Primary magma flow determination using AMS

Interpretation of magnetic fabrics as pertaining to primary magma flow fabrics in tabular sheet intrusions (and lava flows) has a strong historical founding, which has allowed various models to be evaluated and refined. Stacey (1960) provided important insights into the acquisition of AMS fabrics in flowing magma, highlighting that crystals rotated into alignment during magma flow. Initially, magma flow axes were interpreted to parallel K₂, providing the susceptibility axis was statistically valid and oriented within the plane of the dyke (Khan, 1962; Ellwood, 1978). Cañón-Tapia (2004) explains that the particle transport dynamics incorporated to support magma flow parallel to K₂ are restricted in their application and likely do not occur frequently. Knight and Walker (1988) presented the first empirical study of AMS and suggested that K₁, the magnetic lineation, could be equated to the primary magma flow axis with K₃ orthogonal to the intrusion margins (i.e. magnetic foliation is parallel to the dyke plane). Subsequently, many authors have either assumed this relationship is correct (Cañón-Tapia, 2004) or substantiated it through correlations to visible magma flow indicators (e.g. Callot et al., 2001; Aubourg et al., 2002; Liss et al., 2002; Horsman et al., 2005; Morgan et al., 2008). However, several studies have provided examples where magnetic lineations do not correlate to the inferred magma flow trend (e.g. Geoffroy et al., 2002; Cañón-Tapia, 2004; Bascou et al., 2005; Philpotts and Philpotts, 2007: Aubourg et al., 2008). Whilst these may be attributed to variations in magnetic mineralogy (see section 2.2.3), Cañón-Tapia and Chávez-Álvarez (2004) suggest that processes occuring during magma flow may also produce spurious magnetic fabrics. This section provides a brief account of the dynamics of magma flow within sheet intrusions and the potential magnetic fabrics created.

2.2.4:1 Generation of magmatic flow fabrics in sheet intrusions and lavas

Magma flow fabrics in sheet intrusions may be attributed to the hydrodynamic alignment of suspended crystal populations by non-coaxial simple shear or coaxial pure shear, dependent on variations in magma-velocity gradients across the intrusion (Fig. 2.6a) (Correa-Gomes et al., 2001; Callot and Guichet, 2003; Cañón-Tapia and Chávez-Álvarez, 2004). The geometry of the magma-velocity gradients, which define the velocity profile, in a sheet intrusion is dependent on the rheological behaviour (i.e. Newtonian, pseudo-plastic or Binghamian) of magma and friction with the wall rocks (Correa-Gomes et al., 2001). Velocity gradients are greatest at the intrusion margins and impart a simple shear stress, whereas towards the interior of the intrusion the velocity gradient decreases and pure shear dominates (Fig. 2.6) (Correa-Gomes et al., 2001; Callot and Guichet, 2003; Cañón-Tapia and Chávez-Álvarez, 2004;

Féménias et al., 2004).

Cañón-Tapia and Chávez-Álvarez (2004) suggest that cyclical rotation of crystals during magma flow may produce a multitude of possible fabric orientations as K₁, K₂ and K₃ systematically switch. This seems to contradict the consistency in magma flow fabric orientations often observed. Importantly, Cañón-Tapia and Chávez-Álvarez (2004) incorporated no crystal-crystal interaction into their models. Arbaret et al., (1996) showed that >15 % crystallinity, mechanical interactions between crystals prevents cyclicity and promotes alignment parallel to the shear direction. Regardless of magma rheology, the magma velocity-profile at sheet margins and crystal interactions causes fabrics to become imbricated (<30° from the dyke planes; Tauxe et al., 1998) parallel to the symmetrical inclination of the magma velocity profile (Fig. 2.6a) (Knight and Walker, 1988; Correa-Gomes et al., 2001; Geoffroy et al., 2002; Callot and Guichet, 2003; Féménias et al., 2004; Morgan et al., 2008; Philpotts and Philpotts, 2008). This relationship is demonstrated by sheet intrusions containing magnetic fabrics parallel to bulbous sheet terminations, which reflect and preserve the magma-veolocity profile (e.g. Horsman et al., 2005). Importantly, the closure direction of the imbricated fabrics coincides with the magma flow direction (Fig. 2.6a). Geoffroy et al., (2002) suggested imbricated magnetic foliations provide a more reliable sense of magma flow

A

B

Imbricated foliation (or lineation) fabrics mimic frontal sheet geometry

Fig. 2.6: Deformation of flowing magma within a constricted conduit (see text for details) (Féménias et al., 2004). B) Potential velocity profiles for magmas of differing rheology (Correa-Gomes et al., 2001).

direction compared to imbricated magnetic lineations as image analysis, field observation and AMS study of dykes in E Greenland highlighted a strong consistency between imbricated magnetic foliation and other magma flow indicators but a discrepancy in the orientations of magnetic lineations and magma flow interpretations for >50 % of dykes measured. The disparity in magnetic lineation and magma flow direction was attributed to the formation of a strong lineation formed by the intersection between two foliations; a fabric composed of an imbricated foliation sub-fabric and magma flow coaxial sub-fabric (Geoffroy et al., 2002;

Callot and Guichet, 2003). These results suggest samples acquired from intrusion margins, to measure imbricated fabrics, provide the most reliable estimates of magma flow. However, consideration of the study by Philpotts and Philpotts (2007) highlights a potential caveat to this method. Philpotts and Philpotts (2007) measured vesicle and magnetic fabrics from the chilled margin of a camptonite dyke significantly different to those within the sheet core, suggesting two magma flow regimes are recorded (Tauxe et al., 1998).

From a brief consideration of magma flow dynamics, it is apparent that magnetic fabrics can provide good indicators of magma flow and that K₁ may be interpreted as (sub-) parallel to the magma flow axis if there is independent evidence to corroborate it. The imbrication of fabrics may be analysed in an attempt to determine magma flow direction, although samples should be collected from the chilled margins (if present) and in the normal sheet groundmass for comparison.