Fossils in igneous rocks

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

To elaborate on the discussion:

Volcanoclastic sediments (such as ashfalls) are in the borderland between
igneous and sedimentary rocks.  Yes, they are derived directly from a melt,
and by that definition are igneous.  On the other hand, they are deposited
in Earth surface conditions, temperatures, and pressures, and hence are also
sedimentary.  Such is life... er, rock.

As for fossils in granite: extraordinarily unlikely, at least as geologists
use the term "granite" (there is an industrial definition of the term, which
is any hard rock (where almost every soft rock is "marble"), but that is
beside the point).  Granites cool from molten masses which were entirely
underneath the surface of the Earth; they form under temperature conditions
which would obliterate organic tissue, and are molten for up to millions of
years (extreme cases: more generally ten-to-hundreds of thousands).  Hence
they do not form in environments where living things are present.

(One possibility, though, is that the native rock into which the granitic
magma intruded was a sedimentary rock possessing fossils.  A section of the
native rock could break off and be suspended in the magma as a xenolith.
That way you could have a fossil in a sedimentary rock in a granite).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796