Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over ∼400 km 2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation ( k MAX– k INT) parallel to dike walls and a lineation ( k MAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility ( k MAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow ( k MAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced ∼4 km parallel to the spreading axes and ∼6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for ∼240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α 95 = 3.9; mean T UB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α 95 = 8.7; mean T UB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-co