Portal Rock, Queen Alexandra Range

Portal Rock (83°50 S, 165°36 E) is a small mountain at the confluence of the Tillite Glacier with the Lennox-King Glacier in Fig. 13.32. The location of the 

13 Ferrar Group Dolerite Sills and the Dufek Intrusion 

The profiles of SiO, MgO, and Na O in Fig. 13.36 reveal that the concentrations of these elements vary only within narrow limits, except for the lower chilled contact described above. The concentration of SiOj decreases up-section from about 54% to 52%, but the trend reverses at about 100 m above the base of the sill and the concentrations of SiOj increase in the upper 30 m toward 54%. The concentration of MgO ranges from about 6% to 7% and includes only local variations. This profile indicates that the sill at Portal Rock does not contain a hypersthene zone as do the dolerite sills in the ice-free valleys of southern Victoria Land (Gunn 1962 1966). The concentrations of NajO remain virtually constant throughout the body of this sill contrary to the expectation that the crystallization of early-formed anorthite enriched the residual magma in Na O and caused the plagioclase to become enriched in the albite conponent. 

The Rb-Sr and Sm-Nd systematics of six samples analyzed by Hergt et al. (1989b) scatter widely in Fig. 13.37 and do not converge to their respective Rb-Sr and Sm-Nd isochrons. Nevertheless, three samples do form a Rb-Sr errorchron in part A that yields a date of 169 16 Ma and an initial Sr/ Sr ratio of 0.70988 0.00010 (ct). The other samples in this set scatter widely, probably because their Rb/Sr ratios were altered and not because the magma from which they formed had a range of different isotope compositions of strontium. Although this conjecture needs to be tested, we prefer to base conclusions about petrogenesis only on the intercepts of errorchron or isochron lines. 

The Sm-Nd date in part B of Fig. 13.37 approximate an errorchron based on four of the same six samples analyzed by Hergt et al. (1989b) for dating by the Rb-Sr method. The slope of the Sm-Nd errochron corresponds to a date of 185 7 Ma and the intercept is 0.510602 0.0000058 (ct). The Nd/ Nd ratios measured by Hergt et al. (1989b) were multiplied by 1.00154 in order to make them compatible with Nd/ Nd = 0.7219 (the value of the decay constant of Sm is 6.54 x 10 year ). The Rb-Sr and Sm-Nd dates have overlapping 

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Chemical Analyses of Rock Samples from a Measured Section of the Sill of Ferrar Dolerite on Portal Rock, Queen Alexandra Range (J.M. Hergt personal communication to G. Faure, April 27,1987)... 

Fig. 13.32 Portal Rock (aka Portal Peak) at the mouth of the TilUte Glacier in the Queen Alexandra Range exposes the Permian Mckellar Formation (Pm) which overlies the Pagoda TilUte (Pp). The Mackellar Formation at this location was intruded by a sill of the Jurassic Ferrar Dolerite (Jf). The geology of the Queen Alexandra Range was described by Barrett et al. (1986). The elevation of the summit of Portal Rock is 1,990 m a.s.l. which causes it to stand 390 m above the surface of the Lennox-King Glacier. Excerpt of the reconnaissance geologic map by Lindsay et al. (1973)...

Fig. 13.35 The dolerite sill on Portal Rock in the Queen Alexandra Range is a subalkalkic low-Ti quartz dolerite. The rocks of this sill make a small footprint on this diagram because their chemical compositions are remarkably homogeneous. The only deviation is the chilled margin at the base of the sill (sample P-1-3C) which has a low concentration of Si02 (50.8%) and high concentration of K O (5.44%). This specimen is not representative of the chemical composition of the dolerite magma. Data in Appendix 13.6.7 by J.M. Hergt (personal communication 1987)...

Fig. 13.36 The concentration profiles of the dolerite sill on Portal Rock in the Queen Alexandra Range reflect the homogeneity of its chemical composition. The principal exception is the lower chilled margin which has low concentrations of SiO, MgO, Nap, and CaO (0.70%) while AIP3 (21.45%) and Hp+...

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