4. MYRMEKITE FORMED BY Ca-METASOMATISM
Lorence G. Collins
email: lorencec@sysmatrix.net
February 3, 1997
In this fourth presentation, Ca-metasomatism of deformed K-feldspar to produce myrmekite in granite at Alastaro, Finland, is compared with K-metasomatism of deformed plagioclase to produce myrmekite as discussed in presentations 1, 2, and 3. Ca-metasomatism of relatively sodic plagioclase in anorthosite to produce myrmekite is also included.
In southern Finland, 9 km northeast of Alastaro (75 km north-northeast of Turku), pods and veins of megacrystal granite, containing K-feldspar crystals (2 to 3 cm long), occurs in sharp contact with gabbro. Both the granite and the gabbro have undergone deformation so that K-feldspar in the granite and plagioclase is deformed and fractured (Fig. 1). In some veins the deformation has caused the K-feldspar crystals to become aligned.
Under the petrographic microscope, fractures in the K-feldspar megacrysts commonly have a central quartz-ribbon bordered by myrmekite against the K-feldspar (Fig. 2, Fig. 3, and Fig. 4). In more strongly deformed K-feldspar crystals, myrmekite becomes abundant throughout the K-feldspar (Fig. 5) and in some places may almost totally fill the crystals (Fig. 6). The quartz vermicules in all this myrmekite have nearly a constant thickness against the K-feldspar (Fig. 7) rather than tapering toward it as in presentations 1, 2, and 3. Euhedral plagioclase inclusions in the K-feldspar megacrysts may also be bordered by myrmekite or nearly completely filled with myrmekite.
At Alastaro, it is clear that the K-feldspar is primary instead of secondary because it is deformed and fractured, and that the myrmekite is secondary, because it is undeformed in the fractures. Myrmekite is also found on borders of deformed plagioclase in the gabbro, in the interiors of fractured K-feldspar megacrysts in the granite, and in Carlsbad-twinned euhedral plagioclase inclusions in the megacrysts. Ca-bearing fluids have entered following the deformation and produced the myrmekite.
2KAlSi3O8 + Ca+2 = CaAl2Si2O8 + 4SiO2 + 2K+1potassium-feldspar . . . . . . . . myrmekite
The fact that plagioclase in the gabbro is converted to myrmekite indicates that Ca-metasomatism has affected the relatively sodic rims of plagioclase as shown in the following equation.
2NaAlSi3O8 + Ca+2 = CaAl2Si2O8 + 4SiO2 + 2Na+1sodic plagioclase . . . . . . . . myrmekite
The possibility that the myrmekite could be the result of a late-stage magmatic process is essentially eliminated because myrmekite is found only in deformed rock and occurs in both granite and gabbro as would be expected if Ca-bearing fluids were introduced in shear zones that cut both rock types.
Distinctive features of Ca-metasomatism of K-feldspar (as seen at Alastaro) include (1) fractures in the K-feldspar bordered by myrmekite, (2) a central quartz ribbon (Figs. 2, 3, and 4), (3) aggregate masses of myrmekite filling large portions (>60%) of the K-feldspar (Figs. 5 and 6), and (4) the constant thickness of quartz vermicules in the myrmekite against the K-feldspar (Fig. 7). The nearly constant thickness of the quartz vermicules would be expected since the introduced fluids should have a nearly constant Ca content.
These features contrast with K-metasomatism of plagioclase in which (1) the K-feldspar is unfractured, (2) the K-feldspar completely or nearly completely replaces broken, deformed, and strained plagioclase, (3) the K-feldspar is bordered by only minor quantities of myrmekite (generally <1%), and (4) the quartz vermicules in the myrmekite are not uniform but taper and/or branch (feather) toward the K-feldspar contact.
The Alastaro site where Ca-metasomatism has replaced K-feldspar is significant because the replacement of K-feldspar by Ca and Si shows the predicted characteristics for Ca-metasomatism, as discussed in presentation one. The absence of these characteristics in other terranes for which other investigators have claimed that myrmekite has formed by Ca- and Na-metasomatism along borders of K-feldspar indicates that their hypotheses are likely faulty.
Another locality, recently described, which shows Ca-metasomatism forming myrmekite occurs in the Velay granite (Massif Central) in France (Garcia et al, 1996). In this area, replacement of K-feldspar by myrmekite is observed only in the enclaves, not in the host granite (Figs. 8, 9, and 10). This replacement is thought to be driven by the influx of Na-rich fluids from the host granite in the range of 450-650º C. These Na-rich fluids cause subsequent reequilibration with the Na-poorer feldspars in the enclaves (Figs. 11 and 12). Here, the Na replaces Ca in plagioclase, and, in turn, the released Ca replaces K-feldspar to form myrmekite. Cathodoluminescent images are provided by Daniel Garcia.
In leucosomes of Velay migmatitic paragneisses, K-feldspar invades and replaces quartz-plagioclase contacts (Fig. 13), and locally produces a rock containing nearly pure K-feldspar. Because the invading K-feldspar preferentially develops along contacts between plagioclase and quartz, minimal transport of Si and Al is required. This reaction may be viewed on a local scale, as opposite to the replacement of K-feldspar by myrmekite (Garcia, personal communication, 1997).
K-feldspar replacement of the plagioclase without forming myrmekite suggests that the system is wide open so that Ca and Na are easily displaced from the plagioclase. The lack of quartz in the melanosomes may also be a factor.
Perchuk et al. (1994; personal communication, 1997) reports a third locality in Sri Lanka in charnockites where Ca-metasomatism of K-feldspar produces myrmekite.
Ca-metasomatism forming myrmekite in anorthosites has been well documented (Wager and Brown, 1967; De Waard et al., 1977; Ashworth, 1986; Dymek and Schiffries, 1987). Therefore, it is appropriate to compare myrmekite formed by Ca-metasomatism of K-feldspar in granitic rocks with myrmekite formed by Ca-metasomatism of plagioclase in anorthosite. In both kinds of Ca-metasomatism, the quartz vermicules tend to have constant thickness, but locally in anorthosite, vermicules taper toward the primary, non-quartz-bearing plagioclase because of incorporation of Na which requires more silica in the feldspar lattice.
Post Script: An example of Ca- and Na-metasomatism of primary K-feldspar porphyroclasts can be found in a mylonitic contact aureole surrounding a granodiorite in the eastern Alps. The metasomatism is not large scale but local, using Ca and Na released during metamorphism of minerals available in a volume smaller than that represented by a thin section. The metasomatism produces a corona of myrmekite around the K-feldspar. K-feldspar megacrysts lacking myrmekite coronas can be found in undeformed granitic gneisses outside the contact aureole. See the article by B. Cesare, C. Marchesi, and J. A. D. Connolly, 2002, "Growth of myrmekite coronas by contact metamorphism of granitic mylonites in the aureole of Cima de Vila, Eastern Alps, Italy": Journal of Metamorphic Geology, v. 20, p. 203-213, at the following link: http://www.dmp.unipd.it/bernardo/papers.html With the permission of Bernardo Cesare, the following is a link to his Fig. 4d which shows an example of a myrmekite corona around K-feldspar in a mylonite.
Dr. Lorence G. Collins Department of Geological Sciences 18111 Nordhoff Street Northridge, CA 91330-8266 FAX 818-677-2820
