Evidence of Supercontinent Wilson Cycle From the Central Tianshan Arc Terrane, NW China

Geochemistry and geochronology of the Precambrian high-grade metamorphic complex in the Southern Central Tianshan ophiolitic mélange, NW China

Authors:

Wang et al

Abstract:

The Central Tianshan Arc Terrane is one of the major constituents of the Tianshan orogen in the southwestern Altaids. Thus its Precambrian evolution is important for unraveling the geodynamic and continental evolution of the Altaids. Biotite-plagioclase-hornblende orthogneisses intruded by leucogranite dykes are exposed as exotic blocks within the Southern Central Tianshan ophiolitic mélange. Zircon U–Pb age dating of one orthogneiss sample yields an upper intercept age of 2466 ± 51 Ma and a weighted mean 207Pb/206Pb age of 1812 ± 19 Ma around the lower intercept. These ages are interpreted as protolith crystallization age and subsequent metamorphic overprint age, respectively. The orthogneisses have moderate SiO2 contents, high MgO and Na2O contents, high Sr/Y ratios (32–43) and moderately fractionated REE patterns ([La/Yb]N = 7.59–13.64) with negligible Eu anomalies, enriched Sr contents and pronounced negative Th–U, Nb–Ta, Zr–Hf and Ti anomalies resembling high-SiO2 TTGs derived from subducted basaltic slab-melts. Positive ɛHf(t) values (+3.4 to +9.2) and low (176Hf/177Hf)i ratios (0.281304–0.281469) with Neoarchaean to early Paleoproterozoic single-stage Hf model ages (TDM1 = 2431–2652 Ma) suggest that the orthogneisses probably originated from partial melting of juvenile subducted oceanic crust. The orthogneisses were subsequently also affected by collisional orogenic events associated with the assembly of the supercontinent Columbia. In contrast, the zircon U–Pb age of 785 ± 15 Ma obtained for the intrusive leucogranite dykes is consistent with the timing of rifting events associated with the breakup of Rodinia. The leucogranites have a high-Al trondhjemitic composition characterized by extremely low MgO and K2O contents as well as high SiO2, Al2O3 and Na2O contents, dramatically low ∑REE abundances and REE patterns with moderate LREE enrichments ([La/Yb]N = 3.55–8.4) and pronounced positive Eu anomalies (Eu/Eu* = 1.29–2.61). The high zircon initial Hf compositions (0.281670–0.281841) and negative ɛHf(t) values (-21.7 to -15.4), in conjunction with high whole-rock initial 87Sr/86Sr ratios (0.70737–0.70751) and negative ɛNd(t) values (-4.7 to -5.1) suggest that the leucogranite resulted from reworking of ancient lower crust. Based on the presented data we conclude that the Central Tianshan Arc Terrane has undergone three tectonothermal events, namely ∼2.5 Ga continental crustal growth, ∼1.8 Ga collision related to the Columbia assembly and ∼785 Ma crustal reworking due to the Rodinia breakup. The Central Tianshan Arc Terrane, which experienced the same tectonic events as the Tarim Craton during the Precambrian, is considered to be a micro-Precambrian block that separated from the Tarim Craton during the Rodinia breakup.

Isotopic Evidence Suggests North China Craton Formed Prior to NeoArchean

Archean-Paleoproterozoic crustal evolution in the eastern North China Craton: Zircon U-Th-Pb and Lu-Hf evidence from the Jiaobei terrane

Authors:

Wang et al

Abstract:

The Precambrian metamorphic basement of the North China Craton (NCC) records a complex evolutionary history during the Neoarchean and the Paleoproterozoic. The Jiaobei terrane provides one of the best regions to evaluate the early Precambrian crustal growth of the Eastern NCC. Here we report the results of U-Pb zircon geochronology and Hf isotopes of tonalite-trondhjemite-granodiorite (TTG) gneiss and associated mafic rocks from the Jiaobei terrane. SIMS dating of zircons from the TTG gneiss shows that the protolith was formed at 2912 ± 6 Ma and subjected to anatexis at 2488 ± 9 Ma. Zircons from the mafic rocks yield three age populations at 2484 ± 7 Ma, 1855 ± 5 Ma and 1836 ± 3 Ma. Based on the differences in morphology, internal texture, and the U-Th compositions between magmatic and metamorphic zircons in the mafic suite, we interpret the 2484 ± 7 Ma and 1836 ± 3 Ma ages obtained from two meta-mafic intrusions as the crystallization age of their magmatic precursors, and the 1855 ± 5 Ma from a garnet amphibolite as the metamorphic age. Magmatic zircons from TTG gneiss and meta-gabbro yield Hf model ages ranging from 3.2 to 2.8 Ga, suggesting an important period for crustal growth during Mesoarchean. The earlier mafic magmatic event (2.49 Ga) involved magma generation from a depleted mantle, whereas the later phase (1.84 Ga) was derived from differentiation of a parent magma that originated from the partial melting of the Mesoarchean lower curst. The newly grown metamorphic zircons in the garnet amphibolite show initial 176Hf/177Hf and TDMC values from 0.281211 to 0.281343 and from 3.11 to 3.42 Ga, indicating that the Mesoarchean and older crustal materials reworked at 1.86 Ga.

The data reported in this study, coupled with previous geochronological and Lu-Hf isotopic data from other localities of the eastern NCC on the widespread emplacement of the mafic magmas and the coeval high-grade metamorphism or melting events at 2.50-2.48 Ga suggest that these regions were already part of a coherent tectonic unit with a common crustal evolution history during the end of Neoarchean.

Hf-Nd Isotope Decoupling in Neoarchean Sea Water Likely From Weathering of Emerging Continents

Decoupled Hf-Nd isotopes in Neoarchean seawater reveal weathering of emerged continents

Authors:

Viehmann et al

Abstract:

Marine chemical sediments from the Temagami banded iron formation (BIF) in Canada exhibit nonchondritic Zr/Hf and Y/Ho ratios and seawater-like rare earth element patterns, indicating that their Hf and Nd are not detrital, but derived from seawater. This is confirmed by Sm-Nd and Lu-Hf isochron ages of 2605 ± 140 Ma (initial εNd +0.03 ± 4.1) and 2760 ± 120 Ma (initial εHf +7.2 ± 5.3), respectively, that overlap within error the 2.7 Ga U-Pb age of associated igneous rocks. The Temagami BIF is therefore an excellent archive of the Nd-Hf isotopic composition of Neoarchean seawater. Whereas εNd2.7Ga values cluster around +1, εHf2.7Ga values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an εHf-εNd distribution is typical of modern seawater, plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks. The only mechanism known to produce natural waters with decoupled Nd and Hf isotope compositions is the incongruent mobilization of Hf from continental crustal material. Therefore, input of such highly radiogenic Hf into seawater requires substantial amounts of evolved Neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering.

Evidence of Mature Plate Tectonics in the NeoArchean

Neoarchean metagabbro and charnockite in the Yinshan block, western North China Craton: Petrogenesis and tectonic implications

Authors:

Zhang et al

Abstract:

Properly calibrating the magmatic record of mantle and crustal origin in Archean granite-greenstone terranes is crucial for understanding the petrogenetic and geodynamic processes that generated early continental landmasses. This geochronological and geochemical study documents a Neoarchean bimodal metaplutonic suite of metagabbro and charnockite from the Yinshan block, North China Craton. The meta-gabbroic rocks show an SiO2 range from 45.9 to 51.7% and high MgO content from 6.7 to 16.4%, with enrichment in Ba and light rare earth elements (LaN/YbN = 3.11–5.99) and depletion in high field strength elements. Together with their enriched whole-rock Nd (ɛNd(t) = -0.23 to 1.49) and zircon Hf (ɛHf(t) = 0.1 to 6.9) isotopic signatures relative to the Archean depleted mantle in the craton, these rocks are supposed to originate from the second-stage high-temperature partial melting of refractory depleted mantle that experienced prior basaltic magma extraction and subsequent metasomatism by subduction-related fluids. The associated charnockites range in SiO2 from 61.3 to 69.6% and exhibit a magnesian, calc-alkalic and metaluminous character, with variable Sr/Y and LaN/YbN ratios. These elemental features, plus their evolved isotopic compositions (ɛNd(t) = 0.93 to 1.85, zircon ɛHf(t) = -0.3 to 2.5), are consistent with partial melting of newly underplated mafic lower crustal protolith. In combination with widespread occurrence of metasomatized lithospheric mantle- derived magmas (e.g., high-Mg basalts, sanukitoid suites) and juvenile crust-extracted potassic granites in the NCC during Late Neoarchean time, such a mafic and felsic magma association not only attests to the establishment of a craton-scale subduction-related metasomatized sub-continental lithospheric mantle, but also encapsulates a scenario of coupled lithospheric mantle-crust formation at ∼2.7Ga and juvenile crustal reworking at ∼2.5 Ga within a modern-style convergent continental margin possibly featuring episodic slab break-off events.

Evidence of NeoArchean Volcanic Islands Arcs in India

Zircon U-Pb geochronology and Hf isotope of felsic volcanics from Attappadi, southern India: Implications for Neoarchean convergent margin tectonics

Authors:

1. M.N. Praveen (a)
2. M. Santosh (b)
3. Q.Y. Yang (b)
4. Z.C. Zhang (b)
5. H. Huang (b)
6. S. Singanenjam (c)
7. K.S. Sajinkumar (d)

Affiliations:

a. Geological Survey of India, Dharani Bhawan, Manikanteshwaram PO, Thiruvananthapuram 695 013, India

b. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China

c. Geological Survey of India, Rajaji Bhavan, Besant Nagar, Chennai 600 090, India

d. Department of Geology, University of Kerala, Thiruvananthapuram 695 581

Abstract:

The Attappadi area on the south-western flanks of the Archean Dharwar Craton in southern India is located along the E-W trending Bhavani Shear Zone which marks the trace of a Neoarchean suture zone. The dominant rock types in the area include meta-ultramafics, amphibolites, TTG (tonalite-trondhjemite-granodiorite) gneisses, metapelites, and sulphidic banded iron formation (BIF). Here we report the occurrence of felsic volcanic rocks preserving primary textures from the Anaikatti area in eastern Attappadi. The felsic volcanics are interbanded with the BIF, amphibolite, metapelite, metapyroxenite and hornblende gneisses. The felsic volcanics are divided into two types based on their textures. The Type-1 rock is medium grained with gneissic texture and lack unequivocal primary volcanic textures. Type-2 felsic volcanics are thinly laminated, fine grained and at places preserve relict soft-sediment deformation structures. They also contain relict volcanic clasts or lapilli and are interpreted as felsic tuff. Geochemically, the Attappadi felsic volcanics are rhyolitic in composition and have arc-related trace element signatures. They also possess a calc-alkaline volcanic affinity.

We report LA-ICPMS U-Pb ages from zircons in four samples of the felsic tuffs which show weighted mean ages of 2567 ± 18 Ma and (MSWD = 1.4), 2499 ± 19 Ma (MSWD = 0.57), 2555 ± 24 Ma (MSWD = 1.7) and 2576 ± 64 Ma (MSWD = 5.8). The late Neoarchean – early Paleoproterozoic ages obtained in our study correlate well with the zircon U-Pb ages reported in recent studies from ophiolites and other suprasubduction suites from Attappadi and surrounding regions. The zircon εHf values range from -11.1 to 7.6 suggesting heterogeneous source material involving both juvenile and older reworked components. We build a tectonic model for the SW margin of the Dharwar Craton with an oceanic realm characterized by island arcs and widespread submarine tholeiitic as well as komatiitic ultramafic and mafic volcanism during the Neoarchean. This predominantly mafic volcanism on the ocean floor is represented by primitive komatiitic lavas, oxide and sulphide facies BIF. The birth of volcanic arc at the convergent margin is marked by felsic volcanism and the deposition of felsic volcanics and volcano-sedimentary successions. In the final stage of ocean closure, the ocean-plate and continental arc assemblages were brought in juxtaposition including the accretion of the ophiolitic fragments. Our study confirms the recent models of arc-arc and arc-continental accretion to the southern margin of the Dharwar Craton and major continental growth at the end of the Archean.

NeoArchean Tumbiana Formation Conical Stromatolites Were From Phototrophic Microorganisms?

Sedimentology, stratigraphy and geochemistry of a stromatolite biofacies in the 2.72 Ga Tumbiana Formation, Fortescue Group, Western Australia

Authors:

1. J.M. Coffey (a)
2. D.T. Flannery (a)
3. M.R. Walter (a)
4. S.C. George (a, b)

Affiliations:

a. Australian Centre for Astrobiology, School of Biotechnology and Biomolecular Sciences, University of New South Wales, New South Wales 2052, Australia

b. Department of Earth and Planetary Sciences, Macquarie University, New South Wales, 2109, Australia

Abstract:

The 2.72 Ga Tumbiana Formation is a succession of clastic and carbonate rocks outcropping along the southern margin of the Pilbara Craton in Western Australia. It hosts abundant, diverse and exceptionally well-preserved stromatolites and has provided the setting for numerous investigations focussing on the Archean biosphere. Despite its palaeobiological significance, the overall depositional setting of the Tumbiana Formation remains unclear. Here we present the results of stratigraphic, sedimentological and geochemical investigation of the Tumbiana Formation in the well-known Redmont/Knossos area and at several localities in the northwestern Pilbara sub-basin. We suggest these data are best explained by deposition in fluvial and lacustrine environments of an inward-draining continental basin. δ13Corg values vary from -49.9‰ to -15.0‰. Conical stromatolite morphologies, commonly attributed to cyanobacteria, are anomalously little depleted in 13Corg, implying a higher relative contribution of organic matter from phototrophic versus methane cycling metabolisms.

Carbon Dioxide Depleted in Late Archean Sea Water

Decrease of seawater CO2 concentration in the Late Archean: An implication from 2.6 Ga seafloor hydrothermal alteration

Authors:

1. Takazo Shibuya (a, b, c)
2. Miyuki Tahata (d)
3. Yuichiro Ueno (d)
4. Tsuyoshi Komiya (e)
5. Ken Takai (a, b, f)
6. Naohiro Yoshida (g, h)
7. Shigenori Maruyama (e)
8. Michael J. Russell (c)

Affiliations:

a. Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

b. Submarine Hydrothermal System Research Group, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

c. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

d. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

e. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

f. Subsurface Geobiology Advanced Research (SUGAR) project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

g. Department of Environmental Science and Technology, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan

h. Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan

Abstract:

Before continents attained a critical aerial dimension on the early Earth, hydrothermal carbonation of subseafloor crust is considered to have played the dominant role in fixing CO2 from the CO2-rich ocean. However, it is uncertain how and when the seawater CO2 level decreased and the strong carbonation of oceanic crust ceased. Here we report the depth profiles of the volume concentration and the carbon isotopes of calcites in the Late Archean/Paleoproterozoic volcanic rocks (Fortescue and Hamersley groups), exposed in the southwestern Pilbara Craton, Western Australia. The depth profiles indicate that 2.6 Ga seafloor hydrothermal carbonation is well preserved in the study area and that the CO2 content of subseafloor crust per seafloor unit area is estimated to be clearly lower than those in the Early and Middle Archean and similar to the Phanerozoic equivalents. This suggests that the CO2 concentration in seawater decreased from the Middle Archean to the Late Archean. This period broadly corresponds to the time of the first appearance of supercontinent on Earth. The amalgamation of continents has the potential to decrease seawater CO2 concentration due to the removal of platform carbonate to continental interior. Subsequent fragmentation of supercontinent likely cause the carbonate deposition around newly created continental shelves. It is therefore implied that seawater CO2 concentration in the early Earth was lowered by not only the hydrothermal carbonation of subseafloor crust but also through the formation and breakup of supercontinent in the Late Archean.

Suflate Signatures from NeoArchean May be Masking Evidence of Sulfate Reducing Bacteria

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes

Authors:

1. James Farquhar (a,b)
2. John Cliff (b)
3. Aubrey L. Zerkle (c)
4. Alexey Kamyshny (d)
5. Simon W. Poulton (e)
6. Mark Claire (f)
7. David Adams (b)
8. Brian Harms (a)


Affiliations:

a. Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742

b. Centre for Microscopy and Microanalysis, University of Western Australia, Perth, WA 6009, Australia

c. School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom

d. Department of Geological and Environmental Sciences, Faculty of Natural Sciences, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

e. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom

f. School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom

Abstract:

It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.

New Neo-Archaen Microbial Evidence

An analysis of sulfide ore deposits from one of the world's richest base-metal mines confirms that oxygen levels were extremely low on Earth 2.7 billion years ago, but also shows that microbes were actively feeding on sulfate in the ocean and influencing seawater chemistry during that geological time period.

The research, reported by a team of Canadian and U.S. scientists in Nature Geoscience, provides new insight into how ancient metal-ore deposits can be used to better understand the chemistry of the ancient oceans – and the early evolution of life.

Sulfate is the second most abundant dissolved ion in the oceans today. It comes from the "rusting" of rocks by atmospheric oxygen, which creates sulfate through chemical reactions with pyrite, the iron sulfide material known as "fool's gold."

The researchers, led by PhD student John Jamieson of the University of Ottawa and Prof. Boswell Wing of McGill, measured the "weight" of sulfur in samples of massive sulfide ore from the Kidd Creek copper-zinc mine in Timmins, Ontario, using a highly sensitive instrument known as a mass spectrometer. The weight is determined by the different amounts of isotopes of sulfur in a sample, and the abundance of different isotopes indicates how much seawater sulfate was incorporated into the massive sulfide ore that formed at the bottom of ancient oceans. That ancient ore is now found on the Earth's surface, and is particularly common in the Canadian shield.

The scientists found that much less sulfate was incorporated into the 2.7 billion-year-old ore at Kidd Creek than is incorporated into similar ore forming at the bottom of oceans today. From these measurements, the researchers were able to model how much sulfate must have been present in the ancient seawater. Their conclusion: sulfate levels were about 350 times lower than in today's ocean. Though they were extremely low, sulfate levels in the ancient ocean still supported an active global population of microbes that use sulfate to gain energy from organic carbon.

"The sulfide ore deposits that we looked at are widespread on Earth, with Canada and Quebec holding the majority of them," says Wing, an associate professor in McGill's Department of Earth and Planetary Science. "We now have a tool for probing when and where these microbes actually came into global prominence."