Microtextural controls of diagenetic alteration of detrital alkali feldspars; a case study of the Shap Conglomerate (Lower Carboniferous), Northwest England

Abstract

ABSTRACT The diagenetic alteration of detrital alkali feldspar grains in the Lower Carboniferous Shap Conglomerate, which were derived from the nearby Lower Devonian Shap Granite, was strongly influenced by their pre-diagenetic microtextures. Alkali feldspars collected from a working quarry in the Shap Granite are perthites composed of two types of microtextures: primary igneous and deuterically altered. Primary igneous microtextures, which formed during early cooling of the pluton, are composed of albite exsolution lamellae in tweed orthoclase. Small lamellae (platelets) form cryptoperthites, whereas larger lamellae (films) form microperthites. Albite platelets have coherent interfaces with orthoclase (i.e., the crystal structure is strained but continuous across the interface of the exsolution lamellae), whereas larger films are semicoherent (i.e., dislocations have formed to relieve strain at the exsolution interface). Deuteric alteration leads to the development of micropore-rich veins composed of discrete subgrains of albite and microcline, referred to as patch perthites, which cut the cryptoperthites and microperthites. These veins formed during fluid-feldspar interaction within the granite in the later stages of igneous cooling. Subgrains within these veins are semicoherent or are incoherent (i.e., there is no continuity in crystal structure across the subgrain boundary). During diagenesis of Shap Granite-derived alkali feldspars in the Shap Conglomerate, the semicoherent and incoherent albite in microperthites and patch perthites was selectively dissolved or replaced by microcline, whereas coherent albite platelets in cryptoperthites remained largely unaltered. This diagenetic history reflects an interplay between two main determinants of alteration: (i) the nature of the pore fluids, which resulted in albite-rich feldspar being more heavily altered than orthoclase-rich feldspar and (ii) the coherency of interfaces, which determined the specific albite-rich feldspar microtextures that were affected. Fluid-feldspar interaction was favored at semicoherent and incoherent interfaces owing to their high surface and strain energies. Release of energy stored at such interfaces is also the major intragranular driving force of deuteric alteration of alkali feldspars in cooling igneous rocks in general. As a result, petrographically and geochemically similar microtextures may result from both deuteric and diagenetic alteration, and care must be taken to discriminate between them. Results of this study underscore the importance of understanding the nature of alkali feldspars in igneous and metamorphic rocks for an accurate interpretation of their subsequent diagenetic history.