Formation of Rapakivi Feldspars in the Deer Isle Granite Complex, Coastal Maine: <em>In Situ</em> Lead Isotope and Trace Element Analysis

• Main Author: OBrien, William Desmond
• Format: Others
• Published: BYU ScholarsArchive 2017
• Subjects: rapakivi; magma mixing; LA-ICP-MS; Deer Isle Granite Complex; Geology
• Online Access: https://scholarsarchive.byu.edu/etd/6558; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=7558&amp;context=etd

Rapakivi and alkali feldspar phenocrysts from the Deer Isle Granite Complex were investigated using in situ trace element and Pb isotope geochemistry to see if magma mixing or isothermal decompression was responsible for their formation. Pb isotope and trace element profiles, along with CL imagery of quartz phenocrysts, indicate compositional changes in the magma chamber occurred during rapakivi and alkali feldspar growth. Repeated episodes of magma mixing/replenishment by relatively isotopically primitive and LREE enriched magmas (along with hybridized variations with the host magma) created localized disequilibration. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase. Entrainment back into this zone of mixing caused multiple mantles of plagioclase to form on some phenocrysts. For grains more distal to these zones of mixing, complete disequilibration of the grain did not occur and continued growth resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the chambers floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved fluids out from interstitial pores. Small amounts of evolved liquid, trapped interstitial to the cumulate, formed LREE depleted albitic rims on some grains. The crystallization, transportation and juxtaposition of rapakivi, mantled alkali feldspar and plagioclase phenocrysts suggests that a relatively large and active magma chamber, periodically recharged by batches of melt, must have existed.