Contact metamorphism of Cretaceous sedimentary rocks near the Bee Mountain intrusion, Brewster County, Texas



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Bee Mountain, located near Study Butte, Texas, Is a Tertiary soda microsyenite body that has Intruded three Upper Cretaceous marine sedimentary formations. The intrusion of the soda microsyenite occurred at a depth of slightly more than one kilometer; the maximum load pressure was approximately 1/3 kilobar, and the average temperature produced in the country rock at the contact was slightly in excess of 400°C. On the eastern and northeastern sides of Bee Mountain, the intrusive activity has altered the structure, texture, and mineralogic composition of the Boquillas Formation, a series of argillaceous biomicrites and marls, and the Pen Formation, a structureless marl. Unaltered Boquillas and Pen samples were found to consist of calcite, calcium montmorillonite, and various amounts of analcime, volcanic glass, and terrigenous material, Textural alterations of these sedimentary rocks involved the recrystallization of micritic and fossil calcite, redistribution of analcime and iron minerals, and a loss of friability due to the replacement of clay minerals by calcium silicates. Changes in mineralogic composition due to elevated temperatures were studied by examining samples collected at various distances from the intrusive-sedimentary rock contact. These mineralogic changes took place in three distinct stages. Incipient contact metamorphism of the Boquillas and Pen is indicated by the presence of the calcium-bearing brittle mica xanthophyllite, and sodic plagioclase which was derived from analcime and quartz. Experimental studies indicate that sodic plagioclase can be produced from analcime at temperatures as low as 190°C. In the intermediate stage of metamorphism, montmorillonite and xanthophyllite were replaced by prehnite, which is stable at temperatures slightly less than 300°C. The highest-temperature metamorphic mineral assemblage present in the Bee Mountain area consists of grossularite, which is sometimes accompanied by zoisite and epidote. At 1/3 kilobar pH2O, prehnite will decompose to form these minerals at 370°C, a temperature which was most likely attained near the intrusive contacts. Metasomatic replacement of calcite by apophyllite and quartz occurred in some rocks, all of which are near intrusive contacts.