Petrology of Rattlesnake Mountain Sill, Big Bend National Park, Texas

Abstract

Research Article| February 01, 1975 Petrology of Rattlesnake Mountain Sill, Big Bend National Park, Texas MAX F. CARMAN, JR.; MAX F. CARMAN, JR. 1Geology Department, University of Houston, Houston, Texas 77004 Search for other works by this author on: GSW Google Scholar MARYELLEN CAMERON; MARYELLEN CAMERON 2Board of Earth Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar BERNARD GUNN; BERNARD GUNN 3Geology Department, Université de Montréal, Montréal 101, Quebec, Canada Search for other works by this author on: GSW Google Scholar KENNETH L. CAMERON; KENNETH L. CAMERON 2Board of Earth Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar JOHN C. BUTLER JOHN C. BUTLER 1Geology Department, University of Houston, Houston, Texas 77004 Search for other works by this author on: GSW Google Scholar Author and Article Information MAX F. CARMAN, JR. 1Geology Department, University of Houston, Houston, Texas 77004 MARYELLEN CAMERON 2Board of Earth Sciences, University of California, Santa Cruz, California 95064 BERNARD GUNN 3Geology Department, Université de Montréal, Montréal 101, Quebec, Canada KENNETH L. CAMERON 2Board of Earth Sciences, University of California, Santa Cruz, California 95064 JOHN C. BUTLER 1Geology Department, University of Houston, Houston, Texas 77004 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (2): 177–193. https://doi.org/10.1130/0016-7606(1975)86<177:PORMSB>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation MAX F. CARMAN, MARYELLEN CAMERON, BERNARD GUNN, KENNETH L. CAMERON, JOHN C. BUTLER; Petrology of Rattlesnake Mountain Sill, Big Bend National Park, Texas. GSA Bulletin 1975;; 86 (2): 177–193. doi: https://doi.org/10.1130/0016-7606(1975)86<177:PORMSB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.