WILLIAM H. TAUBENECK, 1967. "Petrology of Cornucopia Tonalite Unit, Cornucopia Stock, Wallowa Mountains, Northeastern Oregon", Petrology of Cornucopia Tonalite Unit, Cornucopia Stock, Wallowa Mountains, Northeastern Oregon, William H. Taubeneck
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The earliest unit in Cornucopia stock is a concentrically zoned tonalite that grades from a more mafic margin to a more felsic core. Differentiation resulted from fractional crystallization and diffusion in a volatile-rich magma that crystallized inward from the margins. Minor filter pressing in border rocks may have augmented differentiation. Modal data warrant subdivision of the tonalite into border, intermediate, and interior rocks. A salient of interior tonalite protrudes into country rocks on the southeast, representing lateral movement of residual magma in the core after marginal consolidation.
Most accessory minerals and alteration products increase systematically from interior to border rocks. Distribution of minor minerals reflects outward movement of volatiles during consolidation, in contrast to distribution of minor minerals in zoned plutons characterized by inward concentration of volatiles.
New occurrences of zeolites in granitic rocks are recorded. Heulandite is interstitial, an alteration product of plagioclase, and a vein mineral in microfractures. Traces of chabazite and several unidentified zeolites also occur.
Chemical data confirm a trondhjemitic differentiation trend. Proportions of almost all major and minor elements increase or decrease from mafic border to felsic core. Uncommon relationships of Mn/Fe2, Ba/Rb, Ba/K, Ga/Al, Y/Ca, and K/Rb ratios are explained by the mineral distribution in the intrusion.
Related granitic intrusions in the Wallowa Mountains do not support separation of trondhjemitic and calc-alkaline differentiates by geographic distribution, temporal control, or by a difference in K/Na ratios of parental crustal material.