Abstract
Abstract Granitic plutons worldwide contain ladder structures (LSs) that consist of nested trough-shaped layers alternating between mafic and felsic compositions. LSs and other forms of modal layering have been attributed to crystal accumulation, but their chemical trends differ greatly from those of cumulates and are discordant with chemical variations of their granitic hosts. Mafic layers reach extreme enrichments in transition metals, high-field-strength elements, and incompatible elements, and are extremely depleted in Si and Al. These geochemical characteristics are difficult to explain by crystal accumulation and conflict with sequences of phase appearance during crystallization. They are characteristic of liquid immiscibility, which is an accepted process in the genesis of tholeiitic and alkalic rocks. We propose that ladder structures and other forms of modal layering are markers of immiscibility in calc-alkaline granitic rocks.
Highlights
Structures and textures in plutonic rocks have generally been inferred to reflect dynamic processes in a mixture of crystals and liquid (e.g., Gilbert, 1906; Wager and Brown, 1968; Barbey, 2009)
We propose that liquid immiscibility can produce mafic layering in calc-alkaline granitic rocks
Processes Forming Ladder Structures If the mafic rungs are crystallized from immiscible liquids, one possibility for their formation is that when the melts exsolved, physical separation was on a short length scale; e.g., the less voluminous, lower viscosity, Fe-rich melt formed globules in an emulsion with the more abundant, higher viscosity felsic melt (Veksler et al, 2006)
Summary
Structures and textures in plutonic rocks have generally been inferred to reflect dynamic processes in a mixture of crystals and liquid (e.g., Gilbert, 1906; Wager and Brown, 1968; Barbey, 2009). Ladder “dikes” comprise curving, nested laminae of mafic minerals ( biotite, magnetite, and hornblende with high concentrations of titanite, apatite, and zircon) that form strips typically 0.5–1 m wide when viewed in outcrop (Fig. 1). They are not dikes in the literal sense, and we refer to them hereafter as ladder structures (LSs). 1E and 1F) follow the same chemical trends as LSs but do not reach such extreme compositions (Fig. 2; Reid et al, 1993) Their chemical similarity suggests that LSs and these other forms of mafic layering resulted from similar processes
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