Viscous fingering during replenishment of felsic magma chambers by continuous inputs of mafic magmas: Field evidence and fluid-mechanics experiments

Abstract The U2 group of plutonic rocks constituting the main exposed part of the Corsica-Sardinia batholith (CSB) was emplaced from 308 to 275 Ma (the early Visean U1 group of Mg-K intrusions is not considered here). Field evidence earlier established volcanic-plutonic relationships in the U2 group of calc-alkaline intrusions of the CSB, though detailed chronological data were still lacking. Large outcrops of U2 volcanic formations are restricted to the less eroded zone north-west of the Porto-Ponte Leccia line in Corsica, but volcanic and volcano-sedimentary formations were widely eroded elsewhere since Permian times. They probably covered most of the batholith before the Miocene, as testified by the volcanic nature of the pebbles that form much of the Early Miocene conglomerates of eastern Corsica. U-Pb zircon dating (SHRIMP) was used for deciphering the chronology and duration of different volcanic pulses and for better estimating the time overlap between plutonic and volcanic rock emplacement in the CSB. The obtained ages fit well with field data, showing that most of the U2 and U3 volcanic formations were emplaced within a brief time span of roughly 15 m.y., from 293 to 278 Ma, coeval with most U2 monzogranodiorites and leucomonzogranites (295–280 Ma), alkaline U3 complexes (about 288 Ma), and mafic-ultramafic tholeiitic complexes (295–275 Ma). The same chronological link between deep-seated magma chambers and eruptions was identified in the Pyrenees. These results correlate with U-Pb zircon dating of HT-LP granulites from the Variscan deep crust exhumed along the “European” margin of the thinned Tethys margin in Corsica and Calabria. Here, the peak of the low-pressure/high-temperature metamorphism was dated at about 285–280 Ma. Our results throw light on the condition of magma production during the orogenic collapse in the southern Variscan realm. While juvenile tholeiitic basaltic magma was produced by the melting of spinel mantle lithosphere, all fertile protoliths melted in a brief period during the HT-LP peak in lower continental crust, leading to massive emplacement of large felsic U2 calc-alkaline and minor U3 A-type volcano-plutonic formations over about 15 Ma.

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Mafic-silicic layered intrusions: the role of basaltic injections on magmatic processes and the evolution of silicic magma chambers

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300006647 ◽

1996 ◽

Vol 87 (1-2) ◽

pp. 233-242 ◽

Cited By ~ 79

Author(s):

R. A. Wiebe

Keyword(s):

Basaltic Magma ◽

Mafic Magma ◽

Silicic Magma ◽

Granitic Magma ◽

Magma Chambers ◽

Mechanical Mixing ◽

Diffusive Convection ◽

Type Granite ◽

Mafic Magmas ◽

Silicic Magmas

ABSTRACT:Plutonic complexes with interlayered mafic and silicic rocks commonly contain layers (1–50 m thick) with a chilled gabbroic base that grades upwards to dioritic or silicic cumulates. Each chilled base records the infusion of new basaltic magma into the chamber. Some layers preserve a record of double-diffusive convection with hotter, denser mafic magma beneath silicic magma. Processes of hybridisation include mechanical mixing of crystals and selective exchange of H2O, alkalis and isotopes. These effects are convected away from the boundary into the interiors of both magmas. Fractional crystallisation aad replenishment of the mafic magma can also generate intermediate magma layers highly enriched in incompatible elements.Basaltic infusions into silicic magma chambers can significantly affect the thermal and chemical character of resident granitic magmas in shallow level chambers. In one Maine pluton, they converted resident I-type granitic magma into A-type granite and, in another, they produced a low-K (trondhjemitic) magma layer beneath normal granitic magma. If comparable interactions occur at deeper crustal levels, selective thermal, chemical and isotopic exchange should probably be even more effective. Because the mafic magmas crystallise first and relatively rapidly, silicic magmas that rise away from deep composite chambers may show little direct evidence (e.g. enclaves) of their prior involvement with mafic magma.

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Genesis and evolution of mafic microgranular enclaves through various types of interaction between coexisting felsic and mafic magmas

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300007835 ◽

1992 ◽

Vol 83 (1-2) ◽

pp. 145-153 ◽

Cited By ~ 182

Author(s):

Bernard Barbarin ◽

Jean Didier

Keyword(s):

Chemical Exchange ◽

Magma Chambers ◽

Mixing Processes ◽

Microgranular Enclaves ◽

Mafic Microgranular Enclaves ◽

Mass Fractions ◽

Long Time ◽

Mafic Magmas ◽

The Many ◽

Structural Levels

ABSTRACTThermal, mechanical and chemical exchange occurs between felsic and mafic magmas in dynamic magma systems. The occurrence and efficiency of such exchanges are constrained mainly by the intensive parameters, the compositions, and the mass fractions of the coexisting magmas. As these interacting parameters do not change simultaneously during the evolution of the granite systems, the exchanges appear sequentially, and affect magmatic systems at different structural levels, i.e. in magma chambers at depth, in the conduits, or after emplacement. Hybridisation processes are especially effective in the plutonic environment because contrasting magmas can interact over a long time-span before cooling. The different exchanges are complementary and tend to reduce the contrasts between the coexisting magmas. They can be extensive or limited in space and time; they are either combined into mixing processes which produce homogeneous rocks, or only into mingling processes which produce rocks with heterogeneities of various size-scales. Mafic microgranular enclaves represent the most common heterogeneities present in the granite plutons. The composite enclaves and the many types of mafic microgranular enclaves commonly associated in a single pluton, or in polygenic enclave swarms, are produced by the sequential occurrence of various exchanges between coexisting magmas with constantly changing intensive parameters and mass fractions. The complex succession and repetition of exchanges, and the resulting partial chemical and complete isotopic equilibration, mask the original identities of the initial components.

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