Petrological and geochemical characteristics of the Pleasant Ridge zinnwaldite-topaz granite, southern New Brunswick, and comparisons with other topaz-bearing felsic rocks

AssrRAct The Pleasant Ridge pluton and adjacent tin-mineralized cupola at Bonny fuver, in southern New Brunswick, were emplaced around 360 Ma, well after the cessation of tectonothermal activity associated with the Acadian Orogeny. Granites from the Pleasant Ridge and Bonny River plutons are fine- to medium-grained, equigranular to seriate textured, with essential quartz, alkali feldspar, plagioclase, lithian mica, and topaz. Accessory minerals include zhcon, niobian rutile, columbite-tantalite, cassiterite, uraninite, monazite, and bastniisite. The granites contain primary albite (An0-3) and axe subsolvus in type. Both the topaz (17.6-19.4 wt.tlo F) and lithian mica (5.9-7.4 wt.qo F) are F-rich. The lithian mica Q84 w.Vo Li2O) resembles zinnwaldite. The granites are weakly peraluminous (A/CNK = 1.0-1.2) with normative (Qtz+Kfs+Ab) contents greater than 96t/0, and in the Qtz-Ab-Or system with excess water they plot close to the minimum (T : 690'C) at I kbar with I wt.9o added fluorine. The granites are fluorine-rich (F : 0.7 to 1.0 wt.qo, F/Cl>70), high-silica (SiO2 >75 r't.90) types with extremely elevated contents of Li(703-1124 ppm), Rb (1444-1752 ppm), Y (79-97 ppm), Sn (9-16 ppm), Cs (28-53 ppm), Ta (ll-28 FFm)' W (3-27 ppm), and U (ll-35 ppm), and depletions of TiOr ( 73 wt. q0) contents, and (2) its higher abundances of REE, Y, and Th. It is thus representative of a low-P subtype of topaz granite, which is geochemically distinct from its high-P (P2Oj >0.4 wt.Vo, Al2O3 > 14.5 wt.Vo, SiO2 <73 wt.Vo, very low REE, Y, and Th) counterparts (ag., St. Austell).