Intraplutonic Quench Zones in the Kap Edvard Holm Layered Gabbro Complex, East Greenland

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Abstract
The Kap Edvard Holm Layered Gabbro Complex is a large layered gabbro intrusion (>300 km2) situated on the opposite side of the Kangerdlugssuaq fjord from the Skaergaard Intrusion. It was emplaced in a continental margin ophiolite setting during early Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thickness of >5 km, have a typical four-phase tholeiitic cumulus mineralogy: plagioclase, clinopyroxene, ±olivine, and ±Fe–Ti oxides. The cryptic variation is restricted (plagioclase An81–51, olivine Fo85–66, clinopyroxene Wo43–41 En46–37 Fs20–11) and there are several reversals in mineral chemistry. Crystallization took place in a low-pressure, continuously fractionating magma chamber system which was periodically replenished and tapped. Fine-grained (0⋅2–0⋅4 mm) equigranular, thin (0⋅5–3 m), laterally continuous basaltic zones occur within an ˜1000 m thick layered sequence in the Taco Point area. Twelve such zones define the bases of individual macrorhythmic units with an average thickness of ˜80 m. The fine-grained basaltic zones grade upwards, over a few metres, into medium-grained (>1 mm) poikilitic, olivine gabbro with smallscale modal layering. Each fine-grained basaltic zone is interpreted as an intraplutonic quench zone in which magma chilled against the underlying layered gabbros during influx along the chamber floor. Supercooling by ˜50°C is believed to have caused nucleation of plagioclase, olivine, and clinopyroxene in the quench zone. The nucleation rate is believed to have been enhanced as the result of in situ crystallization in a continuously flowing 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 cryptic 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 opposite trend. Although plagioclase cores are believed to retain their original compositions, olivines re-equilibrated by reaction with trapped liquid. Some plagioclase cores contain relatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones vary over a range from 10 to 18% MgO although the mg-number remains constant at ˜0⋅78. The average composition (47⋅7% SiO2, 13⋅3%MgO, 1⋅57% Na2O+K2O) is taken as a best estimate of the parental magma composition, and is equivalent to a high-magnesian olivine tholeiite. The compositional variation of the quench zones is believed to reflect bursts of nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separate tranquil influxes which flowed along the interface between a largely consolidated cumulus pile and the residual magma. The resident magma was elevated with little or no mixing. At certain levels in the layered sequence the magma drained back into the feeder system; such a mechanism is referred to as a surge-type magma chamber system.

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