HR: 16:00h
AN: V54A-01    [Abstracts]
TI: Sills as Piecemeal Emplacements of a Layered Intrusion: The Ferrar Magmatic System, Dry Valleys, Antarctica
AU: * Marsh, B D
EM: bmarsh@jhu.edu
AF: Dept. Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218,
AB: It is especially difficult to discern for mafic layered systems the initial magmatic conditions giving rise to the end product. What was the style and duration of filling? What was the compositional variation and phenocryst content of the individual magmatic pulses? And, if phenocrysts are present, were they sorted prior to injection during ascension? The Ferrar dolerite magmatic system (~180 ma; > 5,000 km3) contains information fundamental to these questions. The central stack of (at least) four massive sills (~300-750 m thick) represents an episodic or serial loading of a magmatic system with compositionally and temporally contiguous aliquots of phenocryst-free tholeiitic and phenocryst-laden ultramafic magma. The same magmatic-volcanic episode gave rise to the overlying extensive Kirkpatrick basalt (e.g., Fleming et al., 1995; CMP,121;217). Whole rock compositions (e.g., CaO vs MgO) for the suite of sills shows a sweeping pattern very similar to that of Hawaii except that the context of each sample in its natural setting, as opposed to a stack of lavas, is well known. Rock composition can be related to the physical setting of the sample and the spatial setting of the contiguous rock mass. The internal structure of the ultramafic Basement Sill, the youngest and most mafic sill, reflects a long succession of filling events. It is stuffed by a vast tongue of large (1-20 mm) orthopyroxene and much smaller (< 1 mm) plagioclase; both were present in the inflowing magma, with plagioclase growing during transport. Bulk composition (7-20 MgO) and opx size and composition (En40-En90) symmetrically vary vertically through the sill, reflecting sorting during ascent. (The marginal quench zones argue against significant post emplacement re-equilibration.) The size and high concentration of opx has generally allowed little relative post- emplacement settling, but this opx matrix has acted as a sieve to the plagioclase, forming anorthosite stringers and layers pervasively and exclusively in the tongue over a vast region (~104 km3). The Opx Tongue thickness decreases radially throughout the region from Bull Pass. Everywhere within the Tongue, and only within the Tongue, are pervasive signs of crystal sorting and layering. Local ponding in a deeper recess has formed a small (~600 m thick), well-defined, layered body with many of the sorting features of large layered bodies. This is the Dais Intrusion. Rapid cooling over ~2,000 yrs. has preserved diagnostic textural relations that are annealed out in much larger layered bodies. The next highest sill, Peneplain Sill, also shows layering and the incipient development of an Opx Tongue near Bull Pass, but the other upper two sills each show no Tongue and are progressively poorer in MgO. The overall package shows strong differentiation radially and vertically. The lesson: were these same injections to form a single chambered magma, following the enumerated emplacement sequence, beginning with phenocryst-free tholeiites and concluding with heavy inputs of opx-rich ultramafics, the final result would most likely be a major layered body. (Continual stretching with intermittent injection would mimic an ocean ridge system.) Slow cooling would promote an extensive annealing to sharpen and accentuate the initial modal layering and enhance the initial cryptic layering. Initial conditions are exceedingly difficult to uncover, but their importance is fundamental to understanding the final product.
DE: 3618 Magma chamber processes (1036)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
DE: 3642 Intrusive structures and rocks
DE: 3643 Layered magma chambers
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting