On the enigmatic mid-Proterozoic : Single-lid versus plate tectonics

The mid-Proterozoic (ca. 1850-850 Ma) is a peculiar period of Earth history in many respects: ophiolites and passive margins of this age are rare, whereas anorthosite and A-type granite suites are abundant; metamorphic rocks typically record high thermobaric (temperature/pressure) ratios, whereas ultrahigh pressure (UHP) rocks are rare; and the abundance of economic mineral deposits features rare porphyry Cu-Au and abundant Ni-Cu and Fe-oxide Cu-Ag (IOCG) deposit types. These collective observations have been used to propose that a stagnant-lid, or single-lid, tectonic regime operated at this time, between periods of plate tectonics in the Paleoproterozoic and Neoproterozoic. In our reappraisal of the mid -Proterozoic geological record, we not only assess the viability of the single-lid hypothesis for each line of evidence, but also that of the plate tectonic alternative. We find that evidence for the single-lid hypothesis is equivocal in all cases, whereas for plate tectonics the evidence is equivocal or supporting. We therefore find no reason to abandon a plate tectonic model for the mid-Proterozoic time period. Instead, we propose that the peculiarities of this enigmatic interval can be reconciled through the combination of two processes working in tandem: secular mantle cooling and the exceptionally long tenure and incomplete breakup of Earth's first supercontinent, where both of these phenomena had a dramatic effect on lithospheric behaviour and its resulting imprint in the geological record. (c) 2022 British Geological Survey (c) UKRI 2022. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Enhanced U-Pb detrital zircon, Lu-Hf zircon, δ18O zircon, and Sm-Nd whole rock global databases

High-quality global isotopic databases provide Earth scientists with robust means for developing and testing a variety of geological hypotheses. Database design establishes the range of questions that can be addressed, and validation techniques can enhance data quality. Here, six validated global isotopic databases provide extensive records of analyses from U-Pb in detrital zircon, Lu-Hf in zircon, Sm-Nd from whole rocks, and δ18O in zircon. The U-Pb detrital zircon records are segregated into three independently sampled databases. Independent samples are critical for testing the replicability of results, a key requisite for gaining confidence in the validity of a hypothesis. An advantage of our updated databases is that a hypothesis developed from one of the global detrital zircon databases can be immediately tested with the other two independent detrital zircon databases to assess the replicability of results. The independent εHf(t) and εNd(t) values provide similar means of testing for replicable results. This contribution discusses database design, data limitations, and validation techniques used to ensure the data are optimal for subsequent geological investigations

The replication crisis and its relevance to Earth Science studies: Case studies and recommendations

Numerous scientific fields are facing a replication crisis, where the results of a study often cannot be replicated when a new study uses independent data. This issue has been particularly emphasized in psychology, health, and medicine, as incorrect results in these fields could have serious consequences, where lives might be at stake. While other fields have also highlighted significant replication problems, the Earth Sciences seem to be an exception. The paucity of Earth Science research aimed at understanding the replication crisis prompted this study. Specifically, this work aims to fill that gap by seeking to replicate geological results involving various types of time-series. We identify and discuss 11 key variables for replicating U-Pb age distributions: independent data, global sampling, proxy data, data quality, disproportionate non-random sampling, stratigraphic bias, potential filtering bias, accuracy and precision, correlating time-series segments, testing assumptions and divergent analytical methods, and analytical transparency. Even while this work primarily focuses on U-Pb age distributions, most of these factors (or variations of them) also apply to other geoscience disciplines. Thus, some of the discussions involve time-series consisting of εHf, δ18O-zircon, 14C, 10Be, marine δ13C, and marine δ18O. We then provide specific recommendations for minimizing adverse effects related to these factors, and in the process enhancing prospects for replicating geological results

A geochronological review of magmatism along the external margin of Columbia and in the Grenville-age orogens forming the core of Rodinia

A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.publishedVersio

Master of Arts

thesisThe Mineral Range is located in Beaver and Millard Counties In southwestern Utah. The range is about thirty miles long and averages five miles in width. The southern end of the range can be reached via Utah State Highway 21 which connects the towns of Beaver, Adamsville, Minersville, and Milford; there are many dirt roads which also traverse the range. The purpose of the present Investigation is two-fold: (1) to make a thorough study of the relationship between the Precambrian metamorphic complex on the west side of the Mineral Range and the Mineral Range pluton, and (2) to summarize in detail the petrogenesis of the Mineral Range pluton. Liese (1957) and Earll (1957) have described the sedimentary rocks of the Mineral Range; therefore, only a brief summary of the stratigraphy condensed from these two theses is presented here. The Cambrian is represented by the Prospect Mountain quartzite, Pioche shale, and a series of undifferentiated limestones. The Ordovician and Silurian are missing. Resting on top of the Cambrian is a series of undifferentiated lower Paleozoic limestones and dolomites. Devonian sediments have not been found; the Mississippian is represented by the Topache limestone; the Permian by the Coconino(?) sandstone and Kaibab limestone; the Triassic by the Moenkopi formation; the Jurassic by the Navajo sandstone and Carmel formation; the Cretaceous by the Claron-Indianola conglomerate; and the Recent by lake beds and other alluvium. A Laramide thrust is responsible for the Prospect Mountain quartzite resting on top of middle Cambrian limestones in the northern Mineral Range; also, a zone of cataclastic metamorphism north of Cave Canyon is interpreted by the author as a thrust zone. The southern and southeastern part of the Mineral Range is a homocline with beds dipping southeast and striking northeast. A prominent set of high-angle faults strike east-west in the southern and northern part of the range; north trending Basin and Range faults cut all previous structures. The following list summarizes the igneous rocks recognized in the Mineral Range by the author: (1) rhyolite porphyry dikes and plugs In the southern part of the range, (2) dellenite porphyry dikes in the northern part of the range, (3) lamprophyre (spessartite) dikes along the west side of the range, (4) the Minersville volcanics in the southern part of the range, (5) the Ranch Canyon volcanics in the central part of the range, and (6) Pleistocene and Recent basalt flows found around the borders of the range. All of the igneous rocks with exception of the basalt flows and possibly the Ranch Canyon volcanics are Tertiary in age. The Precambrian (?) gneisses, schists, granodiorites and adamellites along the west side of the range are described and named the Wildhorse Canyon series. Also, an extensive zone of cataclastic metamorphism north of Cave Canyon is described as well as small tactite and skarn zones found around the periphery of the Mineral Range pluton and the Lincoln stock. The Mineral Range pluton is the largest body of granite in the state of Utah. It is over twenty miles long and the maximum width is six miles. The Lincoln stock (adamellite) and its apophyses crop out south of Cave Canyon fault on the west side of the range and are thought to be basic apophyses of the Mineral Range pluton. An extensive ""inclusion zone"" found along the western part of the Mineral Range pluton is described and shown on the geologic map. Also, numerous migmatites, pegmatites, and aplites which were found to be gradational with each other are described. The chief criteria used and accepted by most petrologists to distinguish granitized granite from magmatic granite are presented and applied to the Mineral Range pluton. Those criteria diagnostic of magmatic injection were not found in the pluton while all but two out of thirteen major criteria for granitization were present In the Mineral Range pluton or the Lincoln stock. The tectonic setting of the pluton is evaluated in terms of Misch (I949), and the zone of emplacement Is evaluated in terms of Buddington (1959). The following is a summary of the petrologic and tectonic history of the Mineral Range as interpreted by the author: Precambrian sediments and possibly plutonic rocks were metamorphosed and eroded before Paleozoic time. Paleozoic and Mesozoic deposits are relatively thin (12,000 feet) and for this reason it is believed that this area represents the western margin of the Colorado Plateau. Floods of conglomerate were deposited in this area during Coloradoan time; the area was folded and domed (E. Montanan?) and the Mineral Range pluton was formed (L. Montanan?) during late Cretaceous time; thrusting and cataclastic metamorphism occurred during Paleocene time; lamprophyres, rhyolite and dellenite porphyry dikes were intruded during mid-Tertiary time and east-west high-angle faulting commenced; the Minersville volcanics were deposited and Basin-range faulting commenced in Oligocene time; epeirogeny and erosion and finally deposition of the Ranch Canyon volcanics occurred during Plio-Pleistocene time; basalt flows, lake beds and other alluvium were deposited during Pleistocene and Recent time

Plate Tectonics and Crustal Evolution

ix,288 hal,;ill,;30 c

Episodic growth of juvenile crust and catastrophic events in the mantle

Episodic growth of continental crust and supercontinents at 2.7,1.9,and 1.2 Ga may be caused by superevents in the mantle as descending slabs pile up at the 660-km seismic discontinuity and then catastrophically sink into the lower mantle. A superevent cycle involves supercontinent breakup that initiates both slab avalanches and the onset of formation of a new supercontinent; arrival of slabs at the D" layer triggers mantle plumes that rise and bombard the base of lithosphere producing juvenile crust trapped in the growing supercontinent; and shielding of the mantle beneath the new supercontinent results in a mantle upwelling that eventually breaks the supercontinent as the cycle starts over. Superevents comprise three or four events each of 50-80 My duration, each of which may reflect slab avalanches at different locations and times at the 660-km discontinuity. Superplume events in the late Paleozoic and Mid-Cretaceous may have been caused by minor slab avalanches as the 660-km discontinuity became more permeable to the passage of slabs. The total duration of a superevent cycle decreases with time probably reflecting the cooling of the mantle

A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga?

Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more important of these are the following: (1) increasing proportion of basalts with “arc-like” mantle sources; (2) an increasing abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources; (3) onset of a Great Thermal Divergence in the mantle; (4) a decrease in degree of melting of the mantle; (5) beginning of large lateral plate motions; (6) appearance of eclogite inclusions in diamonds; (7) appearance and rapid increase in frequency of collisional orogens; (8) rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9) appearance of ophiolites in the geologic record, and (10) appearance of global LIP (large igneous province) events some of which correlate with global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere, and they may record the gradual onset and propagation of plate tectonics around the planet. To further understand the changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution