Mafic enclaves in the Eastern White Mountain Batholith, New Hampshire
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Abstract
Mafic enclaves (inclusions) are distributed throughout a syenitic ring dike of the Jurassic age White Mountain Batholith (New Hampshire). The enclaves represent droplets of magmas which were injected, and quenched as pillows in the host Albany Syenite. The compositions of the enclaves range from trachyte to trachyandesite. The enclaves formed from a mixed magma suite, as shown by field, petrographic, and compositional data. In the field, some enclaves show light and dark layering; both structures suggest mixing of mafic and silicic parents. Petrographically, the enclaves contain abundant feldspar phenocrysts with resorbed and rimmed layers. Chemically, abundances of many elements in the enclaves fall on reasonable mixing lines. The elements which do not follow mixing lines are likely to have been affected by post-magmatic processes. Crystal fractionation is not consistent with the compositional trends of the enclaves. The composition of the mixing end members is not known for certain, although they may be similar to the most mafic and silicic enclaves. The most mafic enclave has the composition of a trachyandesite (Si02 = 56%, MgO = 2.3%). However, the actual mixing magma may have been more mafic than the most mafic enclave. The most silicic enclave is a trachyte (Si02 = 62%, MgO = 0.5%). Again, it is not known if this was the actual composition of the silicic end member magma. The enclaves represent droplets of the mixed magma suite, injected, and mingled, into a partially molten Albany Syenite magma. The rounded and ellipsoidal enclaves have fine-grained rims and typical quench textures like radiate sprays of amphiboles and acicular apatite grains. The most mafic enclaves typically have very sharp rims, while the more silicic enclaves have diffuse margins. The differences are in accord with predictions of the thermophysical behavior of interacting magmas.
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Thesis (M.A.)--Boston University
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