Dating and melthing icq dating com
This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C.
After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level.
For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex.
A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78).
We thank the CNRS-INSU IT program "Exhumation" for financial support of field and laboratory work, and we are very much indebted to Luisa Carton-de la Cruz for Rb–Sr analytical work.The isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale.The Re–Os system in molybdenite was not affected by subsequent 0.93 Ga contact metamorphism, corresponding to formation of garnet quartz coronitic textures around molybdenite and other minerals in the deposits.In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma.
For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year.
First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation.