Accretionary Orogenesis Research Articles

Timing and spatial variation of deformation along the Kanggur-Huangshan shear zone in the Chinese Tianshan: Implications for regional differential uplift and mineralization

The Chinese Tianshan experienced large-scale transcurrent tectonics, synkinematic emplacement of ultramafic to felsic intrusions, and the formation of various mineral deposits during late Paleozoic accretionary orogenesis. The relationships among the spatial variation of deformation, the distribution of Permian orogenic Au and magmatic Ni-Cu sulfide deposits, and the kinematic evolution of crustal-scale shear zones, however, remain ambiguous. To address these ambiguities, the spatial variation in the degree of deformation in the Kanggur-Huangshan shear zone in the Chinese Tianshan was characterized using detailed structural measurements and zircon U-Pb and muscovite 40Ar/39Ar age data. The new structural data indicate that a prominent spatial variation exists in the style of deformation throughout the Kanggur-Huangshan shear zone; intense ductile deformation structures are dominant in the east, while brittle structures become progressively more dominant toward to the west. Zircon U-Pb and muscovite 40Ar/39Ar age data for syn- and postkinematic intrusions along the Kanggur-Huangshan shear zone indicate that dextral strike-slip shearing occurred between 279 Ma and 249 Ma. The spatial variation in the degree of deformation and exhumation along the Kanggur-Huangshan shear zone was potentially caused by regional differential uplift induced by the collision of the Tianshan and Beishan regions; this was likely responsible for the predominant occurrence of magmatic Ni-Cu sulfide deposits in the eastern portion of the Kanggur-Huangshan shear zone and orogenic Au deposits in the western portion. The identified spatio-temporal relationship between deformation and distribution of orogenic Au and magmatic Ni-Cu sulfide deposits is crucial to the future success of mineral exploration in the Central Asian orogenic belt.

Altaids and accretionary orogenesis: preface

Altaids and accretionary orogenesis: preface

U-Pb geochronology of the Silurian-Devonian Bega Batholith, south-eastern Australia: Insights into the origin and development of I-type granites

Earth’s continental crust is primarily composed of buoyant, felsic igneous (granitic) material that has been extracted from the mantle along convergent plate margins. Establishing the timescales of subduction-related granite batholith assembly is key to understanding the processes of magma generation, segregation, and transport that define the dynamics and rates of crustal growth. The ‘Cordilleran’ Bega Batholith is the largest I-type granite batholith in the Lachlan Fold Belt (LFB) of south-eastern Australia. The product of accretionary orogenesis along the Terra Australis Orogen, it has been extensively characterised by isotopic and geochemical studies yet lacks a robust geochronological framework. We present new ID-TIMS and SHRIMP U-Pb zircon ages for twenty-two plutonic rocks of the Bega Batholith. This geochronological survey indicates a prolonged duration for I-type magmatism, with crystallisation ages ranging from ca. 420 Ma in the west, to 385 Ma in the eastern Bega Batholith. These I-type granites collectively occupy ∼90,000 km3 and were emplaced over a period of ∼20–35 Ma, corresponding to a magma production rate of ∼8–15 km3/Ma km−1, considerably lower than that of S-type granites of the region and other subduction-related granite terranes globally. Fundamentally, the temporal overlap between the Bega I-type granites and S- and A-type granites also argues against a hiatus between each phase of granite magmatism in the LFB, suggesting a continuum between the granite types and derivation from common source components. This new geochronological data provides the first comprehensive compilation of U-Pb ages from across the Bega Batholith, establishing initial constraints on the development and evolution of I-type magmatism in south-eastern Australia. Our results demonstrate that I-type granite petrogenesis is linked with crustal growth processes, requiring the modification of global crustal growth curves during the Phanerozoic to account for new continental crust generated within I-type granite terranes of the Paleo-Pacific Gondwanan margin.

Mesoproterozoic accretionary orogenesis: Evidence from ∼ 1.4 Ga metamorphism on the southeastern margin of the North China Craton

The Mesoproterozoic period has always been thought of as an orogenic quiescence stage. However, contemporaneous magmatism and metamorphism indicate that this period was relatively restless. A Mesoproterozoic metamorphic event (~1.4 Ga) was first discovered in the southeastern part of the North China Craton. Samples, including garnet amphibolite and its countryrock biotite plagiogneiss, were collected from Rushan city on the Jiaodong Peninsula. The major elements of the rock are characterized by high Ti, where the trace elements show slight depletions in Th, U and Sr contents, and the REEs show E-MORB like patterns. The mineral assemblage of the garnet amphibolite includes garnet, hornblende and plagioclase. These metamorphic minerals are also found in zircons as inclusions. Cloudy and homogeneous CL images and flat HREE patterns of zircon trace elements in garnet amphibolite indicate a metamorphic origin, and zircon U-Pb age dating obtained an ∼ 1.4 Ga metamorphic age, which is consistent with the second age group age of the countryrock. Our dating results suggest that the Meoproterozoic metamorphic event in the North China Craton is comparable to metamorphic events reported in the Laurentian and Australian Cratons. Similar magmatism and comparable stratigraphy of these ancient cratons also support the Mesoproterozoic era was not an orogenic quiescence stage. Therefore, we infer a giant orogenic belt that existed along the outer margin of the supercontinent of the Columbia in the Earth’s middle age. Long-lived Mesoproterozoic accretionary orogenesis probably affected the edges of supercontinents in a transition stage from the breakup of Columbia to the assembly of Rodinia.

Early Mesozoic polyphase contractional deformation in the Yanshan belt: Implications for the destruction of the north China Craton

• The northern North China Craton was in a contractional setting in early Mesozoic. • Post-collisional contractional deformation was due to the lithosphere delamination . • A decoupling process dominated the early stage of the destruction of the NCC. After the collision of the Mongolian arc terranes with the amalgamated Bainaimiao arc and North China Craton in the latest Permian to earliest Triassic, the northern North China Craton ended the Andean-type accretionary orogenesis. The nature of the subsequent post-collision tectonic setting in early Mesozoic in the northern North China Craton remains controversial. Some workers proposed that the northern North China Craton evolved into a post-orogenic extensional stage. However, this view was mainly built on geochemistry studies of Triassic plutonic rocks in northern North China Craton while it had never received support from studies of tectonic deformation. In contrast, our recent field mapping combining with geochronological studies in the Yanshan belt suggest that the northern North China Craton was in an overall polyphase multi-directional contractional settings in early Mesozoic. We have identified four stages of contractional deformation (D 1 -D 4 ) in this region. D 1 (250–231 Ma) is the intracratonic response to the collision of the Mongolia arc terranes with the NCC. D 2 (214 ∼ 207 Ma) and D 3 (204 ∼ 195 Ma) are post-collisional intracratonic contractional deformation which occurred synchronously with the lithosphere thinning. D 2 and D 3 and the intervened plutonism might be a crustal response to the delamination and sinking of the over-thickened lithosphere of the northern North China Craton. D 4 is the result of the west-ward subduction of the Izanagi plate beneath the eastern Asian continent plate in the Early Jurassic. These findings together with previous studies indicate that the northern North China Craton experienced a long period of polyphase contractional deformation before its final destruction in the Early Cretaceous. The early stage of the destruction of the North China Craton is characterized by coeval lithosphere thinning and crustal thickening.

Ordovician amphibolite-facies metamorphism in Hainan Island: A record of early Paleozoic accretionary orogenesis along the northern margin of East Gondwana?

• Polyphase metamorphism is observed in the Baoban Complex, Hainan Island. • SIMS zircon U-Pb geochronological dating yielded a metamorphic age of 462 Ma. • The peak metamorphic conditions are 715–650 °C/0.91–0.86 GPa. • The Ordovician metamorphism is associated with accretionary event along the northern Gondwana margin. Understanding the metamorphic evolution of Hainan Island is crucial to constraining its geodynamic history. Here, we report evidence for a hitherto unknown early Paleozoic amphibolite-facies event that affected the Baoban Complex in southwestern Hainan Island and establish its role in the tectonic history of the northeast margin of Gondwana. SIMS and LA-ICP-MS zircon U-Pb dating reveals a polyphase metamorphic history, consisting of three age groups. Group 1 performed on the outermost zircon rim with the lowest concentration of HREE and U, gives a weighted average age of 462 ± 7 Ma that corresponds to the metamorphic stage of the crystallization of garnet. Group 2 displays moderate HREE and low Th/U ratios (less than 0.079). Concordant U-Pb ages range from 1371 to 1017 Ma which record metamorphism in the Mesoproterozoic. Group 3 consists of typical igneous-derived detrital zircons with ages ranging from 3095 to 1380 Ma. The age spectrum is consistent with the siliciclastic protolith lithologies of the Baoban Complex. P-T conditions for the garnet-forming stage (Di + Grt + An + Hb + Q) were estimated by geothermobarometry at 715–650 °C and 0.91–0.86 GPa, which is consistent with 708 ± 44 °C conducted on zircons of Group 1 through Ti-in-zircon thermometry. P-T estimates and zircon U-Pb ages reveal that the amphibolite-facies metamorphism occurred at 462 Ma. The end-Mesoproterozoic metamorphism overlapped in age with the assembly of Rodinia. Combing with regional tectonothermal history, we infer that the Ordovician amphibolite-facies metamorphism formed during accretionary orogenesis along the northern margin of East Gondwana.

The effect of seamount chain subduction and accretion

Seamount chains (oceanic plateaus, submarine ridges) are unique morphologic features on the seafloor, which resulted from the motion of the lithosphere over a convective plume. Seamount chains are being carried along by plate motions, which are eventually subducted beneath continental/oceanic plates or accreted in the accretionary complex, thus causing a series of effects in the convergent margin. In this paper, we briefly review the distribution and features of the typical modern seamount chains, including Hawaiian‐Emperor and Louisville seamount chains, Cocos, Ninetyeast and Caroline ridges, and Ogasawara Plateau, and mainly focus on the effects of seamount chains subduction and accretion. The geological effects mainly include the following: (1) deformation and tectonic erosion of the overriding plate; (2) flat‐slab subduction in the convergent margin; (3) growth of continental crust through time; (4) subduction initiation of plate tectonics; (5) generation of large earthquakes; (6) magmatism of the volcanic arc; and (7) formation of porphyry copper and gold deposits. The subduction of seamount chains resulted in similar effects in the Central Asian Orogenic Belt (CAOB) which is one of the largest and longest‐lived accretionary orogens on Earth. However, there are still many aspects that need further improvements, such as identification of the seamounts in the geological record, and geodynamic effects of subducted seamount chains. A detailed study on these sides of the seamount chains will enable us to further understand the tectonic evolution and metallogenesis of the CAOB, especially in accretionary orogenesis, continental crust growth, and subduction initiation dynamics.

Early Permian Syn-Subduction Extension in the South Tianshan (NW China): Insights From A-Type Granitoids in the Southern Altaids

A-type granite is an important geodynamic indicator because it requires a high melting temperature that is commonly driven by extensional events. Here we report geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of newly identified A-type granitic rocks from the South Tianshan in the southern Altaids. Zircon LA-ICP-MS ages indicate that the granitoids were emplaced at ca. 298–272 Ma. Geochemically, they are metaluminous to slightly peraluminous (A/CNK = 0.95–1.10), and belong to the high-K calc-alkaline to shoshonitic series. They are characterized by relatively high zircon saturation temperatures (824–875°C), K2O + Na2O contents (7.31–9.36%), high field strength elements (HFSE; Zr + Nb + Ce + Y = 365–802 ppm), and Ga/Al ratios (2.8–4.2), which all point to an A-type affinity. In addition, they have slightly enriched Hf isotope compositions (εHf(t) = −10.9 to + 0.6), and corresponding Mesoproterozoic (1,272–1759 Ma) crustal model ages, suggesting they were probably generated by partial melting of mature crust that contained minor mantle-derived magmatic material. The granitoids have distinctive subduction-related trace element signatures, with deep Nb and Ta troughs, elevated large ion lithosphere elements (LILEs), and flat HFSEs patterns, very similar to arc-derived granites in the Lachlan accretionary orogen. Integration of these new sedimentological, structural and geochronological results with relevant published information provides a new data-archive, which indicates that neither the Tarim mantle plume nor post-collisional extension can explain the genesis of these A-type granitoids. Instead, we propose a new more pertinent and robust model according to which they formed due to high temperature gradient in a subduction-related extensional setting probably triggered by southward rollback of the South Tianshan oceanic lithosphere, which caused upwelling of asthenospheric mantle combined with an increased temperature that led to large-scale crustal melting. This process gave rise to a broad magmatic arc in the southern active margin of the Yili-Central Tianshan. Our new data shed light on the retreating accretionary orogenesis of the southern Altaids in the Permian.

The Pre-Grenvillian assembly of the southeastern Laurentian margin through the U–Pb–Hf detrital zircon record of Mesoproterozoic supracrustal sequences (Central Grenville Province, Quebec, Canada)

AbstractThe detrital zircon perspective on the pre-collisional crustal evolution of the Grenville Province remains poorly explored. In this study, we conducted in situ laser ablation U–Pb–Hf isotopic microanalysis on detrital zircon grains from three pre-orogenic (>1 Ga) supracrustal sequences that crop out in the Central Grenville Province (Lac Saint-Jean region, QC, CA). Detrital zircon grains from vestiges of these sequences record three dominant age peaks at c. 1.46 Ga, 1.62 Ga, 1.85 Ga, and a subordinate peak at 2.7 Ga. The 1.46 Ga and 1.62 Ga age peaks are recorded in detrital zircon grains from a quartzite associated with a metavolcanic sequence (i.e. Montauban Group) with a maximum depositional age of c. 1.44 Ga. In contrast, the c.1.85 Ga age peak is observed from recycled zircon grains in metasediments with maximum depositional ages between 1.2 and 1.3 Ga. The suprachondritic Hf isotope composition in detrital zircon grains of the 1.46 Ga and 1.62 Ga age populations records juvenile crustal growth during peri-Laurentian accretionary orogenesis related to the Pinwarian (1.4–1.5 Ga) and Mazatzalian–Labradorian (1.6–1.7 Ga) events. The detrital zircon grains associated with Penokean–Makkovikian (1.8–1.9 Ga) source rocks record reworking of c. 2.7 Ga continental crust derived from a near-chondritic mantle reservoir. Overall, crust-forming and basement reworking events associated with accretionary orogenesis in southeastern Laurentia are retained in the detrital zircon load of Precambrian basins even after the terminal Grenvillian collision and assembly of Rodinia.

Cretaceous Kuching accretionary orogenesis in Malaysia Sarawak: Geochronological and geochemical constraints from mafic and sedimentary rocks

The Kuching zone, the most important tectonic boundary in Borneo, separates the Sibu zone in Central Sarawak from the Schwaner Mountains granitoids in SW Borneo. It potentially provides a key window for probing the paleo-Pacific subduction process. This paper presents new geochronological, elemental and SrNd isotopic data for the Pakong-Serabang and Serian mafic rocks, along with detrital zircon UPb age-data for the associated greywacke in the Sarawak Kuching zone. These mafic rocks are dated at ~77–98 Ma and show distinctive geochemical signatures. The Pakong- Serabang mafic rocks are subalkaline basalt with SiO2 = 46.22–51.75 wt%, TiO2 = 0.49–2.42 wt% and MgO = 6.30–10.54 wt%. They exhibit MORB-like PM-normalized patterns with depletion in LILEs and HFSEs. Their 87Sr/86Sr(t) and εNd(t) range from 0.70285 to 0.70728 and + 7.4 to +9.4, respectively, originating from a slab-derived fluid-modified MORB-like source. The late Cretaceous Serian Volcanics are marked by SiO2 of 51.52–57.90 wt%, MgO of 3.50–5.06 wt% with mg-number = 36–46 and show arc-like geochemical signatures with (Nb/La)n = 0.37–0.47. They have 87Sr/86Sr(t) = 0.71188–0.71417 and εNd(t) = −11.1 ~ −7.3, and were derived from a mantle wedge with the input of recycled sedimentary components. The detrital zircon grains from the associated sandstones give similar age-spectra to those in East Peninsular Malaysia and SE Vietnam. Our data revealed a Cretaceous Kuching trench-arc system. In combination with other geological observations, it is concluded that NW Borneo to the southwest of the Sarawak Kuching zone was a part of the Indochina/East Peninsular Malaysia fragment in SE Asia prior to the Jurassic and re-activated as an active continental margin in the Cretaceous. Widespread accretionary orogenesis along the Kuching zone likely initiated at or before the early Cretaceous and ended no earlier than the latest Cretaceous in response to the westward subduction of the paleo-Pacific slab.

Imaging Neoarchean crustal structures: An integrated geologic-seismic-magnetotelluric study in the western Wabigoon and Winnipeg River terranes, Superior craton

The Neoarchean is considered as a pivotal era during the Earth’s evolution in terms of cratonization and development of plate tectonics. The supporting crustal processes, however, remain elusive due in part to limited comprehensive imaging of crustal structures underlying Neoarchean terranes. This study uses new seismic, magnetotelluric, and geologic data from the Metal Earth Sturgeon transect (70 km) and existing seismic and aeromagnetic data to investigate the Neoarchean crustal architecture underlying the greenstone-dominated western Wabigoon terrane (WWT) and the tonalite-trondhjemite-granodiorite-dominated Winnipeg River terrane (WRT) of the Superior craton. The results suggest that (1) the Sturgeon Lake greenstone belt of the WWT is 5–10 km thick and separated by a thrust fault from the underlying basement, (2) the basement is characterized by gneissic fabrics from the mid- to lower-crust of probable Paleo to Mesoarchean WRT rocks, and that (3) the crust was extensively reworked by Neoarchean post-tectonic magmatism that was characterized by carbon- and/or hydrogen-rich silicates and likely caused by partial melting of mantle/crustal rocks triggered by subduction-related fluids/melts. With the reprocessed Lithoprobe seismic data from the same region, a 3D crustal architecture is reconstructed and reveals a convergent belt between the main WRT to the north and a continental margin promontory of the WRT to the south. This Neoarchean convergent belt is characterized by an allochthonous greenstone belt as a thrust sheet in the upper crust, a collision zone in the mid- to lower-crust, an apparent crustal root of 3–5 km relief, and subcreted crustal rocks beneath a mantle wedge. Published data suggest that an outboard, north-dipping subduction zone associated with the Wawa terrane provided the fluids/melts for the post-tectonic magmatism in the WRT-WWT crust. This study favors a model of subduction followed by collision over a collision-only model, which indicates that multiple subduction zones were operative synchronously in a periodduring the assembly of the western Superior craton. Cratonization by accretionary orogenesis due to plate tectonics in the Neoarchean, therefore, is imaged and characterized in this study using a 3D crustal architecture.

Accretion kinematics and driving mechanism of the eastern Central Asian Orogenic Belt: Insights from seismic tomography and middle Permian–Middle Triassic magmatism in central Jilin Province

The amalgamation of accretionary terranes plays an important role in the evolution of ancient accretionary orogens, but the kinematic process and its driving mechanism are commonly ambiguous. Here we provide new constraints on the accretion kinematics and associated driving mechanism of the eastern Central Asian Orogenic Belt (CAOB), using seismic tomography and magmatism. Permian to Middle Triassic magmatism was linked to two distinct accretionary orogens, that existed along the northern margin of the North China Craton. The geochemistry of middle Permian to Middle Triassic igneous rocks indicates a magmatic arc origin at an active continental margin. Based on the horizontal zoning of magmatism and south-dipping stagnant oceanic slab imaged by seismic tomography, we conclude that southward subduction of the oceanic slab resulted in the accretion and collision of multiple arc systems along the continental margin. Our seismic tomographic models indicate that formation and growth of the eastern CAOB occurred as the result of the continuous accretion and amalgamation of arc assemblages with addition of crustal material. Tomographic models and previous Re-Os geochronological studies indicate the existence of a deep crustal root and ancient sub-continental lithospheric mantle in most regions of the eastern CAOB. Taking into account the magmatic evolution and sedimentary sequences, we conclude that the terrane accretion should be the driving mechanism of accretionary orogenesis in the eastern CAOB.

Refining the Paleoproterozoic tectonothermal history of the Penokean Orogen: New U-Pb age constraints from the Pembine-Wausau terrane, Wisconsin, USA

Abstract An enduring problem in the assembly of Laurentia is uncertainty about the nature and timing of magmatism, deformation, and metamorphism in the Paleoproterozoic Wisconsin magmatic terranes, which have been variously interpreted as an intra-oceanic arc, foredeep or continental back-arc. Resolving these competing models is difficult due in part to a lack of a robust time-frame for magmatism in the terranes. The northeast part of the terranes in northern Wisconsin (USA) comprise mafic and felsic volcanic rocks and syn-volcanic granites thought to have been emplaced and metamorphosed during the 1890–1830 Ma Penokean orogeny. New in situ U-Pb geochronology of igneous zircon from the volcanic rocks (Beecher Formation), and from two tonalitic plutons (the Dunbar Gneiss and Newingham Tonalite) intruding the volcanic rocks, yielded crystallization ages ranging from 1847 ± 10 Ma to 1842 ± 7 Ma (95% confidence). Thus, these rocks record a magmatic episode that is synchronous with bimodal volcanism in the Wausau domain and Marshfield terrane farther south. Our results, integrated with published data into a time-space diagram, highlight two bimodal magmatic cycles, the first at 1890–1860 Ma and the second at 1845–1830 Ma, developed on extended crust of the Superior Craton. The magmatic episodes are broadly synchronous with volcanogenic massive sulfide mineralization and deposition of Lake Superior banded iron formations. Our data and interpretation are consistent with the Penokean orogeny marking west Pacific-style accretionary orogenesis involving lithospheric extension of the continental margin, punctuated by transient crustal shortening that was accommodated by folding and thrusting of the arc-back-arc system. The model explains the shared magmatic history of the Pembine-Wausau and Marshfield terranes. Our study also reveals an overprinting metamorphic event recorded by reset zircon and new monazite growth dated at 1775 ± 10 Ma suggesting that the main metamorphic event in the terranes is related to the Yavapai-interval accretion rather than the Penokean orogeny.

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.

Prototethyan Accretionary Orogenesis Along the East Gondwana Periphery: New Insights From the Early Paleozoic Igneous and Sedimentary Rocks in the Sibumasu

AbstractNew geochronological and geochemical data document a new and widely distributed succession of igneous and sedimentary rocks within the Sibumasu that extend from SW Yunnan through SE Asia as far as Sumatra. Our data revealed the presence of an Ordovician‐Silurian (mainly ∼490–430 Ma) igneous zone. The early Paleozoic granites in the zone display similar elemental and Sr–Nd–Pb–Hf isotopic compositions. They were derived from a common mixed source of ancient crustal materials with a juvenile component and formed in a subduction‐related setting. Detrital‐zircon U‐Pb apparent ages from the previously defined Precambrian Lancang and Ximeng units in the SW Yunnan portion of the Sibumasu are in the range of 3,306–428 Ma and 3,316–512 Ma, with the youngest age‐peaks of 505 Ma and 447 Ma, respectively, indicating the lower Paleozoic (Cambrian‐Silurian) sedimentary sequences. The Tarutao Unit and its equivalents in the SE Asia portion of the Sibumasu yield the U‐Pb apparent ages of 3,177–510 Ma, suggesting the maximum deposited time being Cambrian‐Ordovician. All these detrital zircon U‐Pb ages define two main peaks of ∼554–528 Ma and 964–905 Ma with εHf(t) values similar to those in the Qiangtang, Tethyan Himalaya, and eastern Indochina. The lower Paleozoic detritus in the Sibumasu were likely derived from the India‐Antarctica region of East Gondwana. We propose that the late Cambrian‐Silurian units formed in an outward‐propagating accretionary orogen in response to the consumption of the Prototethyan Ocean in SW Yunnan and SE Asia. Sibumasu might represent a part of the easterly migrating accretionary orogen along the northern margin of East Gondwana in the late Cambrian‐late Silurian.

Early Paleozoic Arc Magmatism and Accretionary Orogenesis in the Indochina Block, Southeast Asia

AbstractSoutheast Asia plays a key role in the evolution of the eastern Tethys, which is characterized by accretion and amalgamation of numerous terranes since early Paleozoic. In the present study...

Age and Petrogenesis of the Gabbros from Tajik South Tianshan: Implications for Early Paleozoic Geodynamic Evolution of the Southwestern Central Asian Orogenic Belt

Abstract Identification of slab window process is important for understanding the nature of the accretionary orogenesis. In this study, we report detailed petrological, geochronological, geochemical, Sr–Nd isotopic, and mineral chemical data for two dyke-like gabbroic intrusions from the South Tianshan belt of Tajikistan, southwestern margin of the Central Asian Orogenic Belt. Both intrusions are composed of coarse- and fine-grained gabbros. U–Pb zircon dating shows that they were emplaced at 431±5 Ma. The gabbroic rocks show relatively large variation in elemental and isotopic compositions, with SiO2 of 40.62–53.97 wt.%, Sr of 333–1261 ppm, and εNdt of +2.5 to +5.8. Especially, the fine-grained gabbros show lower SiO2 and higher MgO but more evolved isotopes than the coarse-grained gabbros for each of the intrusions. All the rocks display OIB-like or transitional OIB-/E-MORB-like geochemical characteristics with no obvious Nb-Ta depletion, indicative of an intraplate affinity. Combined with their mineral chemical compositions, we suggest that these gabbroic rocks were generated by partial melting of asthenospheric mantle in the transitional spinel-garnet stability field, followed by different degrees of fractional crystallization of olivine, clinopyroxene, and plagioclase and mixing with carbonatitic melts. The available data indicate that roll-back of the subducting Turkestan oceanic slab occurred during the Late Ordovician to Early Silurian period. Asthenosphere upwelling due to the opening of slab window resulted from localized slab tearing during slab roll-back may have been responsible for the generation of the studied dyke-like gabbroic intrusions.

Early Paleozoic accretionary orogenesis in the northeastern Indochina and implications for the paleogeography of East Gondwana: constraints from igneous and sedimentary rocks

The Indochina block is one of a number of continental fragments that lay along or outboard of the East Gondwana northern margin. Its early Paleozoic tectonic nature and the relationship with East Gondwana remain poorly-known. This paper shows the general geology of northeastern Indochina and presents a set of new geochronological and geochemical data with respect to its early Paleozoic tectonics in East Laos. These results reveal early Paleozoic (~485-410 Ma) igneous activity in the northeastern Indochina, which are divisible into four geochemical groups. The ~450-434 Ma Group 1 and ~485-434 Ma Group 2 mafic-intermediate igneous rocks were derived from MORB-like and supra-subduction zone wedge source modified by the recycled-sedimentary components, respectively. The Group 3 rhyolites at ~424 Ma represent the melting products of juvenile mafic crust. The Group 4 granitoids and equivalent volcanics formed at ~485-407 Ma and were derived from crustal materials with variable input of juvenile crust. The detrital-zircon age-spectra are marked by peaks at ~560 Ma, ~960 Ma, ~1600 Ma, ~1850 Ma and ~2450 Ma for the pre-Silurian paragneiss in the NE Indochina, reflective its geographical proximity to the India-Antarctica segment of East Gondwana until the earliest Paleozoic. The age-peaks of ~447 Ma and ~780 Ma is additionally given for the detrital-zircon from the Silurian sandstones in the Truong Son zone. An accretionary assemblage model is proposed for the early Paleozoic tectonic pattern of the SW Indochina with Yangtze fragments. Such an orogenesis formed the part of the North Indo-Australie Orogen related to subduction of the Prototethyan Ocean and the subsequent accretion of Asian micro-continental fragments along the Peripheral East Gondwana until earliest Devonian.

Evolution of Late Paleozoic Magmatic Arc in the Yili Block, NW China: Implications for Oroclinal Bending in the Western Central Asian Orogenic Belt

AbstractSubduction transformation (advancing vs. retreating) may be manifested by compositional variations of arc magmas and may result in oroclinal bending. Identifying relevant chemical and physical processes is crucial for understanding accretionary orogenesis and continental crustal evolution. The Northern Yili Block (NYB) was situated on an active margin associated with subduction of the Junggar Ocean (part of the Paleo‐Asian Ocean) and underwent perplexing accretionary orogenesis in late Paleozoic. Two episodes of subduction‐related granitoid magmatism have been identified, the first in the Late Devonian (374–369 Ma) and the second in the Late Carboniferous (ca. 304 Ma). These two episodes of granitoid magmatism exhibit contrasting features; for example, the first episode shows low εNd(t) (−6 to −2) and εHf(t) (−12 to +3) values, while the second episode displays relatively higher values (−2 to +7 and 0 to +20, respectively), suggesting increase contribution of juvenile components in the magma sources. The calculated zircon saturation temperatures are mostly &lt;800°C for the first episode but increased to &gt;800°C for the second episode. The (La/Yb)N ratios of the two‐episode granitoids vary from 2–17 to 1–9, indicative of the NYB crustal thinning in the Late Carboniferous. In addition, the migrations of the magmatic arc and trench in the NYB took place coevally in the Late Carboniferous and were accompanied by development of an accretionary complex and an immature back‐arc basin. We interpret that the arc magmatic variations and migrations reflect geodynamic transition from the Late Devonian advancing subduction to the Late Carboniferous retreating subduction. The retreating subduction of the Junggar oceanic plate possibly resulted in prominent oroclinal bending of the southern Kazakhstan collage system in the Late Carboniferous.

Ordo-Silurian assemblage in the Indochina interior: Geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic constraints of early Paleozoic granitoids in South Laos

AbstractIn order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464–485 Ma, 455–471 Ma, and 427–446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96–1.99 and K2O &amp;gt; Na2O, which are marked by enrichment in large-ion lithophile elements and depletion in high field strength elements with remarkable Nb-Ta, Sr-P, and Ti negative anomalies. Their initial 87Sr/86Sr ratios range from 0.70510 to 0.71559, εNd(t) from −9.5 to −3.0, (206Pb/204Pb)i from 18.65 to 19.72, (207Pb/204Pb)i from 15.66 to 15.80, and (208Pb/ 204Pb)i from 38.84 to 39.79. The corresponding zircon ɛHf(t) and δ18O values are in the range of −10.6 to +1.0 and 6.88‰ to 8.94‰, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined “single-ancient” Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.