Magnetic fabric of the Říčany granite, Bohemian Massif: A record of helical magma flow?

A highly unusual fabric pattern and inferred flow mechanism was discovered in the shallow-level Říčany granite pluton, Bohemian Massif, using an integrated structural and anisotropy of magnetic susceptibility (AMS) study. The pluton consists of an outer strongly porphyritic and an inner weakly porphyritic biotite granite separated by a wide gradational contact. Both varieties share a steep margin-parallel magmatic foliation (defined by K-feldspar phenocrysts and biotite) and magnetic (AMS) foliation carried by biotite. The steep foliation bears a shallowly-plunging magnetic lineation arranged parallel to the circumference of the pluton margin and a steep magnetic lineation in the weakly porphyritic pluton center. We propose a new mechanism of magma flow, in which the bulk ascent along a steep-sided, cylindrical conduit was partitioned into a high-viscosity, phenocryst-rich outer margin flowing helically and a low-viscosity, phenocryst-poor center flowing vertically. This interpretation is supported quantitatively by a simple model of magma flow within a cylindrical pipe, in which the linear decrease in phenocryst content from the pluton margin inwards causes a power-law decrease in the effective viscosity of the magma. According to the Poiseuille equation, such a power-law viscosity distribution across the conduit produces a central zone of high magma ascent velocity. The magnetic (AMS) fabric pattern in the Říčany pluton may thus preserve a record of helical magma ascent driven by viscosity partitioning in a steep-sided conduit, which presumably linked an underlying magma chamber with a volcanic feeder at the surface. Except for one deep-seated granite–migmatite complex, no such helical fabric pattern has been documented in a granite pluton as of yet. We propose that the viscosity-partitioned helical flow may be a more common magma ascent mechanism in shallow-level volcano-plutonic systems than previously envisaged.