Growth and albitization of K‐feldspar in crystalline rocks in the shallow crust: a tracer for fluid circulation during exhumation?

Abstract

A general feature of medium‐ to coarse‐grained, sheet‐silicate bearing, quartzo‐feldspathic rocks of either metamorphic or igneous affinity is the retrograde development of lenses of pure K‐feldspar at the grain boundaries between sheet silicate (0 0 1) faces and original feldspar grains. The growth of these lenses acts to displace and deform the sheet silicate grain by a force of crystallization, although the substrate feldspar and adjacent quartz are not deformed. Subsequent to the growth of the lenses they are replaced to variable degrees by pure albite, which grows into the lens from the substrate feldspar behind an irregular replacement front. The composition and texture of both K‐feldspar and replacive albite suggest a strong affinity with authigenic feldspars, although it is considered likely that the K‐feldspar of the lenses is derived from low‐temperature biotite‐breakdown reactions. A model is proposed whereby the lenses grow into open pores at dilatant sites in response to infiltration of aqueous fluids as the crystalline rocks are exhumed under brittle conditions. Continued circulation of infiltrating fluids in a temperature gradient results in the replacement of K‐feldspar by albite via an alkali exchange process. The lenses point to a significant grain‐scale permeability in crystalline rock at shallow levels in the crust.

Fluids in the Continental Crust

“…These observations are consistent with field observations of crystalline rocks. Retrograde alteration is actually widespread (e.g., Holness, 2003;see Yardley et al, 2014) but is much more common in the field than in the teaching laboratory because of the obsession of generations of petrologists to trim off "altered" material from the edges of their specimens before carrying them home. [Note that this is the opposite of what ore deposit geologists dothey carry home only the altered and mineralised rocks!]…”

Albitization of Granitic Rocks: The Mechanism of Replacement of Oligoclase by Albite

“…Albitization is known to take place over a wide range of P-T conditions from diagenesis (Saigal et al 1988, lee & lee 1998, Holness 2003, lee et al 2003 to greenschist- (Munz et al 1994(Munz et al , leichmann et al 2003 and even amphibolite-facies metamorphism (Clark et al 2005). Metasomatic reactions depend on the compositions of the fluid phases leaving and entering the system, and cannot be thermodynamically treated by merely considering the solid phases in the same way that isochemical metamorphic reactions are traditionally used to determine P-t conditions.…”

A Paleomagnetic and Petrographic Analysis of Unconformity Surfaces in Nevada and Missouri: Possible Implications for Paleoweathering Processes

“…Ricordel et al (2007), Engvik et al (2008, and Parcerisa et al (2010) have described albitization of basement silicate rocks that occurred in reddened rocks, often spatially related to the presence of hematite. Holness (2003) described the albitization of alkali feldspars when exposed to ion-saturated surficial fluids that penetrate to lower rocks after passing through two-feldspar silicates. The Vishnu group schists studied by Brown et al (1979) All samples were cut to standard 2.2 cm specimen length and then measured for natural remanent magnetization (NRM) using a 2G Enterprises three-axis cryogenic magnetometer with DC SQUIDS.…”