Journal of the Virtual Explorer

The Kameruka Granodiorite, the dominant rocktype found within the section, is relatively homogeneous, containing coarse grained, megacrystic K-feldspar together with equigranular plagioclase, quartz and biotite. The granite also contains a high proportion of microgranular mafic enclaves and metasedimentary enclaves. Both enclave types tend to have high aspect ratios with their longest axis aligned parallel to, and also highlighting, a strong foliation defined primarily by the preferred alignment of plagioclase and the K-feldspar megacrysts. The foliation trends between N-S and NNW-SSE, dipping moderately to the east, parallel to the orientation of the pluton country rock contact. Is the foliation a result of compaction or flow – how could we determine the difference? We can address this issue on the outcrop.

Figure 7. Towamba River Rapids

Geological map of the Towamba River Rapids showing the main features we will be examining including 1) Syn-magmatic microgranite feeder dyke, 2) Magmatic mineral alignment, and 3) composite dykes.

The granodiorite is host to a number of dykes. The two main types of dykes that occur in the section are composite dykes and felsic, microgranitoid dykes. The composite dykes are found entirely within the western, lower part of the section.

Several composite dykes strike approximately E-W across the rock exposures (Figure 7). They are up to 2 metres wide and consist of basaltic pillows hosted in a microgranitoid matrix. The pillows have fine crenulate contacts with, or are injected by micro veinlets of, the granite. Large pillows are constrained to elongate bodies by the dyke margins, whereas smaller pillows are more rounded. Veins of felsic granite extend across some of the pillows, either filling straight or curved fractures. The curved fractures locally surround the smaller pillows, showing that the latter have formed virtually in situ from the adjacent larger types.

At one location, a mafic and felsic dyke intersect (Figure 7). The two dykes run parallel to each other for several metres before coming into contact. Approximately one metre eastward (upstream) from the junction, mafic pillows formed and are surrounded by the felsic dyke material. A pre-existing, intermediate-composition hybrid forms a thin discontinuous "film" at the margin of the mafic dyke, which is intruded by the felsic material and the two appear to locally mix. Some mafic material has mingled with the hybrid, forming small mafic enclaves in the matrix. These features suggest that when two dykes of contrasting magma intersect, the more mafic type will form pillows in the more felsic. Presumably, it cools quickly and therefore rapidly becomes more viscous than the host, when it beads into pillows (enclaves).

A single microgranitoid dyke can be traced almost continuously along the river section for several hundred metres to a point where it is observed terminating within the granodiorite (Figure 7). The dyke has a composition varying from granite to monzogranite; it is relatively fine grained and equigranular with grainsize ranging from 1-3mm. Plagioclase occurs predominantly as subhedral grains which display both progressive zoning (An26 core: An24 rim) and oscillatory zoning. K-feldspar also occurs as subhedral grains up to 1mm and exhibits incipient perthitic exsolution and microcline development. Quartz forms anhedral grains with irregular to interstitial grain boundaries and also display varying degrees of undulose extinction. Biotite occurs as evenly distributed, strongly pleochroic (red-brown), tabular to bladed grains. Scattered biotite clots up to 8mm in diameter occur in some of the microgranitoid dykes.

At the western-most exposure, the dyke is approximately 1.5 m in width and trends toward the southeast, or the pluton top. At this point a number of timing relationships are observed. The dyke cuts aligned K-feldspar megacrysts, microgranitoid enclaves and a composite dyke within the granodiorite, indicating that the granite had developed a foliation prior to the intrusion of the dyke.

Approximately 100m up-section, the direction of the dyke abruptly changes from E-to S-trending, adjacent to a large, N-S oriented meta-sedimentary xenolith (Figure 7). At this site, the dyke appears to have followed the lower contact of the xenolith. It can be traced across the river channel, but is offset to the east along a late sinistral fault.

The dyke terminates as a lense-shaped felsic layer, orientated N-S and dipping to the east (Figure 7), sub-parallel to the pluton contact The base of the felsic layer is sharp and slightly lobate downward, whereas the top is gradational to the overlying coarser-grained host-rock (Figure 7). All trace of the original dyke is lost at this stage.

A number of stacked, E-dipping microgranitoid layers closely resemble the lense-shaped dyke termination, and they each represent a discrete stratigraphic unit within the pluton, with a cumulative thickness of 20-30 metres. Although the layers are diffuse, I was able to map about nine layers, each varying in thickness from decimetre to metre-scale (Figure 7). Each layer is defined primarily by a distinct felsic, aplitic unit at the base of each sheet, as shown in Figure 7. The contact between the base of one sheet and the underlying megacrystic granite is generally sharp, but irregular, commonly marked by schlieren of biotite. This contrasts markedly with the upper part of the sheets which are characterised by a gradation from the aplitic phase into megacrystic granodiorite, typical of the Kameruka pluton. At the very top of some layers, a concentration of biotite may be present, which defines the contact between the individual layers. This sequence represents the more common situation but reversals also occur. Overall, the layers are aligned sub-parallel to each other, are irregular and are generally laterally continuous over the mapped area.

With the overall geometry of the Kameruka pluton in mind, the Towamba River section represents a tilted stratigraphic section just above the pluton base. The western-most exposure of the microgranitoid dyke represents a deeper level than the dyke termination which lies up-section and to the east. The microgranite dyke, intruded late in the crystallisation history near the base of the pluton. The granite at this stage must have been rigid enough to enable fracturing and emplacement of the dyke without mixing or mingling along its margins.

Up-section, the dyke swings to the south, where it encountered a rheological contrast, the large xenolith of metasediment (Figure 7). At this point, it migrated laterally along the basal contact of the xenolith, before continuing to intrude vertically (E to SE direction) through the granite. Ultimately, it reached a point where the viscosity of the granodiorite was much lower and the magma spread laterally.

The point of lateral spreading is considered to represent an interface within the pluton where a crystal density contrast existed. More specifically, it is regarded as an interface between an underlying crystal-rich, semi-consolidated magma and an overlying crystal-poor layer, interpreted as the magma chamber floor at the time of dyke injection. At this interface, the lower viscosity of the overlying layer meant that the magma could no longer fracture and the dyke preferentially spread laterally over the crystal-rich, mushy magma chamber floor.

This zone of lateral spreading, in which the sheets are characterised by gradational upper contacts between the dyke and the granodiorite (Figure 7), represents a zone of mixing between the granitic magma sourced from the dyke with the overlying crystal rich magma. Therefore, a single layer may represent the injection of granitic melt from a single pulse, into the magma chamber. Successive pulses of magma resulted in the formation of sheets which were successively formed either below, above or between the pre-existing layers. This zone therefore represents a site of magma chamber replenishment from felsic dykes.

The microgranite dykes are thus interpreted as representing localised feeders to the Kameruka pluton. This is supported by the relative abundance of these dykes decreasing from common near the base of the pluton to sporadic in the upper part of the pluton. Furthermore, dyke compositions lie at the high-Si end of the Kameruka Suite chemical array, and the granites can be geo chemically modelled as an accumulation of crystal within a felsic liquid represented by the average dyke composition.

The process(es) of magma chamber construction is(are) hotly debated. Some of the mechanisms proposed for the way magma migrates from its source (lower crust) to its sink (pluton chamber) include dyke transport, migration as sheets and lenses through active shear zones or as diapiric (inverted tear shaped) bodies which erode (through stoping and downward flow of aureole rocks) their way through the crust. Undoubtedly, the mechanism of pluton construction is case specific, however, we believe that the presence of abundant felsic, microgranitoid dykes throughout the pluton (although more common in the basal rocks) and the termination of these dykes within the pluton is strong evidence that dyke fed construction is a definite processes operating in the Kameruka pluton. Stoping, as well as magma migration along shear zones must also occur – we shall review some of these processes during the two days.