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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.

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