Mafic dykes (Groups A–D) intruded into Mesoproterozoic basement amphibolites, gneisses, and granitoids of the Cape Meredith Complex on the southern tip of West Falkland, provide an important record of at least three periods of lithospheric extension during Palaeozoic and Jurassic times. Group A dykes are calc-alkaline lamprophyres that were generated by partial melting of an enriched subcontinental lithospheric mantle in Cambro-Ordovician times. Group B dykes are Ordovician dolerites derived from an asthenospheric mantle source, perhaps during the same extensional episode as Group A dykes. Group C dykes were also derived from an asthenospheric source and are possibly of Silurian age. The youngest, Group D, dykes are part of the widespread Jurassic Gondwana province. This group contains an oceanic island basalt-like sample and an enriched sample similar to both Group A lamprophyres and to the Jurassic Ferrar province in Antarctica. These correlations have interesting implications for the composition and evolution of mantle sources through time; the co-existence of Cambrian lamprophyres and Jurassic Ferrar-type magmas in the Cape Meredith Complex demonstrate for the first time that the enriched lithospheric mantle source postulated for the Ferrar magmas existed as far back as Cambrian times.

Employing an average vertical sampling interval of 20 cm, oriented samples were collected from 26 m of the loess section at Liujiapo, near Xian, China in order to refine the existing magnetostratigraphic record. Palaeomagnetic results reveal two short reversed-polarity subzones within a profile of predominantly normally magnetised sediments, an interpretation that is at variance with previous results from this section and from other studies of Chinese loess. Investigation of sediment mineral-magnetic properties indicates a close correspondence between down-section variations in intensity of magnetisation, susceptibility and saturation isothermal Journal of @ternary Science remanence (SIRM), and lithology. The ratios IRM-,,~o,,T/SIRM and SIRM/ susceptibility appear to be particularly sensitive indicators of the degree of soil development and hence of palaeoclimate.

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The minerals responsible for the magnetic properties of the Late Triassic Lunde Formation have been examined using a combination of magnetic mineral extraction, microscopy and rock magnetic measurements. The magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis, by a process of differential preservation and modification of the original detrital minerals. The magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material, which contains most of the magnetic minerals. Paramagnetic minerals, both of depositional origin and produced during diagenesis, are largely responsible for the magnetic susceptibility. A suite of residual ferrimagnetic oxides (chromite and Mn-substituted magnetite) is responsible for the remanence properties in grey sandstones, because most of the originally deposited magnetic Fe-Ti oxides have been removed by reducing conditions during diagenesis. However, a small number of reddened horizons preserve a higher proportion of original detrital Fe-Ti oxides (ferrimagnetic ilmenite, ferrimagnetic chromite and magnetite), although authigenic hematite has replaced most of the original mineralogy in the reddened horizons. Magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal, particularly in horizons which have a low content of discrete detrital ferrimagnetic grains, either through diagenetic dissolution, or paucity of the silt and clay fraction. Within the grey sandstones, the Triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates, with a contribution from detrital ferrimagnetic chromite. A model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed.