<p>The Sierra Blanca metamorphic core complex (SBMCC), located 90 miles west of Tucson, is part of the southern belt of metamorphic core complexes that stretches across southern Arizona. The SBMCC exposes Jurassic age sedimentary rocks that have been metamorphosed by intruding Late Cretaceous peraluminous granites and pegmatites. Evidence of this magmatic episode includes polysythetic twinning in plagioclase, albite exsolution of alkali feldspar resulting in myrmekitic texture, and garnet, mica and feldspar assemblages. The magmatic fabric is overprinted by a Tertiary (Miocene?) tectonic fabric, associated with the exhumation of the Sierra Blanca metamorphic core along a low-angle detachment fault, forming the SBMCC. The NW-SE elongated dome of metamorphic rocks forms the footwall of the detachment shear zone, and is separated from the hanging wall, composed of Paleozoic and Mesozoic metasedimentary rocks, by a low-angle detachment shear zone. Foliation is defined by gneissic layering and aligned muscovite, and is generally sub-horizontal, defining the dome. The NNW-SSE mineral stretching lineation is expressed by plagioclase and K-feldspar porphyroclasts, and various shear sense indicators consistent with a top-to the-NNW shear sense. Lineation trends in a NNW-SSE orientation; however, plunge changes across the domiform shape of the MCC. Much of the deformation is preserved in the blastomylonitic gneiss derived from the peraluminous granite, including epidote porphyroclasts, grain boundary migration in quartz, lozenged amphiboles, mica fish, and retrograde mineral alterations. Detailed petrologic observation and microstructural analysis indicate deformation temperatures of 450-575 ? ?C presented here provide thermomechanical constraints on the evolution of the SBMCC.

Understanding crystallisation in magma chambers is a key challenge for igneous petrology. It is particularly important to understand the origins of layering in peralkaline rocks, e.g. the kakortokite (nepheline syenite), Ilímaussaq Complex, S. Greenland, as these are commonly associated with high value multi-element economic deposits. The kakortokite is a spectacular example of macrorhythmic (>5 m) layering. Each unit consists of three layers comprising arfvedsonite-rich (sodic-amphibole) black kakortokite at the base, grading into eudialyte-rich (sodic-zirconosilicate) red kakortokite, then alkali feldspar- and nepheline-rich white kakortokite. Each unit is numbered -19 to +17 relative to a characteristic well-developed horizon (Unit 0), however there is little consensus on their development. This project applies a multidisciplinary approach through field observations combined with petrography, crystal size distributions (CSDs), mineral and whole rock chemistries on Units 0, -8 to -11 and a phonolite/micro-nephelinolite (“hybrid”) sequence that crosscuts the layered kakortokite. Textures and compositions are laterally consistent across outcrop and indicators of current activity are rare. CSDs indicate in situ crystallisation with gravitational settling as a minor process. Chemical discontinuities occur across unit boundaries. The layering developed through large-scale processes under exceptionally quiescent conditions. The discontinuities reflect open-system behaviour; units were formed by an influx of volatile-rich magma that initiated crystallisation in a bottom layer. Nucleation was initially suppressed by high volatile element concentrations, which decreased to allow for crystallisation of arfvedsonite, followed by eudialyte, then alkali feldspar and nepheline to form each tripartite unit. The chemistry of the hybrid indicates mixing between a primitive (sub-alkaline) magma and kakortokite. Thus injections of magmas of varying compositions occurred, indicating a complex plumbing system below current exposure. The lessons learned at Ilímaussaq, which is extremely well exposed and preserved, are relevant to understanding magma chamber dynamics in the more common instances of pervasively altered peralkaline rocks.