Ga Supracrustal Rocks Research Articles

New discovery of 3.84–3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Abstract Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838–3754 Ma trondhjemitic gneisses (4 samples), 3786–3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786–3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4–3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4–56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86–1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2–11.3) and lower Eu/Eu* values (0.62–0.79). The potassic granites have (La/Yb)N ratios of 20.7–55.2 and Eu/Eu* values of 0.75–1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3–3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1–3.5 Ga (mainly 4.1–3.8 Ga) and δ18O of 4.6‰–7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

The Paleoarchean Northern Mundo Novo Greenstone Belt, São Francisco Craton: Geochemistry, U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with εHf(t) = −2.47 to −1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have εHf(t) = −5.40 to −0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between −1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of −0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S ~ 0‰, and Δ36S ~ 0‰. A carbonaceous schist shows positive δ34S (2.1‰–3.5‰) and elevated Δ33S (1.2‰–1.4‰) values, with corresponding negative Δ36S between −1.2‰ to −0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.

Linkage of crustal deformation between the Archaean basement and the Proterozoic cover in the Peräpohja Area, northern Fennoscandia

Reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold-and-thrust belt structural styles and patterns of the entire Palaeoproterozoic Peräpohja Belt in northern Finland show that both the deposition of the 2.4–1.9 Ga supracrustal rocks and the structural evolution during the subsequent Svecofennian Orogeny (1.9–1.8 Ga) have been tightly controlled by (i) the palaeo-topography of the Archaean (>2.5 Ga) basement, and (ii) the reactivation of Archaean basement faults during the multiple stages of rifting and deposition of the supracrustals, as well as during subsequent basin inversion and fold-and-thrust belt development. Folds and thrusts in the supracrustal sequence are found to be largely detached from the underlying basement along the Petäjäskoski Detachment Zone (PDZ) that formed within a mechanically weak stratigraphic unit in the basal part of the Peräpohja Belt stratigraphy. The pattern of contractional structures is complex, but developed during one phase of simple, progressive south-vergent thrusting. Two styles of basement-cover linkage are recognised: The hard-linked style is best illustrated by the reverse Sihtuuna Fault currently defining the northern margin of the E-W to ENE-WSW trending basement horsts and the overlying anticlines, and juxtaposing the stratigraphically lowermost Peräpohja Belt rocks with higher-up stratigraphy, indicating the fault is inherited after a pre-existing basement structure. Based on the distribution and thickness variation of the Palaeoproterozoic supracrustal units, the faults bounding the basement horst were active as extensional faults during deposition. The soft-linked style relationship is evident as abrupt terminations of thrust-related elongate domes and localisation of the compressional overprint into the NW-SE trending central graben. Understanding the basement architecture and basement-cover linkages is highly significant because the majority of the known mineralizations of the belt are located along or close to the basement faults.

A Neoarchean subduction recorded by the Eastern Hebei Precambrian basement, North China Craton: Geochemical fingerprints from metavolcanic rocks of the Saheqiao-Shangying-Qinglong supracrustal belt

The Saheqiao-Shangying-Qinglong supracrustal belt (SSQB) in the northern Eastern Hebei Precambrian basement (EHPB) is located in the northern margin of the Eastern Block (EB) of the North China Craton (NCC). The Shangying terrane constitutes the middle segment of the SSQB and contains primarily metamorphic volcanics and plutonic tonalitic gneisses. The metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites, garnet plagioclase amphibolites, biotite plagioclase amphibole gneisses, and amphibole plagioclase gneisses. Zircon U-Pb-Lu-Hf isotopic analyses reveal that the metavolcanic rocks from the Shangying terrane crystallized at ∼2506–2613Ma with TDM (Hf) values of ∼2541–2944Ma. These metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features. Group I consists chiefly of tholeiites that are characterized by slightly light rare earth element (LREE) depleted patterns and flat multi-element spider diagrams, which are similar to back-arc basin basalt (BABB)-like rocks and were derived from the partial melting of the depleted mantle. The tholeiites in Group II have slightly fractionated rare earth element (REE) patterns without Nb anomalies, exhibit an affinity to Nb-enriched basalt (NEB)-like rocks, and were produced by the partial melting of HFSE-enriched mantle peridotites. Group III is composed of slightly LREE-enriched tholeiites with negative Nb-Ta anomalies that resemble island arc tholeiites. Group IV comprises calc-alkaline basalts and andesites with highly enriched LREEs and evident Nb, Ta and Ti depletions that are geochemically similar to the products of island arcs. The island arc tholeiites and calc-alkaline basalt-andesites originated from the partial melting of sub-arc mantle peridotites that were previously metasomatized by slab-derived fluids/melts with the fractional crystallization of ferromagnesian minerals. Collectively, the BABB-like rocks, the NEBs, arc tholeiites and calc-alkaline basalt-andesites recorded magmatism from a subduction regime. Similarly, NEB, arc tholeiite-andesite-dacites and back-arc related rocks were recognized in the Saheqiao and Qinglong areas in the western and eastern parts of the SSQB (R. Guo et al., 2013, 2015), which further supports the existence of a Neoarchean convergent margin in the Eastern Hebei Precambrian basement (EHPB). Compared to the northwestern EHPB, the southeastern EHPB underwent lower metamorphism, contains arc dacites/rhyolites and greater contributions of the ancient continental crust, implying that the southeastern EHPB might have been or near a back-arc setting. Thus, we conclude that the SSQB experienced a southeastward-dipping subduction, which is consistent with studies of the adjacent regions (e.g., Western Liaoning Province). When combining the ∼2.5–2.6Ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the EB of the NCC, we concluded that ∼2.5–2.6Ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth.

New geochronological and Sm–Nd constraints across the Pajala shear zone of northern Fennoscandia: Reactivation of a Paleoproterozoic suture

Altogether 1130 U–Pb analyses on zircons from 25 samples across the Pajala shear zone in northern Fennoscandia were performed by LA-ICPMS methodology to study the origin of the zone. The samples were also analyzed for their Sm–Nd isotope systematics and geochemically. The new data is tested with existing geological information to elaborate the provenance and maximum depositional ages, the stratigraphic position of studied samples, and finally to assess the tectonic evolution in the Pajala shear zone area.Chromium-enriched metasedimentary rocks are found within and to the east and west of the N-S trending Pajala shear zone. No material originating from Proterozoic felsic igneous (zircons) or mafic (Sm–Nd) rocks was found in these rocks and thus, only Neoarchean or very early Paleoproterozoic ages of deposition are possible. Our new results indicate that the bimodal 1.99–1.97Ga supracrustal rocks of the Rovaniemi Supersuite cover a large area east of the Pajala shear zone. Based on the absence of Archean zircons typical of the metasedimentary rocks covering the Archean basement, it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the Rovaniemi Supersuite. The metasedimentary rocks of the Uusivirka Supersuite, located within the Pajala shear zone, vary from hornblende-bearing metasedimentary rocks, metapelite, and meta-arkose to orthoquartzite. Their characteristic features include a dominant Proterozoic zircon population (65–87%), with a high proportion of analyzed grains yielding ages between 1.96Ga and 1.91Ga, and maximum depositional ages of 1.92–1.91Ga. We record at least three metamorphic events; at ca. 1.92–1.90Ga, ca. 1.85Ga and at 1.82–1.78Ga, of which the youngest can possibly be separated into two events at 1.83–1.82Ga and 1.79–1.77Ga, respectively. We propose a tectonic model in which continental breakup occurred at 2.1–2.05Ga, followed by the development of a magmatic arc (Kittilä arc) at ca. 2.0Ga above a subduction zone. Two Archean continental units (Norrbotten and Karelia) collided at ca. 1.92–1.91Ga, the Kittilä arc was obducted upon the Karelia continent as a foreland fold and thrust belt, and the metasedimentary rocks of the Uusivirka Supersuite were emplaced as foreland deposits. The proposed tectonic model is based on the absence of 2.44Ga layered intrusions, and especially 2.06Ga rocks and 1.99–1.97Ga rocks of the Rovaniemi Supersuite that are found only east of the Pajala shear zone. In our model the Pajala shear zone originated as a divergent plate boundary, and was multiply reactivated after continental collision with both lateral and vertical movements.

Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae craton, Canadian Shield

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3Ga Arrowsmith orogeny, the 1.99–1.93Ga Taltson orogeny, the ca. 1.90Ga Snowbird orogeny and the ca. 1.87–1.80Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to <2.17Ga supracrustal rocks of the Murmac Bay Group at more variable metamorphic grade. Whereas the older package underwent thermal and/or metamorphic events in the Archean (ca. 2.57Ga) and early Paleoproterozoic (ca. 2.34Ga; Arrowsmith orogeny), rocks at higher structural levels bear no record of these events, recording only younger (1.94–1.90Ga) metamorphism interpreted to reflect two further stages of orogenesis. The first stage, correlated with Taltson orogeny, involved tight to isoclinal folding (D1/2) that led to development of the composite ESE-trending (S0/S1/S2) transposition foliation followed by more open folding (D3). The absence of Taltson-age plutonic rocks, except for low-volume crustal melts, suggests that this cycle was driven by crustal thickening, consistent with a clockwise P–T–t path derived for one sample. The high degree of strain along the contact between Archean rocks and the Murmac Bay Group, along with local structural discordance, indicates some degree of translation of cover rocks over basement during this stage. Pelitic rocks at higher structural levels (low- to medium-grade zone) underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels (high-grade zone) attained upper amphibolite facies conditions. Monazite data bracket prograde metamorphism (and related garnet growth) between 1939±6Ma and 1934±5Ma. The second stage, dated most precisely at 1906±7Ma, involved refolding of D1 to D3 structures about NE-trending axes (F4) and coeval dextral shearing and is correlated with Snowbird orogenesis. Rocks of the low- to medium-grade zone were held at roughly the same crustal level whereas rocks in the high-grade zone were uplifted via dextral–oblique displacement along domain-bounding shear zones, instigating decompression (garnet to cordierite) reactions. This study demonstrates that after <2.17Ga deposition of the upper Murmac Bay Group, rocks along the entire west-southwestern Rae margin, from the Northwest Territories/northern Alberta to the Snowbird tectonic zone, were affected by Taltson (1940–1930Ma) orogenesis. The Taltson tectono-metamorphic cycle then appears to have transitioned, as rocks underwent further shortening and uplift in a dextral transpressive regime, into Snowbird tectonism at ca. 1910–1900Ma. Snowbird orogenesis is therefore best viewed as part of a semi-continuous 1940–1900Ma cycle of tectonic burial and exhumation, with its inception at ca. 1910Ma triggered by a change in boundary conditions, as yet not fully understood, but consistent with arrival of the Hearne craton proposed by others. While the Beaverlodge domain has many features in common with the Tantato domain, future work needs to explore why rocks in the Tantato domain lack evidence for Arrowsmith orogeny and failed to record the cycle of 1.94–1.93Ga tectonic burial.

Mojave-Yavapai boundary zone, southwestern United States: A rifting model for the formation of an isotopically mixed crustal boundary zone

The boundary between the Paleoproterozoic Mojave and Yavapai crustal provinces in the southwestern United States is a 75-km-wide zone characterized by complex, isotopically mixed (Pb) crust, similar to other transitional crustal and lithospheric boundaries worldwide. Mojave crust is more isotopically evolved than the juvenile crust of the Yavapai province, indicating an older crustal component. Several areas of isotopically juvenile rocks within and near the boundary zone correspond to exposures of coeval >1.73 Ga supracrustal rocks that contain pillow basalts and bimodal metavolcanic sequences. We suggest that regions of juvenile crust within the Mojave province and its boundary zone represent rift basins, produced by extension or transtension near the eastern margin of the province, into which primitive basaltic lavas and bimodal volcanic sequences were erupted. The largest region of juvenile material within the boundary zone is bound to the west by a series of coincident, N-trending isotopic, geochemical, metamorphic, and geophysical discontinuities that may represent a fundamental, buried crustal structure, perhaps a rift margin. We propose that the distinct isotopic signature of the boundary zone persisted through subsequent deformational events, including the one that juxtaposed the two provinces. If our model for the formation of the boundary zone is correct, its isotopically mixed character is at least partly an artifact of the precollision history of the eastern Mojave province, and may not solely reflect the processes that juxtaposed the two provinces. Formation of new crust by rifting may represent an underappreciated form of crustal growth during the Paleoproterozoic that is distinct from arc accretion. Models for the formation of complex crustal boundary zones must consider the early, prejuxtaposition history of terranes in interpreting their geochemical and geophysical characteristics.

The timing and isotopic character of regional hydrothermal alteration and associated epigenetic mineralization in the western sector of the Kaapvaal Craton (South Africa)

The Kaapvaal Craton of South Africa comprises an Archaean core of ≈3.5 Ga lithospheric and crustal rocks surrounded by younger accreted terrains of ≈3.0–2.7 and ≈2.1–1.9 Ga. The craton is covered by relatively undeformed 3.0–2.4 Ga supracrustal rocks, which show the effects of thermal and hydrothermal interaction. Part of this activity is manifested by a large number of epigenetic Pb–Zn (±Ag, Au, Cu, F) deposits in the cover rocks of the Kaapvaal Craton. These include small volcanic and breccia hosted deposits in mafic and felsic volcanic rocks of the 2.7 Ga Ventersdorp Supergroup and the Mississippi Valley-type (MVT) deposits in the carbonates of the Transvaal Supergroup. MVT mineralization at the Pering (and other Zn–Pb deposits) is hosted in fracture-generated N–S breccia bodies in the Paleoproterozoic carbonate succession of the western Kaapvaal Craton. The fluids carrying the metals were focused in vertical bodies within the fracture zones (FZ), the metals and the sulphur being carried together and precipitated in organic-rich sectors of the basin. Two small Pb–Zn deposits within mafic rocks of the Ventersdorp Supergroup, stratigraphically below the basin-hosted MVTs on the southwestern part of the Kaapvaal Craton have secondary chlorite which is extremely Rb-rich, associated with the mineralization. This chlorite and the associated altered basaltic host rocks give a Rb–Sr date of ≈1.98 Ga, and the associated galena Pb isotope data plot on the same array as those of other Pb–Zn deposits, the radiogenic intercept giving a date of ≈2.0 Ga. We interpret these data to indicate a craton-wide epigenetic fluid-infiltration event, which exploited the Maquassie Quartz Porphyry (MQP) as the aquifer and metal source. Sr isotopic results for the ore-zone gangue minerals show highly radiogenic 87Sr/ 86Sr ratios (>0.710) which support earlier models that the origin of radiogenic Sr isotopic composition in the calcite cements is the felsic tuffs (MQP) of the Ventersdorp Supergroup occurring at deeper levels within the basin. Relationships between δ 18O and δ 13C performed on carbonate cements within the aquifers are complex: the range in δ 13C for some of the cements represents a mixture from two sources and with a progression from heavy carbon in the host to somewhat lighter carbon in the cements. Similarly, the lighter δ 18O values have a narrow range indicative of rapid exchanges between hydrous fluid and rock.

Precambrian crust beneath the Mesozoic northern Canadian Cordillera discovered by Lithoprobe seismic reflection profiling

The Cordillera in northern Canada is underlain by westward tapering layers that can be followed from outcrops of Proterozoic strata in the Foreland belt to the lowermost crust of the orogenic interior, a distance of as much as 500 km across strike. They are interpreted as stratified Proterozoic rocks, including ∼1.8–0.7 Ga supracrustal rocks and their basement. The layering was discovered on two new deep seismic reflection profiles in the Yukon (Line 3; ∼650 km) and northern British Columbia (Line 2; ∼1245 km in two segments) that were acquired as part of the Lithoprobe Slave‐Northern Cordillera Lithospheric Evolution (SNORCLE) transect. In the Mackenzie Mountains of the eastern Yukon, the layering in Line 3 is visible between 5.0 and 12.0 s (∼15 to 36 km depth). It is followed southwestward for nearly 650 km (∼500 km across strike) and thins to less than 1.0 s (∼3.0–3.5 km thickness) near the Moho at the Yukon‐Alaska international boundary. In the northern Rocky Mountains of British Columbia, the upper part of the layering on Line 2 correlates with outcrops of Proterozoic (1.76–1.0 Ga) strata in the Muskwa anticlinorium. At this location, the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for ∼700 km (∼300 km across strike). It disappears as a thin taper at the base of the crust ∼150 km east of the coast of the Alaskan panhandle. The only significant disruption in the layering occurs at the Tintina fault zone, a late to postorogenic strike‐slip fault with up to 800 km of displacement, which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles (∼300 km apart). The base of the layered reflection zone coincides with the Moho, which exhibits variable character and undulates in a series of broad arches with widths of ∼150 km. In general, the mantle appears to have few reflections. However, at the southwest end of Line 3 near the Alaska‐British Columbia border, a reflection dips eastward from ∼14.0 s to ∼21.0 s (∼45 to 73 km depth) beneath exposed Eocene magmatic rocks. It is interpreted as a relict subduction surface of the Kula plate. Our interpretation of Proterozoic layered rocks beneath most of the northern Cordillera suggests a much different crustal structure than previously considered: (1) Ancient North American crust comprising up to 25 km of metamorphosed Proterozoic to Paleozoic sediments plus 5–10 km of pre‐1.8 Ga crystalline basement projects westward beneath most of the northern Canadian Cordillera. (2) The lateral (500 km by at least 1000 km) and vertical (up to 25 km) extent of the Proterozoic layers and their internal deformation are consistent with a long‐lived margin for northwestern North America with alternating episodes of extension and contraction. (3) The detachments that carry deformed rocks of the Mackenzie Mountains and northern Rocky Mountains are largely confined to the upper crustal region above the layering. (4) Accreted terranes include thin klippen that were thrust over North American pericratonic strata (e.g., Yukon‐Tanana), and terranes such as Nisling and Stikinia that thicken westward as the underlying Proterozoic layers taper and disappear. (5) The ages of exposed rocks are not necessarily indicative of the ages of underlying crust, a frequent observation in Lithoprobe interpretations, so that estimates of crustal growth based on surface geology may not be representative.

Geological and isotopic geochemical constraints on the evolution of the Fuping Complex, North China Craton

The Neoarchean to Paleoproterozoic Fuping Complex together with the adjacent Hengshan and Wutaishan complexes form the middle part of the Central Zone that separates the North China Craton into the Eastern and Western Archean continental blocks. The Fuping Complex comprises three major lithological assemblages: the ∼2.50 Ga Fuping trondhjemite–tonalite–granodiorite (TTG) gneisses, the Wanzi paragneisses, and the 2.08 and 2.02 Ga Nanying granitic gneisses. The TTG gneisses contain abundant enclaves of highly-metamorphosed supracrustal rocks, including ∼2.70 Ga mafic granulites and amphibolites with ε Nd t of +3.29 to +3.96 and T DM of 2.72–2.82 Ga, and are characterized by T DM of 2.76–3.04 Ga and a linear correlation between ε Nd t (−1.64 to +0.96) and 1/Nd, indicative of mixing of melts derived from the ∼2.70 Ga metabasic rocks and ∼3.0 Ga crustal materials. Mafic dikes along the Chengnanzhuang shear zone were most likely emplaced at ∼2.31 Ga and have ε Nd t of +2.24 to +4.78 and T DM of 2.34–2.59 Ga. The Nanying granitic gneisses with ε Nd t of −4.04 to −5.24 were most likely related to partial melting of the ∼2.70 Ga supracrustal rocks and/or the ∼2.50 Ga TTG gneisses. These geological relationships and isotopic data suggest that the Fuping Complex probably formed from the following five major tectonomagmatic events: (1) mafic magmatism and early crustal growth at ∼2.7 Ga, with the possible existence of older (∼3.0 Ga) crust(s); (2) collision-related crustal thickening, underplating of basaltic magma, high-grade metamorphism, crustal anatexis and TTG magmatism at ∼2.5 Ga; (3) extensional deformation and emplacement of the mafic dikes and the Nanying granitic plutons along the Chengnanzhuang shear zone (∼2.5 to ∼2.0 Ga); (4) S-type granitic magmatism and associated amphibolite-facies metamorphism (∼2.0 to ∼1.80 Ga); and (5) emplacement of granitic pegmatite dikes at ∼1.80 Ga.

Aspects of the proterozoic geochronology of northern sweden

It has been proved that UPb zircon dating provides reliable formation ages for magmatic rocks affected by medium or lower grade metamorphism. The timing of the main volcanic, plutonic and metamorphic episodes of the early- to mid-Proterozoic evolution of northern Sweden has therefore been verified. A preliminary and schematic chronology of these events is presented. Archaean ortho- and para-gneisses > 2.70 Ga Supracrustal rocks affected by the Svecokarelian orogeny Mafic volcanic phase > 2.20-1.93 Ga Felsic volcanic phase 1.91-1.86 Ga Svecokarelian orogeny 1.88-1.80 Ga Post orogenic migmatization ⩽ 1.80 Ga