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The Caledonian Hasvik Layered Intrusion, North Norway, crystallised from a tholeiitic parental magma, and displays up-section mineralogical (olivine-gabbro, gabbronorite, oxide-gabbronorite, and apatite-oxide-ferronorite) and compositional (plagioclase An% from 72 to 52) changes akin to the Skaergaard intrusion. In contrast to Skaergaard, whole rock initial Sr isotope ratios (Sr0) increase up-section from 0.7038 to 0.7089 and correlate inversely with plagioclase An%. This is explained by coupled assimilation and fractional crystallisation (Tegner et al., 1999; J. Petrol. 40, 363-380).
We have micro-drilled cores and rims of plagioclase crystals in the Upper Zone cumulates as well as plagioclase from metasedimentary country rocks in the metamorphic contact aureole and a xenolith for a detailed Sr isotope study. Sr0 in cores of plagioclase crystals from the apatite-oxide-ferronorites increases up-section from 0.7089 to 0.7094 correlating inversely with plagioclase An% from 47 to 43. Sr0 of rims, from 0.7088 to 0.7093, also correlates inversely with plagioclase An% that varies from 48 to 44, suggesting the same ratio of assimilation and fractional crystallisation when rims crystallised. Laterally along the strike of layering from the central part of the intrusion towards the zone of numerous metasedimentary xenoliths and the margin, Sr0 of plagioclase cores slightly increases from 0.7089 to 0.7098 with a decrease in plagioclase An% from 49 to 47. Similarly to the vertical and core-rim trends, an increase in Sr0 of rims, from 0.7084 to 0.7097, from the central part of the intrusion towards the margin inversely correlates with a decrease in plagioclase An% from 50 to 47, suggesting a coupling between assimilation and fractional crystallisation from the centre to the margin of the magma chamber. The analytical error in the Sr0, using the microdrilling method, is lying between 0.000017 and 0.000020.
Innumerable xenoliths of metasedimentary country rock occur within the intrusion. The sizes and shapes of xenoliths change from small flakes (<10 x 100 cm) to a combination of flakes and large blocks (up to tens of metres across) in the central part of the exposed Upper Zone. The metasedimentary xenoliths are typically banded pyroxene hornfels (dark layers dominated by low-Ca pyroxene and light-coloured layers of leuconorite with or without quartz) that represent refractory restites of an originally fusible sedimentary protolith. Sr0 of a restite xenolith (0.7077) is within error of surrounding ferronorite cumulates and different from the country rock aureole (0.7206-0.7198). This demonstrates complete equilibration of Sr isotopes during the contact metamorphic reaction between magma and xenoliths.
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