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Continental Rifting |
1 Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
2 Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, USA
Basic to intermediate volcanism of the Tertiary Mogollon-Datil volcanic field can be divided petrologically and geochemically into three temporal groups; Pre-30 Ma, 30–20, Ma and Post-20 Ma. The Pre-30 Ma and 30–20 Ma groups are dominated by high-K calc-alkaline andesites and mildly alkaline basaltic andesites respectively. These both have major and trace element characteristics typical of an orogenic origin. In contrast, the late Tertiary, Post-20 Ma lavas are typically alkaline basalts and have geochemical characteristics more consistent with a within-plate setting. The 30–20 Ma basic lavas have high Ba and Sr, and low Nb contents, resulting in high LIL/HFS element ratios (Ba/Nb c. 80). 87Sr/86Sr ratios are > 0.7065. These features are inferred to have been derived from continental mantle lithosphere modified by subduction-related processes in the Proterozoic. The Pre-30 Ma lavas have many similar characteristics but with Rb and Th enriched relative to Ba and Sr, lower 87Sr/86Sr, and more subalkalic parental magmas. In contrast, the Post-20 Ma lavas show a tendency to higher Nb contents (low LIL/HFS ratios), and lower 87Sr/86Sr ratios similar to OIB-like magmas derived from partial melting of the convecting asthenospheric mantle. The overall shift from predominantly lithosphere to asthenosphere-derived magmas with time in the evolution of the Mogollon-Datil volcanic field is consistent with models in which magmatism was triggered by lithosphere extension. It is concluded that calc-alkaline magmas with minor and trace element features similar to those from destructive plate margins were generated in an extensional tectonic setting. In detail, the marked change to within-plate style magmatism took place c. 10 Ma after the period of maximum extension.
