PETROLOGICAL AND GEOCHEMICAL EVIDENCE FOR GRANITOID FORMATION - THE WALDOBORO PLUTON COMPLEX, MAINE

The Acadian-age Waldoboro Pluton Complex (WPC), mid-coastal Maine, con sists of seven granitoid units surrounded by migmatitic, peraluminous gneisses and schists (predominantly Bucksport or Sebascodegan Formatio n). The complex (area >340 km(2)) cross-cuts the westward-vergent St. George thrust fault, which may mark the boundary between the Avalon an d Gander composite terranes. Field and petrologic data indicate in sit u formation of the peraluminous, syntectonic granitoids: contacts with Bucksport paragneisses are transitional and concordant; abundant coun try-rock enclaves show evidence for melting; restitic garnet, biotite, and plagioclase in the granitoids are identical in composition to gar net, biotite, and plagioclase in the country rock. Chemical variations among the main granitoid phases (gneissic granite, granite, and leuco granite) reflect varying degrees of melt-restite unmixing. Major and t race elements define mixing trends between refractory Bucksport lithol ogies and leucogranites which approximate melt compositions. Petrograp hic and whole-rock chemical data are consistent with restitic plagiocl ase, garnet, biotite, tourmaline, zircon, apatite, sphene, and an acce ssory phase such as monazite. Quantitative major-oxide mass-balance mo dels indicate that gneissic granite represents a mixture of 55% melt-4 5% restite whereas granite represents a mixture of 76% melt-24% restit e. Melt-restite proportions calculated from trace element data agree w ith those calculated from major oxide data for the gneissic granite, b ut are different (85% melt-15% restite) from those calculated from maj or oxide data for the granite. This is attributed to inhomogeneous dis tribution of minor phases and the effects of metasomatism. High K2O, R b, Ba, Cs, Li, B, K/Rb, K/Ba, Rb/Sr, and Th/U along the eastern myloni tic margin and elsewhere within the WPC reflect post-solidification me tasomatic processes. Intrusion of mafic magmas during uplift after cru stal thickening appears to have caused high-temperature metamorphism a nd anatexis of Bucksport country rocks at relatively low pressure (0.4 GPa). Dehydration melting of muscovite to produce magmas saturated or nearly saturated with H2O explains the formation of migmatites in the vicinity of the WPC. Formation of granites by 50-60% fluid-absent mel ting of Bucksport source rocks containing 20% biotite requires that fu sion occurred at T greater than or equal to 860 degrees C and consumed all of the biotite in the source rock. Phase equilibrium data and est imated temperatures of formation provide evidence that the granitoids formed at T<860 degrees C, whereas petrographic data indicate that not all biotite in the source rock was consumed during anatexis. Therefor e, the WPC granitoids could have formed by fluid-absent melting if the source rocks contained >20% biotite (the maximum amount observed). Ho wever, it is also possible that influx of aqueous fluid before or duri ng anatexis allowed production of relatively large volumes of melt at T<860 degrees C. Available data do not allow these possibilities to be rigorously tested. The WPC granitoids have many characteristics of S- type granites and preserve a chemical and mineralogical record of thei r source rocks, indicating that granites can image their sources even in tectonically complex regions.