5.1. Chemical changes due to alteration and metamorphism
Whole-rock compositions of major and trace elements were obtained by
ICP-MS analysis of powdered samples fused with LiBO2.
For a subset of samples, whole-rock Th, Nb, Ta, La, Pb, Nd, Sm, Zr and
Hf were also analyzed by high resolution ICP-MS with high-pressure
dissolution and HF–HNO3 digestion. Results are reported
in Appendix E, as well as details of analytical techniques, including
accuracy and precision. In occasions, the ultramafic and gabbroic rocks
of the Rio Boba sequence have been heterogeneously deformed and
metamorphosed to granulite, amphibolite and greenschists facies
conditions. Therefore, the mobility during metamorphism of certain major
(e.g., Si, Na, K, Ca) and trace (e.g., B, Li, Cs, Rb, Ba, U, Sr)
elements may have modified the primary whole-rock geochemistry. However,
the HFSE (Nb, Ta, Zr, Hf, Ti and Y), REE, transition elements (V, Cr, Ni
and Sc) and Th, generally remain unaffected at the scale of
hand-specimen under a wide range of metamorphic conditions (e.g.,
Bédard, 1999; Pearce and Peate, 1995). Accordingly, the following
geochemical characterization of rock samples, calculation of equilibrium
melts and petrogenetic discussion will be based mostly on the HFSE and
REE.