Comb-layering in the Sierra Nevada

Spectacular comb-layering can be observed in the Fisher Lake area, next to Donner Pass in the Tahoe National Forest (Sierra Nevada, California). Comb-layering has been defined in a 1973 paper by Moore and Lockwood. It describes a type of layering were crystals are oriented nearly perpendicular to the planes of layering. It is rather uncommon, but present in a variety of igneous rocks, from granitoids to diorites and gabbros. Comb-layering is related to the crescumulate texture and the harrisitic texture.

At Fisher Lake, outcroping conditions are excellent due to the poor vegetal cover, a consequence of high-altitude, and the quasi-absence of soil. The layered rocks occur near the margin of a hornblende-enstatite diorite pluton. Straight layering can be followed for decameters, although folding and interrupted layers are common. Minerals in the layers include plagioclase, hornblende, olivine, orthopyroxene, and clinopyroxene.

Elongated crystals in the comb-layering structure, as well as other dendritic (s.l.) crystals appear to have formed at high-degree of undercooling (see Faure et al. 2003 for a detailed study on crystal shapes, with the example of olivine). Slow crystal growth in a magmatic liquid generally leds to well-formed, euhedral crystals. Fast growth leads to elongated dendritic crystals or hopper crystals depending on the degree of undercooling. In the case of comb-layering, the fast growth is unlikely to be a consequence of fast cooling, because these structures occur as part of large bodies of plutonic (thus slowly cooled) rocks. The comb-layering structure is thus likely a consequence of strong undercooling. The Fisher lake rocks show numerous evidence for the presence of very-high water content. Water lowers liquidus temperatures and delays nucleation, generating strong degrees of undercooling. When nucleation finally occurs, crystal growth happens very rapidly due to the high undercooling, and produces elongated dendritic crystals.

The strong undercooling delays homogeneous nucleation, so when nucleation is finally forced to occur rapidly, heterogeneous nucleation on existing solid crystals is favored. Nucleation occurs on the walls, producing comb-layering, but also on any piece of mineral or solid rock present in the magma (or fluid). Crystallization around solid fragments produces orbs (orbicular layering), a type of comb layering common in granitic to gabbroic rocks. Large gabbroic orbs (up to 30 cm) are very common at Fisher Lake. Heterogeneous nucleation happens on any solid fragment in the magma. This also means broken pieces of comb-layered gabbro from a previous crystallization step. Similar (but less spectacular) orbs occur in granitic bodies in Brittany (Ploumanac'h, Decitre et al. 2002) and Central France (Signal de Randon, Couturié 1973).

Each layer registered the rapid crystallization of the melt/fluid, and the crystallization sequence follows melt composition (for example, first mafic minerals like amphibole, then plagioclase...) Crystallization then stops until a new magma/fluid batch is undercooled enough to start a new round of heterogeneous nucleation.

Magma or fluid ? Obviously the amount of water is very high here, and drawing a boundary between magma and fluid is not always straightforward. However, Moore and Lockwood (1973) convincingly argue that comb-layering is formed in a low-viscosity, low-density, fast-moving fluid, flowing in pipe-like conduits. Pipes full of orbs form spectacular outcrops near Fisher Lake. Geologists loved the outcrop so much that one of them brought a 5 kg sledge hammer, and left it rusting because I guess he was too loaded with nice samples... Note that structures similar to comb-layering (comb-structures) are found in pegmatites (another fluid-rich environment !) and on the floor/walls of some layered gabbroic intrusions ("harrisitic texture").

To make the area a little more complicated, it is also possible to observe regular layering, very similar to the one observed in Ploumanac'h. This schlieren-layering is usually ascribed to deposition by gravity currents in the cooling pluton.

References:

- Couturié J-P (1973) Un nouveau gisement de granite orbiculaire dans le Massif Central français: le granite du Signal de Randon (Lozère). Contribution to Mineralogy and Petrology 42: 305-312
- Decitre S, Gasquet D, Marignac Ch (2002) Genesis of orbicular granitic rocks from the Ploumanac'h Plutonic Complex (Brittany, France). European Journal of Mineralogy 14: 715-131
- Faure F, Trolliard G, Nicollet Ch, Montel J-M (2003) A developmental model of olivine morphology as a function of the cooling rate and the degree of undercooling. Contribution to Mineralogy and Petrology 145: 251-163.
- Moore JG, Lockwood JP (1973) Origin of comb layering and orbicular structure, Sierra Nevada batholith, California. Geological Society of America Bulletin 84: 1-20.