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Papers by Robert Tucker

A major synmetamorphic Early Devonian thrust and extensional fault system in the Mid Norway Caledonides: relevance to exhumation of HP and UHP rocks

Geological Society, London, Special Publications, 2013

The study of complex orogenic belts commonly begins in the frontal regions with well-defined tect... more The study of complex orogenic belts commonly begins in the frontal regions with well-defined tectonostratigraphy, and relatively simple structure and metamorphism, and proceeds into the progressively more complex hinterland, which nevertheless may contain the best geochronological record of the most intense orogenic events. The northern part of the Western Gneiss Region in the hinterland of the Scandian orogen contains a robust U–Pb zircon geochronological framework on rocks subjected to high-pressure (HP) and ultra-high-pressure (UHP) metamorphism that has implications for tectonic development both there and towards the foreland. HP and UHP eclogite crystallization occurred at 415–410 Ma (Early Devonian, Lochkovian to Pragian), followed by pegmatite crystallization at c. 395 Ma (Late Emsian) during exhumation and return to amphibolite-facies conditions, thus limiting the process to 15–20 myr. The nature and sequence of events are much more complex than in the foreland, causing diff...

Coesite micro-inclusions and the U/Pb age of zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway

Lithos, 2003

... indicated the probable time of eclogite-facies metamorphism, in contrast to previously favour... more ... indicated the probable time of eclogite-facies metamorphism, in contrast to previously favoured interpretations that the major metamorphism of the WGR gneisses and eclogites was at least as old as 17001800 Ma (McDougall and Green, 1964 and Pidgeon and Råheim, 1972). ...

Age and tectonothermal record of Laurentian basement, Caledonides of NE Greenland

Journal of the Geological Society, 1993

Isotope mineral ages provide constraints on the age and tectonothermal record of Laurentian basem... more Isotope mineral ages provide constraints on the age and tectonothermal record of Laurentian basement in Dronning Louise Land, NE Greenland Caledonides. West of the Caledonian thrust zone, U-Pb dates for detrital zircon grains within late Proterozoic–early Palaeozoic foreland cover sequences define an age range of detritus between 3001 Ma and 1700 Ma. Detrital muscovites record 40Ar/39Ar mineral cooling ages of c. 1700–1600 Ma. A reworked granitic orthogneiss east of the Caledonian thrust zone yields a U-Pb zircon age of 1909+6−5 Ma which is interpreted as dating emplacement of the protolith. A lower concordia intercept of 422+28−25 Ma dates Caledonian reworking. The data indicate that the Pre-cambrian basement in NE Greenland forms part of an early Proterozoic (c. 1900–1700 Ma) mobile belt comprising, in part, reworked Archaean rocks. In contrast to central E Greenland, there is no isotopic evidence for regional tectonothermal activity of late Proterozoic age (c. 1000 Ma).

On the location of the boundary between the Nubian Shield and the old African continent: Inferences from U-Pb (zircon) and common Pb data

Geology of northern Penobscot Bay, Maine, with contributions to geochronology

U.S. Geological Survey, Miscellaneous Investigations Series I-2551. Covers parts of the Blue Hill... more U.S. Geological Survey, Miscellaneous Investigations Series I-2551. Covers parts of the Blue Hill, Castine, Vinalhaven, and Deer Isle 15\u27 quadrangles

Reevaluation of the Piermont-Frontenac allochthon in the Upper Connecticut Valley: Restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence

Geological Society of America Bulletin, 2012

ABSTRACT The regional extent and mode and time of emplacement of the Piermont-Frontenac allochtho... more ABSTRACT The regional extent and mode and time of emplacement of the Piermont-Frontenac allochthon in the Boundary Mountains-Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire-Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre-Middle Ordovician section of the anticlinorium was a large allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This similar to 200-km-long allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated "fault" is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged sub marine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry Pb-207/Pb-206 age of 444.1 +/- 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion micro probe Pb-206/U-238 age of 492.5 +/- 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed allochthon. Collectively, these features indicate that the large Piermont-Frontenac allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km(2) klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian-Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.

Age and implications of Early Ordovician (Arenig) plutonism in the type area of the Bay du Nord Group, Dunnage Zone, southern Newfoundland Appalachians

Canadian Journal of Earth Sciences, 1994

In south Newfoundland, an extensive tract of metamorphosed Ordovician metavolcanic, metasedimenta... more In south Newfoundland, an extensive tract of metamorphosed Ordovician metavolcanic, metasedimentary, and granitoid rocks (Bay du Nord Group) lies north of a late Precambrian basement inlier of peri-Gondwanan affinity, separated from the latter by Silurian rocks. In the Bay du Nord Group type area, the metavolcanic and metasedimentary rocks were ductilely sheared and locally fault imbricated with metagabbro prior to emplacement of the Baggs Hill Granite, herein dated at 477.6 ± 1.8 Ma. Some of the volcano-sedimentary strata within this succession, however, contain foliated clasts of Baggs Hill Granite, and these strata must comprise a younger stratigraphic sequence which, in this area, is thrust northwestward over the older rocks.The pre-477.6 ± 1.8 Ma tectonic interleaving of the Dunnage Zone gabbro and stratified rocks is significantly earlier than the Llandovery (early Salinic) recumbent folding and thrusting of Middle Ordovician Exploits Subzone rocks in the east-central Hermitag...

The Merrimack Synclinorium in northeastern Connecticut; discussion

American Journal of Science, 1982

Tectonics, 1993

The Grand Bruit Fault Zone of southern Newfoundland is a fundamental structure within Late Precam... more The Grand Bruit Fault Zone of southern Newfoundland is a fundamental structure within Late Precambrian basement on the Gondwanan margin of the Appalachian orogen. Within the fault zone, a sequence of structures documents changes in the sense of ductile displacement from (1) reverse dip slip, to (2) dextral strike slip, to (3) sinistral oblique slip and finally, to (4) dextral lateral offsets. Fault movements along this structure were punctuated by emplacement of a variety of plutons and minor intrusions which, when precisely dated, allow these movements to be bracketed at between 571 Ma and 564 Ma, 497 Ma and 427 Ma, 424 Ma and 420 Ma, and 421 Ma and 387 Ma, respectively. The tectonic evolution of the Gondwanan inlier of southern Newfoundland is mirrored, in large part, by the record of mylonite development within the Grand Bruit Fault Zone. These tectonic events are attributable to well-constrained, regional orogenic events of both the Pan-African and Appalachian cycles. Newly formed shear zones in the fault zone reactivate parts of much older faults of similar regional orientation and are, in some cases, kinematically indistinguishable from the ancestral structures. Integration of precise geochronological data with the sequence of overprinted fault structures demonstrates that, although the role of progressive deformation in shear zone development was important, the observed disposition of structures and rock units is primarily a function of polyorogenic accretion. As a multiple-reactivated structural lineament in a Gondwanan basement inlier, the fault zone exerted fundamental control over the tectonic development of the leading edge of the convergent southeast margin of the orogen. INTRODUCTION Recognition of the chronological order of the constituent structures within linked fault zones is pivotal to a full appreciation of the complexity of ancient mountain belts that evolved through multiphase accretion of lithotectonic

Mineralogy and Petrology, 2015

The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lav... more The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lava flows, dyke swarms and plugs of Cenozoic age emplaced into the Mesozoic-Cenozoic sedimentary rocks of the Antsiranana basin (NW Madagascar) and above the crystalline basement. The magmatism in the Ampasindava region is linked to a NW-SE trending extensional tectonic setting. New 40 Ar/ 39 Ar age determinations on feldspar separate of alkali granites and basaltic dykes yielded ages of 18.01±0.36 Ma and 26±7 Ma, respectively. Alkali basalts and basanites, nepheline syenites and phonolites, and silica saturated-to-oversaturated syenites, trachytes, granites and rhyolites are the main outcropping lithologies. These rocks have sodic affinity. The felsic rocks are dominant, and range from peraluminous to peralkaline. The mantle-normalized incompatible element patterns of the mafic lavas match those of Na-alkaline lavas in within-plate rift settings. The patterns are identical in shape and absolute concentrations to those of the Bobaomby (Cap d'Ambre) and Massif d'Ambre primitive volcanic rocks. These geochemical features are broadly compatible with variable degrees of partial melting of incompatible element-enriched mantle sources. The mineralogical and geochemical variations are consistent with fractional crystallization processes involving removal of olivine, feldspar, clinopyroxene, amphibole, Fe-Ti oxides and apatite. Removal of small amount of titanite explains the concave upward lanthanide pattern in the evolved nepheline syenites and phonolites, which are additionally rich in exotic silicates typical of agpaitic magmas (eudialyte, F-disilicates).

Petrology and Geochemistry of Cretaceous Mafic and Silicic Dykes and Spatially Associated Lavas in Central-Eastern Coastal Madagascar

Dyke Swarms:Keys for Geodynamic Interpretation, 2011

Abstract Late Cretaceous dykes and lavas from central-eastern coastal Madagascar (Vatomandry and ... more Abstract Late Cretaceous dykes and lavas from central-eastern coastal Madagascar (Vatomandry and Mahatsara districts) span a compositional range from basalt to rhyolite. The dykes strike N–S, parallel to the coast, and intrude the Precambrian schists and gneisses of the area. 40Ar/39Ar age determinations on rhyolite from Sakanila massif (western Vatomandry) gave an age of 86.4 ± 0.3 Ma, which is statistically equivalent to the mean age of eastern rifted-margin rocks of Madagascar. Basaltic dykes are generally tholeiitic varying from olivine-normative to quartz-normative, and characterized by relatively low MgO (4.8–8.0 wt%), Cr (61–341 ppm) and Ni (36–128 ppm) contents. The mantle-normalized diagrams have bell-shaped patterns, with the most Mg-rich dykes having <10 times mantle values. The most Fe-rich ferrobasalts have similar patterns displaced at higher element concentrations, but with evident Sr and Ti troughs. Silicic lavas and dykes are dacites and rhyolites, and are characterized by low CaO, MgO, Fe2O3t, TiO2 and P2O5 and high SiO2, K2O, Ba and Rb. The silicic rocks are enriched in light lanthanides (LREE) and have radiogenic initial 87Sr/86Sr ratios (0.70551–0.70833) and negative initial ɛNdi values (–5.6 to –2.5). The magmatic evolution of the mafic dykes is dominated by fractional crystallization of plagioclase and clinopyroxene with minor olivine. The Vatomandry silicic rocks are the result of prolonged fractional crystallization from basalt parental magmas coupled with small amounts of crustal contamination.

Geosphere, 2012

transform margins of the Argentine Precordillera and Laurentia Ages of pre-rift basement and synr... more transform margins of the Argentine Precordillera and Laurentia Ages of pre-rift basement and synrift rocks along the conjugate rift and Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geosphere www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further

Canadian Journal of Earth Sciences, 2011

The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and ne... more The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U–Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil–Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo–Mesoarchean (3.32–3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55–2.48 Ga), involving emplacement of juvenile igneous rocks, high-g...

The youngest Paleozoic plutonism of the Newfoundland Appalachians: U–Pb ages from the St. Lawrence and François granites

Canadian Journal of Earth Sciences, 1993

The posttectonic St. Lawrence and François granites have long been regarded as the youngest Paleo... more The posttectonic St. Lawrence and François granites have long been regarded as the youngest Paleozoic plutonic suites of the Newfoundland Appalachians. Their U–Pb ages of 374 ± 2 and 378 ± 2 Ma, respectively, define a Middle to Late Devonian magmatic event. Mid-Carboniferous magmatic and (or) rifting events, suggested on the basis of earlier Rb–Sr dating of the St. Lawrence Granite, are not supported by these new data. Both granites intrude major ductile and brittle structures that were active during the early Paleozoic, and they provide a younger age limit for major tectonic activity in the Avalon Zone and on the south coast of Newfoundland. The granites may correlate with plutons of closely similar age reported from the Aspy and Mira terranes of Cape Breton Island, Nova Scotia.

Crustal architecture of northern Madagascar: results from recent geological mapping

Thomas, Robert James; De Waele, Bert; Schofield, David; Goodenough, Kathryn Mary; Key, Roger; For... more Thomas, Robert James; De Waele, Bert; Schofield, David; Goodenough, Kathryn Mary; Key, Roger; Ford, Jonathan Richard; Annells, Richard; Tucker, RD; Bauer, W.; Burton, WB; Walsh, GJ; Howard, K.; Lidke, D.; Rabarimana, M.; Rafahatelo, JM.; Ralison, AV; ...

HPP-07 Late Neoproterozoic orogenic belts and assembly of Gondwana ... New constraints on the ass... more HPP-07 Late Neoproterozoic orogenic belts and assembly of Gondwana ... New constraints on the assembly of the East African Orogen from northern Madagascar ... David I. Schofield, The British Geological Survey (United Kingdom) Robert J. Thomas, The British Geological ...

American Journal of Science, 2007

Pre-Silurian strata of the Bronson Hill arch (BHA) in the Upper Connecticut Valley, NH-VT are hos... more Pre-Silurian strata of the Bronson Hill arch (BHA) in the Upper Connecticut Valley, NH-VT are host to the latest Ludlow Comerford Intrusive Suite consisting, east to west, of a mafic dike swarm with sheeted dikes, and an intrusive complex. The rocks are mostly mafic but with compositions ranging from gabbro to leucocratic tonalite. The suite is truncated on the west by the Monroe fault, a late Acadian thrust that carries rocks of the BHA westward over Silurian-Devonian strata of the Connecticut Valley-Gaspé trough (CVGT). Dikes intrude folded strata with a pre-intrusion metamorphic fabric (Taconian?) but they experienced Acadian deformation. Twenty fractions of zircon and baddeleyite from three sample sites of gabbrodiorite spanning nearly 40 km yield a weighted 207 Pb/ 206 Pb age of 419 ؎ 1 Ma. Greenschist-facies dikes, sampled over a strike distance of 35 km, were tholeiitic basalts formed by partial melting of asthenospheric mantle, with little or no influence from mantle or crustal lithosphere. The dike chemistry is similar to mid-ocean ridge, within-plate, and back-arc basin basalts. Parent magmas originated in the asthenosphere and were erupted through severely thinned lithosphere adjacent to the CVGT. Extensive middle Paleozoic basins in the internides of the Appalachian orogen are restricted to the Québec embayment of the Laurentian rifted margin, and include the CVGT and the Central Maine trough (CMT), separated from the BHA by a Silurian tectonic hinge. The NE-trending Comerford intrusions parallel the CVGT, CMT, and the tectonic hinge, and indicate NW-SE extension. During post-Taconian convergence, the irregular margins of composite Laurentia and Avalon permitted continued collision in Newfoundland (St. Lawrence promontory) and coeval extension in the Québec embayment. Extension may be related to hinge retreat of the northwest directed Brunswick subduction complex and rise of the asthenosphere following slab break-off. An alternative hypothesis is that the basins originated as pull-apart basins between northwest-trending, left-stepping, sinistral strike-slip faults along the southern flanks of the New York and St. Lawrence promontories. introduction The literature of the Paleozoic tectonic history of central New England is dominated by discussion of island arcs and compressional events. In recent years, there has been an increasing awareness of Silurian crustal extension between the Ordovician Taconian and Devonian Acadian orogenies. The existence of mafic dike swarms along the Upper Connecticut River Valley has been known since the mapping of Billings (1937) and White and Billings (1951), but little has been made of the tectonic significance of these rocks. The discovery of sheeted dikes in those dike swarms and the field association of those dikes with a mafic intrusive complex led to our hypothesis that the mafic intrusions were generated during Silurian extension that produced the Connecticut Valley-Gaspé trough (CVGT) and the Silurian tectonic hinge of northwestern Maine (fig. 1). We present the results of a joint study involving field mapping, U-Pb isotopic dating, and geochemistry in order to determine the distribution, age, possible magmatic source areas, and tectonic environment of the suite.

Lithos, 2010

Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoprotero... more Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New U-Pb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen.