Late Archean Rocks Research Articles

Peel-back controlled lithospheric convergence explains the secular transitions in Archean metamorphism and magmatism

The preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the Mesoarchean and Neoarchean (∼3.2–2.5 Ga). These transitions, which are imprinted in the metamorphic and magmatic rock-record, include the emergence of paired metamorphism, an increasing depth of tonalite-trondhjemite-granodiorite (TTG) formation, the widespread appearance of granites and changes in the composition of mafic rock. Previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre-plate tectonic to plate tectonic regimes. However, their tectonic driver remains elusive, in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present-day. Here, we demonstrate that lithospheric convergence driven by peel-back (/peeling) process under warmer mantle conditions – termed as peel-back convergence – may explain the late Archean secular transitions. This tectonic phenomena features large-scale peeling (a form of delamination, but not dripping) of the mantle lithosphere with or without lower crustal rocks during convergence. We simulated numerical models of peel-back convergence under Archean crust-mantle conditions and predicted the pressure-temperature (P-T) conditions of crustal metamorphism and melting. The evolution of peel-back convergent setting features juxtaposition of a colder, compressional regime with thickened crust that forms at the site of peeling, and a warmer, extensional regime with thinned crust forming behind it. The metamorphic and magmatic P-T conditions prevalent in these two tectonothermal sites replicate the features of the late Archean rock record, including the: appearance of coeval high-T/P and intermediate-T/P metamorphic rocks; greater abundance of high-T/P relative to intermediate-T/P metamorphic rocks; dominant formation of TTGs at higher pressures where garnet ± rutile is stable in the residue; reworking of pre-existing felsic rocks/sediments to produce potassic granites; and interaction of crustal melts and mantle to form hybrid granitoids. Thus, peel-back convergence can explain the thermobaric bimodality of late Archean rocks within an asymmetric tectono-thermal framework. However, unlike modern convergent plate boundaries, the asymmetry is controlled by lithospheric peeling.

Stagnant-lid tectonics in early Earth revealed by 142Nd variations in late Archean rocks

A major change in Earth's geodynamics occurred ~3billionyears (Ga) ago, likely related to the onset of modern and continuous plate tectonics. However, the question of how Earth functioned prior to this time is poorly constrained. Here, we find a resolvable positive 142Nd anomaly in a 2.7Ga old tholeiitic lava flow from the Abitibi Greenstone Belt indicating that early-formed mantle heterogeneities persisted at least 1.8Ga after Earth's formation. This result contradicts the expected rapid early (~0.1Ga), as well as the slower present-day (~1Ga) mixing rates in the convecting mantle. Using a numerical modeling approach, we show that convective mixing is inefficient in absence of mobile-lid plate tectonics. The preservation of a 142Nd anomaly until 2.7Ga ago can be explained if throughout the Hadean and Archean, Earth was characterized by a stagnant-lid regime, possibly with sporadic and short subduction episodes. The major change in geodynamics observed around ~3Ga ago can then reflect the transition from stagnant-lid plate tectonics to modern mobile-lid plate tectonics. Solving the paradox of a convective but poorly-mixed mantle has implications not only for Archean Earth, but also for other planets in the solar system such as Mars.

Late Cretaceous volcanic rocks and associated granites in Gyeongsang Basin, SE Korea: Their chronological ages and tectonic implications for cratonic destruction of the North China Craton

The Gyeongsang Basin is the largest exclusively non-marine sedimentary basin in Korean Peninsula, located in southeastern part of the Peninsula. Its sedimentary setting, magma source, tectonic nature, and formation age are important to understand the Mesozoic tectonic evolution of not only the Sino-Korea Craton, but also the entire Northeast Asia. The Gyeongsang Supergroup, the basin fill, is divided into the Sindong, Hayang, and Yucheon Groups in ascending order. The Sindong and Hayang Groups are composed of sandstone, shale, minor amounts of conglomerate, and marl deposited in a non-marine environment, whereas the Yucheon Group is characterized by the dominance of volcanic rocks. Precise isotopic ages and geochemical analyses are scarce for the volcanic elements of this basin, restricting our understanding of its geodynamic setting and evolution. In this study, we present new U–Pb zircon ages from the volcanic rocks of the Yucheon Group and associated granites in the Gyeongsang Basin. Thirteen volcanic samples yielded ages ranging from 94.4 to 78.4Ma, and four granite samples give ages ∼72Ma. These data confirm that a major magmatic event occurred during Late Cretaceous, and the Gyeongsang volcano-sedimentary complex represents a rock association representing a Late Cretaceous island arc. In comparison with the Sambagawa metamorphic belt, the Jurassic Chichibu accretionary complex (AC) with Cretaceous strike-slip basin sediments, and the Cretaceous Shimanto mélange zone in southwest Japan, it is likely that above-motioned complexes constitute the trace of a continuous subduction-related island arc system in SE Korea extending from SW Japan. Although the peak ages of the magmatic rocks in the eastern part of the North China Craton (NCC) are 130±10Ma, indicating an important time of continental lithospheric thinning under the NCC, the ages of the Gyeongsang magmatic rocks are markedly younger. Late Cretaceous magmatism was very weak in the eastern NCC. Therefore, it is probable that Early–Middle Mesozoic lithospheric thinning beneath the eastern NCC is triggered by multiple tectonic factors, not only by the subduction of the Pacific plate suggested by some previous studies. In addition, 20 inherited/xenocrystic zircons from these volcanic rocks give Archean ages ranging from 2.5 to 2.6Ga, suggesting the existence of late Archean rocks in the Yeongnam Massif. The Yeongnam Massif is predominated by the latest Archean crustal growth at ∼2.50Ga, consistent with the characteristics of the North China Craton.

New massif of Archean alkaline syenites in the Murmansk domain of the Kola peninsula

Alkaline magmatism typical of a mature continental crust is among the most important geodynamic markers in the Early Precambrian. The Late Archean alkaline rocks represented by alkali granites, alkali gabbros, syenites, nepheline syenites, and carbonatites are known at the ancient shields [1]. In the Kola Peninsula, such magmatism (with an age of 2.75‐2.61 Ga) in the form of alkali granite and alkali gabbro‐nepheline syenite complexes occurs in the Archean Keivy and Ingozero structures [2, 3]. No Precambrian alkaline complexes have been found yet in the Archean Murmansk domain of the Kola peninsula. The Late Archean Iokan’gskii intrusive complex of porphyritic lepidomelane‐ferrohstingsite granites was previously ascribed by Batieva [2] to the alkali granite complex. However, later studies [4] showed that it belongs to the complex of ancient rapakivilike granites of normal alkalinity. During recent studies of the granitoids from the Barents Sea shore of the Kola Peninsula in the area of Gremikha settlement and Mt. Ostrovnaya, the small Late Archean alkaline massif called Panejavr Massif after an nearby lake was discovered and studied. The Panejavr Massif is located within the Iokan’gskii block of the Murmansk domain, and its position is controlled by the junction of two faults (Fig. 1). The massif is contained among Late Archean migmatite‐granites and metabasite dikes that fill the NE-trending faults. The Panejavr Massif is a complex stock having an autonomous internal structure discordant to the host rocks (Fig. 1) and consisting of two chemically and mineralogically contrasting intrusive phases.

Implications based on the first SHRIMP U–Pb zircon dating on Precambrian granitoid rocks in North Korea

Precambrian basement rocks in North Korea consist of supracrustals and granitoid rocks, of which the former comprise the late Archean Nangrim Group and the Paleoproterozoic Jungsan and Machollyong Groups, whereas the latter include four major types: grey TTG gneisses, garnet/sillimanite-bearing (S-type) granites, massive porphyritic granites, and hornblende-bearing granites. SHRIMP U–Pb zircon geochronology, combined with cathodoluminescence (CL) imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the Precambrian history of North Korea and its relations to North China. SHRIMP U–Pb zircon analyses on two grey gneiss samples (PO501 and K14) reveal that they were emplaced at 2538 ± 14 Ma and 2636 ± 49 Ma, respectively, and metamorphosed at 2462–2433 Ma. These data, for the first time, confirm the existence of late Archean rocks in the Korean Peninsula. Moreover, one grey gneiss sample (K14) contains several zircons with ages of 3.1–3.4 Ga, suggesting possible middle Archean crust in North Korea. Rocks with such ages are widely distributed in North China, implying that North China and North Korea have similar Archean basement. One grey gneiss sample (R53) yielded a SHRIMP zircon age of 1933 ± 12 Ma, indicating that some grey gneisses in North Korea formed during the Paleoproterozoic. Two garnet- and/or sillimanite-bearing S-type granite samples gave 207Pb/ 206Pb ages of 1908 ± 31 Ma and 1903 ± 49 Ma, interpreted to reflect the time of metamorphism of the Paleoproterozoic Jungsan and Machollyong Groups since these S-type granites are considered to be derived from the partial melting of pelitic rocks from these groups. The ∼ 1.9 Ga metamorphic event has also been revealed in Paleoproterozoic rocks in South Korea and North China. SHRIMP zircon data also show that massive porphyritic monzogranites in North Korea evolved over a protracted interval from 1865 Ma to 1843 Ma. Similar-aged porphyritic monzogranites are also found in North China and South Korea, and their origin is considered to be related to a post-tectonic extensional event of regional extent. The last phase of Precambrian magmatism recognized in North Korea is Grenvillian-aged, represented by the hornblende-bearing granite that yields a SHRIMP zircon age of 1195 ± 4 Ma, and is considered to be the source rock of the adjacent Yushulazi Group (North China) that contains numerous 1.1–1.2 Ga old detrital zircons.

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR2) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.

Paleomagnetism of late Archean rocks of Hamersley basin, Western Australia and the paleointensity at early Proterozoic

We report the results of paleomagnetic and rock magnetic measurements of late Archean rocks from Hamersley basin, Western Australia, and the paleointensity determination for early Proterozoic. Basalts and banded iron formations have two to four components of remanent magnetization, and have consistent directions for different localities, indicative of geomagnetic origin. Rock magnetic measurements of basalts reveal that the main magnetic mineral is a fine-grained magnetite present in ∼10 ppm in mass, which do not alter when heated up to its blocking temperature. We interpret the stable component up to ∼390 °C as the post-tilting thermal overprint from uplift at ≃2.0 Ga, and the higher temperature component as the pre-tilting thermoviscous remanence during burial metamorphism. From the Thellier type paleointensity experiments using the thermal overprint component, we obtain a mean virtual dipole moment (VDM) estimate of (1.8–3.6)×10 22 A m 2. This suggests that early Proterozoic was characterized by a 1 weak geomagnetic field of less than one-half of the present.

Late Archean basement in the Bangenhuken Complex of the Nordbreen Nappe, western Ny-Friesland, Svalbard

The rocks of western Ny-Friesland, northern Svalbard, are part of a tectonostratigraphy including four thrust sheets, each composed mainly of orthogneisses overlain by younger metasedimentary rocks. Previous geochronological studies have shown that the orthogneisses are dominated by ca. 1750 Mya granitoids. This study of a quartz-monzonite in one of the thrust sheets, the Nordbreen Nappe, yields a single-zircon U-Pb ion-microprobe age of 2709 O 28 My. This is the oldest rock unit so far reported in the Svalbard Caledonides. However, age-determinations on detrital zircons in the metasediments of western Ny-Friesland have shown that Late Archean rocks were prominent sources. The new ages presented here provide the first evidence of a local source for these sedimentary rocks.

Late Archean basement in the Bangenhuken Complex of the Nordbreen Nappe, western Ny-Friesland, Svalbard

The rocks of western Ny-Friesland, northern Svalbard, are part of a tectonostratigraphy including four thrust sheets, each composed mainly of orthogneisses overlain by younger metasedimentary rocks. Previous geochronological studies have shown that the orthogneisses are dominated by ca. 1750 Mya granitoids. This study of a quartz-monzonite in one of the thrust sheets, the Nordbreen Nappe, yields a single-zircon U-Pb ion-microprobe age of 2709 Ø 28 My. This is the oldest rock unit so far reported in the Svalbard Caledonides. However, age-determinations on detrital zircons in the metasediments of western Ny-Friesland have shown that Late Archean rocks were prominent sources. The new ages presented here provide the first evidence of a local source for these sedimentary rocks.

Origin of the Belomorian Chupa Gneiss in the Light of New Lithogeochemical Data

The stratotype Chupa Sequence, which makes up the synonymous tectonic nappe and is a member of oldest (Late Archean) rocks in the Belomorian Mobile Belt, is scrutinized. The nappe is mainly composed of the migmatized, coarse-grained garnet–biotite gneiss with kyanite. Relic formations are represented by lenses of the fine-grained (possibly primary) garnet–biotite gneiss. Fine-grained gneisses generally represent primary sedimentary formations, while metadacites are rare. The kyanite-bearing gneiss is presumably a restitic rock produced by migmatization. Chupa metasediments are represented by immature (low-differentiated) metagraywackes and are akin to metaterrigenous rocks from Late Archean greenstone belts of Karelia (Kostomuksha) and Canada (Quetico). The provenance of metasediments comprises the following three components: tonalite (dacite) 70%, mafic rocks 20%, and ultramafic rocks 10%. The Chupa metagraywacke presumably deposited in a fore-arc basin.

A Paleoproterozoic A-type Rhyolite

The rhyolites in the upper Luliang Group of Shanxi, China, are Paleoproterozoic weakly alkaline volcanic rocks. They are characterized by high SiO2, Na2O + K2O, Zr, Nb, Ga, Y and REE contents and large FeO* /MgO, Rb/Sr and Ga/Al ratios, and low CaO, Sr and Eu contents, and share much in common with the A-type granitic rocks. They erupted in the rift setting at the continental margin. Chemical features and isotope data, as well as high Nd and low initial Sr ratios, suggest that the original granitic magma was derived from partial melting of Late Archean metamorphic rocks in the lower crust due to the influence of basaltic magma and hot fluid in response to rifting. The A-type rhyolites were finally formed after the fractional crystallization of the dominant mineral feldspar.

Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana palaeogeography

Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform. SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69 Ga, and that the depositional age of the Itremo Group must be less than 1855 ± 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 ± 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland.

Late Archean Mantle Composition and Crustal Growth in the Western Superior Province of Canada: Neodymium and Lead Isotopic Evidence from the Wawa, Quetico, and Wabigoon Subprovinces

Neodymium and lead isotopes are presented for late Archean rocks from the Wawa, Quetico, and Wabigoon subprovinces (Sp) in the Western Superior Province (Ontario, Canada). The isotopic compositions were determined on greenstone volcanic sequences, pre-tectonic (2.73-2.69 Ga) trondhjemite-tonalite-granodiorite (TTG) suites, metasedimentary rocks, and post-tectonic (2.69-2.67 Ga) dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the Western Superior Province. Although derived from a depleted mantle, almost all the greenstone volcanism and pre-tectonic TTG suites record contamination by crustal material as old as 3.2 Ga. Neodymium and lead isotopic compositions of the pre-tectonic TTG can be modeled as products of the melting of amphibolitic crust (e.g., subducted basalts) contaminated by 1-10% of subducted sediments. Moreover, lead isotopes of TTG suites provide evidence that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca. 2.7 Ga juvenile material, in agreement with neodymium isotopic differences recorded by sedimentary units of the Quetico Sp (50:50 mix of young:old crusts) and Wawa Sp (75:25 young:old). Neodymium and lead isotopes of the post-tectonic REE-enriched Archean sanukitoid suites define an isotopic trend which is distinct from that of the TTG suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites. Furthemore, such plutons found in the Wawa Sp have more juvenile neodymium and lead isotopic compositions and higher Nd contents than those intruded into the Quetico and Wabigoon Sp which have more crustal isotopic signatures and lower Nd concentrations. This geographical difference could be due either to differences in Nd contents of initial Archean sanukitoid magmas, lesser assimilation in the Wawa Sp or, more probably, because the Wawa crust is on average more juvenile than the Quetico and Wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in Quetico and Wawa Sp.

A magnetotectonic study correlating late Archean deformation in northwestern Ontario

Late Archean lamprophyre dikes crop out on both sides of the Quetico–Shebandowan subprovince boundary. They post date F1 folds in late Archean rocks (2690 Ma) but their intrusion overlapped with the waning phase of S1 development. S1 developed to varying degrees or is absent in some dikes. Nevertheless, the dikes show a cryptic tectonic fabric in the ferromagnetic minerals revealed by anisotropy of magnetic susceptibility (AMS). This is parallel to S1 in the host metamorphic rocks. Thus, the S1 fabric-forming episode may be correlated from one subprovince to the other using the dike swarm as a chronological marker.

Petrogenesis of a Mid-Proterozoic Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complex: Isotopic and Chemical Evidence from the Nain Plutonic Suite

The Nain Plutonic Suite (NPS), about 1.30 Ga, underlies some 19,000 km², intruding the boundary between Middle to Late Archean rocks in the east (Nain Province) and Late Archean and Early Proterozoic rocks in the west (Churchill Province). Nd isotopic compositions in most igneous units of the NPS reflect their geographic position relative to the inferred boundary between the Nain and Churchill Provinces. Rare exceptions are local intrusions of olivine-bearing basic rocks with highest $$\epsilon_{Nd}$$ @ 1.3 Ga (near -3.0), present in both eastern and western sectors of the NPS. Anorthositic rocks have variable and significant contributions of crustal Nd but notably lower $$I_{Sr}$$ than granitoid rocks. Ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids. Aluminous orthopyroxene megacrysts in NPS anorthositic rocks indicate crystallization pressures of 6.2 to 11.0 kbar, consistent with lower crustal to upper mantle depths. The granitoid rocks crystallized from magmas derived in large part by crustal partial melting. These rocks show the influence of sources similar to the basement gneisses, but were augmented by materials with much shorter crustal residence time, offering support for contemporaneous basaltic underplating of the lower crust. Nd and Sr isotope data, trace element geochemistry, and mineral compositions in major rock units of the NPS, together with knowledge of the timing of intrusive events, permits assessment of the essential features of petrogenesis and development of a paradigm that may have wider application. Separation of early granitoid melts left depleted, hot, plagioclase-pyroxene granulite crustal residues assimilated by mantle-derived basaltic magmas. That process developed large volumes of anorthositic magma in upper levels of deep crustal to uppermost mantle magma chambers by plagioclase flotation. Concurrently, fractionation moved residual liquids toward ferrodiorite compositions. Buoyant upward movement of anorthositic magmas followed crustal paths preheated by earlier passage of granitoid magmas. Only small volumes of high density, Fe-rich, ferrodiorite melts were expelled to higher crustal levels.

Age and composition of Archean crystalline rocks from the southern Madison Range, Montana: Implications for crustal evolution in the Wyoming craton

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the metasupracrustal sequence ([approximately]2.6 Ga)-along with an intercalated meta-andesite ([approximately]2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group ([approximately]3.3 Ga) and a younger, mostly granitic group ([approximately]2.7 Ga). Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crustmore » in their petrogenesis. These comparisons suggest that the juxtaposition of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes. 41 refs., 6 figs., 2 tabs.« less

A review of the Superior Province of the Canadian Shield, a product of Archean accretion

The Superior Province consists of northern and southern high-grade gneiss subprovinces and a central region of alternating granite-greenstone and metasedimentary belts. Subprovinces are commonly fault-bounded and display contrasting lithological assemblages, metamorphic and structural styles, geophysical characteristics, and ages; they are comparable to Phanerozoic suspect terranes. The Middle and Late Archean rocks of the Superior Province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca. 3.1 Ga. High-grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal-rich belts but one, Minnesota River Valley, is part of an older, ca. 3.6 Ga terrane that was juxtaposed late in the tectonic history. Granite-greenstone terranes are characterized by low-grade volcano-sedimentary sequences (tholeiitic-komatiitic submarine lava plain; tholeiitic-calc-alkalic submarine to subaerial central complexes; early platform-type quartz arenite-stromatolitic carbonate; late Timiskaming-type shoshonitic/alkalic volcanic-fluvial sediment) that range in age from ca. 3.0 Ga to 2.7 Ga. The volcanics represent oceanic, island arc, and continental arc volcanism, the quartz arenite-carbonate sequences deposition on relatively stable, early-formed ca. 3.0 Ga crust, and the Timiskaming-type sequences deposition in pull-apart basins developed along strike-slip faults. Late Archean, ca. 2.75-2.70 Ga greenstone sequences occur throughout the Superior Province. Middle Archean, ca. 3.0-2.8 Ga sequences are common in northern and central Superior province, rare or absent in the south. Volcanism was accompanied by mafic to trondhjemitic plutonism. The metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca. 3.0 Ga to 2.7 Ga. The metasediments were deposited after major volcanism and probably represent accretionary prisms. Late Archean orogenesis, accompanied and followed by calc-alkalic plutonism, youngs southward from about 2.73-2.72 Ga to 2.70-2.68 Ga. It involved large-scale north-south compression and dextral transpression and displays thin- to thick-skinned transitions. The contemporaneity of events along the lengths of the belts and their southward younging, coupled with the evidence of major compression, are consistent with subduction-driven oblique accretion of oceanic and continental volcanic arcs, accretionary sedimentary wedges, older microcontinental fragments etc. in a convergent margin setting analogous to parts of the Pacific Rim. Differences (eg. abundance of komatiites and tonalitic plutonic suites) are attributable to a hotter Archean mantle.

Magma mixing in late Archean tonalitic and mafic rocks of the Lac des Iles area, western Superior province

Abstract Plutonic rocks in the Lac des Iles area of the western Superior Province provide a record of contemporaneous late Archean mafic and tonalitic magmatism. This suite of rocks consists of mafic to ultramafic intrusions, mafic to intermediate dikes, hornblendite, and hornblende tonalite. The tonalite contains net-veined and disaggregated mafic to intermediate dikes, mafic schlieren and zones of hornblendite to mela-tonalite with skeletal hornblende. These textures are interpreted to be the result of magma mixing. The hornblende tonalite is geochemically characterized by high Al 2 O 3 , high Na 2 O/K 2 O, low Rb, moderate Sr and fractionated REE. The hornblendite, mafic to intermediate dikes and tonalite show whole-rock trends of decreasing total REE, increasing depletion of HREE and increasing Eu/Eu∗ with increasing SiO 2 . Amphiboles range in composition from edenitic and pargasitic hornblende to hornblende in dike rocks, mela-tonalite and hornblendite, and from hornblende to actinolite in tonalite. The development of skeletal amphiboles with strong normal zoning and resorbed hornblende with weak reverse zoning are attributed to conditions of supercooling and superheating, respectively, in zones of magma mixing. The least fractionated dike rocks geochemically resemble lamprophyres derived from alkali-basaltic magma. Variation of REE among the mafic to intermediate dikes suggests that they fractionated hornblende. Hornblendites and mela-tonalites can be modeled as hornblende cumulates from the dike rocks. The amphiboles probably crystallized rapidly during mixing, leaving a feldspathic residue. Variation in hornblende tonalites is consistent with mixing of SiO 2 -rich tonalite melt with mafic to intermediate dike phases and associated hornblende cumulates and feldspathic residue. Open-system magmatic processes at Lac des Iles result in a continuum of disequilibrium textures reflecting emplacement of mafic magma into a crystallizing felsic host. The plutonic rocks probably formed from a zoned magma chamber with a mafic base and tonalitic top. Breccia zones in the tonalite suggest that explosive degassing of the chamber occurred. Structures similar to those reported here are common in other granitoids in the Superior Province and suggest that mafic magmatism and magma mixing played an important role in the development of late Archean granitoid plutons.

Age and origin of the Tanghu Granite, southeast China: Results from U-Pb single zircon and Nd isotopes

U-Pb single-zircon and Sm-Nd whole-rock isotopic data for the Tanghu Granite of the Zhuguangshan batholith suggest that a basement either composed of or derived from Middle Proterozoic to Late Archean rocks underlies southeast China The granite contains two kinds of inherited zircon components, whole anhedral zircons and small cores within younger magmatic zircons. The U-Pb data for a single analysis of four magmatic zircons yielded a concordant age of 433 ±2 Ma, whereas single zircons yielded two linear arrays with lower intercepts of 435 ±1 and 434 ±14 Ma and upper intercepts of 1415 ±3 and 2516 ±6 Ma, respectively. Xenocrystic zircons of Archean age had not previously been recognized in any rocks from southeast China Two whole-rock samples from the granite yielded ϵNd values of -7.1 and -8.3 at 434 Ma and Nd model ages of ∼1750 Ma relative to a depleted mantle. We conclude from this isotope data that the Tanghu Granite crystallized at 434 ±1.6 Ma (weighted average) and was derived from a source of unknown age, but had an average crustal-residence age of ∼1750 Ma, and was partially composed of a mixture of continental crustal material of at least two ages (∼1400 and ∼2500 Ma).

Stored mafic/ultramafic crust and early Archean mantle depletion

Both early and late Archean rocks from greenstone belts and felsic gneiss complexes exhibit positive ε Nd values of +1 to +5 by 3.5 Ga, demonstrating that a depleted mantle reservoir existed very early. The amount of preserved pre-3.0 Ga continental crust cannot explain such high ε values in the depleted residue unless the volume of residual mantle was very small: a layer less than 70 km thick by 3.0 Ga. Repeated and exclusive sampling of such a thin layer, especially in forming the felsic gneiss complexes, is implausible. Extraction of enough continental crust to deplete the early mantle and its destructive recycling before 3.0 Ga ago requires another implausibility, that the sites of crustal generation and of recycling were substantially distinct. In contrast, formation of mafic or ultramafic crust analogous to present-day oceanic crust was continuous from very early times. Recycled subducted oceanic lithosphere is a likely contributor to present-day hotspot magmas, and forms a reservoir at least comparable in volume to continental crust. Subduction of an early mafic/ultramafic “oceanic” crust and temporary storage rather than immediate mixing back into undifferentiated mantle may be responsible for the depletion and high ε Nd values of the Archean upper mantle. Using oceanic crustal production proportional to heat productivity, we show that temporary storage in the mantle of that crust, whether basaltic as formed by 5–20% partial melting, or partly komatiitic and formed by higher extents of melting is sufficient to balance an early depleted mantle of significant volume with ε Nd at least +3.0.