RE-ASSESSING MINERAL-WHOLE ROCK ISOTOPIC VARIATIONS IN THE KIGLAPAIT LAYERED INTRUSION, LABRADOR

Significant radiogenic isotopic differences, at the time of formation, are increasingly documented between coexisting minerals from mafic layered intrusions. These differences are not related to secondary alteration, thus either the minerals crystallized from magmas of different isotopic compositions or concomitant magmatic processes disturbed their primary isotopic signatures. We are investigating the extent and causes of mineral-whole rock isotopic variability in the ca. 1307 Ma Kiglapait intrusion, the largest troctolitic intrusion of the Proterozoic anorthositic Nain Plutonic Suite, Labrador. The 560 km² bowl-shaped Kiglapait intrusion is 8 km thick and crystallized as a closed system with a differentiated troctolitic Lower Zone (0-84 percent solidified or PCS) and an olivine gabbroic to ferrosyenitic Upper Zone (84-100 PCS). As part of a larger study involving samples from the entire stratigraphy of the Kiglapait intrusion, we report here high-precision trace element concentrations and initial radiogenic isotope compositions (Pb-Hf-Nd-Sr) determined by MC-ICP-MS and TIMS analyses for mineral separates and whole rocks from four stratigraphic locations (PCS = 26.7, 68, 79, 89.3). With the notable exception of 68 PCS, which shows no Pb isotopic variation, the results confirm the existence of Pb isotopic differences between coexisting plagioclase (²⁰⁷Pb/²⁰⁴Pb_i = 14.56-14.68) and mafic minerals (15.18-15.58) and also reveal Nd isotopic heterogeneity (εNd = -3.7 to -4.5 in plag and -4.8 to -6.1 in mafics) within samples, including at 68 PCS. Sr isotopic ratios increase steadily from 26.7 PCS (⁸⁷Sr/⁸⁶Sr_plag = 0.70399; ⁸⁷Sr/⁸⁶Sr_mafics = 0.70405) to 89.3 PCS (⁸⁷Sr/⁸⁶Sr_plag = 0.70428; ⁸⁷Sr/⁸⁶Sr_WR = 0.70438), consistent with small assimilation of roof rocks during crystallization. Based on the contrasting isotopic behavior between 68 PCS and the other samples analyzed to date, mineral-whole rock isotopic variability in the Kiglapait intrusion appears to result from processes that operated during consolidation of the cumulates. Future work will target samples from the early stages of the Kiglapait intrusion (0-20 PCS), the middle part of the Lower Zone (40-60 PCS), and the extreme fractionates at the top of the intrusion (90-100 PCS) just under the roof and the downward crystallized Upper Border Zone.