Reconnaissance isotopic investigations on rocks of an undeformed granite contact within the coesite-bearing unit of the Dora Maira Massif

Recent research on ultrahigh-pressure rocks in Val Gilba and the Brossasco area has revealed the complete textural preservation of large volumes of a premetamorphic granite, together with its former contact aureole, that was subducted to depths of > 100 km, as indicated by coesite pseudomorphs. Petrographic studies indicate that, although most minerals have been affected by metamorphism, biotite and potassium feldspars in the rock are largely preserved except for thin peripheral rimming of biotite by phengite and garnet. Rb-Sr analyses on seven biotite samples fail to produce a valid isochron, indicating either incomplete resetting, or inheritance of radiogenic Sr, during metamorphism. Ar/Ar data from two biotites likewise yield very irregular patterns that are consistent with an episode of severe Ar loss from a pre-Alpine parent rock, followed later by addition of inherited radiogenic Ar. The biotite Kb-Sr and R-Ar systems have therefore been extensively disturbed during Alpine metamorphism in spite of the largely unaltered appearance of the crystals in thin sections. Attempts to measure U-Pb ages with apatite also failed. Although the geochronological studies yielded no reliable ages, Pb and Sr ratios from four potassium feldspar samples are within the ranges found in known Hercynian rocks, providing new evidence that Hercynian rocks played a prominent role as source materials for the Dora Maira rocks. Sr ratios from two apatite samples agree closely with the potassium feldspar values. The ratios are also substantially lower than those found in the whiteschists in completely recrystallized rocks at Parigi, ca. 3 km north of the present rocks. These results indicate that the undeformed rocks in Val Gilba-Brossasco area are less disturbed by Alpine ultrahigh-pressure metamorphism, thereby preserving the record of the protoliths somewhat better than do the completely recrystallized rocks at Parigi to the north. The Pb and Sr isotope data further show that the minerals of the undeformed rocks failed to reach isotopic equilibrium despite subduction to > 100 km and heating to > 700 degrees C. (C) 1997 Elsevier Science B.V.