Abstract

Spatial distribution of isotopic ages of igneous rocks in northern Mexico shows that a magmatic arc commenced at 140 m.y. B.P. near a paleotrench, progressed 1,000 km eastward by 40 m.y. B.P., and then regressed to within 200 km of the continental margin by 18 m.y. B.P. During progression, a calcic and calc-alkaline continental-margin volcano-plutonic regime developed, but farther east, high-K calc-alkaline and alkaline facies formed in northeastern Mexico and west Texas. Following regression, calc-alkaline volcanism was renewed on the Pacific margin at ∼18 m.y. B.P. and continued until cessation of subduction by initiation of transform movements between Pacific and American plates. Benioff zone depth estimates from K57.5 relationships suggest decreasing dip during progression and a variable-dip, variable-depth regime from 140 to 16 m.y. B.P. Both the dip-angle-convergence-rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m.y. B.P., followed by decreasing convergence and sinking rates from 42 to 16 m.y. B.P. Changes in subduction dynamics are attributed to plate reorganization at 44 m.y. B.P. and the approach of the East Pacific Rise. Differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time Cu-(Mo-W) ores (106 to 40 m.y. old) in calc-alkaline magmas formed during progression, whereas Mo and CaF2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths, and Pb-Zn-Ag deposits (49-26 m.y. B.P.) were formed during late arc progression and early regression. These geodynamic variations, plus Sr, Rb, and S isotopic characteristics of the arc terrane, suggest that processes associated with subduction and some crustal contamination, rather than local compositional anomalies in crust or mantle, account for different magma and mineral-deposit suites. Southern Mexico exhibits Fe and Cu zones, whereas the overlapping calc-alkaline trans-Mexican-Chiapanecan arc of late Miocene to Holocene age contains several base- and precious-metal deposits.

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