Basaltic injections into floored silicic magma chambers
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Abstract:
Recent studies have provided compelling evidence that many large accumulations of silicic volcanic rocks erupted from long‐lasting, floored chambers of silicic magma that were repeatedly injected by basaltic magma. These basaltic infusions are commonly thought to play an important role in the evolution of the silicic systems: they have been proposed as a cause for explosive silicic eruptions [ Sparks and Sigurdsson, 1977], compositional variation in ash‐flow sheets [ Smith, 1979], mafic magmatic inclusions in silicic volcanic rocks [ Bacon, 1986], and mixing of mafic and silicic magmas [ Anderson, 1976; Eichelberger, 1978]. If, as seems likely, floored silicic magma chambers have frequently been invaded by basalt, then plutonic bodies should provide records of these events. Although plutonic evidence for mixing and commingling of mafic and silicic magmas has been recognized for many years, it has been established only recently that some intrusive complex originated through multiple basaltic injections into floored chambers of silicic magma [e.g., Wiebe, 1974; Michael, 1991; Chapman and Rhodes, 1992].Keywords:
Silicic
Magma chamber
In order to understand the governing factors of petrological features of erupted magmas of island-arc or continental volcanoes, thermal fluctuations of subvolcanic silicic magma chambers caused by intermittent basalt replenishments are investigated from the theoretical viewpoint. When basaltic magmas are repeatedly emplaced into continental crust, a long-lived silicic magma chamber may form. A silicic magma chamber within surrounding crust is composed of crystal-melt mixtures with variable melt fractions. We define the region which behaves as a liquid in a mechanical sense (‘liquid part’) and the region which is in the critical state between liquid and solid states (‘mush’) collectively as a magma chamber in this study. Such a magma chamber is surrounded by partially molten solid with lower melt fractions. Erupted magmas are considered to be derived from the liquid part. The size of a silicic magma chamber is determined by the long-term balance between heat supply from basalt and heat loss by conduction, while the temperature and the volume of the liquid part fluctuate in response to individual basalt inputs. Thermal evolution of a silicic magma chamber after each basalt input is divided into two stages. In the first stage, the liquid part rapidly propagates within the magma chamber by melting the silicic mush, and its temperature rises above and decays back to the effective fusion temperature of the crystal-melt mixture on a short timescale. In some cases the liquid part no longer exists. In the second stage, the liquid part ceases to propagate and cools slowly by heat conduction on a much longer timescale. The petrological features of the liquid part, such as the amount of unmelted preexisting crystals, depend on the intensity of individual pulses of the basalt heat source and the degree of fractionation during the first stage, as well as the bulk composition of the silicic magma.
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Prominent zones of large globular, fine‐grained, felsic enclaves occur at the highest exposed levels of the Gouldsboro granite along the coast of Maine. The physical characteristics of these enclaves and their chemical relations to the host granite indicate that they represent globules of resident magma that were trapped, probably during eruption, in the crystal mush at the top of the magma chamber. These zones can appropriately be termed “eruption trails.” Enclave compositions provide a record of magma compositions within the chamber at the time of eruption. They fall into two groups (high‐K and low‐K with similar wt % SiO2), suggesting that magma in the chamber consisted of discrete compositional batches. The distribution of these two types of enclaves within the swarms is consistent with the low‐K source magma residing as a layer beneath the high‐K source magma. Chilled and vesiculated inclusions of basalt occur only in the low‐K enclaves. Their presence there suggests that basaltic magma was injected into the base of the chamber just prior to and may have triggered the eruption. The low‐K silicic magma probably developed by selective exchange of alkalies between normal high‐K silicic magma and periodic injections of basaltic magma. Evidence for these injections is seen in the chilled gabbroic sheets at the base of the Gouldsboro granite. Similar large felsic enclaves are known in other shallow‐level granites; many of these may represent centrally derived magma trapped during transport out of the chamber in an eruption rather than, as they have commonly been interpreted, portions of chilled margins engulfed and remobilized during granite emplacement.
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