Petrogenesis of high-alumina tonalite and trondhjemites of the cornucopia stock, Blue Mountains, northeastern Oregon.
Johnson, Kenneth Scott
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The Cornucopia stock is a small composite intrusion comprising five distinct intensive units: a hornblende biotite tonalite, a biotite trondhjemite, and three cordierite-bearing two mica trondhjemites. Dikes of dacitic, granodioritic, and granitic compositions are common throughout the stock. The stock intended greenschist facies metasedimentary and metavolcanic rocks of the Wallowa terrane, remnants of a Permo-Triassic island arc. The age of the intrusion is 116.8±1.2 Ma determined by 40Ar/39Ar incremental heating measurements. Unaltered biotite from the first and last units emplaced yield concordant age plateaus at 116.8 and 116.7 Ma, respectively. The identical ages indicate cooling as a unit and imply coeval emplacement of the tonalitic and trondhjemitic magmas. REE models suggest the tonalite and trondhjemites formed by 15-35% partial melting of a low-K tholeiitic source, in equilibrium with a garnet pyroxene hornblendite residue. Trace element models indicate the source had high Sr and Ba contents, similar to island arc tholeiite of the Wallowa terrane. The high Sr in the rocks, lack of residual plagioclase, abundant residual amphibole, and the H20-rich nature of the rocks suggest that H2O in excess of that produced by amphibole dehydration was present at the site of melting. Residual gamet and hornblende implied by REE models indicate that melting occurred at a shallower depth than envisioned for slab-melting (-10 kbars versus 23-26 kbars). In addition, the Cornucopia rocks do not possess characteristics (e.g., high MgO, Cr, Ni) of a typical slab-melt, precluding involvement of the overlying lithospheric mantle. Results of this study suggest that the tonalitic and trondhjemitic magmas formed by hydrous partial melting of lower island arc crust, possibly as the result of underplating by mafic magmas. Furthermore, these results indicate that high-Al tonalitic and trondhjemitic magmas may be formed by processes other than slab-melting. The Comucopia stock was one of several tonalitic/trondhjemitic plutons emplaced after peak metamorphism associated with the accretion of oceanic terranes to the continental margin during Early Cretaceous time (-128 Ma). Prior to this time, plutons were predominantly granodioritic, and appear to have evolved by AFC processes from mafic, mantle-derived magmas. In contrast, tonalitic/trondhjemitic magmas were apparently generated by partial melting of oceanic terrane rocks. This suggests that the style of magmatism changed, from mantle-derived to crust-derived, as a direct result of terrane accretion.