TY - JOUR
T1 - Petrology, geochemistry, and genesis of high-Al tonalite and trondhjemites of the Cornucopia stock, Blue Mountains, northeastern Oregon
AU - Johnson, Kenneth
AU - Barnes, Calvin G.
AU - Miller, Christopher A.
N1 - Funding Information:
Most samples were packed out of remote areas by Elden and Marge Deardorff, of the Cornucopia Wilderness Pack Station, Inc. We thank Erik Hagen and the late David Little for help in collecting microprobe data, and Fritz Hubacher and Kenneth Foland for 40Ar/39Ar analyses. We appreciate the thorough and constructive reviews by James Beard, Cathryn Manduca, and J. Lawford Anderson, which led to substantial improvement of the manuscript. K–Ar ages were provided by Geochron Laboratories (Krueger Enterprises, Inc.). Neutron activation analyses were obtained from the Radiation Center at Oregon State University through the US Department of Energy Reactor Sharing Program. Funding for portions of this study was provided to K.J. by the Oregon Department of Geology and Mineral Industries, a Penrose Bequest of the Geological Society of America, Sigma Xi, and Texas Tech University, and to C.G.B. through NSF grant EAR-9117103.
PY - 1997/11
Y1 - 1997/11
N2 - The Cornucopia stock, in the Blue Mountains of northeastern Oregon, is a small composite intrusion comprising five distinct intrusive units: a hornblende biotite tonalite, a biotite trondhjemite, and three cordierite-bearing two-mica trondhjemites. The stock was emplaced at shallow levels (<2 kbar) in island arc-related metasedimentary and metavolcanic rocks of the Wallowa terrane. The age of the intrusion is 116·8±1·2 Ma determined by 40Ar/ 39Ar incremental heating of biotite. These ages and volume constraints imply coeval emplacement of the tonalitic and trondhjemitic magmas. Tonalitc and trondhjemitic compositions span a narrow range of SiO2 content (65-74 wt %) and exhibit characteristics of a high Al tonalite-trondhjemite-granitoid (TTG) suite, including light rare earth element (LREE) enrichment, low Y, Nb and Rb/Sr, and high Al2O3 and Sr. These compositions are consistent with an origin by 10-40% partial melting of a low-K tholeiitic source (with a relatively flat REE pattern), similar to metaigneous basement rocks of the Wallowa terrane, in equilibrium with a garnet pyroxene hornblendite residue. High Sr in the TTG rocks, lack of calculated residual plagioclase, and abundant calculated residual amphibole suggest that H2O in excess of that produced by amphibole dehydration was present at the site of melting. Conditions of partial melting are loosely onstrained to pressures ≥10 kbar (residual garnet implied by REE abundances) and temperatures exceeding 900-950°C (magmatic temperatures based on apatite solubility). We suggest the Cornucopia tonalitic and trondhjemitic magmas formed by hydrous partial melting of lower island arc crust, probably as a result of underplating by, or intrusion of, mantle-derioed basaltic magmas. Mantle melting may have been triggered by rapid uplift of Wallowa terrane crust, as the Blue Mountains terrane assemblage collided with the continental margin during Early Cretaceous time. Results of this study suggest that deformation and crustal melting associated with this accretion event were not strictly limited to the suture zone.
AB - The Cornucopia stock, in the Blue Mountains of northeastern Oregon, is a small composite intrusion comprising five distinct intrusive units: a hornblende biotite tonalite, a biotite trondhjemite, and three cordierite-bearing two-mica trondhjemites. The stock was emplaced at shallow levels (<2 kbar) in island arc-related metasedimentary and metavolcanic rocks of the Wallowa terrane. The age of the intrusion is 116·8±1·2 Ma determined by 40Ar/ 39Ar incremental heating of biotite. These ages and volume constraints imply coeval emplacement of the tonalitic and trondhjemitic magmas. Tonalitc and trondhjemitic compositions span a narrow range of SiO2 content (65-74 wt %) and exhibit characteristics of a high Al tonalite-trondhjemite-granitoid (TTG) suite, including light rare earth element (LREE) enrichment, low Y, Nb and Rb/Sr, and high Al2O3 and Sr. These compositions are consistent with an origin by 10-40% partial melting of a low-K tholeiitic source (with a relatively flat REE pattern), similar to metaigneous basement rocks of the Wallowa terrane, in equilibrium with a garnet pyroxene hornblendite residue. High Sr in the TTG rocks, lack of calculated residual plagioclase, and abundant calculated residual amphibole suggest that H2O in excess of that produced by amphibole dehydration was present at the site of melting. Conditions of partial melting are loosely onstrained to pressures ≥10 kbar (residual garnet implied by REE abundances) and temperatures exceeding 900-950°C (magmatic temperatures based on apatite solubility). We suggest the Cornucopia tonalitic and trondhjemitic magmas formed by hydrous partial melting of lower island arc crust, probably as a result of underplating by, or intrusion of, mantle-derioed basaltic magmas. Mantle melting may have been triggered by rapid uplift of Wallowa terrane crust, as the Blue Mountains terrane assemblage collided with the continental margin during Early Cretaceous time. Results of this study suggest that deformation and crustal melting associated with this accretion event were not strictly limited to the suture zone.
KW - Blue Mountains
KW - Crustal melting
KW - Magma
KW - Tonalitte
KW - Trondhjemite
UR - http://www.scopus.com/inward/record.url?scp=0000135533&partnerID=8YFLogxK
U2 - 10.1093/petroj/38.11.1585
DO - 10.1093/petroj/38.11.1585
M3 - Article
AN - SCOPUS:0000135533
VL - 38
SP - 1585
EP - 1611
JO - Journal of Petrology
JF - Journal of Petrology
SN - 0022-3530
IS - 11
ER -