Alternative processes for developing fabric and mineral compositional zoning in intrusive rocks

摘要

We present textural and mineral compositional analyses of samples from previously published experiments examining the behaviour of partially molten silicates in a thermal gradient and/or at a compositional interface. Textural and electron backscatter diffraction (EBSD) analyses demonstrate that, when new minerals grow within a temperature gradient, they develop elongation and in some cases a preferred orientation parallel to the gradient; however, reaction of existing randomly oriented crystals in a thermal gradient does not produce a preferred orientation. For example, amphibole and magnetite crystals growing within the melt-rich hotter end of a thermal migration experiment develop elongate habits. Similarly, when basaltic andesite is juxtaposed with gabbro in a thermal gradient, newly formed crystals of olivine have a preferred crystallographic orientation, whereas the minerals that existed before the reaction remain in their original, random orientation.Compositional analyses and imaging show that olivine crystals in a melt that is slowly increasing in its MgO concentration by diffusion across a compositional interface develop normal zoning patterns, with the cores having overly forsteritic compositions (higher Mg compositions than could have existed in equilibrium with the melt). This finding parallels previous work showing that diffusion across a compositional interface leads to normally zoned plagioclase having overly anorthitic cores. These observations document an alternative process whereby solid solution mineral grains develop normal zoning without ever being exposed to a melt at lower temperature or to a melt having a larger concentration of a lower-temperature component. These alternative processes for producing fabric and mineral zoning could be important in situations where plutons are assembled incrementally by top-down underplating of sills. We use a coupled thermodynamic transport model to show that the mineralogy and compositional stratification of layered mafic intrusions could be produced by such a top-down process of sill injection, followed by compositional differentiation in a moving temperature gradient.