INTRAPLUTONIC QUENCH ZONES IN THE KAP-EDVARD-HOLM-LAYERED-GABBRO-COMPLEX, EAST GREENLAND

The Kap Edvard Holm Layered Gabbro Complex is a large layered gabbro i ntrusion (> 300 km2) situated on the opposite side of the Kangerdlugss uaq fjord from the Skaergaard Intrusion. It was emplaced in a continen tal margin ophiolite setting during early Tertiary rifting of the Nort h Atlantic. Gabbroic cumulates, covering a total stratigraphic thickne ss of > 5 km, have a typical four-phase tholeiitic cumulus mineralogy: plagioclase, clinopyroxene, +/-olivine, and +/-Fe-Ti oxides. The cryp tic variation is restricted (plagioclase An81-51, olivine Fo85-66, cli nopyroxene Wo43-41 En46-37 Fs20-11) and there are several reversals in mineral chemistry. Crystallization took place in a low-pressure, cont inuously fractionating magma chamber system which was periodically rep lenished and tapped. Fine-grained (0.2-0.4 mm) equigranular, thin (0.5 -3 m), laterally continuous basaltic zones occur within an approximate ly 1000 m thick layered sequence in the Taco Point area. Twelve such z ones define the bases of individual macrorhythmic units with an averag e thickness of approximately 80 m. The fine-grained basaltic zones gra de upwards, over a few metres, into medium-grained (> 1 mm) poikilitic , olivine gabbro with small-scale modal layering. Each fine-grained ba saltic zone is interpreted as an intraplutonic quench zone in which ma gma chilled against the underlying layered gabbros during influx along the chamber floor. Supercooling by approximately 50-degrees-C is beli eved to have caused nucleation of plagioclase, olivine, and clinopyrox ene in the quench zone. The nucleation rate is believed to have been e nhanced as the result of in situ crystallization in a continuously flo wing magma. The transition to the overlying poikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typical for an open magma chamber system : olivine (Fo77-68.5) and plagioclase cores (An80-72) show a zig-zag c ryptic variation pattern with no overall systematic trend. Olivine has the most primitive compositions in the quench zones and more evolved compositions in the olivine gabbro; plagioclase cores show the opposit e trend. Although plagioclase cores are believed to retain their origi nal compositions, olivines re-equilibrated by reaction with trapped li quid. Some plagioclase cores contain relatively sodic patches which re tain quench compositions. Whole-rock compositions of nine different qu ench zones vary over a range from 10 to 18% MgO although the mg-number remains constant at approximately 0.78. The average composition (47.7 % SiO2, 13.3%MgO, 1.57% Na2O+K2O) is taken as a best estimate of the p arental magma composition, and is equivalent to a high-magnesian olivi ne tholeiite. The compositional variation of the quench zones is belie ved to reflect bursts of nucleation and growth of olivine and plagiocl ase during quenching. Magma emplacement is believed to have taken plac e by separate tranquil influxes which flowed along the interface betwe en a largely consolidated cumulus pile and the residual magma. The res ident magma was elevated with little or no mixing. At certain levels i n the layered sequence the magma drained back into the feeder system; such a mechanism is referred to as a surge-type magma chamber system.