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Hidden record of mafic enclaves tracks pluton assembly over time

Thermomechanical modelling of enclave deformation demonstrates that plutons grow in response to repeated injections of small pulses of magma

Plutons are large igneous bodies formed from the slow cooling of molten rock in the subsurface. Their construction reflects how magma is produced and transferred from depth, though whether this happens through sudden episodes of magma injection or small pulses of growth is a matter of active research.

Granitic plutons commonly contain mafic enclaves (fragments of less chemically evolved magma suspended in a more evolved host), produced from the intrusion and disaggregation of hotter, more mafic melts into cooler, more felsic magma.  Distortion of these roughly spherical enclaves reflects the strain experienced by different areas of the pluton. The study, from Caricchi and co-authors from the University of Bristol, harnesses this record of deformation to probe the rheological, and hence thermal, evolution of a pluton during its accretion.

Caricchi et al. found that the repeated injection of magmatic pulses into a pluton resulted in expansion of the body, but that enclaves were only deformed in a two narrow temperature windows in which both the host and enclave had a similar viscosity. Knowing this, the team developed a thermomechanical model to simulate how the strain trajectories of enclaves vary as a function of time and distance from the magma injection point. Application of this model to the Lago Della Vacca Complex (LDVC), a 4.5 by 4.7 km section of the Adamello Pluton in Italy, shows that the magma body underwent radial expansion in response to multi-stage growth over 50,000- 150,000 years. Their findings are in agreement with recent geochronological estimates from zircon dating of the structure, and supply evidence for the ‘piecemeal’ nature of pluton assembly.

Caricchi, L, Annen, CJ, Rust, AC & Blundy, JD (2012) ‘Insights into the mechanisms and timescales of pluton assembly from deformation patterns of mafic enclaves’ Journal of Geophysical Research, vol 117, no. B11206. http://dx.doi.org/10.1029/2012JB009325

SCIENTIFIC ABSTRACT

We propose a novel method to constrain both timescales and assembly styles of intrusive bodies using the strain recorded by mafic enclaves, a common component of granitic rocks. Petrology, thermal modeling, and magma rheology are combined to investigate the evolution of strain recorded by enclaves during the piecemeal assembly of a pluton growing at various rates of magma input. The different compositions (and hence phase relations) of host magma and enclaves limits homogeneous deformation of these two materials to restricted temperature ranges, which we term “windows of mutual deformability.” Outside these windows only the less viscous host granite records any appreciable deformation and enclaves are mainly transported as rigid objects. The temporal and spatial development of the windows of mutual deformability reflects the emplacement rate of magmas into the pluton. Consequently, the radial distribution of strain recorded by enclaves can provide a picture of the thermal and rheological evolution of a magmatic body during its construction. Our approach is applied to deformation patterns of mafic enclaves in the Lago della Vacca Complex (LVC) of the Adamello Massif (Italy), to estimate the timescales of pluton emplacement. Our calculations suggest total emplacement timescales of the order 50 to 150 ky, in excellent agreement with recent high precision radiometric dating of zircons from the LVC.

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