Feldspars and fluids in cooling plutons

Abstract

SUMMARY. Alkali feldspars in plutonic igneous rocks vary both in their exsolution textures and in the structural state of their components. The primary factor leading to this diversity is the availability of hydrothermal fluids during their cooling history. In many plutons feldspar variation is related to degree of fractionation as indicated by rock chemistry. The variation reflects build-up of water with magmatic evolution and implies that fluids did not circulate freely in the intrusives in the temperature range of unmixing and ordering. Experimental work bearing on the role of fluids in subsolidus changes in feldspars is reviewed, and points of overlap between crystallographic studies of feldspars and stable isotope studies are discussed. THE factors that control the striking variety of exsolution textures and differences in framework order-disorder in alkali feldspars have been the subject of a number of review articles (e.g. Smith and MacKenzie, I96I;Martin, 1974; Stewart and Wright, I974). The stress in such treatments has been predominantly on thermal history, both with respect to temperature of crystallization and to cooling rate. In this talk, with an eye on petrologists, not feldspar specialists, I wish to concentrate on what is generally agreed to be one of the important subsidiary controls, namelythe presence of aqueous fluids during the cooling of the feldspar from magmatic temperatures, and to suggest that this is pre-eminently the factor that dictates the nature of the feldspar in all but the smallest intrusive igneous bodies provided they have escaped tectonic deformation. I shall show how a few petrographic observations or simple X-ray diffraction measurements may yield a wealth of information about the distribution of fluids during the cooling of a given body of alkali-feldsparbearing rock, and demonstrate that in many ~) Copyright the Mineralogical Society fractionated intrusive complexes 'water' was distributed following a 'magmatic' pattern, building up with progressive fractionation, and remained essentially restricted within the evolving lithological units.The effects of water during ,cooling of igneous bodies is a matter of particular concern at the present time because of the demonstration, by means of stable isotope studies (e.g. Taylor, 1974) of large-scale interactions between plutons and groundwaters. Some of the limited interactions that are demonstrated in the present review contrast strongly with the large-scale, all-pervasive waterrock interactions proposed by Taylor. The crystallographic observations to be described could, under favourable circumstances, be used to limit the thermal range in which water-rock interactions may have occurred, a factor by no means clearly defined in stable-isotope studies.I shall draw a number of examples of feldsparfluid interactions from one small (4 x 3 km stock), very intricate layered syenite pluton, the Klokken intrusion, in the pre-Cambrian Gardar province in South Greenland. The photomicrographs in the following sections illustrate the ran...

The role of fluids in the late-stage evolution of the South Mountain Batholith, Nova Scotia: further geochemical and oxygen isotopic studies

“…This is perhaps most evident in sample 38-1 which may contain a significant amount of intergrown triclinic K-feldspar. How ever, probably the most notable feature is the overall predomi nance of the monoclinic structural state for pegmatitic K-feldspars (e.g., note the sharp (131)K reflection for sample 22C) considering their formation in a fluid-rich environment (Parsons and Boyd, 1971;Parsons, 1977). In contrast to the aforemen tioned samples, development of triclinic phases occurs in samples 29B, 32 and 49, with the latter samples clearly reflecting the presence of both monoclinic and triclinic phases.…”

“…This is found regardless of rock type (i.e., mon zogranite versus pegmatite) and, therefore, the simple but gener alized relationship of increasing ordering with increasing fluid pressure (e.g., Parsons and Boyd, 1971;Parsons, 1977;Parsons and Brown, 1984) does not apppear to strictly apply here. A similar situation exists in other large granitoid batholiths, such as parts of the nearby SL George Batholith of southern New Brun swick (Cherry and Trembath, 1978) or the Wiborg rapakivi massif of southern Finland (Vorma, 1971).…”

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.…”

“…The turbidity in alkali feldspars points to recrystallization driven by infiltration of aqueous fluid. According to Parsons (1978), coarse patch perthite in subsolvus feldspars developed isochemically by water-impelled dissolution and reprecipitation. This suggests that in the course of recrystallization the system remained closed.…”

Evidence from feldspar compositions of high temperatures in granite sheets in the Scourian complex, N.W. Scotland

“…This means that the Scourian mesoperthites probably crystallized within 50~ of the critical temperature of the ternary feldspar solvus at An12. Parsons (1978) estimated that for the constant ratio Ab63Or37 the ternary feldspar solvus increases by 32 ~ % anorthite. This suggests a ternary solvus temperature of 1040~ for the composition (Abo.taOro.37)ssAn12.…”