Oxygen isotopic systematics of an open-system magma chamber: An example from the Freetown Layered Complex of Sierra Leone

摘要

The Freetown Layered Complex of Sierra Leone is a 7-km thick, rift-related tholeiitic intrusion that formed approx 193 Ma ago during the opening of the middle Atlantic Ocean. The Complex was emplaced above Archean basement gneisses, granulites, and schists of the Kasila Group at pressures of 2.8 to 5.1 kbar. The #delta#~(18)O values of bulk cumulates (5.5-6.7), separated plagioclase (5.7-6.0), olivine (5.0-5.7), and clinopyroxene (5.1-5.8) spanning the entire stratigraphic section indicate that the Complex has preserved its primary magmatic oxygen isotopic composition. The #delta#~(18)O values of whole-rocks are uniform in Zone 1 (average = 5.74 +- 0.01) but vary widely in Zones 2 and 3 (average = 6.18 +- 0.52 and 5.71 +- 0.32, respectively). Variations in whole-rock #delta#~(18)O with stratigraphic height correlate well with plagioclase mode, with #delta#~(18)O values being highest in the strongly laminated anorthosites of Zones 2 and 3, and lowest in olivine gabbro from the cyclically layered subzone of Zone 3. The overall pattern of oxygen isotopic variation with stratigraphic height in the intrusion appears to the related to the accumulation of high-#delta#~(18)O, plagioclase-rich rocks that are overlain by low-#delta#~(18)O, olivine or pyroxene-rich rock types. Fractional crystallization in combination with mineral accumulation can explain the observed oxygen isotopic variations in the Complex. Oxygen isotopic thermometry yields equilibration temperatures of 1040 to 1290 deg C, which are similar to temperatures (1045 to 1381 deg C) estimated from the plagioclase-liquid thermometer applied to the Freetown bulk magma for each zone obtained by geochemical summation. The oxygen isotopic temperatures are highest where major influxes of new magmas have occurred, indicating negligible subsolidus resetting of mineral compositions. Contemporary diabase dikes intruding the Complex have whole-rock and plagioclase #delta#~(18)O values of 6.6 and 6.7, respectively, suggesting minimal interaction of the dikes with hydrothermal fluids. Archean gneisses from the Kasila Group have widely varying #delta#~(18)O values of 6.2 to 11.4 for whole rocks and approx 6.8 for quartz. The whole-rock #delta#~(18)O values for the gneisses suggest that the Kasila Group may have been selectively altered by hydrothermal fluids during retrograde (M3) metamorphism associated with the Pan African event. However, the Kasila appears not to have significantly exchanged oxygen with the Freetown magma. The calculated average #delta#~(18)O value for the original Freetown magma is 6.0 +- 0.2, which is similar to the values reported for magmas parental to the Kiglapait, Stillwater, and Duluth Complex intrusions.