Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones.</p

Tracing Deep Carbon Cycling by Zinc Isotopes in a Peralkaline‐Carbonatite Suite

Sedimentary carbonates are known to be carried into the deep mantle by subducted slabs, and studies on mantle‐derived magmas have attempted to trace the recycled carbonate in their mantle source. However, the final depth of storage of recycled carbonate and the role of recycled carbonate in the partial melting of mantle remain controversial. Peralkaline‐carbonatite suites are considered to have been derived from a carbonated mantle source and are windows to evaluate carbon in the mantle. In this study, we report the Zn isotopic compositions of a peralkaline‐carbonatite suite from the Tarim Large Igneous Province (Tarim LIP). The peralkaline‐carbonatite suite has heavier δ66Zn than normal mantle with δ66Zn of 0.34–0.40 ‰ for nephelinite, 0.35–0.47 ‰ for aillikite, 0.51–0.55 ‰ for nepheline syenite, 0.58–0.67 ‰ for calciocarbonatite and 0.38–0.56 ‰ for magnesiocarbonatite. The heavy Zn isotopic compositions of the peralkaline‐carbonatite suite in the Wajilitag complex suggest the incorporation of recycled carbonate‐bearing materials into the deep mantle. We infer that the calciocarbonatite was produced by the initial partial melting of subducted MgSiO3/MgO + C‐bearing carbonated eclogite, whereas the magnesiocarbonatite, aillikite, and nephelinite are considered as reacted melts between carbonated eclogite‐derived melts and peridotite. The heavy Zn isotopic compositions of the nepheline syenite are attributed to fractional crystallization from nephelinite magma in the magma reservoir. Our study highlights the incorporation of carbonated eclogite as an important agent of recycled carbon in the deep mantle and interactions between carbonated eclogite‐derived melts and peridotite lead to the complex lithological heterogeneities in the peralkaline‐carbonatite suite in Tarim LIP.</p

A snapshot of the transition from monogenetic volcanoes to composite volcanoes: case study on the Wulanhada Volcanic Field (northern China)

The transition processes from monogenetic volcanoes to composite volcanoes are poorly understood. The Late Pleistocene to Holocene intraplate monogenetic Wulanhada Volcanic Field (WVF) in northern China provides a snapshot of such a transition. Here we present petrographic observations, mineral chemistry, bulk rock major and trace element data, thermobarometry, and a partial melting model for the WVF to evaluate the lithology and partial melting degree of the mantle source, the crystallization conditions, and pre-eruptive magmatic processes occurring within the magma plumbing system. The far-field effect of India–Eurasia collision resulted in a relatively high degree (10 %–20 %) of partial melting of a carbonate-bearing eclogite (∼ 3 wt % carbonate; Gt/Cpx ≈ 2 : 8, where Gt denotes garnet and Cpx denotes clinopyroxene) followed by interaction with ambient peridotite. The primary melts ascended to the depth of the Moho (∼ 33–36 km depth), crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C with the melt water contents of 1.1 wt %–2.3 wt %. Part of the primary melt interacted with the lithospheric mantle during ascent, resulting in an increase in the MgO contents and a decrease in the alkaline contents. The modified magma was subsequently directly emplaced into the middle crust (∼ 23–26 km depth) and crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C. The primary melts from the same mantle sources migrated upward to the two-level magma reservoirs to form minerals with complex textures (including reverse and oscillatory zoning and sieve texture). Magma erupted along the NE–SW-striking basement fault and the NW–SE-striking Wulanhada–Gaowusu fault in response to the combined effects of regional tectonic stress and magma replenishment. The crustal magma reservoir in the WVF may represent a snapshot of the transition from monogenetic volcanoes to composite volcanoes. It is possible to form a composite volcano with large magma volumes and complex compositions if the magma is continuously supplied from the source and experiences assimilation and fractional crystallization processes in the magma plumbing system at crustal depth.</p