The ∼860 Ma mafic dikes and granitoids from the northern margin ofthe Yangtze Block, China: A record of oceanic subduction in the earlyNeoproterozoic

There are voluminous Neoproterozoic arc-related volcano-sedimentary sequences and small intrusionson the northern margin of the Yangtze Block, South China. The understanding the origin of the Sanli-gang granitoid intrusion and the spatially associated mafic dikes in the region is crucial for unravelingthe tectonic evolution and continental crust growth processes in the Yangtze Block. Zircon U–Pb dat-ing suggests that the mafic dikes (ca. 871 Ma) and granitoids (ca. 860 Ma) are contemporaneous. Themafic dikes have low SiO2(45.37–46.55 wt.%), K2O (0.32–0.82 wt.%) and Na2O (2.01–2.85 wt.%), and arecharacterized by enrichment in large ion lithophile elements (LILEs) and depletion in high-field strengthelements (HFSEs), suggesting that their mantle source was modified by subducted materials. The San-ligang granitoids have intermediate to high SiO2(60.35–71.38 wt.%), intermediate K2O (1.38–3.67 wt.%)and Na2O (3.97–5.33 wt.%), and high MgO (1.03–3.16 wt.%). They show LREE-enriched REE patterns(La/YbN= 7.2–12.3) with no or minor negative Eu anomalies. Their primitive mantle-normalized trace ele-ment patterns are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dikes andgranitoids share similar zircon εHf(t) values (+10.5 to +12.9, +7.9 to +11.7, respectively), whole-rock initial87Sr/86Sr ratios (0.7051–0.7057, 0.7033–0.7041, respectively) and εNd(t) values (+4.0 to +7.1, +3.4 to +4.9,respectively), suggesting that the granitoids were generated by partial melting of juvenile basaltic crust.High Mg# values (49–58) in the granitoids may have resulted from assimilation of residual mafic miner-als in their source region. Based on its arc-related geochemical affinity and contemporaneous arc-relatedmagmatism, the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting duringcrustal growth and reworking. The early Neoproterozoic assemblage from the Sangligang–Sanyang faultbelt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block

Two giants of geology: Kevin Charles Anthony Burke and John Frederic Dewey

This special issue of the Canadian Journal of the Earth Sciences celebrates the career of two of the greatest geologists of our time, who, during the last three decades of the 20th and the first decade of the 21st century, have put their stamp on the tectonic interpretation of the Earth’s behaviour (Burke also extended his efforts into extra-terrestrial space). How fortunate it is for geology that they are both still active and, by all appearances, are likely to remain so for some time to come. It is an immense honour for us as their students and fellow geologists to introduce this special issue with a few lines about them whom we have had the great privilege of knowing closely both as colleagues and as friends

Geochemistry, Nd, Pb and Sr Isotope Systematics, and U-Pb Zircon Ages of the Neoarchean Bad Vermilion Lake Greenstone Belt and Spatially Associated Granitic rocks, Western Superior Province, Canada.

The ca. 2720 Ma Neoarchean Bad Vermilion Lake (BVL) greenstone belt, in the western Superior Province, Canada, is composed of a suite of tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gabbros, and Timiskaming-type siliciclastic sedimentary rocks. The greenstone belt was intruded by Neoarchean granitic rocks, and underwent greenschist facies metamorphism and intense deformation, resulting in mobilization of many elements (e.g., Rb, Ba, Sr, K, U, Pb). The high-field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks, and gabbros are consistent with subduction zone geochemical signatures, suggesting that the BVL greenstone belt formed in a magmatic arc setting. On the basis of lithological associations and trace element systematics, the BVL greenstone belt is defined as a fragment of a Neoarchean subduction-related ophiolite. Three rhyolite samples from the belt have yielded 2722±18 Ma, 2706±13 Ma and 2710±28 Ma U-Pb zircon ages, representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western Wabigoon terrane to the north and the Wawa-Abitibi terrane to the south. The intrusion of the ca. 2671±21 Ma late- to post-tectonic, potassic Ottertail Lake granite marks the end of tectonic accretion in the study area. Both the volcanic rocks and gabbros display large ranges of Nd (143Nd/144Nd=0.511600-0.512849; eNd (2720 Ma) = +0.8 to +4.0), Pb (206Pb/204Pb=13.80-60.67) and Sr (87Sr/86Sr = 0.701481-1.01154) isotopic compositions, suggesting that these isotope systems were variably affected by post-magmatic element mobility. Neither the Sm-Nd (2921±200 Ma) nor Rb-Sr (2130±610 Ma) system has yielded reliable regression (isochron) ages, reflecting the open-system behavior of these systems during metamorphism. Despite large uncertainties, Pb-Pb regression ages yielded by all rock types (2661±60 Ma) and gabbros (2725±83 Ma) agree with the zircon U-Pb ages of the rhyolites, suggesting that the U-Pb system was the most robust among all three systems

Lithological, structural, and geochemical characteristics of the Mesoarchean Târtoq greenstone belt, South-West Greenland, and the Chugach-Prince William accretionary complex, southern Alaska: Evidence for uniformitarian plate-tectonic processes

The Mesoarchean Târtoq greenstone belt, South-West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite and sedimentary rocks, and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite-trondhjemite-granodiorite suites (TTG). Field observations, along with geochemical and zircon U-Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012±4 and 2993±6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach-Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and/or accreted oceanic crust during slab window magmatism. Archean granitoid-greenstone terrains share many geological characteristics of Phanerozoic subduction-accretion complexes such as the Alaskan and Altaid subduction accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of TTG generation in subduction-accretion complexes

Geochemistry and geochronology of mylonitic metasedimentary rocks associated with the Proterozoic Miaowan Ophiolite Complex, Yangtze craton, China: Implications for geodynamic events

Constraining the timing of tectonothermal events in the Proterozoic Miaowan Ophiolite Complex (MOC) and associated rocks in the southern Huangling dome, Yangtze craton, is critical for understanding late Mesoproterozoic and Neoproterozoic tectonic evolution (ca. 1.2–0.9 Ga) of South China and its relationship with the formation of the Rodinian supercontinent. The MOC consists of metamorphosed dunite and harzburgite, isotropic and layered gabbros, diabasic sheeted dike, basalt, plagiogranite, and calc-silicatebearing siliceous and carbonaceous mylonitic rocks. In this study, we present new field, petrographic, geochemical, geochronological and isotopic data for these metasedimentary rocks and mylonitic metasandstone between the ophiolite and underlying flysch sequence, to unravel their origin and tectonic significance for the Yangtze craton. The MOC and associated sedimentary rocks underwent amphibolite facies metamorphism. Geochemical data and field relationships indicate that the mylonitic metasandstone was derived from both the autochthonous rocks of the underlying Yangtze craton and the allochthonous MOC during the accretion of the ophiolite to the craton. The calc-silicate-bearing siliceous and carbonaceous rocks are interpreted as tectonic slices of metasomatized and mylonitized chert and limestone, respectively, deposited on the basaltic crust in a Neoproterozoic ocean. Cores of igneous detrital zircons in the calc-silicate-bearing siliceous and carbonaceous mylonitic rocks have yielded two distinct age groups including a 967–1105 Ma (Mean = 1009 Ma) group and a 1011–1095 Ma (Mean = 1054 Ma) group. The majority of initial eHf(t) values (+9.2 to +14.1) in the zircon cores are similar to those of zircons from the gabbro, diabase and plagiogranite in the MOC, indicating that zircons in the mylonitic rocks were mainly derived from the MOC. Metamorphic overgrowth ages in zircon rims suggest that the latest tectonothermal event in the Precambrian basement rocks of the Yangtze craton took place between 942 and 935 Ma. These metamorphic ages are comparable to those of the Grenvillian-aged collisional events recorded in orogenic belts worldwide. Accordingly, we suggest that this youngest tectonothermal event represents the latest amalgamation time of the heterogeneous tectonic blocks of the Yangtze craton, which were associated with the assembly of the Rodinian supercontinent

Combined bulk-rock Hf- and Nd-isotope compositions of Mesoarchaean metavolcanic rocks from the Ivisaartoq Supracrustal Belt, SW Greenland: Deviations from the mantle array caused by crustal recycling

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHft calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNdt values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon (Polat et al., 2009. Chemical Geology 268, 248-271) which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid restite with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and non-DM isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon

An overview of anorthosite-bearing layered intrusions in the Archaean Craton of southern West Greenland and the Superior Province of Canada: Implications for Archaean tectonics and the origin of megacrystic plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (\u3eAn70) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth

Insights into the tectonic evolution of the North China Craton through comparative tectonic analysis: A record of outward growth of Precambrian continents

Archean cratons have map patterns and rock associations that are diagnostic of the Wilson Cycle, and therefore they can be used to distinguish the principal components of of plate tectonic evolution. The North China Craton (NCC) consists of several distinctly different tectonic units, but the delineation and understanding of the significance of individual sutures and the rocks between them has been hampered by inappropriate definitions of orogens, cratonic blocks, arcs, trapped plateaus, sedimentary basins, accretionary prisms and orogens, and different generations and types of magmatic rocks. The most widely used tectonic division of the craton has been based nearly entirely on the interpretation of metamorphic P-T-t paths and recrystallized zircon morphology, and ignores other familiar elements of the Wilson Cycle. We propose an actualistic tectonic division of the North China Craton based on Wilson Cycle analysis that uses a multi-disciplinary (geology, geochemistry, geochronology, and geophysics) approach to define sutures, their ages, and the nature of the rocks between them, to determine their mode of formation and means of accretion or exhumation, and to propose appropriate modern analogues. The eastern unit of the craton consists of several different small blocks that resemble fragments of accreted arcs from an assembled archipelago similar to those in the extant SW Pacific. These different blocks were assembled between 2.6 and 2.7 Ga ago, although they contain older crustal material dating back to at least 3.8 Ga. A thick Atlantic-type margin developed on the western side of the newly assembled Eastern Block by 2.6-2.5 Ga. A \u3e1,300 km long arc and accretionary prism collided with the margin of the Eastern Block at 2.5 Ga, obducting ophiolites and ophiolitic mélanges onto the block, and depositing a thick clastic wedge in a foreland basin farther into the Eastern Block. This was followed by an arc-polarity reversal, which led to a short-lived injection of mantle wedge-derived melts to the base of the crust that led to the intrusion of mafic dikes and arc-type granitoid (TTG) plutons with associated metamorphism. By 2.43 Ga, the remaining open ocean west of (present geometry) the accreted arc closed with the collision perhaps of an oceanic plateau now preserved as the Western Block with the collision-modified margin of the Eastern Block, causing further deformation to give rise to what has become known as the Central Orogenic Belt. Rifting at 2.4-2.35 Ga of the newly amalgamated continental block led to a rift preserved along its center, and new oceans within the other two rift arms, which removed a still-unknown continental fragment from its (present geometry) northern margin. By about 2.3 Ga an arc collided with a new Atlantic-type margin developed over the rift sequence on the northern margin of the craton, and thus was converted to an Andean margin through arc-polarity reversal. Andean margin tectonics affected much of the continental block from 2.3 to 1.9 Ga, giving rise to a broad E-to-W swath of continental margin magmas, and retro-arc sedimentary basins including a foreland basin superimposed on the passive northern margin. The horizontal extent of these tectonic components is similar to that across the present-day Andes in South America. From 1.88 to 1.79 Ga a granulite facies metamorphic event was superimposed across the entire continental block with high-pressure granulites and eclogites in the north, and medium-pressure granulites across the whole craton. The scale of this event is similar to that of the present day India-Asia collision, which has an across-strike width of 1,000 km, and a duration (~90 Ma), which is similar to that of the on-going Alpine-Himalayan collision that has been active for the last ~50 Ma. The deep crustal granulites and mapped volcanic rocks interpreted to be anatectic melts from deep crustal granulites on the surface today are similar to those that are considered to be presently forming at mid-crustal levels beneath Tibet, with high-grade metamorphism and the generation of partial melts. Analysis of structural fabrics in lower-crustal migmatites related to this event reveals that they flowed laterally parallel to the collision, much like what is hypothesized to be happening in the deep crust of the Himalayan/Tibetan foreland. We relate this continent-continent collision to the collision of the North China Craton with the postulated Columbia (Nuna) Continent. The NCC broke out of the Columbia Continent between 1753-1673 Ma, as shown by the formation of a suite of anorthosite, mangerite, charnockite, and alkali-feldspar granites in an ENE-striking belt across the north margin of the craton, whose intrusion was followed by the development of rift and graben systems, mafic dike swarms, and eventually an Atlantic-type marginal sequence that signaled the beginning of a long period of tectonic quiescence and carbonate deposition for the NCC during Sinian times which persisted into the Paleozoic. The style of tectonic accretion in the NCC changed at circa 2.5 Ga, from an earlier phase of accretion of arcs presently preserved in horizontal lengths several hundred kilometers long, to the accretion and preservation of linear arc blocks several thousand kilometers long with associated oceanic plateaus, microcontinents, and accretionary prisms. The style of progressively younger outward accretion of different tectonic elements is reminiscent of the style of accretion in the Superior Craton, and may signal the formation of progressively larger landmasses at the end of the Archean (perhaps into the time of the Kenorland Continent), then into the Paleoproterozoic, culminating in the assembly of the Columbia (Nuna) Continent at 1.9-1.8 Ga

Median rule and majoritarian compromise

Ankara : The Department of Economics, İhsan Doğramacı Bilkent University, 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references.In this thesis, we analyze the relationship between Majoritarian Compromise (Sertel & Yılmaz, 1984) and the Median Rule (Basset & Persky, 1999). We show that, for the populations with odd size, these two rules are equivalent and we describe the relationship for the case where population size is even. Then, we explore some axiomatic properties of Median Rule. It turns out that Median Rule satisfies all properties that Majoritarian Compromise satisfies in Sertel and Yılmaz (1999) and it fails all properties that Majoritarian Compromise fails in Sertel and Yılmaz (1999). We, then, introduce two axioms which differentiate these rules. We conclude that, the Median Rule can be considered as a viable alternative to Majoritarian Compromise, as it satis- fies all axioms that Majoritarian Compromise is known to satisfy except one particular axiom.Polat, Ali OğuzM.S

Geodynamics of the late Archean Wawa Subprovince Greenstone Belts, Superior Province, Canada

The late Archean (ca. 2.80-2.68 Ga) Schreiber-Hemlo and White River-Dayohessarah greenstone belts of the Superior Province, Canada, are collages of komatiite to tholeiitic, transitional and alkaline basalt ocean plateau sequences; tholeiitic and calc-alkaline basalt to rhyolite volcanic island arc sequences; and arc-derived, syn-kinematic siliciclastic trench turbidites accreted along a SSE-facing convergent plate margin through compressional and transpressional collisions. These subduction-accretion complexes were collectively intruded by the syn-kinematic high-La/Ybn, high-Al, high-Na slab-derived granitoids, syn-kinematic mantle wedge-derived gabbros, and late- to post-kinematic lamprophyres. A rich compositional diversity of komatiites and basalts is present in ocean plateau sequences. The diverse major and trace element compositions of ocean plateau sequences are consistent with a mantle plume that was chemically heterogenous. Mafic to felsic volcanic arc sequences are characterized by coeval tholeiitic and calc-alkaline magma series with variable major and trace element systematics. Both the tholeiitic and calc-alkaline suites were derived from metasomatized subarc mantle wedge sources. Slab melting was a major source of the felsic suite with garnet ± clinopyroxene ± homblende residual in the source. The Schreiber-Hemlo and White River-Dayohessarah greenstone belts have undergone three major phases of deformation. The earliest phase of deformation (D1) is defined by primarily rotated thrust faults: D1 reflects tectonic imbrication of oceanic plateaus, island arcs, and arc-derived turbidites in a subduction-accretion complex. D2 transpression resulted in generation of broken formations and a tectonic melange. The significance of D3 in the study area is thought to be subprovince accretion. The amalgamation processes of the lithotectonic assemblages in the late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts are comparable to those of Phanerozoic subduction-accretion complexes, such as the Circum-Pacific, the western north American Cordilleran, and the Altaid orogenic belts, suggesting that subduction-accretion processes significantly contributed to the growth of the continental crust in the late Archean. The episodic growth of the late Archean Superior Province continental crust may have resulted from major plume activities associated with mantle overturn and major orogenies (MOMO)