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Journal of Petrology, Volume 40, Issue 3: March 1999.
Strontium and neodymium isotopic data for mafic cumulates, chilled margins, and adjacent crustal rocks of the Hasvik Layered Intrusion, North Norwegian Caledonides, are reported together with new mineralogical and whole-rock analytical data to constrain the extent and effect of the assimilation of crustal xenoliths in a basaltic magma chamber. Initial 87Sr/86Sr (700 Ma) of 0·7045 and [epsilon]Nd (700 Ma) of +3·03 for the chilled margin, which has a tholeiitic composition akin to the chilled rocks of the Skaergaard intrusion, demonstrate that the parental magma was derived from a depleted mantle source. The basal cumulates (0-335m) show an up-section decrease in 87Sr/86Sr from 0·7045 to 0·7038 and a correlative increase in [epsilon]Nd from +1·82 to +4·26 suggesting mixing between resident and recharging magma during magma chamber expansion. The overlying fractionated cumulate sequence (335-1550m) shows an uninterrupted tholeiitic crystallization sequence (olivine out, orthopyroxene in, Fe-Ti oxides in, and apatite in) accompanied by a remarkably smooth up-section increase in 87Sr/86Sr from 0·7038 to 0·7089, correlated with decreasing [epsilon]Nd (+4·76 to -3·26), decreasing whole-rock mg-number (0·73-0·30), and changing mineral compositions (e.g. the anorthite component of plagioclase decreases from 0·72 to 0·52). These compositional variations point to steady-state assimilation of crustal rocks accompanied by fractional crystallization (AFC). AFC modelling based on the Sr and Nd isotopic data demonstrates that the rate of assimilation relative to the rate of crystallization was constant at ~0·27. The amount of assimilated crust, ~21% in bulk for the fractionated cumulate section, is close to the upper limit permitted by the thermal budget and places the Hasvik Layered Intrusion among the most contaminated layered intrusions known. Thousands of recrystallized tabular xenoliths of metasedimentary origin enclosed in the cumulates are thought to represent the remnants of the assimilated material. The xenoliths spalled off the roof during magma emplacement, and, together with the elevated temperatures (400-600°C) of the mid-crustal country rocks, led to a high degree of assimilation in the Hasvik magma chamber.
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Pages 363-380