Magnetic characterisation and palaeointensity analysis of rocks from selected I-type granitic plutons
Absolute palaeointensity data are crucial to investigating the behaviour of the geomagnetic field. Long-term global palaeomagnetic databases are compiled with the aim of documenting the evolution of the geomagnetic field. However the temporal and spatial distribution remains limited, partly due to a lack of 'suitable' material in critical time windows, or poor magnetic behaviour of these rocks during experiments. Therefore it would be useful to have other potential sources of palaeointensity data in the surface rock record. Granitic intrusions occur frequently throughout the geological record, are often well exposed and well dated. However, granitic rocks have not previously been fully exploited in palaeointensity studies, due to their perceived lower primary magnetic content and problems associated with the stability of their magnetic signatures. Plutons show an internal grain-size variation and contain mafic enclaves, xenoliths and sheets derived from the primary melt. Therefore a range of magnetic behaviour, and hence a range of potential suitability for palaeointensity work can be expected within a single pluton. To our knowledge, there are no recorded rock magnetic or palaeomagnetic investigations addressing the question of whether granites are generically unsuitable, for palaeointensity work. To test this hypothesis, three I-type granitic plutons were characterised in terms of their magnetic suitability for palaeointensity analysis - including the Ross of Mull Pluton and Loch Doon Complex, SW Scotland and selected intrusions from the Coastal Batholith, Peru. Characterisation was achieved though extensive rock magnetic, palaeomagnetic and SEM analysis which enabled the magnetic properties of the samples to be determined. 180 samples were investigated using the Coe-modified Thellier technique to determine whether reliable palaeointensity data can be obtained. Magnetic characterisation and SEM analysis reveal that the magnetic mineralogy of the granitic intrusions is dominated by multidomain (MD) Ti-poor titanomagnetite. Exsolution textures observed in the Fe-Ti oxide grains are responsible for creating grains with SD-MD grain sizes. Stable univectoral remanences are usually accompanied by narrow unblocking temperatures where most NRM is lost between 560- 580°C. This makes obtaining acceptable PI data difficult, but by no means impossible. PI experiments were of variable success. Experiments including pTRM and pTRM tail checks and standard acceptance criteria were used to assess the quality of the results. The Ross of Mull rocks yielded no PI determinations. However, the Loch Doon Complex and intrusions from the Coastal Batholith were more triumphant with success rates of 28% and 57% respectively. The Loch Doon pluton (408Ma) is calculated to have a pluton average 8.2[mu]T with standard deviation 4[mu]T. The main reasons for failure of PI experiments are weak NRM, alteration and unstable MD behaviour. Experimental success correlates with the perceived geological complexity at each field area, where it is observed that simpler intrusions with less complicated cooling histories yield rocks with a: more stable magnetic signature and allow PI data to be derived. Results indicate that the more mafic rocks sampled in each pluton produced the best quality PI data. PI estimates were made on samples from Peruvian intrusives. The Anta ring dyke and Chasquitambo pluton give a site average of 15[mu]T with standard deviation 13.5[mu]T. Large deviation is caused by a single estimate, which at 46.7±18.4[mu]T is significantly different from the values found from other sites. Remaining sites have PI within error, however it should be noted that according to the K-Ar dates (Wilson, 1975) emplacement of the two separate intrusions occurred millions of years apart, 70.7±1.6Ma and 61±0.7Ma respectively. Samples from the Sayan pluton (61.2±0.4Ma) yield an average PI 25.4[mu]T ±7.7[mu]T. Arai plots are dominated by curvature, pTRM checks and rock magnetic experiments indicate that curvature is not caused by alteration and can be attributed to MD phenomena. PI estimates are made using the end-point method and from linear segments were possible, consistency between the two methods was demonstrated. This research reveals the potential of I-type plutons to provide reliable palaeointensity data.However, the PI data presented here must be considered as preliminary as no cooling rate or anistropy corrections have been applied.