Geology of the Rattlesnake Mountain intrusion, Big Bend National Park, Texas

Date

1969

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Abstract

The Rattlesnake Mountain Intrusion is a zoned sill, 200-300 feet thick, that, is composed of analcime-bearing syenodiorite and that contains sheets and other bodies of syenite. Cross-cutting and chilled relations in the border rocks indicate at least three pulses of magma during emplacement of the intrusion. On the basis of certain internal features, such as apparent layering in the syenodiorite and a regular spatial distribution of certain types of syenitic bodies, the intrusion has been divided vertically into five zones. In ascending order these are: (1) Lower Border Zone, (2) Stair-Stepped Zone, (3) Syenite Cylinder Zone, (4) Syenite Zone, and (5) Upper Border Zone. The bulk of the intrusion is composed of syenodiorite, a fine- to medium-grained rock composed essentially of plagioclase and alkali feldspar with subequal amounts of olivine, augite, biotite, and iron oxides and accessory analcime. Most exposures are weathered to a brown friable state but where fresh, the rock most commonly is dark grey-green as a result of deuteric alteration. The syenite is more resistant to weathering than the syenodiorite, is generally fresher, and is composed principally of a potassium-rich alkali feldspar. On the bases of texture and mineralogy approximately 80% of the syenites can be placed in one of two groups: (1) Coarser- grained syenite, and (2) Finer-grained syenite. The first group contains large ophitic intergrowths of plagioclase and augite, complexly-mantled feldspar crystals with plagioclase cores, and has an average grain size of 2-6 mm. [...]

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Keywords

National parks, Big Bend National Park, Texas

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