Ga Arc Research Articles

The 2.03–2.02 Ga arc–back-arc mafic magmatism from the Zhongxiang Complex, northern Yangtze Block: Implications for Columbia supercontinent assembly

To better understand Paleoproterozoic tectonic processes in the Yangtze Block during the assembly of the Columbia supercontinent, an integrated geochronological and geochemical study of amphibolite facies mafic rocks exposed in the Zhongxiang Complex in the northern Yangtze Block is presented. Based on trace element and rare earth element (REE) characteristics, these mafic rocks can be divided into two coeval groups. The MORB-like mafic rocks show flat REE patterns and are enriched in large-ion lithophile elements (e.g. Rb, Ba, K, Sr). Their zircon ɛHf(t) values range from + 3.43 to + 4.90. The other mafic rocks are arc-like and have elevated REE contents and display relatively light REE enriched patterns, as well as depletion in high field strength elements (e.g. Nb, Ta, Ti). The zircon ɛHf(t) values of the latter group range from + 0.17 to + 2.68. Zircon U-Pb dating yielded a weighted mean 207Pb/206Pb age of 2029 ± 15 Ma for the MORB-like mafic rocks, and 2023 ± 14 Ma for the arc-like mafic rocks, respectively. The elemental and isotopic characteristics indicate that the MORB-like mafic rocks may be derived from a depleted asthenospheric mantle source, whereas its counterpart with arc-like signatures originated from a lithospheric mantle source. The low (La/Yb)CN (1.10–2.84) and Sm/Yb (0.99–1.29) ratios indicate that the mafic rocks are formed by partial melting of spinel lherzolite mantle. Combined with other geological observations, these mafic rocks are inferred to constitute part of a mid-Paleoproterozoic continental arc–back-arc basin system. The 2.03–2.02 Ga mafic rocks from the northern Yangtze Block indicate that the continental margin experienced subduction in response to assembly of the Columbia supercontinent.

Garnet growth and diffusion zoning during the late Paleoproterozoic metamorphism, North Altyn area, SE Tarim Craton, NW China: Implications for a long-lived hot orogen

Metamorphic rocks encode invaluable information on the burial and exhumation history of rocks that is critical to our understanding of orogens. In this work, zircon U−Pb dating, pseudosection, and garnet zoning modeling were performed on samples from the North Altyn area, SE Tarim Craton, NW China. Zircon U−Pb geochronological data indicate that the peak metamorphism occurred at ca. 2.0 Ga. Pseudosection isopleth thermobarometry using garnet growth zoning of the sample from the northern part of the North Altyn area (Kalatashtagh) reveals a decompression and heating path to a peak temperature of 735 °C and 0.74 GPa under the equilibrium crystallization model. The preservation of growth zoning under high temperature requires relatively rapid cooling. In contrast, garnet from the southern part of the North Altyn area (Aktashtagh) preserves diffusion zoning. Metamorphic peak occurs at 670−735 °C and 0.67−0.73 GPa, which suggests a high dT/dP-type metamorphism. Diffusion zoning modeling suggests an extremely slow cooling rate of 1−5 °C/m.y., which indicates prolonged (∼100 m.y.) crustal residence time at high temperature. This requires a long-term heat supply, most likely from radiogenic heat from buried sediments or heat conducted from the asthenosphere. The high dT/dP and slow cooling rate, combined with extensive crustal anatexis from previous studies, indicate that the late Paleoproterozoic North Altyn Orogen is a long-lived hot orogen rather than a subduction−accretionary complex. We propose a tectonic model that involves a ca. 2.1−2.0 Ga arc−back-arc system in the SE Tarim Craton. The back-arc basin was subsequently closed by northward underthrusting due to arc−arc/continental collision at ca. 2.0−1.95 Ga during the integration of the Tarim Craton into the Columbia/Nuna supercontinent.

Late Archean–Paleoproterozoic plate tectonics along the northern margin of the North China craton

AbstractThe North China craton of central Asia is a natural laboratory for investigating early Earth tectonic processes including subduction and large-scale horizontal crustal motions. However, it remains unclear how and when the North China craton formed from the amalgamation of several blocks and orogens including the Archean Western and Eastern blocks, the late Archean Central Orogenic Belt, and the Paleoproterozoic Northern Margin orogen. In this study, we integrated new and existing geological field observations, zircon and baddeleyite U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-C-O isotope analyses along the northern margin of the North China craton to improve our understanding of its late Archean–Paleoproterozoic tectonic evolution. Observations show the Wuchuan ultramafic-mafic complex of the Northern Margin orogen contains a mixture of mylonite gneiss, serpentinized ultramafic blocks, and amphibolite pyroxenite xenoliths. Geochronology and geochemistry results suggest that the ca. 2.55–2.5 Ga development of the Wuchuan ultramafic-mafic complex was associated with subduction, which was followed by ca. 2.39 emplacement of lamprophyre dikes during continental rifting. Subsequent Paleoproterozoic subduction magmatism, continental collision, and post-orogenic rifting between the North China craton and southern margin of the Siberian craton are evidenced by ca. 2.0 Ga arc granitoids, ca. 1.87–1.85 Ga A-type, garnet-bearing granitoids and mafic dikes, and ca. 1.86 carbonatite. Ca. 2.0 Ga collision and ca. 1.87–1.85 Ga rifting were coincident with orogen-normal left-slip shear. Our findings show that the ca. 1.9 Ga North China-Siberia collision was a key event during the assembly of the Columbia supercontinent that occurred via modern plate-tectonic processes.

Neoarchean granitoids and tectonic regime of lateral growth in northeastern North China Craton

The Archean Eastern Liaoning metamorphic basement block is located in the northeastern North China Craton (NCC), in which ∼2.9 to 3.8 Ga lithological assemblages are well developed. The region which exposed these ancient lithological assemblages is usually called as an ancient continental nucleus. Toward north from the northeastern margin of the ancient continental nucleus, ∼2.5 to 2.7 Ga lithological assemblages continuously developed, forming abundant ∼2.5 Ga tonalite- trondhjemite- granodiorite (TTG) and high-K granitoids. Some ∼2.7 Ga TTG gneisses and metamorphic calk-alkaline volcanic rocks have also been documented recently in the Eastern Liaoning to the Southern Jilin of northeastern NCC. Our geological investigations found that many big ∼2.7 Ga TTG gneiss enclaves are preserved in the ∼2.5 Ga granitoids in the northeastern NCC. The early Neoarchean (∼2.7 Ga) TTG gneisses are exposed in the Waitoushan-Baishan areas, together with many contemporaneous subduction-related metamorphic calk-alkaline volcanic rocks in the Helong area of Southern Jilin province, constituting a nearly E-W-trending ∼2.7 Ga island arc belt in the northeastern NCC. However, the ∼2.7 Ga geological record is absent within the Anshan-Benxi-Waitoushan ancient microblock (ABWN) domain, suggesting that the ∼2.7 Ga arc belt was formed far away from the ABWN domain. The petrogenesis of ∼2.52–2.59 Ga TTG and sanukitoid gneisses, integrating with contemporaneous top-to-the-NE thrusting structural features, indicate that the new oceanic crust in the Waitoushan-Weiziyu-Jiubing (WWJ) area subducted to the southwest from ∼2.53–2.60 Ga. At the Archean end, the northern Liaoning-eastern Hebei intraoceanic arc system subducted to the southeast, which led to the final amalgamation among the ABWN, WWJ and northern Liaoning areas. Subsequently (∼2.50–2.53 Ga), the subducted slab broke off and the collisions between the arc and ancient continental nucleus, as well as the arc and arc, resulted in the generation of extensive crust-derived high-K granitoids.

Age and structural constraints on the tectonic evolution of the Paleoproterozoic Saimaa orocline in Fennoscandia

The Paleoproterozoic oroclines of Fennoscandia played a vital role in the making of continental equidimensional crust during the Svecofennian orogeny. We propose that the 1.90–1.88 Ga arc rocks in south-central Finland originally formed a single linear arc system that is now represented by the coupled Bothnian oroclines and their southern continuation, the Saimaa orocline, as defined in this study. A regional large-scale approach with robust geochronological age determinations obtained from metasedimentary rocks and granitoids, and field-based structural studies are used to produce a coherent view on the tectonic evolution of the Saimaa orocline.The pre-orocline tectonic evolution started with a NE-SW collision between the Karelia continent (lower plate) and Proto-Svecofennia (upper plate) at ca. 1.91 Ga (D1). This was followed by subduction reversal and arc magmatism at ca. 1.90–1.88 Ga. Collision or flat subduction at 1.885 Ga caused a widespread collisional event (D2). Our results indicate that the presently E-W oriented D2 structures are not, as is widely proposed, products of N-S contraction but instead originated as a N-S to NNW-SSE oriented belt that was subject to E-W contraction. Dorocline, recorded as the first deformation stage in post-D2 sediments, folded the pre-existing structures into their E-W and ENE-WSW orientation. Following orocline formation, the main shortening direction switched back to the NE-SW (D3) resulting in type-1 and type-2 refolded folds, long wavelength regional folds and bending of the hinge zone of the Saimaa orocline. D4-D5 deformation stages are localized intra-continental tectonic events at 1.83–1.79 Ga. The formation of the Saimaa orocline is bracketed at 1875–1865 Ma and formed by margin-parallel shortening. The Bothnian and Saimaa oroclines form continuous buckles, but whereas the Bothnian coupled oroclines are crustal-scale parallel folds due to more rigid crustal core, Keitele microcontinent, the Saimaa orocline is a crustal-scale isoclinal similar fold.

Structural Controls on Iron Oxide Copper-Gold Mineralization and Related Alteration in a Paleoproterozoic Supracrustal Belt: Insights from the Nautanen Deformation Zone and Surroundings, Northern Sweden

Abstract The Nautanen deformation zone in the Gällivare area of northern Sweden is a highly Cu-mineralized, magnetite-rich, large-scale shear zone with a long-lived (~100 m.y.) deformation, hydrothermal alteration, and mineralization history. This composite structure hosts the Aitik porphyry Cu-Au-Ag ± Mo deposit and several Cu-Au ± Fe ± Ag ± Mo occurrences assigned to the iron oxide copper-gold (IOCG) deposit class. The Nautanen deformation zone was a locus for polyphase deformation and intermittent metasomatic-hydrothermal activity that overprinted middle Orosirian (ca. 1.90–1.88 Ga) continental arc-related volcanic-plutonic rocks. The deformation zone is characterized by intense shearing fabrics that form a series of subvertical to moderately W-dipping, NNW-SSE–trending, first-order shear zones with oblique reverse kinematics and related NNE-SSW–oriented second-order shear zones that control hydrothermal alteration patterns and Cu-Au mineralization. Hydrothermal alteration in the study area formed during several phases. Volcanic-volcaniclastic rocks to the east and west of the Nautanen deformation zone display low to moderately intense, pervasive to selectively pervasive (i.e., patchy zones or bands, disseminations) sericite ± feldspar, amphibole + biotite + magnetite ± tourmaline, and K-feldspar + hematite alteration. Both the amphibole + biotite and K-feldspar + hematite associations occur adjacent to NNW- and NE-oriented deformation zones and are locally associated with minor sulfide. Within the deformation zone, a moderate to intense biotite + amphibole + garnet + magnetite + tourmaline + sericite alteration assemblage is typically associated with chalcopyrite + pyrrhotite + pyrite and forms linear and subparallel, mainly NNW-oriented seams, bands, and zones that locally appear to overprint possibly earlier scapolite + sericite ± feldspar alteration. Late-stage epidote ± quartz ± feldspar alteration (retrograde saussuritization) forms selectively pervasive zones and epidote veinlets across the area and is partly related to brittle faulting. A magnetite-amphibole-biotite–rich, penetrative S1 foliation records shortening during early Svecokarelian-related deformation (D1) and can be related to ca. 1.88 to 1.87 Ga arc accretion processes and basin inversion that overlaps with regional peak metamorphism to near mid-amphibolite facies conditions and a potential initial Cu mineralization event. Folding and repeated shearing along the Nautanen deformation zone can be assigned to a second, late-Svecokarelian deformation event (D2 stage, ca. 1.82–1.79 Ga) taking place at a higher crustal level. This D2 deformation phase is related to late-stage accretionary processes active during a transition to a stage of postorogenic collapse, and it was accompanied by abundant, syntectonic intrusions. D2-related magmatism produced high-temperature and low-pressure conditions and represents a regional magmatic-hydrothermal event that controlled the recrystallization/remobilization of magnetite, biotite, and amphibole. Associated shear zone reactivation during D2 favors the utilization of the Nautanen deformation zone as a fluid conduit, which preferentially controlled the siting and formation of epigenetic Cu-Au mineralization with distinctive IOCG characteristics within second-order shear zones.

Arc-related peridotite blocks exhumed to the Eastern Block of the North China Craton prior to 2.47 Ga

AbstractThe Songling and Majiayu peridotite blocks occur in the Eastern Block, North China Craton (NCC). Geothermobarometry data show that the Songling peridotites began exhumation from a depth of c. 70 km (c. 23 kbar). During exhumation, the Songling peridotites were intruded by an upper-crust-derived, high-δ18O (up to +7.28‰ in zircon) trondhjemitic dyke at 2.47 Ga and experienced granulite-facies metamorphism. The Songling peridotites have hybrid mantle wedge (HMW) -like high SiO2 (> 45 wt%), high FeOt (c. 10 wt%) content, high modal orthopyroxene abundance (> 35%) and high ϵNd(t) (+18.4 to +21.4), which were generated by the reaction between peridotite and eclogite-derived melts. The clinopyroxenes from the Songling peridotites were in equilibrium with a Nb-, Zr- and Ti-depleted arc-like magma. The Majiayu peridotites are characterized by depletion of Nb, Zr and Hf and are highly enriched in FeOt, Th and light rare earth elements (LREEs), which can be interpreted as an open system reaction between hydrous melts and fore-arc mantle peridotites. These two peridotite blocks are considered to be arc-related mantle peridotites that experienced melt extraction and metasomatism in different styles. They were exhumed to the north margin of the North China Craton during the c. 2.47 Ga arc–continent collision along the Zunhua structural belt.

Andean style 1.50–1.35 Ga arc dynamics in the Southeastern Laurentian margin: The rifting and reassembly of Quebecia

AbstractQuebecia is a unique Geon 15 to 13 composite arc belt in the central Grenville Province that preserves record of rifting and subsequent re‐amalgamation of part of the Laurentian margin. Key lithological components include: (a) c. 1.5 Ga metasedimentary sequences indicative of earlier rifting of the Laurentian margin; (b) remnants of 1.50–1.45 Ga peri‐Laurentian island arcs to the south built on rifted crustal slivers; (c) 1.45–1.42 Ga mafic dyke swarms to the northwest, linked to an asthenospheric window or back‐arc opening; (d) a major 1.43–1.37 Ga felsic plutonic belt marking the time of accretion to Laurentia; and (e) 1.38–1.35 Ga AMCG complexes suggesting slab‐breakoff and mafic underplating of the lower crust during arc accretion. Differences between Quebecia and continental arcs of the same age on both sides attest to lateral variations in subduction dynamics under Laurentia comparable to the modern‐day Andean system.

Grenvillian orogeny in the Oulongbuluke Block, NW China: Constraints from an ∼1.1 Ga Andean-type arc magmatism and metamorphism

A systematic petrologic, geochemical, and zircon U-Pb and Lu-Hf isotopic investigation of an ∼1.1 Ga arc magmatism and metamorphism was studied to evaluate the Grenvillian orogeny and assembly of Rodinia in the Oulongbuluke Block in NW China. The peak metamorphic P-T conditions of the paragneiss were restricted to 4.8–5.8 kbar and 652–700 °C based on P-T pseudosection and conventional geothermobarometry. Zircon U-Pb ages of 1132 ± 33 Ma, 1118 ± 9 Ma, and 1124 ± 12 Ma were determined for the metabasite, augen granitic gneiss, and paragneiss, respectively, indicating magmatic intrusion and a metamorphism event in the late Mesoproterozoic. The metabasites exhibited enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depletion of heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs), resembling the geochemical characteristics of Andean arc magmatism. The whole-rock geochemistry and in-situ zircon εHf(t) values of 4.3 and 11.2 suggested that the protolithic mafic magma originated from a subduction-modified lithospheric mantle. The low zircon εHf(t) values of −5.2 and −0.5 also suggested that the calc-alkaline augen granitic gneisses were derived from the partial melting of a late Paleoproterozoic crustal source. The ∼1.1 Ga arc-related magmatism and metamorphism in the Oulongbuluke Block indicated the occurence of a tectonic-thermal event in an Andean-type active continental margin in the late Mesoproterozoic. In this regard, the Oulongbuluke Block was likely temporarily connected to the Qilian Block and Tarim Craton, forming a single continuous block with similar Grenville age during the late Mesoproterozoic to early Neoproterozoic (1.1–0.9 Ga).

Diamondiferous Paleoproterozoic mantle roots beneath Arctic Canada: A study of mantle xenoliths from Parry Peninsula and Central Victoria Island

While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.

1.90-1.88Ga arc magmatism of central Fennoscandia: geochemistry, U-Pb geochronology, Sm-Nd and Lu-Hf isotope systematics of plutonic-volcanic rocks from southern Finland

The earliest Svecofennian magmatism in southern Finland has been dated at 1.90-1.88Ga. As an example of this, the Orijarvi ( ca. 1.89Ga) and Enklinge ( ca. 1.88Ga) volcanic centres comprise bimodal plutonic batholiths surrounded by volcanic rocks of comparable ages and chemical compositions. Here, we report geochemical and Sm-Nd isotope data from intrusive and extrusive samples, combined with zircon U-Pb and Lu-Hf isotopes for granodiorites from both study areas. The samples range from gabbros to granites and indicate a subduction-related continental margin setting. The zircons from the Orijarvi granodiorite define an age of 1892±4Ma whereas the Enklinge granodiorite yields an age of 1882±6Ma. Several inherited ages of 2.25-1.95Ga as well as younger ages of 1.86-1.80Ga were found in the Enklinge granodiorite. The initial e Nd values from the mafic rocks from both locations fall in the range +1.1 to +2.9 whereas the felsic rocks exhibit initial e Nd values of -0.4 to +1.2. The magmatic zircons from the Orijarvi and Enklinge granodiorites show average initial e Hf values of -1.1 (at 1892Ma) and zero (at 1882Ma), respectively, each with a spread of about 7 e-units. The initial e Hf values for the inherited zircons from Enklinge range from +3.5 to +7.6 with increasing age. The Sm-Nd data indicate that the mafic rocks were derived from a “mildly depleted” mantle source while the felsic rocks show larger crustal contribution. Also, the variation in e Hf values indicates minor mixing between mildly depleted mantle derived magmas and crustal sources. U-Pb ages and Hf isotopes for inherited zircons in the Enklinge granodiorite suggest the presence of juvenile Svecofennian “proto-crust” at depth.

Paleoproterozoic (ca. 1.8 Ga) arc magmatism in the Lützow-Holm Complex, East Antarctica: Implications for crustal growth and terrane assembly in erstwhile Gondwana fragments

The Lützow-Holm Complex (LHC) of East Antarctica forms a part of the latest Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb geochronological data on meta-igneous rocks from four localities (Austhovde, Telen, Skallevikshalsen, and Skallen) in the LHC, and evaluate the regional Paleoproterozoic (ca. 1.8Ga) arc magmatism in this terrane for the first time. The geochemical features reveal a volcanic-arc affinity for most of the meta-igneous rocks from Austhovde and Telen, suggesting that the protoliths of these rocks were derived from felsic to mafic arc magmatic rocks. The protoliths of two mafic granulites from Austhovde are inferred as non-volcanic-arc basalt such as E-MORB, suggesting the accretion of remnant oceanic lithosphere together with the volcanic-arc components during the subduction–collision events. The weighted mean 206Pb/238U ages of the dominant population of magmatic zircons in felsic orthogneisses from Austhovde and Telen show 1819±19Ma and 1830±10Ma, respectively, corresponding to Paleoproterozoic magmatic event. The magmatic zircons in orthogneisses from other two localities yield upper intercept ages of 1837±54Ma (Skallevikshalsen), and 1856±37Ma and 1854±45Ma (Skallen), which also support Paleoproterozoic magmatism. The earlier thermal events during Neoarchean to Early Paleoproterozoic are also traced by 206Pb/238U ages of xenocrystic zircons in the felsic orthogneisses from Austhovde (2517±17Ma and 2495±15Ma) and Telen (2126±16Ma), suggesting partial reworking of the basement of a 2.5Ga microcontinent during ca. 1.8Ga continental-arc magmatism. The timing of peak metamorphism is inferred to be in the range of 645.6±10.4 to 521.4±12.0Ma based on 206Pb/238U weighted mean ages of metamorphic zircon grains. The results of this study, together with the available magmatic ages as well as geophysical and lithological data from the region, suggest that the LHC can be divided into three units: Neoarchean (ca. 2.5Ga) unit in the southern LHC (Shirase Orthogneiss or “Shirase microcontinent”), Neoproterozoic (ca. 1.0Ga) unit in the northern LHC, and supracrustal unit in the central LHC with fragments of Paleoproterozoic (ca. 1.8Ga) and minor Neoarchean (ca. 2.5Ga) and Neoproterozoic (ca. 1.0Ga) magmatic arcs. The 1.8Ga arc magmatism inferred in this study has also been reported from adjacent Gondwana fragments such as the Highland Complex in Sri Lanka, and the Trivandrum and Nagercoil Blocks in southern India. Although the ca. 1.8Ga arc-magmatic event is coeval in these regions, the Paleoproterozoic supracrustal unit in the central LHC may not be contiguous with those in the Highland Complex of Sri Lanka because recent studies have shown that the Vijayan Complex in Sri Lanka and the ca. 1.0Ga northern LHC possibly were part of a single crustal unit (northern Lützow-Holm–Vijayan Complex) within the Kalahari Block. The supracrustal unit possibly marks part of a discrete suture formed by the collision of the ca. 2.5Ga southern LHC (Shirase microcontinent) and the ca. 1.0Ga northern Lützow-Holm–Vijayan Complex during the latest Neoproterozoic–Cambrian Gondwana amalgamation, which might be coeval with the collision of the Vijayan and Wanni Complexes and the formation of the Highland Complex in Sri Lanka. Our study provides new insights on crustal growth and terrane assembly in the ancient continental blocks of Gondwana.

Isotopic evidence for Neoarchean continuity across the Snowbird Tectonic Zone, western Churchill Province, Canada

The Snowbird Tectonic Zone (STZ), the ca. 2800km-long boundary between Rae and Hearne domains of the western Canadian Shield, remains of uncertain tectonic significance, but is currently interpreted as a ca. 1.9Ga suture. The eastern Rae domain is well studied and underlain by a >20,000km2 region of high pressure granulites, containing ca. 2.60Ga arc rocks, and granulite to amphibolite-grade structures, which formed at ca 2.57Ga (D1/M1) and 1.90Ga (D2/M2). The 1.85Ga Legs Lake shear zone juxtaposed the high-P granulites of the eastern Rae domain structurally above lower amphibolite- to greenschist-grade supracrustal gneisses of the Hearne domain. Zircon U-Pb and Hf isotopic analyses were acquired from igneous and metasedimentary rocks across the STZ and Rae-Hearne boundary to assess the validity of the suture model, with specific focus given to the >5km wide Legs Lake shear zone. Results indicate that both 2.60–2.55Ga magmatism and deformation continue unimpeded across the STZ and Rae-Hearne boundary. Furthermore, Hf isotopic data from igneous zircon from 2.60Ga plutonic rocks across the STZ display little variation in εHf indicating that no major crustal-break corresponds to the Rae-Hearne boundary, or any of the shear zones that parallel the STZ on the ground. Older plutonic rocks show a greater variability in εHf, which may indicate a disparate origin. Grade, intensity, and timing of Paleoproterozoic features vary along the transect and display little continuity across the Rae-Hearne domain boundary. In conjunction with existing field and isotopic data, we interpret that the Rae and Hearne domains were together by 2.60Ga, and that the STZ was host to repeated Paleoproterozoic reactivation driven by the latter stages of Taltson-Thelon orogenesis, final indentation of the Slave craton into the Rae domain, and accretionary processes on the southern periphery of the Hearne domain from 1.93 to 1.84Ga. Furthermore, assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them.

Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm–Nd, Lu–Hf, O)

The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92–1.85Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U–Pb dating and oxygen isotopes, followed by analyzes for Lu–Hf by LA–MC–ICP–MS.The AC plutonic rocks (1.87–1.85Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1–2.0Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced “subduction-like” Nb–Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite±rutile partly prevailing in the residue (Nb–Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb–Ba–Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks±amphibolites is proposed. The CFGC is characterized by both I-type and A-type (CFGC/A) intermediate and felsic granitoids. The I-type granitoids are divided into two groups at ≥1885Ma and ≤1882Ma, where the latter overlap in age with the CFGC/A granitoids. Both I-type CFGC and CFGC/A granitoids are interpreted to have formed from mixing of Paleoproterozoic SCLM-derived melts with crustal melts from hydrous and dry intermediate-felsic igneous sources, respectively. The geochemistry, dominantly δ18O values below 6.5‰ in zircons and TDM (2.11–2.42Ga) of the CFGC granitoids favor the occurrence of older crust (ca. 2.1–2.0Ga) in their genesis. The BZ granitoids are similar in age but more juvenile with TDM ages between 1.94Ga and 2.16Ga. The 1.92Ga granodiorite in the BB is correlated with juvenile gneissic tonalites and granodiorites found from the AP boundary.We suggest that the present high-velocity lower crust under the CFGC is composed of melt-extracted granulites (crustal source age≥2.0Ga) and mafic cumulates which both formed during 1.90–1.88Ga arc magmatism. The ≤1.88Ga stage represents the end of compression/transpression and is followed by 1.87–1.86Ga buckling, forming the Bothnian Oroclines.

Geochronological Framework and Geodynamic Implications of Mafic Magmatism in the Liaodong Peninsula and Adjacent Regions, North China Craton

AbstractMafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre‐Sinian, although the precise geochronological framework of this magmatism was unclear. Here, we present the results of more than 60 U‐Pb analyses of samples performed over the past decade, with the aim of determining the spatial and temporal distribution of mafic magmatism in this area. These data indicate that Paleoproterozoic–Mesoproterozoic mafic rocks are not as widely distributed as previously thought The combined geochronological data enabled the subdivision of the mafic magmatism into six episodes that occurred during the middle Paleoproterozoic, the late Paleoproterozoic, the Mesoproterozoic, the Late Triassic, the Middle Jurassic, and the Early Cretaceous. The middle Paleoproterozoic (2.1–2.2 Ga) mafic rocks formed in a subduction‐related setting and were subsequently metamorphosed during a ca. 1.9 Ga arc–continent collision event The late Paleoproterozoic (ca. 1.87–1.82 Ga) bimodal igneous rocks mark the end of a Paleoproterozoic tectonothermal event, whereas Mesoproterozoic mafic dike swarms record global‐scale Mesoproterozoic rifting associated with the final breakup of the Columbia supercontinent The Late Triassic mafic magmatism is part of a Late Triassic magmatic belt that was generated by post‐collisional extension. The Middle Jurassic mafic dikes formed in a compressive tectonic setting, and the Early Cretaceous bimodal igneous rocks formed in an extensional setting similar to a back‐arc basin. These latter two periods of magmatism were possibly related to subduction of the Paleo‐Pacific plate.

Zircon U–Pb and Hf – isotopes from the eastern part of the Sveconorwegian Orogen, SW Sweden: implications for the growth of Fennoscandia

Abstract Current models for the growth of Fennoscandia, including the eastern part of the Sveconorwegian Province, are largely based on U–Pb data and do not discriminate between juvenile and reworked crust. Here we present new combined U–Pb and Hf isotopic data, from the Eastern Segment and the Idefjorden terrane of the Sveconorwegian Province, and suggest a revised model of crustal growth. Most of the crystalline basement in this part of the shield formed by mixing of a 2.1–1.9 Ga juvenile component and Archaean crust. Archaean reworking decreases between 1.9 and 1.7 Ga and a mixed Svecofennian crustal reservoir is generated. Succeeding magmatism between 1.7 and 1.4 Ga indicates reworking of this reservoir with little or no crust generation. At c. 1.2 Ga, an influx of juvenile magma is recorded by granite to quartz-syenite magmatism with mildly depleted (ε Hf 1.18 Ga of c. 3) signatures. The amount of recycled crust in the 1.9–1.7 Ga arc system is in contrast to previously proposed models for the growth of the southwestern part of the Fennoscandian Shield. This model agrees with long-term subduction along the western margin of Fennoscandia, but suggests substantial reworking of existing crust and decreasing amounts of <1.9 Ga crustal growth. Supplementary material: The analytical method, U–Pb SIMS table, U–Pb LA-SF-ICP-MS table and Lu–Hf table are available at www.geolsoc.org.uk/SUP18648

Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss), mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750–850 °C, approximately 150 °C lower than those estimated for metasedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ± 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc magmatism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay) by subduction/collision events during the assembly of Gondwana supercontinent, and subsequently underwent ∼850 °C granulite-facies metamorphosed during Neoproterozoic to Cambrian final collisional event.

Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna supercontinent reconstructions

Research Article| December 01, 2010 Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna supercontinent reconstructions Owen P. Shufeldt; Owen P. Shufeldt 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar Karl E. Karlstrom; Karl E. Karlstrom 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels; George E. Gehrels 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Katherine E. Howard Katherine E. Howard 3School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Owen P. Shufeldt 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Karl E. Karlstrom 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA George E. Gehrels 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Katherine E. Howard 3School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia Publisher: Geological Society of America Received: 27 Apr 2010 Revision Received: 12 Jul 2010 Accepted: 22 Jul 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (12): 1099–1102. https://doi.org/10.1130/G31335.1 Article history Received: 27 Apr 2010 Revision Received: 12 Jul 2010 Accepted: 22 Jul 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Owen P. Shufeldt, Karl E. Karlstrom, George E. Gehrels, Katherine E. Howard; Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna supercontinent reconstructions. Geology 2010;; 38 (12): 1099–1102. doi: https://doi.org/10.1130/G31335.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoic Vishnu Schist (Arizona, United States) reveals a bimodal 207Pb/206Pb age probability diagram with peaks at 1.8 Ga and 2.5 Ga. Surprisingly, only 13% of detrital zircon ages overlap with the published depositional age range of 1750–1741 Ma. The similarity of the age distributions in all samples constrains possible suturing of crustal blocks to pre–Vishnu Schist deposition rather than during the peak 1710–1680 Ma deformation. Of all grains analyzed, 15% overlap at 2σ with the 1.84 ± 1 Ga Elves Chasm orthogneiss of the western Grand Canyon. This supports field evidence that the Vishnu Schist was deposited on 1.84 Ga arc basement rather than in a juvenile 1.75 Ga arc setting. Archean grains of 3.8–2.5 Ga compose 30% of all grains. A comparison of the ages older than 2.2 Ga from the Vishnu Schist (495 grains) with compilations of zircon ages from other cratons does not support provenances in the Wyoming, South China, or Siberian cratons; instead sources may be located in Gawler craton of Australia, North China craton, or Antarctica. If the detrital zircons were far-traveled, this is a new constraint for viable reconstructions of the Nuna supercontinent. However, given the high percentage of pre–1.8 Ga zircons, unexposed proximal basement sources are more likely, resulting in a model by which Vishnu sediments were derived from Mojave province crust that consists of Archean and 1.9–1.8 Ga crust, now in the subsurface, that was unroofed during Vishnu deposition. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The extent of juvenile crust in the Grenville Province: Nd isotope evidence

Research Article| May 01, 2010 The extent of juvenile crust in the Grenville Province: Nd isotope evidence A.P. Dickin; A.P. Dickin † School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada †E-mail: dickin@mcmaster.ca Search for other works by this author on: GSW Google Scholar R.H. McNutt; R.H. McNutt School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada Search for other works by this author on: GSW Google Scholar C. Martin; C. Martin School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada Search for other works by this author on: GSW Google Scholar A. Guo A. Guo * School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada *Present address: State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xian 710069, China Search for other works by this author on: GSW Google Scholar Author and Article Information A.P. Dickin † School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada R.H. McNutt School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada C. Martin School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada A. Guo * School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada †E-mail: dickin@mcmaster.ca *Present address: State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xian 710069, China Publisher: Geological Society of America Received: 02 Dec 2007 Revision Received: 04 May 2009 Accepted: 28 May 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2010 Geological Society of America GSA Bulletin (2010) 122 (5-6): 870–883. https://doi.org/10.1130/B26381.1 Article history Received: 02 Dec 2007 Revision Received: 04 May 2009 Accepted: 28 May 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.P. Dickin, R.H. McNutt, C. Martin, A. Guo; The extent of juvenile crust in the Grenville Province: Nd isotope evidence. GSA Bulletin 2010;; 122 (5-6): 870–883. doi: https://doi.org/10.1130/B26381.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A large Nd isotope data set is presented to generate a crustal formation age map for the southwest Grenville Province, placing it in the context of the province as a whole. Previous studies have shown that most crust in the central and eastern parts of the province is pre-Grenvillian, composed largely of juvenile 1.5, 1.7, 1.9, and 2.7 Ga arc terranes, partly reworked by later ensialic arcs and collision zones. In contrast, new Nd data reported here show that juvenile monocyclic crust is restricted to the SW Grenville Province, in part of the Central Metasedimentary Belt interpreted as a failed backarc rift zone. There is a clear isotopic distinction between this rift zone and the pre-Grenvillian crust on the flanks. On the flanks of the rift zone, there is evidence for two crust-forming events, comprising an earlier (1.5 Ga) juvenile Pinwarian event, and a later 1.4–1.3 Ga continental margin arc (the Elzevirian Orogeny). Based on the geographical distribution of Nd model ages in the SW Grenville Province, a revised tectonic model for the Grenville Orogeny is developed. In this model, backarc spreading created a rift zone behind a (pre-Grenvillian) Elzevirian subduction zone that obliquely straddled the edge of a Mesoproterozoic continental margin. As a result of this geometry, the rift zone died out to the north as it propagated into cold Archean crust. Elzevirian subduction was probably halted by the collision of the continental margin with an offshore arc, and later with a major continental block, giving rise respectively to the Shawinigan and the Ottawan orogenic events. These collisional events can be grouped as parts of the Grenville Orogenic Cycle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Proterozoic history and crustal evolution in southwestern Colorado: Insight from U/Pb and Sm/Nd data

New U/Pb age constraints and Sm/Nd isotope data further establish the timing of magmatic and tectonic events from 1.8 to 1.4 Ga in southwestern Colorado, and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of Laurentia. Proterozoic orogenic events in the Needle Mountains led to crustal assembly, thickening and tectonic reworking that was punctuated by periods of relative crustal stability. Pre-1.75 Ga arc successions in southwestern Colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1.73 to 1.71 Ga. Emplacement of post-orogenic plutonic rocks at ∼1.70 Ga was followed by uplift and erosion, and the deposition and deformation of siliciclastic sedimentary sequences. Paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at ∼1.43 Ga that coincided with a pulse of intracratonic deformation. Invasion and melting of first-cycle 1.8–1.7 Ga crust at ∼1.71 and ∼1.43 Ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling. Sm/Nd isotopic signatures of 1.8–1.4 Ga volcanic and plutonic rocks in southwestern Colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust, with minor, if any, contamination by crust older than 1.8 Ga. The Proterozoic magmatic record in southwestern Colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle, followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening.