Clarion Island (CI) represents young (1–1.7 Ma) fracture zone-related volcanism in the east-central Pacific Ocean. The eastern Trans-Pecos magmatic province (TPMP) of west Texas represents continental volcanism that developed during waning Laramide compression, primarily 30 to 38 Ma. Both rock series are alkaline, with the CI rocks conforming to a trend of increased silica undersaturation, and the bulk of the TPMP rocks to a trend of increased oversaturation with respect to silica. A sodic series of alkali basalt–hawaiite–mugearite–benmoreite–trachyte occurs in both regions. On Clarion Island, the evolved rocks tend to be phonolitic. The TPMP contains an additional potassic series, alkali basalt–trachybasalt–tristanite–trachyte, as well as basanites. Modeling and petrography suggest that these sequences were produced by low-pressure crystal fractionation involving olivine, clinopyroxene, plagioclase, Ti-magnetite, amphibole, and apatite. Compositional trends for CI and TPMP rocks are very similar for many elements (e.g., Rb, Th, Ta, Eu, Sc, Ti, Ba, and REE), suggesting similar mechanisms of evolution, possibly similar sources, and an argument for minimal crustal assimilation for the TPMP alkali basalt-trachyte sequence. In addition to these silica-enrichment trends, both regions possess a trend toward silica depletion, possibly the result of fractionation of a high-pressure pyroxene-dominated assemblage. Absent from CI, but important in the TPMP rocks, is the sequence of quartz trachyte to rhyolite/comendite, produced by fractionation of alkali feldspar, clinopyroxene, Ti-magnetite, amphibole, quartz, and apatite, modified by magma replenishment and assimilation of crustal rocks or their partial melts. We regard the nature and thickness of the crust as important controls in the evolution of magmas in these two regions. Mafic magmas are similar in both regions. Low-pressure fractionation beneath Clarion Island’s thin basaltic crust produced the alkali basalt–phonolite trend, whereas fractionation beneath the TPMP’s thicker continental crust led to the alkali basalt-rhyolite trend. The rocks of CI and the eastern TPMP exhibit nearly identical ranges of (Sm/Nd) n . This variation, outside analytical uncertainty, is suggested to be the result of mixing of depleted and enriched mantle, produced during earlier melting or metasomatic events. The rocks of the eastern TPMP have Th/Ta and Rb/Ta ratios that extend from those characteristic of CI to much higher values. This variation is interpreted to have been produced by limited interaction of the ascending melt with the continental lithosphere. In both CI and the eastern TPMP, incompatible elements such as Zr and Nb have nearly constant concentrations over a range of Mg numbers from 70 to 50. This indicates that such Mg numbers may represent primary melts of heterogeneous mantle comprising different proportions of depleted and enriched (veined) mantle.

Peraluminous rhyolites that are chemically somewhat similar to topaz rhyolites and anorogenic granites occur in an orogenic setting near Sierra Blanca in the Tertiary Trans-Pecos magmatic province. The Sierra Blanca rhyolites are even more enriched in most incompatible trace elements than are topaz rhyolites. Some of the extreme enrichments may in part be the result of chemical modification by crystallization from an F-rich vapor phase. The rhyolites were intruded as laccoliths at 36 Ma, during the main phase of Trans-Pecos igneous activity, which is characterized by ash-flow eruptions from numerous calderas and widespread mafic, intermediate, and silicic intrusions. A dominant east-northeast orientation of dikes and veins throughout the region indicates mild compression that was residual from Laramide deformation. This compressive tectonic setting, coupled with concurrent volcanism in Mexico and the east-northeast change in magma chemistry from calc-alkalic in western Mexico through alkali-calcic to alkalic in Texas, suggests that the rhyolites were emplaced in a continental arc. Extension did not begin in Trans-Pecos Texas until after 32 Ma; 31- to 17-Ma dikes are dominantly oriented north-northwest, perpendicular to the direction of extension during early Basin and Range deformation. Thus, the tectonic setting of the Sierra Blanca rhyolites contrasts with that of typical topaz rhyolites, most of which were emplaced during periods of crustal extension. The Sierra Blanca rhyolites are chemical and mineralogic oddities for the region, where most rhyolites are peralkaline or metaluminous. The rhyolites are depleted in the same elements as topaz rhyolites (Mg, Ca, Ti, Sr, Ba) but are more highly evolved than topaz rhyolites. Extreme trace-element enrichments (Li, F, Zn, Rb, Y, Zr, Nb, Sn, Ta, Pb, HREE, Th, U) are accommodated in Li-rich white mica, Zn-rich biotite, Rb-rich feldspars, and numerous trace minerals, including cassiterite, changbaiite, columbite, thorite, xenotime, yttrium- and REE–rich fluorides, and zircon. The rhyolites are large-tonnage, low-grade resources of several rare metals. Also enriched in Be (as much as 180 ppm), the rhyolites are the sources of Be and F in beryllium deposits in fluoritized limestones along the contacts with the laccoliths. Interaction with the limestones probably locally elevated the Ca, Mg, and Sr contents of the rhyolites. Vapor-phase crystallization has modified the original magmatic chemistry of the rocks. Evidence of vapor-phase crystallization includes the presence of minerals typical of pegmatites: cryolite (from 0 to 3 volume percent), alkali feldspars with nearly end-member compositions, polylithionite-zinnwaldite mica, and rutilated quartz, plus fluid inclusions defining quartz overgrowths on magmatic grains. Extreme HREE enrichments (Yb to 72 ppm; chondrite-normalized REE patterns with positive slopes) may also be the result of vapor-phase crystallization.

Igneous rocks of the Sierra Madre Occidental have been studied along two traverses across the range. One is at lat 24°N between Mazatlán and Durango City, where contiguous mapped areas extend across the Sierra; the other is near lat 28°N, where several separate areas west and north of Chihuahua City have been mapped. In these regions the Sierra contains two vast and largely coextensive igneous sequences, both calc-alkalic and both including ignimbrites. The older sequence of rocks, which ranges in age from 45 m.y. to at least 100 m.y., is characterized by abundant batholithic as well as volcanic rocks and is dominantly intermediate in composition. The younger sequence is dominated by rhyodacitic to rhyolitic ignimbrites erupted from large caldera complexes, generally accompanied by rhyolite flows and domes and small outpourings of mafic lavas. Intermediate rocks are rare, and volcanism was largely confined to the interval 34 to 27 m.y. ago, although some activity persisted until 23 m.y. ago. Mid-Tertiary volcanic rocks are exposed eastward across Chihuahua between the Sierra Madre Occidental and the alkalic volcanic province of Trans-Pecos Texas. Volcanic rocks of this region are chiefly calc-alkalic in chemistry, but their alkalinity is intermediate between that of the Sierra Madre Occidental and of Trans-Pecos Texas. Ages are both similar to and older than those in the Sierra. The chronology of both igneous sequences of the Sierra Madre Occidental fits some of the known sea-floor–spreading patterns. The older sequence is similar in age and composition to batholithic terranes of the western United States that were emplaced during Cretaceous–early Tertiary convergence along the western margin of the North American plate. The younger sequence was also emplaced during a period of convergence. Volcanism declined 27 m.y. ago at about the time of reorientation of an east Pacific spreading center and early ridge-trench interaction at the nearest continental margin. The gap in magmatism between 45 and 34 m.y. and the abrupt onset of volcanic activity 34 m.y. ago, however, do not match known sea-floor–spreading events. Furthermore, the brief age span and the bimodal calc-alkalic composition of the younger sequence are not typical for an igneous province at a continental margin.

Alkalic rocks in Trans-Pecos Texas were emplaced in two distinctly different tectonic environments: one compressional (contractional) and one extensional. Rocks (Eocene and early Oligocene) of the older compressional environment can be divided into a western alkali-calcic belt and an eastern alkalic belt. The boundary between the two belts is parallel to the paleotrench that used to lie off the west coast of Mexico. The alkalic rocks were the most inland expression of subduction-generated volcanism. The predominance of east-striking dikes and veins and the orientation of en echelon dikes indicate igneous activity during residual compression remaining from Laramide deformation. As the dip of the subducting slab became gentler with time, calc-alkaline magmatism of Laramide age in Mexico graded eastward into the alkaline magmatism in Texas. Widespread extension and normal faulting began about 24 Ma in the Texas portion of the Basin and Range province. Between 24 and 17 Ma, alkalic basalts were extruded and intruded at several localities, dominantly as north-northwest-striking dikes. Both nepheline- and hypersthene-normative basalts occur in the extensional environment. Rocks of the compressional environment follow two major lines of differentiation: hypersthene-normative basalt to rhyolite and nepheline-normative basalt to phonolite. In contrast, rocks of the extensional environment are apparently limited to basalts. During contraction, magmas rising from the mantle probably formed chambers in which differentiation could occur. During extension, less differentiation occurred, either because tectonically dilated fractures permitted more direct rise of magma to the surface or because the volume of magma was too small. Basalts of the two contrasting tectonic settings are broadly similar in alkalinity and silica saturation. The basalts of the extensional environment are, however, generally richer in magnesium than are the basalts of the compressional environment. This difference is not simply a matter of degree of differentiation but is probably related to the different pressure-temperature regimes of the mantle from which the basalts originated.