GRANITOID STUDIES, ST. JOHNS RANGE (K043)
An area of approximately 100 sq. km in the central St. Johns Range was mapped and 120 granitoid specimens specially sampled for geochemical analysis. The informally named Harker and Seinford Granites form with the Vida the bulk of the granitoids. The Harker is similar to the Vida but has a higher proportion of K-feldspar, a more uniform pink colour and larger grainsize. Chemical differences include higher SiO2, K2O and Rb and lower CaO, Sr and Ba. The Harker Granite intrudes the unfoliated, K-feldspar megacryst-bearing Swinford Granite. The field and limited petrographic and chemical evidence so far available indicates all three granites are I-types.
Gneisses and schists form a remnant basement screen between the Vida, and the Harker and Seinford Granites. Quartz-feldspar-biotite-hornblende paragneiss forms the bulk of the gneiss. Mignatites are developed locally. Of limited extent also, are granite, tonalite and feldspar orthogneisses containing euhedral feldspar phenocrysts. The lower grade rocks consist of quartzo-feldspathic and biotite schists, amphibolites, marbles and calc-silicates.
The gneiss and schists are intimately intruded by a series of reunifying granodiorite dykes. The extent to which these are either a contaminated marginal phase of the Vida, or a separate phase such as the Theseus Granodiorite can not yet be determined. Other dykes comprising at least five different types were mapped and include, pink K-feldspar porphyry, lamprophyre, basalt, granophyre and diorite varieties. Cross-cutting relationships were difficult to ascertain. However the basaltic dykes were considered to be the youngest and may be related to the dolerite sills.
Near Purgatory Pk. a 40m sequence of Altar Mountain Formation and Odin Arkose containing basal conglomerate, grits and sandstones with trace fossils was observed.
Previous studies of granitoids in South Victoria Land have been mainly of a reconnaissance nature and associated with regional mapping (Gunn and Warren, 1962; Findlay, 1983). These studies outlined the gross distribution of granitoids and developed preliminary subdivision of the rocks into several plutonic suites. However granitoid nomenclature is still a problem in South Victoria Land for two reasons.
Firstly the early subdivision into red and grey granites has been carried over to the Irizar Granite and Larsen Granodiorite subdivision. Attempts have been made to lump together granitoids hundreds of kilometers apart. This was brought about by a tendency in the past to attempt to "correlate" various granitoids and was probably the result of early mapping carried out by geologists more versed in stratigraphy than igneous petrology. Granitoids should be mapped and named on a pluton basis, with several plutons comprising a batholith. The names Irizar and Larsen should be restricted to the plutons at the type localities.
Secondly the geological structure and outcrop distribution in South Victoria Land is such that many granitoid boundaries are not well defined because of ice and snow cover. Many granitoid plutons appear to grade in physical appearance and chemical composition from centres to margins. Margins in turn grade into high grade metamorphic rocks and in these areas delineating boundaries becomes very subjective indeed.page 7 page 8
What is now required is for the granitoids to be mapped on a more detailed individual pluton basis and their geochemical and physical characteristics defined. Since many of the granitoids are derived from melting of basement, and may give rise to some of the numerous dykes it is also important that the geochemistry of these be studied too. In a regional study. Palmer (in prep.) has analysed 120 granitoids from South Victoria Land. It is hoped to assign many of these granitoids into the S- and I-type classification of Chappell and White (1974) now in common use.
Our principal objective was to map in detail the distribution of the various granitoids, gneisses, schists and dykes in the St. Johns Range. In addition a representative suite of samples suitable for major and trace element analysis was to be collected to enable petrogenetic relationships between the rock types to be established. To this end an area of over 100 sg. km was mapped and some 120 samples specially collected for analysis.
Several granite bodies were mapped. However the extent to which some of them differ cannot be fully ascertained until detailed geochemical studies are carried out. The Vida Granite crops out in the SW of the area, its description conforming to that given by Allen and Gibson (1961). It is a grey but occasionally pink, fine to medium-grained biotite granite and has a subhedral granular texture. Blocks and xenoliths of schist have been incorporated and partly assimilated around the margins. Xenoliths can be seen sheared out producing intricate flow banding. This has occurred to such an extent that in places the granite is darker in colour and its composition appears to have been modified to that of a granodiorite or tonalite (Plate 1). Away from the margins mafic xenoliths and clots of biotite are not uncommon. At one location a large hornblende-bearing mafic xenolith some 4m across was noted. An analysis of the Vida Granite (Palmer, in prep.) from south of Lake Vida is provided in Table 2 and is particularly notable for the high level of Ba.
A further two granites outcrop extensively, to which the informal names Harker and Swinford Granite have been given after their occurrences on these mountains. The former is a Vida-like granite however it is pinker in colour, coarser-grained and hornblende is virtually absent. Two analyses of this granite from Pond Pk. (Palmer, in prep.) are provided in Table 2. Compared to the Vida, the Barker Granite is enriched in SiO2, K2O and Rb but is depleted in CaO, Sr and Ba. On Pond Pk. and the hills E of Purgatory Pk. bands of intense brick-red alteration occur, probably related to fracture zones. Similar effects have been described elsewhere in South Victoria Land (Craw and Findlay, 1984). The Harker Granite can be observed intruding gneiss and schist at a number of localities. On a ridge E of Schist Pk. angular blocks of gneiss can be seen surrounded by coarse-grained pink granite (Plate 2). On the ridge NE of Mt. Swinford, Harker Granite intrudes the Swinford Granite. This contact is marked by a fine grained chilled margin dipping at about 40 degrees to the SW.
The Swinford Granite is less extensive than the previous two granites. It appears to intrude gneiss and schist at Mt. Swinford and extends NE across the glacier to the next ridge. The granite is particularly conspicuous because large, euhedral K-feldspar megacrysts are found throughout. It generally lacks foliation and contains numerous small mafic xenoliths and less common K-feldspar porphyry xenoliths.page 9
Throughout the belt of gneiss and schist are freely ramifying dykes of microgranodiorite. It is not clear at this stage whether these represent the fine-grained contaminated margin of the Vida Granite or constitute an earlier phase of igneous activity, such as the Theseus Granadiorite. It is hoped that the detailed geochemistry to be carried out on the numerous specimens collected will shed some light on this problem.page 10
Using the limited field, petrographic and chemical data presently available, the Vida, Harker and Swinford Granites would be classed as I-type. All of these granites are relatively unfoliated and show discordant cross cutting relationships with pre-existing rocks. Igneous xenoliths are common and hornblende is often present along with euhedral zircon, sphene and allanite. Muscovite, cordierite and garnet, minerals characteristic of S-types, are absent. The granites have high Na/K ratios. Thus it is thought that these granites are derived from the melting of pre-existing igneous rocks, rather than the melting of rocks which have undergone a sedimentary cycle.
Gneiss and schist, the metamorphic equivalents of the Meserve Member of the Hobbs formation, trend across the area in a NW-SE direction. They appear to remain as a screen of older basement between the Vida and Harker Granites. The schists are strongly deformed with all lithological layering transposed parallel to a dominant S2 foliation. Occasional F3 crenulation folds were noted, especially near the head of the Willis Glacier. This deformation is comparable to that observed by Korsch (1985) on Robertson Ridge. The schist lithologies are also comparable to those noted by Korsch, but the prevalence of amphibolites (metamorphic equivalents of basalts and andesites) throughout the sequence, not only here but elsewhere in South Victoria Land suggests that an island arc sequence is represented rather than a passive continental margin as proposed by Korsch. Several amphibolites were sampled during the present study and it is hoped that from their geochemical signature some evidence of their original tectonic setting can be obtained.
The belt of metamorphic rocks widens to the SE and around Purgatory Pk. gneiss becomes common. Paragneisses that appear to be simply higher grade equivalents of the schists, as well as orthogneisses occur in the sequence. Lenticles and stringers of migmatite are found in the paragneisses. The orthogneisses have been through a melting stage and are characterised by large euhedral feldspar crystals. In the Purgatory Pk. area they comprise granite, tonalite and feldspar gneisses. The feldspar gneiss consists almost entirely of K-feldspar and may represent an accumulation product of the granite, leaving a remainder of tonalite composition. These varieties are comparatively rare.
- Black, basaltic dykes
- Pink, K-feldspar porphyry
- Medium-grained diorite
The dykes range in width from a few cm (aplites) up to 10 or so metres (porphyry). A small (<50m across) body of quartz-feldspar porphyry was mapped 2km SE of Lanyon Pk. Only one diorite dyke was found, occurring as a N trending body some 5m wide at the end of the ridge NE of Lanyon Pk. Generally the dykes are subparallel so that cross cutting relationships were difficult to establish. Low rubbly outcrop in many cases did not contribute to this task. The basaltic dykes resemble the fine-grained margins of the dolerite sills and it is thought they are simply offshoots of these sills. Dykes other than these basaltic types appeared to be absent from the Harker Granite, suggesting that the dykes are related to the Vida Granite and are older than the Harker.page 11
Small remnants of the Beacon Supergroup occur throughout the area generally only as a few meters preserved beneath the peneplain sill. About 2km NW of Purgatory Pk. however, some 40m of conglomerate, grits and sandstone are preserved, dipping at a low angle to the north. The base of the sequence is obscured by scree and snow, but well rounded cobbles (<20cm dia.) of quartz, quartzite and silicic volcanics (?) suggest a basal conglomerate resting on gneiss. Trace fossils occur in fine sandstone near the top of the sequence. Barrett and Webb (1973) describe similar sequences in the Wheeler Valley and at Mt. Suess and correlated them with the Altar Mountain Formation and the Odin Arkose. This is one of the most easterly occurrences of Beacon in the region.
- The preparation of polished thin sections to enable electron microprobe analysis of minerals to be undertaken and the petrographic features of the various rocks to be described.
- X-ray fluorescence analysis of the major and trace elements to allow further refinement of the petrological features above and to enable petrogenetic affinities to be determined.
Whole rock and mineral analytical data will be presented in a Victoria University of Wellington Geology Department publication.
More work is required in South Victoria Land to further distinguish S- and I-type granitoids. Boundaries between these types elsewhere in the world are of fundamental tectonic importance. Detailed geochemical studies are however required in this respect. Further work on granites without taking this into account will only add further to the present confused nature of South Victoria Land basement studies. A future promising line of research stems from observations and preliminary sampling of the amphibolites within the Koettlitz Group. Study of immobile trace element concentrations may enable the pre-metamorphic tectonic characterisation of the unit in a manner analogous to Pearce and Cann, (1973) and Grapes and Palmer, (1984).
The Officer in Charge and Deputy officer in Charge at Scott Base are thanked for their assistance and hospitality. We are grateful to VXE-6 squadron of the U.S. Davy for helicopter transport whilst in the field.
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Barrett, P.J. and Webb, P.N. 1983. Stratigraphic sections of the Beacon Supergroup (Devonian and older (?) to Jurassic) in South Victoria Land. Publication Department of Geology Victoria University of Wellington, 2, Antarctic Data Series 3.page 12
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