The Strontium Cycle of the NeoProterozoic was Driven by Seemingly Unique Paleogeography

Paleogeographic forcing of the strontium isotopic cycle in the Neoproterozoic

Authors:

Goddéris et al

Abstract:

The period spanning from 825 to 540 Ma is characterized by major changes in the surficial Earth system. This extraordinary interval starts with the breakup of the Rodinia supercontinent and eruption of a series of large igneous provinces and ends with the assembly of Gondwana, giving rise to the Pan-African orogenies. This paleogeographic reorganization is accompanied by a global climatic cooling, including the paroxysmal Cryogenian “snowball” glacial events. The 87Sr/86Sr of seawater displays a major long-term rise over this interval that is punctuated by episodic, smaller declines and inflections. We use a coupled deep time climate-carbon numerical model to explore the complex role of tectonics and climate on this distinct evolution in seawater 87Sr/86Sr. We show that the modulation of the weathering of the erupted large igneous provinces by continental drift explains the changes in seawater 87Sr/86Sr from 800 to 635 Ma. The subsequent sharp rise in seawater 87Sr/86Sr from 635 to 580 Ma is the result of erosion of radiogenic crust exposed in the Pan-African orogens. Coeval evolution of atmospheric CO2 displays a decrease from about 80 times the pre-industrial level around 800 Ma to 5 times just before the beginning of the Phanerozoic.

Calymmian MesoProterozoic Nuna/Columbia Supercontinent Reconstructed

Paleogeography of the Congo/São Francisco craton at 1.5 Ga: expanding the core of Nuna supercontinent

Authors:

Salminen et al

Abstract:

The Congo/São Francisco (C/SF) craton, one of the largest cratons in Proterozoic paleogeography, has been lacking reliable paleomagnetic data for the supercontinent Nuna interval (ca. 1600-1300 Ma). Here we provide a new paleomagnetic key pole for this craton from recently dated mafic dykes in the Curaçá (1506.7±6.9 Ma) region of Brazil. The characteristic remanent magnetization (ChRM) direction D=070.6°, I=54.0° (k=22.1 and a95=13.1°) corresponds with a paleomagnetic pole at 10.1°N, 009.6°E (K=15.6, A95=15.8°), which places C/SF craton in moderate paleolatitudes at the time of remanence acquisition. Primary nature of the paleomagnetic remanence is supported by a baked-contact test. A similar ChRM direction was obtained for four Mesoproterozoic mafic intrusions in Chapada Diamantina region. The new pole, only from Curaçá, for C/SF allows us to reconstruct the extended core of the supercontinent Nuna at 1.5 Ga. Based on coeval 1.5 Ga and 1.38 Ga magmatism in Baltica, Siberia and C/SF, we favor the position where Southwest Congo is reconstructed against present South-Southeast (S-SE) Baltica. We explore two alternative 1.5 Ga reconstructions of Nuna’s core. In both of them Baltica and Laurentia are shown in the well-defined NENA (Northern Europe North America) fit, together with Siberia in a tight fit to northern Laurentia. In reconstruction option A, more traditional fit of Amazonia with Baltica is shown, modified from the geologically based SAMBA (South AMerica BAltica) model to accommodate paleomagnetic data. In this option, however, West Africa must be extricated from SAMBA because C/SF has taken its place. For reconstruction option B, Amazonia is shifted to lie adjacent to NE Laurentia and West Baltica. In both options SW Congo is reconstructed against S-SE Baltica, but in option B there is a tighter fit between them, and there is a better match with our new paleomagnetic data for C/SF. In either option, separation of C/SF from Baltica and Siberia probably occurred at 1.38 Ga, the age of pronounced mafic magmatism throughout this sector of Nuna.

Sino-Korean Block was Located at the Northern Margin of East Gondwana Across the Ediacaran/Cambrian Boundary

Detrital zircon geochronology and Nd isotope geochemistry of the basal succession of the Taebaeksan Basin, South Korea: Implications for the Gondwana linkage of the Sino-Korean (North China) block during the Neoproterozoic–early Cambrian

Authors:

Lee et al

Abstract:

The paleogeographic configuration of continental blocks around East Gondwana during the Neoproterozoic–early Cambrian is controversial. This study reports the U–Pb ages of detrital zircons and Nd isotopic composition of the Neoproterozoic–early Cambrian succession developed on the Precambrian basement in South Korea, which formed the southeastern portion of the Sino-Korean block (SKB) in its present configuration. Three stratigraphic units are addressed in this study: the Neoproterozoic Jangsan, early Cambrian Myeonsan, and early–middle Cambrian Myobong Formations. Both the Jangsan (white to pink quartz sandstone) and Myeonsan (dark gray ilmenite-rich sandstone/shale) formations are barren and are unconformably and conformably overlain, respectively, by the dark gray, fossiliferous fine-grained Myobong Formations. The Jangsan and Myeonsan Formations contain zircons with Archean–Paleoproterozoic ages, indicative of detritus derived from the local Precambrian basement. In contrast, the Myobong Formation is dominated by Mesoproterozoic to Neoproterozoic zircons, which are not represented in the local Precambrian basement. The Sm–Nd model ages of the Myobong Formation are younger than those of the underlying strata, indicative of significant changes in provenance during the deposition of this formation. Comparison with coeval sediment having Gondwana signatures in the southern margin of the SKB and the Tethyan Himalayan terrane strongly suggests that the Myobong Formation was derived from orogens in East Gondwana. The results of this study reveal that the timing of the Myobong Formation deposition marks the onset of a sedimentation episode on the southeastern margin of the SKB, which was related to the emergence of a vast source province in East Gondwana, possibly aided by the Cambrian transgression onto the SKB. In comparison with the published literature, we argue for the paleogeographic continuity of the SKB with the northern margin of East Gondwana, possibly between northwestern Australia and northeastern India during the Neoproterozoic–early Cambrian.

The PaleoPosition of the Amazonian Craton in the Calymmian MesoProterozoic

Reassessment of Aguapeí (Salto do Céu) Paleomagnetic pole of the Amazonian Craton and implications for Proterozoic supercontinents

Authors:

D’Agrella-Filho et al

Abstract:

The Aguapei paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U-Pb age on baddeleyites at 1439 ± 4 Ma for the intrusives, constrasts strongly with a previous Ar-Ar age at 981 ± 2 Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm = 208.2°, Im = 68.5° (N = 8, α95 = 6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95 = 10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95 = 7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420-1430 Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

There was no "‘Northern Branch of Neotethys” During the Jurassic

Where are the remnants of a Jurassic ocean in the eastern Mediterranean region?

Authors:

Eyuboglu et al

Abstract:

The subduction polarity of Tethyan oceanic lithosphere during Jurassic is a controversial topic in relation to the geodynamic evolution of the Alpine–Himalayan system. We present new geological, geochemical and zircon U–Pb data from four different regions of the Eastern Pontides Orogenic Belt, a key area of the Alpine–Himalayan system. We discuss the origin of the magmatism and also the existence of an ocean in the eastern Mediterranean region during the Jurassic period. Jurassic intrusions, predominantly gabbro, tonalite and minor diorite, are well exposed in the southern and axial zones of the orogenic belt. Thermobarometry indicates that high-pressure (6–10 kb) crystallization of these intrusions occurred at temperatures of 1183–1250 °C. Zircon U–Pb dating from 10 samples show ages between 195 and 165 Ma, indicating that magmatism occurred between Sinemurian and Callovian time. We characterize the intrusions from electron microprobe, zircon geochronology, and whole rock and Sr, Nd, and Pb isotopes. Our data show that the studied intrusions are broadly tholeiitic, except for two calc-alkaline bodies, and formed in an arc-related setting with minimal involvement of older crust or sediment.

The most widely accepted model proposes that the ultramafic–mafic rocks exposed between the Pontide arc and the Tauride belt are remnants of a Jurassic Penrose-type and/or suprasubduction zone ophiolite. However, new zircon U–Pb age data from mafic lithologies cutting the Kop ultramafic massif do not support this model and clearly indicate that the ultramafic lithologies are Paleozoic or older in age and are not remnants of a Jurassic ocean that known as ‘’Northern Branch of Neotehtys”.

Double Impact Crater Found in Sweden From Floian/Dapingian Ordovician, Possibly Linked to Proposed Ordovician Meteor Event

Double rainbows have nothing on Earth’s newest dynamic duo: the double crater.

Researchers from the University of Gothenburg have uncovered two impact craters in Jämtland, Sweden, which they believe to have occurred simultaneously around 460 million years ago. Double meteor impacts are rare events, and the discovery in Sweden is the first proven instance of its kind.

“Information from drilling operations demonstrates that identical sequences are present in the two craters, and the sediment above the impact sequences is of the same age. In other words, these are simultaneous impacts,” said Erik Sturkell, professor of geophysics at the University of Gothenburg and a member of the team that found the double crater, in a university statement.

Although these two meteorites struck at the same time, that does not mean they are physically alike, however. One crater measures a massive 4.7 miles in diameter, while the other, which was located nearly ten miles away, was a much a smaller 2,300 feet across.

As for how it happened, it all started in the stars.

“Around 470 million years ago, two large asteroids collided in the asteroid belt between Mars and Jupiter, and many fragments were thrown off in new orbits. Many of these crashed on Earth, such as these two in Jämtland,” said Sturkell.

link.

There are several impacts in North America and another in Estonia that roughly line up.  This hypothesized event is termed the "Ordovician Meteor Event."  They are all supposedly within a million years of each other and I wonder though whether or not they line up given the paleogeography. 

Central America Connected to South America Much Earlier During the Middle Miocene Neogene

Middle Miocene closure of the Central American Seaway

Authors:

Montes et al

Abstract:

Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.

Diversity patterns of Devonian Plant Zosterophyllopsida Tied to PaleoGeography

Diversity patterns of the vascular plant group Zosterophyllopsida in relation to Devonian palaeogeography

Authors:

Cascales-Miñana et al

Abstract:

The Zosterophyllopsida originated in the Silurian and became prominent vascular components of Early Devonian floras worldwide. An updated dataset of zosterophyllopsids at species level is analysed to compare the taxic composition of five putative palaeophytogeographic units, Laurussia, Siberia, northwestern Gondwana, Kazakhstan and northeastern Gondwana (i.e. Australia, China and the Shan-Thai block). The high level of endemicity shown by each unit confirms the phytogeographic differentiation and the occurrence of geographical barriers preventing massive floral exchanges between the corresponding regions for the Late Silurian-Early Devonian time interval. Statistical analyses conducted on the three largest datasets, those corresponding to Laurussia, Siberia and northeastern Gondwana, indicate that the diversity dynamics of the group followed the same pattern in these regions. Almost all taxic diversity measures show that specific diversity was greatest in the middle and late Pragian. Diversity dropped significantly thereafter; however, residual diversity reveals genuine regional patterns. From this, we show that the radiation of the Zosterophyllopsida may have stopped earlier in northeastern Gondwana and Siberia than Laurussia. We propose that the onset of these extinctions resulted from the competitive replacement of the zosterophyllopsids by increasingly diversified lycopsids and basal euphyllophytes whose evolution could have been favoured by external factors.

Early Cambrian Palaeobiogeography of the Zhenba-Fangxian Block

Early Cambrian palaeobiogeography of the Zhenba-Fangxian Block (South China): Independent terrane or part of the Yangtze Platform?

Authors:

Yang et al

Abstract:

Early Cambrian small skeletal fossils (SSFs) are studied and revised from the Zhenba-Fangxian Block of the transitional zone between the Yangtze Block and the South Qinling Terrane. The study reveals a diverse fauna with 47 species of various biological affinities, including the new species Gapparodus gapparites sp. nov. The SSFs are assigned to the newly defined Cambroclavus fangxianensis -Rhombocorniculum cancellatum Assemblage Zone. Based on the investigated SSF fauna from Zhenba County, Southeast Shaanxi of China and published data, a palaeobiogeographic study is carried out for the Cambrian Stage 3 (equivalent to the Atdabanian-Botoman of Siberia). A hierarchical Pearson similarity cluster analysis of 295 species from 32 regions of the world indicates a distinct palaeobiogeographic pattern with seven faunal provinces. The result is mostly consistent with existing palaeogeographic reconstructions for the early Cambrian. However, it is also shown that the SSF assemblages of the Zhenba-Fangxian Block have low similarity with those of the Yangtze Block. Instead, they share high similarity with those from Armorica, Tarim and the Karatau-Naryn terranes (South Kazakhstan/ North Kyrgyzstan). The Yangtze Block has a unique SSF assemblage dissimilar to most of other regions. The Terreneuvian-Cambrian Stage 3 sedimentary sequence of the Zhenba-Fangxian Block is more consistent with that of the South Qinling Terrane. Besides, sedimentary Ediacaran manganese ore deposits and Cambrian barite/ witherite deposits have unique distribution pattern on the Zhenba-Fangxian Block. Derived from the profound dissimilarities in faunal composition, sedimentary sequence and distribution of sedimentary ore deposits, we hypothesize that during the Neoproterozoic-Cambrian transition, the Zhenba-Fangxian Block might have been an independent terrane and more distant from the Yangtze Block. The palaeobiogeographic analysis of SSFs also indicates a closer alliance between Avalonia and Siberia. It corroborates the palaeogeographic reconstruction of North China at the margin of Gondwana, in the vicinity of Australia, Antarctica, and Armorica.

Evidence of Rodinia PaleoGeography From Cryogenian NeoProterozoic Madagascar

Geochronology and geochemistry of Cryogenian gabbros from the Ambatondrazaka area, east-central Madagascar: Implications for Madagascar-India correlation and Rodinia paleogeography

Authors:

Zhou et al

Abstract:

Tectonic setting of the ca. 840-760 Ma Imorona-Itsindro Suite throughout central Madagascar is critical to understanding Madagascar-India correlation in pre-Gondwana time and the paleogeography of the rifting Rodinia. Here we present new geochronological and geochemical data for gabbros of this suite from the Ambatondrazaka area in east-central Madagascar. SIMS U-Pb zircon dating reveals that the gabbros emplaced between ca. 797 and 772 Ma. They define a typical tholeiitic trend on the AFM and FeOT/MgO vs. SiO2 plots, which is confirmed by the occurrence of layered Fe-Ti-V oxide mineralization. Owing to the high-K nature, they were previously misinterpreted as a “calc-alkaline” association produced in a mature continental arc. Indeed, it implies in return an intra-plate affinity for tholeiitic rocks. Their high Zr/Y, La/Y and Ti/V ratios differ from those of typical arc and back-arc mafic suites but are compatible with an intra-plate setting. Additionally, they are characterized by low AlZ/TiO2 ratios in clinopyroxene, further supporting an intra-plate environment. The negative ɛNd(t) (-8.4 to -5.1) and zircon ɛHf(t) (-6.84 to -3.08) values, together with enriched trace element signatures, indicate their derivation from an enriched lithospheric mantle source. Considering the initial 87Sr/86Sr ratios (0.705409 to 0.706892) and Nd modal ages (∼2.1-2.4 Ga), we postulate that this mantle reservoir has EM1-type isotopic characteristics and was enriched by a Late Neoarchean “super accretion event”. Taken together, our results question the existence of the Betsimisaraka Suture but support the proposal that the Malagasy Shield and the Dharwar Craton of India are parts of the same entity–the Greater Dharwar Craton. Within this framework, the widely-cited hypothesis that an extensive Andean-type arc was developed along the northwestern margin of the rifting Rodinia is challenged, at least that the magmatic arc did not extend to the Malagasy Shield.

Paleo-position of the North China Craton Within the supercontinent Columbia

Paleo-position of the North China craton within the supercontinent Columbia: Constraints from new paleomagnetic results

Authors:

Xu et al

Abstract:

Several new paleomagnetic and geological studies focused on the reconstruction of the North China Craton (NCC) within the Paleo-Mesoproterozoic Columbia supercontinent. In spite of these new data, there are still widely divergent opinions regarding supercontinental reconstructions. In addition to qualitative correlations of orogenic belts, rift basin ages, stratigraphy and distribution of igneous provinces, paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe. In this paper, we report a detailed paleomagnetic study on the coeval ∼1780 Ma mafic dyke swarm and Xiong’er volcanic province, which extended for more than one thousand kilometers in the central NCC. Rock magnetic studies, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples, indicate that the dominant magnetic carriers are PSD magnetite. Stepwise thermal demagnetization isolated higher-temperature components directed to NNE/SSW with shallow inclinations from 37 sampling sites (16 sites in Yinshan area, 13 sites in Taihang area and 8 sites in Xiaoshan area). A baked contact test conducted on two Yinshan dykes intruded by a younger dyke demonstrates the magnetization in the Yinshan dykes pre-dates 1320 Ma. The existence of dual-polarity magnetizations in both Taihang and Xiong’er areas support our contention that the ChRM was acquired during the cooling of the magma. The primary origin of the ChRM is also supported by a positive fold test on the Xiong’er data, and a coherent regional test between the results from the Taihang and Xiong’er areas. Two different site-mean directions were compiled from these new results along with previous publications. The first direction, from the Taihang and Xiong’er areas, yields Declination (D)/Inclination (I) = 12.4°/−3.7° (κ = 20.5, α95 = 4.3°, N = 57 sites). The second, from the Yinshan area is at (D) 36.7°/(I)−12.4° (κ = 86.8, α95 = 2.7°, N = 32 sites). We argue that the difference is due to Mesozoic and/or Cenozoic vertical-axis rotation of the Taihang and Xiong’er areas with respect to the fixed Yinshan-Ordos basin. The corresponding paleopoles for the Yinshan dykes falls at 245.2°E/35.5°N (A95 = 2.4°). A comparison between the NCC, Laurentia, Siberia and Baltica is consistent with possible links between these four blocks in a perhaps, even larger, continent. The proximity of Australia and India to the NCC is also evaluated.

Cathayasian Block may Have Been Part of Gondwana During Cambrian

The Gondwana connection of South China: Evidence from monazite and zircon geochronology in the Cathaysia Block

Authors:

Zhang et al

Abstract:

The tectonic evolution of the Cathaysia Block in South China has been debated, with limited information to constrain the early Paleozoic geodynamics and paleogeography in the context of the global Gondwana supercontinent. In this study, we report the petrology of mafic magmatic suites and metamorphic units, together with zircon and monazite U − Pb ages from several key areas in Cathaysia. Combined with a comprehensive synthesis of the data reported in previous studies, we argue that the Cathaysia was an early Paleozoic collisional orogen, and speculate the possible linkage of South China with the Gondwana supercontinent. The monazite and zircon U − Pb data presented in this study including the metamorphic ages of the rocks in the Gaozhou complex in Yunkai massif, the Wanquan Group in central Wuyi massif, the garnet-sillimanite gneiss from the Chencai Group, and the garnet amphibolite from the Longyou Group in the northeastern section of Wuyi massif show ages ranging from 447 Ma to 430 Ma. Gabbroic intrusions in the Longyou Group and pyroxenites in the Chencai Group in northeastern Wuyi massif were emplaced at ca. 425 Ma and 470 Ma, respectively. Furthermore, the ca. 470 Ma pyroxenite was overprinted by hydrothermal alteration at ca. 400 Ma. Whole-rock geochemistry and mineral compositions show that the Chencai pyroxenite possesses features similar to that of the Alaskan-type mafic intrusions, whereas the 420 Ma gabbroic intrusions in the Long Group exhibit post-collision signature, which are consistent with the broadly coeval voluminous post-collision granites in Cathaysia. Comprehensive synthesis of zircon and monazite U − Pb ages of the metamorphic rocks and granites in Cathaysia lead us to suggest that the transition from collision to post-collision occurred at ca. 430 Ma.

The early Paleozoic structures, metamorphism and magmatic activity suggest that the NE-extending Cathaysia was a typical collisional orogenic belt rather than an intraplate type. The predominance of late Neoproterozoic – Cambrian detrital zircons in the early Paleozoic granites and pre-Devonian metamorphic rocks in Cathaysia indicates that the at least some clastics of the lower Paleozoic rocks in Cathaysia were derived from the Gondwana continents, suggesting that South China was at the fringe of the Gondwana supercontinent. The Nanhai terrane (including the Hainan massif) could represent the missing link between South China and Gondwana, with the Zhenghe − Dapu fault marking the suture zone between South China and the Nanhai Terrane.

Palaeoproterozoic Granites From India in Africa Have Implications for Gondwana Reconstructions

Palaeoproterozoic ancestry of Pan-African high-grade granitoids in southernmost India: Implications for Gondwana reconstructions

Authors:

Kroner et al

Abstract:

SHRIMP dating of magmatic zircons from granitoid gneisses) and charnockites of the Trivandrum and Nagercoil Blocks in the granulite terrane of southernmost India yielded well-defined protolith emplacement ages between 1765 and ca. 2100 Ma and also document variable recrystallization and/or lead-loss during the late Neoproterozoic Pan-African event at around 540 Ma. Hf-in-zircon and whole rock Nd isotopic data suggest that the granitoid host rocks were derived from mixed crustal sources, and Hf-Nd model ages vary between 2.2 and 2.8 Ga. A gabbroic dyke, emplaced into a charnockite protolith and deformed together with it, only contained metamorphic zircon whose mean age of 542.3 ± 4.0 Ma reflects the peak of granulite-facies metamorphism during the Pan-African high-graqde event. The Sm-Nd whole-rock isotopic system of several granitoid samples dated in this study was significantly disturbed during granulite-facies metamorphism, most likely due to a CO2-rich fluid phase. Whole-rock Nd model ages are consistently older than zircon-derived Hf model ages.

The Trivandrum and Nagercoil Blocks are reinterpreted as a single tectono-metamorphic terrane predominantly consisting of Palaeoproterozoic granitoids interlayered with supracrustal rocks that must be older than ca. 2100 Ma. Ductile deformation, migmatization and anatexis have obliterated the original rock relationships. These blocks probably have their counterpart in the Highland Complex of neighbouring Sri Lanka and in the high-grade Palaeoprotertopzoic terrane of southern Madagascar. We speculate that the southern Indian khondalites may have their counterparts in the khondalite belt of the North China Craton.

Evolution of the Paleo Tethys From the Devonian Through Permian

Devonian to Permian evolution of the Paleo-Tethys Ocean: New evidence from U-Pb zircon dating and Sr-Nd-Pb isotopes of the Darrehanjir-Mashhad “ophiolites”, NE Iran

Authors:

Moghadam et al

Abstract:

Middle to Late Paleozoic ophiolites, which are remnants of the Paleo-Tethys Ocean, are aligned in two main zones in northern Iran: Darrehanjir-Fariman-Mashhad, and Rasht in the north and Jandagh-Anarak ophiolites to the south. Our new U-Pb zircon dating results shows that the ~ 200km long Darrehanjir – Mashhad mafic-ultramafic body is not a single ophiolite but a composite igneous complex composed of Permian pillow lavas and pelagic sediments in fault contact with a small outcrop of Devonian intrusive and ultramafic rocks. Darrehanjir intrusive rocks have U-Pb zircon ages of 380.6 ± 3.7 Ma and 382.9±3.7 Ma respectively, ~ 100 Ma older than published ages for gabbros and radiolarites intercalated with lavas near Mashhad and Fariman. Mantle peridotites from the Devonian complex contain low Cr# spinel, similar to that in MORB-type peridotites. Devonian Darrehanjir gabbros and Permian Mashhad sequences both have boninitic and calc-alkaline signatures, respectively. The δ18Ozircon values from the Devonian ferrodiorite (δ18Ozircon~ 4.6±0.3‰) are slightly lower than the 5.2 to 5.4‰ expected for MORB-type zircons whereas Devonian plagiogranitic zircons mostly have δ18Ozircon < 5‰, perhaps reflecting involvement of hydrothermally altered crust. Similar, strongly positive values of zircon εHf(t) for plagiogranite (av. + 14.9) and ferrodiorite (av. + 13.8) indicate melt derivation from depleted asthenosphere. Darrehanjir-Mashhad ophiolitic rocks can be divided into groups with high εNd (> 10.3) and low εNd (< 5.4) for both Permian and Devonian suites. Most Darrehanjir-Mashhad rocks are characterized by radiogenic 207Pb/204Pb and 208Pb/204Pb, indicating the involvement of subducted terrigenous sediments in the source. The Mashhad-Darrehanjir mafic-ultramafic complex demonstrates that this part of Paleo-Tethys evolved from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. Iranian Paleozoic ophiolites and oceanic igneous complexes along with those of the Caucausus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins that were active from at least Early Devonian to Late Permian time.

The Amazon Basin Drained Through Mexico in the Norian Triassic

Early Mesozoic Southern Mexico-Amazonian connection based on U-Pb ages from detrital zircons: The La Mora Paleo-River in the Mixteca Terrane and its paleogeographic and tectonic implications

Authors:

Silva-Romo et al

Abstract:

The La Mora Formation is the oldest Mesozoic floodplain succession in the Mixteca Terrane of Southern Mexico. The presence of Amazonian detrital zircons in the La Mora Formation and in the overlying volcanic Diquiyú Unit indicates a major fluvial system that drained the Mixteca Terrane. The La Mora Paleo-River crossed the Oaxaquia microcontinent and the Mixteca Terrane prior to the breakup of Pangea, during Late Triassic-Early Jurassic time, when the Acatlán-Oaxaquia block was part of the northwestern portion of the Amazonian craton. Detrital zircons in the La Mora Formation have ages between 3307 ± 31 and 210 ± 12 Ma, which suggest that they originated in Amazonia and Southern Mexico: 33.2% of the detrital zircons have ages that are found only in Amazonian sources; whereas 66.7% of the zircons may be associated with either the Amazonian craton, the Andean basement, or Southern Mexico. We propose that the La Mora fluvial system drained the Amazon basin in a westward direction, with its mouth in central Pangea, and that it most likely fed the Tolimán submarine fan. The inferred location of the Acatlán-Oaxaquia tectonic block at the time of the La Mora fluvial system implies that the basement of Southern Mexico experienced right lateral displacement of at least 2300 km with respect to South America during the Mesozoic.

Evidence of PaleoProterozoic Supercontinent Kenorland From Australia

Paleomagnetism and U-Pb age of the 2.4 Ga Erayinia mafic dykes in the south-western Yilgarn, Western Australia: paleogeographic and geodynamic implications

Authors:

Pisarevsky et al

Abstract:

We present results from a paleomagnetic study of the previously undated Erayinia dykes intruding the south-western Yilgarn Craton. The U-Pb TIMs baddeleyite age of these dykes is now 2401 ± 1 Ma, which is about 10 m.y. younger than the 2418–2410 Ma Widgiemooltha dyke swarm. The paleomagnetic study isolated a stable primary remanence with steep downward direction, and the paleomagnetic pole (22.7°S, 150.5°E, A95 = 11.4°) is similar, but not identical to that of the previously studied Widgiemooltha dykes. We interpret this difference as the result of the movement of the Yilgarn Craton toward the pole at ∼ 1°/m.y angular speed, which is comparable with tectonic plates’ velocities during the Phanerozoic. Paleomagnetic polarities of Widgiemooltha and Erayinia dykes suggest that at least one geomagnetic reversal occurred between these two magmatic events. The estimated amplitude of geomagnetic secular variations at c. 2400 Ma is slightly higher than predicted by the existing models for the last 5 m.y. at the c. 64° latitude. The paleomagnetic data and patterns of c. 2.6–2.1 Ga mafic dyke swarms permit the recently suggested reconstruction of the Paleoproterozoic supercontinent

Was the Grawler Craton a Continental Margin During the NeoArchean Through PaleoProterozoic

SHRIMP U-Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia

Authors:

Reid et al

Abstract:

The Mulgathing Complex within the Gawler Craton, South Australia, preserves evidence for magmatism, sedimentation and metamorphism spanning the transition between the Neoarchean and Paleoproterozoic (ca. 2555–2410 Ma). Prior to this study, limited data were available to constrain the timing of these tectonothermal events. We report SHRIMP zircon U-Pb dating of metamorphosed sedimentary and magmatic rocks from the Mulgathing Complex that shows metasedimentary gneisses (Christie Gneiss) have maximum depositional ages ca. 2480 Ma, in contrast to previous studies that have suggested deposition had occurred ca. 2510 Ma. The oldest metamorphic zircons in our data are ca. 2465 Ma, thus indicating there was a time interval of less than 15 Myr between the cessation of sedimentation and the occurrence of high-grade metamorphism. Metamorphic zircons have a range of ages, from ca. 2465 to ca. 2415 Ma, consistent with a period of ∼50 Myr during which high-grade metamorphism occurred. Mafic and felsic intrusions have ages that range from ca. 2520 Ma to 2460 Ma, indicating magmatism occurred during sedimentation and continued during the early stages of metamorphism and deformation of these rocks. The presence of bimodal magmatism with continental arc affinity (Devils Playground Volcanics) together with mafic intrusions showing temporal overlap with sedimentation within the Mulgathing Complex suggests that the overall tectonic regime prior to deformation likely involved lithospheric extension, possibly related to a continental magmatic arc. The Mulgathing Complex shows similarities in the nature and timing of these tectonothermal processes with several other Neoarchean - Paleoproterozoic terranes, in particular the Terra Adelie Craton in Antarctica, the Sask Craton, Canada, and regions within the North China Craton and the North Australian Craton. It is possible that similarities in process could imply along-strike relationships between these terranes, for example as laterally continuous terranes at the margin of a former continental domain (or domains). Basement rocks with ages of ca. 3250–3150 Ma, ca. 2950–2800 Ma and 2600–2550 Ma within the Gawler Craton, as inferred from magmatic and inherited zircons, are temporally equivalent to some of the main basement units within the Pilbara Craton, and basement rocks in both the Gawler and Pilbara cratons are overlain by sedimentary rocks at ca. 2480 Ma that include banded iron formations. It is possible to envisage the Gawler Craton as representing an extended continental margin adjacent to a larger, more intact continental domain, such as the Pilbara.

Australia's Position in Supercontinents Columbia & Rodinia

Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic

Authors:

Medig et al

Abstract:

Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9-1.3 Ga, and Rodinia from ca. 1.1-0.75 Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499 ± 3 Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493 ± 8 Ma to 1508 ± 4 Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420 Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona.

Greenland Geography From Before the Glaciers Preserved Under the Ice

Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

Authors:

Bierman et al

Abstract:

Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been non-erosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.

Where WAS Iran During the Jurassic? Bookin!

A record of the Jurassic massive plate shift from the Garedu Formation of central Iran

Authors:

Mattei et al

Abstract:

Modern generations of apparent polar wander paths (APWPs) show the occurrence in North American and African coordinates of a major and rapid shift in pole position (plate shift) during the Middle to Late Jurassic (175–145 Ma) that alternative curves from the literature tend to underestimate. This Jurassic massive polar shift (JMPS), of vast and as-yet unexplored paleogeographic implications, is also predicted for Eurasia from the North Atlantic plate circuit, but Jurassic data from this continent are scanty and problematic. Here we present paleomagnetic data from the Kimmeridgian–Tithonian (upper Jurassic) Garedu Formation of Iran, which was part of Eurasia since the Triassic. Paleomagnetic component directions of primary (pre-folding) age indicate a paleolatitude of deposition that is in excellent agreement with the latitude drop predicted for Iran from APWPs incorporating the JMPS. Moreover, we show that paleolatitudes calculated from these APWPs, used in conjunction with simple zonal climate belts, better explain the overall stratigraphic evolution of Iran during the Mesozoic. As Iran drifted from the tropical arid belt to the mid-latitude humid belt in the Late Triassic, carbonate platform productivity stopped while widespread coal-bearing sedimentation started, whereas as Iran returned to arid tropical latitudes during the JMPS, carbonate platform productivity and evaporitic sedimentation resumed. These results illustrate (1) the potent, but often neglected, control that plate motion (continental drift and/or true polar wander) across zonal climate belts exerts on the genesis of sedimentary facies; and (2) the importance of precisely controlled paleogeographic reconstructions for tectonic interpretations, especially during times of fast plate motion like the Jurassic. As a suggestion for future research, we predict that the adoption of Eurasian reference paleopoles incorporating the JMPS may lead to a reconciliation (or reinterpretation) of existing geologic and paleomagnetic data regarding the deformation history of central Asia.