Isotopic and Trace Element Geochemistry of the Kiglapait Intrusion, Labrador: Deciphering the Mantle Source, Crustal Contributions and Processes Preserved in Mafic Layered Intrusions

Abstract

The parent magma and mantle source of the Mesoproterozoic Kiglapait intrusion, the largest and youngest troctolitic intrusion in the Nain Plutonic Suite of coastal Labrador, Canada, are evaluated using an integrated Pb–Sr–Nd–Hf isotopic and trace element framework. The bowl-shaped Kiglapait intrusion crystallized mostly as a closed system, forming an 8 km-thick stack of differentiated cumulates from the troctolitic Lower Zone (0–84% solidified or PCS) through the olivine gabbroic to ferrosyenitic Upper Zone (84–100 PCS). Whole-rocks and mineral separates (plagioclase, clinopyroxene, bulk mafic minerals) were analysed from eight different stratigraphic locations from 5 to 89 3 PCS. This new dataset is complemented by Pb–Sr–Nd TIMS isotopic analyses of samples spanning the entire magmatic stratigraphy of the Kiglapait intrusion (29 samples, 0 12 to 99 99 PCS) and from the Nain Plutonic Suite (26 samples) for regional comparisons. There is no significant change in initial Nd and Hf isotopic ratios from the base to the top of the intrusion. In contrast, initial Sr/Sr steadily increases in the Upper Zone due to progressively increasing small amounts of assimilation of country rock with a composition similar to local Proterozoic supracrustal rocks and Archean gneiss. The Pb isotopic analyses, both MC-ICP-MS and TIMS, reveal differences between less radiogenic plagioclase and more radiogenic mafic separates in nearly all samples. This feature is attributed to cryptic alteration from interaction with a high-temperature, externally-derived, hydrothermal fluid in oxygen isotope equilibrium with the host anorthositic rocks. The Pb isotope composition of the Kiglapait parent magma was recovered through systematic analysis of leached plagioclase separates (and corresponding leachate solutions). Trace element modeling combined with Pb–Sr–Nd–Hf isotopic constraints indicate that the Kiglapait parent magma was derived from depleted mantle ( 95%) with a small contribution ( 5%) from the lower crust assimilated during ponding at the Moho or during transit through the crust. This geochemical model is extended to the origin of other troctolitic and anorthositic magmas in the Nain Plutonic Suite at a regional scale. Most of the Pb–Sr–Nd isotopic compositions of the Nain anorthosites are compatible with crystallization from melts that originated from the mantle and that assimilated variable extents of crustally-derived melts (3–30%). The multi-isotopic and trace element geochemical framework developed for the Kiglapait intrusion in this study can be applied to the investigation of the source, parent magma, differentiation processes and post-crystallization changes in layered intrusions and anorthosites throughout the geological record.