Herwart Helmstaedt

Images of a lower-crustal oceanic slab: Direct evidence for tectonic accretion in the Archean western Superior province

The Archean western Superior province in Canada is the type area for proposed Archean plate tectonics. Seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust. This slab, with inferred garnet amphibolite composition, adds a critical piece of evidence to previous suggestions that Archean subduction was at a shallow angle and that some Neoarchean tonalite-trondhjemite-granodiorite suites, distinct from most modern-day suprasubduction magmas, are melts primarily derived directly from subducted slabs.

Fate of the subducted Farallon plate inferred from eclogite xenoliths in the Colorado Plateau

We present the first finding of the high-pressure mineral coesite in lawsonite-bearing eclogite xenoliths from the Colorado Plateau, United States. The presence of coesite in these xenoliths supports the hypothesis that the eclogite formed in a low-temperature–high-pressure environment such as envisaged inside subducted oceanic lithosphere. Ion-microprobe U-Pb dating of micrometer-scale zircons in the eclogites yields ages ranging from 81 Ma to 33 Ma, the two extremes in age likely indicating the age of crystallization during subduction-related metamorphism and the age of recrystallization by the host magmatic event, respectively. These observations conclusively demonstrate that certain eclogite xenoliths from the Colorado Plateau originated as fragments of the subducted Farallon plate, which had been residing in the upper mantle since the Late Cretaceous. This is the first conclusive evidence that any eclogite xenoliths can be directly linked to a known subducted plate.

Purtuniq ophiolite, Cape Smith belt, northern Quebec, Canada: A reconstructed section of Early Proterozoic oceanic crust

The crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the Early Proterozoic Cape Smith belt in northern Quebec, Canada. The rocks of the ophiolite represent two magmatic suites, each with a mantle source that is Nd isotopically distinct. The older suite comprises pillowed and massive volcanic rocks, sheeted dikes, dominantly gabbroic layered cumulate rocks, and rare plagiogranite sills and dikes in the volcanic sequence. This >5-km-thick tholeiitic suite is ca. 1998 Ma, and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges. The younger suite (>4 km) comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume- generated oceanic islands, such as Hawaii. The composite section is in excess of 9 km. Although the total thickness of the Purtuniq ophiolite may not be typical of that for oceanic crust formed at Early Proterozoic spreading ridges elsewhere, it does suggest that ancient ridge-formed crust was at least as thick as that in modern oceans.

Multiple dikes in Lower Kam Group, Yellowknife greenstone belt: Evidence for Archean sea-floor spreading?

A mafic intrusive-extrusive complex at the base of the Kam Group in the Archean Yellowknife Supergroup at Yellowknife, Northwest Territories, Canada, grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments. Although rock types and primary structures resemble those in upper parts of Phanerozoic ophiolites, the Kam Group lacks an ultramafic base and is much thicker than typical ophiolites. Because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins, however, the Kam Group is viewed as a remnant of Archean oceanic crust rather than as a product of intracontinental rift volcanism. The dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea-floor spreading in an Archean marginal-basin setting.

Geology and Chemistry of the Early Proterozoic Purtuniq Ophiolite, Cape Smith Belt, Northern Quebec, Canada

The two-billion year-old Purtuniq ophiolite comprises pillowed mafic flows, sheeted mafic dykes, gabbros, and minor plagiogranites, and an extensive suite of layered mafic and ultramafic cumulate rocks; depleted mantle rocks have not been observed. The tectonically dismembered ophiolite is similar in most physical and chemical respects to Phanerozoic ophiolites, and represents direct evidence for modern-style plate tectonic processes in the early Proterozoic. The preservedcrustal thickness is of the order of 7. 5–8 km; but may originally have been as much as 9–10 km thick. The pillowed volcanic rocks and sheeted dykes are tholeiitic, with rare earth and other trace element abundances most similar to modern MORBs. The cumulate rocks a,lso follow a tholeiitic trend in major element composition. The mafic nature of the volcanic pile, the absence of extensive pyroclastic rocks, and the presence of a well developed sheeted dyke complex stratigraphically underlying the volcanic rocks supports an oceanic spreading center origin for the ophiolite. Modally graded layers and adcumulate textures observed in thin section suggest that the layered mafic and ultramafic rocks are dominantly the result of cumulus crystallization. Cryptic compositional variations in relict igneous minerals, such as forsterite content in olivine, and chromium and titanium content of clinopyroxene do not vary systematically with stratigraphic height in the cumulate pile. This is thought to record the periodic input of fresh batches of primitive magma into the magma chamber. Nd-isotopic data suggest that two time-integrated depleted mantle sources were responsible for the generation of the ophiolite, one highly depleted (∈Nd + 4. 6 to + 5. 3), the other less so (∈Nd +2. 5 to +3. 6). Each source produced a suite of cumulate rocks and sheeted mafic dykes. The mutually intrusive nature of sheeted dykes from the two suites suggests that the two sources operated simultaneously, and in close proximity to one another. The physical extent of the ocean basin in which the ophiolite was generated is not well constrained.

Zoned sodic amphibole: Petrologic indicator of changing pressure and temperature during tectonism in the Bathurst Area, New Brunswick, Canada

Amphibole, zoned from an actinolite core through a barroisite layer to a crossite or ferroglaucophane rim, coexists with epidote, chlorite, albite, quartz, phengite, sphene, magnetite, and calcite in metabasites of the Tetagouche Group near Bathurst, New Brunswick. Stratigraphic, structural and regional-metamorphic evidence suggests that these rocks were situated in the hangingwall of a SE-dipping subduction zone during the Taconian Orogeny. Using microprobe analyses of the coexisting minerals, a dehydration reaction and a subsequent hydration reaction have been balanced in the system SiO2-Al2O3-TiO2-Fe2O3-(Fe, Mg, Mn)O-CaO-Na2O-K2O-H2O, so as to describe the development of the zoned amphiboles. Relatively large coefficients for magnetite in these reactions accord with the absence of sodic amphibole in associated magnetite-free metabasites, indicating that ferric iron has a significant effect on the transition from greenschist to epidote-blueschist fades. Relatively small residuals of FeO, MgO and MnO show that these components are only weakly partitioned between the reactant and product assemblages. The procedure of Holland and Richardson (1979) has been used to calculate apparentP-T paths that trace the growth histories of the zoned amphiboles from a low-P environment to a high-P environment. Although the apparent pressure-change is implausibly large and the final temperature implausibly low, it is clear that the Bathurst metabasites followed a very differentP-T path than those of the Austrian Alps, despite their identical mineral assemblage.

Timing of the Cenozoic “Great Hydration” event beneath the Colorado Plateau: Th-Pb dating of monazite in Navajo volcanic field metamorphic eclogite xenoliths

Th-Pb dating of monazite in xenoliths of low-temperature metamorphic eclogite facies rocks from diatremes of the Navajo volcanic field in the center of the Colorado Plateau (southwest United States) yields ages of ca. 28 Ma. Because monazite is not a primary phase in basic igneous or metamorphic rocks, but introduced during metasomatism, we suggest that the fluid responsible for monazite growth was derived from prograde metamorphic dehydration reactions in serpentinites and related rocks in the subducted Farallon plate. These fluids hydrated the overlying sub-plateau lithospheric mantle, consuming garnet (thus mobilizing rare earth elements) and lowering mantle density and increasing volume, contributing to the uplift of the Colorado Plateau in early Oligocene time.

Superposed crenulation cleavages resulting from progressive deformation

Abstract Correlation between cleavage formation and growth of syntectonic fibrous minerals is used to show that superposition of crenulation cleavages in Lower Ordovician rocks from the Eastern Townships of Quebec, Canada, resulted from overprinting during progressive deformation. Fibres of calcite, quartz and chlorite in pressure shadows adjacent to pyrite curve through almost 90°, from parallelism with the first to parallelism with the second, superposed crenulation cleavage. Whereas the cleavages mark only the initial and final orientations of the flattening plane, the fibres record the gradual rotation of this plane in the interval between formation of the cleavages. The continuity of the undeformed fibres suggests a single progressive deformation rather than two separate tectonic events.

Stylolitic cleavage and cleavage refraction in Lower Paleozoic carbonate rocks of the Great Valley, Maryland

Abstract Closely spaced stylolitic cleavage is developed in closures of folds in Lower Paleozoic carbonate rocks of the Great Valley, west of Hagerstown, Maryland. Stylolites which are perpendicular to bedding in beds of relatively competent microspar are refracted into a more penetrative cleavage in thin layers of less competent, dolomitic micrite. Cleavage refraction is a result of shear in the micrite layers which accommodated bedding slip between the more competent microspar beds by acting as Ramsay and Graham-type shear zones. The refracted cleavage is not normally parallel to the shear plane (bedding) but may approach parallelism in extremely thin shear zones. Refraction of cleavage between greywackes and slates of the overlying Martinsburg Formation, if due to the mechanism proposed for the carbonate rocks, is incompatible with a pre-lithification origin of the slaty cleavage.

Mantle-derived guyanaite in a Cr-omphacitite xenolith from Moses Rock diatreme, Utah

Abstract Guyanaite, naturally occurring β-CrOOH, has been identified in a xenolith of Cr-rich omphacitite from the Moses Rock diatreme in the Navajo Volcanic Field of the southwestern United States. It occurs as the dominant phase in small clusters of accessory minerals, intergrown with kosmochlor-rich omphacite, zincian chromite, eskolaite, and carmichaelite. The assemblage is interpreted as the result of metasomatism of chromite-bearing serpentinite by slab-derived fluids during subduction of the Farallon Plate in Laramide time. At the time of entrainment of the xenolith, the rock was undergoing prograde metamorphism, with guyanaite dehydrating to eskolaite plus water. This reaction, and the coeval dehydration of the inferred accompanying host serpentinites (which would have been much more volumetrically significant), provided water for hydration of the subcontinental upper mantle, contributing to uplift of the Colorado Plateau. Recognition of guyanaite as a component of a subducted slab supports recent proposals, based on laboratory experiments, that high-pressure polymorphs of other common crustal oxy-hydroxide minerals such as boehmite and goethite (i.e., high-pressure δ-AlOOH and ε-FeOOH) can also transport and store water in the mantle.