B_1 basaltic lavas in Vestofold-Jeloya area,central Oslo rift:derivation from initial melts formed by progressive partial melting of an enriched mantle source

摘要

The earliest(B_1)lavas in Vestfold and Jeloya,in the central part of the Permo-Carboniferous Oslo rift have been divided into four main groups:high Ti(HT)basalts (TiO_2>4.2wt precent),low Ti(LT)basaltic rocks(TiO_2<3.7 wtprecent),phonotephrites and ignimbrites.They are all highly enriched in strongly incompatible elements and have high La/Yb rations.Initial #epsilon#_(Nd) and #epsilon#_(Sr) ratios of whole-rock samples nd clinopyroxene separates(assumed age=295Ma)are +6.0 to -1.6 and -17 to +57,respectively.Their trace element and isotopic compositions imply that,during ascent,the magmas were subjected to varying degrees of fractionaly crystallization,mixing between evolved and mafic magmas,and crustal contamination in the upper crust.Crustal contamination was most likely caused by rocks similar to the metasediments now exposed in the Bamble sector,south of the Oslo rift.The preserved lava sequenced represent interfingering of lavas from at least three eruption centers.After extrusion,the lavas suffered post-magmatic alteration that led to mobilization of Rb,U and K,and increased ~(87)Sr/~(86)Sr rations.Interpretation of magmatic signatures,inferred to have been preserved through contamination and alteration,allow us to make the following conclusions concerning the mantle sources of the Vestfold and Jeloy B_1 basalts.The formation of the HT-basalts involves highly Ti-P-rich,garnet-bearing mantle source rocks,e.g.clinopyroxenites,wehrlites,websterites,and amphibolites.The HT and LT basalts may have originated by progressive partial melting of a veined,enriched,garnet-phlogopite-bearing peridotite at the base of the lithosphere.Progressive melting of this complex source initially produced magnesian lavas with high REE and HFSE contents,followed by less enriched,more voluminous lavas.The early basalts were derived from an isotopically depleted source similar to the PREMA source.Contribution of older lithospheric materials to the later basalts produced more enriched Sr-Nd isotopic signatures.As some of the most primitive basalts found within the European Permo-Carboniferous rifting event,these lavas are key to understanding the nature of the sublithospheric mantal benearth Europe at this time.