¹EARTH SCIENCES DIVISION, LAWRENCE BERKELEY NATIONAL LABORATORY, UNIVERSITY OF CALIFORNIA,MS90-1116, BERKELEY, CA 94720, USA ²DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF OREGON, EUGENE, OR 97403, USA

RECEIVED MAY 27, 1996; REVISED TYPESCRIPT ACCEPTED NOVEMBER 14, 1997

During the middle stages of crystallization of the Skaergaard Layered Series large numbers of blocks became detached from the Upper Border Series and settled into the mush of crystals on the floor. It has been recognized for some time that these blocks now have compositions and textures that differ markedly from those of the units from which they came. They tend to be more plagioclase rich and seem to have lost mafic components to the surrounding gabbro. Numerical simulations coupling crystallization, melting, and heat and mass transfer for a multicomponent system show how the blocks reacted with the mush in which they were emplaced. Enhanced cooling and crystallization of a compositionally stratified mush adjacent to the blocks resulted in patterns of melt compositions similar to those of layering around the blocks. Volume changes during crystallization and melting induced convection of the interstitial melt leading to changes in the bulk compositions of the blocks and the surrounding mush. Inhomogeneities such as inclusions are likely to facilitate the onset of compositional convection in a chemically stratified solidification zone.

Keywords: assimilation;convection; reaction-transport modeling; Skaergaard Intrusion; solidification zones

INTRODUCTION

Various types of inclusions are found in the Skaergaard Intrusion, but the most conspicuous are angular blocks that settled into the crystallizing Layered Series (Fig. 1a). It was clear even to the earliest workers (Wager & Deer, 1939; Wager & Brown, 1968) that these blocks must have fallen from the zone of crystallization under the roof. The Upper Border Series has large missing sections at levels that correspond to those parts of the Layered Series in which most blocks are found. On closer examination, however, several puzzling anomalies were recognized. First, the plagioclase-rich blocks have densities substantially less than that of the magma through which they must have fallen. Moreover, their compositions differ in important ways from those of the units from which they came, namely UBS-[alpha] and UBS-[beta] (Table 1). They have lost FeO*, TiO₂, and P₂O₅, and their minerals no longer have their original compositions but are closer to those of their host rocks. In view of the density differences, these changes must have occurred after the blocks had reached the floor. This interpretation is reinforced by the textural characteristics of the blocks. Although their angular outlines appear sharply defined in outcrops, under the microscope one sees no discontinuity at the boundary of the blocks other than a change in modal proportions. Foliation, which is especially strong in plagioclase-rich blocks, has the same orientation as that of their host and crosses the boundary without any apparent deflection (McBirney & Hunter, 1995). Most baffling of all, a large block in middle zone has a set of four or five layers that appear to be prolongations of a corresponding set in the adjacent gabbro (Fig. 1b).

Table 1. Average major-element compositions of the Upper Border Series and the zone (UBS-[bgr]) from which most of the blocks seem to have fallen (Naslund, 1984); these can be compared with the average present composition of the blocks and their mafic selvages; also shown are the average composition of host rock of Middle Zone (McBirney, 1989) and the experimentally determined composition of its interstitial liquid (McBirney & Naslund, 1990)

Figure 1. (a) A swarm of blocks in Middle Zone. (b) A very large block in Middle Zone has very prominent layering around it, and some of the adjacent layers appear to have been propagated through theblock.

We have attempted to explain these anomalies as the results of a vaguely defined process of metasomatism (McBirney & Sonnenthal, 1990; McBirney, 1995) but until now could not offer a plausible quantitative model. An improved understanding of reaction-transport mechanisms associated with porous flow, together with the development of numerical codes to treat flow and reaction in porous media now permit us to analyze the evolution of these blocks in a more convincing way.

We have already described the general features of the intrusion in earlier parts of this series (McBirney, 1989; Boudreau & McBirney, 1997; McBirney & Nicolas, 1997). In the pages that follow we first elaborate on the geological setting and compositional relations of the blocks then model the physical and chemical processes by which they evolved.

OCCURRENCE AND COMPOSITION

Of all the inclusions in the intrusion, by far the most numerous are the anorthosites and felsic gabbros that fell in swarms into the lower part of the Layered Series. A few blocks of basalt have been found but xenoliths of the gneiss and amphibolite that form most of the lower margins of the intrusion are very rare (Kays et al., 1981), probably because they are now altered beyond recognition. Ultramafic xenoliths are locally abundant but only near the base of the intrusion (Kays & McBirney, 1982).

The blocks that are our main concern here are concentrated in the Lower and Middle Zones; they are rare in Upper Zone a and none at all have been found in Upper Zone c and the upper part of Upper Zone b. In their study of the stability relations of a zone of crystallization under a roof, Brandeis & Jaupart, (1987) concluded that fragments and individual crystals would become detached and sink through the magma in widely spaced clusters. Beneath the blocks the layering is depressed, and at the edges it is truncated and deflected (Fig. 2a). Layering draped over the blocks appears to have been laid down after they had come to rest (Fig. 2b). Some of the blocks, particularly large ones, split on impact with the floor. The cracks are filled with iron-titanium oxides, pyroxene and olivine but little if any plagioclase. The gabbro caught between large blocks is also very mafic and has a strongly laminated fabric (Fig. 3).

Figure 2. (a) Layering depressed under and draped over a small block in Middle Zone. (b) A small block in Middle Zone appears to have started to rise after its density decreased as the result of metasomatic alteration. (Note the pegmatitic gabbro filling the space below the block.)

Figure 3. Strongly layered mafic gabbro compressed between blocks.

A very large block of anorthosite in Middle Zone has a set of four or five layers that appear to be the prolongation of a similar set in the adjacent gabbro. The faint internal layers are apparent only when seenfrom a distance and in the most favorable light. Because the block is exposed high in the near-vertical wall of a glacial cirque, we have not been able to examine it closely.

The gabbro within and around swarms of blocks tends to be more mafic than the average for the zone to which it belongs. It is also more strongly layered. This layering is thought to be due in large part to the disturbance and compaction that resulted from the blocks falling into the crystal mush (Boudreau & McBirney, 1997).

Individual blocks tend to be structurally and compositionally homogeneous but they may differ markedly in texture and composition from close neighbors in the same swarm (Table 2; Fig. 4). Most are anorthosites, and although some are more mafic, few if any approach the bulk composition of their mafic host rock. The mineral compositions are closer to those of the host gabbro than to the original units of the Upper Border Series from which they are thought to have fallen. The average composition of plagioclase is An_56·2, as opposed to An_43·8 for the corresponding part of the Upper Border Series, and their Mg ratio is 0·352, whereas that of the original rocks of the UBS is 0·236. Part of the mafic fraction missing from the anorthosites can be accounted for in the iron-rich selvages and veins in and around the blocks (Fig. 5; Table 1). The remainder may be in the large areas of unusually mafic layered rocks surrounding the blocks.

Table 2. Compositions of representative samples of foundered anorthositic blocks

Figure 4. Plane-table map of a swarm of blocks in Middle Zone, Kraemers Island.

Figure 5</