The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest ... more The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of >7 kbar and >7...
Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. Howeve... more Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. However, the rock-scale processes that govern the efficiency of deep volatile transfer are not fully understood. Garnets from subduction zone rocks commonly have fine-scale, oscillatory elemental zoning that is typically considered to record external fluid ingress/transfer. Elemental and oxygen-isotope zoning in garnets from five exhumed subduction zone complexes show that in subduction zone rocks these records are not necessarily coupled; oxygen isotope evidence of ingress of buffering fluids, obvious only in rare cases, is decoupled from shorter length scale elemental and oxygen isotope zonings (which also show no coupling with each other). This finding suggests multiple mechanisms of internal chemical transfer operate at the grain and rock scale during subduction, and that rocks may commonly experience only limited interaction with external fluids. The results presented are consistent with ...
The Morro do Onça suite is a package of ultramafic rocks in the southern São Francisco Craton, Br... more The Morro do Onça suite is a package of ultramafic rocks in the southern São Francisco Craton, Brazil. Owing to the complete alteration of primary silicate minerals, the Morro do Onça suite has been the subject of diverging interpretations, including: part of a retrogressed eclogite-facies Paleoproterozoic ophiolite/accretionary wedge or an Archean komatiite flow(s). In this paper, we utilize spinel-group mineral chemistry—alongside field mapping, petrography, silicate mineral chemistry and bulk-rock geochemistry—to decipher the magmatic and metamorphic evolution of the Morro do Onça suite. A komatiite origin is supported by the identification of spinifex-textured layers, as well as primary spinel-group mineral compositions (mean Cr-number = 79; mean TiO2 = 0.3 wt%) and chondrite-normalized bulk-rock rare-earth element values ([Sm–Lu]N = 1.2–5.1). Metamorphism reached mid/upper amphibolite-facies and was likely responsible for Al mobility on mineral and bulk-rock scales, demonstrating that the bulk-rock Al/Ti proxy commonly used to classify komatiites is susceptible to alteration. Thus, the Morro do Onça komatiites record variable bulk-rock Al/Ti values (17–47) that overlap with both Munro- (Al/Ti = 22) and Weltevreden-type (Al/Ti = 30) komatiites. Based on immobile trace-element ratios ([Gd/Lu]N = 0.6–2.0) and primary spinel-group mineral compositions, we classify the Morro do Onça komatiites as Weltevreden-type. Mesoarchean komatiites related to mantle plume-magmatism are now recognized throughout the southern São Francisco Craton, potentially suggesting that its precursor lithosphere was continuous during this Era.
The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest ... more The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of >7 kbar and >7...
Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. Howeve... more Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. However, the rock-scale processes that govern the efficiency of deep volatile transfer are not fully understood. Garnets from subduction zone rocks commonly have fine-scale, oscillatory elemental zoning that is typically considered to record external fluid ingress/transfer. Elemental and oxygen-isotope zoning in garnets from five exhumed subduction zone complexes show that in subduction zone rocks these records are not necessarily coupled; oxygen isotope evidence of ingress of buffering fluids, obvious only in rare cases, is decoupled from shorter length scale elemental and oxygen isotope zonings (which also show no coupling with each other). This finding suggests multiple mechanisms of internal chemical transfer operate at the grain and rock scale during subduction, and that rocks may commonly experience only limited interaction with external fluids. The results presented are consistent with ...
The Morro do Onça suite is a package of ultramafic rocks in the southern São Francisco Craton, Br... more The Morro do Onça suite is a package of ultramafic rocks in the southern São Francisco Craton, Brazil. Owing to the complete alteration of primary silicate minerals, the Morro do Onça suite has been the subject of diverging interpretations, including: part of a retrogressed eclogite-facies Paleoproterozoic ophiolite/accretionary wedge or an Archean komatiite flow(s). In this paper, we utilize spinel-group mineral chemistry—alongside field mapping, petrography, silicate mineral chemistry and bulk-rock geochemistry—to decipher the magmatic and metamorphic evolution of the Morro do Onça suite. A komatiite origin is supported by the identification of spinifex-textured layers, as well as primary spinel-group mineral compositions (mean Cr-number = 79; mean TiO2 = 0.3 wt%) and chondrite-normalized bulk-rock rare-earth element values ([Sm–Lu]N = 1.2–5.1). Metamorphism reached mid/upper amphibolite-facies and was likely responsible for Al mobility on mineral and bulk-rock scales, demonstrating that the bulk-rock Al/Ti proxy commonly used to classify komatiites is susceptible to alteration. Thus, the Morro do Onça komatiites record variable bulk-rock Al/Ti values (17–47) that overlap with both Munro- (Al/Ti = 22) and Weltevreden-type (Al/Ti = 30) komatiites. Based on immobile trace-element ratios ([Gd/Lu]N = 0.6–2.0) and primary spinel-group mineral compositions, we classify the Morro do Onça komatiites as Weltevreden-type. Mesoarchean komatiites related to mantle plume-magmatism are now recognized throughout the southern São Francisco Craton, potentially suggesting that its precursor lithosphere was continuous during this Era.
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Papers by George Guice