Petrogenesis of a migmatite, Penobscot County, Maine: ultrametamorphism or intrusion?

Abstract

The Clifton Migmatite crops out between the biotite grade Bucksport Formation (predominantly calcareous pelitic quartzite) on the west, the Lucerne Pluton (biotite granite) on the southeast, and the Parks Pond Pluton (mafic granite to quartz syenite) on the northeast. The Parks Pond crystallized at >1000°C.

The leucosomes are I-type (metaluminous, hornblendebearing inclusions, primary titanite, allanite, hornblende, and apatite included in biotite or hornblende), or S-type (tourmaline and red-brown biotite). They are conformable with melanosome (Bucksport) layering, and vary in outcrop from 7-88% areally, and in composition from tonalite to alkali granite.

Igneous textures in leucosomes argue against a solid-state origin, while biotite and plagioclase analyses indicate that leucosomes were not generated by in situ melting. Compositions of plagioclase (An-content 33-28 in melanosome, 38-15 in adjacent leucosome) and biotite (positive gradient of Ti02 in biotite of melanosome with approach to leucosome) demonstrate disequilibrium. It is argued that the Lucerne was not involved, based on chronology and mineralogy, and leucosomes are not injections of Parks Pond magma, because they lack clinopyroxene, orthopyroxene, and biotite from the Parks Pond. In the leucosomes, different maximum An-contents of plagioclase, four different orders of crystallization, and different compositions suggest that they are multiple melts. 4/5 of analyzed leucosomes are well described as Bucksport Formation minus restite.

Conclusions: 1) The Parks Pond partially melted the Bucksport at depth. 2) Melts intruded by lit-par-lit injection to present exposures. 3) Caution should be exercised in applying I- and S-type criteria to melts of calcareous sediments.