Tectonic Significance Research Articles

Metamorphic evolution and tectonic significance of Neoarchean high-pressure mafic granulites in the Central Limpopo Belt, South Africa

ABSTRACT The Sand River area is in the northeastern part of the Central Zone (CZ) of the Limpopo Belt, and represents a key terrane to reveal early Precambrian tectonic processes in southern Africa. Based on petrology, U–Pb zircon ages, and major and trace element data of high-pressure (HP) mafic granulites from the Sand River area, the tectonic evolution of the Central Zone of the Limpopo Belt is re-evaluated here that allows revision of previous interpretations. The protoliths of the HP mafic granulites and amphibolites are sub-alkaline tholeiitic basalts similar to those formed at a mid-ocean ridge and intruded at 2.8 Ga. Based on the distinct mineral assemblages, three metamorphic stages (M1–M3) are recognized in the studied samples: (1) a peak HP granulite facies stage M1 (~1.17–1.32 GPa/~825–870°C), (2) a post-peak near-isothermal decompression stage M2 (~0.70–0.73 GPa/~835–880°C), and (3) a later, independent metamorphic stage M3 (~0.53–0.69 GPa/~793–801°C). M1 and M2 define together a clockwise P–T path. It reveals important information related to the Neoarchean collisional orogeny, and its later overprinted by an independent orogeny with subsequent exhumation and cooling. U–Pb zircon dating of two mafic granulites samples yielded protolith formation of the mafic granulites is prior to ~2.72 Ga, and the two metamorphic age peaks of ~2.72 (M1) and ~2.0 Ga (M3), which present the peak and post-peak cooling stages, respectively. The metamorphic evolution of HP mafic granulites in the Sand River area as reconstructed in this study suggests the Neoarchean collision of the Kaapvaal and Zimbabwe cratons and a discrete Paleoproterozoic orogenic overprint followed by post-orogenic exhumation and cooling.

Geochemistry and tectonic significance of metamorphosed mafic ophiolitic rocks in the upper high-grade basement unit of the eastern Rhodope Massif (Bulgaria–Greece)

Geochemistry and tectonic significance of metamorphosed mafic ophiolitic rocks in the upper high-grade basement unit of the eastern Rhodope Massif (Bulgaria–Greece)

Landslides triggered by the 10 June 2022 Maerkang earthquake swarm, Sichuan, China: spatial distribution and tectonic significance

Landslides triggered by the 10 June 2022 Maerkang earthquake swarm, Sichuan, China: spatial distribution and tectonic significance

Petrogenesis and tectonic implications of the early Paleozoic granites in the Lincang granitic batholith, southwestern China: Constraints from geochronology, geochemistry and Hf isotopes

This study presents the recently discovered Wulao Shan Ordovician granite within the Lincang granitic batholith, discusses its geochemical affinity, genesis, and tectonic significance, and interprets the proto-Tethys orogenic model in southwest Yunnan. LA-ICP-MS zircon U-Pb analyses indicate a crystalline age of 473.0 ± 2.9 Ma for the Wulaoshan gneissic granite. Geochemical analyses show that the Wulaoshan granite exhibits relatively high SiO2 concentrations (73.4–77.3 wt.%), strong peraluminous properties (ASI > 1.1), strongly Eu-negative anomalies, light and heavy rare earth fractionation, and relative Ba, Nb, Ta, Sr, P, and Ti depletion. SiO2 and P2O5 exhibit a strong negative correlation, whereas Rb and Y-Th with a positive correlation. The magmatic zircons have negative εHf(t) values of -5.47∼2.74. Petrographic observations and whole-rock geochemical and isotopic analyses imply no aluminum-rich minerals such as cordierite, indicating that the Wulaoshan granite is a highly fractionated, high-temperature I-type granite, which may have formed via partial melting of the magnesian-iron igneous crust. Additionally, K-feldspar, plagioclase, and biotite within the granite samples may have undergone intense fractional crystallization. The Wulao Shan gneissic granite is an arc granite associated with subduction, indicating that the Lincang Block underwent proto-Tethys subduction. This finding provides evidence for proto-Tethys subduction down to the Lincang-Simao block and further supports the bidirectional subduction model of the proto-Tethys.

Seismic reflection stratigraphy of Lac Simard, Quebec, Canada: mass flow sedimentation in glacial Lake Barlow-Ojibway

Lac Simard is a glaciated Shield basin situated in the Western Quebec Seismic Zone, an area of recurrent intraplate seismicity encompassing several densely populated areas. Previous works nearby have demonstrated that sediments preserved in lake basins hold the potential to serve as valuable archives of paleoseismicity. Correspondingly, a high-resolution chirp seismic reflection survey was conducted in Lac Simard, with results showing four acoustic facies (Af-1 to Af-4) within two stratigraphic successions (SS-1 and SS-2). The lowermost SS-1 is dominated by high-amplitude, laminated Af-2, interpreted as lateglacial varves deposited by suspension from primarily overflows and interflows during glacial lake ponding following deglaciation. Suspension deposition of Af-2 was episodically interrupted by stacked, chaotically bedded, weakly graded to opaque, channelled Af-3 interpreted as either debris flows or high-density turbidity currents. Presence of silt-clast breccias in Af-3 also suggests downslope collapse and reworking of varves from basin sidewalls. The overlying SS-2 is dominated by weakly reflective, laminated, high-frequency varved Af-4, suggesting a continuing seasonal control on postglacial sedimentation but with relative sediment starvation. A basin-wide erosional unconformity separating SS-1 and postglacial SS-2 records the abrupt drainage of glacial Lake Barlow-Ojibway around 8 000 years ago. Considering the history of lake floor disturbance in the region, mass flow facies Af-3 may be seismogenic, and thus, have regional tectonic significance. However, we cannot rule out the possibility of mass flows triggered by abrupt inflows of meltwater and sediment from a highly dynamic retreating Laurentide Ice Sheet prone to surging.

Micro-Earthquake Seismicity and Tectonic Significance of the 2012 Ms 4.9 Baoying Earthquake, Jiangsu, China

Micro-Earthquake Seismicity and Tectonic Significance of the 2012 Ms 4.9 Baoying Earthquake, Jiangsu, China

The Rossano–San Nicola Fault Zone evolution impacts the burial and maturation histories of the Crotone Basin, Calabrian Arc, Italy

This work addresses the tectonic significance of a NW–SE-trending strike-slip fault zone in the Calabrian Arc of southern Italy, the Rossano–San Nicola Fault Zone (RSFZ). High-quality seismic reflection and 1D forward models of exploration boreholes and pseudo-wells show that the RSFZ experienced multiple Miocene phases of contractional/transpressional tectonics. These were followed by crustal extension during the Pliocene, which occurred in association with the oceanization of the Tyrrhenian Sea, Apennine orogenesis, and collision between the Calabrian Arc and adjacent tectonic plates. Such a setting had a profound influence on the Crotone Basin and its economic potential: (1) tectonic reactivation allowed reservoir units of the Crotone Basin to be charged by gas derived from Triassic/Lower Jurassic source rocks; and (2) source rocks reached their maximum depth and remained in the gas generation window after the emplacement of a large mass-transport complex in the Pliocene. In the surrounding areas, tectonic activity near the RSFZ contributed to source-rock maturation by enhancing local sedimentation rates, particularly during Langhian (Middle Miocene) and Zanclean (early Pliocene) tectonics. This work is important as it demonstrates that the tectonostratigraphic evolution of the Crotone Basin was closely related to the structural evolution of the RSFZ. Crucially, the study area reveals the first example of a gas field fully sealed by a large mass-transport complex. As a corollary, we tie the Late Cenozoic geological history of the Crotone Basin to the geodynamic evolution of the central Mediterranean region, namely the Ionian and Tyrrhenian seas. We identify new prospects in the Crotone Basin, and provide a time frame for gas generation and accumulation in southern Italy.

Petrogenesis and tectonic significance of the Early Cretaceous Xizhelimu diorite in Keyouzhongqi, Inner Mongolia, China

To determine the emplacement age, petrogenesis, and geodynamic setting of the Xizhelimu diorite in Keyouzhongqi, Inner Mongolia of northeastern China, a detailed study of the petrography, geochronology, and whole-rock geochemistry has been conducted. Geological and petrographic studies show that the Xizhelimu diorite is zoned: the central lithofacies zone is composed of medium-fine-grained monzodiorite and quartz diorite, and the marginal lithofacies zone is fine-grained diorite. The zircon U–Pb dating results show that the ages of the central and marginal facies are 133.5 ± 1.9 and 133.4 ± 1.4 Ma, respectively. The whole-rock rare earth and trace element characteristics of the Xizhelimu diorite show an O-type adakite affinity. Combining the analysis of zircon Hf isotope composition ( εHf(t) values of +7.7 to +10.0), the geochemical features of whole rock, and the results of partial melting modeling, we suggest that the parental magma of the Xizhelimu diorite was derived from the partial melting of altered oceanic crust mixing with subducting sediments at shallow depths. In the early stage of early Cretaceous, the Xizhelimu diorite originated in an extensional setting, mainly related to the closure of the western part of the Mongol–Okhotsk Ocean. The upwelling asthenospheric flow in this extensional setting induced partial melting of the paleo-oceanic crust to form the parental magma of the Xizhelimu diorite.

Linking accretionary orogens with continental crustal growth and stabilization: Lessons from Patagonia

The origin of the continental crust and the tectonic significance of Paleozoic magmatic rocks of Patagonia (southernmost South America) remain one of the main enigmas in the history of the Gondwana supercontinent. Here, new whole-rock geochemistry together with coupled zircon U-Pb and Lu-Hf isotopic data of Devonian to Permian intrusive rocks of northern Patagonia are integrated with a revised geochemical and isotopic database of the region, providing a novel model for the tectonomagmatic evolution and related crustal growth mechanisms. The development of a Devonian retreating accretionary orogen associated with crustal thinning was succeeded by a late Carboniferous to Permian advancing orogen and crustal shortening, resulting from slab shallowing. The latter was related to the Gondwanide Orogeny, a major transpressional tectonic event that led to the maturation and stabilization of the continental crust of Patagonia due to widespread magmatism and crustal thickening, which culminated with Permian-Triassic slab break-off. Addition of juvenile mantle-derived magmas is more significant during the Devonian, whereas a progressively increase in crustal reworking is documented from the late Carboniferous to the Permian due to crustal thickening. Therefore, this non-collisional model favors an in situ middle to late Paleozoic crustal growth of Patagonia during changing dynamics of accretionary orogens at the proto-Pacific margin of southwestern Gondwana.

Early Precambrian mafic dykes in western Shandong province, North China Craton: constraints on the chronology, genetic model and tectonic significance

AbstractIn the Late Neoarchaean, the lithosphere of the North China Craton (NCC) experienced a strong extensional event, which is of great significance for understanding the evolution of the continental crust in the Precambrian. In this study, a suite of mafic dykes from Shandong province in the northeastern NCC were investigated to determine the nature, timing and source of rift-related magma activities using zircon U–Pb data, whole-rock geochemistry and Nd–Hf isotopes. Zircon U–Pb dating of four dolerites by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) yielded weighted mean 207Pb/206Pb ages in the range 2509 ± 6.1 to 2537 ± 6.2 Ma (2σ, 95 % confidence interval). The mafic dykes are classified as alkaline rocks based on their K2O + Na2O contents (6.78–7.21 wt %) and belong to the shoshonitic series according to their K2O contents (3.23–3.36 wt %). The dolerites show low concentrations of light rare earth elements ((La/Yb)N between 7.17 and 8.55), positive Eu anomalies (Eu/Eu* between 1.12 and 1.27), positive Ba, K, Pb, Sr, Eu, Dy and Lu anomalies, and depleted U, Nb, Pr, Ta, P, Nd and Ti anomalies. The dykes are characterized by low initial (87Sr/86Sr)i (∼0.6969), positive εNd(t) values (0.2–0.8) and εHf(t) values (0.5–8.6) and relatively old mean Nd and Hf model age (2.73 Ga). Collectively, the data suggest that the mafic dykes were derived from the partial melting (10–20 %) of an isotope-depleted garnet–lherzolite mantle source that was hybridized through interaction with subducted lower crustal material. The parental magmas of these dykes underwent a certain number of crustal contaminations during magma ascent. The mafic magmatism represented in the form of the dyke swarms is considered to be a response to widespread lithospheric extension which affected the NCC at c. 2.5 Ga during the Neoarchaean.

Age and tectonic significance of the Benton pluton, Eel River area, west-central New Brunswick, Canada

The Benton pluton, located in the Eel River area of west-central New Brunswick, Canada, consists of two components—the Caldwell Brook quartz monzonite and Dugan Road monzogranite. Field relations suggest that the two are essentially coeval, the Dugan Road being slightly younger. The Benton pluton intrudes Cambrian to Early Ordovician sedimentary rocks of the Woodstock Group and overlying Early to Middle Ordovician calc-alkaline rhyolitic, andesitic, dacitic, and basaltic rocks of the lower Meductic Group. A new U–Pb (LA-ICP-MS) zircon age of 467 ± 2. Ma indicates that the Dugan Road monzogranite component of the Benton pluton was emplaced during the Middle Ordovician (late Dapingian to early Darriwilian). The Dugan Road monzogranite is ca. 7 million years younger than the nearby arc-related Connell Mountain tonalite and Gibson granodiorite, and ca. 13 million years younger than arc- related volcanic rocks of the Porten Road Formation, the oldest unit of the Meductic Group. The Benton pluton is interpreted to have been emplaced in an active extensional arc setting, coinciding with a shift in the focus of subduction-related volcanism from the Eel River area of New Brunswick to the Greenfield area of adjacent Maine, USA.

Petrology and Tectonic Significance of Epidote Blueschist-Facies Rocks from the Northern Margin of the Pelagonian Unit in the Republic of North Macedonia

Abstract In the Republic of North Macedonia, the northern marginal part of the Pelagonian Unit comprises several isolated slivers of low-grade blueschist-facies rocks. These rocks were metamorphosed at conditions of the epidote-blueschist facies with rare transitions to the lawsonite-epidote blueschist facies. If present, lawsonite occurs as relic inclusions in albite, suggesting a local metasomatic enrichment of Na. No aragonite or fibrous calcite was detected, suggesting that the rocks were equilibrated at pressures below the aragonite-calcite boundary. When present, calcite + quartz coexist with glaucophane, epidote, and titanite suggesting the presence of a metamorphic H2O-rich fluid phase with very low XCO2 values (< 0.002). Most samples contain chlorite and only some have garnet, which is nearly free of Mg, but contains an andradite component. Albite only occurs in garnet-free rocks. A core-to-rim decrease of Fe3+/(Fe3++Al3++Cr + Mn3+) in epidote, VIFe3+/VI(Fe3++Al) in blue Na-rich amphibole, and Fe3+/(Fe2++Fe3+) in garnet grains suggest a decrease of ƒO2 during metamorphism. The general absence of magnetite and the presence of very small amounts of hematite in some samples suggest that the hematite–magnetite buffer curve was probably not crossed. It is thought that the investigated blueschist-facies rocks were part of the once highly oxidized sedimentary cover of the subducting oceanic plate. The underlying basaltic crust and oceanic mantle, however, were probably relatively reduced. Aqueous fluids which left this part of the lithosphere during subduction (dehydration) partially reduced the overlying oxidized metasediments. Concerning metamorphic P–T conditions, a pressure range of 0.7–0.9 GPa is defined by the presence of glaucophane + epidote + quartz at the lower boundary and the absence of aragonite at the higher boundary. The temperature range of 320–420°C is defined by the absence of Fe-Mg-pumpellyite at the lower side and the presence of lawsonite + albite at the higher side. These conditions can be narrowed down by using calculated equilibrium mineral assemblage diagrams (pseudosections) for three different bulk-rock compositions, resulting in P = 0.74–0.85 GPa and T = 340–370°C. It must, however, be emphasized that many phases (garnet, glaucophane, epidote, and phengite) show significant chemical zonation, which makes the use of pseudosections problematic.

Tectonic significance of the shear zones in Southern Granulite Terrane: An integrated geophysical study

• The crustal models derived reveal Moho depth at ∼40 km near MBSASZ. • The increased crustal thickness near MBSASZ can be related to collision. • MBSASZ and PCSZ have different tectono-magmatic history. • Eastward marine prolongation of MBSASZ and PCSZ up to the COB. • The tectonic events in SGT are vital to understand the Precambrian shields. The tectonics of Southern Granulite Terrane (SGT), one of the most important Archean- Neoproterozoic terrain, is unveiled by the gravity-magnetic signatures associated with the complex shear zone system. The significance of gravity and magnetic anomalies and the comparison between the two anomalies elucidate the structural complexity of the region. Though Moyar Bhavani Salem Attur Shear Zone (MBSASZ) and Palghat Cauvery Shear zone (PCSZ) have been identified as the major crustal scale shears, it shows significant difference in their anomaly pattern. A four-layer crustal model derived from gravity and magnetic anomalies is further constrained by seismic information. Along the PCSZ, gravity gradient was subdued and magnetic gradient shows a low in contrast to the magnetic high associated with the MBSASZ suggesting that it might belong to two different crustal terranes. The increase in crustal thickness along MBSASZ also could be explained on the context of the compressional tectonic regime. It is evident that the major shear zones of SGT is extending to the offshore region which suggest that deformation might have extended to the adjacent continents in the geologic past. This leads to an important conclusion that the tectonic events portrayed by SGT is not confined to the terrane alone, but plays an important role in understanding the tectonics in the context of the East Gondwana fragmentation and dispersal.

U-Pb Neoproterozoic age and petrogenesis of a calc-alkaline shoshonitic lamprophyre from Simdega area, Chhotanagpur Gneissic Complex (Eastern India): Implication for the evolution of the Central Indian Tectonic Zone and Rodinia tectonics

The age of emplacement and geochemistry of the lamprophyres are of tectonic significance as they have potential to unravel global scale geodynamic processes. We present petrology, U-Pb SHRIMP apatite and titanite ages and bulk-rock Sr-Nd isotope data for an unmetamorphosed and undeformed lamprophyre dyke from the Simdega area of the Chhotanagpur Gneissic Complex (CGC) which is a component of the E-W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose polarity of their subduction is a contentious issue. The lamprophyre exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Based on combined mineralogy and geochemistry, the lamprophyre is classified to be of calc-alkaline variety (minette) with shoshonitic affinities. Mg# (70.7–78.2) contents highlight the primitive melt character whereas incompatible trace element ratios exclude crustal contamination and are indistinguishable from those of the subduction-related global as well as Eastern Dharwar craton (southern India) calc-alkaline lamprophyres. U-Pb dating of apatite gave an emplacement age of 944 ± 82 Ma which is indistinguishable, within the error limits, from the U-Pb titanite age of 942.1 ± 5 Ma demonstrating a Neoproterozoic magmatic emplacement age of the lamprophyre synchronous with the Rodinia assembly. Bulk-rock 87Sr/86Srinitial (0.707239 and 0.708910) and ɛNdinitial (-8.9 to -8.3) highlights the involvement of an enriched mantle source. Calculated Paleoproterozoic model ages (Nd depleted mantle) of 2.1 Ga of the lamprophyre are indistinguishable from those of the co-spatial amphibolite dykes. Petrogenetic modeling involving rare earth elements reveals that the derivation of the lamprophyre magma from 2 to 3% partial melting of a mixed garnet (70%) and spinel (30%) lherzolitic mantle source with minor phlogopite. Our study highlights that the western part of the CGC was less affected, relative to the eastern part, by the M3 regional amphibolite grade metamorphic event (ca. 920-880 Ma) and also supports the geodynamic models involving northward-directed subduction of the Southern Indian block under the Northern Indian block.-

A review of the geophysical knowledge of the Borborema Province, NE-Brazil, and tectonic implications

This review summarizes the results of nearly 50 years of application of geophysical data to study tectonic problems in Borborema Province (BP), Northeastern Brazil. Much information has been obtained from the interpretation of magnetic, gamma-ray spectrometry, gravity, seismological, magnetotelluric, deep seismic and heat flow data. The resulting picture is that, in comparison with other Brazilian geological domains, BP has a thin crust, a lithosphere with small effective elastic thickness, the presence of predominantly positive free-air gravity anomalies, besides the most expressive geoid anomaly, in addition to relatively high heat flow and intraplate seismicity. These facts reinforce the hypothesis that the BP lithosphere suffered a strong impact in Gondwana fragmentation. Predominantly positive free-air gravity anomalies reveal that part of the province has not yet reached isostatic equilibrium. Gravity data established the boundaries of the BP crustal block with neighboring blocks; in particular, the crustal boundary with the São Francisco Craton is located in the innermost regions of the marginal fold belt, tens of kilometers at the north of the conventional limit. Magnetic data allowed to infer the continuity of basement structures beneath sedimentary basins, in particular that BP continues to the Transbrasiliano Lineament, underneath the sedimentary of the Parnaíba Basin. In addition, enhanced magnetic maps made it possible assessing the amount of deformation associated with the major shear zones and identified a continental-scale Cretaceous dike system crossing the BP. Gamma-ray spectrometry data allowed extensive geological mapping of metasedimentary sequences, granitic intrusions and basement complexes. Seismological data allowed to recognize that major shear zones in BP have lithospheric significance. Deep seismic data evidenced a well-defined contrast between the upper and lower crust in BP (Conrad discontinuity). Magnetotelluric data revealed several deep geoelectrical anomalies in BP, some of them interpreted as relics of subduction zones. Intraplate seismicity studies allowed inferring that the current stress state in the BP upper crust is predominantly transcurrent. The list of open problems in BP is also expressive. Notable examples are: a) it has not yet been possible to define which type of orogeny (accretionary or intracontinental?) was dominant in the Neoproterozoic; b) the cause of the expressive geoid anomaly is unknown; c) the tectonic significance of the Transbrasiliano Lineament (collisional suture? Just an intracontinental shear zone?) is also unknown. Regarding economic applications, the use of geophysical data to prospect new ore deposits is still relatively poor. We hope that this review can contribute to increase the familiarity and acceptance of geophysical results by the geological community.

Petrogenesis and Tectonic Significance of Late Triassic A1-Type Granite from the West Section of North Qinling Orogenic Belt: Constraints from Geochronology and Geochemistry

The North China Block and the South China Block collided in the Middle Triassic, but there is still a lack of consensus regarding the end of collisional orogeny and the closure time of the Paleo-Tethys. In this paper, we report zircon U–Pb ages and geochemistry for the Shimen pluton in the northern margin of the West Qinling Orogenic Belt to investigate its genesis and tectonic environment. The new findings allow to constrain the end time of the Triassic orogeny in the Qinling Orogenic Belt and the closure time of the Paleo-Tethys. The weighted average 206Pb/238U ages of the Shimen pluton are 218.6 ± 1.5 Ma and 221.0 ± 1.7 Ma. Thus, we suggest that the Shimen pluton crystallized at the 218.6 Ma and 221.0 Ma and was formed during the Late Triassic (Norian). The Shimen pluton is mainly syenogranite and has alkaline dark minerals aegirine–augite. It is composed of 73.45 to 77.80 wt.% SiO2, 8.28 to 9.76 wt.% alkali, and 11.35 to 13.58 wt.% Al2O3, with A/CNK ranging from 0.91 to 1.02 and 10,000 Ga/Al ranging from 2.39 to 3.15. These findings indicate that the Shimen pluton is typical A-type granite. The plutons have low rare earth element contents, ranging from 73.92 to 203.58 ppm, with a moderate negative Eu anomaly. All the samples are enriched in large-ion lithophile elements, such as Rb, Nd, Th and U, and light rare earth elements, and are depleted in high field strength elements, such as Nb, P, Zr, Ba, and Sr. The depletion of Ba, Sr, and Zr may be related to the fractionation and evolution of the granite. According to the petrological and geochemical characteristics, the Shimen pluton is an A1-type granite formed in an anorogenic extensional environment. Combined with its tectonic characteristics and petrogenesis, the Shimen pluton was probably formed by the partial melting of the crust under high temperature and low pressure in the intraplate environment after the subduction of the South China Block beneath the North China Block. This observation indicates that the Triassic orogeny in the Qinling Orogenic Belt had ended and the Paleo-Tethys-Mianlve Ocean had also closed by the Late Triassic (Norian).

Opening of the Sumdo Paleo-Tethys Ocean and rifting of the Lhasa terrane from Gondwana: Insights from early Carboniferous magmatism in southern Tibet

The late Paleozoic tectonic evolution of the Lhasa terrane and opening of the Sumdo Paleo-Tethys Ocean remain controversial due to limited data and the ambiguous tectonic significance of Paleozoic magmatism in the southern Lhasa subterrane. Here, we explore the petrogenesis of meta-basalt from the Tangjia area and meta-gabbro from the Sumdo area to provide new constraints on the evolution of the Lhasa terrane. The geochemical characteristics of meta-gabbro are similar to those of oceanic island basalt and have positive whole-rock εNd(t) values (+0.08 to +1.03) and zircon εHf(t) values (+3.4 to +4.9), which suggests that they originated from asthenospheric mantle with minor contributions from lithospheric mantle in an intracontinental rift setting. The meta-basalt yields positive whole-rock εNd(t) values (+6.47 to +6.60) and zircon εHf(t) values (+14.2 to +15.2) and has enriched mid-oceanic-ridge−like geochemical characteristics; we suggest that the meta-basalt was sourced from the depleted mantle and interacted with enriched components in an initial oceanic basin setting. Zircon U−Pb geochronology of meta-basalt and meta-gabbro yielded crystallization ages of ca. 338 Ma and ca. 345 Ma, respectively. We suggest that the Sumdo Paleo-Tethys Ocean opened in the early Carboniferous. Negative buoyancy of Paleo-Tethys oceanic slab during subduction toward Eurasia drove the opening of the Sumdo Paleo-Tethys Ocean. Given that the Sumdo Paleo-Tethys Ocean was a limited oceanic basin, the Lhasa terrane was not completely separated from the northern margin of Gondwana before the Permian.

Mesoproterozoic–Neoproterozoic chronostratigraphic framework and provenance analysis in the southeastern North China Craton and its tectonic significance

The Sishilichangshan area of southwestern Xuhuai Basin, located in southeastern North China Craton (NCC) and to the north of the Qinling–Dabie Orogenic belts, is a key, optimal area to study the regional evolution of the NCC because of its stratigraphic differences within the NCC. An integrated approach of the field investigation, element geochemistry, and detrital zircon U–Pb–Hf isotope analysis reveals essential information on the Mesoproterozoic–Neoproterozoic tectonic‐sedimentary evolution of the Xuhuai Basin. The youngest graphical peak ages of detrital zircons from the Shouxian Formation at the top of the Bagongshan Group, the Jiuliqiao Formation at the bottom of the Huainan Group, and the Anyangshan Formation are 1003 ± 25 Ma, 991 ± 80 Ma, and 832 ± 23 Ma, respectively. New age constraints and stratigraphic correlations show that the traditional Neoproterozoic strata in the southern Xuhuai Basin consist of Late Stenian to Early Tonian Bagongshan and Huainan groups, and the Late Tonian Anyangshan Formation, which overlapped unconformably with mid‐late Ediacaran or Cambrian Series 2 strata. The Anyangshan Formation is first termed in this study, and its equivalent strata occur sporadically in the southeastern, southwestern and northwestern NCC. The Tonian Shouxian, Jiuliqiao, and Anyangshan formations are characterized by detrital zircons of multi‐stage Mesoproterozoic ages and predominantly positive εHf(t) values, with differences in mineral composition, whole‐rock geochemistry, and the Early Neoproterozoic detrital zircon age population. Their possible provenance was a Grenvillian orogen extending from the North Qinling Terrane (NQT) to the central Korean Peninsula, supporting Late Mesoproterozoic to Early Neoproterozoic initial accretion related to the continental arc between the NCC and NQT. Tectonic transition from Late Stenian to Early Tonian (~1.18–0.94 Ga) collisional basin to mid‐late Tonian (0.94–0.74 Ga) rift basin is interpreted as the response of the NCC to the assembly and extension of the Rodinia Supercontinent.

Neoproterozoic amphibolite-facies metamorphism of the Douling complex in the northern Yangtze Craton and its tectonic implications: Constraints from petrology and zircon U-Pb-Hf-O isotopes

Neoproterozoic evolution of the Yangtze Craton has long been debated. The Douling complex, representing one of the oldest crystalline basements, records Neoproterozoic high-grade metamorphism and may provide pivotal clues into a reliable Neoproterozoic configuration of the Yangtze Craton. However, its metamorphic evolution and tectonic significance have not been well constrained. In this contribution, we conduct an integrated study of petrography, zircon U-Pb age and Hf-O isotopic compositions for two garnet amphibolites and one gneiss from the Douling complex. Phase equilibrium modeling and conventional geothermobarometry calculation define peak P-T conditions of 9–10 kbar and 670-690 °C, and shape a clockwise P-T path. Zircon grains from the gneiss mostly reveal core-rim structure. The cores display oscillatory zoning, high Th/U ratios and trace element contents, as well as HREE-enriched patterns, high 176Lu/177Hf ratios and δ18O values, consistent with a magmatic origin. They yield a weighted mean 206Pb/238U age of 868 ± 9 Ma, which is taken as the protolith age. The cores have strongly positive ƐHf(t) values of 7.0–15.6 and young two-stage Hf model ages of ca. 1310–760 Ma, indicating that the protolith was dominantly sourced from prompt reworking of Meso-Neoproterozoic juvenile crust. On the other hand, the zircon overgrowth rims in the gneiss exhibit no zoning patterns, low Th/U ratios and trace element contents, flat HREE patterns and pronouncedly negative Eu anomalies, variable 176Lu/177Hf ratios and comparable ƐHf(t) values to the magmatic cores, indicating their coeval formation with garnet and plagioclase under amphibolite-facies metamorphism and might be generated by dissolution-reprecipitation. Zircon crystals from the two amphibolites perform weak zoning patterns, low Th/U ratios and trace element contents, high formation temperatures, flat to weakly enriched HREE patterns, insignificantly and/or clear negative Eu anomalies, and low 176Lu/177Hf ratios and ƐHf(t) values, suggesting that they may form near the peak stage of amphibolite-facies metamorphism. Metamorphic zircon in the three samples yield homogeneously high δ18O values (10.81 ± 0.17‰, 11.45 ± 0.40‰, 10.92 ± 0.08‰), indicating the participation of external fluid originated from surrounding metasedimentary rocks. These metamorphic zircon grains together yield a weighted mean age of 817 ± 4 Ma (MSWD = 0.63), which is taken as the best estimated age for the upper amphibolite-facies metamorphism in the Douling complex. Considering the widespread contemporaneous magmatism in the northern Yangtze Craton, we propose that the Douling complex recorded a tectonic regime switch from compression to extension at ca. 830–820 Ma, in response to outboard oceanic subduction on the periphery of the Yangtze Craton and then tectonic extension and collapse of the thickened lithosphere.

Neoarchean Magmatic Events in the Western Songnen Massif, Central Asian Orogenic Belt: Timing and Tectonic Significance

AbstractWhether a Neoarchean basement existing in the Songnen massif is currently debated. Identification of Archean magmatism from the Songnen Massif is helpful to resolve this issue. Here, we report newly discovered Neoarchean Shanquan pluton in the Western Songnen Massif. These Neoarchean Shanquan pluton are mainly composed of granites that are exposed near the town of Shanquan in Heilongjiang Province. LA‐ICP‐MS zircon U‐Pb dating reveals that the sample 2015TW1 has an upper intercept age of 2801 ± 69 Ma and a weighted mean age of 2708 ± 18 Ma, while samples LJ27QY1 and LJ27QY2 have upper intercept ages of 2677 ± 57 Ma and 2653 ± 18 Ma, and weighted mean ages of 2649 ± 10 Ma and 2653 ± 15 Ma, respectively. This indicates that these granites were formed at ~2.7 Ga. Most of the ~2.7 Ga zircons have older TDM2 ages of 2762–3326 Ma with positive εHf(t) values ranging from 0 to 6.4, while a few of the zircons have negative εHf(t) values ranging from −8.1 to −11.5 and older TDM2 ages varying from 3158 to 3264 Ma. The zircon Hf isotopes indicate that Paleo‐Mesoarchean crusts might once existed in the Songnen Massif, and the studied Neoarchean magmas were principally derived from partial melting of these Paleo‐Mesoarchean ancient crust. Based on the geochronological spectrum of magmatic and detrital zircons, the Songnen, Erguna, Jiamusi, Bureya massifs may have a common basement prior to the Neoproterozoic and may even be linked with the Triam Craton.