Evidence of the First Continental Kernels From Archean China

U-Pb age and Hf isotopes of detrital zircons from the Southeastern North China Craton: Meso- to Neoarchean episodic crustal growth in a shifting tectonic regime

Auhtors:

Liu et al

Abstract:

The North China Craton (NCC) represents one of only a few cratonic nuclei on the globe with a geological history extending back to the Eoarchean. However, extensive ca. 2.5 Ga crustal reworking has destroyed a considerable portion of the pre-existing crustal record, hindering the investigation of tectonothermal evolution prior to 2.5 Ga. The Huoqiu Complex (HQC), located at the southeastern margin of the NCC, preserves the vestiges of crustal components that survived the ca. 2.5 Ga tectonothermal events, which provide the opportunity to investigate the Meso- to Neoarchean episodic crustal evolution of the NCC. Here we present results from in-situ detrital zircon U-Pb dating and Hf isotope analyses on zircons from three paragneisses in three drill cores that cut through the basement of the HQC. In combination with published data, the concordant age spectra of the detrital zircons in the paragneisses yield 207Pb/206Pb ages of 2343-3997 Ma that cluster into two principal age populations with peaks at 3015 and 2755 Ma. One zircon grain dated at 3997 ± 8 Ma with 98% concordance provides new evidence for 4.0 Ga components in the NCC. The εHf(t) values of all zircons range from -5.2 to + 6.5, with most of the spots (n = 31 of 47) showing positive values, indicating at least two episodes of juvenile continental crustal growth at 3.01 Ga and 2.75 Ga. The older episode is recorded only in few ancient cratons, suggesting limited crustal accretion occurred globally at a time of subdued mantle-derived magmatism. In contrast, the younger episode is coincident with a global rise in magmatic activity in the early Neoarchean. The geochemical and geochronological data suggest that the 3.01 Ga juvenile crust was likely generated in an island-arc subduction system, whereas the 2.75 Ga crustal rocks were probably formed during magmatic underplating and subsequent partial melting of lower crustal mafic rocks. Consequently, a tectonic transition is suggested from a compressive to an extensional setting along the southeastern margin of the NCC between 3.01 and 2.75 Ga. This sequence of events heralds a shift, from a mixture of net crustal growth and crustal reworking during multiple short-lived magmatic pulses, to fragmentation and dispersal of the early continental nucleus within 260 Ma.

Tectonic Implications From Ural Russia for the Columbia Supercontinent

U–Pb zircon geochronology and geochemistry of Paleoproterozoic magmatic suite from East Sarmatian Orogen: tectonic implications on Columbia supercontinent

Authors:

Terentiev et al

Abstract:

Extensive Paleoproterozoic magmatism occurred in the East Sarmatian Orogen (ESO), located at the junction of the Sarmatian and Volga–Ural segments of the East European Craton, generating numerous felsic to mafic-ultramafic complexes. Here we report geochemical and zircon U-Pb age data from a suite of ultramafic–mafic and diorite–granodiorite intrusive rocks from this orogen. SHRIMP U–Pb dating of zircons yielded the following concordant ages for the magmatic suites: (1) 2085 ± 17 to 2093 ± 20 Ma (sub-volcanic high-Mg intrusions of SHMB-like melanorite–melaquartz-diorite–melagranodiorites and quartz-diorite–tonalite–granodiorites); (2) 2072.4 ± 8, 2073 ± 8.3, and 2068 ± 13 Ma (ultramafic–mafic–diorite layered intrusions); (3) 2053 ± 14 to 2058 ± 22 Ma (magnesian–ferroan quartz-diorite–tonalite–granodiorite intrusions) and (4) 2035 ± 18 Ma (ferroan norite–diorite intrusions). The trace and rare earth element geochemistry indicate arc-related features. The enrichment in LREE and LILE in the mafic-ultramafic and diorite-granodiorite suites is correlated with melting of metasomatised lithosphere mantle along with involvement of crustal components in their petrogenesis. The third magmatic event conforms to the age of the post-collision S- and A-granitoids of the region. Our data indicate two major magmatic events in the post-collisional setting, the former immediately after metamorphism with the formation of schistosity in the host rocks, and the latter two during the culmination of the orogenic cycle in the Voronezh Crystalline Massif.

The duration of the Paleoproterozoic orogeny during the collision of the Sarmatian and Volga–Ural segments of the East European Craton is estimated to be approximately 100 million years from ∼2140 to ∼2035 Ma. We propose that the layered orogenic plutons were derived from relatively deeper mantle source, whereas the sub-volcanic rock series are derivatives of the slightly shallow mantle source. The Paleoproterozoic collision in the region broadly coincides with timing of amalgamation of the global supercontinent Columbia.

Microbes Caused the RIse of the First MicroContinents?

Microbes, Mineral Evolution, and the Rise of Microcontinents—Origin and Coevolution of Life with Early Earth

Authors:

Grosch et al

Abstract:

Earth is the most mineralogically diverse planet in our solar system, the direct consequence of a coevolving geosphere and biosphere. We consider the possibility that a microbial biosphere originated and thrived in the early Hadean-Archean Earth subseafloor environment, with fundamental consequences for the complex evolution and habitability of our planet. In this hypothesis paper, we explore possible venues for the origin of life and the direct consequences of microbially mediated, low-temperature hydrothermal alteration of the early oceanic lithosphere. We hypothesize that subsurface fluid-rock-microbe interactions resulted in more efficient hydration of the early oceanic crust, which in turn promoted bulk melting to produce the first evolved fragments of felsic crust. These evolved magmas most likely included sialic or tonalitic sheets, felsic volcaniclastics, and minor rhyolitic intrusions emplaced in an Iceland-type extensional setting as the earliest microcontinents. With the further development of proto-tectonic processes, these buoyant felsic crustal fragments formed the nucleus of intra-oceanic tonalite-trondhjemite-granitoid (TTG) island arcs. Thus microbes, by facilitating extensive hydrothermal alteration of the earliest oceanic crust through bioalteration, promoted mineral diversification and may have been early architects of surface environments and microcontinents on young Earth. We explore how the possible onset of subseafloor fluid-rock-microbe interactions on early Earth accelerated metavolcanic clay mineral formation, crustal melting, and subsequent metamorphic mineral evolution. We also consider environmental factors supporting this earliest step in geosphere-biosphere coevolution and the implications for habitability and mineral evolution on other rocky planets, such as Mars.

Pilbara Craton Controversy: Were There or Were There NOT Hadean Continents?

Hf isotopes in detrital and inherited zircons of the Pilbara Craton provide no evidence for Hadean continents

Authors:

Kemp et al

Abstract:

Predictions of large volumes of stabilized continental crust by the early Archaean stand in stark contrast to the actual amount of pre-3.5 Ga rocks presently exposed on Earth's surface. The Pilbara Craton of Western Australia, one of the best preserved Paleoarchean crustal blocks on Earth, is believed to have developed on a cryptic, possibly ≥3.8 Ga continental basement. If substantiated, this could support the notion of a widespread and enduring Hadean (ca. 4.5–4.0 Ga) felsic–intermediate crust. To test this, and to elucidate the earliest evolution of the Pilbara Craton, we report Hf isotope data from previously dated detrital zircon grains, and inherited zircon crystals hosted by granitic gneisses, the crystallization ages (3.80–3.55 Ga) of which substantially exceed those of the oldest exposed igneous rocks of the craton (∼3.52 Ga). The Hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the Pilbara Craton being extracted from near chondritic mantle between ∼3.7 and 3.6 Ga, with little or no input from significantly older crust. These new data suggest either that the Pilbara Craton developed remote from the isotopic influence of the putative Eoarchean to Hadean continental masses, or that the stabilized volumes of the earliest continents have been overestimated. The latter scenario would be consistent with the extreme scarcity of greater than 3.9 Ga rocks and minerals, and the dominantly chondritic Hf isotope composition of the oldest continental rocks in Earth's most ancient Archaean cratons.

First Evidence of MesoArchean Continental Crust?

Evolution history of the Neoproterozoic eclogite-bearing complexof the Muya dome (Central Asian Orogenic Belt): constraints from zircon U-Pb age, Hf and whole-rock Nd isotopes

Authors:

Shatsky et al

Abstract:

U-Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite-gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet-biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet-biotite-muscovite schist (Pl + Kfsp + Bt + Mu + Grt + Qtz) were dated using the LA-ICP-MS technique. Based on U-Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576-680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U-Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88-2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a Pb207/206Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of ɛHf(t) values (from +13.9 to -15.3) and View the MathML sourceTDMC model ages (0.82-3.86 Ga). Zircons from other samples have a much narrower ranges of ɛHf(t) (+11.6 to -0.7) and View the MathML sourceTDMC (0.85-1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with ɛNd(t) values ranging from -3.5 to +3.6 and tNd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.

The Emergence of Continents was Crucial for Great Oxidation Event

Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis

Authors:

Lalonde et al

Abstract:

The Great Oxidation Event (GOE) is currently viewed as a protracted process during which atmospheric oxygen increased above ∼10−5 times the present atmospheric level (PAL). This threshold represents an estimated upper limit for sulfur isotope mass-independent fractionation (S-MIF), an Archean signature of atmospheric anoxia that begins to disappear from the rock record at 2.45 Ga. However, an increasing number of papers have suggested that the timing for oxidative continental weathering, and by conventional thinking the onset of atmospheric oxygenation, was hundreds of million years earlier than previously thought despite the presence of S-MIF. We suggest that this apparent discrepancy can be resolved by the earliest oxidative-weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere, such as biological soil crusts and freshwater microbial mats covering riverbed, lacustrine, and estuarine sediments. We calculate that oxygenic photosynthesis in these millimeter-thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox-sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant S-MIF signature. As continental freeboard increased significantly between 3.0 and 2.5 Ga, the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass-balance perspective. These observations help reconcile evidence for pre-GOE oxidative weathering with the history of atmospheric chemistry, and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the GOE.

Evidence of Episodic Crustal Formation From the Hadean to the NeoArchean From China

Hadean to Neoarchean episodic crustal growth: Detrital zircon records in Paleoproterozoic quartzites from the southern North China Craton

Authors:

Zhang et al

Abstract:

The Archean terranes exposed in several regions of the North China Craton (NCC) provide important winodws to evaluate the Hadean-Archean continental crustal evolutoin history. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Paleoproterozoic quartzites to probe the early crustal evolution in the southern NCC. Our data show episodic magmatism during the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean and Paleoproterozoic as inferred from the concordant ages of 3.6 Ga and 2.2 Ga and the well-defined upper intercept ages of 3404 ± 30 Ma, 2919 ± 31 Ma, 2772 ± 9.5 Ma, 2698 ± 4.7 Ma, 2652 ± 6.7 Ma, and 2532 ± 5.2 Ma. The Hf and oxygen isotopic compositions of Eoarchean-Paleoarchean zircons demonstrate that the cratonic nucleus of the NCC was built in the Hadean (∼4.0 Ga), similar to the timing of formation of the nuclei of old cratons elsewhere on the globe such as the Yilgarn Craton, Western Australia, and the Slave Craton, Canada, with the Eoarchean-Paleoarchean (3.8 Ga, 3.6 Ga and 3.4 Ga) marking the first phase of crustal reworking of the Hadean crust. The salient aspect of our data is that the Meoarchean to early Neoarchean (3.0-2.6 Ga with a peak at 2.7 Ga) was a significant period for crustal growth from the mantle throughout the NCC, comparable with the event of major Archean crustal growth worldwide. The latest Neoarchean (∼2.5 Ga) marks a period of significant crustal reworking that led to remelting of the newly-formed juvenile mafic crust (∼2.7 Ga). Integration with the data obtained from the TTG (tonalite-trondhjemite-granodiorite) and granitoid rocks of the Archean terranes and the lower crustal granulite xenoliths from various parts of the NCC suggests that the episodic Eoarchean via Paleoarchean and Mesoarchean to Neoarchean crustal growth as well as crustal reworking intensely destroyed the ancient cratonic nucleus, resulting in the scarce preservation of the Hadean and Eoarchean remnants in both the upper and lower crust of the NCC. Following the Paleoproterozoic sedimentation of the Archean zircons, apart from Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, our detrital zircon data provide convincing evidence for the repeated crustal growth and reworking of the NCC in the Archean.

Evidence of Continental Growth in the North China Craton From Neoarchean (MesoArchean?) to Statherian Paleoproterozoic

Neoarchean to Paleoproterozoic continental growth in the southeastern margin of the North China Craton: Geochemical, zircon U-Pb and Hf isotope evidence from the Huoqiu complex

Authors:

Liu et al

Abstract:

The Huoqiu complex in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean grey gneisses, amphibolites and voluminous metasediments. Here we report the occurrence of Neoarchean to Paleoproterozoic rocks from drill core samples. The gneisses are similar to TTG (tonalite-trondhjemite-granodiorite) in composition and show close spatial association with amphibolites. Geochemical characteristics such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICP-MS U-Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76-2.71 Ga. Subsequently, widespread migmatization took place at 1.91-1.82 Ga, generating voluminous migmatites and high-K granites. Hf isotopic compositions of zircon grains from the amphibolite and gneiss show εHf(t) values of 2.4-15.5 and − 3.0-1.5, respectively. The tDM2(Hf) model ages of the gneisses range from 2.87 to 3.14 Ga, and are identical to the tDM1(Hf) ages of amphibolites (2.84-3.16 Ga) within analytical uncertainty, suggesting that the gneisses formed by partial melting of amphibolite, and attest to large-scale reworking of the ancient continental crust during Neoarchean. The zircon grains from the granites define two groups with regard to their Hf isotopic composition. The older group (1916 ± 42 Ma) has εHf(t) values and tDM2(Hf) ages of − 10.5-2.4 and 2.40-3.20 Ga, respectively, whereas the younger one (1823 ± 41 Ma) shows a large variation in εHf(t) values ranging from − 18.1 to 12.5, with tDM2(Hf) model ages of 1.70-3.59 Ga. A couple of zircon grains from the younger group display consistent U-Pb ages and tDM2, indicating accretion of juvenile crust from depleted mantle sources during 1.82 to 1.91 Ga. However, the dominant Hf isotope features are consistent with the reworking of preexisting continental crust. We therefore infer that only limited accretion of juvenile crust occurred during this time, and that the Paleoproterozoic (1.82 to 1.91 Ga) tectonics in the southeastern margin of the NCC witnessed extensive reworking of older continental crust.