Date of Award

8-2003

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

Advisor

Daniel R. Lux

Second Committee Member

David Gibson

Third Committee Member

Edward Grew

Abstract

Magma mixing is a common event in the petrogenesis of many igneous rocks. Many systems, such as Vinalhaven pluton, show evidence for mixing in field relationships. The Deer Isle Pluton, coastal Maine, however, shows cryptic evidence for magma mixing in the forms of enclaves, mineral and rock textures, and chemical trends. Enclaves found in the pluton are generally more famic than the ohst granite and contain mineral textures that suggest a mixing origin, such as zoned plagioclase and titanite, acicular apatite, and amphibole-rimmed quartz. Enclaves decrease in abundance and size from Oak Point (OP) where they are up to a meter in diameter, to Stonington (ST) where they are less than 20 cm and nearly absent. In OP granite, the enclaves are flattened and aligned with the foliation, while they are rounded and found in clusters or groups in ST granite. Plagioclase mantled alkali feldspar also decrease in abundance over the same area. The rock texture changes from the feldspar and quartz supported frameowrk of OP, to slightly porphyritic granite ST. In the Settlement Quarry area, the granite has a heterogeneous texture made up of domains of feldspar segregations, fine-grained granite, and schlieren. Linear trends suggestive of mixing are also observed in the major and trace element composition of the granite and enclaves. The isotopic signature of these units plot in the mantle array and also suggests a mixing event in the evolution of the magma. Using the data that was collected, a simple model for the magma evolution, from melt to solidification, can be developed. Initially, crustal thickening due to the Acadian orogeny promotes melting of the mantle, adding heat to the lower crust. The lower crust, which is of Avalonian affinity, melts and collects in a lower crustal magma chamber. This magma is then invaded by a mafic mantle-derived melt. This input forces the future Deer Isle magma to ascend to the lower crust. During ascent, shearing along the margins of the magma sorts previously formed crystals to the center and liquid to the margins of the magma. This sorting effectively develops the future facies of the granite. The crystal rich center becomes the OP granite and the liquid portion becomes the ST granite. When the magma reaches its future location in the upper crust, due to increased viscosity, the crystal rich portion is first to stop and the liquid is allowed to continue moving and fill in above. The magma starts to cool and crystallizes along a solidification front forming the Crotch Island facies. Compaction of the OP magma develops a foliation and flattens the previously rounded enclaves. The center of the pluton, the Settlement Quarry area, remains liquid the longest and cools the slowest. This preserves the flow related enclave clusters and schlieren layers that are diagnostic to this facies. Finally, late stage residual melt is filter pressed out of the crystal mush and collects to form the felsic aplite dikes.

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