One of the introductory-level bits of information people learn about fossils
is that fossils are found in sedimentary rocks, not igneous rocks or
metamorphic rocks. This works as a first-order approximation, but... it's not
strictly true. For example, among igneous rocks, there are some that overlap
sedimentary rocks, e.g., pyroclastic flows, ash beds, and so on. You can
certainly find fossils in those. There are also occasional impressions of
things in basalt flows;
tree trunk molds
are most common, and at least one rhino (the famous "Blue Lake rhino"; e.g.,
Beck 1937) has been captured this way. Tree molds are also interesting for
testing our conventional definitions of paleontology in other ways: Because of
the rapid cooling and hardening of the basalt, tree molds are essentially
instant fossils. The rock can't get much more lithified, after all (especially
compared with Quaternary sedimentary slop), yet modern examples haven't put in
the time that frequently quoted definitions insist on for fossils, such as
10,000 years. In my day job we talk about paleontological resources as
evidence of life in a geological context, so there is no cutoff. Where was I?
Oh, yes. Basically, you can get fossils in igneous rock if the rock was
playing by sedimentary rules when it formed. Don't expect them in intrusive
igneous rocks, though, except in
xenoliths
of sedimentary host rock (e.g., McCracken et al. 2000, also featuring
cores!).
For metamorphic rocks, most of us with paleontological experience will make
exceptions for low-grade metamorphics: quartzites, slates, low-grade marbles,
things like those. In fact, a lot of rocks commonly called "marbles" are just
limestones that can hold a polish. Get much beyond that, and surely the
temperature, pressure, chemically active fluids, and mineralogical changes
will destroy any unsuspecting crinoid or brachiopod, right?
Well...
Obviously, the answer is "not always", otherwise there wouldn't be a post.
Reports of fossils in metamorphic rocks even as far gone as
eclogite
(Cavrgna-Sani et al. 2010) or
migmatite
(Hill 1985; the rock you get when it stops screwing around with metamorphism
and gets down to serious melting, but doesn't finish) pop up every so often in
the literature as a thin thread of geological
quasi-Forteana. The "thin thread" is because they are not common. If they *were* common,
they wouldn't be worthy of comment. You don't get a paper out of announcing
that there are crinoid columnals in the Decorah Shale. Bucher (1953) is
perhaps the most significant reference on the subject, and includes a good
bibliography, but through no fault of its own is now more than 70 years old
and likely to only get older. Labora-López et al. (2015) adds a few more
recent references in its discussion, although it has its own purposes that do
not include being a thorough review and update of Bucher (1953).
Aside from being a curiosity, fossils in metamorphic rocks are useful for
providing age controls that are notoriously absent from many metamorphic
units, and sometimes can help with correlations. Other interpretations are
limited by distortion or general poor preservation of the fossils, and by the
topic of Labora-López et al. (2015), the loss of information from the
sedimentary matrix itself.
One interesting aspect evident from Bucher's lists is that the fossils are not
necessarily confined to animals with hard parts that are particularly
resistant to replacement (e.g., crinoids). There are recrystallized bivalves,
for example. Of course, bivalve replacement is hardly unusual in itself, as
shells of bivalves and other mollusks are prone to replacement anyway thanks
to the poor stability of aragonite over geologic time, which is why their
fossils are so often molds and casts. Some fossils have been replaced by more
exotic materials, including echinoderm ossicles replaced by
diopside
and
epidote
in a contact metamorphism zone.
Why have fossils in metamorphic rocks not attracted much attention? A
not-insignificant part of the problem is people haven't generally been looking
for them. As noted by Bucher (1953), paleontologists weren't looking because
they assumed that metamorphism destroyed fossils, metamorphic rocks are a pain
in the neck to work with compared to sedimentary rocks, and the fossils that
have been found in metamorphic rocks are not of great quality, so why bother?
Meanwhile, the hard-rock specialists weren't looking because they were
hard-rock specialists and weren't interested in fossils.
Still, though, even if this means that fossils are actually some order of
magnitude more abundant in metamorphic rocks than generally thought, they
aren't exactly falling out of schists and gneisses. Bucher had an elegant
proposal: he suggested that most metamorphic rocks of appropriate age to have
fossils of things like brachiopods, crinoids, and mollusks are derived from
rocks that were deposited in settings with few fossils to begin with. The
exact language used is more complicated and bound up in pre-plate-tectonic
thought and geosynclines (well, it was 1953, after all), but you get
the point: It's hard to have metamorphosed fossils if you don't have fossils
to start with. Another rule of thumb Bucher proposed is that fossils can
persist if metamorphism does not involve mechanical effects and the fossils
are much larger than the size of newly formed minerals. The chances aren't
great, but if you're face to face with a metamorphic rock that began as a
sedimentary rock of
Phanerozoic
age, there might just be a stubborn brachiopod or crinoid in there.
References
Beck, G. F. 1937.
Remarkable west American fossil, the Blue Lake Rhino. The Mineralogist 5(8): 7–8, 20–21.
Bucher, W. H. 1953. Fossils in metamorphic rocks: a review. Bulletin of the
Geological Society of America 64: 275–300.
Cavargna-Sani, M., J. L. Epard, and W. L. Taylor. 1997. Discovery of fossils
in the Adula nappe, new stratigraphic data and tectonic consequences (Central
Alps). Bulletin de la Société vaudoise des Sciences Naturelles 92: 77–84.
Hill, M. L. 1985. Remarkable fossil locality: crinoid stems from migmatite of
the Coast plutonic complex, British Columbia. Geology 13: 825–826.
Laborda-López, C., J. Aguirre, and S. K. Donovan. 2015. Surviving
metamorphism: taphonomy of fossil assemblages in marble and calc-silicate
schist. PALAIOS 30: 668–679.
McCracken, A. D., D. K. Armstrong, and T. E. Bolton. 2000. Conodonts and
corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario
and Quebec. Canadian Journal of Earth Sciences 37(12): 1651–1663.
