Development of High-Silica Adakitic Intrusions in the Northern Appalachians of New Brunswick (Canada), and Their Correlation with Slab Break-Off: Insights into the Formation of Fertile Cu-Au-Mo Porphyry Systems

• Published in: Geosciences
• Main Authors: Fazilat Yousefi, David R. Lentz, James A. Walker, Kathleen G. Thorne
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2024
• Subjects: high-silica adakite; New Brunswick; northern Appalachians; slab break-off; fertile Cu-Au-Mo porphyry systems; Geology; QE1-996.5; Blue Mountain; Canada; Gulch; North Pole; Sugar Loaf; Tower Hill; Verte
• Online Access: https://doi.org/10.3390/geosciences14090241; https://doaj.org/article/df1b3d5848444d209a1a1424678bba58

High-silica adakites exhibit specific compositions, as follows: SiO 2 ≥ 56 wt.%, Al 2 O 3 ≥ 15 wt.%, Y ≤ 18 ppm, Yb ≤ 1.9 ppm, K 2 O/Na 2 O ≥ 1, MgO < 3 wt.%, high Sr/Y (≥10), and La/Yb (>10). Devonian I-type adakitic granitoids in the northern Appalachians of New Brunswick (NB, Canada) share geochemical signatures of adakites elsewhere, i.e., SiO 2 ≥ 66.46 wt.%, Al 2 O 3 > 15.47 wt.%, Y ≤ 22 ppm, Yb ≤ 2 ppm, K 2 O/Na 2 O > 1, MgO < 3 wt.%, Sr/Y ≥ 33 to 50, and La/Yb > 10. Remarkably, adakitic intrusions in NB, including the Blue Mountain Granodiorite Suite, Nicholas Denys, Sugar Loaf, Squaw Cap, North Dungarvan River, Magaguadavic Granite, Hampstead Granite, Tower Hill, Watson Brook Granodiorite, Rivière-Verte Porphyry, Eagle Lake Granite, Evandale Granodiorite, North Pole Stream Suite, and the McKenzie Gulch porphyry dykes all have associated Cu mineralization, similar to the Middle Devonian Cu porphyry intrusions in Mines Gaspé, Québec. Trace element data support the connection between adakite formation and slab break-off, a mechanism influencing fertility and generation of porphyry Cu systems. These adakitic rocks in NB are oxidized, and are relatively enriched in large ion lithophile elements, like Cs, Rb, Ba, and Pb, and depleted in some high field strength elements, like Y, Nb, Ta, P, and Ti; they also have Sr/Y ≥ 33 to 50, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Ta/Yb > 0.3, Sm/Yb > 2.5, Gd/Yb > 2.0, Nb + Y < 60 ppm, and Ta + Yb < 6 ppm. These geochemical indicators point to failure of a subducting oceanic slab (slab rollback to slab break-off) in the terminal stages of subduction, as the generator of post-collisional granitoid magmatism. The break-off and separation of a dense subducted oceanic plate segment leads to upwelling asthenosphere, heat advection, and selective partial melting of the descending oceanic slab (adakite) and (or) suprasubduction zone lithospheric mantle. The resulting silica-rich adakitic magmas ascend through thickened mantle lithosphere, ...