Flood basalts of Vestfjella: Jurassic magnetism across an Archaean-Proterozoic lithospheric boundary in Dronning Maud Land, Antarctica

Continental flood basalts (CFBs) of Jurassic age make up the Vestfjella mou ntains of western Dronning Maud Land and demonstrate an Antarctic extension of the Karoo large igneous province. A detailed geochemical study of the 1 20-km-long Vestfjella range shows the CFB suite to consist mainly of three intercalated basaltic rock types designated CT1, CT2, and CT3 (chemical typ es 1, 2 and 3) that exhibit different incompatible trace element ratios. CT 1 and CT2 of north Vestfjella record wide ranges of Nd and Sr isotopic comp ositions with initial epsilon(Nd) and epsilon(Sr) ranging from +7.6 to -16. 0 and -16 to +65, respectively. The southern Vestfjella is dominated by CT3 with near-chondritic epsilon(Nd) (+2.0 to -4.1) and epsilon(Sr) (-11 to +1 9). A volumetrically minor suite of ocean island basalt (OIB-)like CT4 dyke s (epsilon(Nd) +3.6, epsilon(Sr) + 1) cuts the lava sequence in north Vestf jella. The pronounced isotopic differences suggest different magmatic plumb ing systems for the heterogeneous CT1 and CT2 suites and the relatively hom ogeneous CT3 lavas. This is further supported by the palaeoflow directions, which point to major source regions to the north (CT1 and CT2) and east (C T3) of Vestfjella. These source regions can be associated with two contempo raneous major lithospheric thinning zones that permitted magma emplacement and controlled the melting of upper-mantle sources in the Jurassic Dronning Maud Land. The CT1 and CT2 magmas utilized the northern zone of thinning a nd were emplaced into the 3 Ga Grunehogna craton, whereas the CT3 magmas we re emplaced through thinned Proterozoic Maud Belt lithosphere. Trace elemen t and isotopic studies of the identified magma types reveal a complex histo ry of fractionation and contamination at different lithospheric levels. All extrusive rock types show evidence of crustal contamination but this had r ather small impact on their diagnostic trace element ratios. Much stronger overprint, in the CT1 and CT2 suites, resulted from contamination with vein ed Archaean lithospheric mantle, which produced wide ranges of isotopic and highly incompatible element ratios. CT3, in turn, does not show evidence o f interaction with the Proterozoic lithospheric mantle. The high-epsilon(Nd ) endmembers of CT1, CT2 and CT3 probably closely resemble uncontaminated m antle-derived magmas and indicate three different mantle sources. The CT2 p rimary magmas were derived from light rare earth element (LREE)-depleted, s lightly large ion lithophile element (LILE)-enriched sources, whereas data on the volumetrically preponderant CT1 and CT3 point to variably LREE-enric hed, strongly LILE-enriched sources. The sources of CT1, CT2 and CT3 may re cord large-scale lateral heterogeneity generated by subduction-contaminatio n of the Gondwanan upper mantle. The OIB-like CT4 dykes probably reflect as thenospheric heterogeneities that were unrelated to the proposed subduction -contamination.