Wright Intrusives

The Asgard Formation of Wright Valley described by McKelvey and Webb (1962) consists of interbed-ded marbles, homfels, and schists that grade locally into paragneiss and migmatite close to contacts with plutons of the Wright Intrusives. The type locality of this formation is the south wall of Wright Valley between Mt. Loke and Mt. Valkyrie. 

Fig. 3.14 Rb-Sr isochron defined by whole-rock samples of two Vanda Porphyry dikes (R1 and R2) and K-feldspar concentrates (FI and F2). The date of 460 7 Ma is the crystallization age of these dikes which were intruded into the metasedimentary rocks of the Asgard Formation, and into the plutonic igneous rocks of the Granite Harbor Intrusives in Wright Valley during the Middle Ordovician Epoch after the Ross Orogeny (Plotted from data by Jones and Faure 1967)...

These isotopic data combined with the chemical compositions published by Borg et al. (1986) strongly support the conclusion that the Granite Harbor Intrusives of northern Victoria Land are S-type granitoids. This conclusion applies also to the Wright and Vanda Intrusives of the ice-free valleys of southern Victoria Land and perhaps to some of the Cambro-Ordovician granitic plutons elsewhere in the Transantarctic Mountains. 

Fig. 13.1 The Wright Valley in southern Victoria Land contains three sills of Ferrar Dolerite The Basement sill intruded the Granite Harbor Intrusives of the local basement, the Peneplain siU lies between the granitic basement and the overlying sandstones of the Beacon Supergroup, and the uppermost sill was injected...

Fig. 13.46 The initial Sr/ Sr ratios of the Dufek intrusion overlap the range of initial Sr/ Sr ratios of the Ferrar Dolerite indicating that both bodies of rock contain excess radiogenic Sr compared to mantle-derived basalts in the oceans and on the continents (Faure 2001). Evidently, the Dufek magma could have originated from sources in the subcrustal mantle that contained subducted crustal rocks having higher Rb/Sr ratios than the rocks of the mantle. The initial Sr/ Sr ratios of the Ferrar Dolerite are the intercept values of Rb-Sr errorchrons for sills in Wright Valley (Fig. 13.5), Roadend Nunateik (Fig. 13.8), Mt. Achernar (Fig. 13.27), and Ported Rock (Fig. 13.37). The initial Sr/ Sr ratios of the Dufek intrusion are based on single samples analyzed by Faure et al. (1972) emd Ford et ed. (1986) whose Rb/Sr ratios could have been edtered after crystedUzation...

The association here with an area of volcanic activity is obvious, but even with all of these mentioned studies the causal relationship is still debated. Experience from divergent boundaries on land, in Iceland and East Africa, appears to indicate that both explanations apply. Dike intrusions associated with rifting do produce earthquake swarms (Wright et al. 2012), but swarms that do not seem to be associated with magmatic activity are also common. 

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