Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 2-10
Presentation Time: 11:35 AM


BUDDINGTON, Andrew M., Science Department, Spokane Community College, 1810 N. Greene Street, Spokane, WA 99217, WANG, Da, School of the Environment, Washington State University, Pullman, WA 99164 and ROGERS, Chris, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada

The Pend Oreille Gneiss (POG) is a 2.65 Ga migmatitic orthogneiss that occurs within a culmination of the Priest River core complex (PRC), near Priest River, ID. The POG is calc-alkalic, and tonalitic to granodioritic in composition. Within the POG occur subrounded amphibolite inclusions up to 1 m in length. The inclusions are deformed and occur parallel (lengthwise) to foliation in the orthogneiss.

Two inclusions were sampled for petrography, geochemistry, and U-Pb (LA-ICP-MS ) age dating. The inclusions are basaltic to basaltic andesite with 51.6% (AI-1) and 53.8% (AI-2) SiO2 compositions. The mineralogy of the inclusions includes hornblende, plagioclase, quartz, biotite, with accessory titanite, apatite, zircon, and opaque minerals. In general, the inclusions have variable major and trace element compositions relative to each other. Trace element plots show moderate enrichment in LILE’s, with moderate LILE:HFSE slopes; (La/Lu)n = 18.6, 4.8 respectively. Both samples show negative spikes in Nb, Ta, and Ti, but differ; AI-2 is more enriched in REE with (La/Lu)n = 18.6 and has a small negative Eu anomaly. AI-1 is less enriched with (La/Lu)n = 4.8 and exhibits a small positive Eu anomaly. Geochemically, Al-2 shares more similarities with the POG than Al-1.

We report new U-/Pb (zircon) ages of 1.87 Ga and 1.84 Ga for the inclusions. We interpret these inclusions to represent mafic dikes that intruded the POG during the Paleoproterozoic and were later dismembered during core complex development. These ages compare well with Paleoproterozoic ages of the granitic orthogneisses in the Cougar Gulch area of the southern PRC (40 miles south), as well as granitic orthogneisses in the Clearwater complex approximately 100 miles to the southeast. These new dates from Priest River extend the occurrence of Paleoproterozoic igneous rocks northward and support the significance of a Paleoproterozoic magmatic event in this portion of Laurentia.

  • Inclusions_talk_2019_final.pdf (6.1 MB)