Proposed assembly of the Rodinia supercontinent in the late Mesoproterozoic involved the collision of the Amazon craton with some portion of the southern or eastern margin of cratonic North America. Previously reported paleomagnetic data from the SW Amazon craton suggest a paleogeographic link between "Grenvillian" deformation of the SW Amazon craton and late Mesoproterozoic tectonometamorphism in southern Laurentia. A structural, geochronological, and petrological investigation of the western Amazon basement rocks (Rondônia, Brazil) was carried out in order to document evidence of a Grenvillian collision connecting the Amazon to Laurentia. Integration of 40Ar/39Ar data and feldspar thermometry data from regionally extensive strike‐slip mylonitic shear zones (Ji‐Paraná shear zone network) indicates that deformation took place at 450°–550°C between 1.18 and 1.15 Ga. An older, ca. 1.35‐Ga event found exclusively in less‐deformed basement rocks is interpreted as recording cooling from an earlier metamorphic episode (650°–800°C indicated by feldspar thermometry) unrelated to the Grenville collision. The style of deformation in the SW Amazon craton contrasts with that observed in southern Laurentia, where extensive crustal thickening accommodated by deep‐seated thrust sheets resulted in widespread thermal resetting of isotopic systems during exhumation and postorogenic cooling. In contrast, the predominantly strike‐slip activity observed in the Amazon resulted in age resetting through strain‐induced recrystallization, not regional‐scale thermal resetting. Consequently, the ages recorded by hornblende in the SW Amazon craton are slightly older than the cooling ages preserved in southern Laurentia. Differences in structural style and geochronological record are interpreted as indicative of an exhumed, asymmetric crustal structure similar to that of modern orogens.
Grenville rocks from a 2500 km 2 area centered on Otter Lake, Quebec (some 75 km northwest of Ottawa) are in the uppermost amphibolite to lower granulite facies; orthopyroxene occurs occasionally in both metabasic and charnockitic rocks. The temperature of metapmorphism was approximately 675 °C, based upon oxide, feldspar, and garnet–clinopyroxene thermometry. Little thermal gradient could be detected across the area. Carbonate thermometry, using reintegrated calcite compositions, yielded lower temperatures of 600 °C (maximum), while garnet–biotite and other K d thermometers yielded scattered and for the most part unreasonable results. Metamorphic pressure, calculated from the reaction anorthite = grossular + sillimanite + quartz, was 5.0 ± 0.5 kbar(500 ± 50 MPa). Similar calculations based upon the reactions garnet + quartz = anorthite + orthopyroxene and garnet + quartz = anorthite + clinopyroxene yielded pressures of 5.5–7.0 kbar (550–700 MPa). Pressure calculations based upon assemblages of cordierite–garnet–sillimanite–quartz were less precise, but agreed with the outer estimates. Similar metamorphic temperatures and slightly lower pressures have been estimated for the Adirondack Lowlands of New York. In the Morin Highlands, 100 km east of Otter Lake, and in the Adirondack Highlands, 100 km east of the Adirondack Lowlands, temperatures of metamorphism (700–800 °C) and pressures of metamorphism (6–9 kbar (600–900 MPa)) are both higher. Thus it appears that over an approximate 300 km north–south direction nearly constant metamorphic conditions prevailed at Grenville time. In the east–west direction significant variations in metamorphic grade are recorded; both temperature and pressure markedly increase to the east.
Research Article| February 01, 1980 Update on feldspar and oxide thermometry in the Adirondack Mountains, New York STEVEN R. BOHLEN; STEVEN R. BOHLEN 1Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48109. Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar ERIC J. ESSENE; ERIC J. ESSENE 1Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48109. Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar KAREN S. HOFFMAN KAREN S. HOFFMAN 1Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48109. Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (2): 110–113. https://doi.org/10.1130/0016-7606(1980)91<110:UOFAOT>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 STEVEN R. BOHLEN, ERIC J. ESSENE, KAREN S. HOFFMAN; Update on feldspar and oxide thermometry in the Adirondack Mountains, New York. GSA Bulletin 1980;; 91 (2): 110–113. doi: https://doi.org/10.1130/0016-7606(1980)91<110:UOFAOT>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 Recently obtained feldspar and oxide temperature data for orthogneiss and paragneiss in the Adirondack Mountains, New York, further constrain and generally support the regional thermometry previously established. Peak metamorphic temperatures were 700 to 750 °C throughout most of the Adirondack Highlands and 750 to 800 °C in the High Peaks region during the Granville metamorphic event. 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.
Research Article| January 01, 2006 Unusually Cu-rich magmas associated with giant porphyry copper deposits: Evidence from Bingham, Utah Daniel P. Core; Daniel P. Core 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Search for other works by this author on: GSW Google Scholar Stephen E. Kesler; Stephen E. Kesler 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Search for other works by this author on: GSW Google Scholar Eric J. Essene Eric J. Essene 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Daniel P. Core 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Stephen E. Kesler 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Eric J. Essene 1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48019, USA Publisher: Geological Society of America Received: 23 Apr 2005 Revision Received: 08 Sep 2005 Accepted: 18 Sep 2005 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2006) 34 (1): 41–44. https://doi.org/10.1130/G21813.1 Article history Received: 23 Apr 2005 Revision Received: 08 Sep 2005 Accepted: 18 Sep 2005 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Daniel P. Core, Stephen E. Kesler, Eric J. Essene; Unusually Cu-rich magmas associated with giant porphyry copper deposits: Evidence from Bingham, Utah. Geology 2006;; 34 (1): 41–44. doi: https://doi.org/10.1130/G21813.1 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 SocietyGeology Search Advanced Search Abstract Mass-balance constraints indicate that formation of giant porphyry copper deposits (PCDs) requires either highly efficient collection of Cu from large volumes of magma or unusually Cu-rich parent magmas. Support for the second of these possibilities has been discovered in the form of mafic enclaves with abundant bornite and chalcopyrite in the Last Chance stock, one of the parent intrusions of the giant Bingham PCD, Utah, United States. Mineral assemblages and compositions indicate that the Last Chance enclaves are autoliths consisting of phases that crystallized from the intrusion, and that the intrusion was unusually enriched in Cu. One possible mechanism for generating Cu-rich magmas is fractional crystallization during pyrrhotite undersaturated conditions. The high fO2 conditions observed for the Last Chance stock may have allowed such an evolution. Alternatively, if the magma has not undergone significant fractionation, early crystallization of chalcopyrite and bornite from the magma would indicate that the lower-crustal source region for the magma probably contained Cu-Fe sulfides. Possible Cu-rich source regions are a subcrustal mafic intrusion with sulfide cumulates, or a deeply buried metamorphic terrane containing Cu deposits such as those in the Curaca Valley (Brazil) or Okiep (South Africa). Heterogeneous distribution of Cu-Fe sulfides in an Okiep-type source terrane would produce local PCDs such as Bingham, or large accumulations of Cu-Fe sulfides, possibly in the form of cumulates in subcrustal intrusions at convergent margins, could produce giant PCD provinces such as those in Indonesia, Papua New Guinea, and central Chile. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
