NEODYMIUM AND STRONTIUM ISOTOPIC AND TRACE-ELEMENT COMPOSITION OF A MESOZOIC CFB SUITE FROM DRONNING-MAUD LAND, ANTARCTICA - IMPLICATIONS FOR LITHOSPHERE AND ASTHENOSPHERE CONTRIBUTIONS TO KAROO MAGMATISM

Mesozoic Karoo-related lavas and dikes from north Vestfjella, Dronning Maud Land, comprise four magma types (CTI, CT2, CT3, and CT4) that oc cur intimately interspersed in a 300-m-thick lava succession and assoc iated dike swarm. The CT1 lavas and dikes exhibit low initial epsilon( Nd) (-11.0 to -15.8) and high initial epsilon(Sr) (+36 to +52), belong to Gondwana low-Ti tholeiites and are grossly similar to the Southern Lebombo basalts of Karoo. They show strong geochemical affinity to co ntinental lithosphere and were probably derived from Archean lithosphe ric mantle. The CTI tholeiites are characterized by higher epsilon(Nd) (-6.7 to +2.5) and lower epsilon(Sr) (-4 to +37), resemble the MORE-l ike Rooi Rand magma type of Karoo, and are distinct from typical CFBs. They were probably derived from an isotopically heterogeneous, mildly LILE-enriched source with MORE-affinity. The CT3 basalts have positiv e epsilon(Nd) (+1.5 and +1.9), relatively low epsilon(Sr) (+7 and +11) , and a trace element signature approaching that of island are basalts . Their mantle source was depleted in terms of neodymium isotopic comp osition but high in LILE, possibly due to subduction-related enrichmen t in the Mesoproterozoic. The CT4 dolerites show OIB-like geochemistry , chondritic epsilon(Sr) (+1), and the highest epsilon(Nd) (+3.6) so f ar reported for Antarctic Mesozoic CFBs. Thus they represent the first Karoo-related rock type with unambiguous plume-affinity. In the epsil on(Nd) vs. epsilon(Sr) space, the Vestfjella CFBs span a wide range fr om the oceanic mantle array towards a composition between EMI and EM2. Overall, their trace element and isotopic variation reflects billion- year-scale evolution of the upper mantle at an Archean-Proterozoic lit hospheric boundary. Copyright (C) 1998 Elsevier Science Ltd.