The age and origin of the South Mountain Batholith (Nova Scotia, Canada) as constrained by zircon U–Pb geochronology, geochemistry, and O–Hf isotopes

Abstract

The South Mountain Batholith (SMB; Nova Scotia, Canada) is the largest composite batholith exposed in the Appalachians and lies entirely within the most outboard Meguma terrane. In-situ and CA-TIMS U-Pb dating and in-situ isotopes (Lu-Hf, O) and geochemistry for zircon from all phases of the SMB constrain its source as well as its evolution. CA-ID-TIMS for zircon yield emplacement (autocryst) ages indicating a transition from granodiorite (378.7 ± 1.2 to 375.4 ± 0.8 Ma) to leucogranite (375.4 to 371.8 ± 0.8 Ma) over several million years. Furthermore, in situ SHRIMP, LA-MC-ICP-MS, and SIMS analyses of distinct zircon domains reveal: 1) abundant ancient xenocrysts (~420 Ma to 2.2 Ga); 2) antecryst ages ca. 3–15 million years older than SMB emplacement; 3) autocryst δ18O values between +7.3 and +9.1‰ (V-SMOW); 4) similar isotopes, REE signatures, and derived fO2 values among antecrysts and autocrysts; and 5) εHf values from the 371.8 ± 0.8 Ma Davis Lake Pluton (DLP) autocrysts that are higher (+1.74 to +4.38) than the rest of the SMB (-2.99 to +1.68). Collectively, these data suggest a protracted magmatic evolution for the SMB with melt generation and assembly from ~390 to 370 Ma via melting of a metasomatized mantle source followed by contamination, first from the structurally underlying Avalonian terrane and later by metasedimentary wall rocks of the Meguma terrane. The most southwesterly part of the SMB (i.e., DLP) represents a petrogenetically distinct magmatic phase that underwent less overall contamination than the rest of the SMB.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

“…In terms of other redox estimates for the SMB, Bickerton et al (2022) documented fO2 ranging from FMQ to FMQ-5 based on the Ce-in-zircon oxygen barometer of Smythe and Brenan (2016) for a 13 similar suite of samples to those investigated in this study. This is in broad agreement to the results documented here, in terms of samples being relatively reduced, although FMQ-5 is close to the lower stability limit of Fe-rich biotite (Eugster and Wones, 1962).…”

“…Early (stage 1) biotite granodiorites to monzogranites were emplaced between ~379 to 375 Ma and later (stage 2), more chemically-evolved leucomonzogranite to leucogranite yield ages spanning ~375 to 372 Ma (autocrystic zircon U-Pb zircon; Bickerton et al, 2022). Earlier antecrystic zircon yields ages up to 15 Ma older than the SMB emplacement, which together with age data from autocrystic grains suggests a ~20 Ma window for magmatic activity related to SMB evolution (Bickerton et al, 2022). Some of the later stage 2 plutons are associated with polymetallic mineral deposits, most notably at East Kemptville (Sn-Zn-Cu-Ag; Kontak, 1990;Halter et al, 1995;Bickerton, 2021) and New Ross (Cu-Mo-Mn-Sn-U-W; Carruzzo et al, 2000;.…”

“…Geochemical and petrographic information from SMB samples is consistent with graphite playing a role in controlling magma redox state. Sr-Nd-Pb-Hf isotopic evidence indicates that SMB magmas are in part formed by melting or assimilation of so-called Tangier felsic crust considered to be Avalon zone basement rocks (Erbez et al, 1991;Dostal et al, 2004;Dostal and Chatteree, 2010;Bickerton et al, 2022). The proposed felsic crust is represented by Group A felsic granulite facies xenoliths, containing abundant graphite (Owen et al, 1988), which occur in late-Devonian mafic intrusive bodies cross-cutting the SMB and surrounding lithologies (Tate and Clarke, 1995).…”

Architecture of lacustrine deposits in response to early Carboniferous rifting and Gondwanan glaciation, Nova Scotia, south‐east Canada

“…Caputo & Santos (2020). either from slab detachment or a mantle plume (Keppie & Krogh, 1999;Murphy et al, 1999;Bickerton et al, 2022). The subduction likely terminated during the late Devonian (ca 360 Ma) and the back-arc region would have experienced crustal thinning and extension with high heat flow (Hyndman & Currie, 2011).…”

“…The peri‐Gondwanan Avalon and Meguma terranes drifted away from Gondwana in the early Palaeozoic and converged towards the Euramerica continent during the Silurian and Devonian (Murphy et al ., 2004), with northward subduction of the Rheic Ocean. Enhanced Andean‐type magmatism in the Meguma terrane in the mid Devonian resulted either from slab detachment or a mantle plume (Keppie & Krogh, 1999; Murphy et al ., 1999; Bickerton et al ., 2022). The subduction likely terminated during the late Devonian ( ca 360 Ma) and the back‐arc region would have experienced crustal thinning and extension with high heat flow (Hyndman & Currie, 2011).…”