Igneous petrology and geochemistry of the southern Heritage Range, Ellsworth Mountains, West Antarctica

Igneous rocks exposed in the southern Heritage Range include two diabasic sills (100 m and 300 m thick), three gabbroic stocks (outcrop areas each <0.5 km2), a spessartite lamprophyre plug (200 m diameter), a 10-km2 sanidine quartz phytic dacite stock, and numerous aphanitic to porphyritic dikes and lava flows. Lava flows that are largely basaltic, but range in composition from basalt to alkali rhyolite, compose about 10 to 15 percent of the 1,000-m thickness of the Liberty Hills Formation of the Heritage Group. Eighty-four samples have been analyzed for major, minor, and trace elements by x-ray fluorescence. Field relations and comparison of chemical composition of intrusive and extrusive rocks suggest that all igneous rocks in the southern Heritage Range except the dacite stock were emplaced in Cambrian time; all have undergone pumpellyite-actinolite or greenschist facies metamorphism. A four-point Rb-Sr isochron yields a mid-Devonian (369 ± 18 Ma) emplacement age for the dacite stock. The extrusive igneous rock suite is dominated by basalts but also includes rhyolites, dacites, and sparse andesites and is thus bimodal. The relict primary mineralogy, distribution of alteration-resistant trace elements (Nb, Ti, Y, Zr), and incorporation within a thick sequence of carbonate and clastic sedimentary rocks all suggest that the mafic igneous rocks are continental basalts whose original chemical compositions were largely mildly tholeiitic or transitional between tholeiitic and alkali basalt; some calc-alkaline traits are also evident. Discrimination diagrams based on alteration-resistant trace elements and distribution of large ion lithophile elements (Ba, Rb, Sr) suggest that the felsic and intermediate rocks also possess tholeiitic, calc-alkaline, and alkaline traits. Age relations and comparison of chemical compositions indicate that the southern Heritage Range igneous rocks are not correlative with either the Jurassic Ferrar Super-group or the Jurassic Kirwan Volcanics of the Transantarctic Mountains. The most likely possible correlatives might be diabasic sills and dikes and subordinate plugs and sills of felsic porphyry intrusive into the upper Precambrian Patuxent Formation, which crops out in the Neptune Range of the Pensacola Mountains 500 km to the southeast. Tholeiitic suites with contemporaneous calc-alkaline and alkaline traits are unusual but have been reported from the Yellowknife volcanic belt in Canada’s Northwest Territories and from southeastern Maine. We conclude that the bimodal basaltic-rhyolitic suite of igneous rocks from the southern Heritage Range was emplaced in a continental area undergoing extensional tectonism and does not represent an island arc-continental margin subduction zone complex.

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