Conventional field, petrological and mineralogical
studies of igneous and metamorphic rocks need to be supplemented
by major and trace element geochemistry and by isotope tracer studies
(see Geochemistry and Geochronology subsections) to provide an adequate
chemical description of the rocks.
With these additional geochemical data, it is possible to characterize
the geochemistry of the tectonic setting in which the granitic and
volcanic rocks were formed. It is particularly important to capitalize
on exposures of lower crustal rocks by undertaking petrological
and geochemical studies to elucidate lower crustal petrogenesis
(i.e., the processes by which rocks in the lower crust formed).
Pressure and temperature data from metamorphic mineral assemblages
and their fluid inclusions are being used to estimate rates of uplift
and erosion. This may be achieved by comparing P-T (pressure-temperature)
data for the time the rocks were originally metamorphosed with P-T
data for the time the same rocks were known to be at a higher crustal
level (calibrated, for example, from the time when subsequent granitic
plutons or basic dykes intruded them) and finally from the time
the rocks were brought to the surface and made available to erosion.
Complicated? You bet! But, this kind of information not only reveals
the rate at which crustal thickening and subsequent uplift took
place but also suggests further lines of inquiry regarding the subsidence
histories of sedimentary basins which received the products of erosion
related to the uplift of the metamorphic terranes.
|
