Electronic Supplementary Materials

Parker and White

Whole rock chemical analyses

We report 55 whole-rock major and trace element analyses from the Davis Mountains (5 Basin and Range basalt; 2 Tertiary mafic lava; 3 Buckhorn trachyte; 4 volcanic domes; 10 Gomez Tuff; 9 Adobe Canyon Formation; 5 “other” Adobe Canyon Formation; 17 Star Mountain Formation and one plagioclase rhyolite cobble) and 17 from Pantelleria (representative analyses given in Supplemental Table 1.; entire data set also included as separate table). Davis Mountain samples were mostly collected in the 1980’s (Parker, 1986; Henry et al., 1989) with ten new samples collected in the late 1990’s. Thirty seven samples were analyzed for major and trace elements by WD-XRF at Baylor University; of these, 17 were also analyzed by INAA at ACTLABS, Canada (and a few at Kansas State University). For this study, 21 Davis Mountain samples, including a number of older samples that had not previously been analyzed, were selected for analysis by ICP-OS (majors) and ICP-MS (trace elements) at ACTLABS, Canada. C.D. Henry and E.W. James graciously added three unpublished ICP analyses of Basin and Range basalt. Seventeen Pantelleria samples were collected in two visits (1998 and 2003); these include 11 ICP analyses and six XRF analyses; five of these 17 were reported in White et al. (2003). Replicate analyses of several samples by different methods were in good agreement; a comparison of these methods was given in White et al. (2003).

Mineral Chemistry

Electron probe analysis (361 points) of Davis Mountain samples was accomplished at the University of Texas at Austin using either a JEOL 733 or an older ARL instrument, or, for the 1990 series samples, using a CAMECA Camebax instrument at Baylor University (Supplemental Table 2.xls). About half of these data were reported in Parker (1986) and Henry et al. (1989). Conditions of analysis were similar and duplicate analyses of points were in good agreement. Additional point analyses (~170) from Pantelleria samples were accomplished using the Baylor facility. We also report an average analysis of Pantelleria richterite done at the University of Texas at El Paso Cameca SX-50 facility.

Mineralogy

Northern Davis Mountains rocks contain relatively simple phenocryst mineralogies. Mafic units (basaltic trachyandesite and trachyandesite) contain plagioclase (Ab28-53), augite, biotite and magnetite. Trachyte typically is strongly glomeroporphyritic with ternary feldspar mantled by anorthoclase. Quartz trachyte and low-silica rhyolite of SMF contain anorthoclase (Or21-46) with An up to 12 weight percent. Anorthoclase of peralkalic rhyolite (Gomez Tuff – Or37+1; ACF – Or36-43) has a more limited compositional range with very low An content (Appendix Table 2). Partially resorbed quartz phenocrysts make up about 1 percent of the typical 10 volume percent phenocrysts in Gomez Tuff; phenocrystic quartz is absent from all other NDM units. Pyroxene in trachyte porphyry and mafic quartz trachyte is ferroaugite whereas quartz trachytes and rhyolite contain ferrohedenbergite. Fayalite was identified in three samples (two SMF

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and one Gomez Tuff). Magnetite is the typical FeTi oxide in most NDM rocks; ilmenite occurs rarely in trachyte and quartz trachyte along with magnetite; Gomez Tuff and ACF contain only ilmenite. Groundmasses of Gomez Tuff and the (nonporphyritic) dome samples contain sodic pyroxene ranging up to nearly pure acmite, arfvedsonite, and aenigmatite intergrown with alkali feldspar (Or42-46) and quartz. Groundmasses of SMF and ACF units are dominated by alkali feldspar poikillitically enclosed by quartz (Anderson, 1969), and where not oxidized, sodic amphibole. Rare vitrophyre has been located for most silicic units.

Pantelleria silicic rocks are also dominated by alkali feldspar, making up greater than 85 percent of total phenocrysts (White et al., 2005). Quartz phenocrysts occur only in the most evolved pantellerities. Mafic phenocrysts are dominated by clinopyroxene; fayalite occurs in pantelleritic trachyte and also occurs in pantellerite with A.I. < 1.61. Ilmenite is present in most samples, and co-exists with magnetite in samples with lower peralkalinity (<1.34 A.I). Aenigmatite phenocrysts occur in samples with A.I. >1.50; ferrorichterite was only found in the Green Tuff (Supplemental Table 2).

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