Interpretation of Megaxenolith Emplacement

The evidence discussed above indicates that the Candelo Tonalite is a large xenolith that was rigid and capable of fracturing during emplacement at the base of the Pericoe pluton. Several interpretations have been made concerning the development of the features surrounding the megaxenolith, and of which relate back to the position of the xenolith at the base of the pluton and the mechanism by which it was deposited/incorporated. The Pericoe pluton is basin-shaped and lies in the inverted saddle of a megascopic south-plunging synform (Figure 2 and Figure 6 X-section). The eastern contact dips steeply to the southwest whereas the western contact, where the xenolith is located, dips moderately to the south east. The xenolith can therefore be considered as being located on the shallow, e-tilted floor of the pluton, and is interpreted as a large stoped block of overlying Candelo pluton that has sunk through the Pericoe pluton coming to rest on the pluton floor. Candelo Suite granites are the earliest emplaced pluton in the Bega Batholith, followed by syntectonic intrusion of Kameruka Granodiorite and, soon after, by the Pericoe pluton. The latter was emplaced into the regional synform that was produced by syn-plutonic compressive deformation. Country-rock along the upper contact of the Pericoe pluton must have included solidified Candelo Tonalite, a large fragment of which as was stoped off and sunk through the less dense, partially crystalline magma chamber. Other stoped material included migmatites and gabbroic bodies.

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 6. Characteristic Features

Characteristic Features

Details of some of the characteristic features we will be observing on this trip. The top cross-section is applicable to stops on the first day in the Towamba River and Lower Wog Wog River. The lower cross section shows features observed on the second day in the upper Wog Wog River.

These may have been stoped at the same time, or represent blocks accumulated over a period of time. The latter interpretation is supported by the presence of blocks at various levels of the 600m thick zone. If so, it is an excellent example of an "elephants graveyard", the postulated zone of deposition of large stoped blocks at the base of plutons. The base of the pluton at this time was also accumulating a layer of mafic enclaves, possibly representing a MASLI-type deposit. As the megaxenolith collided with the bottom of the chamber, it disrupted the MASLI layer and formed a trap for escaping volatiles and felsic liquids which ponded beneath it. Presumably, the megaxenolith then rotated into a position of repose, resulting in the formation of unusual convection currents in the directly overlying magma chamber. The thermal disturbance and turbulence in the magma chamber, generated by sinking and settling of the xenolith, resulted in the formation of the overlying compositionally layered granite, which was disturbed and redeposited as 'cross bedding'.

Figure 23. Additional Figure

Additional Figure

We will not have time to visit this spectacular outcrop, however, it is worthy of a short discussion. The site lies some 800m downstream and sits within the Pericoe pluton. Here enclaves have accumulated in a number of cresent-shaped ‘lenses’. This figure shows only a small portion of these enclave swarms. The lenses have arcuate, concave upwards shapes which are defined by a dense accumulation of emclaves. Here the enclaves are closely packed and mould around underlying enclaves and exhibit classic ‘pillow’ shapes. Away from the contact (toward the southwest) the enclaves become dispersed into the overlying granite. We have interpreted these structures as enclave-rich channels where mafic material (enclaves) have come to rest on a crystal-mush boundary within the pluton. Classic MASLI type structures. Successive deposits overly previous accumulations so that the channels appear stacked; oldest at the base, youngest at the top. These features closely resemble a cross-section through an aggrading valley floor deposit.