193 / 352
177
Another explanation for the chemical
variation shown by this sample could be that the
protolith of this rock was not really an igneous
rock but a sedimentary one. In fact, this sample
was collected assuming it was a non-altered
metasedimentary rock instead of an orthogneiss
(see field photograph, Fig. 57). This confusion
may look like a novels geologist mistake but
the truth is that the rocks outcropping in this
high-grade terrain are frequently cryptic in
terms of rock classification. This rock has been
classified as an orthogneiss because it has no
inherited zircon, but doubts arise, since in arc
environments, a sedimentary rock can contain
a single zircon population from a unique source
area and an igneous rock can have a huge number
of inhered zircon grains due to a complex igneous
development.
7.4. ORTHOGNEISSES