The Moosehorn Plutonic Suite, southeastern Maine and southwestern New Brunswick: age, petrochemistry, and tectonic setting

Abstract

The Moosehorn Plutonic Suite in the coastal Maine magmatic province covers an area of approximately 250 km 2 in the area of Calais, Maine, and St. Stephen, New Brunswick. Based on a compilation of previous work combined with new fi eld mapping, geochronology, and petrochemical data, the Moosehorn Plutonic Suite (MPS) is interpreted to consist mainly of fi ve approximately contemporaneous plutons: Staples Mountain Gabbro, St. Stephen Gabbro, Calais Quartz Diorite, Baring Granite, and Elliott Mountain Diorite. The layered, sill-like Staples Mountain Gabbro is mainly mafi c, whereas the larger St. Stephen Gabbro consists of a core of dunite and troctolite, surrounded by olivine gabbro and gabbro layers. The latter unit grades to quartz diorite of the Calais Quartz Diorite, the most extensive component of the MPS. The Baring Granite consists of medium-grained biotite monzogranite, which is widely mingled with quartz diorite and diorite of the Calais Quartz Diorite. The latest pluton of the MPS appears to be the Elliott Mountain Diorite, which consists mainly of texturally varied dioritic rocks. Each pluton of the MPS is interpreted to have formed by magma differentiation to produce a range of derived compositions. Evidence for mingling and mixing between magmas is also widespread in the MPS, but was not investigated during this study. A sample from the Baring Granite yielded a U-Pb (zircon) crystallization age of 421.1 ± 0.8 Ma) and phlogopite from the olivine gabbro unit of the St. Stephen Gabbro yielded a 40 Ar/ 39 Ar cooling age of 421 ± 4 Ma. The gabbroic parts of the St. Stephen Pluton and the Calais Quartz Diorite are similar in petrochemistry to mafi c and intermediate parts of the Bocabec Pluton of the Saint George Batholith whereas the Baring Granite is similar to the granitic parts of the Bocabec Pluton. Plutons of the MPS generally have calc-alkalic to within-plate chemical characteristics, and their slightly negative (-0.4) to moderately positive (+3.4) ε Nd values are consistent with formation by melting of primitive lower crust, such as may have been present below the Mascarene and Ellsworth terranes. Melting likely occurred in a back-arc setting related to juxtaposition in the late Silurian between these terranes and more outboard terranes of the northern Appalachian orogen. RÉSUMÉ Le cortège plutonique de Moosehorn dans la province magmatique et côtière du Maine couvre une superfi cie d'environ 250 kilomètres carrés dans la région de Calais, au Maine, et de St. Stephen, au Nouveau-Brunswick. Selon une compilation de travaux antérieurs combinés à de nouvelles données pétrochimiques, géochronologiques et cartographiques obtenues sur le terrain, le cortège plutonique de Moosehorn (CPM) est interprété comme un ensemble principalement composé de cinq plutons plus ou moins cinq syngénétiques : le gabbro du mont Staples, le gabbro de St. Stephen, la diorite quartzite de Calais, le granite de Baring et la diorite du mont Elliott. Le gabbro stratifi é en fi lons-couches du mont Staples est principalement mafi ...

Geological relationships and laser ablation ICP-MS U-Pb geochronology of the Saint George Batholith, southwestern New Brunswick, Canada: implications for its tectonomagmatic evolution

“…Similar magma mingling has been observed in the main part of the Bocabec Gabbro in the Oak Bay area ( Fig. 1) farther to the southwest and in the Moosehorn Plutonic Suite in adjacent Maine (Fyffe 1971;McLaughlin et al 2003). Note, however, that the new mapping suggests that a sedimentary screen exists between the Utopia and Bocabec plutons ( Fig.…”

Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians

“…In southern New Brunswick, the Acadian-related plutonic rocks comprise an Upper Silurian suite dominated by intermediate to granitic rocks with minor associated gabbro (McLaughlin et al 2003). They also include the c. 423 Ma volcanic rocks of the Mascarene back-arc (Van Wagoner et al 2002).…”

“…The Gaspé and New Brunswick dataset, which includes both Silurian to Devonian plutonic and volcanic rock analyses, was compiled from Bédard (1986), Dostal et al (1989Dostal et al ( , 1993, McLeod (1990), Whalen (1993a, b), van Wagoner et al (2001,2002), McLaughlin et al (2003), Yang et al (2003) and Wilson et al (2005). The Meguma zone dataset consists exclusively of plutonic rocks and is mainly from the large (1153 samples) compilation by Tate & Merrett (1994), which was filtered for samples that included Rb, Y and Nb data, giving 948 samples.…”

Pressure-temperature paths and exhumation of Late Ordovician-Early Silurian blueschists and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians

“…The start of the Acadian orogeny in southernmost New Brunswick and adjacent coastal Maine (Robinson et al, 1998) is defi ned by inversion of the Silurian Mascarene back-arc basin (Fig. 1), situated immediately behind the Silurian coastal Acadian arc (Barr et al, 2002;McLaughlin et al, 2003; van Staal, 2007). This inversion occurred virtually coeval with the onset of closure of the Silurian La Poile back-arc or intra-arc basin in southern Newfoundland (O'Brien et al, 1991), which occupied an equivalent tectonic setting (Lin et al, 2007).…”