Abstract
The Granite Harbour Intrusives of the Taylor Valley and Ferrar Glacier regions comprise two distinct suites. The names Dry Valleys 1 suite and Dry Valleys 2 suite are informally proposed.
The elongate, northwest trending Bonney Pluton of the Dry Valleys 1 suite is the largest intrusion in the study area, and intrudes upper amphibolite to granulite facies metasediments of the Koettlitz Group and it's intercalated orthogneisses. This pluton exhibits a wide compositional variation from monzodiorite through to granite, and is flow-foliated and variably K-feldspar megacrystic. Broad, irregular zones of interdigitating granitoid within this pluton formed by mingling of magmas of contrasting compositions and viscosities. The intense foliation at the margin of the Bonney Pluton adjacent to the contact with the Koettlitz Group most likely resulted from ballooning of the pluton during emplacement. The Dry Valleys 2 suite intrudes the Dry Valleys 1 suite, and comprises dike swarms and small intrusions. The intrusions are non-foliated, and contacts with the country rock are undeformed, planar, and chilled, indicating lower emplacement temperatures than the granitoids of the Dry Valleys 1 suite.
The two suites have distinctive chemistries. The Dry Valleys 1 suite is calc-alkaline, whereas the Dry Valleys 2 suite is alkali-calcic. The Dry Valleys 1 suite has a higher TiO2, MgO, CaO, FeO, Cr, Ni, V, and Sc concentration, and a lower K20, Rb, and Zr concentration than the Dry Valleys 2 suite. The higher mafic mineral content and lower K-feldspar content of the Dry Valleys 1 suite accounts for the different chemistry compared to the Dry valleys 2 suite. Both suites are peralkaline I-type granitoids, and are not products of partial melting of Koettlitz Group metasediments, as previously suggested. The major, trace, and Rare Earth Element chemistries of the Dry Valleys 1 and Dry Valleys 2 suites suggest that each suite evolved by fractional crystallization of different basic magmas.
The Dry Valleys 1 suite has many characteristics of Cordilleran I-type granitoids, and is therefore considered to be derived by partial melting of the upper mantle and/or deep continental crust in a continental arc. The granitoids of the Dry Valleys 2 suite however, resemble Caledonian I-type granitoids, which post-date major tectonic events and are not subduction-related.
The model presented invokes subduction of oceanic crust beneath the Antarctic craton resulting in the formation of the Dry Valleys 1 suite. A change in tectonic environment resulted in cessation of subduction. The subducted slab continued to sink, resulting in upwelling of hot aesthenospheric material towards the base of the crust, causing high heat flow in this region. Subsequent extension and thinning of the crust resulted in emplacement of the mantle-derived dikes and lower crustal-derived granitoids of the Dry Valleys 2 suite.
Orbicules crop out in zones within the Bonney Pluton near Koettlitz Group contacts. They exhibit a variety of forms, and commonly comprise euhedral hornblende laths surrounding cores of broken orbicules, enclaves, and metasedimentary xenoliths. The orbicules crystallized in melt, and are not products of metasomatism between metasedimentary xenoliths and host magma, as previously suggested. Plagioclase shells appear to have resulted from later growth of microscopic plagioclase inclusion rings arranged concentrically within the hornblende shells.