INVESTIGATION OF CORDIERITE AND SILLIMANITE TEXTURES AS A TEST OF EMPLACEMENT MODELS FOR THE FOSDICK GNEISS DOME, ANTARCTICA

Unusual textures of cordierite [Crd] and sillimanite [Sil] offer a means to assess the emplacement mechanism for the Fosdick gneiss dome, a migmatite culmination emplaced at 105 to 99 Ma during Cretaceous opening of the West Antarctic rift system. The dome cooled at the dramatic rate of 75ºC/m.y. Such rapid cooling could arise from buoyant ascent of partial melt-rich dome rocks to shallow depths, as in a diapir [Fayon et al. 2004].

Crd and Sil exist in varied textural settings that potentially track the translation of dome materials from deeper to shallower crustal levels. Typical paragneiss melanosome contains both phases in the invariant assemblage garnet-Crd-Sil-Kfeldspar-biotite-quartz, equilibrated at P≈500 MPa and T exceeding 725°C. Crd and garnet are unzoned. Five distinct morphologies of cordierite occur, three of which are aligned with the predominant foliation and two that are texturally late. The focus of this study is the late Crd, found as nodular and dendritic Crd-Qz-Kfs aggregates that overprint peak assemblages and fabrics. The delicate, branching forms are viewed as a disequilibrium growth texture formed late in the history of the dome. Also studied is prismatic, strongly preferentially oriented Sil forming mm- to cm-scale, nearly monomineralic shear zones that overprint aligned biotite in both metaplutonic and metasedimentary gneisses.

Petrographic and electron microprobe study is underway, to characterize the Crd and Sil textures and investigate the role of fluids in the Fosdick dome. The unusually high localized concentrations of aligned prismatic Sil and of the branching forms of Crd may both be induced by melt migration and presence of acidic fluids along shears or tensile fractures, with leaching of preexisting silicates. The Sil textures are being used for kinematics, to characterize the fluids present during dome emplacement, and to evaluate the role of small-scale shear zones in material transfer. Crd is being used to assess the interpretation of Barbey et al. [1999] that dendritic cordierite is a unique decompression texture formed at P ≤ 240 MPa. If such low pressures were achieved in the Fosdick dome while high temperatures were sustained, then diapirism likely played a role in dome emplacement.