INCLUDED BLOCKS (AND BLOCKS WITHIN BLOCKS) IN THE SKAERGAARD INTRUSION - GEOLOGIC RELATIONS AND THE ORIGINS OF RHYTHMIC MODALLY GRADED LAYERS

The early Eocene Skaergaard intrusion of Greenland includes enormous n umbers of rocks of both exotic and cognate origins. The lower parts of the Marginal Border Series contain abundant fragments of feldspathic peridotite that are possibly autoliths, intermixed with occasional xen oliths of Precambrian gneiss and metasomatized Cretaceous-Paleocene se diments derived from adjoining country racks. The Upper Border Series includes one exceptionally large block of gneiss (several hundred mete rs across), and numerous smaller fragments, these originating from the intrusion's footwalls, plus a few pieces of peridotite. The Layered S eries contains countless autoliths of troctolite, gabbroic anorthosite , and oxide (magnetite-ilmenite) gabbro, broken from parts of the Uppe r Border Series that have otherwise been lost to erosion; at the upper midlevel of its western half, it contains a fem xenoliths of basalt, derived probably from the now-eroded (Eocene) roof of the intrusion. A distinctive postintrusion composite basaltic dike at one place contai ns 40 or more xenoliths of troctolite, olivine gabbro, and gabbroic an orthosite that may represent parts of the Layered Series still hidden at depth. The Layered Series autoliths range from fragments a few cent imeters on a side to blocks more than 400 m across, and they typically are coarser grained than their host cumulates, being in this respect more like Upper Border Series rocks. The autoliths are spread stratigr aphically through the lower 70% of the exposed 2500 m thickness of the Layered Series and are generally concentrated in three broad stratigr aphic zones. Their physical relationships to their host rocks-particul arly the way they indent older layers beneath and are covered by young er layers above-provide abundant evidence that there was generally a s harp, well-defined interfere between the top of the cumulate pile and the main body of magma in the intrusion while the Layered Series was f orming. The distribution of the autoliths between and through the well -known, rhythmic, thin, modally graded layers shows that these layers were spread by magmatic currents; and their relations to the more exte nsive macrorhythmic layering suggest that it too was significantly sha ped by currents. Many of the larger autoliths are crudely layered inte rnally, and in places it is evident that their stratification existed before they broke loose; therefore, it must have formed in the Upper B order Series. One particularly large block of oxide gabbro exhibits ex traordinarily well-developed modal and textural layering and includes small troctolitic autoliths of an earlier generation, and it provides evidence that currents also spread crystalline materials across the to p of the magma body. Many of the very small autoliths in the Layered S eries are highly anorthositic in composition, apparently because they were leached of mafic minerals, and some of the larger blacks show loc al patchy internal replacement by anorthosite. Most large blocks show little sign of postaccumulation modification, and some have thin, fine -grained augite-rich rims or rinds, demonstrating that even though the y were out of thermal and chemical equilibrium with their host cumulat es, they still were effectively armored against extensive chemical cha nge. Also documented is a large block that was cut by several early ba saltic dikes before it broke free from the top of the intrusion; these early dikes transgress small anorthositic replacement pipes in the bl ock, showing that the replacement process also occurred in the upper b order environment. Two mechanisms are described whereby graded cumulat e layers can be sorted and deposited by magmatic crystal-liquid suspen sion currents. One, involving density surge currents, has been advocat ed previously; the other is a new concept based on boundary now separa tion and reattachment vortex cells. The two mechanisms are used in com plementary ways to illustrate the formation of (1) some of the princip al Skaergaard structures involving blocks and layers; (2) modally grad ed layers in the Layered Series that rhythmically alternate with unifo rm layers; and (3) modally sorted layers in the Upper Border Series fe aturing ''underside draping'' beneath small included blocks. Explanati ons are provided for (1) why plagioclase did not neat away from the to ps of graded layers even though it was less dense than the liquid, and (2) how the liquid part of a current was fractionated away from the c rystalline materials. Modal and grain-size data from Skaergaard intrus ion graded layers are shown to be in excellent accord with characteris tics predicted for layers sorted by currents; a synthesis diagram is p resented illustrating how all the above processes may have functioned in concert in the intrusion.