Report on participation in GANOVEX VII Expedition, Terra Nova Bay 1992/93

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Skinner, D.N.B. 1993 Report on participation in GANOVEX VII Expedition, Terra Nova Bay 1992/93. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 93/12 28 p.

Abstract: Out of 35 days at Gondwana Station, work was possible on only 20 days, and then often for just a few hours or even less. The weather was particularly bad with heavy snow fall and extremely strong winds. Field work was concentrated on the Deep Freeze Range between Mt Levick on the Priestley Glacier, Mt Dickason on the Campbell Glacier and Terra Nova Bay. A reconnaissance flight enabled all Priestley Formation outcrops on the east side of the Campbell Glacier (western Southern Cross Mountains) to be sampled, to compare with Priestley Formation rocks from west of the Snowy Point Gneiss Complex belt along the eastern Deep Freeze Range. 315 rock samples have been collected for dating, petrological and structural analysis, and geochemistry. In spite of the inclement weather, traverses were completed through the Mt Dickason massif gneiss and granulite sequence, and the Wishbone Ridge (Boomerang Glacier) Priestley Formation, gneiss and granulate succession. These traverses served to fix the tectonic contact between polymetamorphic gneiss (Snowy Point Gneiss Complex) and metasedimentary rocks of Priestley Formation - The Boomerang Thrust. The effects of this thrusting on the two lithodemic rock units were confirmed, especially the occurrence of a zone of near pure shear strain in the overlying gneiss plate, which manifests itself as a zone of flattening and superimposed schistosity on the gneiss. Within the contact zone and the gneiss, granitic and mafic intrusions abound and are strained by the thrust tectonism and metamorphosed with the gneiss and Priestley Formation metasediments. The trace of the Boomerang Thrust itself is not exposed north of the Corner Glacier, but its probable line of continuance was traced into the granites of the Howard Peaks - Mt Burrows area on the Tourinaline Plateau where an appropriately oriented brittle cleavage is common and a significant proportion of mylonitised Dickason Granite blocks lie within a small moraine. Migmatitic and mafic (enderbitic) granulate facies rocks characterised by garnet and pyroxenes were mapped within the gneisses of the Mt Dickason massif. There is a common occurrence of low angle zones of pyroxene-garnet migmatite that have the appearance of mafic inclusion-rich intrusions of anatectic ''granite''. The field observations suggest that after amphibolite facies migmatisation of the gneiss, intrusion of mafic sills caused a high temperature remelting of the earlier migmatite and gneiss and back melting into the mafic intrusives. Several increments of deformation preceded the mafic intrusions, creating a gneiss pile where the dominant foliation forms a series of flat-lying stacked recumbent folds, separated from smaller zones of steep foliation and isoclinal folds by monocanal hinges with brittle cleavage. However, at Gondwana Station, the dominant foliation is tightly folded into steeply plunging isoclinal folds that post date mafic dike/sill intrusion. On the Mt Levick massif, the upper low angle lim b of a 2 km sized overturned fold is refolded and intruded by swarms of deformed granite dikes and plugs; this was accompanied by a rapid rise northwards in metamorphic grade. Further to the north, the bedded but schistose Priestley Formation passes into gneiss within a zone of intense granite intrusion and strain. The actual contact lies within a 1 km section at the north end of the ridge adjacent to Tourmaline Plateau but could not be walked over in the time available. A most significant discovery, to be confirmed petrologically, is that within schists of Priestley Formation previously mapped as low grade metamorphics on the western slopes of Mt Levick, pelitic schist contains andalusite (chiastolite), but boudinaged quartz veins and saddle reefs in isoclinal folds contain disthene (kyanite) crystals up to 5 cm long that appear to be changing to sillimanite! If confirmed, this seems to be an occurrence of the rare alumino silicate triple point, albeit within different lithologies. It is also clearly a sign of high pressure - perhaps related to the Boomerang Thrust - and certainly will require a re-think of the distribution of kyanite in North Victoria Land, where it is previously known from only north and east of the Aviator Glacier. (auth)