Wallrock Melting and Reaction Effects along the Higganum Diabase Dike in Connecticut: Contamination of a Continental Flood Basalt Feeder |
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| Authors: | PHILPOTTS ANTHONY R; ASHER PRANOTI M |
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| Institution: | Department of Geology and Geophysics and Institute of Materials Science, University of Connecticut Storrs, Connecticut 06269 |
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| Abstract: | The question of whether continental flood basalts are contaminatedwith crustal material during ascent through the lithosphereis addressed through a study of the feeder dike to the firstof the Mesozoic flood basalts in the Hartford basin of Connecticut.Faulting associated with basin formation exposed the dike, notonly where it connects with the flood basalt, but also at levelsthat may have been as deep as 10 km at the time of crystallization.Wallrocks at all levels of exposure and along the entire 250-kmlength of this 50–60 m wide dike show evidence of partialmelting and contamination of the diabase magma. Melting of the wallrocks took place on grain boundaries betweenquartz and feldspar, especially where these boundaries werefluxed with water released from the breakdown of biotite. Themelts were simple mixtures of quartz and the particular feldsparinvolved rather than equilibrium minimum compositions, probablybecause heating rates were faster than melting rates. The meltscrystallized to form granophyre. Feldspars in direct contactwith the diabase partly melted, with plagioclase developingthe fingerprint texture and orthoclase being converted to anexceptionally fine-grained mixture of plagioclase and quartz. Melts generated in the wallrocks were forced into the marginsof the dike by the volume expansion caused by melting. The earliestmelts to enter the dike are preserved as felsic wisps in thechilled margins, but at distances of more than a few centimetersfrom the edge of the dike they appear to have been assimilatedby the diabase magma. Melts that entered the dike after themargins had solidified were intruded as crosscutting granophyreveins. Chemical profiles across the dike reveal that the contaminantswere not restricted to narrow marginal zones but entered themain body of the dike. Thus, by the time this magma had risento the surface and erupted as the Talcott basalt it had assimilateda significant amount of crustal material, which accounts forthe change from olivine normative compositions in the deepestlevel exposures of the dike to quartz normative ones in thebasalt. Mass balance calculations indicate that the Talcottbasalt may have had {small tilde}6% granophyre added to it duringthe magma's ascent through the last 10 km of the crust. Theamount assimilated in traversing the entire continental crustwould therefore be greater than this. |
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