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The New Cornwall Syenogranite, Nova Scotia: Petrology and Geochemistry G Document generated on 10/01/2021 7:25 p.m. Atlantic Geology The New Cornwall syenogranite, Nova Scotia: petrology and geochemistry G. Pe-Piper and S. Ingram Volume 37, Number 2-3, Summer 2001 Article abstract The New Cornwall syenogranite is a 1.7 km2 intrusion at the southern margin URI: https://id.erudit.org/iderudit/ageo37_2_3art01 of the Whale Lake mon7ogranitc of the South Mountain Batholilh of southwestern Nova Scotia- It has a mean SiO> content of 75 8%, and contains See table of contents accessory amounts of tourmaline, andalusite, and primary muscovitc. Ratios such as A-'CNK (- 1.25). Rb/K (-215), and NbH'a (- 3.5) show that the syenogranitc is not highly fractionated. Although binary clement plots for some elements Publisher(s) show the same regular trend for both monzogranite and syenogranite, variations in trace elements such as Rb, Ha, Th and LRI'E show that the Atlantic Geoscience Society syenogranite is not derived from the monzogranite by fractional crystalli7ation, as the syenogranite has lower Eu and HREE compared with the ISSN monzogranite. Similarities in LREI', Nd isotope composition (eNJ - -2) and other geochemical indicators between monzogranite and syenogranitc suggest that 0843-5561 (print) they were derived by partial melting of a common source Both the Whale Lake 1718-7885 (digital) monzogranite and the New Cornwall syenogranite then evolved independently by fractional crystallization and late fluids played only a minor role in the Explore this journal further evolution of the syenogranitc. This represents a third mode of development of mineralized Icucogranitc m the South Mountain Batholith, in addition to the previously recognized "associated" and "independent" Cite this article Icucogranitc Pe-Piper, G. & Ingram, S. (2001). The New Cornwall syenogranite, Nova Scotia: petrology and geochemistry. Atlantic Geology, 37(2-3), 133–151. All rights reserved © Atlantic Geology, 2001 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ Atlantic Geology 133 The New Cornwall syenogranite, Nova Scotia: petrology and geochemistry G. Pe-Piper and S. Ingram Department of Geology, Saint Mary's University, Halifax, NS B3H 3C3 <gpiper@stmarys. ca> Date Received: January 18, 2001 Date Accepted: December 11, 2001 The New Cornwall syenogranite is a 1.7 km2 intrusion at the southern margin o f the Whale Lake monzogranite o f the South Mountain Batholith o f southwestern Nova Scotia. It has a mean S i0 2 content o f 75.8%, and contains accessory amounts of tourmaline, andalusite, and primary muscovite. Ratios such as A/CNK (~ 1.25), Rb/K (~ 215), andNb/Ta (~ 3.5) show that the syenogranite is not highly fractionated. Although binary element plots for some elements show the same regular trend for both monzogranite and syenogranite, variations in trace elements such as Rb, Ba, Th and LREE show that the syenogranite is not derived from the monzogranite by fractional crystallization, as the syenogranite has lower Eu and HREE compared with the monzogranite. Similarities in LREE, Nd isotope composition (eNd — 2) and other geochemical indicators between monzogranite and syenogranite suggest that they were derived by partial melting o f a common source. Both the Whale Lake monzogranite and the New Cornwall syenogranite then evolved independently by fractional crystallization and late fluids played only a minor role in the further evolution o f the syenogranite. This represents a third mode o f development o f mineralized leucogranite in the South Mountain Batholith, in addition to the previously recognized “associated” and “independent” leucogranite. Le syenogranite de New Cornwall constitue une intrusion de 1,7 kilometre carre sur la limite meridionale du granite monzonitique du lac Whale, lequel fait partie du batholithe du mont South, dans le sud-ouest de la Nouvelle-Ecosse. II a une teneur moyenne en S i0 2 de 75,8 %, et renferme des quantites accessoires de tourmaline, d’andalousite et de muscovite primaire. Les rapports d’A/CNK (~ 1,25), de Rb/K (~ 215) et de Nb/Ta (~ 3,5) revelent que le syenogranite n’est pas extremement-fraetionne. Meme si les representations graphiques des elements binaires de certains elements revelent la meme tendance reguliere dans le cas du granite monzonitique et du syenogranite, les variations des elements traces comme le Rb, le Ba, le Th et les elements de terres rares legers signalent que le syenogranite ne provient pas du granite monzonitique par cristallisation fractionnaire, car le syenogranite possede des teneurs moindres en Eu et en elements de terres rares lourds comparativement au granite monzonitique. Les similarites existantes en ce qui conceme les elements de terres rares legers, la composition en isotopes de Nd (cNd — 2) et d’autres indicateurs geochimiques entre le granite monzonitique et le syenogranite permettent de supposer qu’ils proviennent d’une fonte partielle d’une source commune. Le granite monzonitique du lac Whale et le syenogranite de New Cornwall ont ensuite tous deux evolue independamment par cristallisation fractionnaire et les fluides tardifs ont seulement joue un role secondaire dans revolution ulterieure du syenogranite. II s’agit la d’un troisieme mode de developpement du leucogranite mineralise dans le batholithe du mont South, qui s’ajoute aux leucogranites « associes » et « independants » deja reconnus. Traduit par la redaction Introduction unit (Home 1992) and intrudes, with an exposed intrusive contact, the Goldenville Formation of the Meguma Group at The Devonian South Mountain Batholith (SMB) of the southern margin of the SMB, on the north side of Caribou southern Nova Scotia (Fig. 1) is a composite plutonic suite of Lake (Fig. 1). It contains Mo, Cu, and Bi mineral occurrences principally granodiorite and monzogranite in the Meguma (Home 1992). terrane of the northeastern Appalachian orogen. Siliceous The purpose of this study is to better understand the origin rocks, variously termed leucogranite, leucomonzogranite or of this unique syenogranite through a detailed mineralogical syenogranite, form kilometre-scale intrusive bodies that were and geochemical study, and to compare this syenogranite with emplaced mostly late in the evolution of the batholith. Using leucogranite and leucomonzogranite elsewhere in the SMB. the definition of Streckeisen (1976), “leuco” applies to igneous rocks with <6% ferromagnesian minerals. In the SMB, MacDonald et al. (1992) distinguished leucogranite P rev io us w ork with <2% biotite from leucomonzogranite with 2-7% biotite. MacDonald et al. (1992) mapped the SMB and described The New Cornwall syenogranite is a leucogranite in the the petrography, geochemistry, and age. They recognized 13 sense of Streckeisen (1976), although not in the terminology component plutons, grouped into early biotite-bearing of MacDonald et al. (1992) and Home (1992), and is the only monzogranite and granodiorite and later muscovite ± biotite syenogranite body in the SMB. It has an area of about 1.7 km2, monzogranite - leucomonzogranite - leucogranite. The New outcrops at the southern part of the Whale Lake monzogranite Atlantic Geology 37, 133-151 (2001) 0843-5561/01/020133-19$3.85/0 134 Pe-Piper and Ingram S h A A A A |A"X‘ A A A A A AIA A ~ 7 r ~ + T 1 ---T—TT T + a) 64° 34' + + + a a / 64°32' A A A A A A A \ + + + + ^AAAAAAAAAAA J + + + + + ' + + + + + |/\ --\W\i A /\A /\A /\A /\A /\A /\A - + Cb + + + 4- + + + + + < "k "k + + + + + + + + + + '-f-F’ + + + + + + - r k + + + + + + + Lake + + + + + + + + + + + + + + + + ^Misery + + + + + + + + + + + + + + + + + + V + + + + + + + + + + + + + + + + + + + + ( + + + + + + + + + + + + + + + + + + + + + -ft + + + + + + + + + + + + + + + + + + +r + + + + + + + + + + + + + Northfielders + + + 3< + + + + + + + + + + + +__ Laken + A- + S LakepV^+ + + + + + T— + ^ ] + + ^Bagpipe- + + + -Lake + + + + + Cxi + CO f + + V - K f + > x— a New Cornwall Syenogranite ^ + —^-l(DCsgNC) TTTri New Ross Leucomonzogranite (D C Im N R ) 1+— 3 Whale Lake Monzogranite L _ J (DCmgWL) Goldenville Formation (•COG) Geological boundary defined 0 - - approximate - - assumed km b) Fig. 1. Geological map (after Home 1992) showing a) location o f the New Cornwall syenogranite and b) the sampled locations. Rock unit codes follow Home (1992). SMB on inset location map = South Mountain Batholith. Cornwall syenogranite and Whale Lake monzogranite form occur as larger discrete bodies with sharp contacts with other part of the New Ross late-stage pluton. The various plutons rocks of the batholith and represent highly fractionated melts. were generated by melting of protoliths with differing Although the volume of leucogranite in the SMB is small compositions (Smith 1979). (0.7% according to MacDonald et al. 1992), it is host to many The petrography of the New Cornwall syenogranite and of the mineral deposits. its mineralization are given in Home (1987, 1992). MacDonald et al. (1992) included the syenogranite in their fine-grained leucomonzogranite, which makes up 6.8% of the F ield observations batholith. Clarke et al. (1993) presented a comprehensive We re-examined both the New Cornwall syenogranite
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