"Rock of the Month # 239, posted for May 2021" ---

Nature of the rock

Kentallenite is defined (Le Maitre et al., 1989, p.79) as "a local term for a melanocratic variety of monzonite composed of olivine, augite, zoned plagioclase, brown and green micas and interstitial alkali feldspar". The "family name" monzonite refers to a plutonic rock with subequal amounts of plagioclase and alkali feldspars, and minor amphibole and/or pyroxene. With more plagioclase the rock would be a diorite; with more alkali feldspar, a syenite. Some monzonites are silica-saturated (quartz monzonite), but not this one! It is part of the so-called, locally-named appinite suite.

The Caledonian appinite suite, as found in the area of Kentallen, is described in detail by Bowes and colleagues (1964, 1967, 1993). Kentallenite itself is described, at least in part, as an augite diorite (see Bowes and Wright, 1967). Note that no thin section is available for this sample, and it is hard to discern the proportions of feldspars and other minerals that determine, for instance, whether this particular sample is a monzonite or a diorite. For more on the appinite suite, see the long-ago Rock of the Month 32. Appinite (Le Maitre, 1989, p.48) is "a local, general term for medium- to coarse-grained, meso- to melanocratic rocks with conspicuous hornblende in a base of oligoclase-andesine and/or orthoclase (feldspars) with or without quartz". Subvolcanic equivalents of appinites belong to the lamprophyre clan.

Local it may be, yet these terms have been exported on occasion. As an example, the Spanish quentalenita (Long, 1991) may be cited as evidence - an obscure and obsolete term for a Scottish augite olivine monzonite, far from home! These are, as noted above, tough igneous rocks, and as such - like the wider granite clan - could be used as aggregate in road construction (Pearce, 1994), as well as for potential, more glamourous uses as decorative stone.

For a much more elegant and extensive documentation of kentallenite, complete with location map and no less than 45 photomicrographs, please see the web site of my erudite Italian friend Alessandro di Mommio. See also the photographic atlas of Mackenzie et al. (1982).

The Appin district hosts numerous unusual breccia pipes, studded with clasts that were plastic when incorporated into the breccia: these breccias "represent the rarely-preserved boundary zone between active magma and breccia systems" (Platten, 1984). In this lithologically diverse suite, "andesine is the typical plagioclase in all these rocks, so they are classified modally as diorites" (Platten, 1991). A combination of zircon and titanite (sphene) U-Pb age dating of appinitic and associated granitic rocks indicates a relatively brief, Silurian episode of melt emplacement, with dates on appinites, granodiorite and diorite in the range 430-420 Ma (Rogers and Dunning, 1991). The middle Silurian was a time of major transcurrent fault motion and voluminous magmatism, seen also in Newfoundland, Canada. The site of Rubha Mor, just southwest of Kentallen, includes rocks such as kentallenite and pyroxenite, lamprophyre and augite-biotite diorite. The overall magmatic succession was early kentallenite, followed by lamprophyre, and then by granites.

Fig. 2: Obverse side of sample from Figure 1.