TY - JOUR
T1 - Petrology of iron-rich magmatic segregations associated with strongly peraluminous trondhjemite in the Cornucopia stock, Northeastern Oregon
AU - Johnson, K.
AU - Barnes, C. G.
AU - Browning, J. M.
AU - Karlsson, H. R.
N1 - Funding Information:
Acknowledgements We thank the owners of the Cornucopia Wilderness Pack Station, Inc. for packing samples out of remote areas. Instrumental neutron activation analyses were provided by the Radiation Center at Oregon State University through the US Department of Energy Reactor Sharing Program. We thank Kip Miller for help with microprobe analyses, and Sharla Salisbury, Joel Boyd, Josh Rodgers, Shannon McGuire, and Melanie Barnes for assistance with sample preparation and ICP-AES analyses. The thorough and constructive comments of two anonymous reviewers are greatly appreciated. Funding for portions of this study were provided to K.J. by the Oregon Department of Geology and Mineral Industries, a Penrose Bequest of the Geological Society of America, and a Sigma Xi Grant-in-Aid of Research, and to C.G.B. through NSF grant EAR-9117103.
PY - 2002
Y1 - 2002
N2 - The Middle Cretaceous Cornucopia stock in the Blue Mountains of northeastern Oregon is a small composite intrusion consisting of hornblende biotite tonalite, biotite trondhjemite, and three cordierite two mica trondhjemite units. Unusual magnetite + biotiterich tonalitic rocks are associated with the Crater Lake cordierite trondhjemite, the youngest of the intrusions. Oxide-rich tonalites are characterized by high Fe (∼47-68 wt% total Fe as FeO), low SiO2 ( < 36 wt%), and enrichments in HFSE and REE (La(N) = 361-903). Oxide-rich tonalites appear in a variety of forms, including composite dikes and sheets, in which they are associated with leucocratic tonalite. Leucotonalite is lower in SiO2 (60-72 wt%) than Crater Lake trondhjemite, and generally has ΣREE contents and Eu anomalies intermediate between the oxide-rich tonalite and Crater Lake compositions. Oxide-rich tonalites crosscut, and are crosscut by, shear zones in the host trondhjemite, indicating their emplacement late in the pluton's crystallization history. Granitic dikes crosscut the composite dikes in all localities. Geochemical considerations and sedimentary-like structures, such as load casts and bedding of magnetite-rich assemblages in the composite dikes and sheets, are suggestive of crystal settling from an Fe-rich parental magma. The Fe-rich liquid parental to the oxide-rich tonalite-leucotonalite pairs formed by extensive, in-situ, plagioclase + quartz-dominated crystallization of strongly peraluminous trondhjemite. Early magnetite saturation in the trondhjemite was suppressed, either because the parental trondhjemitic magma had a lower initial total Fe content or because it had a lower ferric-ferrous ratio, possibly reflecting a lower oxygen fugacity. Accumulation of magnetite from Fe-rich residual magma is a viable mechanism for the concentration of iron, and the subsequent formation of Fe-rich rocks, in calcic siliceous intrusions. Apparently, Fe-enrichment can occur locally in calcic magmas, and is not restricted to rocks of mafic tholeiitic or anorthositic affinity.
AB - The Middle Cretaceous Cornucopia stock in the Blue Mountains of northeastern Oregon is a small composite intrusion consisting of hornblende biotite tonalite, biotite trondhjemite, and three cordierite two mica trondhjemite units. Unusual magnetite + biotiterich tonalitic rocks are associated with the Crater Lake cordierite trondhjemite, the youngest of the intrusions. Oxide-rich tonalites are characterized by high Fe (∼47-68 wt% total Fe as FeO), low SiO2 ( < 36 wt%), and enrichments in HFSE and REE (La(N) = 361-903). Oxide-rich tonalites appear in a variety of forms, including composite dikes and sheets, in which they are associated with leucocratic tonalite. Leucotonalite is lower in SiO2 (60-72 wt%) than Crater Lake trondhjemite, and generally has ΣREE contents and Eu anomalies intermediate between the oxide-rich tonalite and Crater Lake compositions. Oxide-rich tonalites crosscut, and are crosscut by, shear zones in the host trondhjemite, indicating their emplacement late in the pluton's crystallization history. Granitic dikes crosscut the composite dikes in all localities. Geochemical considerations and sedimentary-like structures, such as load casts and bedding of magnetite-rich assemblages in the composite dikes and sheets, are suggestive of crystal settling from an Fe-rich parental magma. The Fe-rich liquid parental to the oxide-rich tonalite-leucotonalite pairs formed by extensive, in-situ, plagioclase + quartz-dominated crystallization of strongly peraluminous trondhjemite. Early magnetite saturation in the trondhjemite was suppressed, either because the parental trondhjemitic magma had a lower initial total Fe content or because it had a lower ferric-ferrous ratio, possibly reflecting a lower oxygen fugacity. Accumulation of magnetite from Fe-rich residual magma is a viable mechanism for the concentration of iron, and the subsequent formation of Fe-rich rocks, in calcic siliceous intrusions. Apparently, Fe-enrichment can occur locally in calcic magmas, and is not restricted to rocks of mafic tholeiitic or anorthositic affinity.
UR - http://www.scopus.com/inward/record.url?scp=0036174335&partnerID=8YFLogxK
U2 - 10.1007/s00410-001-0311-z
DO - 10.1007/s00410-001-0311-z
M3 - Article
AN - SCOPUS:0036174335
SN - 0010-7999
VL - 142
SP - 564
EP - 581
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 5
ER -