MINERALOGY AND ORIGIN OF SPOTS IN SPOTTED SLATE FROM THE MALAGUIDE COMPLEX, BETIC CORDILLERAS, SPAIN: AN XRD, EMPA AND TEM-AEM STUDY

We have studied the nature of spots occurring in spotted slates from the Maláguide Complex, in the Betic Cordilleras of southern Spain, by optical microscopy, X-ray diffraction, electron-microprobe analysis and transmission electron microscopy. These spots developed during an episode of contact metamorphism after the Alpine regional event. They are optically and mineralogically zoned, the extent of the zones being dependent upon both the composition of the slates (or of the different bands in these) and the degree of spot development. The mineral zonation is clearly correlated with the chemical variations, as revealed by electron-microprobe analyses obtained across the spots. In the best-developed spots, an inner isotropic zone, characterized by Al enrichment relative to the slate, is followed by an intermediate quartz- or mica-rich zone showing a relative enrichment in either Si or in Si, K, and Na; an outer yellow to reddish zone shows a notable increase in Fe. TEM–AEM studies indicate that the innermost isotropic zone mainly consists of “amorphous ” Si–Al or Si–Al–Fe phases that contain numerous phyllosilicate inclusions. Two main phyllosilicate associations have been identified in the spots: muscovite + berthierine ± chlorite and muscovite + biotite + chlorite. The first assemblage is characterized by the development of Al-poor oxidized berthierine, which preferentially developed in Fe-rich spots, whereas the second association is better developed in micaceous spots. Even where both assemblages coexist in a single spot, they are restricted to different microdomains. Textural features indicate that berthierine formed as a product of the reaction of both chlorite and muscovite. The first transformation occurred through disordered polytypes of chlorite