Tectonic significance of a supra-ophiolitic sedimentary cover succession, Unst, Shetland, Scottish Caledonides: insights from the U–Pb–Hf detrital zircon record
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The tectonic significance of the Muness Phyllite, which overlies the Unst–Fetlar ophiolite in Shetland, Scottish Caledonides, is poorly understood. U–Pb analyses of detrital zircons show that it was deposited after c . 469 Ma. Early Paleozoic grains have Keywords:
Phyllite
Laurentia
Orogeny
Conglomerate
Shetland
Laurentia
Orogeny
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Recent advances in our understanding of Palaeozoic tectonics, and in the precise dating of tectonic events require exact definitions of terminology. The Caledonian Orogeny is here redefined to include all the Cambrian, Ordovician, Silurian and Devonian tectonic events associated with the development and closure of those parts of the Iapetus Ocean, which were situated between Laurentia (to the NW) and Baltica and Avalonia (to the SE and east). We suggest that the term ‘Caledonian Orogeny’ be restricted in this geographic sense, but that (as in modern usage) it continues to encompass a series of tectonic, or orogenic, phases (related to arc–arc, arc–continent and continent–continent collisions as Iapetus was closing). Many of these phases have been named; these and many more unnamed events are defined as orogenic phases (local components) of the Caledonian Orogeny. Some of these phases were synchronous over long distances, whereas others were diachronous. The whole Caledonian Orogeny occupied a time interval of around 200 Ma. Thus the term Caledonian should not be used to indicate age.
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Baltica
Laurentia
Devonian
Diachronous
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ABSTRACT The Mesoproterozoic is a controversial time within the Earth’s history, and is characterized by high temperature/pressure ratios in metamorphic rocks, a large volume of extensional plutons, very few economic mineral deposits, and possibly a slowdown in plate tectonic processes. In Laurentia, ca. 1.48–1.35 Ga is well known as a time of voluminous ferroan magmatism, which led to conflicting tectonic interpretations that range from continental extension to convergent margin settings. Recently, a ca. 1.50–1.35 Ga orogenic belt was proposed that spanned Laurentia from present-day eastern Canada to the southwestern United States. Unlike the preceding Paleoproterozoic Yavapai/Mazatzal orogenies and the subsequent late Mesoproterozoic Grenville orogeny, the early–mid-Mesoproterozoic Picuris orogeny in the southwestern United States was relatively unrecognized until about two decades ago, when geochronology data and depositional age constraints became more abundant. In multiple study areas of Arizona and New Mexico, deposition, metamorphism, and deformation previously ascribed to the Yavapai/Mazatzal orogenies proved to be part of the ca. 1.4 Ga Picuris orogeny. In Colorado, the nature and extent of the Picuris orogeny is poorly understood. On this trip, we discuss new evidence for the Picuris orogeny in the central Colorado Front Range, from Black Hawk in the central Colorado Front Range to the Wet Mountains, Colorado. We will discuss how the Picuris orogeny reactivated or overprinted earlier structures, and perhaps controlled the location of structures associated with Cambrian rifting, the Cretaceous–Paleogene Laramide orogeny, and the Rio Grande rift, and associated mineralization. We will also discuss whether and how the Picuris orogeny, and the Mesoproterozoic in general, were unique within the Earth’s history.
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Laurentia
Mountain formation
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An outcrop of the basal Deadwood Formation was measured and described south of Lead, South Dakota along Whitewood Creek. At the base of the Deadwood is a fluvial and tidally reworked, matrix-supported conglomerate unit with sub-rounded to angular pebbles that are imbricated in a northwest orientation. This, along with their composition, suggests a southeast provenance of quartzite, vein quartz, and phyllite. Overlying the conglomerate is compositionally and texturally mature quartz sandstone with bedding typical of beach deposition. Regional studies have shown that steeply dipping Precambrian quartzite and phyllite units formed topographic ridges and valleys that were flooded by a transgressing Late Cambrian sea. These topographic highs and lows controlled the distribution and thickness of the basal congolmerate unit.
Conglomerate
Phyllite
Outcrop
Pebble
Deposition
Lithology
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Conglomerate
Phyllite
Pebble
SLATES
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The fault breccia along the Max Meadows overthrust in the Draper Mountain area, Virginia, consists of three cataclastic zones: an upper autoclastic breccia composed of large, angular blocks and smaller tabular fragments of quartz sericite phyllite; a lower autoclastic zone consisting of angular blocks and fractured masses of Elbrook limestone and dolomite; and an intervening zone of crush conglomerate composed chiefly of rounded fragments of limestone, dolomite, and phyllite. Contacts of the upper and lower breccias with contiguous wall rocks are gradational. Crush conglomerate, which grades into the autoclastic breccias above and below, was formed by mingling, crushing, and rolling out of parts of the autoclastic breccias. The crush-conglomerate zone contains fragments of several types of rocks which are not found in the adjacent autoclastic breccias. These must have been acquired down the original dip of the Max Meadows fault surface where it cuts Lower Cambrian and pre-Cambrian rocks. Parts of the Max Meadows fault breccia have been squeezed into fractures of the rocks on both sides of the fault zone and occur in the form of dikes of crush conglomerate. Of the several modes of origin discussed, only strictly tectonic processes could have produced the breccias and crush conglomerate occurring along or near the Max Meadows fault in this area.
Breccia
Conglomerate
Phyllite
Cataclastic rock
Dolostone
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Underthrusting of Laurentia by the continental margin of Baltica during Caledonian orogeny resulted in the lateral emplacement of Iapetus Ocean-related terranes of the Upper Allochthon at least 500 km onto Baltica. The underlying Lower and Middle allochthons of the Baltoscandian margin mostly comprise Cryogenian, Ediacaran and Cambro-Silurian sedimentary successions; basement to these formations are present only as minor, isolated fragments, except at the base of the Middle Allochthon and within the underlying windows. The upper parts of the Middle Allochthon are notable for the presence of early Ediacaran dyke-swarms and other components of the Baltoscandian continent–ocean transition zone (COT). New data are presented here on the c. 610 Ma age of the COT-related dolerites in the Kalak Nappe Complex in Northern Norway and also on detrital zircons in the underlying Laksefjord and Gaissa nappes. The former confirms that the Baltoscandian COT has a similar age along the length of the orogen; the latter shows that the detrital zircon signatures in the Lower and Middle allochthons are comparable throughout the orogen. These sedimentary rocks have dominating populations of Mesoproterozoic to latest Palaeoproterozoic zircons similar to those from southern parts of the orogen, where Sveconorwegian complexes comprise the basement to the Caledonides. Thus, they help define the probable character and age of the crystalline basement that existed along this outer margin of Baltica during the Neoproterozoic, continental lower crust that was partly subducted during Ordovician continent-arc collision and subsequently lost beneath Laurentia during the 50 million years of Scandian collisional orogeny.
Baltica
Allochthon
Laurentia
Orogeny
Basement
Continental Margin
Passive margin
Rodinia
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Abstract The Longobucco area displays a secondary sedimentary basin transgressive on a granite-phyllite basement. The sedimentary sequence begins with lower Jurassic quartzose conglomerate containing granite and phyllite. The conglomerate is overlain by a homogeneous calcareous marl which forms a large part of the basin and has been divided into lower, middle and upper Liassic (lower Jurassic) by dating with brachiopods, pelecypods and cephalopods. The middle Jurassic consists of red limestones of the Torrente Colognati valley. Cretaceous rocks are not present, probably because of erosion rather than non-deposition. Tertiary rocks begin with either the Paleocene or Eocene as evidenced by the abundance of Distichoplax in the microfauna.
Conglomerate
Phyllite
Marl
Basement
Transgressive
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