Unmetamorphosed Early Eocene sediments and volcanic rocks of the Trinity Hills and Enderby Cliffs yield K–Ar dates of 42–49 Ma. These overlie high-grade gneisses yielding K–Ar ages on biotites, muscovites, and hornblende ranging from 47 to 60 Ma. The Eocene sediments and volcanics rest nearby on low-grade phyllites, greenstones, and schists yielding dates from 83 to 155 Ma. The gneiss dates are regarded as reset by some Late Cretaceous to earliest Cenozoic thermal event that did not affect, at least to the same degree, the nearby less metamorphosed basement rocks. A thermal history has been constructed to account for the decreasing apparent ages of biotite (assumed blocking temperature of 250 °C) with increasing depth below the sub-Eocene unconformity, for the greater ages of hornblende and muscovite in the same rocks (blocking temperatures of 500 and 350 °C), as well as for thermal changes associated with high vitrinite reflectance from coal at one site in the covering sediments. Very rapid stripping (something like 5 km in 12 Ma) is inferred for the areas of reset gneisses, but not for the schist areas, in early Cenozoic time.

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