Multiple Generations of Granite in the Fosdick Mountains, Marie Byrd Land, West Antarctica: Implications for Polyphase Intracrustal Differentiation in a Continental Margin Setting

Production of granite in the middle to lower crust and emplacement into the middle to upper crust at convergent plate margins is the dominant mechanism of crustal differentiation. The Fosdick Mountains of West Antarctica host migmatitic paragneisses and orthogneisses corresponding to the middle to lower crust and granites emplaced as dikes, sills and small plutons, which record processes of intracrustal differentiation along the East Gondwana margin. U-Pb chronology on magmatic zircon from granites reveals emplacement at c. 358-336 Ma and c. 115-98 Ma, consistent with a polyphase tectonic evolution of the region during Devonian-Carboniferous continental arc activity and Cretaceous continental rifting. The gneisses and granites exposed in the Fosdick migmatite-granite complex were derived from Early Paleozoic quartzose turbidites of the Swanson Formation and Ford Granodiorite suite calc-alkaline plutonic rocks, both of which are widely distributed outside the Fosdick Mountains and have affinity with rock elsewhere in East Gondwana. Granites of both Carboniferous and Cretaceous age have distinct chemical signatures that reflect different melting reactions and accessory phase behavior in contrasting sources. Based on whole-rock major and trace element geochemistry and Sr-Nd isotope compositions, Carboniferous granites with low Rb/Sr are interpreted to be products of melting of the Ford Granodiorite suite. Extant mineral equilibria modeling indicates that the Ford Granodiorite suite compositions produce melt volumes > 10 vol. % at temperatures above biotite stability, involving the breakdown of hornblende + plagioclase, consistent with the high CaO and Na2O contents in the low Rb/Sr granites. The Carboniferous low Rb/Sr granites show a sequence from near-melt compositions to compositions with increasing amounts of early crystallized biotite and plagioclase and evidence for apatite dissolution in the source. Carboniferous granites derived from the Swanson Formation are scarce, suggesting that the significant quantities of melt produced from the now residual paragneisses were emplaced at shallower crustal levels than are now exposed. The Cretaceous granites are divided into two distinct groups. An older group of granites (c. 115-110 Ma) has compositions consistent with a dominant Ford Granodiorite source, and characteristics that indicate that they may be less evolved equivalents of the regionally exposed Byrd Coast Granite suite at higher crustal levels. The younger group of granites (c. 109-102 Ma) has distinct light rare earth element depleted signatures. The chemical and isotopic data suggest that these granites were derived from partial melting of both fertile and residual Swanson Formation and had low water contents, indicating that the source rocks may have been dehydrated prior to anatexis as the Byrd Coast Granite suite magmas were transferring through and accumulating at higher crustal levels. The Cretaceous granites derived from the Swanson Formation make up a prominent horizontally sheeted leucogranite complex. The accumulation of these melts probably facilitated melt-induced weakening of the crust during a well-documented transition from regional shortening to regional extension, the formation of a detachment structure, and rapid exhumation of the Fosdick migmatite-granite complex. These multiple episodes of melting along the East Gondwana margin resulted in initial stabilization of the continental crust in the Carboniferous and further intracrustal differentiation in the Cretaceous.