Chemical modification of east Greenland Tertiary  magmas by two-liquid interdiffusion  Minik T. Rosing*  Department of Geology, Stanford University, Stanford, California 94305  Charles E. Lesher  Lamont-Doherty Geological Observatory, Palisades, New York 10964  Dennis K. Bird  Department of Geology, Stanford University, Stanford, California 94305  ABSTRACT  The macrodikes of the east Greenland Tertiary igneous province  partially melted Precambrian basement along their contacts. Field evi- dence suggests that silicic liquid produced by partial melting of the  basement buoyantly migrated roofward in the intrusions, where it  ponded, and kinetic and/or rheological barriers prevented the crust- derived melt and mafic magma of the macrodikes from intimately  mixing. However, extensive redistribution of mobile elements oc- curred locally between these reservoirs. Close agreement between  element partitioning in the various lithologic units in the macrodikes  and experimental observations involving liquid immiscibility and sil- icate liquid interdiffusion suggests that chemical equilibrium between  the magma reservoirs was approached through diffusive element  exchange.  INTRODUCTION  Magmas ascending through the crust may be modified by interaction with the crustal lithologies. Recent studies have emphasized the geochemi- cal effects on the mantle-derived magmas. However, the loss of crustal components to mafic magmas also modifies the crustal reservoir. The conceptually simplest chemical interaction is bulk assimilation eventually combined with crystal fractionation (DePaolo, 1981), a process that modi- fies the ascending magma but leaves the residual crust unfractionated. Variations in the mobility of chemical species and the melting behavior of different crustal lithologies probably make contamination of ascending magmas selective in most cases (Maury and Bizouard, 1974; Kays et al., 1981). Equilibrium and incongruent partial melting lead to assimilation of specific mineral components and will leave refractory residues in the crust, thus fractionating the crustal reservoir by differential loss of chemical components. As discussed by Yoder (1973), anatexis caused by intrusion of mafic magmas will produce silicic magmas that may form separate domains in the magma chambers. Contrasts in density, viscosity, and other physical properties inhibit homogenization, although the two liquids are thermody- namically miscible, and permit selective chemical interaction. Igneous rock suites often comprise rocks at different stages of interaction, but usually lack one or both end members as well as exposures of the setting of interaction. The east Greenland Tertiary igneous province was formed during the early stages of opening of the North Atlantic Ocean. The magmatic succes- sion spans the transition from continental to oceanic volcanism (Brooks and Nielsen, 1982a). The role of preexisting sialic crust in the petrogenesis of magmatic rock suites therefore can be evaluated. East Greenland basal- tic macrodikes and associated remobilized granitic basement gneisses compose a natural example, where there is coexistence of large volumes of 68°I5'N - •Present address: Geological Museum, Öster Voldgade 5-7, DK-1350 Keben- havn K, Denmark.  32°OOW 45'  RECENT GLACIAL DEPOSITS  MACRODIKES a SILLS  M SKAERGAARD INTRUSION  LOWER TERTIARY BASALTS  PRECAMBRIAN GNEISS  AIRSTRIP  Figure 1. Generalized geologic map of Miki Fjord area, central east  Greenland, showing Kraemer Island and Vandfaldsdalen macrodikes  and Eskimonaes and Sodalen segments of Miki Fjord macrodike.  mafic and silicic magma, and the source areas and conduits for the silicic end member are exposed. Magma reservoirs that can be interpreted as representing modified crust-derived magma and virtually unmodified mantle-derived and crust-derived magmas have been recognized in the dike complex. The modified mafic components may be represented by small late-stage alkaline dikes, which appear to tap a reservoir in the main dike complex. Because the modified magma compositions cannot be reached by linear mixing combinations of the unmodified end mem- bers, we try to explain the redistribution of chemical components between the crust- and mantle-derived magmas of the macrodikes by other mechanisms. 626 GEOLOGY, v. 17, p. 626-629, July 1989