ISOTOPIC DISEQUILIBRIUM IN LAYERED INTRUSIONS: REVISITING THE RADIOGENIC ISOTOPIC VARIATIONS OF THE KIGLAPAIT INTRUSION

Stratigraphic variations in radiogenic isotopic compositions within layered intrusions are used to document compositional changes (e.g., magma recharge , mixing, contamination) associated with the filling, crystallization, and cooling of crustal magma chambers. Recent studies are revealing pronounced isotopic disequilibrium between minerals from nearly every major layered intrusion investigated, including Pb isotopes in coexisting plagioclase and sulfide from the Bushveld (Mathez, 2003, GCA) and the Stillwater (McCallum, 1999, CMP), Sr and Nd isotopes in whole rock, plagioclase, and clinopyroxene from the Skaergaard (McBirney, 2003, JP), and Sr isotopes in single plagioclase crystals from the Rum layered intrusion (Tepley, 2003, CMP). Except for the Stillwater intrusion, this isotopic disequilibrium is not related to low-temperature secondary alteration. Instead, slow cooling (105-106 years) of these large bodies from near-liquidus to subsolidus temperatures appears to allow for complex geochemical evolution of partially molten systems.

To this list of intrusions that record mineral-mineral isotopic disequilibrium, we can add the large troctolitic Kiglapait layered intrusion in the ca. 1.3 Ga Nain Plutonic Suite. Plagioclase separates and whole rocks show significant differences in both measured and initial Pb isotopic compositions. Bulk mafics and separated minerals (apatite, magnetite, augite and olivine) give internal Pb-Pb and U-Pb isochron ages consistent with the crystallization age. Measured ²⁰⁶Pb/²⁰⁴Pb is highest in apatite (30-73), indicating that apatite is the main U/radiogenic Pb carrier. The clear distinction between the Pb isotopic composition of plagioclase and mafics persists after correction for the crystallization age for in situ U decay. A similar distinction is observed in Sr-isotope initial ratios. All minerals appear in textural equilibrium at mm to mm scales and the isotopic disequilibrium has existed since the time of crystallization of the intrusion. The mafic components in the Kiglapait may have acquired their relatively evolved Pb-isotopic compositions from late apatite-, Fe-rich residual liquids that have interacted with the enveloping anorthositic rocks of the Nain Plutonic Suite.