Layered Mafic Intrusions and Stratigraphy in Plutons

Sheet-like bodies of relatively mafic magmatic rock occur in many granitic plutons (Wiebe, 1974; Barbarin 1988; Michael 1991; Blundy & Sparks 1992; Chapman & Rhodes 1992; Wiebe 1993,1994; Fernandez & Gasquet, 1994; Coleman et al. 1995). Such bodies vary from km-scale to m-scale, and may be represented as enclave swarms or merely as dispersed, aligned enclaves, such as in many parts of the Kameruka pluton – we are visiting the Kameruka pluton (Figure 1) to view some of these structures.

Figure 1. Bega Batholith

Bega Batholith

Geological map of the Bega Batholith

Individual sheets or lenses may be chilled on the base, which commonly displays prominent convex downward lobate structures that closely resemble sedimentary load cast structures (Wiebe 1974, 1993). These structures range from metres to tens of metres in diameter. The base is commonly perforated by flame structures and veins of leucogranite, which provide way-up indicators (Wiebe 1974). This felsic material appears to represent interstitial liquid, which was filter-pressed from the underlying granitic crystal mush and rose upward into the overlying mafic body. Compaction of the underlying crystal mush may lead to the development of feldspar-rich cumulates, in which individual crystals are moulded into the basal part of the overlying mafic layer (Figure 3b). In some mafic layers, pipe structures have formed as cylindrical diapirs of crystal mush, which rose vertically from the underlying granitic crystal mush into the overlying unconsolidated mafic layer (Chapman & Rhodes 1992). Where they have not been affected by slumping or magmatic flow in the host mafic layer, they appear to provide a record of the vertical during consolidation of the mafic layer and, hence, can be used to indicate the initial dip of the floor and the amount of tilting since deposition (Wiebe 1993).

Figure 2. Map and location of stops visited on this trip

Map and location of stops visited on this trip

The cross section gives a general overview of the structure of this part of the pluton. Apparent ‘folding’ of the pluton has resulted in exposure of the base of the pluton in a number of localities, two of which we shall be visiting.

Figure 3. Flat lying mafic enclaves

Flat lying mafic enclaves

3a) Flat lying mafic enclaves of varying compositions hosted within the Kameruka Granodiorite close to the eastern margin of the pluton. 3b) Feldspar rich cumulate at the base of a mafic layer showing crystals moulding onto the base. 3c) Classic cresent-shaped channel deposit of enclaves in Kameruka pluton.

Upper margins of sheets may be sharp gradational to the overlying granite (Wiebe, 1974). Thick mafic layers generally have unchilled tops and commonly show evidence for mechanical mixing with the overlying granite. For example, the size and proportion of feldspar or quartz xenocrysts commonly increases toward the top of the mafic layer (Wiebe 1974). It is most likely that convective stirring in both layers led to mechanical mixing and break-up of the top of the mafic layer (Wiebe 1974). In the overlying granite, the occurrence of enclaves that decrease in size and abundance upward from the top of the mafic layer provide further evidence for convection and mixing (Wiebe, 1974; Barbarin 1988).

The contrasting character of the bases and tops of individual mafic sheets strongly suggests that thick sequences of parallel mafic sheets in granite formed by sequential deposition of each mafic layer at the interface between a crystal-rich base of a granitic chamber (an aggrading chamber floor) and a crystal-poor, liquid interior. Because these basally chilled mafic layers rest on rocks ranging from gabbro to granite (Wiebe 1993, 1994), their level of emplacement into the granitic bodies cannot be related to neutral buoyancy. Instead, the level of emplacement is probably controlled by the rapid change in rheology from a crystal-rich material beneath the active chamber to a crystal-poor liquid in the interior. These were described by Wiebe (1993, 1994) as Mafic And Silicic Layered Intrusions (MASLI).

Wiebe and Collins (1998) provided a general model for the formation of MASLI systems in granite dominated magma chambers (Figure 4). After the mafic injection enters the magma chamber, it spreads laterally at a level of rheological contrast (Figure 4). Once the sheet begins to cool its density increases and it sinks into the underlying crystal mush, generating overlying "eddy" currents which are capable of ripping mafic magma globules off the upper surface (Figure 4). With progressive sinking, load casts and flame structures develop in the mafic sheet (Figure 4), which tends to develop a flat top during settling (Figure 4). Further cooling results in complete solidification of the mafic sheet and entrainment in an enclave-rich granitoid crystal mush. Flow in the overlying magma chamber produces stretched enclaves and a magmatic foliation (Figure 4) in the mush. With repeated injections, the pluton aggrades as a sequence of crystal mushes that provide a stratigraphic record within the intrusion (Figure 4).

Figure 4. Stages of formation of MASLI system

Stages of formation of MASLI system

Model showing the progressive stages of formation of a MASLI system (from Weibe and Collins, 1998)

In occurrences that they studied, Wiebe and Collins (1998) saw no evidence to suggest that these sheets were emplaced as sills at random levels in the package of layers. Thus, they considered that such a sequence of inter-layered mafic and granitic rocks preserves a stratigraphic record of magma chamber processes that were active during the crystallization of the granite intrusion. Indeed, the apparent sedimentary character of some plutons is enhanced by the presence of channel deposits, which are crescent shaped accumulations of poorly sorted enclaves that are convex-down (Figure 4).