TRENDS OF VERY-LOW-GRADE METAMORPHISM IN THE AROOSTOOK-MATAPEDIA BELT OF NORTHEASTERN MAINE (AROOSTOOK COUNTY) USING GRAPTOLITE AND VITRINITE REFLECTANCE

Graptolite (GRo) and vitrinite (VRo) reflectance (Malinconico, 1992, 1993, unpub. data) with previously published conodont alteration indices (CAI) is used to detail very-low-grade metamorphic trends in the analcime-prehnite-pumpellyite (PPA) grade (Richter and Roy, 1976) Ordovician-Lower Devonian rocks of the Aroostook-Matapedia (AM) belt in northeastern Maine (Aroostook county). Graptolite reflectance, like the reflectance of vitrinite (fossil wood), is a useful diagenetic to very-low-grade metamorphic (maturity) indicator in marine rocks that pre-date the appearance of land plants. The main study area includes the Munsungun/Winterville-Pennington (MWI) inliers of the Bronson Hill-Popelogan belt on the west, the New Brunswick (NB) border on north and east, and Presque Isle city on the south. The Red Indian Line (boundary between Laurentia and Ganderia) is commonly drawn on western side of the MWI.

Like the PPA metamorphism, pre-Acadian CAI and mean maximum GRo increase from the western arm of the Winterville-Pennington Mt (WPM) inlier (analcime; GRo 1.1-1.5%; VRo 1.0%; CAI 1.5) SE towards Presque Isle (pumpellyite-actinolite; GRo 4.2%; CAI 4). Within this area is a narrow high reflectance belt (GRo>5%) trending NE and widening into NB. Plotting GRo on regional cross-sections shows folding of isoreflectance contours and suggests maturity is due to pre-Acadian depth of burial in a deep AM basin. Additionally, thinning of pre-Seboomook Siluro-Devonian (S-D) formations west over the WPM, and low maturity of Ordovician-to-pre-Acadian formations along western arm of WPM implies that structure was a S-D topographic high and Acadian deformational buttress.

South of Presque Isle, scattered GRo measurements increase south from Houlton (GRo 5.5%) through Millinocket (10.6%) to GRo 12.7%, just inside the biotite isograd in Piscataquis county.

VRo was also measured on post-Acadian Middle Devonian formations: redbeds/greenbeds of Wang (2020) along the MWI (1.4%), Mapleton (1.4%), Trout Valley (1.6-2.0%). These formations have been considered unmetamorphosed, but VRo indicates paleo-burial of 3-5 km. Questions: 1) were these just deep post-orogenic extensional basins or was there also blanketing post-orogenic (Mid-Late Devonian-Carboniferous) cover; 2) what was age of exhumation/erosion?