Cretaceous isochron ages from K-Ar and 40Ar / 39Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K-Ar ages of 50-87 Ma. The pelitic schist with paragonite and phengite yielded K-Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite-phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial 40Ar / 36Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K-Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.