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Frontiers and Opportunities in Antarctic Geosciences * Certosa di Pontignano * 29-31 July 2004
A Major Crustal Discontinuity in George V Land: New Petrological and 40Ar39
Ar Data and New Research Opportunities
F. TALARICO1*, G. DI VINCENZO 2 & G. KLEINSCHMIDT3
1
Dip. Scienze della Terra, Università di Siena, Via del Laterino 8, I-53100 Siena - Italy
2
Istituto di Geoscienze e Georisorse, CNR, Via G. Moruzzi, I-56124 – Pisa – Italy
3
Geol.-Paläont. d. Universität Frankfurt, Senckenberganlage 32, D-60054 Frankfurt/Main – Germany
*Corresponding author ([email protected])
George V Land and Adelie Land are key regions for Gondwanaland reconstructions providing precise correlations of
Paleoproterozoic terrains across the Southern Ocean between Australia and Antarctica (Oliver & Fanning, 1997). This
coastal segment of East Antarctica is also important to gain new insights into the tectonic relations between East
Antarctic Craton (EAC) and the Ross Mobile Belt. Furthermore, it potentially retains records of the younger (?late
Mesozoic-Cenozoic) crustal structures and tectonic evolution of a key region of East Antarctica at the northernmost
edge of the Wilkes Subglacial Basin at the backside of the Transantarctic Mountains.
During the GANOVEX VIII-BACKTAM 1999/2000, a major result of field investigations in the Mertz Glacier area
(Correll Ntk, Aurora Pk, Mt Murchison) was the finding of a prominent mylonitic zone (the “Mertz Shear Zone”,
MSZ), with steeply dipping ~NNW-SSE oriented foliation planes and shear sense indicators indicating dextral sense of
ductile shear expedition (Talarico & Kleinschmidt, 2003). Enderbitic orthogneiss (often retrograded and transformed
into hornblende-biotite augen gneisses) and minor sillimanite-garnet felsic granulites, mafic granulites and amphibolites
are the main lithologies of the MSZ. The MSZ preserves a series of progressive and overprinting shear structures, with
mylonites to ultramylonites developed under changing metamorphic conditions (from early low P granulite- to lower
amphibolite- and greenschist-facies conditions), which were locally overprinted by brittle faulting with
pseudotachylytes, but all with similar kinematics (dextral shearing). The MSZ is located within the mainly ice-covered
boundary zone between two domains with different lithological assemblages and contrasting P-T evolution. A low-P
high-T metamorphic event is documented in low- to high-grade metasediments occurring as rafts within dominant
strongly peraluminous granites in the area between Mertz and Ninnis Glaciers (Eastern Domain). In contrast, a
decompressional trajectory from medium-P granulite-grade conditions is recorded in the rock units along the western
margin of the Mertz Glacier (Western Domain), where a complex Late Archean to Paleoproteozoic evolution is
documented, apparently without evidence of reactivation during the Ross tectono-metamorphic cycle (Fanning et al.,
2002).
Consistently with such a major geological subdivision, an 40Ar-39Ar laserprobe investigation of selected samples from
the MSZ, and from adjacent rock units from both Western and Eastern Domains (Garnet Point, Penguin Point) provides
conclusive evidence that: (1) both amphibolite-grade mylonitic fabrics of the MSZ and amphibolite-grade reequilibration of mafic granulites at Garnet Point, formed during Paleoproterozoic time; (2) biotite and muscovite from a
strongly peraluminous granite of the eastern Domain, from the closest outcrop to the MSZ, gave ages of ~488 Ma,
which are consistent with the 490-505 Ma SHRIMP ages on zircons reported by Fanning et al. (2002) for granitoids
from the Mertz-Ninnis Gl. region. Moreover (3), dating of single-grain detrital white micas from a metasandstone
(forming together with minor muscovite-bearing granites the dominant rock types of erratics at Horn Bluff) indicates a
dominant Pan-African source, similarly to the low-grade metasedimentary sequences from the western Wilson Terrane
in northern Victoria Land (Priestley Formation – e.g. Calonaci et al., 2002). The new geochronological data confirm a
Paleoproterozoic age for the MSZ and further corroborate its correlation with shear zones with comparable ages and
kinematics, such as the pre-1610 Ma Kalinjala Mylonite Zone within the Gawler Craton in southern Australia.
Surprisingly, the samples from the western Domain and the MSZ do not show any record of the Ross tectonometamorphic events in spite they were sampled only few tens of kilometres away from the Ross-age granites.
These results point to a renewed interest for a further research effort along the George V Land transect aimed to place
new geological and geophysical constrains on the location, geometry and timing of the boundary between the EAC and
the Ross Mobile Belt, as well as a better imaging of the crustal structure at the northernmost tip of the Wilkes
Subglacial Basin. We shall present a research plan for a geological – geophysical survey focused on the Mertz Glacier
area and based on the prosecution of the aeromagnetic survey performed during the GANOVEX VIII-BACKTAM
1999/2000 expedition (Damaske et al., 2003) and on field and laboratory ( 40Ar-39Ar, magnetic and metamorphic
petrology) investigations.
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Damaske D., Ferraccioli F. & Bozzo E., 2003. Terra Antartica, 10, 85-96.
Calonaci B., Di Vincenzo G., Ricci C.A., Talarico F., 2002. Abstracts volume, Worhshop on Ant. Earth Sci, Siena 30/11-2/10, 51-52.
Fanning C.M., Menot R.P., Pecaut J.J. & Pelletier A., 2002. Abstracts 16th AGC, Adelaide: 67, 224.
Talarico F. &. Kleinschmidt G., 2003. Terra Antartica, 10, 229-248.
Oliver R.L. & Fanning C.M., 1997. Terra Antartica Publications, 163-172