Mafic cumulus processes and chemical evolution in an open magma chamber: The Newark Island Layered Intrusion, Labrador, Canada

The Newark Island Layered Intrusion (NILI) is a Middle Proterozoic (1305 +/ - 5 Ma; U-Pb zircon) mafic pluton on the eastern edge of the Nain Anorthosi te Complex, Labrador. The NILI has been subdivided by Wiebe (1988) into a s tratigraphically lower, earlier, layered series (LS) and a later hybrid ser ies (HS). Although grossly similar in mineralogy, the REE and Sr isotope sy stematics of the LS and HS cumulates differ dramatically. All but one of the LS cumulates exhibit low total REE (La = 5 to 8 x chondr ites) and a large positive Eu anomaly, indicative of an adcumulus to mesocu mulus origin, with plagioclase and olivine as the expected major cumulus ph ases. Analyzed HS cumulates and other nonchilled rocks also contain princip ally olivine and plagioclase as cumulus phases, yet exhibit much higher abu ndances of the REE (La = 40 to 90 x chondrites), are LREE enriched ((Ce/Sm) (n) = 1.78), and show only a small positive Eu anomaly that diminishes with increasing REE abundances, No assemblage of mafic minerals alone could hav e generated these patterns and abundances. This suggests that HS mafic cumu lates must contain a substantial amount of trapped liquid, and therefore ar e classified as orthocumulates. Modeling of the compatible element-incompat ible element ratio Sr/Ba confirms the need for a substantial trapped liquid component in HS nonchilled mafic rocks. Sr isotopic studies indicate a restricted range for the HS cumulates (Sr-87 /Sr-86(initial) = 0.7036 to 0.7040), which is indistinguishable from chille d mafic pillows (= liquids) found from within the HS. LS cumulates exhibit an extremely broad range (Sr-87/Sr-86(initial) = 0.7050 to 0.7081), which i s elevated relative to any mafic rock found in the HS. The relatively eleva ted and variable Sr isotopic initial ratios for LS cumulates may indicate s elective contamination with a more radiogenic component (crust?) in the ear ly stages of NILI magmatism. With the intrusion of TTS magmas, magmatism re ached its xenith and contamination was no longer a factor. Early LS magmati sm may have "armored" the conduit(s) that fed the NILI chamber and allowed later magma pulses, such as those that gave rise to the HS, to arrive at th e level of the intrusion relatively uncontaminated. This scenario is analog ous to what has been hypothesized for successive volcanic eruptions in the Aleutian Islands, Alaska (e.g., Myers et al., 1985; Meyers and Frost, 1994) .