Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif

Abstract

Multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR) methods in a domain of mafic–felsic magma mingling at the northwestern margin of the Sázava pluton, Central Bohemian Plutonic Complex, Bohemian Massif. Gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles (mostly 40–70°) whereas in their tonalitic host lineations plunge from vertical to horizontal angles, but mostly less than 40°. The magnetic lineations in both rock types spread along a ~ NNE-SSW steep plane that could be simplified as representing an “average” margin-parallel magmatic foliation in the pluton concordant with an “average” regional cleavage in the wall-rock. The plane-confined lineations are interpreted as having recorded the heterogeneous superposition of two processes: (1) vertical stretching during emplacement and magma mingling which left behind the steep lineations; and (2) regional tectonic stretching, which progressively rotated the mineral grains in rheologically weaker domains (chiefly in the host tonalite) to form the sub-horizontal lineation. The average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin-perpendicular shortening during emplacement, overprinted by coaxial regional tectonic shortening. This example reaffirms that (1) magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation; and that (2) separating magmatic (and also magnetic) fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized.