2 Cordilleran Tectonics from the Basin and Range to Alaska and the Arctic I: A Celebration of Elizabeth Miller’s Career (2018 GSA Structure and Tectonics Division Career Contribution Award)

Wednesday, 15 May 2019
8:00 AM - 12:00 PM
Room B117-119

AMPHIBOLITE INCLUSIONS WITHIN THE 2.65 GA PEND OREILLE GNEISS, PRIEST RIVER COMPLEX, NORTHERN IDAHO

Paper number: 2-12
Program: Theme Sessions
Time: 11:35 AM – 11:55 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329583

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