Lead-isotope investigations in the Minnesota River Valley—Late-tectonic and posttectonic granites
B. R. DOE, M. H. DELEVAUX, 1980. "Lead-isotope investigations in the Minnesota River Valley—Late-tectonic and posttectonic granites", Selected Studies of Archean Gneisses and Lower Proterozoic Rocks, Southern Canadian Shield, G. B. Morey, Gilbert N. Hanson
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In the Minnesota River Valley, an epizonal, anorogenic granite that is often referred to as the granite of section 28 (lat 44°49.73′N, long 95°33.90′W) has been found to have a Pb-Pb age of 1.84 ± 0.05 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. The Th-Pb age of the feldspar-whole-rock pair is 1.9 b.y., which is in satisfactory agreement with the Pb-Pb age; but the U-Pb age is greater than the Pb-Pb age and probably indicates that uranium has been leached from the whole rock within the past several hundred million years, perhaps as a consequence of dilatancy resulting from uplift and erosion. Another granite, the mesozonal, late-tectonic Precambrian Sacred Heart Granite in the Minnesota River Valley (lat 44°41.3′N, long 95°21.5′W) is found to have a Pb-Pb age of 2.605 ± 0.006 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. Both the Th-Pb and U-Pb isochron ages are much older than the Pb-Pb age. An older age was expected for the U-Pb system as it had been previously found in the epizonal granite, but to also find it for the Th-Pb system was surprising.
As is predicted for these kinds of granites in this type of tectonic environment on the basis of Mesozoic and Cenozoic analogues, the initial leads in the granites indicate that they were derived from source material having values of 238U/204Pb <9 normalized to the present day. This feature is common in Mesozoic and Cenozoic igneous rocks penetrating Precambrian terranes but is rarely observed in pre-Mesozoic igneous rocks. The Sacred Heart Granite is the oldest igneous rock known to show this effect and is the first representative of a mesozonal granite. The uranium depletion event appears to have been a granulite-facies metamorphism, but the age of that metamorphism cannot be determined from the available data. The model-lead-age information, however, suggests that it occurred before 2.78 b.y. ago. The source materials for both granites also underwent an earlier stage of extensive but unknown duration during which 238U/204Pb >9. In Phanerozoic rocks, such values are characteristic of ensialic tectonic environments. Similar development of ensialic environments was apparently occurring also in perancient times.