Download Microfabrics of ultra-high pressure metamorphic rocks in the Dora

5
General conclusion
5.
General conclusion
94
Microfabrics of UHP metamorphic rocks of the Dora Maira Massif, western Alps,
provide information on the deformation and stress history of deeply subducted
continental crust. The implications of specific microstructures are grouped into
three sections, which are represented by the thesis.
Microfabrics examined in the Brossasco metagranite as well as in surrounding
granite gneisses and mylonites (Chapter 2) are fine grained foam structured
quartz with random crystallographic orientation, as well as quartz grains showing
highly irregular grain shape, sutured high-angle grain boundaries, undulatory
extinction, low angle boundaries, and a marked SPO and CPO, respectively.
These microfabrics indicate that all deformation took place after transformation of
coesite to quartz, at a late stage of exhumation and at greenschist facies
conditions. There is no evidence of earlier deformation prior to or during UHP
metamorphism in the investigated metagranite. Therefore, the diameter of
internally undeformed slices of continental crust, that experienced UHP
metamorphism, can well exceed the diameter of the presently exposed
undeformed Brossasco granite.
In the pyrope quartzites, and locally in the Brossasco granite prominent
microstructures (Chapter 3) are elongate quartz grains with sutured grain
boundaries and no CPO in the matrix. These compare well to radially arranged
small elongate quartz grains in inclusions in UHP minerals, and are therefore
expected to likewise result from growth of quartz after coesite. The preservation
of such elongate crystal shapes indicates that locally recrystallisation and grain
growth during annealing were insufficient to wipe out the primary coarse-grained
palisade microstructure in the matrix. In addition, any significant deformation of
quartz in the regime of dislocation creep during the later stages of exhumation
can be ruled out.
In jadeite-kyanite-quartz rocks (Chapter 4) mechanical twinned and undeformed
jadeite crystals, both with random crystallographic orientation occur within a
coarse grained foam structured quartz matrix. Mechanical twins in jadeite
indicate a differential stress above 0.3 GPa. The random orientation of twinned
and undeformed jadeite crystals indicate a highly inhomogeneous stress field. It
is proposed that the high stresses at inhomogeneous stress field are the result of
volumetric strain and a very rapid transformation of coesite to quartz. The foam
microstructure of quartz would develop during annealing and grain growth.
Locally sutured quartz high angle grain boundaries occur adjacent to jadeite
largely transformed to albite. This reflects local deformation related to the volume
change during transformation of jadeite to albite and quartz.
The general conclusions of the thesis are consistent with previous findings, i.e.
the entire absence of deformation observed in the UHP metamorphic Brossasco
granite. Also, the hypothesis of a very low interplate shear stress (below a few
5
General conclusion
95
MPa) at UHP metamorphism and during early stages of exhumation, prior to
reaching the upper crust, was confirmed. The induced peak differential stress of
more than 0.3 GPa and the associated inhomogeneous stress field indicates very
high rates of coesite to quartz transformation, comparable to laboratory
experiments. The structural and microstructural record of exhumed UHP
metamorphic rocks provides important information on subduction zone rheology
and the general geodynamics of convergent plate boundaries. This information
can be used in simulations and in the interpretation of results of geophysical field
studies on present-day subduction zones.
Fig. 5.1 P-T path with microstructural record related to the stages of exhumation.