Petrogenesis of the Cambrian Liv Group, a bimodal volcanic rock suite fromthe Ross orogen, Transantarctic Mountains

Cambrian volcanic rocks of the Liv Group, defined here as including the Wya tt, Ackerman, Taylor, Fairweather, and Leverett Formations, occur along the paleo-Pacific margin of Gondwana, in the Queen Maud Mountains, Transantarc tic Mountains. The Ackerman and Wyatt Formations are dominated by massive d acite lava flows and were erupted ca, 525 Ma. The Taylor, Fairweather, and Leverett Formations form a bimodal assemblage of basalts and rhyolites and were erupted ca, 515 Ma. The dacites of the Ackerman and Wyatt Formations a re the most light rare earth element (REE) and large ion lithophile element (LILE) enriched rocks (La-N/Yb-N = 6.6-10.2; Th/Nb = 0.9-1.8) of the Liv G roup. They have the lowest epsilon Nd-i (-1.8 to -3.1) of all the Liv Group volcanic rocks and are interpreted to be partial melts of continental crus t, Sm-Nd model ages suggest that some of this crust may be as old as 1.5 Ga . The volumetrically minor basalts and basaltic andesites of the Taylor, Fa irweather, and Leverett Formations are variably light REE and LILE enriched (varying from La-N/Yb-N = 1.5 to 6.0) and have epsilon Nd-i between 5.7 an d -1.1, The most depleted of these basalts are transitional between normal midocean ridge basalt (MORB) and enriched MORB and are interpreted as melts of asthenospheric mantle that were variably enriched in light REE and LILE by melts from lithospheric mantle and/or continental crust, The rhyolites of the Taylor, Fairweather, and Leverett Formations have La-N/Yb-N = 2.8-6. 0, Th/Nb = 1.0-1.6, and epsilon Nd, between 2.1 and -2.8 and are interprete d as mixtures of fractionated mafic magma and crustal partial melt. The Liv Group rhyolites were probably generated in response to the mafic magmatism , The most likely tectonic setting for the Liv Group was in an extensional rift environment within or behind an active volcanic are.