Oxygen isotope evidence for the genesis of Upper Paleozoic granitoids from southwestern Nova Scotia

The oxygen isotope ratios for 127 rocks and coexisting minerals from Paleozoic granitoids and clastic metasedimentary rocks of southwestern Nova Scotia have been measured. The whole-rock δ18O values for samples of the South Mountain batholith range from 10.1–12.0‰.But discrete granitoid plutons, located to the south of the South Mountain batholith, have lower δ18O values (7.8–10.4‰). Coexisting minerals from the Nova Scotia granitoids are near isotopic equilibrium, indicating that the whole-rock δ18O values primarily reflect the δ18O of the magma, rather than secondary alteration processes. The Meguma Group clastic metasedimentary rocks that host the Nova Scotia granitoids range in δ18O from 10.1–12.9‰. These clastic metasedimentary rocks show no systematic geographic variation in δ18O. The greenschist facies Meguma Group rocks that host the South Mountain batholith have similar δ18O values to the amphibolite facies equivalents located about the southern discrete plutons. Large scale isotopic exchange between the Meguma Group and the South Mountain batholith, or the southern plutons, is not evident.The relatively high δ18O values of the peraluminous South Mountain batholith (10.1–12.0‰) indicate that it formed by anatexis of 18O-rich clastic metasedimentary rocks. The southern plutons were also derived by partial melting of clastic metasedimentary rocks, but their lower δ18O values reflect exchange of the source material with a low 18O reservoir (mafic magmas?) prior to, or during anatexis.The sheared Brenton pluton is much lower in δ18O (5.0‰) than any of the other rocks, probably because of exchange with low 18O fluids during shearing.

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TRACING GARNET ORIGINS IN GRANITOID ROCKS BY OXYGEN ISOTOPE ANALYSIS: EXAMPLES FROM THE SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA

The Canadian Mineralogist ◽

10.3749/canmin.49.2.417 ◽

2011 ◽

Vol 49 (2) ◽

pp. 417-439 ◽

Cited By ~ 17

Author(s):

J. S. Lackey ◽

S. Erdmann ◽

J. S. Hark ◽

R. M. Nowak ◽

K. E. Murray ◽

...

Keyword(s):

Nova Scotia ◽

Oxygen Isotope ◽

Isotope Analysis ◽

The South ◽

South Mountain Batholith ◽

Granitoid Rocks

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Crystal-melt ± fluid equilibria versus late-stage fluid - rock interaction in granitoid rocks of the South Mountian Batholith, Nova Scotia: whole rock geochemistry and oxygen isotope evidence

Atlantic Geology ◽

10.4138/1643 ◽

1988 ◽

Vol 24 (1) ◽

Cited By ~ 1

Author(s):

Daniel J. Kontak ◽

David F. Strong ◽

Robert Kerrich

Keyword(s):

Nova Scotia ◽

Oxygen Isotope ◽

Late Stage ◽

The South ◽

Rock Interaction ◽

Isotope Evidence ◽

Granitoid Rocks

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The interplay of regional structure and emplacement mechanisms at the contact of the South Mountain Batholith, Nova Scotia: floor-down or wall-up?

Canadian Journal of Earth Sciences ◽

10.1139/e01-029 ◽

2001 ◽

Vol 38 (9) ◽

pp. 1285-1299 ◽

Cited By ~ 7

Author(s):

Nicholas Culshaw ◽

Pradeep Bhatnagar

Keyword(s):

Mechanical Properties ◽

Nova Scotia ◽

Country Rock ◽

Wall Rock ◽

Ductile Deformation ◽

Linear Structures ◽

The South ◽

Metasedimentary Rocks ◽

South Mountain Batholith ◽

Differential Movement

In southern Nova Scotia, the Devonian South Mountain Batholith was emplaced into metasedimentary rocks of the Cambro-Ordovician Meguma Group at ca. 370 Ma. The contact of the eastern end of the South Mountain Batholith transects at a high angle the trace of subhorizontal, upright Acadian (mid-late Devonian) folds formed in the Meguma Group. At two locations, where the contact is well exposed, there are contrasting structures in the country rocks adjacent to Acadian anticlinoria and synclinoria, respectively. Regional folds are affected by ductile deformation where anticlinoria abut the batholith but are undisturbed at the synclinoria. At the anticlinorial contacts, the metasedimentary bedding youngs towards the granite, and granite side-down shear resulted in a belt in which bedding is transposed to a new contact-parallel fabric. Deflection of linear structures that were initially horizontal in the Acadian folds (e.g., intersection lineations) illustrates the granite side-down shear. The reorientation of initially horizontal linear structures gradually diminishes as the contact is followed from the anticlinoria to the synclinoria, where the regional fold geometry is preserved right up to the contact, showing that there is no granite side-down shear in the synclinoria at the present level of erosion. Two models that potentially explain this variation in contact structure are discussed. In the first, it is explained as an artifact of emplacement of the batholith late in the growth of the Acadian folds, in which the horizontal, upright anticlinoria amplified and moved upward relative to the pluton. A shear zone was formed parallel to the contact along the thermally softened tip of the anticlinoria. The synclinoria remained fixed vertically and there was no differential movement between granite and country rock. Thus, regional structures and evidence for stoping are most widely preserved in the synclinoria, where they were not overprinted by the marginal shearing. The second model invokes floor-down emplacement of magma into folds of layered sediments with contrasting mechanical properties. The erosion surface within the synclinoria intersects slates of the Halifax Formation with mechanical properties that favour emplacement predominantly by dyking and stoping. Below the level of erosion, the stratigraphically underlying Goldenville Formation, having different mechanical properties than the Halifax, presumably is displaced downwards predominantly by ductile deformation (pure and simple shear). Within the anticlinoria, where the Goldenville Formation is exposed, the requirement of a level pluton floor necessitates that downward deflection is accompanied by relatively high ductile strains in the wall rock. A third possible model that combines features of the syntectonic and floor-down models is an obvious option.

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