The Henry Mountains represent one of three major igneous centers of the Colorado Plateau interior during mid-Tertiary (25--30 Ma) time. These intrusions occur ~1000 km from the paleotrench and are unrelated to subduction in a classical sense, despite many compositional similarities to orogenic magmatic rocks. Most of the intrusive volume (95%) consists of plagioclase-hornblende porphyry of intermediate composition, with minor syenite making up the remainder, and both suites appear to have cooled rapidly as evidenced by the fine-grained texture of their groundmass. Although many elemental variations can be explained by fractional crystallization, Sr, Nd, Pb, and O isotope systematics require open system interaction of mantle derived magmas and Proterozoic amphibolite crust.

The plagioclase-hornblende porphyry evolved via assimilation fractional crystallization (AFC) in deep-crustal magma chambers. Syenite porphyry evolved in two distinct stages. First, AFC produced a shallow, zoned, subintrusive magma chamber containing Ne-normative syenite magma. Second, as magma was withdrawn, batches of Ne-normative magma mixed with tonalitic melts produced by fusion of amphibolite country rock to produce Q-normative syenite porphyry. Trace element abundance patterns suggest that both plagioclase-hornblende and syenite porphyry were derived from the same mantle source. A higher degree of partial melting may explain the silica-oversaturated character and lower trace element abundances in the plagioclase-hornblende porphyry relative to the syenite porphyry. Trace-element systematics also indicate that high large ion lithophile (LILE) to high field strength element (HFSE) ratios, typical of subduction-derived magmas, are characteristic of the source and have not been imposed solely by crustal contamination. Therefore, the porphyries of the Henry Mountains appear to be related to contemporaneous voluminous regional magmatism of the western United States as part of a large-scale igneous system with arc-like affinities. The relatively minor volume of igneous rocks implies that the Colorado Plateau acted as a structural barrier to the ascent of magma to high crustal levels. ¿ American Geophysical Union 1993