The Tancítaro-Nueva Italia region (~4400 km 2 ) is located within the west-central Mexican volcanic belt and spans ~80 km of arc length along the volcanic front. On the basis of 65 40 Ar/ 39 Ar ages, it is shown that a total volume of ~326 ± 57 km 3 of magma has erupted from >200 vents since 1.2 Ma. Nearly a third of this volume (103 ± 5 km 3 ) was erupted as crystal-rich andesite from a large stratovolcano, Volcán Tancítaro, whereas the remaining two-thirds (~223 ± 52 km 3 ) span the complete range from 51 to 63 wt% SiO 2 , are relatively crystal-poor, and were erupted from peripheral vents. The total proportions of erupted lava types are ~6%-7% basalt, ~18% basaltic andesite, and ~74%-76% andesite; there is no rhyolite and only a trace of dacite.There is no evidence that any signifi cant magma differentiation occurred in the upper crust along this arc segment. Based on phenocryst abundances and modes, majorand trace-element data, as well as phaseequilibrium experiments from the literature, the following mechanisms for formation of andesite in this region can be ruled out:(1) crystal fractionation of basalt/basaltic andesite in the upper crust, (2) magma mixing of basalt/basaltic andesite with dacite/ rhyolite, and (3) assimilation of granitoid by basalt/ basaltic andesite. Instead, the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting (>20%) of hornblende-rich (~40%) gabbronorite. The basaltic andesites are required, owing to their elevated Ni concentrations, to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two-pyroxene fractionation in the deep crust.
The Mars Hill terrane (MHT), a lithologically diverse belt exposed between Roan Mountain, North Carolina-Tennessee, and Asheville, North Carolina, is distinct in age, metamorphic history, and protoliths from the structurally overlying Eastern Blue Ridge and underlying Western Blue Ridge. MHT lithologies include diverse granitic gneisses, abundant mafic and sparse ultramafic bodies, and mildly to strongly aluminous paragneisses. These lithologies experienced metamorphism in the granulite facies and are intimately interspersed on cm to km scale, reflecting both intrusive and tectonic juxtaposition. Previous analyses of zircons by high-resolution ion microprobe verified the presence of Paleoproterozoic orthogneiss (1.8 Ga). New data document a major magmatic event at 1.20 Ga. Inherited and detrital zircons ranging in age from1.3 to 1.9 Ga (plus a single 2.7-Ga core), ubiquitous Sm-Nd depleted mantle model ages ca. 2.0 Ga, and strongly negative ε ε Nd during Mesoproterozoic time all attest to the pre-Grenville heritage of this crust that was suggested by previous whole-rock Pb and Rb-Sr isotope studies. A single garnet amphibolite yielded a magmatic age of 0.73 Ga, equivalent to the Bakersville dike swarm, which cuts both the MHT and the adjacent Western Blue Ridge. Zircons from this sample display 0.47-Ga metamorphic rims. Zircons from all other samples have well-developed ca. 1.0-Ga metamorphic rims that date granulitefacies metamorphism. Silica contents of analyzed samples range from 45 to 76 wt %, reflecting the extreme diversity observed in the field and the highly variable protoliths. The MHT contrasts strikingly with basement of the adjacent Eastern and Western Blue Ridge, which comprise relatively homogeneous, 1.1-to 1.2-Ga granitic rocks
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