Tectonothermal effects of mid- and upper-crustal magmatism




Beam, Eric Christopher, 1966-

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This study considers the effects of the addition of heat and fluids to the earth's middle and upper crust by magmatism, as well as methods by which these effects may be distinguished in rock fabrics. A series of axisymmetric finite element transient heat conduction models were constructed to estimate crystallization rates, a proxy for fluid production. Crystallization rate varies from a high of ~80 km³/ky to a low of ~10 km³/ky. The shape of the crystallization rate histories is also highly variable, with thinner batholiths having high, sharp, early peaks. High fluid production rates will favor pooling of magmatic fluids in cupolas at the tops of stocks. In some cases this may trigger explosive eruptions, but in others could lead to economic mineralization. A computer program was written to simulate the formation of inclusion trails in porphyroblasts growing in deforming rocks. This program models synkinematic porphyroblast growth as a series of steps of growth and rotation. Resultant inclusion trails are complex. This complexity is a result of the variable relative rates of rotation of the inclusion and the cleavage. In some cases foliations are generated which could easily be interpreted as an included crenulation cleavage, other cases give an apparent sense of rotation opposite to the actual rotation. Theory describing the rotation of rigid inclusions is applied to biotite, garnet, and amphibole porphyroblasts in order to evaluate the sense of shear, magnitude of strain, and strain path (pure vs. simple shear) in deformed metamorphosed rocks adjacent to a tonalite sill in the Maclaren Glacier Metamorphic Belt (MGMB), south-central Alaska. Mean shear strains determined from biotite populations in thin sections range from γ = 2.4 to 3.3. Coupled temperature-displacement finite element models were constructed to simulate a cooling pluton in a zone of simple shear, using a realistic power law rheology. From these models it is clear that the thermal anomaly associated with a cooling pluton can concentrate deformation not just in the pluton, but into a laterally extensive zone running through the pluton. This may explain the deformation observed in the MGMB.



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