et al., The ultramafic xenoliths are dispersed in 2008). The rapid variation in splitting delay times from Llano uplift to southeastward might the basanite, where they comprise 5%–10% of be either due to different degree of alignment of the crystals’ fast axes or to difference in the total rock volume. They have subspherical thickness of the anisotropic layer (Satsukawa et al., 2010). APM—apparent plate motion; or ellipsoidal forms, with long axes ranging BIP—Balcones Igneous Province. from 1 to 6 cm. They are usually well rounded, although some are polygonal. Host rock-xeno- lith contacts are sharp in most cases, but some show reaction rims suggesting interaction
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