TRACE-ELEMENT AND ISOTOPIC CHARACTERISTICS OF SMALL-DEGREE MELTS OF THE ASTHENOSPHERE - EVIDENCE FROM THE ALKALIC BASALTS OF THE ANTARCTIC PENINSULA

Miocene-Recent continental alkalic basalts were erupted along the Anta rctic Peninsula as a result of decompressional melting of the asthenos phere caused by the formation of slab-windows beneath the continental margin following the cessation of subduction. The basalts appear not t o be related to a period of major lithospheric attenuation, nor were t hey formed as a result of the influence of a mantle plume. They exhibi t strong trace-element and isotopic affinities with OIB, Sr- and Nd-is otope compositions ranging from 0.70269 to 0.70343 and 0.512863 to 0.5 1300, respectively, similar to the composition of HIMU OIB. However, n ew Pb-isotope analyses show that Pb-206/Pb-204 ratios (18.79-19.28) fa ll within the range for E-type MORB with DELTA8/4 and DELTA7/4 varying from -28 to +26 and from +1 to +10, respectively. DELTA8/4-values, Sr -isotope ratios and some LILE/HFSE ratios exhibit negative covariation s with La(n)/Yb(n) and Nb/Y ratios implying some control of degree of partial melting on geochemical composition. Nb/U ratios (14-40) are co nsiderably lower than most OIB and MORB. The basalts also have unusual ly low absolute abundances of Rb and Ba and high K/Ba and K/Rb ratios (50-140 and 400-1500, respectively). Correlated Pb-Sr-Nd isotope and t race-element behaviour suggests that the asthenosphere from which thes e basalts were derived was subjected to multiple melt extraction/deple tion events. One period of melt extraction was ancient (approximately 1.7 Ga) and similar to that affecting MORB source mantle, and was foll owed by a more recent (?Mesozoic) event. This more recent event result ed in increased U/Pb, U/Nb and U/Th ratios and further depletion in ul tra-incompatible element such as Rb and Ba, causing high K/Rb and K/Ba ratios in the erupted lavas. This implies that the asthenosphere bene ath the Antarctic Peninsula is heterogeneous on a small scale. Small-d egree melts are capable of sampling geochemically, and possibly minera logically, distinct mantle domains from larger-degree melts. During la rger degrees of partial melting, the scale of melting approaches the s cale of heterogeneity and integration of melts from different geochemi cal domains occurs.