27 Igneous and Metamorphic Geology

Friday, 28 March 2008
1:00 PM - 4:20 PM

LATE STAGE MAGMATIC FLUIDS AND TEXTURAL AND CHEMICAL EVIDENCE OF EARLY MAGMATIC PROCESSES IN A BIMODAL PLUTONIC COMPLEX, COASTAL MAINE MAGMATIC PROVINCE

Paper number: 27-4
Program: Technical Sessions
Time: 2:00 PM – 2:20 PM

Authors

Abstract

Melt inclusions (MI) in kfs, qtz, hbl, plag, and ol within the lowermost horizons of the bimodal Gouldsboro pluton reveal a complex record of magmatic fluid generation during multiple felsic and mafic injections. Recent isotopic work (Waight et al., 2007) suggests that extensive interaction of contemporaneous crustally derived and mantle magmas and a protracted crystallization history drastically modified the Sr and Nd systems in this pluton. Disequilibrium textures within qtz, kfs and plag, grain boundary modification and high concentrations of MI, suggest that magmatic fluids mobilized both major and trace elements throughout the course of crystalization. MI in qtz from the middle levels of the system preserve non-devitrified MI that show elevated total OH- concentration. Regions of host crystals adjacent to devitirified MI in kfs and hbl show higher concentrations of structural water than crystalline areas free of MI, as well as recrystallization to more hydrous phases along grain boundaries. Gabbros at the base of the chamber preserve MI in ol cores that have not been destroyed by alteration to opx, cpx or hbl.

Both magmatic and secondary epidote (ep) are present within this system. Secondary ep, from saussuritization of plag, commonly fills miarolitic cavities and interstitial spaces nearing the roof; however, magmatic ep is present in granodiorites and ol+opx+cpx gabbros at the base of the system. The presence of ep, typically armored by kfs ± secondary bt aggregates, suggest that crystal slurries formed at higher pressures (up to ~6-7 Kb) in a deeper crustal chamber and were subsequently brought rapidly to a shallow level of emplacement (~3-5 Kb pressures) via dikes, which manifest as sheets at shallow levels. Partial dissolution of ep occurred before the relatively more rapid growth of hbl and kfs protected ep crystals from further degradation. It is also plausible that generation of magmatic fluids by repeated infusions of heat created anomalously high vapor pressures at a shallow crustal level.

These observations support previous regional interpretations about the presence of a large pene-contemporaneous mafic body underlying the Gouldsboro. This data also supports the concept that both crustal melts and mantle sources were tapped during the early Devonian portion of the Acadian orogeny at this latitude.