Nd and Pb isotopic constraints on the origin of the Purtuniq ophiolite and Early Proterozoic Cape Smith Belt, northern Québec, Canada

摘要: Abstract We report Pb isotopic compositions for 35 samples and Sm-Nd data 44 volcanic rocks from the Purtuniq ophiolite, continental Povungnituk oceanic Chukotat groups in Cape Smith Belt. Pb-Pb whole-rock dating of sheeted dike complex cumulate which are preserved separate thrust sheets, yield identical isochron ages 1.97 ± 0.04 0.11 Ga, respectively, supporting a contemporaneous origin. Combined layered cumulates, sheets dikes pillow basalts constrain age ophiolite to 1.98 0.03 Ga (μ1 = 8.0) good agreement with previously reported 1.998-Ga U-Pb zircon age. In contrast, ∼1.96−1.92-Ga related transitional continental-oceanic Group young dates 1.89 1.72 may record tectonothermal events collision. indicate highly heterogeneous sources (ϵNd(t) + 1.0to 6.1) form mixing array corresponding an anomalously old ∼2.2 Ga. A similar ∼2.3 is displayed by LREE-enriched associated, more depleted, Group. Magma contamination Late Archean basement can be ruled out we attribute excessively melting two distinct mantle sources. The dominant magma source ϵNd(t)-values 3 5 depleted Early Proterozoic mantle. subordinate component displays as low 1 enrichment LREE relative It have resulted relatively event that produced high Nd/Sm moderately radiogenic Nd at 2 At other extreme, it reflect much older little LREE. this case strong some volcanics would due fractionation during melting, assumed isotopically evolved had been stored outside long time, order avoid its destruction convection. evidence compatible origin plum pudding type or plume. heterogeneity suggests slowly spreading ridge immature ocean basin.