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Preliminary Petrographic, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana

1975; Geological Society of America; Volume: 86; Issue: 3 Linguagem: Inglês

10.1130/0016-7606(1975)86 2.0.co;2

1943-2674

E‐an Zen, Richard F. Marvin, Harald H. Mehnert,

Geochemistry and Geologic Mapping

Research Article| March 01, 1975 Preliminary Petrographic, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana E-AN ZEN; E-AN ZEN 1U.S. Geological Survey, Reston, Virginia 22092 Search for other works by this author on: GSW Google Scholar R. F. MARVIN; R. F. MARVIN 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar H. H. MEHNERT H. H. MEHNERT 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1975) 86 (3): 367–370. https://doi.org/10.1130/0016-7606(1975)86 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 MailTo Tools Icon Tools Get Permissions Search Site Citation E-AN ZEN, R. F. MARVIN, H. H. MEHNERT; Preliminary Petrographic, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana. GSA Bulletin 1975;; 86 (3): 367–370. doi: https://doi.org/10.1130/0016-7606(1975)86 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 Petrographic and chemical data on five types of intrusive rocks of the so-called Pioneer batholith in the northeastern Pioneer Mountains of southwest Montana show that the rocks range from quartz diorite to granite. The volumetrically most important body is a coarse-grained biotite-hornblende “granite” that superficially resembles the Butte Quartz Monzonite of the Boulder batholith, some 60 km to the northeast. Their K2O content puts the rocks into Tilling's “sodic series” of the Boulder batholith. K-Ar dating on biotite and hornblende shows that the date of intrusion of all but the quartz diorite is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its biotite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Formation (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg batholiths and show the same tendency for the less mafic rocks to be younger. 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.