Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

Abstract

Alkali feldspar phenocrysts (bulk composition Or75.oAb24.6Ano.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto-and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ~< 590~ by heterogeneous nucleation of thin albite films which coarsened to > 1 ~tm length. Cryptoperthites developed at < 400~ by homogeneous nucleation of sub-~tm long platelets between films. Platelets are coherent, but the coarser microperthite lameUae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite--orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous Ixm to sub-~tm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lameUae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystaUization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400~ the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of cryptoand micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

Origin and hydrothermal alteration of rare-metal granites in the Al-Hamra area, northeastern Arabian Shield, Saudi Arabia

“…Several studies showed that abundance of exsolution textures in alkali feldspar indicates extensive structural rearrangement of magmatic alkali feldspar at low (=400 °C) temperature (e.g. Parsons 1978;Brown and Parsons 1989;Worden et al 1990;Lee et al 1995;Walker et al 1995). The turbidity in alkali feldspars points to recrystallization driven by infiltration of aqueous fluid.…”

Three-dimensional cooling pattern of a granitic pluton I: The study of exsolution sub-solidus reactions in the Toki granite, Central Japan

“…The front of the turbid (patchperthite) zone intrudes and cuts the pristine zone, which contains semi -coherent microperthite lamellae. The Ab -rich lamellae in microperthite are more thread -like than the Ab -rich patches in patchperthite, and the interface between the albite lamellae and the alkali feldspar primary phase in microperthite is sharp (semi -coherent), whereas the interface in patchperthite is irregular (incoherent; Lee et al, 1995). The Ab -rich patches of patchperthite occur in either a vein form or a bead form.…”

“…1B); Ms -Bt granite, muscovite -biotite granite; Hb -Bt granite, hornblende -biotite granite; Bt granite, biotite granite (Yuguchi et al, 2010). croscopy (SEM) have shown that alkali feldspar phenocrysts in the Shap granite of Cumbria, northwest England, have undergone both an exsolution reaction to form "coherent" cryptoperthite and a deuteric reaction to form "incoherent" patchperthite (Worden et al, 1990;Waldron and Parsons, 1992;Waldron et al, 1994;Lee et al, 1995;Walker et al, 1995;Lee and Parsons, 1997). Hashimoto et al (2005A) suggested that an exsolution reaction in alkali feldspar is responsible for the formation of "semi -coherent" microperthite with micrometer -scale Ab -rich lamellae and cryptoperthite with a nanometer -scale Ab -rich phase (zigzag or lozenge form).…”

Microtexture and compositional variation of alkali feldspars from the Kakkonda granitic pluton, northeast Japan: Implications to the formation processes of granitic texture

“…On the contrary, most of alkali feldspars in granites are reorganized to form patch microperthites with pervasive turbidity over a grain (e.g., Lee et al, 1995;Lee and Parsons, 1997;Hashimoto et al, 2005a;Yuguchi and Nishiyama, 2007), although relatively pristine part of cryptoperthite is still few remained Lee and Parsons, 1997). Thus, perthitic textures of alkali feldspars become a very important tool to clarify the cooling histo-ries of host plutons during magmatic to hydrothermal stages (Brown and Parsons, 1994;Parsons and Lee, 2009).…”