Collisional Orogenesis Research Articles

Crustal maturation and cratonization in response to Neoarchean continental collision: The Suizhong granitic belt, North China Craton

The late Archean (3.0–2.5 Ga) is a pivotal period, when properties of the continental crust significantly changed and a fundamental shift of geodynamic processes took place. As the major felsic component in early Archean (> 3.0 Ga) crustal records, tonalite-trondhjemite-granodiorite (TTG) suites are compositionally different to the present-day felsic upper continental crust. But it remains unclear how and to what degree the felsic part of the continental crust evolved during the late Archean. The 250 km long Neoarchean Suizhong granitic belt in the North China Craton (NCC) is mainly composed of K-rich granitoids and compositionally distinct to the TTG-dominated early Archean upper continental crust. This belt is a natural laboratory to explore how the early Archean upper continental crust evolved in the late Archean. Combined with our new data (zircon U-Pb ages, bulk-rock geochemistry and zircon Hf isotopes), we provide an overview of the temporal and compositional features of the Neoarchean Suizhong granitic belt. Granitic magmatism in the Suizhong granitic belt mainly took place within 100 Myrs around the end of the Archean, and these rocks are primarily K-rich granitoids with minor TTGs and mafic magmatic enclaves (MMEs). The K-rich granitoids include sanukitoid-like granites and potassic granites; the former are hybrids between melts from Mesoarchean enriched mafic crust and enriched mantle, and the latter are melts from Hadean–Mesoarchean crustal lithologies. The volumetrically minor TTGs were sourced from Mesoarchean low-K mafic crust. MMEs are either cumulates or inclusions of captured enriched mantle melts. Overall, these Neoarchean granitoids are essentially intracrustal reworking products of ancient continental nuclei and are compositionally and temporally similar to syn- and post-collisional magmatism in Phanerozoic collisional orogenic belts. The Neoarchean crustal reworking process was most likely to be induced by amalgamation of micro-continents through collision. Through the collision-induced reworking, a variety of K-rich granitoids were introduced to the Neoarchean upper continental crust. As a result, layering of felsic upper and mafic lower crust was developed, and the maturity of Archean continental crust was significantly enhanced. Collisional orogenesis played a critical role in maturing the Archean continental crust and initial cratonization of the NCC during the late Archean.

Neoproterozoic tectonic shift from collisional orogenesis to intraplate extension in the Yili Block, southern Central Asian Orogenic Belt

Understanding the tectonic evolution of the many micro-continents in the Central Asian Orogenic Belt is a complex and controversial issue. One such micro-continent, the Yili Block is characterized by Neoproterozoic magmatic rocks, which can provide important constraints on the tectono-magmatic history of the region. Herein, we present a systematic investigation of the petrology, whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes on newly identified leucogranite intrusions and gabbroic dikes in the Wenquan district of the northern Yili Block. Two leucogranite samples yielded zircon U-Pb ages of 983 ± 9 Ma and 905 ± 9 Ma, respectively. Both of muscovite-bearing leucogranites have S-type affinities and show distinctive geochemical compositions similar to peraluminous granites. Furthermore, the leucogranites exhibit low Rb/Sr ratios (0.10–0.49) and elevated zircon εHf(t) values (+3.04 - +13.59), indicating that they were derived from partial melting of supracrustal pelitic and mafic rocks. In contrast, one gabbroic dike sample yielded a zircon U-Pb age of 779 ± 5 Ma. The gabbroic samples have distinctly high Th/Yb (1.10–1.68) and low Nb/La (0.37–0.46) ratios, with enrichment in large ion lithophile elements and depletion in Nb, Ta and Ti, implying that they were sourced from a metasomatic subcontinental lithospheric mantle. Integrated with other available data, we propose that the Yili Block underwent continuously collisional orogenesis during the early Neoproterozoic (ca. 983–780 Ma), with a subsequent tectonic shift from collisional to an intraplate extensional setting at ca. 779–776 Ma. A comparable magmatic evolution also occurred in adjacent micro-continents of the Kazakhstan region and northern Tarim Craton, indicating that these micro-continents may have shared a common Neoproterozoic geodynamic evolution.

Early Paleozoic granodiorites and gabbros related to asthenosphere upwelling in the eastern Wuyi-Yunkai Orogen, South China

The early Paleozoic Wuyi-Yunkai Orogen (WYO) developed in the inner part of the South China Block (SCB) and underwent large scale felsic magmatism. Different tectonic models have been proposed to explain the evolution of the WYO, including intracontinental and collisional orogenesis. The debates focus on whether the asthenospheric materials were involved during evolution of the WYO. We conducted detailed petrographic and geochemical analyses on gabbros from the Qiaotou pluton and granodiorites and the hosted MMEs from the Xinsi pluton in the eastern WYO, and present the first evidence of asthenosphere upwelling that induced significant lithosphere remobilization in the SCB during early Paleozoic. Granodiorites from the Xinsi pluton (441 ± 2 Ma) contain abundant amphibole and are metaluminous, which originated from the igneous basement in the Cathaysia Block. The hosted MMEs (440 ± 2 Ma) have variable SiO2 and enriched Sr-Nd isotopic compositions with high zircon δ18O values, which were formed through hybridization of felsic melts and mafic magmas from the enriched lithosphere mantle. Gabbros from the Qiaotou pluton (448 ± 8 Ma) have low K2O with insignificant Nb-Ta-Ti anomalies and show variable Sr-Nd-O isotopic compositions. They were derived from the depleted asthenosphere and underwent crustal assimilation. The asthenosphere upwelling may be induced by slab breakoff during the collision between the western Cathaysia Block and an unknown block near northern Gondwana. The continental collision possibly transferred compressional stress and caused crustal uplifting along the weak zone in the inner SCB. The asthenosphere upwelling provided tremendous heat for the lithosphere remobilization and voluminous magmatism in the WYO.

Timing of UHT metamorphism and cooling in south Indian granulites: New P-T-t results from a sapphirine granulite

The Precambrian granulite terrane of south India is known for the preservation of regional-scale high to ultrahigh-temperature (HT-UHT) metamorphic rocks. In the Madurai block, the largest crustal block in the terrane, a significant proportion of these HT-UHT rocks occur along a narrow belt referred to as the Kambam ultrahigh-temperature belt. Understanding the P-T-t evolution of these HT-UHT granulites is vital in decoding the spatial and temporal evolution of the Madurai block in relation to other crustal blocks. Here we present the petrology, mineral chemistry, phase equilibrium modelling, and accessory mineral (zircon, monazite, rutile and apatite) geochronological and geochemical data for a hitherto unreported sapphirine granulite from the Kambam ultrahigh-temperature belt. Combined mineral reaction and phase equilibrium modelling indicate extreme P-T conditions up to 1130 °C at 11 kbar. The geometry of the P-T path is clockwise with initial isothermal decompression followed by near isobaric cooling. LA-ICPMS U-Pb geochronological studies on accessory phases report two distinct thermal events: (i) Paleoproterozoic high-T event and associated crustal anatexis at ∼2.5 Ga from zircons and (ii) Neoproterozoic UHT event at ∼550 Ma from monazites. The monazite ages suggest a 50–60 Ma prolonged thermal event, attributed to Ediacaran-Cambrian collisional orogenesis. Additional metamorphic pulses are also identified, demonstrating the polydeformed crustal evolution history of the terrane. U-Pb age dating of rutile and apatite in the sample provide ages of ∼458 Ma and ∼392 Ma and cooling rates of ∼6 °C/Ma and ∼0.3–2 °C/Ma respectively. This suggests a complex history of slow cooling followed by ultraslow cooling. The results bring forth the significance of the Kambam ultrahigh-temperature belt in understanding the tectonothermal evolution of Madurai block and provide additional evidence for long-lived Neoproterozoic UHT orogenesis in south India.

Evaluating preservation bias in the continental growth record against the monazite archive

Abstract Most recent models of continental growth are based on large global compilations of detrital zircon ages, which preserve a distinctly episodic record of crust formation over billion-year timescales. However, it remains unclear whether this uneven distribution of zircon ages reflects a true episodicity in the generation of continental crust through time or is an artifact of the selective preservation of crust isolated in the interior of collisional orogens. We address this issue by analyzing a new global compilation of monazite ages (n >100,000), which is comparable in size, temporal resolution, and spatial distribution to the zircon continental growth record and unambiguously records collisional orogenesis. We demonstrate that the global monazite and zircon age distributions are strongly correlated throughout most of Earth history, implying a link between collisional orogenesis and the preserved record of continental growth. Our findings support the interpretation that the continental crust provides a preservational, rather than generational, archive of crustal growth.

Pressure–temperature–time–deformation (P–T–t–d) path for Devonian forearc deposits in the Imjingang Belt, South Korea: Implications for Permian–Triassic collisional orogenesis on the eastern margin of Eurasia

AbstractThe Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo‐feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling–Dabie–Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure–temperature (P–T) path for the formation is characterized by a high ΔP/ΔT prograde part and subsequent near‐isothermal decompression with peak P–T conditions of 600–620°C and 9.0–10.5 kbar, based on observations of mineral assemblages and phase equilibria modelling. Strong compressional deformation (Dn) produced a N–S‐trending Sn foliation that transposed the earlier Sn–1 and isoclinal Fn folds and was coeval with an episode of medium‐ to high‐pressure amphibolite‐facies metamorphism. The NNE–SSW‐trending left‐lateral Ryeseonggang Fault played a key role in producing the N–S distribution of the metamorphosed forearc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276–250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U–Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P–T–t (time)–d (deformation) path for the Taean Formation indicates rapid burial during Dn followed by rapid exhumation during the Permian–Triassic collision between the Qinling microcontinent and the North China Craton.

Tectonic Accretion Controls Erosional Cyclicity in the Himalaya

AbstractThe evolution of Earth's climate over geological timescales is linked to surface erosion via weathering of silicate minerals and burial of organic carbon. However, methodological difficulties in reconstructing erosion rates through time and feedbacks among tectonics, climate, and erosion spurred an ongoing debate on mountain erosion sensitivity to tectonic and climate forcing. At the heart of this debate is the question of whether late Cenozoic climate cooling has increased global erosion rates or not. The Himalaya plays a prominent role in this debate as its erosion produces a large fraction of global sediments delivered to ocean basins. We report a 6‐Myr‐long record of ‐derived erosion rates from the north‐western Himalaya, which indicates that erosion rates in this region varied quasi‐cyclically with a period of ∼1 Myr and increased gradually toward the present. We hypothesize that the observed pattern of erosion rates occurred in response to the tectonic growth of the Himalaya by punctuated basal and frontal accretion of rocks from the underthrusting Indian plate and concomitant changes in topography. In this scenario, basal accretion episodically changes rock‐uplift patterns, which brings landscapes out of equilibrium and results in quasi‐cyclic variations in erosion rates. We used numerical landscape evolution simulations to demonstrate that this hypothesis is physically plausible. We attribute the long‐term increase in erosion rates to the erosional response of topography due to frequent basal accretion relative to frontal accretion. Because tectonic accretion processes are inherent to collisional orogenesis, they likely confound climatic interpretations of erosion rate histories.

Earth's new tectonic regime at the dawn of the Paleoproterozoic: Hf isotope evidence for efficient crustal growth and reworking in the São Francisco craton, Brazil

Abstract A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive εHf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative εHf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism.

Construction of the Lesser Himalayan–Subhimalayan thrust belt: The primary driver of thickening, exhumation, and high elevations in the Himalayan orogen since the middle Miocene

Abstract Documenting the structural evolution of the Himalayan orogen is fundamental for understanding the dynamics of collisional orogenesis. We argue that the importance of deformation in the frontal, Lesser Himalayan–Subhimalayan (LH-SH) portion of the Himalayan thrust belt for driving crustal thickening over the past ~15–13 m.y. has long been overlooked. To quantify its contribution to thickening, we measured parameters from 22 published cross sections that span the length of the orogen. The mean structural uplift accomplished by the LH-SH thrust belt increases from 10–15 km in the eastern half of the orogen to 15–23 km in the western half. An antiformal culmination constructed by LH duplexing is observed across the orogen and increases in structural height (to as much as 15–20 km) and north-south width moving westward. Construction of the culmination was the primary mechanism for building and maintaining wedge taper. The westward scaling of culmination size is accompanied by doubling and tripling of LH-SH shortening and accretion magnitude, respectively; when combined with a consistent orogen-wide modern taper angle (11° ± 2°), this indicates that duplexing facilitated the growth of an overall larger orogenic wedge moving westward. Following the initial southward propagation of deformation into LH rocks at ca. 15–13 Ma, the Himalayan orogenic wedge has been characterized by stacking of multiple thin, small-displacement thrust sheets to develop a high-taper orogenic wedge. Thus, LH-SH deformation has had a profound effect on driving thickening, exhumation, and the attainment of high elevations since the middle Miocene.

Deciphering paleogeography from orogenic architecture: Constructing orogens in a future supercontinent as thought experiment

Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid <i>a priori</i> dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis9 that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.

Absence of evidence for Palaeoproterozoic eclogite-facies metamorphism in East Antarctica: no record of subduction orogenesis during Nuna development

The cratonic elements of proto-Australia, East Antarctica, and Laurentia constitute the nucleus of the Palaeo-Mesoproterozoic supercontinent Nuna, with the eastern margin of the Mawson Continent (South Australia and East Antarctica) positioned adjacent to the western margin of Laurentia. Such reconstructions of Nuna fundamentally rely on palaeomagnetic and geological evidence. In the geological record, eclogite-facies rocks are irrefutable indicators of subduction and collisional orogenesis, yet occurrences of eclogites in the ancient Earth (> 1.5 Ga) are rare. Models for Palaeoproterozoic amalgamation between Australia, East Antarctica, and Laurentia are based in part on an interpretation that eclogite-facies metamorphism and, therefore, collisional orogenesis, occurred in the Nimrod Complex of the central Transantarctic Mountains at c. 1.7 Ga. However, new zircon petrochronological data from relict eclogite preserved in the Nimrod Complex indicate that high-pressure metamorphism did not occur in the Palaeoproterozoic, but instead occurred during early Palaeozoic Ross orogenesis along the active convergent margin of East Gondwana. Relict c. 1.7 Ga zircons from the eclogites have trace-element characteristics reflecting the original igneous precursor, thereby casting doubt on evidence for a Palaeoproterozoic convergent plate boundary along the current eastern margin of the Mawson Continent. Therefore, rather than a Palaeoproterozoic (c. 1.7 Ga) history involving subduction-related continental collision, a pattern of crustal shortening, magmatism, and high thermal gradient metamorphism connected cratons in Australia, East Antarctica, and western Laurentia at that time, leading eventually to amalgamation of Nuna at c. 1.6 Ga.

Evidence from the U–Pb–Hf signatures of detrital zircons for a Baltican provenance for basal Old Red Sandstone successions, northern Scottish Caledonides

The provenance of Devonian Old Red Sandstone (ORS) continental successions in the northern Scottish Caledonides is poorly known: were they derived locally or from more distal sources? The integration of U–Pb and Hf isotope analyses in detrital zircon crystals reduces potential ambiguities arising from non-unique age populations and yields information on the crustal evolution of the source terranes. Samples of basal ORS successions yield zircon U–Pb age groupings of c. 1800–1500 and c. 1200–900 Ma, with minor Neoarchean, Tonian, Ediacaran and Ordovician contributions. SW Baltica provides the best match for detrital zircon ages and Hf isotopic signatures and much of the &gt;900 Ma zircon population was probably recycled from Neoproterozoic successions. ε Hf ( t ) values in c. 1800–1000 Ma grains reflect the assembly of Nuna, the development of a long-lived retreating subduction system along its margin and Grenville collisional orogenesis. These basal ORS successions were probably deposited within the same regional fluvial system as coeval sedimentary rocks in the Midland Valley, draining an area of positive relief in SW Baltica where continental convergence continued through the Early Devonian. Supplementary material : Isotopic data, statistical comparison of selected samples and TIMA heavy mineral data are available at https://doi.org/10.6084/m9.figshare.c.5301780

Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

Ultra-high temperature (UHT) metamorphism plays an essential role in the development and stabilization of continents through accretionary and collisional orogenesis. The Napier Complex, East Antarctica, preserves UHT metamorphism, and the timing is still debated. U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The zircons are analyzed using a sensitive high-resolution ion microprobe (SHRIMP). The cathodoluminescence observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains (Group I), metamorphic domains (Group II), and U–Pb system disturbed domains (Group III). The REE patterns of Group II are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the above classification. The 207Pb*/206Pb* ages of Group II have an age peak at 2501 Ma, therefore, the gneiss experienced high temperature metamorphism at 2501 Ma. δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests re-equilibration of oxygen isotopes after metamorphism at ca. 2501Ma under dry UHT conditions.

Paleoproterozoic multiple magmatic-metamorphic events in the Dunhuang Block, eastern Tarim Craton: Implications for assembly of the Columbia supercontinent

The evolution of the Dunhuang Block associated with the assembly of the Columbia supercontinent is poorly constrained. The geochemical composition and temporal evolution of magmatic and metamorphic rocks may hold key information regarding critical geodynamic processes that occurred during the evolution of the Dunhuang Block. In this paper, we present a systematic petrographic, geochemical, and zircon U-Pb and Hf isotopic investigation on tonalitic gneiss, carbonatite, granitic dyke, mafic granulite and A-type granite in the Dunhuang Block. The results suggest that tonalitic gneiss was derived from partial melting of a thickened oceanic plateau crust or island-arc crust at ~3.67 Ga and then it recorded an important subduction-collision orogenesis in the Paleoproterozoic (~2.02 Ga to ~1.96 Ga). The ~1.94 Ga carbonatite and ~1.85 Ga granitic dyke were formed by partial melting of marble and Meso- to Neoarchean basement rocks, respectively. Both of them were formed in post-collisional setting. The ~1.82 Ga mafic granulite recorded high-pressure granulite facies metamorphism followed by a medium–low pressure granulite facies metamorphic overprint, suggesting a clockwise P-T path and implying an environment of collisional orogenesis. The ~1.76 Ga A-type granite resulted from the reworked Neoarchean crust and was most likely related to a post-collisional episode. Combined with previous studies, we identify two Paleoproterozoic orogenic belts of the Dunhuang Block: the ~2.00–1.96 Ga South Orogen and ~1.85–1.80 Ga North Orogen. According to the coupling of these two orogenies with the Columbia orogeny, we conclude that the Dunhuang Block was associated with the assembly of the Columbia supercontinent.

A tectonic transition from closure of the Paleo-Asian Οcean to subduction of the Paleo-Pacific Plate: Insights from early Mesozoic igneous rocks in eastern Jilin Province, NE China

The timing of tectonic transition from the Paleo-Asian oceanic regime to the Paleo-Pacific regime in the easternmost Central Asian Orogenic Belt remains controversial. To constrain the event, we present new zircon U-Pb ages, Hf isotopic compositions, and whole-rock major- and trace-element data for early Mesozoic igneous rocks from eastern Jilin Province, NE China. These new ages, together with existing age data, indicate that early Mesozoic magmatism in the Kaiyuan–Yanji area can be subdivided into four stages: Early Triassic (ca 250 Ma), Early–Middle Triassic (249–235 Ma), Late Triassic (229–208 Ma) and latest Triassic to Early Jurassic (207–175 Ma). The Early Triassic igneous rocks constitute a bimodal igneous rock association that formed in an extensional environment caused by break-off of the subducting slab during final closure of the Paleo-Asian Ocean. The Early–Middle Triassic magmatism is dominated by adakitic granitoids whose primary magmas were derived from partial melting of thickened mafic lower crust, suggesting that the North China Craton and Songliao–Xilinhot Block were still undergoing collisional orogenesis at 249–235 Ma. The Late Triassic igneous rocks comprise a typical bimodal association of rocks in an E–W-trending belt, formed in a post-orogenic extensional environment related to final closure of the Paleo-Asian Ocean. The latest Triassic to Early Jurassic igneous rocks have calc-alkaline characteristics, were emplaced as a NE–SW-trending belt parallel to the eastern margin of Eurasia, and formed in an active continental margin setting associated with initial subduction of the Paleo-Pacific Plate. We conclude that the tectonic transition between the Paleo-Asian and the Paleo-Pacific regimes took place during the latest Triassic.

Adakitic rocks and A‐type felsic dykes in the Changlingzi area, NE China: Constraints on multistage tectonism in the southern Great Xing'an Range

New LA–ICP–MS zircon U–Pb geochronology, whole‐rock major and trace element geochemistry data, and petrographic observations are presented for three granitic dykes in the Changlingzi area of NE China. These data provide precise age and petrogenesis information with respect to the late Palaeozoic–Mesozoic intrusions in the area, which further constrain the geodynamic evolution of the southern Great Xing'an Range. Our zircon U–Pb data denote that the felsic dykes were emplaced during the Late Permian (~254 Ma) and Late Jurassic (143–142 Ma). Geochemically, the Late Permian monzonite porphyries are characterized by high Al2O3 (16.52–16.92 wt%) and Sr (490–579 ppm) contents and low Yb (0.52–0.57 ppm), Y (5.7–6.3 ppm), and heavy rare earth element (HREE) contents, which indicate an adakitic affinity. The Late Jurassic felsic dykes are characterized by high FeOt/MgO (8.2–11.9) and Ga/Al (3.3–3.8) ratios, high zircon saturation temperatures (888–929°C), and remarkable negative Eu (Eu/Eu* = 0.25–0.42) anomalies, indicating they are akin to A‐type granites. The Late Permian adakitic magma resulted from the partial melting of thickened lower crust, and was formed during collisional orogenesis related to subduction of Palaeo‐Asian Ocean crust. The Late Jurassic A‐type magmas were probably generated by the partial melting of amphibolite‐facies lower crustal materials. The melting occurred in response to the upwelling of asthenospheric material caused by collapse of thickened crust, due to gravitational instability during closure of the Mongol–Okhotsk Ocean. Subsequently, the latest stage of magmatic activity in this area and elsewhere in NE China was controlled mainly by subduction of Palaeo‐Pacific Ocean crust.

Constraining paleoproterozoic (∼1.7 Ga) collisional orogenesis between the Eastern Dharwar and Bastar cratons: New Sm–Nd garnet isochron and Th–U-total Pb monazite chemical ages from the Bhopalpatnam orogen, central India

The timing of assembly into India by amalgamation of cratonic fragments, is controversial with estimates ranging from Tonian, earliest Mesoproterozoic to Neoarchean. All models have as the nucleus the South Indian Block of Dharwar, Bastar and Singhbhum cratons. Although the time of assembly of the Bastar and Singhbhum cratons is uncertain, the implication of a proposed 1891–1883 Ma-aged Large Igneous Province, straddling the Bastar and Eastern Dharwar cratons is that their amalgamation occurred by Paleoproterozic time (≥1.9 Ga ago) and is at variance with the ages reported from the Bhopalpatnam granulite belt between 1.7-1.6 Ga ago. To test whether this belt formed by convergent collisional tectonics along the margins of these two cratons, petrochronologic analysis was conducted on high-grade rocks of this belt using garnet Sm–Nd and texturally-controlled Th–U-total Pb monazite chronometry.The whole rock-garnet Sm–Nd isochron from garnet-clinopyroxene-amphibolite in migmatite (M1a-stage), yielded an isochron age of 1692±13Ma with Ndi of 0.51042±0.00006 (2σ, MSWD = 2.5 n = 4) whereas a whole rock-garnet Sm–Nd isochron from interlayered garnet-sillimanite-rutile metapelite (M1b-stage), yielded an isochron age of 1675±7 Ma with Ndi of 0.50997±0.00003 (2σ, MSWD = 1.9 n = 3) Monazite inclusions within garnet porphyroblasts in this metapelite have a weighted mean age of 1703±28 Ma (2σ, MSWD = 0.95, n = 8), within error of the age of monazite inclusions within matrix biotite at 1665±16 Ma (2σ, MSWD = 0.56, n = 17). Post M1b stage lower temperature hydration-retrogression events likely occurred by late Mesoproterozoic time. The metamorphism followed a clockwise P-T path (high-pressure amphibolite-facies stage M1a P ≥ 12 kbar at ∼750 °C, followed by M1b granulite-facies stage with T ≥ 850 °C at lower pressures), interpreted as a response to crustal thickening during Paleoproterozoic continental collision. We propose that the Bhopalpatnam orogen is the exhumed locus of the leading edge of the Archean Bastar craton that was thrust under the EDC craton.

New insights into Paleoproterozoic tectonics of the Yangtze Block in the context of early Nuna assembly: Possible collisional granitic magmatism in the Zhongxiang Complex, South China

The newly discovered Zhongxiang Complex preserving extensive Paleoproterozoic granitoids plays a pivotal role in understanding the early tectonic processes of the Yangtze Block, South China, in the context of the Nuna supercontinent cycle. New zircon U-Pb age and Hf-isotope coupled with whole-rock chemical and Nd-isotope results from three granitic intrusions in the southern Zhongxiang Complex document a magmatic record of significant late Paleoproterozoic tectonic convergence in the region. Geochronological data show that granites from these intrusions were emplaced in a relatively long period at 2.0–1.93 Ga. The granites have variably high SiO2 contents of 74.01–77.19 wt%, and are weakly to strongly peraluminous with high ASI (alumina saturation index) values (>1.05). They are generally magnesian, showing fractionated S-type granite affinities. They have negative zircon εHf(t) of −9.69 to −17.11 and negative whole-rock εNd(t) of −7.43 to −11.69, corresponding to two-stage Hf model ages of 3.24–3.67 Ga and two-stage Nd model ages of 3.0–3.37 Ga, respectively, indicating a derivation of their parental magmas from remelting of ancient crustal components (i.e., the Yangpo “Group” paragneisses). The moderate to strong LREE/HREE fractionation ((La/Yb)N > 22, average 73.15) and the relatively low Sr/Y and negative Eu anomalies of the least felsic samples further imply a residual assemblage probably involving both garnet and plagioclase, corresponding to middle-lower crustal levels. This magmatic event most likely occurred at a convergent continental margin related to continental collision, which, in conjunction with broadly coeval metamorphism and the succeeding A-type granitic magmatism in the region, delineates a possible late Paleoproterozoic tectonic transition from compression to extension, signifying a cycle from collision (2.0–1.93 Ga) to post-collision (1.85 Ga) in the Zhongxiang Complex. Such a collisional orogenesis was potentially linked to the Paleoproterozoic internal assembly of the eastern part of the Yangtze Block, presumably as part of the early assembly of the Nuna supercontinent.

Deformed continental arc sequences in the South Tianshan: New constraints on the Early Paleozoic accretionary tectonics of the Central Asian Orogenic Belt

The tectonic affinity and structural evolution of the South Tianshan are key issues to understand the Paleozoic accretionary or collisional orogenesis in the southwestern Central Asian Orogenic Belt, but remain controversial. Geological and structural investigations on the Early Paleozoic low-grade metamorphic volcanic-sedimentary sequences around the Bayinbuluk basin demonstrate that they were prevalently reworked by north-vergent ductile deformation and overlain unconformably by Devonian to Carboniferous carbonates. Zircon U–Pb dating indicates that the volcanic rocks erupted during the Middle Ordovician to Early Devonian time (460–410 Ma), and contain Neoarchean to Neoproterozoic zircon xenocrysts (~2.75 Ga, ~2.46 Ga, ~1.60 Ga, 997–963 Ma and 827 Ma). Their whole-rock elemental, Sr–Nd isotopic (εNd(t) = −5.0 to +3.6) and zircon Lu–Hf isotopic (εHf(t) = −13.5 to +11.6) features resemble those of Andean-type continental arc volcanic rocks. The coexisting tuffaceous sedimentary rocks indicate a depositional environment amidst a continental volcanic arc at 421 to 404 Ma. The newly obtained and available geochronological, geochemical and structural data suggest that: (1) a wide active continental margin covering the South and Central Tianshan and northern Tarim was formed due to Early-Mid Paleozoic southward subduction of the Paleo-Tianshan Ocean beneath the Tarim Craton; and (2) a Mid-Paleozoic transition from advancing to retreating accretionary orogeny was followed by opening of the South Tianshan back-arc basins and break-up of the northern Tarim continental margin. The South Tianshan was subsequently subjected to regional deformation during the successive diachronous closure of the major and back-arc oceanic basins, and locally reworked by the ductile strike-slip faulting (307–255 Ma).

Metamorphism and the evolution of subduction on Earth

AbstractSubduction is a component of plate tectonics, which is widely accepted as having operated in a manner similar to the present-day back through the Phanerozoic Eon. However, whether Earth always had plate tectonics or, if not, when and how a globally linked network of narrow plate boundaries emerged are matters of ongoing debate. Earth's mantle may have been as much as 200–300 °C warmer in the Mesoarchean compared to the present day, which potentially required an alternative tectonic regime during part or all of the Archean Eon. Here we use a data set of the pressure (P), temperature (T), and age of metamorphic rocks from 564 localities that vary in age from the Paleoarchean to the Cenozoic to evaluate the petrogenesis and secular change of metamorphic rocks associated with subduction and collisional orogenesis at convergent plate boundaries. Based on the thermobaric ratio (T/P), metamorphic rocks are classified into three natural groups: high T/P type (T/P &gt; 775 °C/GPa, mean T/P ~1105 °C/GPa), intermediate T/P type (T/P between 775 and 375 °C/GPa, mean T/P ~575 °C/GPa), and low T/P type (T/P &lt; 375 °C/GPa, mean T/P ~255 °C/GPa). With reference to published thermal models of active subduction, we show that low T/P oceanic metamorphic rocks preserving peak pressures &gt;2.5 GPa equilibrated at P–T conditions similar to those modeled for the uppermost oceanic crust in a wide range of active subduction environments. By contrast, those that have peak pressures &lt;2.2 GPa may require exhumation under relatively warm conditions, which may indicate subduction of young oceanic lithosphere or exhumation during the initial stages of subduction. However, low T/P oceanic metamorphic rocks with peak pressures of 2.5–2.2 GPa were exhumed from depths where, in models of active subduction, the slab and overriding plate change from being decoupled (at lower P) to coupled (at higher P), possibly suggesting a causal relationship. In relation to secular change, the widespread appearance of low T/P metamorphism in the Neoproterozoic represents a “modern” style of cold collision and deep slab breakoff, whereas rare occurrences of low T/P metamorphism in the Paleoproterozoic may reveal atypical localized regions of cold collision. Low T/P metamorphism is not known from the Archean geological record, but the absence of blueschists in particular is unlikely to reflect secular change in the composition of the oceanic crust. In addition, the premise that the formation of lawsonite requires abnormally low thermal gradients and the postulate that oceanic subduction-related rocks register significantly lower maximum pressures than do continental subduction-related rocks, and imply different mechanisms of exhumation, are not supported. The widespread appearance of intermediate T/P and high T/P metamorphism at the beginning of the Neoarchean, and the subsequent development of a clear bimodality in tectono-thermal environments are interpreted to be evidence of the stabilization of subduction during a transition to a globally linked network of narrow plate boundaries and the emergence of plate tectonics.