Rifting-related (sub)alkaline magmatism in the Bamble sector (Norway) during the 'Gothian'-Sveconorwegian interlude

Results of whole-rock chemistry, mineral chemistry, and Nd and Sr isotope studies are reported on a newly identified dyke swarm in the Bamble sector, south Norway. The Blengsvatn dyke swarm consists of alkalic gabbros to monzogabbros and monzodiorites. Hornblende is the dominant mafic mineral in the alkalic gabbro to monzogabbro dykes. These dykes have Mg# between 47 and 62, Cr 90-132 ppm, Ni 43-222 ppm, Ba 56-151 ppm, and ΣREE between 45 and 116 ppm. La_CN/YB_CN ratios vary from 1.5 to 3.7. With the exception of one sample, the dykes are both olivine and nepheline normative, some contain slightly K-deficient relict biotite phenocrysts. The dominant mafic mineral in the monzodiorite dykes is biotite. These dykes have Mg# between 25 and 43, Cr and Ni ≤10 ppm, and P₂O₅ (0.69-1.68 wt.%) and high Ba (>2100 ppm). They have ΣREE of about 755 ppm and La_CN/ Yb_CN ratios of about 10. The dykes are olivine normative. Both types of dykes together yield a Sm-Nd whole-rock regression line of 1.47±0.07 Ga (MSWD 8.2), with an initial ε_Nd of +3.5. The alkalic gabbro to monzogabbro dykes alone yield a Sm-Nd regression line of 1.57±0.17 Ga (MSWD 5.8, ε_Nd +3.9). The dykes are the first reported evidence of magmatic activity in this part of the Baltic Shield during this period (c. 1.5 Ga), and are interpreted as being the products of a phase of incipient rifting during the period prior to the Sveconorwegian orogeny. The Rb-Sr isotopic system has been disturbed, with data scattering around an 1.1 Ga reference line. As the Blengsvatn dykes intruded the Blengsvatn gabbro and supracrustals of the Nidelva quartzite complex, they provide independent evidence for the occurrence of a suite of pre-Sveconorwegian ('Gothian') gabbros, and demonstrate that the Nidelva quartzite complex was deposited prior to c. 1.5 Ga.