PETROGENETIC INFORMATION GLEANED FROM GABBROIC CUMULATES: AN EXAMPLE FROM THE FISH LAKE COMPLEX, NE OREGON

The ~158 Ma Fish Lake Complex (FLC) is a collection of small gabbroic intrusions in the Blue Mountains Province in northeastern Oregon, formed during collision of the Wallowa and Olds Ferry island arcs (159-154 Ma; Schwartz et al., 2011). These rocks are coarse-grained assemblages of plag + cpx + opx ± ol and exhibit an igneous lamination, particularly along the margins of the intrusions. Late hbl encompassing pyx occurs in some gabbroic rocks. Dikes of hbl andesite, tonalite, and trondhjemite (~158-154 Ma) occur throughout the intrusions.

Gabbros have 46.8 to 52.5 wt.% SiO₂, low K₂O (up to 0.53 wt.%), and variable Al₂O₃ (14.5-21.1 wt.%). In addition, they have low ∑REE, with most samples having Eu/Eu*>1.0. With one exception, all gabbros plot within “cumulate polygons” bound by plag, cpx, opx, and ol compositions. Rhodes diagrams indicate cpx, opx, and ol were not in equilibrium with melts having their respective whole rock compositions. All evidence suggests the gabbroic rocks are largely cumulates.

Calculated melt compositions in equilibrium with late hbl (Zhang et al., 2017) from the “non-cumulate” gabbro are dacitic, with SiO₂=67-70 wt.% and K₂O=2.5-2.8 wt.%. Major element mass balance calculations show that this melt composition can be produced by crystallization of the cumulate phases and resulted from ~70% crystallization, and that the calculated cumulate is similar in composition to the gabbroic rocks. Equilibrium crystallization calculations using rhyolite-MELTS (Gualda et al., 2012) are consistent with mass balance results, and show the liquid line of descent results in andesitic and dacitic residual liquids. In addition, calculated crystallization temperatures of ol, cpx, and opx predicted by rhyolite-MELTS are similar to those obtained using the thermometers of Beattie (1993; for ol) and Putirka (2008; for cpx and opx).

Results of this study suggest that equilibrium crystallization of a basaltic magma can produce evolved dacitic residual liquid. Low-K₂O tonalitic and trondhjemitic dikes probably represent localized crustal melts, rather than the results of basalt differentiation. Although dacites have not been observed in the FLC, we suggest the dacitic melt effectively separated from the cumulates late in the history of this magmatic system, which is consistent with the well-developed lamination observed in most samples.