Assimilation of crustal xenoliths in a basaltic magma chamber: Sr and Nd isotopic constraints from the Hasvik Layered Intrusion, Norway

Strontium and neodymium isotopic data for mafic cumulates, chilled margins, and adjacent crustal rocks of the Hasvik Layered Intrusion, North Norwegia n Caledonides, are reported together with new mineralogical and whole-rock analytical data to constrain the extent and effect of the assimilation of c rustal xenoliths in a basaltic magma chamber. Initial Sr-87/Sr-86 (700 Ma) of 0.7045 and epsilon(Nd) (700 Ma) of +3.03 for the chilled margin, which h as a tholeiitic composition akin to the chilled rocks of the Skaergaard int rusion, demonstrate that the parental magma was derived from a depleted man tle source. The basal cumulates (0.335 m) show an up-section decrease in Sr -87/Sr-86 from 0.7045 to 0.7038 and a correlative increase in epsilon(Nd) f rom +1.82 to +4.26 suggesting mixing between resident and recharging magma during magma chamber expansion. The overlying fractionated cumulate sequenc e (335-1550 m) 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 Sr-87/Sr-86 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. t he anorthite component of plagioclase decreases from 0.72 to 0.52). These c ompositional variations point to steady-state assimilation of crustal rocks accompanied by fractional crystallization (AFC). AFC modelling based on th e Sr and Nd isotopic data demonstrates that the rate of assimilation relati ve to the rate of crystallization was constant at similar to 0.27. The amou nt of assimilated crust, similar to 21% is bulk for the fractionated cumula te section, is close to the upper limit permitted by the thermal budget and places the Hasvik Layered Intrusion among the most contaminated layered in trusions known. Thousands of recrystallized tabular xenoliths of metasedime ntary origin enclosed in the cumulates are thought to represent the remnant s of the assimilated material. the xenoliths spalled off the roof during ma gma emplacement, and, together with the elevated temperatures (400-600 degr ees C) of the mid-crustal country rocks, led to a high degree of assimilati on in the Hasvik magma chamber.