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Crustal growth by magmatic underplating: Isotopic evidence from the northern Sherman batholith

¹ Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

It is accepted that continental growth takes place both in intraplate and convergent margin settings, but the relative importance of crustal growth in these two tectonic environments is the subject of ongoing debate. In this study we suggest that magmatic underplating in continental interiors results in significant increases in continental volume. We maintain that A-type, or anorogenic, granites are derived from young, underplated mafic crust. Magma sources for A-type granites typically are difficult to identify due to the lack of isotopic contrast between mantle and crustal sources. However, the northern portion of the Mesoproterozoic Sherman batholith, southeastern Wyoming, intrudes Archean gneiss. Isotopic data from these granites preclude derivation from felsic crust, and instead require the involvement of a mantle or mantle-like isotopic reservoir. The data are analogous to those for eruptive equivalents of A-type granites, the fayalite rhyolites of Yellowstone, which also ascended through Archean felsic crust but carry little Archean isotopic signature. Anorogenic granites thus may represent a middle to upper crustal record of magmatic underplating at depth.

Key Words: A-type granites • isotopes • Proterozoic • Wyoming province • crustal origin

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