Petrogenesis of Mesoproterozoic granitic plutons, eastern Llano Uplift, central Texas, USA

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• 作者：R.K. Smith ; Walt Gray ; Tyson Gibbs ; M.A. Gallegos
• 关键词：Mesoproterozoic ; Grenville ; Llano Uplift ; Geochemistry ; Granite ; A-type ; Texas
• 刊名：Lithos
• 出版年：2010
• 出版时间：August 2010
• 年：2010
• 卷：118
• 期：3-4
• 页码：238-254
• 全文大小：2958 K

文摘

The Llano Uplift of central Texas is a gentle structural dome exposing  1370 to 1230 Ma metaigneous and metasedimentary rocks of Grenville affinity along the southern margin of Laurentia. The metamorphic rocks were subsequently intruded by  1119 to 1070 Ma late syn- to post-tectonic granites collectively known as the Town Mountain Granite (TMG). The eastern most of the TMG, the Marble Falls (MF), Kingsland (KL), and Lone Grove (LG) plutons, are metaluminous to marginally peraluminous, high-K, calc-alkaline, ferroan, biotite-calcic amphibole granites [Fe/(Fe + Mg) = 0.71–0.92 and 0.78–0.91 for biotite and calcic amphibole, respectively] displaying distinct variation trends with increasing silica content. They are chemically and texturally zoned and have mineralogical and chemical characteristics similar to A-type granites; i.e., 1) Fe-rich biotites, calcic amphiboles, accessory fluorite, and sporadic rapakivi texture, 2) high K₂O (> 4 wt. % ), 3) low Al₂O₃ (< 16 wt. % ) and CaO (< 3 wt. % ), 4) high Fe/(Fe + Mg), 5) enrichments in Zr, Nb, REE, Ga/Al, and 6) depleted Eu. However, in contrast to typical A-type granites (having low Sr and Ba) the MF, KL,and LG plutons are enriched in Sr and Ba; i.e., up to 229 ppm and 1090 ppm, respectively.

On granite discrimination diagrams [(K₂O + Na₂O)/CaO vs. Zr + Nb + Ce + Y (ppm) and Zr (ppm) vs. Ga/Al10,000] the KL and MF plutons plot within the A-type field, whereas the LG pluton compositions are divided between A-type and fractionated granite fields (I-, S- and M-types). On tectonic discrimination diagrams (Y vs. Nb ) the MF and KL granites plot in the “within-plate” granite field, but the LG pluton plots across several fields including “within-plate” and “volcanic arc plus syn-collisional” fields. Consequently the tectonic classification on a geochemical basis for the LG pluton is unclear.

Based on thermal metamorphic mineral assemblages, normative Q–Ab–Or plots, and Q–Ab–Or–H₂O experimental data (Johannes and Holtz, 1996), crystallization temperatures and pressures are estimated to range from 750 to 850 °C and 200 to 500 MPa, respectively. The assemblage of titanite + magnetite + quartz suggests crystallization at low f_O2 [confirmed by Fe/(Fe + Mg) vs. ^[4] Al microprobe analyses of calcic amphibole] and a water content of less than 1.5 wt. % (Wones, 1989). Like other Town Mountain-type plutons, the MF, KL, and LG granites display comparable iron contents at similar alkali and silica enrichments.

Melting models (Ba vs. Sr) suggest the MF, KL, and LG plutons may have evolved from the partial melting (anatexis) of juvenile, tonalitic, lower crustal rocks, followed by plagioclase and pyroxene dominated fractionation. Nd isotopic data for the MF pluton (ε_Nd = + 3.4 at 1.06 Ga; Patchett and Ruiz, 1989) and whole-rock δ¹⁸O values for the MF, KL, and LG plutons (+ 7.0 < δ¹⁸O >+10.1‰; Rangel et al., 2008) suggest that the magmas in the eastern Llano Uplift may contain a significant mantle component, whereas relatively high δ¹⁸O values (+ 9.3 to + 9.7‰; Bebout and Carlson, 1986) for other coeval TMG rocks suggest that a significant crustal component is involved.

Whole-rock and trace-element chemistry indicate that the MF and KL plutons, along with the coarser grained textures of the LG pluton, are ‘A-type’ granites. However, with no coeval mafic dikes, syenitic compositions, or volcanic rocks it is clear that the TMG plutons do not represent anorogenic granites. The available evidence is most compatible with emplacement of the TMG plutons in a post-orogenic (Grenville), relaxation and extensional (i.e., slab breakoff) setting.