Thiel Mountains

The Thiel and Pensacola mountains were mapped by geologists of the US Geological Survey (Schmidt and Ford 1969). The geologic map of the Shackleton Range was published by Clarkson (1982) and by Clarkson et al. (1995) based on field work of geologists from many countries. The Theron Mountains were discovered and described by members of the Commonwealth Trans-Antarctic Expedition (1955-1958 Section 1.5.4 Stephenson 1966). 

Faure and T.M. Mensing, The Transantarctic Mountains Rocks, Ice, Meteorites and Water, DOI f0.f007/978-90-48f-9390-5 8, Springer Science+Business Media B.V. 20f0 

The rocks exposed in the Thiel Mountains belong to the basement complex of the Transantarctic Mountains. The sedimentary rocks of the Beacon Supergroup that uncon-formably overlie the basement rocks elsewhere in the Transantarctic Mountains do not occur in the Thiel Mountains either because they have been removed by erosion or because they were not deposited there. The stratigraphy of the basement rocks of the Thiel Mountains in Table 8.1 is deceptively simple compared to the basement rocks elsewhere in the Transantarctic Mountains (e.g., the Queen Maud Mountains and the Wisconsin Range). 

Schmidt and Ford (1969) mentioned that a thin layer of marble in the Mt. Walcott Formation contains stromatolites. However, Storey and Macdonald (1987) and Pankhurst et al. (1988) later stated that these stro-matohtes are actually composed of interlaminated calcareous siltstone and limestone that underwent soft-sediment deformation. The same authors did find abundant echinoderm plates and sponge spicules similar to Protospongia fenestrata which occurs in the Middle Cambrian Nelson Limestone of the Pensacola Mountains (Weber and Fedorov 1980, 1981). The Mt. Walcott Formation also contains trace fossils (Planolites and Chondrites). Although Planolites has been found in rocks that are 1,000 milhon years old, the earhest appearance of Chondrites is in rocks of Cambrian age. Therefore, Pankhurst et al. (1988) concluded that the paleontological evidence supports a Phanerozoic age for the Mt. Walcott Formation, whereas Schmidt and Ford (1969) had previonsly assigned it a probable Neoproterozoic age. 

Reed Ridge Granites Light gray and pink, coarse-grained, biotite granitic rocks in plutons and smaller intrusives 

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Ishimaru, S. Yoshikawa, K. (2000) The weathering of granodiorite porphyry in the Thiel Mountains, Inland Antarctica. Geografiska Annaler A82, 45-57. 

J. S. Delaney and M. Prinz, in Field and Laboratory Investigations ofMeteorites from Victoria Land and the Thiel Mountain Region, Antarctica, 1982-1983 and 1983-1984 (ed. U.B Marvin and G. J. MacPherson), Smithson. Contrib. Earth Sci. 28, p. 65 (1989). 

Pankhurst RJ, Storey BC, Millar IL, Macdonald DIM, Vennum WR (1988) Cambrian-Ordovician magmatism in the Thiel Mountains, Transantarctic Mountains, and implications for the Beardmore Orogeny. Geology 16(3) 246-249 Peacock SM, Goodge JW (1995) Eclogite facies metamorphism preserved in tectonic blocks from a lower crustal shear zone, central Transantarctic Mountains, Antarctica. Lithos 36 12-13... 

The igneous rocks of the Wyatt Formation occur at many locations within the Queen Maud Mountains, as well as in the Horlick and Thiel Mountains. In spite of these extensive exposures, the origin of the Wyatt Formation is still uncertain because in some places it clearly intruded folded metasedimentary rocks of the LaGorce Formation, whereas in other places it over-lies them conformably. In addition, hydrothermal alteration of the rocks and the effects of regional metamorphism described by Minshew (1967) have disturbed the isotopic composition of strontium of the rocks, which has caused data points to scatter on Rb-Sr isochron diagrams and caused large uncertainties in age determinations of the Wyatt Formation listed in Table 6.5. 

Eord and Aaron 1962) and their findings were incorporated by Schmidt and Ford (1969) into the set of geologic maps edited by Craddock (1969 a, b). More recently in 1983/84, a joint US-British airborne geology field party landed at the Thiel Mountains and made observations which resulted in publications by Storey and Dalziel (1987), Storey and Macdonald (1987), and Pankhurst et al. (1988). 

Fig. 8.1 The eastern end of the Transantarctic Mountains consists of the Horlick Mountains, the Thiel Mountains, the Pensacola Mountains, the Shacldeton Range, and the Theron Mountains. These mountain ranges are separated from each other by large gaps covered by the East Antarctic ice sheet and in this way occupy about 1,300 km of the total length of the Transantarctic Mountains. AU of these distant mountains have been mapped by geologists (e.g., Mirsky 1969 Davis and Blankenship 2005 Schmidt and Ford 1969 Clarkson et al. 1995) (Adapted from Craddock 1982)...

Thiel Mountains Porphyry Dark-gray, cordierite-and hypersthene-bearing porphyry probably forming a thick siU... 

Table 8.2 Average abundances ofminerals (modal composition) of 11 specimens of the Thiel Mountains Porphyry determined by Ford (1964)...

Ford (1964) reported that the Thiel Mountains Porphyry contains several types of granuhte xenohths most of which are less than 2 cm in diameter but one large xenolith of gneissic granuhte measured 150 by 60 cm. The presence of xenoliths composed of hyper-sthene-cordierite granulites is an important clue to the origin of the Porphyry. 

The Thiel Mountains Porphyry is essentially a massive body of igneous rocks with locally-developed faint strahflcation which Ford (1964) attributed to the flow of magma that began to crystallize at depth before it was either intruded at shallow depth in the crust or was erupted at the surface where it cooled rapidly. Although Ford (1964) originally favored the explanation that the Porphyry formed from magma that was intruded at shallow depth. Ford and Sumsion (1971)... 

Ford and Himmelberg (1976) also pubfished the average chemical composition of 27 specimens of the Thiel Mountains Porphyry, which they classified as a rhyodacite, as well as chemical analyses of the cordierite and the orthopyroxene aU of which are fisted in Appendix 8.5.1. The chemical composition of the Thiel Mountains Porphyry closely resembles the chemical compositions of the metavolcanic rocks of the Wyatt Formation on Mt. Gardiner in the Scott Glacier area and on Metavolcanic Mountain at the head of the Reedy Glacier (Appendix 7.8.1). 

Many investigators who have worked on the rocks of the Wyatt Formation (Sections 6.3.2, 6.4.1, and 7.1.2) and on the Thiel Mountains Porphyry have commented on the similarities of their chemical composition, their texture, their massive structure, their modes of emplacement, and their relation to underlying clastic sedimentary rocks (e.g., Vennum and Storey 1987a Pankhurst et al. 1988 Stump 1995). Even Ford and Sumsion (1971) pointed out that the Thiel Mountains Porphyry and the rocks of the Wyatt Formation are both of pyroclastic origin and formed during an episode of silicic volcaiuc activity on an immense scale because the volume of pyroclastic rocks in the Thiel Mountains alone seems to have exceeded 5,000 kml... 

However, even if the Thiel Mountains Porphyry and the pyroclastic rocks of the Wyatt Formation formed during the same episode of silicic volcanic/magmatic activity, cordierite has only been identified in the Thiel Mountains Porphyry. The presence of this mineral as... 

The first age determinations of the Thiel Mountains Porphyry and of the Reed Ridge Granites pubfished by Ford et al. (1963), Aaron and Ford (1964), Ford (1964), and Sclmudt and Ford (1969) were based in part on the lead-alpha method of dating that was described by Gottfiied et al. (1959) and sununarized by Faure (1986). The Pb-alpha dates of zircon in the Thiel Mountains... 

Porphyry ranged from 670 50 to 530 60 Ma, which caused Schmidt and Ford (1969) to assign the Thiel Mountains Porphyry a Neoproterozoic age. 

The lead-alpha dates of zircon in the Thiel Mountains Porphyry appeared to be confirmed by Eastin (1970) who obtained a whole-rock Rb-Sr date of 632 102 Ma and an initial Sr/ Sr ratio of 0.7086 0.0059 for specimens of the Porphyry. This date and the initial Sr/ Sr was derived by an error-weighted regression procedure applied to the analytical data in Appendix 8.5.3. The same data were used by Faure et al. (1977) to calculate a date of 660 79 Ma and an initial ratio of 0.7069. The large uncertainty ( 102 x 10 year) of the date calculated by Eastin (1970) arose because the data points do not constrain the slope and intercept of a straight line well enough to calculate a geologically useful crystallization date of the Thiel Mountains Porphyry. 

Eastin (1970) also analyzed two samples of biotite granite and three of quartz monzonite from the Thiel Mountains for dating by the Rb-Sr method (Appendix 8.5.3). He used these data to determine the crystallization date of the granite but was unable to fit a straight line to all five data points. Therefore, we excluded a biotite-rich sample of quartz monzonite from Smith... 

Fig. 8.3 The crystallization age of the granitic basement rocks of the Thiel Mountains was calculated from analytical data of Eastin (1970) listed in Appendix 8.5.3. The rock samples were provided by Arthur B. Ford. Solid circles = quartz monzonite open circles = biotite granite. One of the five samples ( 499) analyzed by Eastin (1970) was excluded because it lost radiogenic Sr. The value of the decay constant of Rb used to calculate the date from the slope of the isochron was A = 1.42 x 10 " year A The slope and intercept of the isochron were determined by unweighted linear regression...

Knob (499 A.B. Ford 7.12 61.64) which had lost radiogenic Sr. The four remaining samples of granitic basement rocks closely define an isochron in Fig. 8.3 that yields a date of 530 6 Ma and an Sr/ Sr ratio of 0.71153 0.00057 (la). This date and the initial Sr/ Sr ratio associate the granitic rocks of the Thiel Mountains with the Granite Harbor Intrusives and thereby indicate that this remote mountains range is an extension of the Transantarctic Mountains. 

The most recent and most successful whole-rock Rb-Sr age determinations of the basement rocks of the Thiel Mountains were published by Pankhurst et al. (1988) based on analyses of 45 specimens of igneous and sedimentary rocks. Appendix 8.5.4 includes these age determinations and initial Sr/ Sr ratios as well as the results published earlier by Schmidt and Ford (1969) and by Eastin (1970). However, the lead-alpha dates of zircon and whole-rock Rb-Sr dates of Schmidt and Ford (1969) were excluded from this tabulation. 

Fig. 8.4 The granitic plutons of the nunataks located in a belt in West Antarctica between the Thiel Mountains and the Ellsworth Mountains were intruded during the Jurassic Period. The dates of these plutons are listed in Appendix 8.6.5 (Adapted from Vennum and Storey 1987a)...

The Thiel Mountains are part of the far-eastern mountains that extend from the Ohio Range of the HorUck Mountains to the east coast of the WeddeU Sea. The rocks exposed along the north-facing escarpment of the Thiel Mountains consist of undeformed sedimentary rocks of low metamorphic grade (Mt. Walcott Formation) that are overlain by the cordierite-bearing hypersthene-quartz-monzonite porphyry of the Thiel Mountains Porphyry. This unit is a massive deposit of crystal-rich silicic tuff that contains cordierite and hypersthene as xenocrysts and in small granuhte xenoliths. The presence of cordierite in this volcanic or shallow intrusive... 

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