Mafic Crust Research Articles

Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

The Oytag pluton in the West Kunlun orogenic belt of China represents the rare exposed Carboniferous granitoids associated with the early-stage evolution of the Paleo-Tethys Ocean, and its origin is controversial. New zircon U-Pb data suggest an emplacement age of 327–324 Ma for trondhjemite, which slightly predated tonalite (∼314 Ma). These results, along with available published ages, indicate several episodes of magmatism to build the Oytag pluton. Both trondhjemite and tonalite have positive whole-rock Nd isotopic compositions (ε Nd (t) = +4.56 to +8.01 for trondhjemite; ε Nd (t) = +4.68 to +6.15 for tonalite), which positively correlate with whole-rock MgO values. Given the presence of Ordovician inherited zircon grains, variations of whole-rock Nd isotopic compositions are attributed to addition of <10% crustal contaminants during assimilation and fractional crystallization (AFC) processes. Interestingly, trondhjemite and tonalite follow distinct AFC trends, consistent with their different ages. Zircons from two trondhjemite samples have overlapping U-Pb ages but show distinctly different trace elements and internal structures (sector-zoned vs. oscillatory-zoned). The sector-zoned zircons have a less evolved signature of lower Hf contents, higher Eu/Eu* and Th/U ratios than the oscillatory-zoned ones, indicating significant crystal fractionation processes for their parental melts. The more variable Hf contents of the oscillatory-zoned zircons than those of the sector-zoned ones reflect a more dynamic environment where they crystallized. We propose that the sector-zoned zircons were crystallized from locked, interstitial melt in a mush and the oscillatory-zoned zircons from extracted melt. In situ crystal-liquid segregation in a mush chamber thereby could be an efficient approach to the differentiation of Oytag trondhjemite. On the other hand, zircons from tonalite have similar Hf contents to the sector-zoned zircons from trondhjemite, but display slightly higher Eu/Eu* ratios and much higher Th/U ratios. This indicates that tonalite should have experienced much different AFC processes from trondhjemite. Both the episodic magmatism feature and the distinct differentiation processes argue for rises of different batches of magmatism to generate trondhjemite and tonalite in Oytag. Along with the large variations of Hf isotopes (5–6 epsilon units) in zircons, we propose a partial melting of juvenile mafic crust model for the Oytag pluton. Heat from upwelling of asthenosphere may have triggered the partial melting of wet, mafic underplates during the opening of a Paleo-Tethyan back-arc basin. • The Oytag pluton comprises predominant trondhjemite with minor tonalite. • Oytag trondhjemites (327–324 Ma) formed earlier than tonalites (∼314 Ma). • The Oytag pluton was built by different batches of magmas via partial melting.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

Multispectral discrimination of spectrally similar hydrothermal minerals in mafic crust: A 5000 km2 ASTER alteration map of the Oman–UAE ophiolite

Multispectral remote sensing of hydrothermal alteration in volcanogenic massive sulfide (VMS) ore systems in mafic crust is relatively uncommon, in part due to the short-wave infrared spectral similarity of several key alteration minerals: epidote, chlorite, actinolite, and serpentine. In this study, we developed regional mosaic generation and classification workflows for Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery to discriminate these minerals over the entire crust of the Semail ophiolite (Oman–UAE). Spectral discrimination was achieved through adaptation of the ASTER (pre-)processing workflow to the specific mapping targets, available datasets, and location of this study. Necessary steps included the pre-selection of ASTER scenes without residual atmospheric water features, mosaic normalization based solely on overlapping target outcrops, correcting cross-mosaic ramp errors, and alteration map classification based on image-derived reference data. The resulting alteration map, validated through comparison with field mapping and sampling, is the most areally extensive continuous survey of hydrothermal alteration yet presented for oceanic crust, providing a renewed framework for research and mineral exploration of Earth's largest ophiolite. Our map confirms that the vast majority of the upper oceanic crust is regionally altered to a spilite type secondary mineral assemblage. Localized areas of epidosite alteration, marking focused hydrothermal flow paths, are confined to the upper oceanic crust, whereas areas of previously unrecognized but intense actinolite alteration are common in both the lower and upper oceanic crust. Our methodological developments expand the standard considerations necessary for regional geological mapping using infrared image mosaics. They further demonstrate the underappreciated capability of multispectral data for mapping spectrally similar rock types. Although the specifics of the method are necessarily optimized for the Oman–UAE ophiolite, re-optimization based on local reference data should allow similar results to be achieved in other well-exposed mafic-hosted VMS districts.

Chronological framework of Precambrian Dantazi Complex: Implications for the formation and evolution of the northern North China Craton

The Dantazi Complex in the North Hebei Terrane is located in the middle segment of the northern North China Craton, and plays a crucial role for understanding the early Precambrian crustal evolution of the craton. The Dantazi Complex is composed mainly of late Neoarchean tonalites-trondhjemites-granodiorites, potassic granites, sanukitoid granodiorites-monzodiorites and quartz syenites, which were emplaced by Paleoproterozoic gabbroic stocks. SHRIMP and LA–ICP–MS zircon dating analyses have revealed that the Neoarchean diversified granitoids of the Dantazi Complex were formed in ∼2.52–2.47 Ga, and preserved ∼2.60–2.53 Ga inherited/captured zircon ages. All these rocks experienced early metamorphism and deformation at ∼2.40 Ga, later granulite facies metamorphism at ∼1.80 Ga, and alterations at ∼1.73 Ga. Whole-rock chemical characteristics and Sm–Nd–Lu–Hf isotopes indicate that (1) The ∼2.52 Ga tonalites-trondhjemites-granodiorites were derived from partial melting of juvenile mafic crust; (2) the ∼2.51 Ga sanukitoid monzodiorite-granodiorite series came from slab-mantle interaction with different melt/rock ratios; (3) The ∼2.50–2.47 Ga potassic granites were generated by intracrustal recycling of metagreywacks that sourced from the mixture between juvenile and ancient crustal materials; and (4) The ∼2.50 Ga quartz syenites were originated from fractional crystallization of mantle-derived magmas. These petrogenetic processes reveal that the subduction-related crust-mantle interaction is the key reason for the diversity of late Neoarchean granitoids in the Dantazi Complex. Regional comparison indicates that the Precambrian North Hebei Terrane show distinct structure in the chronology and lithological assemblages from other basement terranes in the trans-North China Orogen and Western Block, implying that they were formed in different crust-mantle dynamic systems. Therefore, the Precambrian North Hebei Terrane was probably an independent micro-continental block at least until the early Paleoproterozoic. In the Archean crystalline basement range of the North Hebei–West Liaoning–East Hebei, from NW to SE, the magmatic crystallization ages gradually became older, and the granitoid assemblages transformed from sodium-rich to potassium-rich, the spatio-temporal evolution of chemical composition and lithological assemblages further indicates that the North Hebei Terrane was most likely developed in an arc environment with continuous retreat subduction caused by the slab roll-back.

Tungsten isotopic fractionation at the Mariana arc and constraints on the redox conditions of subduction zone fluids

Fluid release from subducting slabs during recycling of oceanic lithosphere plays an important role in concentrating select elements near the Earth’s surface. Yet the extent to which these fluids are oxidising or reducing (fO2), one of the key characteristics that controls element mobility, remains poorly understood. Additionally, it is unclear whether the fO2 signature of arc fluids is generated deep within the slab during subduction or if it reflects later, shallower subarc crustal processes. Previous experimental work has shown that the extent to which highly incompatible W partitions into aqueous fluids depends on the temperature and fO2 conditions of the fluids. In the subducting slab, W partitions into rutile with a different coordination number to that which it exhibits in fluids, a process which may generate stable W isotopic fractionation. Therefore, if the competition between partitioning into aqueous fluids versus rutile is controlled by the conditions within the subducting slab, the elemental and stable W isotopic systematics of arc lavas may provide a novel tool with which to investigate subduction zones. We present isotopic compositions of a well characterised suite of arc lavas from the central island province of the Mariana arc (W. Pacific), and rutile separates from exhumed fragments of subducted, depleted, mafic oceanic crust from the Raspas Complex, Ecuador. Our data show that lavas from Guguan that exhibit the strongest geochemical signature of subduction zone fluids (e.g., high Ba/Th, Mo/Ce, Pb/Ce), contain the highest abundances of W compared to similarly magmatically incompatible Th, in agreement with anticipated fluid mobile behaviour. These Guguan lavas have distinct, heavy stable W isotopic compositions (δ186/184W = +0.134 to +0.156 ‰) compared to MORB (δ186/184W = +0.078 to +0.099 ‰) and those from sediments-dominated islands like Uracas and Agrigan (δ186/184W = +0.080 to +0.111 ‰). The W isotopic compositions of sediments off-board the Mariana arc are more variable, with the volcaniclastic sediments having δ186/184W values closest to those of the sediment dominated arc lavas. We show that rutile from representative subducting mafic crust incorporates isotopically light W, consistent with a coordination change from tetrahedral in fluids and melts to octahedral in the structure of rutile. We suggest that isotopically heavy fluids, complementary to the residual rutile in the mafic crust, account for the high δ186/184W of the fluid-dominated Guguan lavas, a process previously invoked to control Mo systematics in the same samples. Combining our new elemental and isotopic W data with these existing Mo data, we model the influence of oxygen fugacity on fluid compositions in equilibrium with the mafic crust, and the fraction of such fluids required to be added to the mantle wedge to reproduce the Mo-W systematics of erupted arc lavas. Our models show that more fluid is required (F = 6 %) than can be generated internally in the mafic oceanic crust, and that this fluid must be oxidised, perhaps as high as FMQ +5. We suggest that these requirements document the interaction of subducted mafic crust with an external source of oxidising fluids sourced from dehydration of underlying serpentinites.

Archean crustal evolution of the Saglek-Hebron Complex, Northern Labrador, revealed from coupled 147−146Sm-143−142Nd systematics

The Saglek-Hebron Complex in Northern Labrador, Canada, hosts some of the oldest rocks on Earth. Recording a protracted crustal history over 1 billion years, the complex includes multiple generations of felsic rocks formed through episodic magmatic activity spanning the whole Archean Eon, between ∼3900 Ma and ∼2700 Ma. Coupled 147−146Sm-143−142Nd systematics from these granitoids show a complex crustal history, including melting of older mafic crustal precursor sources and reworking of Eoarchean felsic crust. Eoarchean granitoids are characterized by high initial ε143Nd up to +4 and μ142Nd up to +15. Combined long- and short-lived Sm-Nd isotope systematics suggest the crustal precursor source of the oldest Saglek-Hebron granitoids was derived from an early incompatible element depleted reservoir formed between 4200 Ma and 4300 Ma, evolving with a 147Sm/144Nd ratio of ∼0.265. The source involved in the formation of the Saglek-Hebron Eoarchean felsic crust appears to be younger and more depleted compared to that of the Saglek-Hebron mafic-ultramafic rocks, but similar to the source of the Eoarchean granitoids from the Itsaq gneiss complex in SW Greenland. This may suggest the formation of a Hadean highly depleted mantle reservoir at the scale of the North Atlantic Craton, perhaps resulting from the extraction of Hadean mafic crust. Paleoarchean ∼3300 Ma granitoids from the Illuilik magmatic episode exhibit initial ε143Nd between -1.6 and -1.3, which could be consistent with the reworking of the Saglek-Hebron Eoarchean felsic crust. However, the whole-rock geochemical composition of some Iluilik samples rather suggests derivation from a mafic crustal source, and thus their initial ε143Nd values imply derivation from an Hadean mafic precursor. This scenario would also account for the lower μ142Nd of ∼+6 found for the Iluilik granodiorite, compared to the older TTG, and consistent with derivation from a distinct crustal source. Most granitic rocks exhibit initial ε143Nd and μ142Nd consistent with reworking of the oldest Saglek-Hebron TTG, throughout the Archean, between ∼3700 Ma and ∼2700 Ma. Finally, coupled 147−146Sm-143−142Nd systematics suggest a shift in precursor sources at the end of the Paleoarchean, with the ∼3200 Ma Lister gneiss exhibiting more juvenile initial ε143Nd and no deviation of μ142Nd from the terrestrial standard, as opposed to most other Saglek-Hebron granitoids. This shift to more juvenile precursor material in the Paleoarchean has also been observed in other Archean cratons, suggesting a planetary-scale transition for crustal formation processes.

Size and Composition of the MORB+OIB Mantle Reservoir

AbstractMost efforts to characterize the size and composition of the mantle that complements the continental crust have assumed that the mid‐ocean ridge basalt (MORB) source is the incompatible‐element depleted residue of continental crust extraction. The use of Nd isotopes to model this process led to the conclusion that the “depleted MORB reservoir” is confined to the upper ∼30% of the mantle, leaving the lower mantle in a more “primitive” state. Here, we use Nb/U and Ta/U to evaluate mass and composition of the mantle reservoir residual to continent extraction and find that it exceeds 60% of the total mantle. Thus, the (Nb, Ta)/U‐based mass balance conflicts with the ε(Nd)‐based mass balance, and this invalidates the classical 3‐reservoir silicate Earth model (continental crust, depleted mantle, and primitive mantle). Including the combined MORB + ocean island basalt (OIB) sources in the ε(Nd)‐based mass balance does not reconcile the conflict as it would require their average ε(Nd) to be ≤3.0, much lower than observed MORB + OIB ε(Nd) averages. We resolve this conflict by invoking an additional, “early enriched reservoir” (EER), formed prior to extraction of significant continental crust, but now hidden or lost. This EER differs from EERs previously invoked by having no Nb‐Ta anomaly. We suggest that it originated as an early mafic crust, which had unfractionated (Nb, Ta)/U but fractionated Sm/Nd ratios. The corresponding “early depleted” reservoir generated the present‐day continental crust and the “residual mantle” MORB‐OIB reservoir, which occupies at least 63% of the present‐day mantle and is only moderately depleted in incompatible trace elements.

Cambrian intra-oceanic subduction within the southern branch of the Proto-Tethyan Ocean: Constraints from rhyolites in the Lajishan suture, NE Tibetan Plateau

The South Qilian suture in the Qilian Orogen is an early Paleozoic belt that developed as a result of closure of the southern branch of the Proto-Tethyan Ocean. It is characterized by widespread Cambrian intermediate-basic volcanic rocks which record the early-stage evolution of the paleo-ocean basin. However, Cambrian evolution of the paleo-ocean basin remains controversial due to the lack of research on the age and tectonic affinity of these volcanic rocks. In this study, we identified rhyolites from the intermediate-basic volcanic rocks in the South Qilian suture. They occur as pebbles in the breccias, or as veins and interlayers within the intermediate-basic volcanic rocks. Zircon U-Pb datings yield ages of 514.0 ± 2.8 Ma, 521 ± 4 Ma, 523 ± 5 Ma, and 513 ± 9 Ma for four rhyolites, respectively. The Cambrian rhyolites show variable La/Sm ratios and Rb contents and positive zircon εHf(t) values (+6.8 to +14.6) and whole-rock εNd(t) values (+2.1 to +4.6), suggesting that they were derived from partial melting of juvenile mafic crust. They also have high Al2O3 and CaO contents, as well as low K2O and trace element concentrations, which are similar to those of oceanic arc rhyolite. These early Cambrian oceanic arc rhyolites, along with the SSZ-type ophiolite (ca. 530–520 Ma) to the north and late-stage granitoid intrusions (ca. 474–450 Ma), indicate that the southern branch of the Proto-Tethyan Ocean experienced southward intra-oceanic subduction during the early Cambrian-early Ordovician period.

Thermodynamic constraints on the composition of orogenically thickened lower crust

Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system. Our modeling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (TMoho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO2 (>50.2 wt%), XMg (>0.70), XCa (>0.49), or low XAl (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modeling indicates that if TMoho reaches 900–1000 °C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust, and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low Vp and Vp/Vs values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.

Mechanism of crustal thickening and exhumation of southern Lhasa terrane during the Late Cretaceous: Evidence from high-pressure metamorphic rocks of the Eastern Himalayan Syntaxis

AbstractThe mechanism of Late Cretaceous crustal thickening and exhumation of the southern Lhasa terrane is critical for understanding the tectonic evolution of the Tibetan Plateau. High-pressure metamorphic rocks from the lower crust are good candidates for addressing this issue. In this study, we focus on Late Cretaceous, high-pressure, garnet-bearing amphibolites from the Nyingchi Complex of the Eastern Himalayan Syntaxis and present an integrated study of geochronology, petrography, mineral chemistry, and thermodynamic modeling. Petrographic data determine three metamorphic stages (M1–M3). The M1 stage is characterized by a peak mineral assemblage of garnet + hornblende + albite + rutile + muscovite + quartz, which is followed by a post-peak (M2) assemblage of garnet + hornblende + plagioclase + epidote + biotite + rutile + quartz. The late retrograde stage (M3) is defined by hornblende + plagioclase symplectites surrounding garnet porphyroblasts. Mineral chemistry, with thermodynamic modeling, constrains the P-T conditions of the M1–M3 stages to 14–19 kbar/660–720 °C, 8–10 kbar/650–660 °C, and &amp;lt;7 kbar/&amp;lt;600 °C, respectively. Metamorphic zircons yield a concordant age at 90 Ma, which indicates the formation of garnet-bearing amphibolites. These results indicate a P-T-t path involving near-isothermal decompression for garnetbearing amphibolites, which suggests that the Nyingchi Complex underwent peak-pressure metamorphism (M1) at 90 Ma, followed by rapid exhumation to the depth of 32–26 km along the subduction channel. Moreover, the garnet-bearing amphibolites are considered to be the product of high-pressure metamorphism of mafic crust at the base of the Gangdese belt. Hence, the crust of the Gangdese belt experienced significant crustal thickening of up to 60 km at 90 Ma.

Crustal maturation and cratonization in response to Neoarchean continental collision: The Suizhong granitic belt, North China Craton

The late Archean (3.0–2.5 Ga) is a pivotal period, when properties of the continental crust significantly changed and a fundamental shift of geodynamic processes took place. As the major felsic component in early Archean (> 3.0 Ga) crustal records, tonalite-trondhjemite-granodiorite (TTG) suites are compositionally different to the present-day felsic upper continental crust. But it remains unclear how and to what degree the felsic part of the continental crust evolved during the late Archean. The 250 km long Neoarchean Suizhong granitic belt in the North China Craton (NCC) is mainly composed of K-rich granitoids and compositionally distinct to the TTG-dominated early Archean upper continental crust. This belt is a natural laboratory to explore how the early Archean upper continental crust evolved in the late Archean. Combined with our new data (zircon U-Pb ages, bulk-rock geochemistry and zircon Hf isotopes), we provide an overview of the temporal and compositional features of the Neoarchean Suizhong granitic belt. Granitic magmatism in the Suizhong granitic belt mainly took place within 100 Myrs around the end of the Archean, and these rocks are primarily K-rich granitoids with minor TTGs and mafic magmatic enclaves (MMEs). The K-rich granitoids include sanukitoid-like granites and potassic granites; the former are hybrids between melts from Mesoarchean enriched mafic crust and enriched mantle, and the latter are melts from Hadean–Mesoarchean crustal lithologies. The volumetrically minor TTGs were sourced from Mesoarchean low-K mafic crust. MMEs are either cumulates or inclusions of captured enriched mantle melts. Overall, these Neoarchean granitoids are essentially intracrustal reworking products of ancient continental nuclei and are compositionally and temporally similar to syn- and post-collisional magmatism in Phanerozoic collisional orogenic belts. The Neoarchean crustal reworking process was most likely to be induced by amalgamation of micro-continents through collision. Through the collision-induced reworking, a variety of K-rich granitoids were introduced to the Neoarchean upper continental crust. As a result, layering of felsic upper and mafic lower crust was developed, and the maturity of Archean continental crust was significantly enhanced. Collisional orogenesis played a critical role in maturing the Archean continental crust and initial cratonization of the NCC during the late Archean.

Geochronology, geochemistry, and isotopic composition of the early Neoproterozoic granitoids in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China: Petrogenesis and tectonic implications

The early Neoproterozoic granitoids exposed in the Bikou Terrane preserve vital information for understanding the Tonian tectonic evolution along the northwestern margin of the Yangtze Block, South China. However, their accurate forming age, petrogenesis, and tectonic attributes remain ambiguous. This study reports new results on zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Baiquesi and Bahaihe quartz monzonite, Shilingou monzogranite, and Maliupu granodiorite from the Bikou Terrane. Detailed zircon U–Pb dating results reveal that the quartz monzonite and monzogranite were formed at ca. 860 Ma, whereas the granodiorite was emplaced at ca. 825 Ma. The quartz monzonite and monzogranite samples exhibit certain linear correlations in the Harker diagram, congruent distribution patterns of rare earth elements, and consistent isotopic compositions, indicating that they shared the same magma source, whereas the monzogranite was formed by differentiation crystallization of primitive quartz monzonite melt. The quartz monzonite samples are calc-alkaline in composition and have high ratios of Sr/Y and (La/Yb)N, coinciding well with the adakitic rocks. The relatively high contents of MgO, Ni, Cr, and Sr, low contents of Yb and Y, low ratios of La/Ce, Rb/Sr, and Th/La, and positive values of εHf(t) (+4.8–+6.7) and εNd(t) (+1.7–+2.1) indicate that they were formed by the interaction between slab-derived melt and mantle wedge. This mechanism may be triggered by a slab rollback process in an arc-related setting. The Maliupu granodiorite samples are classified as calc-alkaline I-type granitoids, and have low concentrations of MgO, Cr, Ni, and Y, and low ratios of Sr/Y and (La/Yb)N. They are featured by highly evolved εHf(t) values from −15.0 to −10.9 and εNd(t) values from −11.8 to −11.9, with Hf and Nd model ages ranging between 2.63 and 2.20 Ga, implying they were derived from partial melting of the ancient lower mafic crust and were formed in a subduction-related setting. Therefore, the new data, combined with those from previous studies in the Bikou Terrane, provide an essential link in the evidence chain for the hypothesis that the northwestern periphery of the Yangtze Block was in a persistent subduction-related tectonic regime in the early Neoproterozoic.

Neodymium Isotope Constraints on the Origin of TTGs and High-K Granitoids in the Bundelkhand Craton, Central India: Implications for Archaean Crustal Evolution

Abstract The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica–high Mg (sanukitoids and hybrids) and high silica–low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid, and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages and geochemical signatures as well as εNd(t) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and preexisting crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.

Permeability of subducted oceanic crust revealed by eclogite-facies vugs

Abstract We report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western European Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements, including Cr, Mn, and rare earth elements. Thermodynamic modeling revealed that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite, and chlorite between 505 °C and 525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geologic and geochronologic constraints indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 m.y. These observations can only be explained by the presence of extremely low permeability values (&amp;lt;10−22 m2) to keep the fluid confined at the meter scale within vugs on such time scales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into Earth's mantle, with implications for volatile recycling budgets.

High-resolution Ge-Si-Fe, Cr isotope and Th-U data for the Neoarchean Temagami BIF, Canada, suggest primary origin of BIF bands and oxidative terrestrial weathering 2.7 Ga ago

Unique deposits that formed in the Precambrian oceans are the Banded Iron-Formations (BIFs) which are chemical sediments comprised of alternating layers of iron- and silica-dominated precipitates. The origin of this enigmatic banding is still debated, with most favoring a primary sedimentary and a few others arguing for a secondary diagenetic origin. We here use a high-resolution integrated multi-proxy approach and report on the Ge-Si-Fe and Th-U systematics and on the isotopic composition of Cr in exceptionally pure adjacent magnetite and metachert bands from the ∼2.7 billion years old Temagami BIF, Ontario, Canada.While each of the geochemical proxies alone may be controversial, our multi-proxy approach sheds new light on their applicability, as the two types of adjacent BIF bands are each characterised by a distinct geochemical signature. The magnetite bands show super-crustal (i.e. fractionated) Ge/Si ratios approaching those of modern marine high-temperature hydrothermal fluids, unfractionated Th/U ratios similar to those of crustal rocks, and unfractionated Cr isotope ratios similar to “Bulk Silicate Earth”. Adjacent metachert bands, however, show lower (i.e. unfractionated) Ge/Si ratios close to those of modern seawater, sub-crustal (i.e. fractionated) Th/U ratios, and fractionated Cr isotope ratios above those of Bulk Silicate Earth. The composition of the magnetite bands suggests that the original iron-oxyhydroxide precipitated during periods when the dissolved Ge, Th, U, and Cr inventory of ambient seawater was dominated by input from mafic crust via anoxic high-temperature hydrothermal fluids. In contrast, the composition of the metachert bands indicates that these formed during times when ambient seawater was dominated by low-temperature riverine input from an evolved landmass that despite its Neoarchean age had provided environments for oxidative Cr and U mobilization during terrestrial weathering. This suggests that habitats with oxygenic photosynthesis existed on land at least 200 million years before the Great Oxidation Event. The systematic and internally consistent difference between the geochemical signatures shown by the two types of adjacent BIF bands also demonstrates that the prominent banding of BIFs is a primary depositional feature reflecting precipitation from different watermasses and is not the result of post-depositional separation of an initially homogeneous chemical sediment.

Pre-plate tectonics and origin of continents

<p indent="0mm">Established in the mid-1960s and regarded as a revolution in Earth sciences, the plate tectonics theory can reasonably interpret nearly all geological phenomena, processes, and events that happened during post-Archean time (from 2.5 billion years ago to the present), and has also been applied to explain the formation and evolution of continents. According to the plate tectonics theory, the mafic lower crust and the felsic upper crust of continents can develop from an island arc that formed by subduction of one oceanic crust beneath another, where the mafic lower crust of continents can be extracted from the mantle through the partial melting of the mantle wedge in the subduction zone, whereas the felsic upper crust of continents can form by the partial melting of the already-formed mafic lower crust. However, such an island arc model under a plate tectonic regime cannot well explain the magmatic, metamorphic and structural features of Archean (&gt;2.5 billion years) continents. For example, island arc models fail to explain the presence of ~1600°C komatiites, absence of andesites that is dominant in post-Archean arcs, nearly coeval and craton-scale emplacement of tonalite-trondhjemite-granodiorite (TTG) rocks, dominancy of dome-and-keel structures, and lack of ultrahigh-pressure rocks, paired metamorphic belts and ophiolites, etc. All of these imply that Archean continents may not have been formed under plate tectonic regimes, but were originated from some pre-plate tectonics (non-plate tectonics). So far, researchers have proposed a number of pre-plate tectonics models, of which the most representative ones are mantle plumes, sagduction, heat-pipes and stagnant lids. Although each of these pre-plate tectonics models can satisfactorily interpret some features of Archean cratons, none of them is successful in explaining all lithological, structural and metamorphic features of Archean cratons. For example, although the mantle plume-derived oceanic plateau models can well explain many features of Archean cratons, oceanic plateaus formed by mantle plumes may not provide enough water (H₂O) for aqueous partial melting of basaltic rocks to create TTG magmas. It is the same case with heat-pipe models. As the sagduction models assume the existence of an old felsic continental crust, they are not suitable for discussing the origin of Archean continents. As for stagnant lids, they just describe the state of a single lithosphere plate which itself cannot provide any geodynamic mechanisms for making a mafic crust be partially melt to form TTG magmas. Therefore, none of available pre-plate tectonics models proposed so far has been successfully applied to interpret the origins of Archean continents. Thus, a number of research groups in the world are conducting extensive and comprehensive investigations on this important scientific conundrum. Although Chinese geologists missed a chance to have made contributions to establishing plate tectonic theory in the 1960s, they have a great potential for breakthroughs in establishing a pre-plate tectonics theory since they have done tremendous work on the early geodynamic mechanisms for the formation and evolution of Archean continents and produced large amounts of new data and competing interpretations in the past four decades.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Diorite enclaves and host granite of the early Miocene Gorontalo pluton in the North Sulawesi Arc, Indonesia: Implications for recycled oceanic crust and crust-mantle interaction

• Granites from the Early Miocene Gorontalo pluton were derived from a juvenile crust. • Diorite enclaves originated from mixing of depleted mantle- and juvenile crust-derived magmas . • Sub-normal mantle δ 18 O values suggest the involvement of recycled altered oceanic crust . • Early Miocene crust-mantle interaction occurred in the North Sulawesi Arc. Zircon Hf-O isotopes, combined with whole-rock Sr-Nd-Pb isotopes, can provide unambiguous evidence for crustal recycling and crust-mantle interaction processes. The North Sulawesi Arc (Indonesian) was traditionally regarded as an intra-oceanic arc, and the diorite enclaves and host granites of the Gorontalo pluton provide excellent opportunities for evaluating these issues, based on our zircon U-Pb geochronological and Hf-O isotopic, and whole-rock geochemical analyses. This study firstly reports the zircon U-Pb ages of ∼20.1–22.4 Ma for the host granites and ∼19.5–20.4 Ma for the diorite enclaves. The host granites have high SiO 2 , and low 10000*Ga/Al, A/CNK and Rb/Sr, classified as unfractionated I-type granites. They are characterized by high εNd (t) (from +4.8 to +5.7), high zircon εHf (t) (from +11.1 to +16.4), sub-normal mantle zircon δ 18 O (4.6–5.4‰), moderate ( 206 Pb/ 204 Pb) i (18.29–18.45), and young two-stage Hf model ages (52–394 Ma), deriving from partial melting of a juvenile mafic crust. The diorite enclaves show typical magmatic textures and contain abundant acicular apatite and plagioclase megacrysts with an obvious rim-core texture. They also have high εNd (t) (from +4.8 to +5.1), high zircon εHf (t) (from +10.7 to +15.2), sub-normal mantle zircon δ 18 O (4.1–5.1‰) and moderate ( 206 Pb/ 204 Pb) i (18.3), interpreted as the products of mixing of magmas derived from a depleted mantle and a juvenile crust. The diorite enclaves have sub-normal mantle δ 18 O values, positive εNd (t) and zircon εHf (t) , suggesting recycled altered oceanic crust into depleted mantle prior to magma generation. The presence of the host granites and diorite enclaves provides robust evidence for early Miocene (or prior to early Miocene) crust-mantle interaction in the North Sulawesi Arc.

The early Eocene (48\xa0Ma) Qaladeza trondhjemite formed by wet partial remelting of mafic crust in the arc-related Bulfat Igneous Complex (Kurdistan, Iraq): constraints on the timing of Neotethys closure

Events in the tectono-magmatic framework of the Eocene Bulfat Igneous Complex in the Kurdistan region of northeastern Iraq are revealed through the integrated field, petrographic, geochemical and zircon petrography and U–Pb geochronology of the Qaladeza trondhjemite sheets that cut gabbros of the complex. The trondhjemite sheets are variably deformed, with the cataclasis of large plagioclase feldspars. The trondhjemites are albitic, and geochemical features such as strong enrichment of the light REE relative to the heavy REE and low Ti and REE content show that they did not form as late differentiates of a MORB-like source, but instead by hydrous partial melting of mafic rocks, possibly also involving some more evolved crustal source as well. Qaladeza trondhjemite oscillatory-zoned magmatic zircon has an age of 48.0 ± 0.6 Ma (95% confidence). These domains have been etched to form voids in the zircons, the walls of which have thin coatings of Th- and Ca-bearing species. Subsequent partial recrystallization of the zircons obliterated the voids and increased the Th/U of the recrystallized zircon, which mostly formed at 38.74 ± 0.74 Ma. The 48 Ma domains relate to recycling of mafic crust in a proto-arc tectonic setting and are congruent with Eocene proto-arc magmatic activity within the Neotethyan oceanic basin. The 39 Ma domains relate to later tectonic-induced recrystallisation events coeval with the closure of Neotethys resulting from the late Eocene collision between the Arabian plate and Eurasia in Kurdistan.

Subduction initiation of the western Paleo-Asian Ocean linked to global tectonic reorganization: Insights from Cambrian island-arc magmatism within the West Junggar, NW China

Abstract The subduction initiation associated with the beginning of accretionary orogens has been thought to be related to global plate reorganization. To characterize the initial subduction within the western Central Asian Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide range (511–488 Ma) of ages with older ages up to 514–511 Ma. The felsic rocks exhibit variable SiO2 (53.0–77.4 wt%) and K2O (0.05– 2.24 wt%) contents and can be classified as diorite, granodiorite, trondhjemite, and tonalite. On the basis of their low TiO2 (0.12– 0.71 wt%) contents and characteristic trace element trends as well as high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ18O (5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret that the Cambrian felsic rocks have diverse origins, involving differentiation of arc basalts and partial melting of subducted oceanic crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which are lower than those of igneous zircons in equilibrium with mantle and thus reflect modification of their mantle source by hydrothermal fluids with seawater-like oxygen isotopes at high temperature. Combined with regional data, we propose that the West Junggar arc represents the extending of the Boshchekul-Chingiz arc in the Early Cambrian, defining a long (&amp;gt;1000 km) E-W–trending subduction zone. The earliest island-arc tholeiitic felsic plutons in the West Junggar took place at ca. 514–511 Ma, which, coupled with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages of subduction of the western Paleo-Asian Ocean probably occurred in the Early Cambrian. The simultaneity between the initial subduction of the western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests a causal link between the three, collectively correlated to a global plate adjustment event.