HR: 16:30h
AN: V14C-03    [Abstracts]
TI: Discovery of a Funnel-like Deep Feeder Zone for the Ferrar Dolerites, McMurdo Dry Valleys, Antarctica
AU: * Marsh, B D
EM: bmarsh@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Hersum, T G
EM: hersum@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Simon, A C
EM: adam.simon@ccmail.nevada.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Charrier, A D
EM: charrier@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Souter, B J
EM: bsouter@mindspring.com
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AB: The nature of the deeper zones that feed major sill complexes has long been a mystery. Often early dike swarms are found to be clearly connected chemically to the sill complex, but actual physical connections are unclear or not found at all; the dikes are almost always cut by the sills and the dikes most always seem much too small to have generated any single major sill. Feeder zones have been speculated to be deep-seated regional plumes or local stocks and fissures that fill individual sills and interconnect a stack of sills to form a massive complex like the Ferrar Dolerites of the McMurdo Dry Valleys (McDV). Although local flow indicators and AMS techniques offer good promise of resolving this fundamental issue, the process is tedious and vulnerable to local complexity. The Basement Sill of the Ferrar Dolerites offers an exceptional opportunity to trace the general flow field of emplacement due to the presence of a vast and massive tongue of entrained phenocrysts of orthopyroxene (opx). The physical characters of these tongues, their geographic distribution and mass, and the detailed character of the attendant phenocrysts, their spatial variations in size, shape, and composition, provide a detailed dynamic record of the process of ascent and sill emplacement. A tongue of mainly large orthopyroxene crystals (large cpx is also present with much smaller plag.) of an areal extent of perhaps 10,000 km occupies much of the 330 m thick Basement Sill, the basal sill of the ~4 km thick Ferrar Dolerite sill sequence. The spatial variation in Tongue thickness (250 to 20 m) is a direct indication of the feeding location, which is near Bull Pass in Wright Valley where it fills nearly the entire sill. The Tongue systematically thins 40 km north to Wheeler Valley, 60 km south to Cathedral Rocks, and dramatically 10 km east down Wright Valley. Yet, even though the general location of emplacement can be inferred, it has proven perplexing to identify the exact form and nature of the feeder zone. Recent detailed mapping and synthesis of earlier mapping now shows that the Basement Sill is formed from a massive funnel-like feeding zone ~10 km in diameter and centered in the mountains containing Mt. Orestes to the east of Bull Pass. Bull Pass itself forms the western rim of this feature. The funnel itself is characterized by a steeply inward dipping circular pattern of the Basement Sill (and its attendant layering) whose inner zone often shows multiple chilled margins, sometimes separated by large slices of granitic wall rock. The funnel rim also shows local `climbing' contacts where upward advancements of the sill have been aborted and further emplacement has extended horizontally to form extensive regional lobes of Basement Sill. One lobe has topped out beneath the overlying Peneplain Sill and then continued horizontally to the south and another lobe has descended westward to pond and form the Dais Layered Intrusion. There is also some indication that this complex may have formed a much larger, perhaps fissure-like feeding zone extending SW some 40 km to the area of Solitary Rocks.
DE: 1036 Magma chamber processes (3618)
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
DE: 9310 Antarctica (4207)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005