Physical, chemical, and thermal interactions in the Pleasant Bay Layered Gabbro-Diorite Intrusion, Maine
Abstract
The approximately 3 km thick Pleasant Bay Layered Intrusion (PBLI) is interpreted to have formed (420 Ma) by repeated intrusions of gabbroic magma into a partly solidified dioritic magma chamber (Wiebe, 1993; Waight et. al., 2001) during the earliest stage of the Acadian orogeny (Tucker et. al., 2001). Typical field relationships in the PBLI include gradational gabbro-diorite layers of variable thicknesses with pipes of silicic composition intruding along the chilled lower contacts of gabbroic layers, chilled gabbroic blobs encased within diorite, and composite dikes consisting of intermingled gabbro blobs of rounded and/or angular geometries within a dioritic or granitic matrix. Detailed studies of similar relationships in the nearby Isle au Haut Igneous Complex (IHIC) have indicated that where diorite underlies gabbro, residual silicic melt was extracted from the underlying partly solidified diorite by compaction and migrated upwards to form a thin layer of buoyant melt that underwent a Rayleigh-Taylor (R-T) type instability producing the silicic pipes which were subsequently arrested by solidification of the overlying gabbro (Chapman & Rhodes, 1992; Patwardhan & Marsh, 2011). Whereas in the IHIC pipes typically are 11-12 cm in diameter and spaced ~30 cm apart, instability features in the PBLI occur at varying scales with pipe diameters commonly ranging from 4 to 100 cm and pipe spacing varying between 20 to 150 cm. Strong compositional differences between the gabbro (~48 wt.% SiO2), diorite (~57 wt.% SiO2), and pipes (~66 wt.% SiO2) correspond to significant differences in estimated density, liquid viscosity, and temperature (near liquidus gabbro: 2.68 g/cm3, 102.3 poise, 1210 oC, partly crystallized diorite: 2.61 g/cm3, 990 oC, and silicic melt derived from partly crystallized diorite: 2.36 g/cm3, 105 poise). The thickness of the buoyant silicic layer and the ascent rate of the pipes emanating from this layer are calculated using estimated viscosity ratios and density differences together with field measurements of pipe radii and spacing. Rapid solidification of the gabbro along its lower contact accompanied by an increase in viscosity slowed down the ascent of the pipes and ultimately arrested them in their present state. Petrographic analyses of samples taken across the gabbro-diorite contacts and pipe-gabbro contacts indicate significant differences in the occurrence and abundance of hydrous phases, notably biotite and hornblende, between the chilled gabbro contacts and bulk of the gabbro, the underlying diorite, and the pipes. SEM-EDS analyses also reveal changes in mineral compositions within a few mm of the pipe-gabbro and gabbro-diorite contacts (e.g. drop in plagioclase An content in chilled gabbro surrounding pipes). These observations suggest transfer of water from the diorite and/or pipes into anhydrous gabbro as well as physical transfer of crystals across these boundaries. Quantifying the mineral and chemical differences along the pipe-gabbro and gabbro-diorite contacts in the context of the timescales of R-T instability and solidification reveal the in situ rates of physical and chemical exchange between these magmas. (Special thanks to Dr. Robert Wiebe for assistance with fieldwork, and to Dr. Chris Daniel for assistance with SEM analyses.)
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V11C2769P
- Keywords:
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- 3613 MINERALOGY AND PETROLOGY / Subduction zone processes;
- 3640 MINERALOGY AND PETROLOGY / Igneous petrology;
- 3642 MINERALOGY AND PETROLOGY / Intrusive structures and rocks;
- 3643 MINERALOGY AND PETROLOGY / Layered magma chambers