Abstract

Alkali feldspars usually exhibit optical or sub-optical intergrowths of albite and K-feldspar (known generically as perthite) which have morphologies and crystallographic characteristics that are distinctive of the igneous or metamorphic environment in which they grew and cooled to surface temperatures. We review the current terminology and understanding of how the microtextures form and discuss their role in feldspar dissolution during weathering, transport, and diagenesis. We show how microtextures can be used as provenance indicators in arkosic siliciclastic rocks, using examples from the Fulmar Formation, a reservoir rock in the Upper Jurassic Humber group in the Central North Sea, in which feldspar dissolution is a major source of secondary porosity. We describe the most effective techniques for routine characterization, of which the most practical is back-scattered electron imaging in a scanning electron microscope, often coupled with cathodoluminescence. We provide an atlas of the main types of perthitic intergrowth likely to be encountered in igneous and metamorphic rocks worldwide, and show how many of the microtextures can be matched in clastic grains in the Fulmar Formation. Grains which are preferentially preserved have microtextures that make them relatively unreactive in aqueous fluids at low temperatures, and we conclude that provenance has a major impact on reservoir quality. Detrital grains in the Fulmar exhibit authigenic albitization, K-feldspar overgrowths, and replacement, and we discuss how diagenetic features can be distinguished from replacive features developed in the protolith.

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