Accretionary Belt Research Articles

Mesoproterozoic (∼1.4 Ga) magmatism in northern Alxa, China: Implications for tectonic affinity of microcontinents in the southern Central Asian Orogenic Belt

The origin and paleogeographic reconstruction of Precambrian microcontinents in the Central Asian Orogenic Belt (CAOB) are crucial for understanding continental crustal evolution, mechanisms of accretionary orogeny, and assemblage and breakup of the supercontinent during the Earth’s early history. The existence and basement nature of the South Gobi microcontinent within the central segment of the southern CAOB remain controversial. This study represents the first work to unveil the presence of Mesoproterozoic (∼1.4 Ga) intrusive rocks in the Zhusileng-Hangwula tectonic zone that is the central segment of the southern CAOB, and is distributed along the border areas between China and Mongolia. It offers pivotal evidence for the existence of the South Gobi microcontinent.Three alkali feldspar granites samples collected from the Zhusileng-Hangwula tectonic zone yielded crystallization ages ranging from 1371 ± 17 Ma to 1420 ± 29 Ma, while detrital zircons from a two-mica quartz schist xenolith yielded similar 206Pb/238U ages clustered around ∼1.4 Ga. The ∼1.4 Ga zircons display depleted Hf isotopic signatures (εHf(t) = 3.2–6.0) and relatively young two-stage Hf model ages (1.9–1.7 Ga), suggesting rapid reworking of juvenile mantle-derived materials. These findings indicate that the Zhusileng-Hangwula tectonic zone constitutes a microcontinental fragment with Mesoproterozoic basement, rather than a Paleozoic accretionary complex as previously postulated. Combining the contemporaneous magmatism data from the microcontinents in the southern CAOB, the 1.48–1.32 Ga arc-related magmatism exhibits a coherent trend of geochemical evolution, potentially indicating a geodynamic transition from a convergent to an extensional environment. Moreover, the temporal, tectonic evolution, and petrological similarities imply a tectonic affinity between these microcontinents and Fennoscandia, and they may have originated from an accretionary belt along the edge of Nuna during the Mesoproterozoic.

Reply to the comments on “Late Archean sedimentary basins in the northeastern Superior Province, Canada: Plume-generated crustal tears or syn-convergent accretionary belts?” of

Reply to the comments on “Late Archean sedimentary basins in the northeastern Superior Province, Canada: Plume-generated crustal tears or syn-convergent accretionary belts?” of

U-Pb zircon and Sm-Nd geochronology of the dom Silvério group, SE Brazil: Implications for the evolution of the superposed Araçuaí and Minas-Bahia orogens

The Araçuaí orogen, the South American portion of the Brasiliano/Pan-African Araçuaí-West Congo orogenic system (AWCO), formed during the Ediacaran-Cambrian assembly of West-Gondwana. The basement of a substantial portion of the Araçuaí orogen comprises Archean and Paleoproterozoic rocks involved in the 2.1–2.0 Ga Minas-Bahia orogen. The Dom Silvério Group, which consists of mica schists containing layers and lenses of mafic, ultramafic, and Mn-rich rocks, as well as quartzites, graphite schists, marbles, and banded iron formations, defines a ca. 10 km-wide and 150 km-long belt in the southwestern portion of the Araçuaí orogen. This succession, currently interpreted as deep marine deposits or an ocean-floor assemblage of uncertain age - Paleoproterozoic or Neoproterozoic-, potentially records processes involved in critical evolutionary stages of the superposed Minas-Bahia and Araçuaí orogens. Aiming to characterize the age and tectonic significance of the Dom Silvério Group, we performed a geochronological and geochemical investigation on samples collected from selected lithotypes. U-Pb (LA-ICP-MS) determinations on detrital zircons revealed two distinct age spectra. Zircons extracted from the lower Dom Silvério schists define a spectrum characterized predominantly by Neoproterozoic age clusters with a maximum depositional age of 601 ± 10 Ma. Quartzites from the lower and upper sections of the group do not contain zircons younger than 1.9 Ga and show unimodal age distributions around 2067 Ma. The maximum depositional age estimated for these rocks is 2020 ± 6 Ma. The spectrum of the lower section schist indicates the turbidites of the Ediacaran Salinas Formation as their most probable correlative. In contrast, the quartzites and schists of lower and upper sections correlate with the Orosirian Itacolomi Group, the post-collisional assembly of the Minas-Bahia orogen. Sm-Nd isotope analyses and whole-rock geochemistry allow the discrimination of two generations of mafic rocks. The older generation, showing TDM model ages = 1.92 to 3.6 Ga and ƐNd(t) = -12.93 to +4.63, represents components of the basement. The younger generation, with TDM model ages = 1.0 Ga to 1.25 Ga and ƐNd(t) = +0.05 to +2.29, is similar to the ophiolitic rocks that occur imbricated with Ediacaran metasedimentary successions in the central portion of the Araçuaí orogen defining the Ribeirão da Folha accretionary belt. They show Th/Yb and Nb/Yb ratios of the EMORB-OIB mantle array and Nb/Y and Zr/Y ratios compatible with OIB and oceanic plateau basalts. The Dom Silvério Group is thus a complex that involves members of the Araçuaí and Minas-Bahia orogens. Their Ediacaran components materialize the southern extension of the Ribeirão da Folha accretionary wedge and, as such, provide further evidence for oceanic consumption during the development of the southern Araçuaí orogen.

Intermittent terrane arrival induces pulses of inland tectonic cycles

Pulsing volcanotectonic cycles characterized by short-lived (10 s Myr) switches in magmatic and tectonic patterns have been broadly identified in active margins. However, the specific mechanism that causes these switches remains ambiguous, i.e., whether the subduction continuity and/or terrane arrival (accretion, underthrusting, or subduction of buoyant continental/oceanic blocks with a thicker crust than their surrounding oceanic plate) plays a crucial role in controlling the observed volcanotectonic cycles remains controversial. Here, by modeling subduction processes involving the sequential arrival of buoyant terranes, we show that 1) in scenarios where the oceanic plate is weakly coupled with terranes, the entraining of the terrane into the subduction induces slab breakup to occur between the partially subducted terrane and its adjacent oceanic slab, with the terrane then rebounding and moving away from the trench. This evolution causes switches in the magmatic and tectonic patterns within the overriding plate. 2) In these models, the exposed terrane materials can preserve characteristic pressure-temperature-time trajectories, i.e., nearly isothermal compression to isothermal decompression and/or isobaric heating after partial exhumation. 3) slab breakup does not guarantee the occurrence of a trench jump; only when the terrane has a medium scale (∼300 km) will a new trench tend to develop prominently behind the rebounded terrane. 4) in scenarios where the composite slab (terrane and oceanic portions) resists yielding deformation and the terrane density is close to that of the oceanic plate (<0.6% density contrast), continuous subduction will occur. In this latter scenario, slab deformation (revealed by subduction angle and slab curvature), instead of slab breakup, will control the magmatic and tectonic patterns in the overriding plate. By further comparing model results with observations, we demonstrate that intermittent subduction interrupted by the subduction of terranes can be a tectonic driver for episodes of compression-to-extension transformations and magmatism (or piston-like volcanotectonic cycles) in two representative accretionary belts — with or without trench jumps (exemplars in south-central-Tibet and eastern-Mediterranean). In contrast, continuous subduction with strongly coupled upper and subducting plates could have contributed to similar cycles in the example without accretion (e.g., the Altiplano in the Central Andes). Therefore, over 10 s of Myr, the scale and frequency of terrane arrivals could essentially control the specific motion pattern of the subducting plate, creating the observed short-lived volcanotectonic switches at these three subtypes of active margins.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

Case study of the large-scale Mo-W mineralization in Eastern Qinling, China: Geology and genesis of the Yechangping porphyry-skarn system

Eastern Qinling Orogen is the largest Mo province in the world, hosting nine giant Mo deposits. Four giant Mo deposits contain economic W mineralization, while the mechanism of W mineralization remains unclear. The Yechangping deposit is a representative porphyry-skarn Mo-W deposit, with four paragenetic stages of prograde skarn, retrograde skarn, polymetallic sulfides and carbonate stage. Magmatism at Yechangping includes early barren monzogranite porphyry at 159.0 ± 1.5 Ma and late ore-related granite porphyry at ca. 145 Ma, and the latter porphyry shows a higher magmatic fractionation than the early barren monzogranite porphyry. Both barren and ore-related porphyries have similar high magma oxidation states, which may efficiently extract Mo during partial melting and hinder subsequent molybdenite crystallization until the porphyry-skarn Mo mineralization. Three types of scheelites are distinguished by their texture, paragenetic relationship and geochemistry, including Sch-1a in the retrograde skarn stage, and Sch-1b and Sch-2 in the polymetallic sulfides stage. Scheelite displays decreasing MoO3 contents from 7.01–51.59 % in Sch-1a, through 3.15–17.51 % in Sch-1b, to 0–1.51 % in Sch-2. The decreasing MoO3 contents in scheelite are attributed to the transformation of dominant Mo-bearing phases, i.e., from retrograde skarn stage powellite to polymetallic sulfides stage molybdenite. We suggest that the Mo-W mineralization is triggered by the decreasing oxygen fugacity, pH, and temperature, supported by thermodynamic modeling. In addition, the precipitation of fluorite and destabilization of fluorine complexes may have also promoted the Mo-W mineralization. Zircon Hf isotopes yielded εHf(t) values of −14.9 to −7.5 and TDM2 ages of 1671–2147 Ma, indicating that Yechangping porphyries were primarily sourced from the ancient basement of the Huaxiong Block, with additional contribution from the North Qinling Accretionary Belt. Skarn W mineralization in the southern part of Huaxiong Block may be controlled by both magmatic fertility and carbonate wall-rocks.

Recognizing different types of ocean plate stratigraphy in the southeastern South Qilian Accretionary Belt, northeastern Tibet Plateau

Dissection of relicts of ocean plate stratigraphy (OPS) in orogens could provide important insights into oceanic evolution, orogenic formation and continental growth. Here we describe a study of petrology, geochronology, geochemistry and Sr–Nd isotopes for OPS in the South Qilian Accretionary Belt with the aim of providing new constraints on the evolution of the Proto-Tethys Ocean (PTO). Three types of OPS units were recognized by their differing petrological, geochemical and isotopic features. Type 1 OPS, representing relicts of ocean islands or seamounts, comprises ocean island basalt and gabbro ( t = 525 ± 4 Ma, ( 87 Sr/ 86 Sr) i = 0.7035–0.7039 and ε Nd ( t ) = +3.63 to +4.16) and (siliceous) carbonate rocks. Type 2 OPS, representing fragments of normal oceanic crust, consists of normal mid-ocean ridge basalt (N-MORB) and basaltic andesite (( 87 Sr/ 86 Sr) i = 0.7059–0.7063 and ε Nd ( t ) = +6.33 to +7.44), chert and siliceous mudstone. Type 3 OPS could be oceanic plateau-derived and comprises enriched (E)-MORB and dolerite (( 87 Sr/ 86 Sr) i = 0.7051–0.7064 and ε Nd ( t ) = +4.81 to +6.24), carbonate–siliceous mudstone and carbonate rock. These OPS units witnessed evolution of the PTO. Using available studies on regional arc-type magmatism and U–Pb geochronological data for detrital zircon in this study, a model of ‘trench jam accompanying subduction flip induced by accretion of the OPS’ is put forward. Supplementary material : Zircon U–Pb data, whole-rock major and trace element data and a supplementary figure are available at https://doi.org/10.6084/m9.figshare.c.6764637 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

NEW EVIDENCE OF THE INDEPENDENT PRECAMBRIAN TECTONIC HISTORY OF THE FADDEY TERRANE OF THE CENTRAL TAIMYR ACCRETIONARY BELT

New geostructural, petro-geochemical, U-Pb geochronological and paleomagnetic data for sills of metagabbro-dolerites of the Severnobyrranga and Yasnenskiy complex specify the paleotectonic position of the Faddey terrane of the Central Taimyr accretionary belt. The structural position of the sills in the rocks of the Oktyabr and Zhdanov formations is indicative of their intrusion before deformations caused by the accretion of the Central Taimyr belt. The U-Th-Pb age of crystallization for the metagabbro-dolerites has been established as 1357 ± 9 Ma. The primary paleomagnetic record reflecting the time of intrusion has not been preserved, but two metachronous components have been identified. The first one corresponds to crust formation processes at ca. 840 Ma due to collision, probably of continent – island arc type involving the cratonic block that included the Faddey and Mamont-Shrenk terranes. These collisional events took place several hundreds of kilometers from the Siberian margin. The second component corresponds to thermal events on the Paleozoic-Mesozoic boundary and reflects the strike-slip deformation in the Kara orogen.

Microcontinental block amalgamation in the northern Tibetan Plateau and its significance for understanding the closure of the Proto-Tethyan Ocean

The Qilian-Qaidam Orogenic Belt is a component of the western Central China Orogenic Belt, and records a nearly complete evolutionary history of the South Qilian and North Qaidam oceans (branches of the Proto-Tethyan Ocean). However, the timing and process of amalgamation of the microblocks of the western Central China Orogenic Belt and the closure time of the South Qilian and North Qaidam oceans remain uncertain. The Olongbuluke Terrane (OLT), located in the central of the Qilian-Qaidam Orogenic Belt, contains well-developed Precambrian to Paleozoic sedimentary successions that shed light on the tectonic history of these ocean basins. In this paper, detrital zircon UPb ages combined with ophiolitic, magmatic and high-pressure to ultrahigh-pressure metamorphic records and field investigations, indicate that the OLT was part of a passive continental margin during the early Paleozoic and there was a microblock, named the North Qaidam Terrane (NQT), lying between the Qaidam Block and the OLT. A marked provenance change occurs in the OLT after ca. 517 Ma, which is inferred to record the collision between the OLT and NQT. Later, the North Qaidam oceanic plate subducted under the NQT, and the North Qaidam Ocean closed at ca. 450 Ma. The similar detrital zircon UPb age spectra of contemporaneous deposit flysch sequences in the Shihuigou Formation (OLT) and the Balonggonggaer Formation (the South Qilian Accretionary Belt) indicates the onset of collision between the OLT and Central Qilian Terrane occurred at ca. 458 Ma. The detrital zircon UPb age spectra of microblocks from the early Paleozoic strata of the northern Tibetan Plateau share a North Indian affinity, indicating that the closure of the Proto-Tethyan Ocean may relate to the formation of the Gondwanan landmass.

New Evidence of the Independent Precambrian Tectonic History of the Faddey Terrane of the Central Taimyr Accretionary Belt

New geostructural, petro-geochemical, U–Pb geochronological and paleomagnetic data for sills of metagabbro-dolerites of the Severobyrranga and Yasnenskiy complex specify the paleotectonic position of the Faddey terrane of the Central Taimyr accretionary belt. The structural position of the sills in the rocks of the Oktyabr and Zhdanov formations is indicative of their intrusion before deformations caused by the accretion of the Central Taimyr belt. The U–Th–Pb age of crystallization for the metagabbro-dolerites has been established as 1357 ± 9 Ma. The primary paleomagnetic record reflecting the time of intrusion has not been preserved, but two metachronous components have been identified. The first one corresponds to crust formation processes at ca. 840 Ma due to collision, probably of continent – island arc type involving the cratonic block that included the Faddey and Mamont-Shrenk terranes. These collisional events took place several hundreds of kilometers from the Siberian margin. The second component corresponds to thermal events on the Paleozoic-Mesozoic boundary and reflects the strike-slip deformation in the Kara orogen.

Channelised magma ascent and lithospheric zonation beneath the Songliao Basin, Northeast China, based on surface-wave tomography

The Songliao Basin in Northeast China is located within the eastern part of the Central Asian Orogenic Belt and to the north of the accretionary belt of the North China Craton. The lithosphere beneath the basin and the surrounding region has undergone a series of complex deformation processes due to sequential interactions among different tectonic plates since the Palaeozoic. Here we construct a three-dimensional lithospheric shear-wave (S-wave) velocity model for the region via surface-wave tomography to better understand the deep dynamic processes that shaped the Songliao Basin. The S-wave velocity model highlights strong linkages between the surface geology and mantle lithospheric structures. The Baicheng–Changchun uplift (or Song-Liao watershed), which divides the basin into two river systems, is underlain by a dome-like area of uplifted crust. The central Songliao basin is underlain by a weak, thin lithosphere that is ∼80 km thick, whereas the eastern Great Xinggan Range to the northwest is underlain by a normal crust and lithosphere that is ∼100 km thick. The northern North China Craton and southern Songliao Basin are also underlain by normal crust and a lithosphere that is up to ∼150 km thick, with a low-velocity zone penetrating the high-velocity mantle lithosphere at ∼80–100 km depth. Such structural features imply that the far-field effect of Pacific Plate subduction since the Cenozoic varies across Northeast China and is dependent on the lithospheric structures that existed prior to subduction.

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.

Sources and oxidation state of the Permian arc magmatic rocks of SW Jilin Province in the eastern Central Asian Orogenic Belt: evidence from Li, Hf isotopes and oxygen fugacity

AbstractThe late Palaeozoic continental-arc magmatic rocks in the Gongzhuling area are located in the Liaoyuan Accretionary Belt. Here we present new zircon U–Pb ages, whole-rock major- and trace-element compositions, Li and zircon Hf isotopic compositions and oxygen fugacity of these rocks with an aim to constrain the lithium isotopic composition of the source region and origin of the magmas. These rocks were formed during 269–258 Ma in middle–late Permian time. The dioritic rocks were formed through mixing processes, with the mafic melts originating from a metasomatized mantle wedge and the felsic melts from the lower crust of a Neoproterozoic arc. The mantle wedge has been metasomatized by Li-rich fluids derived from subducted oceanic crust, as indicated by the δ7Li values of +0.4 ‰ to +3.5 ‰ and positive ϵHf(t) values (+0.7 to +13.1). Redox-sensitive Ce in the zircons indicates the fO2 of the magmas to be low to intermediate (FMQ−2.2 to FMQ+2.6; FMQ is the fayalite–magnetite–quartz redox buffer), precluding large-scale porphyry Cu–Mo mineralization. The middle–late Permian magmatic rocks represent the terminal magmatic record of the subduction of the Palaeo-Asian oceanic crust, meaning that the final closure of the Palaeo-Asian Ocean in the eastern Central Asian Orogenic Belt occurred at the end of the Permian Period. Recent identification of Mesoproterozoic (c. 1400 Ma) granites suggests some Palaeoproterozoic crustal fragments still exist in the Liaoyuan Accretionary Belt, but only in a small amount; therefore, it is concluded that the crustal growth of the Liaoyuan Accretionary Belt occurred mainly during the Neoproterozoic period.

IBM-type forearc magmatism in the Qilian Orogen records evolution from a continental to an intra-oceanic arc system in the Proto-Tethyan Ocean

Forearc magmatism in IBM-type intra-oceanic arc systems is crucial for understanding the processes of subduction initiation and subsequent evolution of the arc system. Here we report a forearc volcanic sequence exposed in the Danghe-Nanshan, at the westernmost end of the South Qilian Accretionary Belt of the Qilian Orogen, NW China. Based on petrography and bulk-rock composition, three types of volcanic rocks (boninite, high-Mg basaltic andesite and andesite) and arc-related felsic plutons are identified. These rocks show correlating trends of trace elements and isotopes. The high-Cr spinels and clustered Ti/V ratios indicate that the boninite and high-Mg basaltic andesite magmas were derived from highly refractory peridotite sources in a forearc setting, whereas the andesite and felsic plutons were formed by fractionation of clinopyroxene and plagioclase from primary magma. Notably, slab-derived components have limited involvement in magma generation. Variation of fluid mobile/immobile element contents and Sr-Nd isotopic mixing calculations suggest less than 1% incorporation of subducted slab-derived fluids into the boninite source, whereas ∼1–3% fluids and ∼3–7% partial melts from the slab contributed to the high-Mg basaltic andesite and andesite, respectively. Therefore, an abnormally high mantle potential temperature (>1429 °C) was required for boninite production because there were little aqueous fluids involved. Zircon U-Pb dating reveals these forearc magmas formed at ∼450 Ma. Combined with the regional tectonics and published data in this region, we suggest that these magmas record a tectonic process of arc system from active continental margin to newly formed intra-oceanic subduction zone during trench retreat of the Proto-Tethys Ocean.

Carbonatitic pockets in intra-ocean arc volcanics (Qilian orogen): Petrogenesis and implications for carbon recycling in subduction zones

Carbonatites in island arcs are crucial for understanding the processes of magmatism and carbon recycling in subduction zones. Here we report carbonatitic pockets in Ordovician intra-oceanic island-arc volcanic rocks and in Ocean island basalts (OIB) of Cambrian ophiolites in the South Qilian Accretionary Belt, NW China. All these carbonatitic pockets are composed of relatively pure calcites, some of which have accessory minerals of apatite, sphene, hematite, magnetite and allanite. The 87Sr/86Sr(i) ratios of calcite range from 0.703833 to 0.706732, concentrated at about 0.705, which is close to the composition of Enriched Mantle 1 (EMI) affected by fluids from the subduction zone. The C-O isotopic compositions show trends that evolve away from primary carbonatite (δ13CPDB = −9.0 ~ −1.0 ‰, 18OSMOW = 10.1– 16.5 ‰), probably caused by calcite fractional crystallization. High Sr, Th, U, Pb and low Nb, Zr, Hf, Ti contents and 87Sr/86Sr(i) ratios indicate that these carbonatitic pockets derived from carbonatite magma, and may have been altered by subduction-zone fluids. Carbonatitic pockets in ankaramites represent primary mantle-derived carbonate melts/liquids with low REE (rare earth elements) abundances (<10 ppm) and δ13CPDB. Elevated REE contents and LREE/HREE (light / heavy rare earth elements) ratios suggest that carbonatitic pockets in high-Al andesite may have experienced fractionation crystallization. The EMI-type carbonatitic pockets occurrence in the two different rock assemblages may reflect an interaction between an oceanic arc and an oceanic plateau, deriving from the recycling of Proterozoic marine carbonates through the lower mantle.

Ridge subduction and episodes of crustal growth in accretionary belts: Evidence from late Paleozoic felsic igneous rocks in the southeastern Central Asian Orogenic Belt, Inner Mongolia, China

Abstract The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and preserves evidence that more than 50% of this orogen represents juvenile crustal growth over an extended period of some 750 million years from ca. 1000 Ma to ca. 250 Ma. However, the mechanism of crustal growth is controversial, as implied by a variety of proposed models ranging from contributions of mantle-derived basaltic underplating in a post-collisional extensional setting to subduction-related processes in an island arc setting. To distinguish among these models, we report petrological, geochemical, whole-rock Sr-Nd isotope and zircon U-Pb and Hf isotope analyses of late Paleozoic felsic igneous rocks from the northern Inner Mongolia region, southeastern CAOB. New zircon U-Pb analyses of three plutonic and extrusive magmatic suites yield Late Carboniferous to Early Permian ages of 319–279 Ma. The Xi Ujimqin granodiorites have low-K tholeiitic to calc-alkaline, metaluminous to weakly peraluminous compositions, and are magnesian I-type granitoids. These granitoids are also characterized by relatively high MgO and Sr contents, high Mg# values, mid-ocean ridge basalt (MORB)-like Nd-Hf isotope compositions and young Nd-Hf model ages of 600–298 Ma. These features indicate that the parental magmas of the Xi Ujimqin I-type granitoids originated from a depleted lithospheric mantle that had been metasomatized by fluids released from a subducting slab. By contrast, the Xilinhot alkali-feldspar granites and Dashizhai rhyolites display geochemical signatures of ferroan granites. These, together with their moderately depleted Nd-Hf isotope compositions and young Nd-Hf model ages of 809–277 Ma, suggest that they were produced by re-melting of juvenile lower crust via underplating of mantle-derived magmas. The coexistence of an association of boninite-adakite-high-Mg andesite-Nb-enriched basalt, and MORB to ocean island basalt-type mafic rocks in the northern Inner Mongolia region implies a dramatic change in composition from fluid-related calc-alkaline arc magmatism to melt-related and mantle-derived magmatism initiated by upwelling asthenosphere. We infer that ridge subduction may have occurred in this region during Late Carboniferous to Early Permian. Combined with previous studies, there are two ridge subduction events during the early and late Paleozoic in Inner Mongolia within the southeastern CAOB, which coincide with two large-volume magmatic flare-ups (300 ± 20 and 450 ± 20 Ma). Furthermore, our model calculations suggest that most of the juvenile crust in the southeastern CAOB was generated in a relatively short interval (~20–40 m.y.) during each long-lived tectonic cycle (&amp;gt;140 m.y. spanning oceanic rifting, subduction, and collision) of magmatic activity. Ridge subduction may have played a significant role in the rhythmic growth of juvenile crust in the CAOB during the Phanerozoic. Repeated cycles of punctuated, rapid growth of juvenile crust associated with long-lived subduction systems represent a pattern that may be generalized to other Phanerozoic accretionary belts on Earth.

Early depositional and magmatic history of the Beardmore-Geraldton Belt: Formation of a transitional accretionary belt along the Wabigoon-Quetico Subprovince boundary in the Archean Superior Craton, Canada

The Beardmore-Geraldton Belt (BGB) is a major gold camp along the boundary between the granite-greenstone eastern Wabigoon subprovince and the metasedimentary Quetico subprovince of the Archean Superior Province, Canada. New detrital zircon geochronology suggests that the metasedimentary rocks of the BGB and northern Quetico subprovince were derived from the erosion of the eastern Wabigoon subprovince that is dominated by ca. 2900–2680 Ma rocks but also comprises crust as old as > 3200 Ma. Uplift of the eastern Wabigoon subprovince as a result of convergence and compression along its southern margin exposed those rocks to erosion and shed sediments in the Quetico basin to the south. Sedimentation in the Quetico basin is bracketed between ca. 2700 Ma, the age of the youngest detrital zircon population, and 2694.0 ± 1.0 Ma, the crystallization age of cross-cutting feldspar-quartz porphyries (FQP). Continued convergence facilitated the thrust-imbrication of the metavolcanic rocks of the eastern Wabigoon subprovince with the metasedimentary rocks of the Quetico basin resulting in the assembly of the BGB. This resulted in crustal thickening, the introduction of hydrated mafic rocks at the base of the crust, and partial melting of these rocks to form TTG suite melts at depths >50 km, as suggested by the geochemical characteristics of the FQP intrusions. Imbrication and thrusting ended with the emplacement of the stitching, 2690 ± 1 Ma Croll Lake Stock, which has a high transition elements and ferromagnesian oxides signature of sanukitoid suite rocks. It formed during delamination of the lower crust or slab break-off from magmas generated by mixing of hot, mantle material with TTG suite melts at depths > 50 km. The early geological evolution of the Geraldton gold camp is broadly similar to that of other major Archean gold camps associated with major faults and/or terrane boundaries, where deposition of fluvial conglomerate and turbiditic sandstones in fault-bounded basins overlying older volcanic rocks is accompanied by magmatic activity and the upward migration of gold-bearing hydrothermal fluids, while differences between the Geraldton and other camps may be explained by craton-scale variations in geodynamic processes.

Unwrapping reworked crust at the Columbia supercontinent margin within central southern Amazon Craton using multi-source geophysics and geochronology data synergy

The geometry and evolution of pre-existing basement in accretionary belts bordering supercontinents are often unclear. Integrative interpretation of long-wavelength potential field satellite data can image deep crust structure, improving our understanding of lithospheric processes that formed these margins bottom-up. Here, we present a multidisciplinary interpretation of the lithospheric architecture of the central southern Amazon Craton, a fragment of an accretionary belt at the southwestern Columbia supercontinent margin. Satellite-borne gravity and magnetic data, airborne magnetic data, passive seismic (Vp/Vs ratio, crustal thickness) and seismic tomography data reveals that basement terranes from the interior of the craton extend into the accretionary margin of Columbia. We demonstrate a vertically heterogeneous structure with an underlying strongly reworked pre-Columbia tectonic wedge that sustained prolonged modification during the supercontinent assembly as corroborated by Nd isotope and geochronology data. Nd isotope data suggest that the protracted orogenic wedge was influenced by subduction angle shifts over time, including addition of substantial juvenile material during slab retreat events. This interplay promoted Craton growth at the supercontinent margin while keeping a subtle record of the pre-existing framework. Our findings point to the possible misrepresentation of basement extension and geometry of supercontinent margins elsewhere.

The Early Paleozoic intrusive magmatism and tectonic thermal evolution in the Hala Lake area, southern Qilian, NW China

南祁连增生杂岩带作为祁连造山带的构造单元之一，是研究祁连造山带与柴达木地块构造演化及二者耦合关系的关键地区，得到了国内外学者的广泛关注。前人对南祁连增生杂岩早古生代以来的构造热演化历史研究相对较少，且缺少相对准确的年代学数据约束。本文通过对南祁连增生杂岩带哈拉湖地区阿腊郭勒岩体二长花岗岩开展LA-ICP-MS锆石U-Pb定年、岩石地球化学特征、锆石和磷灰石裂变径迹年龄测试及热历史模拟，并结合野外地质调查和构造演化特征，揭示南祁连增生杂岩带哈拉湖地区的构造热演化历史和山脉隆升过程。结果显示：（1）南祁连哈拉湖地区在中志留世发生一期岩浆侵入事件（425~429Ma），其形成的岩体具有壳源花岗岩特征，产出于同碰撞的构造环境，说明该期岩浆事件是与祁连洋洋壳俯冲结束后的柴北缘地区大陆碰撞过程中的岩浆活动有关；（2）该岩体经历了中志留世至晚泥盆世的岩浆侵位与快速冷却阶段以及晚泥盆世-侏罗纪的构造平稳与缓慢冷却阶段；（3）早白垩世以来的中低温冷却和快速隆升的构造热演化历史。此外，祁连山地区自始新世以来经历了多期与印度欧亚-板块碰撞有关的构造变形。;As an important tectonic unit of the Qilian orogenic belt, the southern Qilian accretionary belt plays a significant role in studying the tectonic evolution and their coupling relationship between the Qilian orogenic belt and the Qaidam block. Although wide attention has been paid to this belt by geologists all over the world, the tectono-thermal evolution of the southern Qilian accretionary belt since Early Paleozoic is still unclear due to the lack of relatively precise geochronological data constraints. In this paper, we integrated field obervations, LA-ICP-MS zircon U-Pb age, geochemical characteristics, low-temperature themochronometry (apatite- and zircon-fission track ages) and thermal history modeling to provide constraints on the tectono-thermal history and range growth of the Hala Lake area in the southern Qilian accretionary belt. The results suggest that the southern Qilian accretionary belt experienced a multi-phase tectono-thermal evolution history, including:(1) Early Paleozoic intrusive magmatism and rapid cooling due to the Qilian orogeny from Middle Silurian (425~429Ma) to Late Devonian, with continental crust resource characters and generating from syn-collisional tectonic setting of the monzonitic granites, which indicates that this period of magmatism was related to the continental's collision of southern Qilian accretionary belt and the Qaidam block after the ocean crust subduction of Qilian Ocean; (2) tectonic quiescence and slow cooling during Late Devonian to Jurassic; (3) mid-low temperature cooling and rapid uplift since Early Cretaceous. Furthermore, the Cenozoic Qilian Shan thrust belt also experienced multi-phase structural deformations since Eocene as a result of the India-Asia collision.

Ca. 835–823 Ma doming extensional tectonics in the west Jiangnan accretionary orogenic belt, South China: Implication for a slab roll-back event

The Neoproterozoic Jiangnan accretionary orogenic belt recorded the accretion and collision of the Yangtze and Cathaysia blocks to form a stablized South China Block, but related geometry and kinematics is poorly constrained, leading to largely varied tectonic models. Here, we present detailed field investigation and kinematic analysis of the plutonic-metamorphic complexes in the Yuanbaoshan and Sanfang areas of the west Jiangnan orogenic belt, which enables identification of extensional granite-cored domes. In the dome margins, down-dipping lineations display a radial pattern and dome dominated foliations are extensively developed. The shearing structures within the plutonic-metamorphic complexes display extensional shearing surrounding the Yuanbaoshan and Sanfang granitic dome cores. Gneissic granites and massive ones from both the Yuanbaoshan and Sanfang plutons yield comparable crystallization ages of ca. 835–823 Ma that are within age errors of each other, as are the sheared recrystallized asymmetric quartz veins and mylonites dated at 831 Ma. Overall ages of the deformed Sibao Group and the undeformed overlying Danzhou Group, along with those of the granite plutons and mylonites, suggest formation of the granite-cored domes at ca. 835–823 Ma, coeval to the timing of emplacement of the granitic plutons. Locally, top-to-the-E thrusting structures are also observed in the west Yuanbaoshan and Sanfang areas and are inferred as at ca. 860–835 Ma, coinciding well with E- or SE- directed structures developed elsewhere in the Jiangnan orogenic belt, but in contrast with doming extensional shearing structures. Therefore, overall geometry and kinematics in the west Jiangnan belt indicate development of granitic dome related extensional ductile shearing deformation dated at ca. 835–823 Ma and a possible top-to-the-E compressional ductile thrusting deformation within 860–835 Ma. Given the previously inferred regional geology observations, along with age and chemical data across the Jiangnan orogenic belt, the dominant extensional shearing deformation in the region argue for a slab roll-back event within an accretionary belt, typical of domes-and-basins structures formed in accretionary convergent continental margin. The top-to-the-E thrusting is here interpreted as corresponding to compressional regime generated by the west directed subduction of oceanic crust beneath the northern Guangxi continental margin arc in the west Jiangnan orogenic belt.