MICROCLINE MEGACRYSTS OF THE LEXINGTON BATHOLITH, CENTRAL MAINE, USA: IMPLICATIONS FOR MAGMATIC HISTORY

Microcline megacrysts are a prominent feature of the granite that makes up the central lobe of the Devonian Lexington batholith of central Maine, USA. Megacrysts are 3-20 cm long, tabular, and perthitic. Long axes are typically aligned, defining the flow lineation of the granitic magma. Like K-feldspar megacrysts described by other workers, those in the central lobe of the Lexington batholith host one to four concentric zones of inclusions of biotite, plagioclase, and quartz, and display oscillatory barium zoning. Biotite, plagioclase, and quartz, as well as microcline, also occur in the groundmass of the granite. Biotite diorite enclaves are abundant in the central lobe of the Lexington granite, and hornblende diorite enclaves are also present but are relatively rare. Nd isotopic analyses of the granite, microcline megacryst separates, and the enclaves, suggest that the granite records differing degrees of hybridization between a partial melt of the central metasedimentary belt crust and magma now represented by the hornblende enclaves. Later mixing between the granitic magma and magma represented by the biotite enclaves may have occurred at the emplacement level. A model suggested by Cox et al. (1996, Jnl. Geol. Soc. London) for the growth of megacrysts in the Shap granite of Cumbria, England accounts for many of the features of megacrysts of the Lexington batholith. This model involves dissolution of the margins of microcline megacrysts when hotter magma, preserved as enclaves, invaded the granitic magma chamber. In the Lexington granite, plagioclase inclusions in the megacrysts have calcic cores and sodic rinds, suggesting that liberation of Na during dissolution of microcline may have provided Na for rind growth on the inclusions before their incorporation into the megacrysts. Plagioclase glomerocrysts appear to be cumulates in which crystal margins dissolved and reprecipitated, perhaps as a result of introduction of hot magma into the chamber. Barium zoning in the megacrysts may be a result of dissolution and crystallization of biotite in response to invasions of hotter magma into the chamber. Variations in water concentration in the microcline megacrysts might be a remnant to dehydration events in the chamber resulting from influxes of hot, drier mafic magmas.