Evidence of a Suducting Crustal Slab From EoArchean Sulfur Isotopes

Multiple sulfur isotope signature of early Archean oceanic crust, Isua (SW-Greenland)

Authors:

Siedenberg et al

Abstract:

The Archean sulfur cycle was different from the present-day cycle, as the emission of volcanogenic sulfurous gases was the dominant process in the anoxic environment of the early Earth.This emitted sulfur exhibits mass-independently fractionated sulfur isotopes (MIF-S), resulting from photochemical reactions in the atmosphere, and it differs substantially from unfractionated sulfur in the mantle. So far, the main focus of multiple sulfur analyses (32S32S, 33S33S, 34S34S and 36S36S) was placed on the sedimentary part of the Archean sulfur cycle. In order to constrain the magmatic part of the sulfur cycle, we analyzed the sulfur isotopic composition of oceanic crustal rocks from the ca. 3.7–3.8 Ga Isua Supracrustal Belt (ISB). Differently altered samples were taken from two units:(1) the Undifferentiated Amphibolites (UA) and (2) the younger Amphibolites with Boninitic affinity (AB). The mean values are:δ34SCRS=+0.01±0.65‰δ34SCRS=+0.01±0.65‰ (values range from −0.87 to 1.37‰; CRS = chromium-reducible sulfur),Δ33SCRS=+0.02±0.12‰Δ33SCRS=+0.02±0.12‰ (values range from −0.17 to 0.26‰), Δ36SCRS=-0.47±0.06‰Δ36SCRS=-0.47±0.06‰ (values range from −0.56 to −0.38‰). Thus, the mean isotope values support the assumption that the sulfur isotopic signature reflects the expected near-zero signature of their mantle origin. However, differences in Δ33SCRSΔ33SCRS values are discernible and non-zero suggesting that different sources are contributing to the isotopic signature. An influence of alteration is excluded for all samples as different alteration-sensitive geochemical parameters do not show any correlation with the multiple sulfur isotope signatures. Further, it is unlikely that the small magnitudes in Δ33SCRSΔ33SCRS are generated by microbial mass-dependent processes because of the narrow range of δ34SCRSδ34SCRS values. Possible sources contributing an atmospheric MIF-S signature include seawater sulfate (negative Δ33SCRSΔ33SCRS values) through hydrothermal circulation, the assimilation of ocean floor sediments during the ascent of the melt and/or a mantle source contamination by subducted oceanic slab.

Subduction or sagduction in Scotland's Archean ultramafic–mafic bodies

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

Authors:

Johnson et al

Abstract:

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Palaeoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognized that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

More Evidence for Active Plate Tectonics and Subduction From NeoArchean/PaleoProterozoic North China

Petrology and geochemistry of the Guyang hornblendite complex in the Yinshan block, North China Craton: Implications for the melting of subduction-modified mantle

Abstract:

Ma et al

Abstract:

The hornblendite complexes hosted in the Guyang granite-greenstone terrane form part of the Neoarchean basement in the Western Block of the North China Craton. In this study, we focus on the largest one from this block, previously named the Guyang komatiite, and present results from lithological, geochronological and geochemical studies. The dominant lithology in the Guyang hornblendite complex is greenschist facies hornblendite, and can be divided into clinopyroxene hornblendite (∼75%) and olivine-orthopyroxene hornblendite (∼25%). The oldest calculated Re depletion model ages(TRD) of these hornblendites is of 2454 Ma, and the zircon U-Pb age of the wallrock is 2480 Ma, with single-stage depleted mantle Nd model ages TDM1(Nd) varying from 2.61 to 2.88 Ga. These data suggest that the Guyang hornblendite complexes formed during Neoarchean-Paleoproterozoic time. The hornblendites show low SiO2, high MgO contents and are enriched in Cr, Ni, and LREE with negative Ce anomalies and depleted in Ti, Nb and Ta. Their 187Os/188Os ratios range from 0.11145 to 0.11279, with γOs(2.5Ga) varying from −3.9 to +1.4. Geochemically, the olivine-orthopyroxene hornblendite shows little variation with typical cumulate feature. In contrast, the clinopyroxene hornblendite shows a large range of chemical variation. In the CaO vs. MgO and CaO/Al2O3 vs. MgO diagrams, the clinopyroxene hornblendite shows Opx fractionation trend. Combined with lithological information, we infer that the hornblendite magma was emplaced as a crystal mush, with orthopyroxene as cumulus phase. The Os and Nd isotope composition, negative Nb, Ta, Ce anomalies, and the relationships in Th/Yb-Nb/Yb diagram suggest that the Guyang hornblendites formed from a source mantle that was modified by subduction-related melts/fluids derived from a seawater-altered basaltic slab. Compared with typical Archean komatiites, the rocks of present study show significant differences in lithological and geochemical characteristics, and thus it cannot be named as komatiite. To explain these characteristics, we propose a geodynamic model involving ridge subduction and slab window mechanism to account for the formation of the Guyang hornblendite complex.

Evidence of an Arc or Back Arc System From NeoArchean North China

A Neoarchean arc–back-arc system in Eastern Hebei, North China Craton: constraints from zircon U–Pb–Hf isotopes and geochemistry of dioritic–tonalitic–trondhjemitic–granodioritic (DTTG) gneisses and felsic paragneisses

Authors:

Bai et al

Abstract:

Dioritic–tonalitic–trondhjemitic–granodioritic (DTTG) gneisses are widely exposed in the Zunhua–Qinglong microblock of Eastern Hebei, North China Craton (NCC). LA–ICP–MS zircon U–Pb isotopic dating for the DTTG gneisses in the Qinglong area reveals that their magmatic precursors were emplaced between 2517 ± 10 and 2505 ± 23 Ma. Zircons of these DTTG gneisses exhibit positive ɛHf(t) values of 0.0 to +6.5, with adakite-like geochemical features represented by relatively high MgO, Mg# and (La/Yb)N ratios. Petrogenetic studies reveal that dioritic magmas were derived from depleted mantle peridotites that were strongly modified by slab-derived melts/fluids and that the tonalitic–trondhjemitic–granodioritic (TTG) magmas were formed by a high degree of fractional crystallization in the dioritic magmas with hornblende as the primary fractionated phase.

LA–ICP–MS zircon U–Pb isotopic analyses reveal that the protoliths of the felsic paragneisses in the Northern Zunhua area were deposited in the Late Neoarchean with a maximum depositional age of 2517 ± 20 Ma. Most of the detrital zircon grains display apparent 207Pb/206Pb ages from 3432 ± 20 to 2502 ± 21 Ma, with negative ɛHf(t) values of −15.4 to −2.4 and calculated TDMC ages between 3995 and 3368 Ma, suggesting that they were formed from the recycling of Eo- to Paleoarchean crustal materials closely related to the southeastern ancient continental crust of the interior Eastern Block. These data together with previous investigations reveal that the basement rocks of Zunhua–Qinglong microblock formed in a Neoarchean arc–back-arc system with the subduction direction from present northwest to southeast.

More Evidence of NeoArchean Subduction From the North China Craton

Discovery of Neoarchean suprasubduction zone ophiolite suite from Yishui Complex in the North China Craton

Authors:

Santosh et al

Abstract:

The Archean tectonic realm of the North China Craton (NCC) is considered in recent models as a collage of several microblocks which were amalgamated along zones of ocean closure during late Neoarchean. Here we report the finding of a dismembered ophiolite suite from the southern margin of the Jiaoliao microblock in the interior of the unified Eastern Block of the NCC. The suite is composed of lherzolite, pyroxenite, noritic and hornblende gabbro, and hornblendite intruded by veins and sheets of leuco granite. Together with transposed layers and bands of metavolcanics and amphibolites, banded iron formation (BIF), and diabase dykes in the adjacent locations, the Yishui complex corresponds well with a dismembered suprasubduction zone ophiolite suite. Clinopyroxene in the pyroxenite and gabbroic rocks are Mg rich and range in composition from augite to diopside. Among orthopyroxenes, those in lherzolite shows the highest XMg of 0.84-0.85. Plagioclase in hornblende gabbro shows high anorthite content (An50 -64). Calcic amphiboles in the gabbroic rocks range in composition from ferropargasite to ferro-edenite, edenite and pargasite. Spinel inclusions in lherzolite are Cr-rich magnesiospinel. Geochemically, the mafic rocks from Yishui complex show subalkaline basaltic source, whereas the granitoids show volcanic arc affinity. The hornblende gabbro and gabbro, lherzolite and hornblendite show compositional similarity to E-MORB and N-MORB respectively. The lherzolite and hornblendite possess arc-related ultramafic cumulate nature, with overall features straddling the fields of IAT and IAT-MORB. The geochemical features are consistent with evolution in a suprasubduction regime with no significant crustal contamination. The majority of zircon grains in the Yishui suite exhibit magmatic texture and high Th/U ratios. Zircon grains from hornblendite define 207Pb/206Pb upper intercept age of 2538 ± 30 Ma. Zircons from the granite show ages of 2538 ± 16 Ma and 2503 ± 21 Ma, and those from the gabbros yield ages of 2503 ± 16 Ma and 2495 ± 10 Ma. The well defined major age peak at 2500 Ma is broadly coeval with Neoarchean ages reported from the microblocks in the North China Craton. The zircon Lu–Hf data from the Yishui suite display εHf(t) values between -2.5 and 5.0, with corresponding model ages suggesting magma derivation from Neoarchean juvenile sources together with limited reworked Paleo-Mesoarchean crustal components.

Our study is the first report of Neoarchean suprasubduction-type ophiolites from a locality far from the margins of the major crustal blocks and suture zones in the NCC and strengthens the concept that the craton is a mosaic of several microblocks with intervening oceans that closed along multiple subduction zones. We envisage that the amalgamation between the Xuhuai and the Jiaoliao microblocks resulted in the accretion of the Yishui suprasubduction zone ophiolitic assemblages onto the southern margin of the Jiaoliao microblock. The Neoarchean microblock amalgamation in the North China Craton provides new insights into continental growth in the early Earth and confirms that modern style plate tectonics might have been initiated early in the history of our planet.

Australia's Lake Eyre Caused by Fossil Subduction Zone, to Disappear in 30 Million Years

Geoscientists have, for the first time, discovered the origins of Australia's two largest basins: Lake Eyre and the Murray-Darling Basin. The research also implies that in 30 million years' time both basins will cease to exist.

Monash University geoscientist Associate Professor Wouter Schellart, and his colleague Professor Wim Spakman from Utrecht University, have discovered how the floor of an entire ocean basin that was destroyed 70 to 50 million years ago off the North coast of New Guinea is currently located at 800-1200km depth below Central and South-eastern Australia.

Using supercomputers, the researchers found that this dense piece of ocean floor material (called a lithospheric slab) is slowly sinking into the Earth's mantle and is responsible for the formation of the Lake Eyre Basin, one of the Earth's largest internally drained basins and home to the lowest point in Australia at 15m below sea level, as well as the Murray-Darling Basin, home to the largest river system in Australia. With a combined surface area exceeding 2 million square kilometres, both basins are located directly above the deep mantle slab.

The research also predicts that in 30 million years from now, when Australia has moved about 1500km northwards, the fossil slab will be located below the Southern Ocean and, as a consequence, the Lake Eyre Basin and Murray Darling Basin will cease to exist.

Using geological and geophysical data from the New Guinea region, Schellart was able to reconstruct the geological evolution of the region over the last 70 million years, including the motion of the tectonic plates and plate boundaries. He discovered that the occurrence of deep ocean floor rocks, volcanic rocks and deformed rocks, which are currently found in the mountain ranges of New Guinea, point to the existence of a 4000km wide subduction zone. At subduction zones such as these, an oceanic tectonic plate sinks (subducts) into the Earth's interior, the mantle.

With these plate tectonic reconstructions Schellart was able to predict where the fossil subduction zone was during its lifetime some 50-70 million years ago, and therefore where the lithospheric slab disappeared into the mantle. With a global seismic tomography model that makes use of seismic waves to map the internal structure of the Earth's mantle, Schellart and Spakman were able to identify the fossil slab structure below central and south-eastern Australia at a location and depth predicted by the reconstructions.

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Evidence of Ridge Subduction From Ediacaran NeoProterozoic Namibia

Fingerprints of late Neoproterozoic ridge subduction in the Pan–African Damara belt, Namibia

Authors:

Meneghnini et al

Abstract:

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

Evidence of Rapid Sediment Recycling From PaleoProterozoic

Paleoproterozoic S-type granites in the Helanshan Complex, Khondalite Belt, North China Craton: Implications for rapid sediment recycling during slab break-off

Authors:

Dan et al

Abstract:

S-type granites, typically derived from the rapid recycling of sedimentary rocks, are sometimes accompanied by contemporary mafic magmatism and granulite metamorphism. However, the geodynamic context for such rock suites is often highly disputed, with various model proposed, including back-arc basin opening, lithospheric delamination, mantle plume and continental rifting. The Paleoproterozoic Khondalite Belt in the North China Craton provides an example of synchronous mafic and felsic magmatism that was accompanied by granulite-facies metamorphic events for which the tectonic affinities of these rocks remains unclear. This study integrates in situ zircon Hf–O isotope analyses, whole-rock geochemistry and Nd isotope results for the earliest two-mica granites (ca. 1.95 Ga) in order to provide constraints on the above issues. The granites are strongly peraluminous (A/CNK value greater than 1.1), and characterized by high zircon δ18O values of 7.3 to 10.6‰, corresponding to calculated magmatic δ18O values of 9.1 to 12.3‰, similar to those of typical S-type granites. They have relatively high and homogeneous ɛNd(t) values of -1.1 to +0.9 and highly variable zircon ɛHf(t) values ranging from -1.0 to +8.3. In situ zircon Hf-O isotopic compositions indicate that the S-type granites may contain some mantle or juvenile crustal components in addition to a sediment component. Based on the new results and published data, a slab break-off model is proposed to explain the rapid recycling of sedimentary precursors and the generation of the ca. 1.95 Ga S-type granites

Subduction-Driven Plate Tectonics MAY Date From MesoArchean

Petrogenesis of Neoarchean TTG rocks in the Yangtze Craton and its implication for the formation of Archean TTGs

Authors:

Wu et al

Abstract:

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U-Pb age, Hf-O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U-Pb dating for two gneiss samples yielded weighted mean ages of 2509 ± 21 and 2496 ± 15 Ma. The gray gneisses have high SiO2 (63.64-73.74%), Na2O (3.66-6.21%), but low K2O (0.76-2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb) N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6-46.0 and 14.5-143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of -3.21 and -3.39 and Hf model ages of 3194 ± 26 and 3205 ± 24 Ma, indicating their derivation from ca. 3.2 Ga juvenile mafic rocks. Their δ18O values of 6.41 ± 0.12 and 5.44 ± 0.12 ‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U-Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8-3.4 Ga zircons exhibit ɛHf(t) values of -5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2 Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2 Ga in the Yangtze Craton.

Evidence of a Continental Collision/Subduction at the NeoArchean/PaleoProterozoic Boundary

Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong terrain, North China Craton: Petrology, zircon age determination and geological implications

Authors:

Liu et al

Abstract:

The North China Craton is an ideal place for studying the transition of the Earth's thermal structure and tectonics at the Archean-Proterozoic boundary due to its good preservation of the ~ 2.5 Ga tectono-thermal events. We report the discovery of a high-pressure mafic granulite from the Jiaodong Terrain in the North China Craton. The mafic granulite occurs as garnet-clinopyroxene-orthopyroxene-hornblende gneiss enclaves within a late-Archean trondhjemite-tonalite-granodiorite (TTG) gneiss. Typical high-pressure mineral assemblage of garnet - clinopyroxene - plagioclase - quartz ± rutile has been identified. Plagioclase + clinopyroxene ± orthpyroxene ± hornblende symplectite surrounding garnet ("white eye") is also observed. Using the conventional geothermobarometry and the pseudosection modeling, a clockwise metamorphic P-T path with the peak conditions at ~ 17 kbar and ~ 880 °C was determined. Zircon U-Pb analyses (SHRIMP) on the overgrowth rim of zircon grains of two samples from the same outcrop yielded a metamorphic age of 2473 ± 6 Ma (MSWD = 0.8). The analyses on magmatic core gave a probable magmatic age of 2527 ± 12 Ma (MSWD = 1.9). The high-pressure granulite facies metamorphism corresponds to a collisional event between the ~ 2.5 Ga crust and ~ 2.9 Ga crust at the dawn of Paleoproterozoic in the North China Craton. It also represents a new but rare case of a subduction-collision tectonics at the Archean-Proterozoic transition and provides insight into the change of the Earth's thermal structure.