The redox state of granitoids relative to tectonic setting and earth history: The magnetite–ilmenite series 30 years later

Abstract

The redox state variation of orogenic granitoids along convergent plate margins is examined in the Phanerozoic Circum-Pacific Belt and in some Cryptozoic terranes. The Phanerozoic granitoids of the NW and NE Pacific Rims can be divided into reduced ilmenite series occurring in the accretionary terranes with compressional tectonic setting, and oxidised magnetite series intruding crystalline basements under extensional to intermediate regional stress regime. The ilmenite-series granitoids have negative but the magnetite series have positive δ34S values, which show a positive correlation with magnetic susceptibility of the granitoids. The negative δ34S sulphur originated in biogenic sulphur from accreted pelitic sediments and positive δ34S values show that sulphate sulphur migrated from seawater through subduction processes. The whole rock δ18O values are higher than 8 permil in the ilmenite series, but lower than 8 permil in the magnetite series, and as a whole show negative correlation with the magnetic susceptibility of the granitoids. The higher δ18O values reflect those of accreted sediments, whilst the lower δ18O values represent magmatic values of an oxidised mafic protolith at depth.The predominance of ilmenite-series granitoids of the NW Pacific rim can be explained by well-developed accretionary terranes in which mafic magmas from depth mingled with felsic magmas from the accretionary complex to form granodioritic magmas, whilst that of magnetite-series granitoids is postulated to be oxidised igneous sources for the magma generation and an extensional and/or intermediate tectonic setting for the magma ascent. The absence of the accretionary wedges by tectonic erosion and/or no fore-arc sedimentation also helped to form magnetite-series granitoids. Potassic granitoids are generally of oxidised type. A-type granites in late orogenic environments also belong to the magnetite series. Adakitic high-Sr/Y granitoids are oxidised in the Mesozoic–Cenozoic but are reduced in the Archaean TTG, reflecting the redox state of the then-current sea-floor environment. The oldest magnetite-series granite so far known is the 3105 Ma-old biotite granite of the Nelspruit batholith, South Africa.

Paleomagnetism and Geochronology of the Early Cretaceous Dipilto Batholith (NW Nicaragua): Chortís Block Large Rotation With Respect to SW North America

“…However, field observations from this study indicate cross‐cutting relationships and petrographic evidence indicating that the batholith underwent a second postbatholithic mafic magmatic event (Figure d), independent of the magmatic mingling recognized by Buriánek and Žáček () that was also observed in this study (DI‐06 site; Table and supporting information S1). Likewise, the magnetic susceptibility response related to the content of Mag has allowed us to differentiate both pulses in the ilmenite‐ and magnetite‐series, as well as the tectonic setting (Ishihara, , ). Geologic mapping and petrographic determinations indicate that the magmatic pulse that generated the ilmenite‐series is distributed throughout the study area, while the magnetite‐series only appears in the western part of the batholith, together with microdiorite/diabase dikes.…”

“…A common problem in the paleomagnetic studies of the ilmenite‐series intrusives and the S‐type granites is the noisy signal. This problem lies mainly in the scarce magnetic mineralogy in these rocks (e.g., Ishihara, ). However, obtaining an NRM and a subsequent demagnetization behavior with a ChRM allow verifying the reliability of the paleomagnetic signal.…”

“…Tectonics Ishihara, 2004). However, obtaining an NRM and a subsequent demagnetization behavior with a ChRM allow verifying the reliability of the paleomagnetic signal.…”

Composite Sunrise Butte pluton: Insights into Jurassic–Cretaceous collisional tectonics and magmatism in the Blue Mountains Province, northeastern Oregon

“…The bulk (volume) magnetic susceptibility is a measure of magnetization of a rock in the applied magnetic fi eld and thus directly relates to the proportion, type, and composition of magnetic minerals (e.g., Hrouda and Kahan, 1991;Tarling and Hrouda, 1993). The bulk susceptibility has also proven to be a sensitive indicator of changes in chemical composition (e.g., Aydin et al, 2007) and redox state of granitoid magmas and has been used to defi ne the oxidized magnetite and reduced ilmenite series of granitoid rocks (e.g., Ishihara, 1977Ishihara, , 2004.…”

“…Red biotite in rocks from the Desolation Creek unit is higher in total Al than biotite from the Sunrise Butte unit, and this is thus also suggestive of reducing conditions in the magma (Lalonde and Bernard, 1993). The magnetic characteristics of the magmas could have been inherited from their sources, recording a transition from a reduced, mantle-derived source to a deep crustal, more-oxidized source with time (e.g., Ishihara, 2004). The paramagnetic signature of the Desolation Creek unit could also have been further enhanced through in situ assimilation of the surrounding serpentinitic wall rocks and/or graphite-bearing metasedimentary rocks of the Badger Creek unit (e.g., Ague and Brimhall, 1988;Malvoisin et al, 2012;Tomkins et al, 2012).…”