Plagioclase mantles on sanidine in silicic lavas, Clear Lake, California: Implications for the origin of rapakivi texture

Abstract

At Clear Lake, California, mixed dacite lavas contain sanidine mantled by oligoclase that closely resemble rapakivi feldspars in granites. Resorption and mantling of sanidine by plagioclase is one of many disequilibrium textures in these lavas resulting from maficfelsic magma interaction. Mantling of sanidine by plagioclase may occur directly in mixed magmas due to shifts in composition and temperature, or indirectly in crystal-rich silicic magmas influenced by thermal under-plating, or mafic-felsic hybridization in adjacent regions of the magmatic system. Other silicic laves containing mantled sanidine also show evidence for mafic-felsic magma interaction. Textural and compositional similarities between rapakivi texture in extrusive and intrusive rocks suggest that they have a common origin. Moreover, modification of early-formed textures due to protracted crystallization, subsolidus reactions, and metasomatic-hydrothermal alteration in the plutonic environment accounts for most of the observed differences between extrusive and intrusive examples. The association of coeval mafic rocks and evidence for mafic-felsic magma interaction in many plutons containing rapakivi exture confirm that magma mixing plays a key role in its formation in some intrusive texamples. Evidence for a compositional control and multiple episodes of mantle formation are also compatible with repeated cycles of recharge and mixing. Regardless of the exact cause of K-feldspar instability, textural evidence from volcanic rocks and experiments indicates that development of rapakivi texture is controlled by cation diffusion in a dissolution boundary layer developed on K-feldspar. Mantles appear to form in a two-stage process that involves initial epitaxial nucleation of plagioclase on sanidine, followed by simultaneous dissolution of sanidine and inward growth of plagioclase.