Structural and Magmatic Evolution of a Magma Chamber: The Newark Island Layered Intrusion, Nain, Labrador

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Abstract
The Newark Island Layered Intrusion occurs in the Proterozoic Nain anorthosite complex of Labrador. It contains an exceptional suite of cumulates ranging from troctolites and gabbros to quartz monzonites and intermediate hybrid rocks. These layered rocks formed in a chamber that was periodically fed by a wide range of basic and acid magmas, the compositions of which are preserved in numerous feeder dikes. Where basic magmas commingled with cooler granitic magma, they commonly formed chilled pillows. Because of periodic injections of both acid and basic magmas the magma chamber was compositionally stratified for much of its existence. At times, granitic cumulates formed along the chamber walls while mafic to intermediate hybrid cumulates formed on the floor. Stratigraphic and structural relations indicate that the magma chamber grew upward during deposition, and that it evolved from a west-dipping sheet to a north-plunging synform. Three major episodes of expansion can be linked to injections of large (e.g., 20km3) volumes of acid magma. The entry of this acid magma into the chamber disrupted previously formed cumulates, creating enlarged feeders down which resident basic magma collapsed. The resultant structures (troughs) contain strongly chilled pillows of resident basic magma that existed near the bottom of the chamber at the time of acid replenishment.

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