Stop 1 - Wog Wog Mafic Complex

This outcrop lies immediately below the campsite in the Wog Wog River.

The Wog Wog Mafic Complex (WWMC) is a lense shaped, sheeted to strati form body some 2.5 km long and up to 600m wide, striking to the NE and dipping ~50° to the SE. It consists of multiple, metre scale layers of fine to medium hornblende-gabbro layers and coarser grained to oikocrystic, homblende-rich variants. Relict cores of both orthopyroxene and clinopyroxene are locally present in the finer grained phases. Fine grained homblende bearing basaltic dykes cut the complex: some are sub-parallel to the layering and others are oblique at low angles. The latter are easily confused with younger basalts of transitional alkaline character.

A 65m-long representative section of the WWMC exists directly upstream from the Fulligans Track river crossing, immediately below the campsite (Figure 16). Here, the gabbroic layers dip between 30-60° to the SE.

The lower units of the section (to the west) vary up to two metres in thickness and consist of fine to medium basaltic rocks at the base, grading to medium grained, lighter coloured dioritic rocks at the top. Small fragments of very fine grained basalts, representing “rip-up" clasts (Figure 16), are common at this locality. Some of the clasts are also metasedimentary in composition and indicate that some layers, which are now totally enclosed within the mafic complex, must have been in contact with the surrounding metasedimentary countryrock. In other units, more rounded dioritic enclaves appear to have been partially solidified when incorporated.

Figure 16. Map of Rockbar in front of the Campsite

Map of Rockbar in front of the Campsite

Map of the southwestern end of the rockbar in front of the campsite. This is the type section of the Wog Wog Mafic Complex. It exhibits characteristic layering and stuctures that resemble plumes. The layering is defined by variations in grain size and composition. The base of individual layers is typically fine-grained and basaltic while the tops of the sheets are coarse-grained and doritic to gabbroic.

Farther up the sequence, in the middle section, massive coarse-grained gabbro layers contain large (5 cm diameter) oikocrysts of hornblende set within a matrix dominated by plagioclase. This distinctive unit has a transitional contact with underlying medium grained gabbros, although local intrusive contacts with finer grained units also occur. Much of this layer is obscured by a flat-lying felsic dyke, which links with other smaller felsic dykes and veins throughout the section.

Above the oikocrystic layers, a distinctive gabbro breccia unit forms an irregular body that transgresses the layered complex. Fragments consist of basaltic and dioritic clasts, some of which contain fine laminations that are either planar or convolute. The general angular nature of the clasts, and the discordant contacts of the breccia unit, indicate that it represents a later pulse of gabbroic magma that intruded the more solidified layered sequence, ripping up fragments and transporting them to the site of emplacement. These relations support evidence that the complex was constructed through progressive injection of parallel (sub-horizontal) sheets of mafic material.

The uppermost part of the sequence reflects a return to initial conditions. Metre-scale layered units again show chilled bases and coarsening upward, concomitant with differentiation to more dioritic compositions. Some of the units have a basal section that contains rip-up clasts. Generally, the upper mafic units are coarser grained and more shallowly dipping than their lower equivalents at the bottom of the section. Aplitic and mafic dykes cut the layers.

Way-up indicators

The basal contacts of individual layers tend to be planar and fine grained, suggesting strong under cooling against the underlying unit. Individual units commonly contain "rip-up" clasts of igneous fragments, which suggest derivation from underlying units (Figure 16). The units commonly grade upward into coarser, more felsic (dioritic) variants, illustrating a type of magmatic "graded bedding". This feature and the rip-up clasts indicate that way-up is to the SE, and the whole complex is "right-way-up".

Another major "way-up" indicator are diapir-like structures of medium to coarse-grained dioritic material that have intruded through the layered rocks. In the lowest part of the "type-section" (Figure 16), a discordant intrusive contact between diorite and basaltic layers outlines a 3m-high coliform structure emanating from one of the lowermost coarse-grained dioritic units (Figure 16 and Figure 17). We interpret this structure as a diapir rising from a buoyant differentiated layer in the complex. Another diapiric structure with 2m-high crown is recognised several metres higher in the sequence, and a set of at least three emanate from another feIsic layer approximately three quarters of the way up through the section. Elsewhere, narrow layers of coarse-grained gabbro transitional to pegmatite, located directly below basal chilled contacts, reflect accumulation of rising volatiles from the underlying unit trapped against an impermeable upper barrier (Figure 17c). These structures confirm that younging is to the SE.

Figure 17. "Rip-up" clasts of basaltic fragments

"Rip-up" clasts of basaltic fragments

a) “Rip-up” clasts of basaltic fragments in different units. Photo also illustrates the sheeted or layered nature of the WWMC. b) Plume-like structure of felsic “diapir ” piercing through several layers. c) Entrapment of risine volatiles against an impermeable, chilled overlying mafic layer.