Arc Accretion Research Articles

Prolonged arc accretion during growth of juvenile crust in the Arabian Nubian Shield: Insights from the granitoids of northern Ethiopia

The Arabian Nubian Shield (ANS) is one of the largest juvenile continental crust formed after the Archean. To determine the timing and understand the mechanism of its early crustal growth, we conducted geochronological and geochemical studies on granitoids from northern Ethiopia. A plagiogranite, dated at 929.7 ± 2.2 Ma, represents one of the early granitoids from the ANS. It shows low K2O, TiO2, and REE contents, flat REE pattern and absence of Eu anomaly. Moreover, it exhibits (87Sr/86Sr)i, ƐNd(t) and ƐHf(t) values of 0.70341, +5.0 and +8.4, respectively. Its zircon δ18O value is 4.8‰, lower than the mantle value. These characteristics suggest that it was derived from partial melting of altered oceanic crust. The oxygen fugacity obtained from zircon trace elements is ΔFMQ+1.3, consistent with a subduction setting.Other plutons have much younger ages ranging from 860 to 840 Ma and are mainly medium K, I-type granitoids. These granitoids show low MgO, Ni and Cr contents, low Sr/Y, (La/Yb)N, Nb/Ta and Zr/Sm ratios. They exhibit whole-rock (87Sr/86Sr)i values of 0.70236–0.70304, ƐNd(t) values of 4.2–5.2, ƐHf(t) values of 5.8–7.6 and zircon ƐHf(t) values of 5.5–10.4, slightly lower than coeval depleted mantle. Furthermore, they exhibit fractionated REE patterns, enrichments in LILE and depletion in HFSE. These geochemical features suggest that these granitoids were generated by remelting of arc crust. Compiled data show that the associated basaltic rocks are of arc affinity, which confirms the coeval development of arc magmatism in the southern ANS. Our results suggest that the growth of juvenile crust in the southern ANS started from the early Neoproterozoic and peaked at ca. 880 to 800 Ma, much earlier than those in the northern ANS. The prolonged history accounts for the growth of the vast juvenile crust in the ANS.

Accretion of microcontinents and arcs in the southern Central Asian Orogenic Belt: insights from provenance analyses of early Paleozoic sedimentary records in Beishan

The Central Asian Orogenic Belt involves amalgamation of numerous magmatic arcs, accretionary complexes and continental fragments during long-lived subduction and accretion. However, the nature of key microcontinental terranes, as well as their continuity, remains unclear. This study conducted outcrop observation, heavy mineral assemblage, whole-rock geochemistry, detrital zircon U–Pb dating and Lu–Hf isotope analysis on sandstones in the Beishan orogenic belt. The findings from field observations, petrographic analysis and weathering indices show that the tectonic setting of the sandstone is a forearc sedimentary basin. The maximum depositional ages (MDAs) of sandstones are 503.8 ± 6.9 Ma and 431.9 ± 6.0 Ma. The provenance of the sandstone is complex, which probably originates from the Shuangyingshan and Hanshan blocks. By comparing the detrital zircon U–Pb ages with adjacent terranes such as Central Tianshan, Dunhuang and Tarim, it was observed that the Shuangyingshan block shares similar provenance with the Dunhuang terrane, while the Mazongshan–Hanshan shows affinity to the Central Tianshan terrane. This study highlights the complex nature of sediment provenance in the Beishan orogenic belt, influenced by tectonic processes involving thrusting, accretionary complexes and the presence of multiple terranes.

Paleoaltimetry and paleotectonic reconstruction using triple oxygen and hydrogen isotopes: Depleted δ18O and δD values in ignimbrites of Verkhneavachinskaya caldera record collisional uplift during Miocene arc accretion to Kamchatka

Here we use δ18O and δD values as tools to investigate the paleo-altitude and the origin of large-volume (120 km3, 10 × 12 km) ignimbrites of Verkhneavachinskaya caldera (cf. Verkhne-Avachinskaya) (VC) in eastern Kamchatka, formed the in Late Miocene 5.8 Ma. The basaltic-andesitic intracaldera ignimbrite deposit exhibits low δ18O values, reaching −5.03‰, and δD values of −182‰ across a 1.2 km depth range in several sampled sections. The results support a massive meteoric-hydrothermal system throughout the cooling history of the thick intracaldera ignimbrite deposit. Using triple oxygen isotope data we estimate that the δ18O values of altering meteoric water are as low as −19‰ to −23‰, much lower than modern precipitation of −14‰, or − 16‰ estimated for the 2–3 °C warmer Kamchatka climate of the late Miocene. Therefore, the VC meteoric-hydrothermal system depended on high-altitude precipitation and glaciers, and altitudinal isotopic lapse rates suggest a paleo-altitude of 3.5 km at 5.8 Ma during caldera formation. These elevations exceed the modern by 1.5 km and provide a unique snapshot into an evolving landscape and paleo-environment of eastern Kamchatka at 6–5 Ma as the dynamic outcomes of accretion of the Kronotski arc. In particular, the existence of such a high plateau is in line with contemporary accretionary tectonics: accretion of the Shipunsky peninsula of this accreting arc terrain, ∼120 km to the east of the VC, and evidence of contemporaneous exhumation of high-grade Ganal amphibolites to the west. We conclude that the eruption of large-volume mafic ignimbrites that formed VC caldera was syncollisional or intracollisional in nature, likely requiring delamination of thickened crust to account for both uplift and magmatism.

Zircon U[sbnd]Pb geochronology and geochemistry of hot subduction volcanic suite in the deep bore-hole well-core KBH-5: Evidence of Neoarchean Shimoga greenstone belt beneath the Cretaceous Deccan Traps, India

The continents across Earth have grown through accretion and amalgamation of plume-generated oceanic plateaux and subduction-originated magmatic arcs. The evidence of such evolutionary processes is well preserved in the greenstone belts of Canada, South Africa, China, Australia and India. Unlike accretion that leads to lateral growth of the continents, flood basalt volcanism results in the superposition of large volumes of dense mafic material above relatively less dense sialic upper continental crust. The resulting density inversion is liable to generate tectonic stresses resulting in faulting of under burden along weak planes. The nature of the crust obscured beneath flood basalt provinces that have accumulated over the past 500 My in Large Igneous Provinces (LIPs) has remained enigmatic, largely because of the voluminous lava pile that deters access to the concealed basement, unless otherwise retrieved through deep continental drilling. Here, we present in situ zircon UPb geochronology and bulk-rock geochemistry of a volcanic sequence discovered at −400 m m.s.l in the deep bore-hole well-core KBH-5 of the southwestern Deccan Traps, India. The ∼300 m thick metavolcanic succession includes a basalt – Nb-enriched basalt – andesite – Mg-andesite – dacite – adakite suite. The zircons yielded a 2.58 Ga U-Pb age for the suite, which also incorporates entrained zircon xenocrysts of 2.7 Ga age from a preexisting juvenile material during its emplacement. The bore-hole KBH-5 is located on an NNW-SSE trending normal fault, paralleling a series of major faults west of the Western Ghat Escarpment. Although the crustal block plummeted ∼1000 m downward along the fault plane, the geochemical attributes of the metavolcanic rocks are thoroughly comparable to those reported in the Medur Formation of the Shimoga greenstone belt, ∼300 km south of KBH-5 in the western Dharwar craton. Our study provides the first physical evidence of Dharwar greenstone belt(s) extending further beneath the Deccan Traps, which was previously only speculative based on short-wavelength gravity highs. The metavolcanic sequence evolved in a subduction-related back-arc tectonic regime and accreted to the active continental margin of India in the Neoarchean. Unveiling of potentially similar scenarios in cratons elsewhere entrapped beneath the vast canopy of continental flood basalts, would reveal new crustal growth events during the Archean -Proterozoic transition across the Earth.

Early Ordovician to Early Permian tectonic evolution of the northern Western Kunlun Orogen (NW China): Insights from the Omixia Complex and surrounding rocks

The Palaeozoic tectonic evolution of the Western Kunlun Orogen plays an important role in deciphering the Tethyan tectonic evolution. This study provides new geochronology, geochemistry and Lu–Hf isotopic data of igneous rocks from the Omixia Complex in the eastern section of the northern Western Kunlun, as well as detrital zircon ages from turbidites and meta‐sediments north of it. From the Omixia Complex, the determined ages of five ultramafic–mafic rock samples are approximately 470, 456, 429, 401 and 382 Ma. Two samples of acidic rocks from the same complex yielded ages of approximately 438 and 378 Ma. One pegmatitic plagiogranite sample exhibits feature of accretionary arc granites, while other samples show geochemical characteristics of island arc tholeiite and E‐MORB. Additionally, four turbidite matrix and two limestone samples, with a major peak around ca. 480–500 Ma, have the youngest zircon ages ranging from ca. 481 to 387 Ma, paralleling the age range of igneous rocks in the ophiolitic mélange. The εHf(t) values of these samples reveal a broad spectrum of crustal and mantle processes. The youngest zircon ages of five meta‐sedimentary rock samples north of the Omixia Complex range from ca. 581 to 535 Ma, with peak ages concentrated around ca. 0.9–1.0 Ga. Their provenance characteristics differ from the turbidite matrix in the southern Omixia Complex and from the meta‐sediments in the northern Tiklik terrane with peak ages of ca. 0.8 Ga. A younger limestone sample yielded youngest zircon age of ca. 294 Ma, which is unconformably overlain the Omixia Complex and surrounding older rocks. Based on these new results, combined with previous data, we propose a new tectonic model for the eastern section of the northern Western Kunlun Orogen, suggesting a continuous evolution process of multi‐terrane subduction–accretion collage from the Early Ordovician to the Middle Devonian in the Paleo‐Tethys Ocean, which evolved into an Andean‐type active margin in the Early Permian, contributing to the substantial continental growth of the southern Tarim Craton.

San Albino, Nicaragua: A Low-Angle, Thrust-Controlled Orogenic Gold Deposit

Abstract The San Albino deposit is an orogenic gold occurrence hosted by a low-angle thrust that is the site of a new open-pit mine in northern Nicaragua. The deposit is hosted in greenschist facies rocks of the Jurassic metasedimentary Neuvo Segovia Formation. The schist was uplifted and exposed during arc accretion and Cretaceous thin-skin deformation, forming the NE-striking Colon fold-and-thrust belt. Deformation included emplacement of the 119 to 113 Ma NE-trending Dipilto batholith into the regionally metamorphosed clastic rocks about 5 km northwest of the San Albino deposit. Mineralization is dominated by three laminated quartz vein systems (i.e., San Albino, Naranjo, Arras) that broadly follow shallowly dipping (approx. 30°) carbonaceous shears roughly concordant to schistosity along the limbs of a doubly plunging antiform. The three main parallel shears are each separated by about 90 m and individually reach a maximum thickness of about 8 m. Maximum thickness of ore zones is where post-ore local folding and reverse motion along the shallow shears has duplicated the laminated low-angle gold-bearing veins (D2 and early D3). Additional gold was added to the veins, with abundant sulfides, during a subsequent brecciation event of the early formed quartz veins that accompanied progressive thrusting (late D3). This predated boudinage of the veins during continued compression and thrust loading (D4); high gold grades are particularly notable along pyrite- and arsenopyrite-bearing stylolites formed during D4 pressure solution. The D2 to D3 gold event is likely coeval with Albian uplift of the Dipilto batholith and with back thrusting in the schist aided by the stress inhomogeneities provided by the igneous complex. Low-angle thrust-controlled orogenic gold deposits may represent world-class exploration targets because of their large linear footprints, although they are traditionally looked at as less favorable exploration targets relative to gold systems developed more commonly along high-angle reverse faults. Our case study of the San Albino deposit shows that although low-angle deposits are not inherently misoriented for failure like the more common subvertical reverse fault-related deposits, they may be sites of significant pressure buildup due to hydrothermal mineral precipitation during initial water-rock interaction or slight temperature decreases along the low-angle flow path. Resulting fluid cycling may lead to thick laminated vein development, such as seen at San Albino, where especially high-grade zones may be associated with local steepening and/or dilational zones within the broader, low-angle vein-hosting shear system.

Rodinia to Gondwana evolution record, South Sinai, Egypt: Geological and geochronological constraints

The geological history of Sinai is complicated and differs from the other parts of the Arabian Nubian Shield. Some rocks in Sinai have a signature from the Grenvillian crust, as evidenced in Wadi Seih area. The ultimate objective of this research is to clarify the progression of episodic island arc accretion events from the amalgamation of Rodinia to the assembly of Gondwana supercontinent.Geochemical, mineralogical, U-Pb zircon geochronological, and thermobarometric studies are considered to constrain the nature and tectonic evolution of Wadi Seih gneisses. The protoliths of the orthogneisses are tonalite, trondhjemite, granodiorite, and granite, which were derived from metaluminous-calc-alkaline magma and formed during island-arc accretion. The paragneisses originated from immature Fe-sand and arkose that were deposited in active continental margin and back-arc basin. The orthogneisses and orthopyroxene-free paragneisses were metamorphosed under amphibolite to low granulite-facies conditions with temperatures from 530 to 775 °C at pressures of 3–7 kb, while the orthopyroxene-bearing paragneisses were metamorphosed under granulite-facies at a temperature of 870 °C and pressures of 5–7 kb. Zircon U-Pb geochronological data of the orthogneisses yields three distinct ages (i.e., the oldest ages are 994 ± 6 Ma and 976.6 ± 6.2 Ma, the second age is 740 ± 6.4 Ma, and the third age is 619.4 ± 4.2). Similar ages are obtained from detrital zircons in paragneisses: 1039 ± 57 Ma, 643 ± 49 Ma, 628 ± 3.7 Ma, and 627 ± 6.6 Ma. These new U-Pb data delineate three episodic Mesoproterozoic to Neoproterozoic island arc events at 1050–900 Ma, 850–700 Ma, and 650–580 Ma. The oldest 1050–900 Ma arcs potentially formed during the Rodinia supercontinent-arc collision, while the 850–700 Ma arcs may have formed during the closure of the Mozambique Ocean. The youngest 650–580 Ma arcs could reflect the Gondwana assembly. The age of metamorphism in this study is constrained by the low Th/U ratio of a zircon grain at 633 ± 10 Ma, which is consistent with other previous studies in other areas in Sinai.

Neoarchaean DTTGs from the Dunhuang Block, Tarim Craton: insights into petrogenesis and crust–mantle interactions

ABSTRACT Earth’s first continental crust is formed by Archaean and mainly consisted of tonalite–trondhjemite–granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust–mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U‒Pb dating and Lu‒Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14–71.70 wt%), low MgO contents (0.65–1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t) = -5.77–0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80–3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t)= −3.99–3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72–59.11 wt%) but high MgO (3.51–4.52 wt%) contents with positive Nd-Hf isotopes [ƐHf (t) = -0.16–4.17 and ƐNd (t) = 2.00–4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust–mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction.

Origin, nature, and evolution of the northern Pontiac subprovince, Canada: Insights from the intrusive record

The metasedimentary subprovinces of the Superior Province provide critical insights into the tectonic style and thermal state prevailing during its Neoarchean cratonic assembly. The Pontiac subprovince consists of metamorphosed turbiditic sequences with minor interlayered mafic volcanic rocks that are intruded by abundant mafic to felsic plutonic rocks of different geochemical affinities, and for which the source, timing, deformation, and relationship to metamorphism and gold endowment are poorly constrained. Petrological, geochemical, isotopic (Lu-Hf zircon), and geochronological (U-Pb zircon and monazite) data reveal three plutonic events consisting of 1) c. 2764 Ma medium-pressure TTG gneiss, interpreted as the local basement to the Pontiac supracrustal sequences; 2) syn- to late- sedimentary c. 2690–2670 Ma juvenile (mean εiHf = 4.02 ± 1.12) monzonite sanukitoid and highly potassic calc-alkaline sills and plutons; and 3) voluminous c. 2670–2620 Ma S-type granite sourced by protracted partial melting of sedimentary rocks at depth. This succession in the intrusive record combined with syn-depositional volcanism and subsequent Barrovian metamorphism of the metasedimentary host rock is consistent with a transition from a continental rift setting to crustal thickening via terrane assembly and challenges early models of arc accretion.

Geochemistry of the Neoproterozoic ophiolite from the Bensa-Girja area, southern Ethiopia: Petrogenesis and paleotectonics

The Bensa-Girja area is located in northernmost part of the Neoproterozoic Adola belt, southern Ethiopia. It is largely constituted of mafic rocks (amphibolite and metagabbro), ultramafic suite (serpentinite/serpentinized peridotite, talc schist and subordinate pyroxenite), metasedimentary (metapelitic to semi-metapelitic schists, minor marble and metasandstone) and metavolcaniclastic rocks. Geochemical analysis was carried out on the mafic–ultramafic rocks to characterize their petrogenesis, magma source and paleotectonic setting. The results indicate that the mafic rocks have tholeiitic to boninitic composition and represent primitive to evolved magma. The current serpentinite and serpentinized peridotite were formed after harzburgites that are interpreted as a refractory mantle wedge peridotite, and pyroxenites are considered as cumulate rocks crystallized from partial melts of the peridotite. The geochemical data further suggest that the mantle sources of the mafic-ultramafic rocks were modified by both slab-derived fluids and melts in a supra-subduction zone. The rocks thus display geochemical characteristics typical of island arc system with slight MORB-like affinity, inferred to have formed in a sub-arc or spreading forearc setting. It is interpreted that the studied rocks are representing remnants of a fragmented oceanic lithosphere. This together with field geological data supports the arc accretion model postulated for the crustal growth of the Arabian-Nubian Shield.

From arc accretion to within-plate extension: Geochronology and geochemistry of the Neoproterozoic magmatism on the northern margin of the Yangtze Block

Early Neoproterozoic arc magmatism and arc accretion processes are fundamental issues in reconstructing the Rodinia supercontinent. In this study we present new data of zircon U-Pb geochronology, Hf-O isotopes and whole-rock geochemistry for the magmatic rocks (including 12 felsic volcanic rocks, 3 granodioritic intrusions, 8 basaltic extrusive rocks and 12 gabbroic intrusions), which are spatially independent, in the Dahongshan area on the northern margin of the Yangtze Block. The investigated intermediate-felsic rocks are dated at ca. 970 Ma, ca. 885–865 Ma, ca. 840–830 Ma and ca. 810–790 Ma by in situ ion probe and laser zircon U-Pb dating methods. The mafic rocks are dated at ca. 870 Ma by in situ ion probe. Combined with previous age data, our new geochronology results suggest continuous magmatic activities from ca. 970 Ma to ca. 790 Ma in the Dahongshan area with a magmatic flare-up at ca. 840 Ma. The ca. 970–840 Ma mafic rocks, as well as the contemporaneous intermediate-felsic rocks, show enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high-field strength elements (HFSEs), representing arc-related tectonic settings. Zircon grains from these rocks generally have positive εHf(t) values from +7.3 to +14.7 and slightly elevated δ18O values from +5.59 to +6.89, indicating a dominant juvenile source with minor supracrustal incorporation. In contrast, the ca. 830–790 Ma mafic rocks are characterized by E-MORB- (enriched mid-ocean ridge basalt) and OIB- (oceanic island basalt) like and the contemporaneous felsic rocks show characteristics of A2-type granites, implying a within-plate extension environment. Zircon grains from the ca. 830–790 Ma felsic rocks display broad εHf(t) values from −5.7 to +15.3 and low mantle δ18O values from +3.81 to +6.11, implying multiple magma sources for their generation and the imprinting of high-temperature hydrothermal alteration. The relative contributions of the mature crustal component in the Dahongshan magmas decreases from 10–20 % in the ca. 970–840 Ma felsic rocks to 0–10 % in the ca. 830–790 Ma felsic rocks. The Neoproterozoic magmatic activity on the northern margin of the Yangtze Block recorded an integrated evolution from accretion of previous oceanic arc (ca. 970–870 Ma) at ca. 870–840 Ma to within-plate extension (ca. 830–790 Ma) in response to the convergence and divergence of the supercontinent Rodinia.

Accretion of an early Paleozoic Alaska-type arc onto northern North China: Implications for continental growth of the Central Asian orogenic belt

The accretionary orogenesis of the Central Asian orogenic belt is essential for understanding the reconstruction and growth of the Asian continent. The origin and accretion of the Bainaimiao arc along the northern margin of North China remain controversial. Here, a comprehensive study of field geology, geochemistry, and geochronology was performed on plutons and an ophiolite-bearing accretionary complex in the Ganqimaodu region, Inner Mongolia. Geochemical data indicate that ca. 445−431 Ma plutons were generated in an intra-oceanic arc and represent the western extent of the Bainaimiao arc. Field mapping and zircon U-Pb dating revealed that the ophiolitic mélange contains ca. 470−435 Ma tectonic blocks of ultramafic rocks, gabbros, plagiogranites, and metavolcanic and siliceous rocks in a matrix of intensely foliated quartz schist. The pillow basalt−limestone sequence might have originated from a seamount and then been incorporated into the accretionary complex. Northward subduction of the South Bainaimiao Ocean was responsible for the early Paleozoic tectonic evolution of the Bainaimiao arc, based on the spatial-temporal configuration of arc magmatism and accretionary complexes. Integration of new data with previous studies indicates that the Bainaimiao arc was an Alaska-type arc with various components of intra-oceanic and continental arcs. We suggest that a scissor-like arc-continent collision led to the accretion of the Bainaimiao arc onto the northern margin of North China, which played a fundamental role in the continental growth of the southern Central Asian orogenic belt.

The proto-type basin and tectono-paleogeographic evolution of the Tarim basin in the Late Paleozoic

The Tarim basin is a large composite and superimposed sedimentary basin that has undergone complex multi-period and polycyclic tectonic movements. Understanding the proto-type basin and tectono-paleogeographic evolution of this complex superimposed basin is important for understanding the basin-mountain coupling and dynamical mechanisms of the Paleo-Asian and Tethys tectonic systems as well as hydrocarbon exploration and development. Based on previous works, together with the recent exploration, and geological evidences, three global plate tectonic pattern maps, four Tarim proto-type basin maps (in present-day geographic coordinates) and four regional tectono-paleogeography maps (in paleogeographic coordinates) during the Late Paleozoic are provided in this paper. Based on these maps, the proto-type basin and tectono-paleogeographic features of the Tarim basin during the Late Paleozoic are illustrated. The Devonian to Permian is an important period of terranes/island-arcs accretion and oceanic closure along the periphery of the Tarim block, and a critical period when the polarity of Tarim basin (orientation of basin long-axis) rotated at the maximum angle clockwise. During the Late Paleozoic, the periphery of the Tarim block was first collisional orogeny on its northern margin, followed by continuous collisional accretion of island arcs on its southern margin: on the Northern margin, the North and South Tianshan Oceans closed from East to West; on the South-Western margin, the Tianshuihai Island Arc gradually collided and accreted. These tectonic events reduced the extent of the seawater channel of the passive continental margin in the Western part of the basin until its complete closure at the end of the Permian. The Tarim basin was thus completely transformed into an inland basin. This is a process of regression and uplift. The Southwest of the Tarim basin changed from a passive to an active continental margin, through back-arc downwarping and eventually complete closure to foreland setting. The intra-basin lithofacies range from shelf-littoral to platform-tidal flat to alluvial plain-lacustrine facies. The tectonic-sedimentary evolution of the Tarim basin is strongly controlled by peripheral geotectonic setting.

Tracing tectonic processes from Proto- to Paleo-Tethys in the East Kunlun Orogen by detrital zircons

The East Kunlun Orogen (EKO), a crucial Tethys composite accretionary belt in the northern Tibetan Plateau, experienced a polyphase tectonic history during an orogenic evolution from the Proto-Tethys to the Paleo-Tethys oceans. It is thus an ideal natural laboratory for tracing the Tethys orogenic cycles. Although numerous studies have been done on the geologic evolution of the East Kunlun Orogen, the timing of initial subduction, final closure, and the transition from the Proto-Tethys to the Paleo-Tethys still remain controversial. This study presents U-Pb-Hf isotopic analyses of 794 detrital zircons from three modern river drainage samples and two sedimentary samples in the EKO. The analysed zircons cluster in five age groups: 2900–2100 Ma, 1900–1300 Ma, 1200–730 Ma, 550–370 Ma, and 290–200 Ma, in which almost 67 % are of Phanerozoic age. The εHf(t) values of the 550–370 Ma detrital zircons have a broad range from −28.8 to + 15.9 and exhibit a trend towards positive values, suggesting that subduction of the Proto-Tethys Ocean initiated at ca. 550 Ma and accretion of the continental arc occurred with increasing addition of mantle material. The εHf(t) values (−8 to + 13) of 290–200 Ma detrital zircons have no obvious evolutionary trend, reflecting the continuous crust-mantle interaction during the evolution of Paleo-Tethys arc. Our results, combined with previous research, reveal two discontinuous and distinct orogenic cycles from the Proto-Tethys to the Paleo-Tethys in the EKO: the Proto-Tethys orogenic cycle from oceanic subduction to continental collision and orogenic collapse at ca. 550–370 Ma, and orogenesis of the Paleo-Tethys Ocean at ca. 290–200 Ma in the EKO, with a gap of ∼ 80 Myr, during which time there was no subduction, and the extensional setting resulted in rare identifiable zircon record.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

Two-Stage Orogenic Cycle of the Eastern Paleo-Asian Ocean from Early Palaeozoic to Early Triassic: Constraints from Magmatic Rocks of the Southeastern Central Asian Orogenic Belt

The evolution of the eastern Paleo-Asian Ocean (PAO) has controlled the formation of the southeastern Central Asian Orogenic Belt (CAOB). However, the evolution history and final closure time of the eastern PAO still remain controversial, which greatly restricts understanding of the formation process of the CAOB. To address these issues, we provide detailed zircon chronology and Hf isotope and geochemical data of Paleozoic to Triassic magmatic rocks in the southeastern CAOB. We have identified four periods of magmatism as evidenced by: Early Silurian quartz diorites (434.7 Ma), Early Devonian monzogranites (394.2 Ma), Middle Permian granites (260.2–264.5 Ma) and Late Permian-Early Triassic syenogranite (250.8–253.6 Ma). These rocks have features of low MgO and mantle-compatible elements, are enriched in Th, U, K, Pb, Sr, Zr and Hf and depleted in Nb, Ta, La, Ce, P, and Ti. The quartz diorites belong to the medium-K calc-alkaline series with εHf(t) values of −0.76 to 2.21, indicating that they may be derived from partial melting of mafic lower crust with minor contribution of mantle magma. The monzogranites and syenogranite have high Zr + Nb + Ce + Y (260–390; 261–461 ppm, respectively), total alkali contents (9.98–10.80; 8.46–9.29 wt.%, respectively), and high zircon saturation temperature (807–840; 810–885 °C). They can be classified as A-type granites. Monzogranites have εHf(t) values (between −1.20 and +3.34); hence, we believe that they were derived from the crust modified by mantle-derived fluids or melts. Syenogranite have high εHf(t) values (5.49–11.36), and we suggest that they were derived from the juvenile lower crust that originated from the depleted mantle. The granites have high Sr/Y ratios (118–257), low Y (1.42–2.82 ppm), and Yb (0.31–0.41 ppm), consistent with the features of adakite. Considering the εHf(t) isotopic values (2.99–8.50), we suggest that they originated from thickened juvenile lower crust. Combining the results from our own and previous studies, we propose a new evolution model of the eastern PAO from Paleozoic to Triassic. It can be divided into two stages: (1) Late Cambrian to Early Devonian; (2) Early Permian to Triassic. The first stage is the evolution of the Bainaimiao ocean (secondary ocean basin of the PAO), which closed in the Late Silurian and led to the Bainaimiao arc accretion to the North China Cratons. The second stage is the final closure of the eastern PAO during the Late Permian (~254 Ma).

Terminal stage of divergent double subduction: Insights from Early Cretaceous magmatic rocks in the Gerze area, central Tibet

Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong–Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong–Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response. • Manifold magmatism from the Gerze area, central Tibet, is active at ca. 105 Ma. • The ca. 105 Ma Gerze magmatic rocks record a slab breakoff event. • Slab breakoff plays an important role in the divergent double subduction system.

Neodymium Isotope Constraints on the Origin of TTGs and High-K Granitoids in the Bundelkhand Craton, Central India: Implications for Archaean Crustal Evolution

Abstract The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica–high Mg (sanukitoids and hybrids) and high silica–low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid, and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages and geochemical signatures as well as εNd(t) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and preexisting crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.

Extremely rapid up-and-down motions of island arc crust during arc-continent collision

Mountain building and the rock cycle often involve large vertical crustal motions, but their rates and timescales in unmetamorphosed rocks remain poorly understood. We utilize high-resolution magneto-biostratigraphy and backstripping analysis of marine deposits in an active arc-continent suture zone of eastern Taiwan to document short cycles of vertical crustal oscillations. A basal unconformity formed on Miocene volcanic arc crust in an uplifting forebulge starting ~6 Ma, followed by rapid foredeep subsidence at 2.3–3.2 mm yr−1 (~3.4–0.5 Ma) in response to oceanward-migrating flexural wave. Since ~0.8–0.5 Ma, arc crust has undergone extremely rapid (~9.0–14.4 mm yr−1) uplift to form the modern Coastal Range during transpressional strain. The northern sector may have recently entered another phase of subsidence related to a subduction polarity reversal. These transient vertical crustal motions are under-detected by thermochronologic methods, but are likely characteristic of continental growth by arc accretion over geologic timescales.

The tectonic context of hafnium isotopes in zircon

The assembly and dispersion of continental crust are first-order controls on paleogeography and geochemical cycles. The associated reworking of Earth's crust can be tracked with zircon initial hafnium (εHfT) through space and time. Here we apply a new method of quantitative analysis using εHfT density estimates based on a compilation of 155,329 εHfT values. Investigation of the global database reveals significant geographic and temporal bias in the εHfT record associated with sampling and regional tectonic events. Recent research has attempted to address global εHfT bias using resampling methods to augment gaps of low εHfT data density, which in turn obfuscates tectonic signals and artificially weights outliers. Instead, we evaluate εHfT density patterns for both igneous and detrital zircon on eight continental zones demarcated by Paleozoic sutures: Africa, Antarctica, Asia, Australia, Baltica, North America, Peri-Gondwana, and South America. Pairwise two-dimensional quantitative comparison highlights similarity in timing and εHfT values between zones, all of which can be linked to documented shared regional tectonism. Integration of all pairwise comparisons reveals that peak similarity corresponds to the timing of supercontinent amalgamation, and that the associated εHfT differs depending on the style of supercontinent amalgamation, particularly internal versus external orogenesis. The three most recent supercontinents produced distinctive εHfT signals, shared by the constituent continental zones. The supercontinents Rodinia and Pangea were constructed through collisions of marginal arc terranes, peripheral to ancient crust, and did not produce highly enriched εHfT values. In contrast, Ediacaran to Cambrian formation of the Gondwana supercontinent was largely the product of internal Pan-African orogens that formed directly after Neoproterozoic Rodinia rifting and arc accretion forming the Arabian Shield. The final assembly of Gondwana was dominated by continent-continent collisions of old radiogenic crust without establishment and accretion of extensive intervening depleted arc terranes, resulting in a more enriched distribution of εHfT values compared to prior and subsequent supercontinent formation. The secular εHfT record is the product of spatiotemporally biased sampling and preservation of specific orogenic belts with predictable εHfT data arrays, modulated by the amalgamation, tenure, and breakup of supercontinents through time.