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

it is;accepted that continental growth takes place both in intraplate and c onvergent 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 resul ts in significant increases in continental volume. We maintain that A-type, or anorogenic, granites are derived from young, underplated mafic crust. M agma 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, southeaster n Wyoming, intrudes Archean gneiss, Isotopic data from these granites precl ude derivation from felsic crust, and instead require the involvement of a mantle or mantle-like isotopic reservoir. The data are analogous to those f or eruptive equivalents of A-type granites, the fayalite rhyolites of Yello wstone, which also ascended through Archean felsic crust but carry little A rchean isotopic signature. Anorogenic granites thus may represent a middle to upper crustal record of magmatic underplating at depth.