Admiralty Intrusives

The Admiralty plutons in the Robertson Bay Terrane are sharply discordant with chilled contacts against the country rocks but their contact-metamorphic aureoles are only a few tens of meters wide. They are also unfo-hated except close to contacts and are thought to have been intraded at shallow depth after the tectonic deformation of the country rocks. 

Chemical and isotopic data by Borg et al. (1986, 1987) indicate that the Admiralty Intmsives are primarily 1-type granitoids in contrast to the Granite Harbor hitrusives which include both 1-type and S-type rocks. Nevertheless, regional trends in the chemical compositions of the Admiralty Intmsives suggest that cmstal rocks of sialic composition contributed to their petrogenesis and that the involvement of cmstal rocks increased in a northeasterly direction across the Robertson Bay Terrane. 

The K-Ar dates of hornblende in Fig. 4.21b range similarly from older than 345 Ma to less than 370 Ma and are also concentrated in the Famennian Stage. The good agreement between K-Ar dates of mica and 

A small number of Rb-Sr dates of micas of Admiralty plutons at Cooper Spur (70°38 S, IbS OS E) and 

The whole-rock Rb-Sr method has had limited success for dating the Admiralty Intrusives because data points scatter on the isochron diagrams. For example, only four of the nine whole-rock samples of the Yule Bay Batholith (Pennell Coast) in Fig. 4.22a analyzed by Kreuzer et al. (1987) define a straight line that yields a date of 364 5 Ma and an initial Sr/ Sr ratio of 0.71495 0.00015. The other five rock samples scatter above and below the line because their initial Sr/ Sr ratios at 360 Ma range from 0.7125 to 0.7184. 

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Fig. 4.2 Northern Victoria Land consists of three terranes that are separated from each other by the Leap Year and Lanterman faults. Each terrane is composed of characteristic assemblages of metasedimentary and metavolcanic rocks of Neoproterozoic and Cambrian ages. These basement rocks were locally intruded by plutons of the Granite Harbor Intrusives and by the younger Admiralty Intrusives. Rocks of the Beacon Supergroup occur only in a few places in northern Victoria Land including in the Morozumi Range and HeUiwell HiUs, in the adjacent Freyberg Mountains, and in the Lichen HiUs. The Kukri Peneplain, which is so prominent in southern Victoria Land, is only preserved in these locations (Adapted from Stump 1995)...

Fig. 4. 21 K-Ar and Rb-Sr dates of hitotite, muscovite, and hornblende of the Admiralty Intrusives in the Robertson Bay Terrane. These dates sharply define the time of crystrillization between 355 and 365 Ma during the Famennian Stage of the Late Devonian Epoch (lUGS 2002). The good agreement of the K-Ar biotite and hornblende indicates that the rocks cooled rapidly presumably because they were intruded at shallow depth (Data from Kreuzer et al. 1987)...

The age of the felsic volcanics at Gallipoli Heights was determined by Faure and Gair (1970) by means of a two-point whole-rock Rb-Sr age determination based on samples collected by S.J. Carryer. The results yielded a Middle-to-Late Devonian date of 370 40 Ma (recalculated to A, = 1.42 x 10 year ) and an initial Sr/ Sr ratio of 0.70548. This value of the initial Sr/ Sr ratio is typical for continental volcanic rocks (e.g., rhyolites of Central America and Mexico Faure 2001), but it is lower than the initial ratio (at360Ma) of most of the 27 specimens of Admiralty Intrusives analyzed by Kreuzer et al. (1987). 

Andesites of the Black Prince Volcanics have whole-rock K-Ar dates of 323 6 and 375 6 Ma (Late Devonian to Early Carboniferous) reported by Adams et al. (1986). These dates are in good agreement with age determinations of the Admiralty Intrusives in Fig. 4.21. The lava flows at Mount Black Prince are interbedded with sedimentary rocks that contain plant fossils of Middle Devonian to Early Carboniferous age (Findlay and Jordan 1984). 

T T I Admiralty Intrusives metarmorphism LP, MP, HP = low/medium/high pressure, g = greenschist facies... 

S.4 Admiralty Intrusives, Yule Batholith, Yule Bay, Pennell Coast, K-Ar ... 

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