Paleoproterozoic Orogeny Research Articles

Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M1), and the peak stage (M2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M3a) and final cooling stages (M3b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende ± biotite ± clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P–T paths, including isothermal decompression and cooling from the peak stage (M2: 10.4–10.8 kbar/830–860 °C and 9.6–11.2 kbar/850–930 °C, respectively) to the retrograde stages (M3a and M3b: 7.5–8.0 kbar/840–870 °C and 7.8–8.2 kbar/650–700 °C, respectively; M3: 7.2–8.6 kbar/670–800 °C). Zircon U–Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5–2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51–2.45 Ga and c. 1.90–1.85 Ga. The c. 2.51–2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95–1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive εHf(t) value of 7.64 and 2.8–2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8–2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8–2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC.To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P–T–t paths with near-isothermal decompression segments and low geothermal gradients (c. 21–26 °C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95–1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.

A modified paleoplacer model for the metaconglomerate-hosted gold deposits at Jacobina, Brazil

Evidence is presented that the metaconglomerate-hosted Jacobina gold deposits in Brazil represent paleoplacers that became partly remobilized during later metamorphic overprint analogous to Witwatersrand-type deposits elsewhere, notably in South Africa. This includes strong lithological and sedimentological control on the gold, presence of detrital minerals with gold inclusions, and detrital gold particles. Detrital, synsedimentary, and post-depositional pyrite types can be differentiated. Whereas the first two types can be linked to gold accumulation, the latter was associated with gold dispersion. Synsedimentary pyrite has the highest Au content, from which elevated Au concentrations in Archean rivers can be inferred. The nature and extent of post-depositional alteration, mainly in the course of the Paleoproterozoic Orogeny, distinguishes the Jacobina deposits from other Witwatersrand-type gold deposits. Phase equilibria and Zr-in-rutile thermometry indicate peak metamorphic temperatures of ca. 600 °C. Both Mg-chlorite and Fe-chlorite formed in disequilibrium at approximately 280–340 °C during retrograde metamorphism. An igneous signature in the chemistry of some of the tourmaline and the remobilization of gold associated with Fe-oxides, near intrusive rocks, point at a local magmatic influence on the post-depositional mineralization stage. Whether magmatic hydrothermal fluids added Au to the system at that stage remains to be determined. Remobilization during regional metamorphism was insufficient to form substantial ore bodies but led to purification of the initially detrital gold particles that now contain relatively little Ag and Cu and lack Hg.

Exhumation of an Archean Granulite Terrane by Paleoproterozoic Orogenesis: Evidence from the North China Craton

Abstract Granulites represent high-grade metamorphic rocks of the deep continental crust. The metamorphism and exhumation of granulites from Archean terranes provide insights into the crustal evolution of Archean cratons and shed light on the formation and reactivation of cratons. We present petrology, U–Pb geochronology (zircon, rutile, and titanite), and pressure–temperature (P–T) paths for metadiabase dikes in an Archean granulite terrane of the North China Craton. Garnet (Grt) coronae in the metadiabase dikes are developed between plagioclase (Pl) and clinopyroxene (Cpx) via the reaction Pligneous + Cpx → Pl1 + Grt ± quartz. The reaction proceeds inward within the plagioclase, progressively consuming Pligneous (XAn = 0.65–0.69) and leaving Ca-poor Pl1 (XAn = 0.46–0.53) as the residue. Geothermobarometry and P–T pseudosections suggest peak conditions for garnet formation at ~800°C and 10–13 kbar. During retrograde metamorphism, Grt broke down to Ca-rich Pl2 (XAn = 0.73–0.74), and ilmenite replaced rutile. Geothermobarometry and Zr-in-titanite temperatures constrain the P–T conditions of retrograde metamorphism at 700–750°C and 4.5–7.5 kbar. Zircon and titanite U–Pb geochronology shows that the protolith of the metadiabase dike was formed at 2.4 Ga and underwent granulite-facies metamorphism at 1.86 Ga. The intrusion of mafic dikes into the Archean granulite terrane indicates that the Archean basement was also heated and buried in the Paleoproterozoic. The metadiabase dikes and the hosting Archean basement underwent Paleoproterozoic granulite-facies metamorphism at a depth of ~40 km, followed by near-isothermal decompression and subsequent near-isobaric cooling (cooling rate of 1–3°C Myr−1) at depths of 15–25 km. Crustal shortening and thickening may have been caused by the underplating of the Khondalite series beneath the Archean basement during the amalgamation of supercontinent Columbia. The Paleoproterozoic orogeny induced a second generation of metamorphism of the Archean basement along the margin of the craton and drove the exhumation of the Archean granulite terranes to the middle crust (~15 km).

Geochemistry, geochronology and metamorphism of high-pressure mafic granulites in the Huai'an Complex, North China Craton: Implications for the tectonic evolution of the Paleoproterozoic orogeny

The Paleoproterozoic was a critical tectonic transition stage for the North China Craton (NCC), with several collisional orogenic belts identified. However, the tectonic boundaries, temporal-spatial evolution, and geodynamic mechanisms of these orogens remain controversial. In this contribution, we document a newly identified largest high-pressure (HP) mafic granulite outcrop at Xizhaojiayao in the Huai'an Complex, which is composed of metamorphosed supracrustal (volcano-sedimentary) sequences, including HP mafic granulites (metabasalts), marbles (metalimestones), quartzites (metasilicalites), and pelitic granulites (metapelites). They occur as tectonic slices within Neoarchean migmatized tonalitic gneisses. The Xizhaojiayao HP mafic granulites show different field occurrences and rock assemblages from the dike-type HP mafic granulites that are widely documented in the Huai'an and Hengshan Complexes. Microstructural studies, geothermobarometer calculations, and phase equilibrium modeling of the HP mafic granulites defined a clockwise pressure–temperature (P–T) path with prograde evolution from amphibolite-facies (M1, 5.2–5.5 kbar/538–668 °C) to HP granulite-facies (M2, 11.2–12.2 kbar/850–880 °C), followed by a significant near-isothermal decompression to medium-low-pressure granulite-facies (M3-1, 8.4–9.1 kbar/800–900 °C; M3-2, 4.9–5.6 kbar/779–805 °C). Zircon/titanite U–Pb dating constrains peak HP metamorphism, granulite-facies decompression, and amphibolite-facies cooling retrograde at 1902–1909 Ma, ∼1830 Ma, and < 1818 Ma, respectively. Whole-rock major and trace-element compositions, as well as positive εNd(t) (+2.0 to + 4.9) and zircon εHf(t) values (+2.9 to + 8.8) of these HP mafic granulites, indicate a mid-ocean ridge basalt geochemical affinity, whereas an island-arc signature is common for the dike-type HP mafic granulites. The Xizhaojiayao supracrustal sequences, which represent dismembered and metamorphosed upper oceanic crustal fragments, were subducted from the upper crust to depths of 35–40 km, followed by rapid exhumation to the middle-shallow crust and were tectonically involved in the melt-weakened tonalitic crust. These tectono-metamorphic scenarios illustrate the Paleoproterozoic orogenic evolution of a consecutive subduction-collision-exhumation process from 1.95 to 1.80 Ga, which led to the final assembly of the NCC.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton

Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-T−t history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

Diamond formation beneath the Coromandel area, southwestern São Francisco Craton – The role of re-fertilization and subduction

The lithospheric mantle underpinning the Alto Paranaíba Igneous Province , southwestern São Francisco Craton , was investigated through a study of 82 diamonds and their mineral inclusions from the Coromandel (Douradinho River), Verde River (northern Coromandel), Abaeté, Frutal, and Romaria deposits. Mineral inclusion abundances show that lherzolite is the main diamond source-rock, followed by eclogite , harzburgite and minor websterite. The limited chemical depletion recorded in lherzolitic inclusions suggests a post-Archean origin or modification of the lithospheric mantle beneath this part of the São Francisco craton. Sinusoidal rare earth element patterns for lherzolitic garnet inclusions indicate variable but overall low degrees of metasomatism by fluids or silico‑carbonatitic (proto-kimberlitic) melts. These results contrast with previous studies on mineral inclusions in diamonds (Canastra range, southern Coromandel; Rio da Prata system, northeastern Coromandel; our new data on the Frutal area, southwestern Coromandel), where a dominance of harzburgitic inclusions documents a more depleted, likely Archean, cratonic root. Bulk rock compositions reconstructed from eclogitic and websteritic inclusions establish a clear link between subduction processes and diamond formation in the Coromandel region. Calculated bulk rare earth element patterns match typical sections of upper oceanic crust , where plagioclase accumulation is uncommon. Pyroxene inclusion-based geothermobarometry indicates that lherzolitic and websteritic diamonds formed and last resided along a ∼ 39 mW/m 2 model geotherm. Projection of eclogitic garnet-clinopyroxene pairs onto this 39 mW/m 2 geotherm places their origin near the base of the lithosphere (> 180 km depth). Low nitrogen contents and high aggregation states (up to 100 %B) indicate that eclogitic, websteritic, and part of the lherzolitic diamond populations experienced extended (2.0 Byr) mantle residence at high temperatures (> 1200 °C). The similarity in mantle residence temperatures and the shared presence of minor negative Eu anomalies for eclogitic and websteritic diamonds and their inclusions may be indicative of a common origin, following emplacement of subducted oceanic crust beneath the São Francisco Craton margin likely during a Paleoproterozoic orogeny (2.2–1.9 Ga). Based on our results, future exploration efforts in the Alto Paranaíba region should shift their focus from harzburgitic to lherzolitic and eclogitic indicator minerals to find new diamond deposits and kimberlites in this area. • Lherzolite and eclogite main diamond substrates in Alto Paranaíba Igneous Province. • Moderate olivine Mg# and rare G10 garnets indicate mild SCLM depletion. • Lherzolitic substrates affected by fluid or silico‑carbonatitic melt metasomatism. • Eclogitic and websteritic diamonds derived from base of lithosphere. • Eclogitic diamonds linked to Paleoproterozoic (2.2–1.9 Ga) subduction.

Late Paleoproterozoic orogenic evolution of the northern Tarim Craton, NW China: Insights from phase equilibrium modeling and zircon U-Pb geochronology of metapelitic granulite in the Kuluketage area

Global Paleoproterozoic orogenic evolution can provide important constraints on the Columbia (Nuna) supercontinent assembly. Zircon U-Pb chronology studies on the northern Tarim reveal that a paleoproterozoic tectono-thermal event was developed, which is related to the assembly of the Columbia supercontinent. However, limited metamorphic investigations have hindered the recognition of Paleoproterozoic orogenic evolution in the northern Tarim, further affecting the understanding of the relationship between Tarim Craton and the Columbia supercontinent. The Xingditage Group in northern Tarim is the result of Paleoproterozoic orogeny which can shed light on the Paleoproterozoic orogenic evolution of the northern Tarim Craton. Our study conducted petrography, zircon U-Pb geochronology, and phase equilibria modeling of metapelitic granulite collected from the Xingditage Group. Petrographic and phase equilibria modeling indicate a clockwise P-T path involving the peak metamorphic stage (M1) at ∼840 °C/7 kbar, the near-isothermal decompression stage (M2) at ∼840 °C/3 kbar and followed near-isobaric cooling stage (M3) at ∼785 °C/2.5 Kbar. The ages of metamorphic zircons in metapelitic granulite range continuously from 1831 ± 13 Ma to 1782 ± 16 Ma. Petrographic observation shows almost all zircons grow in quartz and biotite (Bi-2), indicating they were recrystallized during the isobaric cooling stage (M3) after decompression. Thus, the ca. 1.85–1.80 Ga metamorphic age in northern Tarim records a regional post-Tmax cooling event after exhumation rather than a collisional event of crustal thickening. We further propose a tectonic evolution scenario of the northern Tarim Paleoproterozoic orogenic belt and suggests the northern Tarim has collided with an unknown block (perhaps Sharyzhalgai uplift in southwestern Siberia Craton) and became a peripheral part of the Columbia supercontinent prior to ∼1.85 Ga.

Structural elements and evolution of the Yangmuchuan dome: Constraints on the tectonics of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton

The nature of assembly of the Columbia supercontinent is well-characterized by the orogenic belts that were formed during amalgamation of continental blocks. The Jiao-Liao-Ji Belt (JLJB) in the eastern North China Craton possesses complex deformation structures that exemplify the Paleoproterozoic orogeny. A series of granitic gneiss domes in the southern JLJB characterizes the structural deformation in the Paleoproterozoic orogenic belts.Located in the middle segment of the JLJB, the Yangmuchuan dome is composed of three major elements, i.e., the Liaoji granites in the core, and sheared metamorphic rocks in the mantle and outer shell. Bedding-parallel ductile shear zones and weakly deformed metamorphic rocks between them are well-preserved in the latter two elements. Structural analysis reveals that the Yangmuchuan dome mainly experienced two deformation stages (D1 and D2). The D1 deformation is characterized by pure shear-dominated deformation structures with occurrence of penetrative foliations, symmetrical boudinages, mineral lineations and rootless folds in all the rocks from the mantle and outer shell. Dominated by simple shearing, the D2 deformation is the key stage contributing to the formation of the dome structure in the area. The D2 deformation is manifested as occurrence of A-type folds, S-C fabrics, mylonitic foliation S2, mineral stretching lineation L2 in granites, and in layered rocks in the mantle and outer shell. They were superposed on and transformed, along high strain zones, D1 deformation structures. Magnetic fabrics of the Liaoji granites from the core show a generally consistent orientation with the foliations and stretching lineations in the sheared granites. LA-ICP-MS zircon U-Pb dating on syn-tectonic leucogranitic dykes indicates that the peak deformation (D2) of the Yangmuchuan dome occurred from >1887 to 1808 Ma. Geometrical and kinematic analysis from the structural, microstructural and magnetic fabric data reveal that formation of the Yangmuchuang gneiss dome resulted from dominant subhorizontal flow and subsidiary vertical flow in the middle and lower crust as the consequence of orogenic activity along the JLJB during the assembly of the Columbia supercontinent.

Long-lived Paleoproterozoic eclogitic lower crust

The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

Diversity of zircon U-Pb geochronology of meta-sedimentary rocks from the Gaixian Formation, South Liaohe Group, Jiao-Liao-Ji belt: Implications for different provenance and crustal evolution

Comprehensive studies of depositional age and source material of the Liaohe Group are significant to the tectonic evolution of the Jiao-Liao-Ji Belt (JLJB). The Gaixian Formation is widely distributed and stratigraphically defined as the uppermost formation of the Liaohe Group. However, our new work reveals that not all meta-sedimentary rocks of the Liaohe Group deposited before the late Paleoproterozoic orogeny. Based on the geochronological results, these meta-sedimentary rocks can be subdivided into two categories. Detrital zircon from the category A meta-sedimentary rocks yield U-Pb age peaks at ca. 2.50 Ga, 2.18 Ga and 2.06 Ga. The youngest age peak of ca. 2.06 Ga, together with the metamorphic age of 1.95–1.84 Ga yielded by pervious studies, indicate their depositional ages of 2.06–1.95 Ga. Compared to the category A, detrital zircon from the category B yield similar 207Pb/206Pb age peaks at ca. 2.54 Ga, ca. 2.17 Ga and ca. 2.05 Ga, but importantly also at ca. 1.87 Ga which was never observed for the category A. The late Paleoproterozoic zircon (~1.87 Ga) include magmatic cores and weakly oscillatory zoned or unzoned rims. These rims yield two age peaks of ca. 1.88 Ga and ca. 1.80 Ga, which are interpreted as inherited partial melting ages. Therefore, the category B meta-sedimentary rocks deposited at some time after ca. 1.80 Ga. These new geochronological data reveal that the category B meta-sedimentary rocks are significantly different with those from the traditional Gaixian Formation. In combination with regional stratigraphy data, it is recommended that the category B meta-sedimentary rocks should be ascribed to the Changhai Group rather than the Liaohe Group.

Multiple tectonomagmatic reactivation of the unexposed basement in the northern Siberian craton: from Paleoproterozoic orogeny to Phanerozoic kimberlite magmatism

ABSTRACT Zircon xenocrysts from two diamond-barren kimberlite pipes (Leningrad and Ruslovaya) in the West Ukukit kimberlite field opened a ‘window’ to the buried crustal basement in the northern Siberian craton. Zircon U-Pb ages reveal a close affinity of the basement of the Khapchan belt to the Archaean Anabar province and a significant tectonomagmatic reworking in the Paleoproterozoic (~2.1–1.8 Ga) due to collision between the Anabar province and the Olenek province. The West Ukukit kimberlite field experienced multiple tectonomagmatic reactivation from ~670 to 144 Ma, which can be attributed to interaction of the deep crust with mantle-derived melts. Hf isotope composition of zircon xenocrysts reveals significant addition of juvenile material into the crust during the Paleoproterozoic orogeny in diamond-barren kimberlite fields, which is different from the reworking crust in the southern Yakutia diamondiferous kimberlite fields. Eruption of the Leningrad and Ruslovaya pipes were constrained as the Late Jurassic, much later than the well-known Late Silurian-Earth Devonian kimberlites in the West Ukukit kimberlite field. A NE-trending, >2000 km long kimberlite corridor is proposed to account for a prolonged lithospheric channel for episodic eruption of kimberlites in the Siberian craton. The diamond storage in the lithosphere beneath the West Ukukit kimberlite field may have been largely reduced by the Paleoproterozoic orogeny and Phanerozoic reworking.

Garnet perspectives on the metamorphic history and tectonic significance of Paleoproterozoic high-pressure mafic granulites from the northern Hengshan, North China Craton

Reconstructing the pressure-temperature (P-T) evolution of early Precambrian high-pressure granulites and eclogites is critical for decoding the tectono-metamorphic evolution of the early Earth. However, debates continue regarding the P-T history of these rocks because of various modifications by peak and retrograde rehydration metamorphism. As a significant rock-forming mineral in high-grade metamorphic rocks, garnet not only grows with increasing pressure or temperature and develops variably zoned patterns along given P-T loops, but its properties are stable subsquent to its formation when compared to other minerals in the matrix. Consequently, garnet is excellent for monitoring the thermo-metamorphic history of its host rock during orogeny. Selected garnet grains from the Paleoproterozoic high-pressure mafic granulites in the northern Hengshan area of the North China Craton, which exhibit variable microstructural and chemical zoning, present an excellent example for extracting metamorphic evolution of the host rocks. Microstructural zoning displays a zonal distribution of frozen inclusion-type minerals in garnet. Chemically, the garnet shows pronounced compositional zoning, with bell-shaped XSps, initially flat and then increasing/decreasing XPyr and Fe#, initially increasing and then decreasing XGrs from core to mantle, and resorbed rims. Both zoning patterns document multiple generations of radial growth history. The progressive evolution of the host rocks can be further divided into early prograde (M1–1), late prograde (M1–2), near peak (M2–1), and peak (M2–2) stages. The resorption textures indicate the succeeding decompression (M3) and cooling (M4) stages.The pseudosection modeling indicates P-T conditions from M1–1 to M2–1 of 6–9 kbar/600–700 °C, 9–11.2 kbar/680–740 °C and 12–15 kbar/735–785 °C, respectively. Although the peak metamorphic information of M2–2 is lost because of the breakdown of the garnet rim, plausible P–T results are estimated at ~15–17 kbar/~790–810 °C. These conditions prevailed to the transitional eclogite-granulite facies through qualitative reconstruction of the original composition of clinopyroxene, modal proportion, and rim composition of the garnet. The subsequent conditions of decompression (M3) and cooling (M4) stages were roughly estimated at ~7–10 kbar/~800–850 °C and 3–5 kbar/ 650–680 °C. Together, they define a clockwise P-T path in response to the Paleoproterozoic orogeny involved in the assembly of the North China Craton. The zoned garnet suggests that the host mafic granulite documents a relatively complete progressive metamorphic history, which probably correlates with subduction processes. The resorbed rim sequentially records exhumation and cooling processes. Furthermore, the preservation of growth zoning in granulitic garnet and near-isothermal decompression P-T path suggests that the host rocks must have experienced a very short peak metamorphism and a rapid exhumation process to the middle crust. These findings suggest that the tectonic processes involved in the formation of the Paleoproterozoic northern Hengshan granulite terrane might be similar to those of the Phanerozoic continental collisional orogens under modern plate tectonic regimes; however, they record relatively higher apparent thermal gradients (~14–16 °C /km).

Crustal evolution of the Paleoproterozoic Ubendian Belt (SW Tanzania) western margin: A Central African Shield amalgamation tale

The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8 Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U–Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85 Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89–1.85 Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0 Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570 Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate–Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.

Local Rapid Exhumation and Fast Cooling in a Long-lived Paleoproterozoic Orogeny

Abstract The exhumation and cooling rates of high-grade metamorphic rocks are crucial for inferring orogenic processes and understanding the regimes of heat transport in the Earth’s crust. Quantification of these rates remains challenging for Precambrian terranes, because the temporal resolution of geochronology becomes coarser in deeper geological time. This limitation is partly reflected by a striking lack of Proterozoic or older short-duration events (&amp;lt;10 Myr); most documented cases of fast metamorphism are confined to the Phanerozoic. In this study, we use garnet geospeedometry to explore the metamorphic rates of Paleoproterozoic high-grade rocks from two representative areas within the long-lived (1·95–1·80 Ga) Jiao-Liao-Ji orogenic belt, North China Craton. The pelitic granulites in the Taipingzhuang area record high-pressure granulite-facies (HPG) metamorphism of ∼12 kbar and ∼800 °C, followed by a fast decompression-cooling to ∼5 kbar and ∼600 °C within ∼5 Myr, at c. 1·87 Ga. The pelitic granulites in the Rizhuang area document a brief (&amp;lt;1 Myr) thermal excursion to ultrahigh-temperature (UHT) metamorphism of ∼8 kbar and ∼940 °C at c. 1·85 Ga, followed by a fast cooling to ∼600 °C within 1–5 Myr. In light of available geological data, the fast decompression-cooling of HPG granulites is interpreted as the syn-collisional exhumation of thickened lower crustal segments at c. 1·87 Ga, most probably through tectonic extrusion. The thermal excursion transiently reaching UHT conditions is inferred to be triggered by localized syn-metamorphic mafic intrusions in association with magmatic underplating during post-collisional extension at c. 1·85 Ga. These metamorphic pulses were interspersed within the protracted Paleoproterozoic orogenesis and require geodynamic processes resembling modern plate tectonics. Notably, these ancient rapid events are beyond the temporal resolution of commonly used in situ geochronology, which tends to yield apparent longer durations given errors and uncertainties. We therefore note that most ancient metamorphic rates might be underestimated using geochronological data, and recommend garnet geospeedometry as a promising alternative approach. The largely similar rates recorded by Paleoproterozoic and Phanerozoic orogens, as well as high-pressure metamorphism at 1·9–1·8 Ga, support the operation of modern plate tectonics in Paleoproterozoic time.

Possible imprints of late Paleoproterozoic orogeny in the Dunhuang terrane, NW China: Constraints from igneous and metapelitic rocks

The Dunhuang terrane is a NEE trending tectonic belt confined by the Tarim Craton, Beishan Orogen, Alxa Block and Altyn Tagh Fault. To constrain the late Paleoproterozoic geotectonic background of this terrane, a synthetic research on petrology, geochemistry, zircon and monazite geochronology, and zircon Lu-Hf and whole-rock Nd isotopes of late Paleoproterozoic (ca. 1.9–1.8 Ga) igneous rocks and metapelites was conducted. The Sanweishan-Dongshuigou Na-rich rhyolite and granite formed at ca. 1.84–1.82 Ga exhibit enriched light rare earth elements (LREEs), depleted heavy rare earth elements (HREEs) and insignificant Eu anomalies, with low Y (3.44–6.39 ppm) and Yb (0.29–0.53 ppm) but relatively high Mg# (54.5–58.5) and Sr (368–643 ppm), and variable Cr (5.58–29.7 ppm) and Ni (2.82–15.6 ppm) contents, as well as negative to positive εHf(t) (−6.1 to +6.4) values, indicating subduction-related arc magmatism accompanied by crustal-mantle interaction. The Shuwozi granitic gneiss crystallized at ca. 1.81 Ga displays S-type granite affinity (A/CNK > 1.1) and has low Sr/Y (1.0–1.3) and (Gd/Yb)N (1.50–1.98) ratios, and negative εHf(t) (−20.3 to −5.1) and εNd(t) (−4.3) values, suggesting partial melting of crustal rocks at shallow crustal depth. The Hanxia amphibolite with protolith age of ca. 1.86–1.84 Ga is featured by depleted LREE and flat HREE patterns, insignificant Eu anomalies, low Nb/Ta (11.0–12.0), Ta/Yb (0.02–0.04), Th/Yb (0.01–0.04) and Th/Ta (0.7–1.3) but high Ti/V (22.9–26.7) ratios, and positive εNd(t) (2.7–4.0) values, resembling depleted mantle-derived N-MORB- or BABB (back-arc-basin basalt) geochemical affinities. The metapelite, characterized by mineral assemblage of garnet, biotite, sillimanite, plagioclase, K-feldspar, quartz and rutile, documented ca. 1.81 Ga upper amphibolite-facies metamorphism with peak metamorphic P-T conditions being restricted to 690–810 °C and 5.3–6.4 kbar based on conventional geothermobarometry. These magmatic and metamorphic rocks were variously altered and deformed during Paleozoic (ca. 430–360 Ma) tectonothermal event. Integrated with published data, it can be inferred that the Dunhuang terrane underwent ca. 1.89–1.81 Ga subduction-collisional orogeny related to late Paleoproterozoic (ca. 2.0–1.8 Ga) global assembly of the Columbia supercontinent, similar as those recorded in the northern Tarim Craton, Alxa Block and Quanji massif, and then overprinted Paleozoic (ca. 440–360 Ma) orogenic process, corresponding to the previously proposed composite orogeny in the Dunhuang terrane.

Petrology and geochemistry of TTG and K-rich Paleoproterozoic Birimian granitoids of the West African Craton (Ghana): Petrogenesis and tectonic implications

We conducted petrologic, geochemical and isotopic studies on Paleoproterozoic Birimian granitoids that intrude the Bole-Nangodi Belt in northern Ghana, in southeastern West African Craton (WAC), to constrain the geodynamic evolution of the Birimian Supergroup. The rocks are potassic (K)-rich granitoids (KRGs), tonalities, trondhjemites, granodiorites (TTGs), pegmatite and aplite. The K2O contents of the KRGs are up to 5 wt%, with generally high concentrations of Rb, Ba, Sr, Cr and Ni. They are highly fractionated calc-alkaline I-type granitoids, enriched in LILE and LREE relative to HREE and HFSE. Chondrite-normalized REE diagrams indicate fractionated LREE (average LaN/SmN = 5.05) and HREE (GdN/YbN = 4.56) patterns, with slight negative Eu anomalies (average Eu/Eu* = 0.75). This may indicate an evolved magma source with varying degrees of plagioclase fractionation. The KRGs share similar geochemical characteristics with the TTGs into which they intrude. Both rock types are characterized by Ba and Th enrichment and depletion in Nb‐Ta, Zr‐Hf, and Ti, which are typical characteristics of subduction-related magmas. The pegmatite-aplite association is slightly geochemically different and may have formed at a time later than the KRGs. The ԐNd(2.1 Ga) values range from + 0.90 to + 1.24 and −0.86 to + 1.37 respectively for the TTGs and KRGs, with TDM1 ages of 2.34–2.38 Ga and 2.36–2.53 Ga and TDM2 ages of 2.21–2.24 Ga and 2.22–2.39 Ga respectively. These indicate their juvenile character, possibly a depleted mantle source with minor contributions from a pre-Birimian (or Archean?) crustal material in their source material(s). The TTGs and KRGs are volcanic arc granites (VAG) and syn-collisional (Syn-COLG) granites, with formation temperatures and pressures of ~640–750 °C and ~2–6 kbar respectively. Based on the high temperatures, I-type signature, calc-alkaline nature and other geochemical characteristics, we infer that the TTGs, KRGs and pegmatite-aplite association developed under high oxygen fugacity conditions in an orogenic tectonic setting related to subduction. Thus, our geochemical and isotopic results are consistent with an island arc tectonic setting arising from subduction–accretion processes, which are typical for the Paleoproterozoic terranes of the WAC. These processes may have contributed to the amalgamation of the Columbia (Nuna) supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogeny.

Lower crustal contribution to the magma formation of the Damiao massif-type anorthosite, North China Craton: Evidence from zircon Hf-O isotopes

The formation of massif-type anorthosites and the associated mangerite, charnockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the “Earth’s Middle Age” (~1.70–0.75 Ga), and the magma sources of massif-type anorthosites are highly debated between the depleted mantle and the mafic lower crust. The ~1.7 Ga Damiao massif-type anorthosite in the North China Craton consists mainly of anorthosite and leuconorite with minor melanorite, mangerite, Fe-Ti-(P) ores and abundant ferrodioritic or gabbroic dykes. In-situ zircon Hf-O isotopic data from anorthosite reveal decoupling of mantle-like δ18O values (5.1–7.4) and ancient crustal-like εHf(T) values (−4.0 to −8.9). Since the significant 177Lu/176Hf reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative εHf(T) values after long period of time (e.g., 0.5–1.0 Ga). In contrast, if no supracrustal materials were added, these mafic crustal materials would retain the mantle δ18O features. Thus, the decoupling of mantle-like δ18O and ancient crustal-like εHf(T) values suggest a lower crustal origin of the Damiao massif-type anorthosite. Compilation of Sr-Nd-O isotopic data of massif-type anorthosites reveal decoupling of εHf(T), initial 87Sr/86Sr and δ18O values, which signifies important lower crustal input in magmas sources of most massif-type anorthosites. We proposed that zircon U-Pb age of the Damiao massif-type anorthosite (~1.70 Ga) and regional age correlation suggest that the anorthosite was emplaced in an intraplate, anorogenic extensional setting after the Paleoproterozoic orogeny had ended.

Precambrian Plate Tectonics in Northern Hudson Bay: Evidence From P and S Wave Seismic Tomography and Analysis of Source Side Effects in Relative Arrival‐Time Data Sets

AbstractThe geology of northern Hudson Bay, Canada, documents more than 2 billion years of history including the assembly of Precambrian and Archean terranes during several Paleoproterozoic orogenies, culminating in the Trans‐Hudson Orogen (THO) ∼1.8 Ga. The THO has been hypothesized to be similar in scale and nature to the ongoing Himalaya‐Karakoram‐Tibetan orogen, but the nature of lithospheric terrane boundaries, including potential plate‐scale underthrusting, is poorly understood. To address this problem, we present new P and S wave tomographic models of the mantle seismic structure using data from recent seismograph networks stretching from northern Ontario to Nunavut (60–100∘W and 50–80∘N). The large size of our network requires careful mitigation of the influence of source side structure that contaminates our relative arrival time residuals. Our tomographic models reveal a complicated internal structure in the Archean Churchill plate. However, no seismic wave speed distinction is observed across the Snowbird Tectonic Zone, which bisects the Churchill. The mantle lithosphere in the central region of Hudson Bay is distinct from the THO, indicating potential boundaries of microcontinents and lithospheric blocks between the principal colliders. Slow wave speeds underlie southern Baffin Island, the leading edge of the generally high wave speed Churchill plate. This is interpreted to be Paleoproterozoic material underthrust beneath Baffin Island in a modern‐style subduction zone setting.

Precambrian Tectonic Discontinuities in Western Laurentia: Broadband Seismological Perspectives on the Snowbird and Great Falls Tectonic Zones

AbstractThe Snowbird Tectonic Zone (STZ) in southwestern Canada and the Great Falls Tectonic Zone (GFTZ) in northern Montana are two structural lineaments with major implications for the formation and evolution of the western North American craton. In this study, we examine the origins of these two proposed Proterozoic sutures using broadband seismic data from Alberta and USArray stations. We find substantial spatial variations in crustal structure and thickness across both the STZ and GFTZ based on an integrated analysis of P‐to‐S receiver functions, shear velocities, and potential field measurements. The STZ marks the transition from deep (NW, ~44 km) to shallow (SE, ~39 km) Moho in central Alberta. This steep Moho relief, in conjunction with elevated Vp/Vs ratios, provide compelling evidence for Proterozoic subduction and magmatism. In northern Montana, the Moho is depressed by ~6 km within the fold‐and‐thrust belts of the Trans‐Montana Orogen, where high (&gt;1.84) Vp/Vs ratios spread along the collisional suture of the Dillion Shear Zone and terminates sharply along the reworked margin of the Wyoming craton. These observations, coupled with widespread lower crustal conversions, are concordant with a Paleoproterozoic orogeny resulting from collision(s) between Archean cratons. On the other hand, a flat Moho under major northeast striking potential field lineaments in the northernmost Montana favors an origin as a ductile shear zone, rather than suture, in the orogenic hinterland. We find evidence for crustal underplating beneath the GFTZ, though this anomalous layer is smaller than previous estimates and appears to be confined to the Trans‐Montana Orogen.

3D MODEL OF THE DEEP STRUCTURE OF THE SVECOFENNIAN ACCRETIONARY OROGEN: A GEODYNAMIC INTERPRETATION

The results of this study are reported in two related successive publications. The article “3D model of the deep structure of the Svecofennian accretionary orogen based on data from CDP seismic reflection method, MT sounding and density modeling” (M. V. Mints, E. Yu. Sokolova, LADOGA Working Group) presented a 3D model of the deep structure of the Late Paleoproterozoic Svecofennian accretionary orogen. The model is based on harmonized data of geological mapping, FIRE-2-2a-1 and FIRE-3-3a seismic reflection profiling, using sections of the 3D crustal density model, results of magnetotelluric surveys along the Vyborg-Suojarvi profile in the Northern Ladoga area and materials from MT surveys previously carried out by Finnish specialists in Southern Finland. In this paper these results were used as the baseline for the construction of the evolutionary model and determination of the position of the Svecofennian orogen in the structure and history of the Paleoproterozoic Lauro-Russian intracontinental orogen. This Paleoproterozoic orogeny swept the predominant part of Lauroscandia, a paleocontinent that united the North American and the East European cratons. Interpretation of the structural features and evolution of the Lauro-Russian orogen leads to the conclusion that its onset and evolution had a causative relation to the development of the Paleoproterozoic superplume which, in turn, initiated plate tectonic events. The evolution of the Lauro-Russian orogen included three intensive development periods (~2.5; 2.2–2.1 and 1.95–1.87 Ga). In the axial zone of the intracontinental orogen there appeared the Svecofennian Pre-Labrador Ocean, and the accretionary Svecofennian and Pre-Labrador orogens formed along its continental margins after its closure.