Isotopic Studies of Processes in Mafic Magma Chambers: I. The Kiglapait Intrusion, Labrador

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Abstract
The initial Nd and Sr isotopic ratios of cumulate rocks and minerals of the Kiglapait intrusion are pertinent as indicators of the processes that affected the Kiglapair magma while it was resident in the crust. A Sm–Nd mineral isochron indicates that the crystallization age of the intrusion is 1305±22 million years. The initial ɛNd values range from—1·6 in the Lower Zone to 6 in the Upper Zone and correlate with the anorthite content of plagioclase. The initial 87Sr/86Sr ratios are 0.70407 to 0.70433 in the Lower Zone, and increase monotonically up-section (decreasing plagioclase anorthite content) to 0.7068 in the Upper Zone. These variations are attributed to assimilation of roof rock concurrently with crystallization. It is evident that replenishment of the chamber with uncontaminated magma was important during the accumulation of the lower zone rocks, but did not occurduring crystallization of the Upper Zone. A mathematical model relating isotope ratio shifts to the relative rates of crystallization, assimilation, and replenishment is presented. It is estimated that the rate of assimilation was between 0.01 and 0.04 of the crystallization rate, and that during the accumulation of the Lower Zone, the rate of replenishment was about half of the crystallization rate. A formulation for ‘assimilation efficiency’is presented that related the actual amount of assimilation (determined from the isotope data) to the thermodynamic maximum allowable amount. The assimilation efficiency of the Kiglapait intrusion is not tightly constrained, but appears to be of the order of 0.01 near the end of crystallization and perhaps as high as 0.07 near the beginning. Further quantitative estimates of assimilation efficiency in different intrusions emplaced under different conditions may aid in understanding assimilation mechanisms and in assessing the role of assimilation in magmatic evolution in general. Many layered intrusions studied to date have initial Nd isotope ratios close to the chondritic value (ɛNd= 0), suggestive of mantle magma sources that are chemically undepleted, and thus different from the sources of mid-ocean ridge basalt and island-arec basalt.

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