Late Archean Magmatism Research Articles

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable εHf(t) values from -2.8 to 11.0 and δ18O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling.

Late Neoarchean potassic high Ba–Sr granites in the Taishan granite–greenstone terrane: Petrogenesis and implications for continental crustal evolution

A series of Late Neoarchean high Ba–Sr granites with a potassium-rich signature has been identified from the Taishan granite–greenstone terrane (TSGT), within the Eastern Block of the North China Craton (NCC). LA-ICP-MS zircon dating shows that the Wanghailou, Hujiazhuang and Xiajiazhuang granitic intrusions were emplaced at ~2.54Ga, ~2.52Ga and ~2.47Ga, respectively. Geochemically, they are alkali-rich, with high K2O contents and high K2O/Na2O ratios, revealing their high-potassium calc-alkaline nature. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to typical A-, I- and S-type granites, and show negative anomalies in HFSE (Nb, Ta, Ti) and weak Eu anomalies. Their geochemical and isotopic characteristics, such as extremely low Y and Yb concentrations, high Sr/Y and (La/Yb)cn ratios, strongly fractionated REE patterns and elevated εNd(t) and εHf(t), imply that the parental magmas were derived directly from partial melting of the sub-arc enriched mantle at depths >15kbar with residual garnet in the source. This mantle was previously depleted lithospheric mantle and was subsequently metasomatized by subduction-related fluids and/or melts. Fractional crystallization and crustal assimilation of these magmas during ascent resulted in their variable geochemical and isotopic characteristics. Taking into account the space–time patterns of late Archean magmatism in the area, an active subduction regime is favored to account for the generation of these potassic high Ba–Sr granites in the TSGT.The two-stage Nd and Hf model ages of these granites suggest that continental crust older than 2.80Ga is preserved beneath the TSGT. In combination with previous data from the region, it is noted that, apart from rapid crustal growth at ~2.7Ga, other additions of mantle-derived magma, such as minor 2.6–2.5Ga sanukitoids, siliceous high-Mg basalts (SHMBs) and high Ba–Sr granites, also made a significant contribution to continental crustal evolution in the TSGT.

Petrogenesis and geodynamics of Late Archean magmatism in eastern Hebei, eastern North China Craton: Geochronological, geochemical and Nd–Hf isotopic evidence

The eastern part of the North China Craton contains large volumes of Late Archean hornblenditic, tonalitic, dioritic, granodioritic and granitic rocks. LA-ICPMS zircon U–Pb dating of three plutons from eastern Hebei, eastern North China Craton, record emplacement ages of 2526–2515 Ma and metamorphic ages of 2500–2490 Ma for the diorite, granodiorite, and monzogranite and 2440 Ma for the K-feldspar granite. Hornblendites have high MgO (8.6–10.6 wt.%), Cr (674–1126 ppm), Ni (125–159 ppm) and V (224–315 ppm) at low (basic) silica contents (SiO 2 = 45.7–49.0 wt.%). They are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with positive ɛ Nd( t) values (+1.2 to +2.0), indicating they were derived from an enriched mantle source. The monzogranites have high Sr and Ba contents and Sr/Y and La/Yb ratios, low Y and HREEs abundances, and positive ɛ Nd( t) and zircon ɛ Hf( t) values with Nd and Hf model ages less than 3.0 Ga, reflecting that they were derived from partial melting of a juvenile lower crust at high pressure. However, the K-feldspar granites, with low Sr and Ba concentrations and high HREE and Y contents, are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels. The diorites and granodiorites have variable major and trace element concentrations. Magmatic zircons from these rocks have heterogeneous ɛ Hf( t) values but record uniform homogeneous U–Pb ages, indicating they resulted from mixing of mafic and felsic magmas, coupled with fractional crystallization. Because of the widespread distribution of Late Archean magmatism and evidence for magma mixing, possibly related to mafic magma underplating, it appears likely that this event in the eastern North China Craton may relfect mantle plume activity, that resulted in significant growth of continental crust during the Late Archean.

A MORB-arc basalt–adakite association in the 2.5 Ga Wutai greenstone belt: late Archean magmatism and crustal growth in the North China Craton

The Wutai, Fuping and Hengshan Complexes form part of a major 2.5Ga late Archean arc system in the northern part of the North China Craton. The Wutai Complex is a well-preserved greenstone belt dominated by mafic, intermediate and felsic volcanic rocks, with a mid-ocean ridge basalt (MORB)-arc-back arc, basalt–adakite association. Multi-element plots for MORB-type mafic volcanic rocks are characterised by depletion of Th, Nb (Ta) and La, nearly flat or LREE-depleted REE patterns, and Nb/U and Nb/Th ratios greater than primitive mantle values. Back arc basin basalt (BABB)-type rocks have nearly flat trace element patterns with Nb, Ti and Y troughs, and LREE-depleted or near-flat REE patterns. Island arc basalt (IAB)-type rocks are characterized by negative anomalies of Nb (Ta), Zr (Hf), Ti and Y, and by LREE-enrichment. The felsic volcanic rocks conform to most compositional criteria for Cenozoic adakites, including low Y and Yb, high La/Smn, La/Yb and Zr/Sm ratios, and similar trace element and REE patterns. Trends defined by the association are consistent with magma mixing between MORB-like melts and subduction (slab)-related melts. Intra-oceanic subduction, coupled with contemporary MORB-type mantle upwelling related to back arc basin extension, can account for the interaction between the two components. Subduction–accretion or back arc basin collapse led to accretion of the Wutai intra-oceanic arc-back arc complexes with the magmatic arcs of the Fuping and Hengshan terranes, that are dominated by high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses. This geodynamic interpretation is different from many tectonic models for Archean greenstone belts in which magmatism and crustal growth were dominated by mantle plumes, or interaction between subduction zones and mantle plumes. The final disposition of the arc complexes was controlled by the collision of the eastern and western blocks of the North China Craton during the Paleoproterozoic, resulting in extensive tectonic interleaving within a major orogenic belt that extended throughout the central portion of the North China Craton.

Early Archean to Mesoproterozoic evolution of the Wyoming Province: Archean origins to modern lithospheric architecture

Local preservation of 3.6–3.0 Ga gneisses and widespread isotopic evidence for crust of this age incorporated into younger plutons indicates that the Wyoming Province was a [Formula: see text] 100 000 km2 middle Archean craton, which was modified by late Archean magmatism and tectonism and Proterozoic extension and rifting. On the basis of differences in late Archean histories, the Wyoming Province is subdivided into five subprovinces: three in the Archean core, (1) the Montana metasedimentary province, (2) the Bighorn subprovince, and (3) the Sweetwater subprovince, and two Archean terrains that may be allochthonous to the 3.0 Ga craton, (4) the Sierra Madre – Medicine Bow block, and (5) the Black Hills – Hartville block. A thick, fast lower crustal layer, imaged by Deep Probe, corresponds geographically with the Bighorn subprovince and may be an underplate associated with ca. 2.70 Ga mafic magmatism. The Sweetwater subprovince is characterized by an east–west tectonic grain that was established by three or more temporally related, late Archean, pulses of basin development, shortening, and arc magmatism. This tectonic grain, including the 2.62 Ga Oregon Trail structure, controlled the locations and orientations of Proterozoic rifting and Laramide uplifts. The present-day lithospheric architecture of the Wyoming Province is the result of cumulative processes of crustal growth and tectonic modification; lithospheric contrasts have apparently persisted for billions of years. If there has been any net crustal growth of the Wyoming Province since 3.0 Ga, it has involved a combination of mafic underplating and arc magmatism.

The Nevoria gold skarn deposit in Archean iron-formation, Southern Cross greenstone belt, Western Australia; I, Tectonic setting, petrography, and classification

The Nevoria gold skarn deposit is located in the Southern Cross belt of the Archean Yilgarn craton, Western Australia, and occurs in amphibolite facies greenstones between two dome-shaped granitoid batholiths. Regional pressure estimates suggest that the terrane is deeply eroded and that the skarn formed at a 10- to 15-km depth. The orebodies are confined to the folded contacts of three beds of silicate facies iron-formation and extend over a strike length of more than 1 km. Iron-formation-hosted skarn contains 6 to 7 g/t gold, 2 to 3 g/t silver, and less than 0.1 wt percent base metals. The gangue is calcic, highly reduced, and composed of two distinct mineralogic types: hedenbergite-actinolite and almandine-hornblende skarn. The calc-silicates are intergrown with abundant pyrrhotite (10 vol %) and with accessory (< or =0.5%) pyrite, arsenopyrite-loellingite, and chalcopyrite. Native gold is enclosed in hedenbergite, actinolite, almandine, and quartz and occurs together with the alloy maldonite (Au 2 Bi) and a suite of bismuth tellurides. Skarns replacing amphibolite and metakomatiite adjacent to calcic skarn in banded iron-formation form broad (up to 17 m) contact zones of outer biotite-rich alteration. These zones are composed of numerous alternating bands of reduced diopside-hornblende, almandine-hornblende, and almandine-cummingtonite-plagioclase skarn. Veins of oxidized grossular-diopside skarn occur in the amphibolites between and adjacent to the banded iron-formations. The skarns are cut by barren pegmatite dikes and are underlain by a massive pegmatite-granite pluton. These postskarn intrusions, in turn, are displaced by brittle-ductile faults, which control zones of mesothermal muscovite-chlorite-calcite alteration replacing amphibolite. These zones lack pyrite and gold mineralization. The Nevoria gold skarn is part of a larger group of reduced skarn deposits, which formed in a deep midcrustal environment (10-15 km) during late Archean magmatism, possibly in broad continental arcs at convergent plate margins. The Archean skarns are distinct from the copper-gold skarns in Cenozoic continental margins and from gold-rich iron-copper skarns in Cenozoic to Mesozoic island-arc terranes, which all formed at much shallower depths of 2 to 5 km.

Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: evidence from UPb systematics in zircon

The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically complex assemblage of lithotectonic units that include: (1) a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant UPb ages of 3.5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this rock: (1) an apparently protracted period of high-grade metamorphism and/or intrusion of additional magmas at ∼ 3.25 Ga; and (2) growth of hydrothermal zircon at ∼ 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ∼ 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on late Archean magmatism as well.

Geochronological constraints on late Archean magmatism deformation and gold–quartz vein mineralization in the northwestern Anialik River greenstone belt and igneous complex, Slave Province, N.W.T.

Three phases of the Anialik River igneous complex (ARIC) give U–Pb zircon ages in the range −2705–2683 Ma, and three units from the adjacent northwestern Anialik River greenstone belt (ARGB) give ca. 2678 Ma ages. Titanite from unsheared ARIC rocks crystallized during localized metamorphism and deuteric alteration between 2693 and 2683 Ma. Hydrothermal titanite in wall rocks to gold-mineralized shear zones crystallized during early shear zone development (2670 ± 1 Ma) and was subsequently locally altered to rutile, with gold occurring within the rutile-bearing assemblage. Unaltered second-generation hydrothermal titanite, overgrowing the rutile assemblage, crystallized during later brittle–ductile movement (2656 ± 2 Ma) and provides a minimum age for gold mineralization. Relatively high 207Pb/204Pb ratios of Pb in gold-associated galena suggest that it was partly derived from significantly older crustal material, possibly underlying the igneous complex and greenstone belt. This interpretation is consistent with other evidence for the existence of &gt; 3.0 Ga crustal rocks to the west of the study area. A late crosscutting granite gives an age of [Formula: see text] and is therefore part of the Pan-Slave tectono-thermal event. These results, and other data for the Slave Province, indicate temporal variations in the development and deformation of predeformational greenstone belts. The new ages show that regional deformation and metamorphism in the northwestern Slave Province followed shortly after major magmatism, and that gold mineralization might have occurred during the late Archean accretion of the greenstone belt and igneous complex to an older crustal domain to the west.

U–Pb ages for late magmatism and regional deformation in the Shebandowan Belt, Superior Province, Canada

Five precise U–Pb zircon (and titanite) ages from different lithologic units of the Shebandowan greenstone belt in the western Wawa Subprovince of the Superior Province put tight constraints on the time of late Archean magmatism and of two major deformation events.A porphyry sill from the older supracrustal sequence has an age of 2733 ± 3 Ma. Another porphyritic rock, a trondhjemite occurring as a clast in a conglomerate of the unconformably overlying Timiskaming-type supracrustal sequence, formed 2704 ± 2 Ma ago and defines a maximum age for the deposition of the Timiskaming-type sequence. An alkalic volcanic rock from this sequence has been directly dated at [Formula: see text], in accord with the above constraint and with another probable maximum age of deposition given by the date of 2696 ± 2 Ma for the Shebandowan Lake Pluton. A first deformation event (D1) was related to a predominantly vertical tectonic regime and occurred during or before intrusion of the Shebandowan Lake Pluton at 2696 ± 2 Ma. The second deformation event (D2) was caused by northwesterly-directed compression and occurred after [Formula: see text] ago, the age of the Timiskaming-type volcanic rocks. A minimum age for the D2 deformation event, which also affected the adjacent Quetico metasedimentary belt and was probably related to the development of major transcurrent fault systems throughout the Superior Province, is provided by an age of [Formula: see text] for the undeformed, late-kinematic Burchell Lake Pluton.

The age and geological history of the Wollaston, Peter Lake, and Rottenstone domains in northern Saskatchewan: Reply

Three U–Pb zircon and four K–Ar mineral age determinations from national topographic system (NTS) area Geikie River (SE) in northern Saskatchewan are presented and the geology of the region is described. The area lies within the Churchill Province of the Canadian Precambrian Shield and includes parts of the Wollaston, Peter Lake, and Rottenstone domains; the relationship between the geological histories of these domains and the age determinations is discussed. Zircon age measurements suggest a period of late Archean magmatism ca. 2500 Ma was responsible for both the Wollaston domain granitic basement and a suite of basic plutons in the adjoining Peter Lake domain.The overall lithological differences of the presumed lower Proterozoic supracrustal rocks in the region are examined in the context of a plate tectonic model. This invokes late Archean continental rifting accompanied by limited volcanism close to the present Needle Falls Shear Zone, followed by continental separation and the generation of an Aphe...

The age and geological history of the Wollaston, Peter Lake, and Rottenstone domains in northern Saskatchewan

Three U–Pb zircon and four K–Ar mineral age determinations from national topographic system (NTS) area Geikie River (SE) in northern Saskatchewan are presented and the geology of the region is described. The area lies within the Churchill Province of the Canadian Precambrian Shield and includes parts of the Wollaston, Peter Lake, and Rottenstone domains; the relationship between the geological histories of these domains and the age determinations is discussed. Zircon age measurements suggest a period of late Archean magmatism ca. 2500 Ma was responsible for both the Wollaston domain granitic basement and a suite of basic plutons in the adjoining Peter Lake domain.The overall lithological differences of the presumed lower Proterozoic supracrustal rocks in the region are examined in the context of a plate tectonic model. This invokes late Archean continental rifting accompanied by limited volcanism close to the present Needle Falls Shear Zone, followed by continental separation and the generation of an Aphebian ocean basin. Deposition of shelf and deeper water sediments along the trailing edge of the Wollaston domain continental margin was followed by reversal of plate movement, oceanic subduction, and the formation of an island-arc complex. This marked the start of the Hudsonian Orogeny and was accompanied by the development of successive generations of granitic material, including the Wathaman batholith. The regional emplacement of this batholith ca. 1865 Ma ago is compared to other volcano-plutonic belts described elsewhere along Cordilleran-type continental margins.