The timing of ups and downs of metamorphism

An enlargeable topographic map of Sri Lanka

Complex topography of Sri Lanka via Wikipedia

As the temperature and pressure affecting crustal rocks go up and down, as for instance in the thickening of crust when two continents collide and then erosion strips off the cover so that the rocks slowly rise, the rocks undergo progressive changes in their mineral content; in both cases they are metamorphosed. Rising intensity of conditions gives rise to a prograde metamorphic sequence, and when they wane retrograde metamorphism takes place as the elements that combine in minerals react to adjust to new conditions. In some cases it is possible to use the mineral assemblages, specifically the proportions of different elements that are shared between two or more minerals, to chart the changes in temperature and pressure. That reveals the path taken by the rock through temperature- and pressure space, which is effectively a measure of the crustal processes involved and the geothermal conditions under which they acted: a P-T path. Adding the timing to give a sort of movie to all the changes has been hit-or-miss up to now, and based on radiometric ages from igneous rocks formed and emplaced during the metamorphic evolution. Thanks to the finely targeted mass spectrometry that an ion microprobe an achieve, adding the ‘t’ dimension is now possible from the metamorphic rocks themselves (Sajeev, K. et al. 2010. Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature  metamorphism exemplified by Sri Lankan granulites. Geology, v. 38, p. 971-974). Minerals based on the element zirconium (Zr), such as zircon and monazite are extremely resistant to the effects of temperature as regards the radioactive and radiogenic elements that they contain, specifically uranium (U) and thorium (Th) and the lead (Pb) isotopes that form when 235U, 238U and 232Th decay. Both these minerals become zoned as successive layers grow during metamorphism, and the ion microprobe can measure the isotopic composition on a later-by-layer and therefore event-by-event basis. The famous granulites (charnockites) of the island of Sri Lanka (Ceylon) reached the peak of their metamorphism (1050°C and 0.9 GPa) at ~570 Ma and began to retrogress about 20 Ma later around the start of the Cambrian. Previously it was not possible to separate metamorphic ages from those when the original rocks formed in the Archaean and early Neoproterozoic.  Such high temperatures are very difficult to attain in the crust under normal geothermal conditions unless extra heat is added by large volumes of basaltic magma ponding at the base of the crust during crustal thickening.


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