New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships

Abstract

Research Article| December 01, 1992 New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships STEPHEN T. NELSON; STEPHEN T. NELSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar JON P. DAVIDSON; JON P. DAVIDSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar KIM R. SULLIVAN KIM R. SULLIVAN 2Department of Geology, Brigham Young University, Provo, Utah 84602 Search for other works by this author on: GSW Google Scholar Author and Article Information STEPHEN T. NELSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 JON P. DAVIDSON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 KIM R. SULLIVAN 2Department of Geology, Brigham Young University, Provo, Utah 84602 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (12): 1547–1560. https://doi.org/10.1130/0016-7606(1992)104<1547:NADOCC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEPHEN T. NELSON, JON P. DAVIDSON, KIM R. SULLIVAN; New age determinations of central Colorado Plateau laccoliths, Utah: Recognizing disturbed K-Ar systematics and re-evaluating tectonomagmatic relationships. GSA Bulletin 1992;; 104 (12): 1547–1560. doi: https://doi.org/10.1130/0016-7606(1992)104<1547:NADOCC>2.3.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 Published K-Ar ages of laccoliths of the Henry and La Sal Mountains range from 23 to 56 Ma, with a bimodal distribution in the Eocene and late Oligocene epochs. Eocene ages require a period of magmatism during the Laramide Orogeny, a time of little or no igneous activity in the western United States at these latitudes. On the basis of 40Ar/39Ar and fission-track ages, we conclude that intrusive activity occurred during middle to late Oligocene and early Miocene time (31-20 Ma) in the Henry, La Sal, and Abajo Mountains. These results imply a genetic link to magmatism in the Reno-Marysvale and San Juan volcanic fields, which lie to the west and east of the Colorado Plateau, respectively. We interpret the small volume of the laccoliths, relative to the surrounding volcanic fields, to reflect differences in crustal composition and tectonic history of the plateau and adjacent areas.Eocene K-Ar ages result from excess 40Ar or contamination by xenocrysts. Excess argon is manifested by trapped argon with 40Ar/36Ar ratios greater than atmosphere (295.5), resulting in total-gas or apparent K-Ar ages that are too old. Laser-fusion experiments and microprobe data show that some samples with Eocene or older total-gas ages and uncorrelated inverse correlation plots are affected by radiogenic argon residing in incompletely outgassed hornblende xenocrysts. All such samples contain middle- to late-Oligocene magmatic hornblende crystals as revealed by laser-fusion experiments. Xenocrysts may be derived from amphibolite xenoliths common in these rocks. Mixing calculations indicate that only 1%-4% contamination may be required to increase K-Ar ages by 10-15 Ma for xenocrysts outgassed as much as 75%. The 40Ar/39Ar method is capable of identifying small degrees of magmatic contamination by older K-bearing phases. 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.