Dear Mauricio,
Stratigraphy is dealing with stratified, it is supracrustal rocks, which are sedimentary or volcanic. And as far as you are dealing with stratigraphy you should estimate the age of formation of these rocks it is time of sedimentation or volcanism.
Metamorphism is particular event which age is also important. But what is the age of metamorphism is also problem, because this is long lasting process, it has progressive branch, peak metamorphism and regressive branch. Moreover there might be few stages of metamorphism (it is polimetamorphism). What time you mean then?
Different problem is the age of metamorphic rocks, completely created in course of metamorphism, as metasomatic rocks, or polimyct cataclasites, or impact rocks (if they are regarded as metamorphic).
Again special case is if you analyze set (suite, complex) or rocks formed in course of certain orogeny or its certain stage. This set might involve metamorphosed supracrustals, metamorphic and igneous rocks under one umbrella with stratigraphic mark. But this is conditional, and recognition of protolith of supracrustals and estimation of their age as certain stratigraphic level remains any way.
 Best regards,
Gediminas Motuza
Vilnius University
Lithuania

Mauricio Silva <[log in to unmask]> rašė:

Hello dear professors and colleagues.

First I would like to congratulate Prof. Yardley for the award. As already written in the latest emails, it was well-deserved for all his work on metamorphism.

This week I had an interesting discussion with one of my colleagues at the Geological Survey about where high-grade metamorphic rocks should lie in a stratigraphic column.
At the Geological Survey and in the schools of geology in Brazil, there are opposing ideas about this subject. Some argue that we should use the age of the protolith, while others say that the age of metamorphism should represent the "real" age of the rock.
This discussion arose because several lithological units composed of orthoderived rocks from Ribeira Belt (Southern Brazil) has Paleoproterozoic ages for the protolith and Neoproterozoic ages for the high-grade metamorphic event.

Since I started to work in Geological Survey, I have used the "protolith age" to define the position of the unit. But now I'm reviewing if I have been doing this "correctly"....

I would like to "hear" your opinion about this subject.

Cheers,

Mauricio Pavan