The Brant Lake Gneiss is exposed in the cores of two structural domes in the northwestern portion of the Bolton Landing, New York, quadrangle (15′). Overlying the gneiss, there are metasedimentary rocks which may be correlated respectively with the “Older Paragneisses” (lowest), the Paradox Lake Marble, and the Treadway Mountain Formation (highest) of Walton (Walton and deWaard, 1963).
Rb-Sr isochrons from whole-rock samples yield the following: (1) core-rocks of the larger dome are 1119 ± 39 m.y. old with (Sr87/Sr86)o = 0.7073 ± .0021; (2) core-rocks of the smaller dome are 1144 ± 11 m.y. old with (Sr87/Sr86)o = 0.7055 ± .0006; (3)samples of the “Older Paragneisses,” collected from the same outcrop, are 1210 ± 45 m.y. old with (Sr87/Sr86)o = 0.7023 ± .001.5. All errors are given at 80 percent confidence limits.
The rocks in the cores of the domes are clearly not the basement upon which the precursors of the “Older Paragneisses” were deposited. They are younger and probably represent intrusive masses; their initial Sr87/Sr86 ratios suggest derivation from pre-existing crustal material, perhaps by anatexis or by metamorphic processes which resulted in re-crystallization and homogenization of Sr isotopes. If the average Rb87/Sr86 of the “Older Paragneisses” is representative of the rock mass which was melted or metamorphically homogenized about 127 ± 90 m.y. would be required for Sr87/Sr86 in these rocks to change from an assumed initial value of 0.7023 to a value of 0.7064 at a time 1130 m.y. ago. Considering the uncertainties in these values, it is reasonable t o believe that the intrusive bodies were formed about 1130 m.y. ago from rocks like the “Older Paragneisses,” which formed about 1210 m.y. ago and whose Sr87/Sr86 ratio at that time was about 0.702.