Brian L. Cousens

This study investigates the provenance, weathering history, and tectono-sedimentary evolution of lower Ediacaran siliciclastic rocks of the Imiter Formation (Saghro Group, Imiter Sub-inlier, Anti-Atlas, Morocco) deposited along the northern margin of Gondwana. An integrated approach combining petrography, whole-rock major and trace element geochemistry, rare earth elements (REEs), Sm–Nd isotopes, and organic geochemistry (TOC and δ¹³C_org) was used to constrain sediment sources and depositional conditions. Geochemical proxies, including Th/Sc, La/Sc, and Zr/Sc ratios, together with REE distribution patterns, indicate that the sediments were mainly derived from felsic-to-intermediate rocks of the upper continental crust, with only minor sediment recycling. The negative εNd(t) values (−8.5 to −6.2) and Paleoproterozoic Nd model ages (1.6–2.1 Ga) further suggest the erosion of evolved crustal sources related to the West African Craton. Weathering indices (CIA, CIW, PIA) suggest weak-to-moderate chemical weathering under predominantly arid conditions. Redox-sensitive proxies (V–Ni, V/Cr, V/(V + Ni)) and low TOC content (0.1–0.3 wt.%) indicate deposition under mainly oxic-to-dysoxic conditions, with only transient reducing episodes. Tectonic discrimination diagrams, supported by regional magmatism, point to sedimentation within an extensional basin evolving from active-margin to continental rift conditions during the late Pan-African orogeny. The Imiter Formation records a system dominated by crustal recycling, syn-rift tectonics, and dynamic redox conditions in a shallow marine environment.

This study investigates the provenance, weathering history, and tectono-sedimentary evolution of Lower Ediacaran siliciclastic rocks of the Imiter Formation (Saghro Group, Imiter Sub-inlier, Anti-Atlas, Morocco) deposited along the northern margin of Gondwana. An integrated approach combining petrography, whole-rock major and trace element geochemistry, rare earth elements (REE), Sm–Nd isotopes, and organic geochemistry (TOC and δ¹³Cₒᵣg) was used to constrain sediment sources and deposi-tional conditions. Geochemical proxies, including Th/Sc, La/Sc, and Zr/Sc ratios, to-gether with REE distribution patterns, indicate that the sediments were mainly derived from felsic to intermediate rocks of the upper continental crust, with only minor sedi-ment recycling. The negative εNd(t) values (−8.5 to −6.2) and Paleoproterozoic Nd model ages (1.6–2.1 Ga) further suggest erosion of evolved crustal sources related to the West African Craton. Weathering indices (CIA, CIW, PIA) suggest weak to moder-ate chemical weathering under predominantly arid conditions. Redox-sensitive proxies (V–Ni, V/Cr, V/(V+Ni)) and low TOC contents (0.1–0.3 wt.%) indicate deposition under mainly oxic to dysoxic conditions with only transient reducing episodes. Tectonic dis-crimination diagrams, supported by regional magmatism, point to sedimentation within an extensional basin evolving from active margin to continental rift conditions during the late Pan-African orogeny. The Imiter Formation records a system dominat-ed by crustal recycling, syn-rift tectonics, and dynamic redox conditions in a shallow marine environment.

Mid- to late-Miocene continental arc volcanism on the North Island of New Zealand is found in the Coromandel Peninsula, the Kiwitahi volcanic chain and the Taranaki Basin (Kora Volcano) offshore the western margin of the North Island. Coeval oceanic arc volcanism is also found along the offshore Colville Ridge/Kermadec Ridge north of New Zealand. This Pb–Sr–Nd–Hf isotopic study aims to evaluate mantle sources and potential crustal contaminants along these sections of the Miocene arc system. The Colville/Kermadec Ridge and Kora lavas have the lowest Sr (0.7029–0.7045) and highest Nd (0.51305–0.51292) ratios; the Coromandel and Kiwitahi lavas overlap (Sr = 0.704–0.706; Nd = 0.51268–0.51296). The Colville/Kermadec Ridge, Kora, and Coromandel/Kiwitahi rocks form three distinct arrays on Pb–Pb plots, all above the Northern Hemisphere Reference Line, but none trend towards local Mesozoic basement greywackes. Isotopic and trace element ratio variations suggest that subducted sediments are a component in Coromandel/Kiwitahi mafic lava sources. The younger, southern Kiwitahi lavas have a more depleted mantle source than that for the older, northern Kiwitahi chain. Evolved lavas commonly have interacted with Waipapa basement rocks. Kora rocks have compositions similar to those of back-arc lavas and have been emplaced as sills in a rift environment above a distinct subduction-modified mantle.

The NE-trending multiphase Late Devonian Eagle Lake granite (ELG) in southwestern New Brunswick is mineralized, consisting of hypabyssal porphyritic stocks and dikes that intruded Silurian metabasic volcanic rocks; however, its various phases, ages, and associations with notable stockwork Cu–Mo–Au mineralization and alteration have yet to have been studied. The ELG suite is predominantly composed of phenocrysts and a microcrystalline groundmass of quartz, K-feldspar, and plagioclase, with minor biotite and accessory minerals. In situ LA ICP-MS U–Pb zircon dating of this pluton yielded 360 ± 5 Ma (Late Devonian), so this pluton is considered part of the Late Devonian granitic series in southwestern New Brunswick. The isotopic analysis of two granitic samples yielded an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512164 and 0.512184, initial ⁸⁷Sr/⁸⁶Sr of 0.70168 and 0. 70675, and initial ¹⁷⁶Hf/¹⁷⁷Hf of 0.282619 and 0.282631. The εNd (360 Ma) is −0.37 to +0.03, whereas the εHf (360 Ma) values are +2.1 and +2.5. Pb isotopic analysis yielded a ²⁰⁶Pb/²⁰⁴Pb of 18.49 and 18.72, ²⁰⁷Pb/²⁰⁴Pb of 15.62 and 15.63, and ²⁰⁸Pb/²⁰⁴Pb of 38.26 and 38.37, indicative of a relatively radiogenic source contaminating a primitive mantle melt. Potassic alteration and pyrite-quartz stockwork Cu–Mo–Au veining is evident in some parts of these porphyries. Petrographic and geochemical evidence indicates that this composite pluton is a low-T, I-type granite with zircon saturation temperatures between 720° and 825 °C, with emplacement depths of 10.3 to 4.4 km. ELG was emplaced along a major structural trend manifested by contemporaneous faults and shear zones, i.e., the Belleisle Fault Zone in southern New Brunswick.

The Pescadero Basin Complex (PBC) in the southern Gulf of California comprises three distinctive stretched rhomboid pull-apart basins separated by several short transforms. Multibeam and Autonomous underwater vehicle (AUV) bathymetry data collected at 40-m and 1-m resolution, respectively, combined with the processing and interpretation of three 2-D high-resolution multichannel seismic reflection profiles, were used to characterize the architecture of the entire PBC, as well as the internal structure of the northern Pescadero basin. Detailed mapping and cross-sectional kinematic modeling based on multichannel seismic images of the northern Pescadero basin reveals a highly evolved pull-part geometry, characterized by a well-defined ~1.8 km wide axial graben stretching ~32 km in an NNE-SSW direction. Both finite and incremental strain analyses carried out in this study point out that the PBC developed under sustained transtensional deformation, where the relative motion of the crustal blocks is oblique and divergent to the transforms or principal displacement zones (PDZ's), and subsidence is likely being accommodated by one of more décollement layers located at the bottom of a broad negative flower structure. We also present new geochemical data of lava flows with a N-MORB composition outcropping on the NE segment of the northern Pescadero axial graben, and lava-flow samples of E-MORB composition from an uplifted sediment hill on the western margin of the southern Pescadero basin. MORB samples from the PBC represent the northernmost surface flows known in the Gulf of California, highlighting that the PBC has evolved beyond being a pull-apart complex to having initiated seafloor spreading with new oceanic crust formation in response to the opening of the Gulf of California.