The Rattlesnake Mountain intrusion in the Big Bend region of Texas is an early Tertiary analcime-bearing monzonite sill about 80 m thick that was injected at shallow depth and underwent differentiation in place. It contains sheets, lamellae, cylindroidal masses, and ocelli of syenite that show systematic distribution within the monzonite. Lamellae occur in the contact rocks, cylinders are confined to the lower central zone, and sheets are concentrated in the upper central zone. Ocelli are found only in a narrow zone near the top of the sill.

All rocks of the sill display similar mineral assemblages with strong compositional zonation of individual mineral species. In particular, feldspars range from intermediate plagioclase, through anorthoclase, to Na-rich sanidine, and finally, to albite plus K-feldspar. Pyroxenes show a general trend from essentially normal augite to increasingly iron-plus-sodium–enriched varieties.

Over-all crystal settling did not cause formation of syenitic rocks, but the central zone of the sill contains a relatively mafic monzonite that is complementary in composition and amount to the enclosed bodies of syenite. Border zones have little syenite (1 percent), and the monzonite composition is the same as a mixture of the more mafic central monzonite and enclosed syenitic rocks in the proportions in which they occur in the central zone.

Crystal fractionation played a dominant role in generating syenitic rest liquids that could be aggregated into the structures found in the sill; rifting of a partially solid crystal mesh explains the formation of lamellae and many sheets, but the cylindroidal masses and ocelli are more enigmatic.

The rocks of the Rattlesnake Mountain sill, as well as others of Big Bend, show distinct chemical similarities to alkalic oceanic suites such as those of the Azores. The syenite of the sill is typical of the undersaturated felsic rocks found in many shallow mafic intrusions in Big Bend, and it is in contrast to oversaturated quartz-bearing microsyenite, microgranite, and extrusive equivalents that occur as separate masses unconnected with more mafic rocks. There appear to be two differentiation trends related, perhaps, to undersaturated and saturated basaltic lava flows known in the area and possibly also affected by depth at which differentiation occurred.

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