Arc Accretion Research Articles

The Rio Piranhas-Seridó Domain, Borborema Province, Northeastern Brazil: Review of geological-geochronological data and implications for stratigraphy and crustal evolution

This paper presents a review of available lithostratigraphic and isotopic data, in addition to other information collected from projects of the Geological Survey of Brazil (SGB-CPRM), for the Rio Piranhas-Serido Domain (PSD). This domain is one of the tectonostratigraphic compartments of the central-northeast portion of the Borborema Province (Northeast Brazil), bound by the Picui-Joao Câmara, Patos and Portalegre shear zones. More recent isotopic determinations (U-Pb dating of zircon) and geological maps (usually on a 1:100000 scale) have clearly detected and/or redefined several units, e.g., the Amarante, Campo Grande, Granjeiro and Saquinho complexes, in addition to the Serra do Inga body (Archean), Arabia Complex (Siderian), Caico Complex and Serra da Formiga Suite (Rhyacian), Poco da Cruz Suite (Orosirian-Statherian), Serido Group (Ediacaran) and Ediacaran to Early-Cambrian magmatic suites, in addition to pegmatites and granitic pegmatites. With respect to the supracrustal rocks of the Serido Group, the deposition in the Ediacaran is based on field relations and on the ages of detrital zircons, which showed a greater contribution from Mesoarchean to Neoproterozoic sources, the vast majority of which may come from the PSD itself. Current records indicate that the evolution of the PSD started in the Eoarchean, and peaked in the Neoproterozoic with the Brasiliano event, which is characterized by sedimentation of the Serido Group, intrusion of plutonic bodies, deformation and metamorphism (especially at the regional level), in addition to the formation or reactivation of important shear zones. Such evolution is acknowledged on the basis of the accretion of oceanic arcs during the Paleoproterozoic (containing Archean fragments), including the generation of juvenile magmas and a protocrust (Caico Complex). The Neoproterozoic record, associated with the Brasiliano event, is considered by several authors as reworking with no or little contribution of juvenile material, with deposition of the Serido Group, emplacement of several Ediacaran to Early-Cambrian magmatic suites, accompanied by the formation of shear zones, predominantly in the southern and eastern portion of the domain.

Tectonic evolution and multi–episodic metallogenesis of the Sanjiang Paleo-Tethys multi-arc-basin-terrane system, SW Tibetan Plateau

The Sanjiang Tethys region in SW China lies in the tectonic junction of the Tethys domain, and represents a key precious/base metal province in China. Based on a compilation of studies from the past decades on the formation, evolution, and metallogenesis of the tectonic units in the region, we proposed a multi-arc-basin-terrane (MABT) structural framework for the Paleo-Tethys Sanjiang region. Convergence between South China and Indochina-Pu'er blocks had led to the accretion of many island arcs and micro-continental terranes, which is different from the traditionally direct convergence between two continents. Such prolonged terrane accretion process likely formed the various metallogenetic belts that contain diverse deposit types and mineral assemblages. In this contribution, we divided the Sanjiang region into ten metallogenetic belts, according to their characteristic lithologic assemblages, structures, and interpreted tectonic settings . Subsequently, we discussed the regional tectonic-metallogenic evolution of these ten ore belts and propose their metallogenetic models. This study reveals a genetic link between the MABT system and the regional metallogenesis, which is important for future mineral exploration in this region.

Deciphering crustal growth in the southernmost Arabian Shield through zircon U-Pb geochronology, whole rock chemistry and Nd isotopes

ABSTRACT New U-Pb zircon geochronology using high-spatial resolution secondary ion mass spectrometry fills a data gap and provides crystallization ages for granitoids from the Asir composite terrane in the southernmost Arabian Shield of Saudi Arabia. Ages of c. 810–685 Ma, c. 663–636 Ma, and 625–610 Ma reflect oceanic island arc genesis, subduction-related arc accretion (syn-collisional), and post-collisional stabilization, respectively. All samples have juvenile εNd(t) compositions with no evidence of older material being involved in their genesis, indicating that this part of the Arabian Shield grew through juvenile magmatic addition and that assimilation by syn- and post-tectonic magmatism involved an isotopically juvenile component(s). The crustal thickness derived from the (La/Yb)N proxy indicates significant thickening from 10–20 km to c. 70 km at c. 650 Ma, consistent with timing of orogenic uplift and increasing crustal thickness post-dating peak Nabitah orogeny. The age of an intrusion cross-cutting the Atura formation, when combined with other data, provides a well-constrained depositional age of c. 646–625 Ma for the Atura formation and indicates that erosion of the orogenic edifice in this part of the Arabian Shield began at latest by 625 Ma. Our new data indicate that denudation occurred 80–100 m.y. before the development of the prominent sub-Cambrian peneplain, consistent with previous assertions that major pulses of denudation occurred prior to the waning stages of Nabitah orogenesis.

Sulfur isotope evidence for surface-derived sulfur in Eoarchean TTGs

The conditions giving rise to the first evolved melts preserved on Earth remain a subject of debate. These melts formed tonalite-trondhjemite-granodiorites (TTGs) comprising the first cratonic cores. Constraining the nature of the rocks that melted to form these TTGs is crucial to understanding the mechanism that produced the first continents. Previous studies have indicated that TTGs' source rocks are hydrated mafic lithologies similar to modern arc tholeiitic basalts, comparable to amphibolites embedded in greenstone belts within Archean cratons.To elucidate the geodynamic setting of TTG formation, we investigated 3.9-3.6 Ga TTGs and amphibolites from the Itsaq Gneiss Complex (IGC), southern West Greenland using multiple sulfur isotope signatures, as well as textural and compositional analysis of selected sample sulfides. Small but significant nonzero Δ33S and Δ36S values were measured in the TTGs, with Δ33S values from 0.00‰ to +0.30‰, and Δ36S values from -0.13‰ to +0.80‰. Amphibolites yielded Δ33S values of -0.01‰ and +0.14‰, and Δ36S values of +0.08‰ and +0.23‰. These values are consistent with the presence of sedimentary sulfur, likely introduced to the sources of the TTGs' precursor rocks via horizontal tectonics. Sulfur in the TTGs was likely subject to mass independent fractionation on Earth's surface and may have experienced subsequent 34S enrichment during metamorphism in the TTGs' source rocks. Relative enrichment in 34S and 36S in TTGs may be explained by incorporation of fluid sampling hydrothermal deposits within the thickened mafic crust, released during arc accretion. This fluid may have triggered partial melting, forming the TTGs. Hence, the sulfur isotope composition of the TTGs represents a mixture of material derived from the source rocks by melting and from fluid released from other rocks bearing hydrothermally derived sulfur. Our results add weight to existing models of modern-like tectonic processes active in the Eoarchean.

Metamorphic evolution of the juvenile Serrinha forearc basin in the southern Brasiliano Orogen

Understanding the accretion of an oceanic island arc to a craton is a major issue in unravelling the evolution of the continents. P-T-t conditions registered in metamorphic rocks, the boron isotopic compositions of tourmaline provide essential information to delimit the accretionary processes. The Tonian-Cryogenian São Gabriel Terrane is a prominent juvenile structure in the southern Brasiliano Orogen that records key information about accretionary processes. Limited previous studies have investigated the São Gabriel Terrane, with little presently known about the metamorphic conditions. To pursue the objective, we combined multiple methods, including pseudosection modeling, U-Pb garnet geochronology and δ11B isotopes of tourmaline applied to one key sample from the Serrinha Formation of the São Gabriel Terrane. The recorded peak P-T conditions occurred at 4.5–5.1 kbar and 536–555 °C. U-Pb dating of low-U (<1 ppm) garnet yielded a lower intercept age of 721 ± 14 Ma for the bulk garnet growth. Dravite isotopic δ11B (+0.51 to + 3.39) compositions indicate oceanic crust components present in the island arc rocks. The calculated P-T conditions correspond to the metamorphic peak from heat pulses during burial events. Collision and thrusting of the São Gabriel arc against and over the Rio de La Plata Craton occurred at 721 ± 14 Ma. The evaluation of the parameters obtained in this study indicate a Barrovian-type evolution and a Late Tonian age of metamorphism for the Serrinha Basin. The sedimentation of the Serrinha Basin occurred in a forearc basin of the São Gabriel island arc. Collision of the oceanic arc with the craton is thus better constrained. Our study contains one point in the evolution of the protracted, complex amalgamation of the supercontinent Gondwana during the Brasiliano Orogeny.

Structural Controls on Iron Oxide Copper-Gold Mineralization and Related Alteration in a Paleoproterozoic Supracrustal Belt: Insights from the Nautanen Deformation Zone and Surroundings, Northern Sweden

Abstract The Nautanen deformation zone in the Gällivare area of northern Sweden is a highly Cu-mineralized, magnetite-rich, large-scale shear zone with a long-lived (~100 m.y.) deformation, hydrothermal alteration, and mineralization history. This composite structure hosts the Aitik porphyry Cu-Au-Ag ± Mo deposit and several Cu-Au ± Fe ± Ag ± Mo occurrences assigned to the iron oxide copper-gold (IOCG) deposit class. The Nautanen deformation zone was a locus for polyphase deformation and intermittent metasomatic-hydrothermal activity that overprinted middle Orosirian (ca. 1.90–1.88 Ga) continental arc-related volcanic-plutonic rocks. The deformation zone is characterized by intense shearing fabrics that form a series of subvertical to moderately W-dipping, NNW-SSE–trending, first-order shear zones with oblique reverse kinematics and related NNE-SSW–oriented second-order shear zones that control hydrothermal alteration patterns and Cu-Au mineralization. Hydrothermal alteration in the study area formed during several phases. Volcanic-volcaniclastic rocks to the east and west of the Nautanen deformation zone display low to moderately intense, pervasive to selectively pervasive (i.e., patchy zones or bands, disseminations) sericite ± feldspar, amphibole + biotite + magnetite ± tourmaline, and K-feldspar + hematite alteration. Both the amphibole + biotite and K-feldspar + hematite associations occur adjacent to NNW- and NE-oriented deformation zones and are locally associated with minor sulfide. Within the deformation zone, a moderate to intense biotite + amphibole + garnet + magnetite + tourmaline + sericite alteration assemblage is typically associated with chalcopyrite + pyrrhotite + pyrite and forms linear and subparallel, mainly NNW-oriented seams, bands, and zones that locally appear to overprint possibly earlier scapolite + sericite ± feldspar alteration. Late-stage epidote ± quartz ± feldspar alteration (retrograde saussuritization) forms selectively pervasive zones and epidote veinlets across the area and is partly related to brittle faulting. A magnetite-amphibole-biotite–rich, penetrative S1 foliation records shortening during early Svecokarelian-related deformation (D1) and can be related to ca. 1.88 to 1.87 Ga arc accretion processes and basin inversion that overlaps with regional peak metamorphism to near mid-amphibolite facies conditions and a potential initial Cu mineralization event. Folding and repeated shearing along the Nautanen deformation zone can be assigned to a second, late-Svecokarelian deformation event (D2 stage, ca. 1.82–1.79 Ga) taking place at a higher crustal level. This D2 deformation phase is related to late-stage accretionary processes active during a transition to a stage of postorogenic collapse, and it was accompanied by abundant, syntectonic intrusions. D2-related magmatism produced high-temperature and low-pressure conditions and represents a regional magmatic-hydrothermal event that controlled the recrystallization/remobilization of magnetite, biotite, and amphibole. Associated shear zone reactivation during D2 favors the utilization of the Nautanen deformation zone as a fluid conduit, which preferentially controlled the siting and formation of epigenetic Cu-Au mineralization with distinctive IOCG characteristics within second-order shear zones.

Early Paleozoic arc-accretion in the northern branch of the Proto-Tethys Ocean: New insights from detrital zircon U[sbnd]Pb ages and geochemistry of paraschists from the Kuanping Complex, North Qinling Orogenic Belt, China

The Proto-Tethys Ocean existed from the Neoproterozoic through early Paleozoic during the breakup of Rodinia and was closed with the subduction and accretion of numerous East Asian blocks. The North Qinling Orogenic Belt is thought to have formed upon the closure of the Kuanping Ocean between the North China Craton and the Qinling Block. However, the timing of this closure is widely debated between Neoproterozoic and Paleozoic. To date, this controversy remains as a barrier in understanding the ocean-continent configuration and closure process of the Proto-Tethys Ocean. The Kuanping Ocean is best represented by the Kuanping Complex, which is the northernmost unit in the North Qinling Orogenic Belt and consists of a wide range of meta-sedimentary rocks. Here we present a systematic study of the petrology, whole-rock geochemistry and geochronology of paraschists from the complex. The paraschists yield new detrital zircon UPb ages from 500 Ma to 3584 Ma with two main peaks at 798 Ma and 958 Ma and a less dominant peak at 2460 Ma. Based on our investigations we infer that the protoliths of the paraschists were dominantly wackes and minor litharenites and shales with maximum depositional ages from 578 to 500 Ma, which represents different levels of a sedimentary sequence in an arc-related basin. Their clastic input was sourced mainly from the Qinling Block, with minor addditions from the North China Craton. Combining our results with previous data, we also infer that the Kuanping Complex represents an early Paleozoic accretionary wedge assemblage with existence of late Paleoproterozoic to early Neoproterozoic continental nuclei. Initial intra-oceanic subduction in the Erlangping-Kuanping Ocean occurred at ca. 578 Ma with a northward subduction polarity towards the continental nuclei, resulting in the formation of the Erlangping island arc and the protoliths of meta-sedimentary rocks in the Kuanping Complex in an arc-related basin until ca. 500 Ma. During the accretion of the Erlangping arc and the Kuanping Complex, oceanic crust subducted underneath this continental amalgam at 500–490 Ma. Northward continental subduction of the Qinling Block beneath the consolidated Erlangping-Kuanping arc terrane at 490–460 Ma and their final accretion to the southern margin of the North China Craton at ca. 440 Ma marked the closure of the northern branch of the Proto-Tethys Ocean. However, sedimentation continued in few of the remaining residual oceanic basins until 436 Ma. Our new model suggests that the Erlangping-Kuanping Ocean was part of the northern branch of the Proto-Tethys Ocean and closed in the early Paleozoic with a northward subduction polarity.

Appinitic and high Ba[sbnd]Sr magmatism in central Brazil: Insights into the late accretion stage of West Gondwana

Large amounts of high BaSr magmatism can be produced as the result of late accretion or decompression in the final stages of continental arc evolution in orogenic systems and leads to continental crustal growth. This type of magmatism often occurs associated with mega-shear zones and displays geochemical signatures that indicate magmas deriving from the melting of enriched lithospheric mantle. Such enrichment may be an indication of slab-derived fluid and/or melt interaction, released from subducted material. The high BaSr granitoids of the Brasiliano Paraguay Belt, Tocantins Province, central Brazil, comprise two suites that crystallized approximately 40 My apart from each other. Both suites are composed of metaluminous to slightly peraluminous rocks (ASI = 0.64–1.09) with magnesian character, and present high Ba (~1430 ppm), Sr (~800 ppm), K2O (~4 wt%), LREE and LILE contents. The negative εNd(T) values (−6.32 and − 6.65) associated with high contents of incompatible elements indicate an evolution from partial melting of mantle-derived magma previously metasomatized by subducted slab fluids. Crystallization ages of 557 ± 5.7 Ma and 554 ± 2.9 Ma have been obtained for the older, high MgK calc-alkaline suite. It evolved from high Cr and Ni mantle-derived appinitic magma to coeval granodiorites by crystal fractionation accompanied by minor crustal contamination. The younger, 515 ± 3.6 Ma-old suite displays an elevated concentration in some HFSEs, show alkali-calcic affinity and is represented by high-K monzogranites. A model proposed for the two hybrid suites is based on a slab break-off and a subsequent lithospheric delamination after 40 My, where the heat flux generated by mantle upwelling led to a partial melting of the enriched subcontinental lithospheric mantle beneath the continental crust. The older calc-alkaline suite represents the end process of magmatic arc accretion in the central Tocantins Province. The younger one is compatible with a transitional shoshonitic to A-type affinity, associated to post-collisional collapse stage of the orogen at Cambrian times. The petrogenesis of the investigated rocks requires an ocean closure phase at ca. 590 Ma, which could be linked to the consumption of the Clymene Ocean toward Amazonian Craton, recording a new collisional event in the central West Gondwana.

The Late Carboniferous to Early Permian high silica magmatism in the Southern Mongolia: Implications for tectonic evolution and continental growth

Tremendous granites were produced during the late-stage evolution of the Central Asian Orogenic Belt and important to understanding the tectonic processes from subduction to collision. Here, we present zircon U-Pb-Hf, and whole-rock geochemical, and Sr-Nd isotopic data for a suite of Carboniferous-Permian granites from the Khan-Bogd area of the southern Mongolia. U-Pb age dating of zircons indicates that the alkaline feldspar granites and quartz monzonite-monzodiorites on the southwest of the Khan-Bogd town were formed at ~315–310 Ma and the alkaline feldspar granites on its south at ~290 Ma. The ~315–310 Ma alkaline feldspar granites possibly result from crustal anatexis of the Devonian-Carboniferous juvenile arc crust there, evidenced by their intensive continental geochemical signals and roughly similar Nd and Hf isotopes to these arc crust. The coeval quartz monzonite-monzodiorites with limited outcrop show arc affinity. Thus, these Late Carboniferous granites indicate a sharp decrease of mantle-crust interaction from earlier arc accretion. Combined with previous work, this change was possibly triggered by a low-angle subduction. The ~290 Ma alkaline feldspar granites show A-type petrological and geochemical features. In addition, they have more variable Nb/Ta (~10–18), Nd and Hf isotopes indicative of various continental crust involvement in their magma source. The lack of adakite and arc-type magma deprives the possibility of lithosphericdelamination, ridge subduction and slab rollback. As a significant mantle input evidenced by coeval bimodal magmatism, this crust-derived granite was likely induced by a slab breakoff (~305–290 Ma), rather than backarc extension. Combined with previous work, this slab breakoff probably indicates a tectonic transition of post collision (~315–290 Ma) from subduction-accretion and marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) of the Paleo-Asian Ocean. Moreover, it plays an important role for the vertical crustal growth of the CAOB.

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental crust.

Late Cambrian calc-alkaline magmatism during transition from subduction to accretion: Insights from geochemistry of lamprophyre, dolerite and gabbro dikes in the Dzhida terrain, Central Asian orogenic belt

The Central Asian orogenic belt (CAOB) is one of the largest orogens on Earth characterized by complex and long-term history resulted from the development of the Paleo-Asian Ocean. In Late Cambrian–Ordovician, closure of Paleo-Asian basins and accretion of microcontinents, island arcs and back-arc basins to the southern margin of the Siberian continent resulted in large-scale magmatism recorded in Southern Siberia and Mongolia. The timing of terminal stage of subduction and start of accretion-collision stage is a matter of discussion for different parts of this extensive collage of terrains. In this paper, we report new data on whole-rock and mineral composition and 40Ar/39Ar age of calc-alkaline dolerites, lamprophyres and gabbros of Bayangol river area (Northern Mongolia) from the Dzhida terrain of CAOB. The Bayangol calc-alkaline mafic rocks were previously considered as a member of boninite-basaltic sequence originated in a primitive island arc. We found that the Bayangol calc-alkaline mafic rocks compose late mafic dikes, cutting terrigenous sediments and serpentinite mélange with boninite blocks, and, therefore, calc-alkaline rocks are not related to boninites. We obtained 40Ar/39Ar age of 498.9 ± 7.7 Ma for amphibole from Bayangol lamprophyre. The calc-alkaline rocks preserve clinopyroxene (diopside, Mg# of 76–90) and amphibole (mainly pargasite and magnesiohastingsite, Mg# of 38–92), which crystallized at 1170–1210°С, 2.2–9.5 kbar and 760–1030°С, 2.0–7 kbar, respectively. Dolerites, lamprophyres and gabbros show variations in major oxides (3.3–14.9% MgO, 10.1–19.4% Al2O3, 0.09–0.92% P2O5) and immobile trace element systematics (e.g., Th and LREE enrichment, HFSE depletion) corresponding to island arc calc-alkaline series. Isotope compositions of Nd (εNd(T) = −0.7 to +2.5) and Sr (87Sr/86Sr(T) = 0.7049–0.7078) suggest 1–5% addition of recycled terrigenous sediment melts to mantle wedge source of the Bayangol mafic rocks. Relations of trace element and SiO2 abundances and NdSr isotope ratios are resulted from fractional crystallization and do not support in situ crustal contamination of Bayangol magmas. Within the Dzhida terrain, the Bayangol mafic rocks have Nd isotope compositions transitional between those of Late Cambrian island arc intrusions with juvenile signatures and Early Ordovician collisional granitoids with crustal characteristics. Therefore, the Bayangol calc-alkaline mafic rocks mark transition from subduction to the accretion-collision regime during accretion of Dzhida island arc to the Siberian continent as a result of Paleo-Asian Ocean closure in SW Transbaikalia and Northern Mongolia.

Preliminary discussion on tectonic setting of the magmatic-metamorphic complex in the northern Dunhuang region: Insight into southward extension style of the southern Central Asian Orogenic Belt

中亚造山带南缘如何向南扩展，对深入理解增生型造山作用和大陆地壳生长机制以及中亚构造域与特提斯构造域的衔接具有重要科学意义。作为中亚造山带南缘的关键构造单元，敦煌构造带大地构造属性长期备受关注且颇有争议。传统观点认为敦煌构造带是古亚洲洋南侧的前寒武纪稳定大陆地块，以刚性块体的形式参与了中亚造山带南缘的最终拼贴过程。然而，近年来研究认为敦煌构造带卷入了古亚洲洋南部的俯冲增生造山过程，属于中亚造山带南缘的增生系统。显然，这一争议限制了对中亚造山带南缘向南扩展方式及增生造山过程的理解。敦煌北部三危山地区出露一套古生代岩浆-变质杂岩，是解开这一争论的关键。本文综合前人研究基础及新的资料，归纳了这套岩浆-变质杂岩的野外岩石-构造组合、地球化学和年代学等方面特征：该岩浆-变质杂岩整体显示二元结构特征，即较老的增生杂岩为基底，弧岩浆岩侵入或不整合覆盖其上；其中岩浆岩属于中钾-高钾钙碱性系列中酸性岩浆岩，富集大离子亲石元素（LILE）和轻稀土元素（LREE），亏损高场强元素（HFSE），与典型的弧岩浆岩类似，并且微量元素组成特征反映中酸性岩浆的源区与俯冲沉积物部分熔融有关；岩浆作用大致归为510Ma、460~410Ma和370~360Ma三期。岩浆岩中结晶锆石不一致的ε_Hf（t）值（既有正值，又有负值）以及继承锆石的存在表明，岩浆源区既有古老地壳物质的加入，也有新生地壳物质的形成。以上这些特征与发育在增生杂岩之上的增生弧十分类似，因此本文提出敦煌北部岩浆-变质杂岩的属性为古生代增生弧，并且该增生弧与其南部的红柳峡俯冲增生杂岩共同勾勒出敦煌构造带自北向南增生弧-增生杂岩的基本构造格架，即敦煌构造带的大地构造属性实为造山带而非稳定地块。结合区域地质背景及敦煌地区与北山地区古生代至早中生代构造-热事件的对应关系，认为敦煌造山带属于中亚造山带中段南缘的增生系统，中亚造山带中段以增生弧-增生杂岩的形式向南扩展至敦煌地区。;The southward extension style of the southern Central Asian Orogenic Belt (CAOB) is significant for understanding the process of accretionary orogeny and growth mechanism of continental crust, as well as the connection between the Central Asian tectonic domain and Tethys tectonic domain. As a key tectonic unit located in the southern CAOB, the tectonic attribution of Dunhuang tectonic belt has long been concerned and controversial. Traditionally, the Dunhuang tectonic belt is considered to be a Precambrian stable continental block on the southern side of the Paleo-Asian Ocean (PAO), which participated in the final collage process of the southern margin of the CAOB in the form of rigid plates. However, in recent years, studies have suggested that the Dunhuang tectonic belt is involved in the subduction-orogenic process in the southern PAO, and belongs to the accretionary system of the southern CAOB. This controversy has limited the understanding of the southward expansion mode and accretive orogenic process of the southern margin of the CAOB. The focus of the debate is whether the Dunhuang tectonic belt is a Precambrian block or a Phanerozoic orogenic belt. The Paleozoic magmatic-metamorphic complex in the Sanweishan area is the key to reveal the tectonic attribution of Dunhuang tectonic belt. Integrating the published and new data, this paper sums up the field rock-structure association, geochemistry, and geochronology of the magmatic-metamorphic complex. In the field, the magmatic-metamorphic complex show dual structure, namely the older accretionary complex as the basement, and the arc magmatic rocks intrude or unconformity overlie on it. The magmatic rocks belong to medium potassium-high potassium calc-alkaline series, enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE), which resembles the arc magmatic rocks. And the trace elements compositions indicate that the magma sources are perhaps related to partial melting of subducted sediment. The magmatism can be roughly classified into three periods: 510Ma, 460~410Ma, and 370~360Ma. The discordant ε_Hf(t) values (both positive and negative) of crystalline zircons in magmatic rocks and the existence of inherited zircons indicate that both the reworking of ancient crust and juvenile material were added to the magmatic sources. These characteristics are very similar to the accretionary arcs, so we propose that the magmatic-metamorphic complex in the northern Dunhuang region represent a Paleozoic accretionary arc. This accretionary arc and the southside Hongliuxia subduction-accretion complex collectively outline the basic tectonic framework of the Dunhuang orogenic belt. In combination with the relations of tectono-thermal events between Dunhuang and Beishan regions during Paleozoic to Early Mesozoic eras, it is considered that the Dunhuang orogenic belt belongs to the accretionary system of the southern margin of the middle section of CAOB. The middle section of CAOB extends southward to Dunhuang region in the form of accretionary arc-accretionary complex.

Evidence for a complex accretionary history preceding the amalgamation of Columbia: The Rhyacian Minas-Bahia Orogen, southern São Francisco Paleocontinent, Brazil

The Minas-Bahia orogeny juxtaposed Archean crustal fragments and Paleoproterozoic magmatic arcs to form the São Francisco-Congo Paleocontinent by the Rhyacian (ca. 2.05 Ga). Unravelling the Minas segment of the Minas-Bahia Orogenic Belt (MBO) is an important key to understanding the role of the São Francisco-Congo Paleocontinent in the construction of the Columbia Supercontinent. The Orosirian (ca. 1.9 Ga) final amalgamation of Columbia was preceded by a complex history of accretion of Archean nuclei and Proterozoic magmatic arcs. We present new whole-rock element geochemistry and isotopic (Rb-Sr, Sm-Nd) data and U-Pb ages for granitoid rocks of the main basement complexes located in the southern part of the São Francisco cratonic tip, which displays varied degrees of Neoproterozoic reworking related to the Brasiliano orogeny. Published data for the Campo Belo Complex, the Mineiro Belt and the Piedade Block are combined with the new data set to propose an integrated model for the tectonic evolution of the Minas segment of the MBO. This evolutionary model documents a complete Paleoproterozoic orogenic cycle, from subduction with terrane accretion to collision, followed by late-orogenic collapse. Subduction started diachronously between ca. 2.4 Ga and 2.2 Ga involving various Archean nuclei and Paleoproterozoic magmatic arcs that were later amalgamated during two collisional events at ca. 2.10 and 2.05 Ga. The oldest tonalite-trondhjemite-granodiorite (TTG) to sanukitoid magmatic suites transition are of Neoarchean age in the Piedade block, and of Paleoproterozoic age in the Mineiro belt and Mantiqueira complex, apparently indicating a diachronous onset of plate tectonic processes in different crustal segments. The petrogenesis, geochronology and isotopic signatures of these granitoid rocks provide important evidence towards understanding the periodicity of tectonic processes associated with the supercontinent cycle throughout Earth history.

The accretion history of the South China Block at its northwest margin in the Neoproterozoic: Records from the Changba complex in the Mianlue zone

To constrain the accretion history of the South China Block (SCB), a geochronological and geochemical study was carried out for the Changba Complex within the Mianlue zone in the northwestern margin of the SCB. Zircon U-Pb dating reveals that the metagabbros and metatonalites were formed at ca. 985–950 Ma, while the bt-amp gneisses have a younger protolith age of ca. 830 Ma. The metagabbro and metatonalite are low- to medium-K tholeiitic series and are significantly enriched in LILEs (Rb, Ba, Th, U, K) and slightly enriched in LREEs, but depleted in HFSEs such as Nb, Ta and Ti. In addition, they have highly positive zircon εHf(t) values (+11.3~ +16.9) and whole-rock εNd(t) values (+3.9~ +5.5) and εHf(t) values (+11.0 ~ +11.8). These geochemical features resemble to those of the magmatic rocks in juvenile oceanic arcs. Zircon δ18O value ranges from 3.0 to 6.2‰ for metagabbro and 4.6 to 5.34‰ for metatonalite, which are slightly lower than the mantle values. Hydrothermally altered gabbros in the lower oceanic crust often have lower δ18O values than mantle and can be primary components of arc lower crust. Therefore the Changba tonalite is interpreted to have been formed by partial melting of the hydrothermally altered gabbroic lower crust. The ca. 830 Ma gneisses are enriched in LREEs and incompatible elements and depleted in HFSEs, similar to the trace element pattern of the average bulk continental crust. They have enriched Sr-Nd-Hf isotopic compositions (0.7055 ~ 0.7060, −5.1 ~ −6.3, and −8.5 ~ −8.8, respectively), which is distinct from the juvenile arc crust. Besides, variable zircon εHf(t) values of −40.6 to 0 and slightly low to elevated zircon δ18O values of 4.2 to 6.6‰, suggesting derivation from reworking of a heterogeneous crustal source. In this regard, the Changba complex records the accretion of juvenile island arc and subsequent reworking of both juvenile and ancient crustal materials, which highlights the crustal growth and geodynamical evolution of the northern margin of the South China Block during the Neoproterozoic in response to the assembly of Rodinia.

Structural diversity within a thrust complex reflecting progressive overprinting during a protracted orogenic process of terrane accretion, Natal belt, South Africa

The Tugela Terrane, forming the northern segment of the Mesoproterozoic Natal belt, South Africa, represents an example of accretionary tectonics that were initiated by closure of the Tugela Ocean basin, and culminated in accretion of magmatic arcs against the southern margin of the Archaean Kaapvaal Craton during the assembly of Rodinia. A detailed structural analysis of the Madidima thrust sheet, part of the Tugela Terrane, reveals five distinct deformation events, D1 to D5. Deformation varied between dominantly non-coaxial during D2, D3 and D5, and dominantly coaxial during D4, the latter being interpreted as a phase of gravitational collapse. While D2 ductile shearing characterizes the most pervasive event that defines the tectonic grain of the sheet, most, if not all, of the structural imprints can be understood as representing consecutive stages of deformation overprinting during a protracted orogenic process of arc-continent convergence. The observed structural complexity is partly due to this multiple overprinting in the course of orogeny, but also due to the broadly synchronous formation of different types of folds within the thrust sheet, and the corresponding variation of foliation orientations. The persistently north-east-directed tectonic transport during at least three of the deformation events is accompanied by intense transposition of early-formed structures, multiple foliation reactivation and formation of transpression-related structures.

Petrogenesis of early Carboniferous bimodal-type volcanic rocks from the Junggar Basin (NW China) with implications for Phanerozoic crustal growth in Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the world's largest and longest-lived accretionary orogens that has been regarded as an important area of crustal growth during the Phanerozoic, yet the growth mechanism is still under debate. A combined study of petrology, geochronology and geochemistry for drilling-sampled early Carboniferous volcanic rocks from the Junggar Basin (NW China), southwestern CAOB, aims to constrain their petrogenesis and implications for Phanerozoic crustal growth. The volcanics show a bimodal-type suite, dominated by basalts, basaltic andesites and subordinate rhyolites, and have consistent zircon U–Pb ages of 334–336 Ma. The mafic rocks exhibit arc-like trace-element distribution patterns characterized by enrichment of LILEs (e.g., Pb) and LREEs but depletion of HFSEs (e.g., Nb, Ta and Ti). They have juvenile isotopic signatures of zircon ɛHf(t) (+11.4 to +15.4) and whole-rock ɛNd(t) (+7.8 to +8.5), with initial (87Sr/86Sr)i of 0.7040–0.7054 and (206Pb/204Pb)i of 17.681–17.817. These geochemical features, together with their variable Ba/La (4.28–44.2) but low (Tb/Yb)N (1.24–1.85) and Sm/Yb (1.65–3.44) ratios, suggest that the mafic volcanics could be derived from a main spinel-bearing mantle source metasomatized by subduction-related fluids. In contrast, the felsic samples are geochemically equivalent to A2-type granite, with high contents of SiO2, Zr, Ga, and show strong depletions in Eu, Sr and Ti. They have zircon Hf (ɛHf(t) = +12.0 to +15.3) and whole-rock Sr-Nd-Pb isotopic compositions ((87Sr/86Sr)i = 0.7032–0.7038; ɛNd(t) = +7.8 to +8.1; (206Pb/204Pb)i = 17.973–18.004) similar to the coeval mafic rocks. These observations, along with the model calculations using MELTS, favor a derivation of felsic rocks from the mafic magma by fractional crystallization. We suggest that the early Carboniferous bimodal-type volcanism formed in a localized back-arc extension within an overall convergent setting, presumably triggered by slab roll-back of the subducting Paleo-Junggar Ocean plate. This bimodal-type magmatism recorded a significant vertical crustal growth event in the Junggar Basin during the Phanerozoic. We propose that oceanic subduction (flare-ups) not only leads to the lateral accretion of arcs, but also results in the vertical growth of mantle-derived materials and subsequent magma differentiation during slab roll-back (flare-lulls); this may considerably change our view of the models of crustal growth in CAOB.

Remnants of Earth's Oldest Continental Crust Formed by Subduction

AbstractEoarchean (3.6 – 4.0 Ga) tonalite – trondhjemite – granodiorite (TTG) are the major component of Earth's oldest persevered continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths (e.g., Moyen and Martin, 2012), but whether this requires subduction remains enigmatic (e.g., Palin et al., 2016). Recent studies show that most early Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction (e.g., Johnson et al., 2017). We have identified a suite of Eoarchean tonalitic gneisses dated at ∼3.7 Ga from the Tarim Craton, northwestern China (Ge et al., 2018). These rocks are probably the oldest high‐pressure TTGs so far documented worldwide. Thermodynamic and trace element modelling demonstrates that the parent magma may have been generated by water‐fluxed partial melting of moderately enriched arc‐like basalts at 1.8–1.9 GPa and 800–830 °C, indicating an apparent geothermal gradient (400–450°C GPa ‐1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high‐pressure TTGs were generated by partial melting of a subducted proto‐arc during arc accretion. Our model implies that modern‐style plate tectonics was operative, at least locally, at ∼3.7 Ga and was responsible for generating some of the oldest continental nuclei.

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo‐Asian Ocean

AbstractAs the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt (CAOB) have always been one of the hot topics among geologists (Sengör et al., 1993, 1996; Jahn et al., 2000a; Badarch et al., 2002; Windley et al., 2007; Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino‐Korean and Tarim paleo‐continents and the final collision between these continental margins because of the subduction of the Paleo‐Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite (serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the “ophiolite trinity”, were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U‐Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian (275–280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan (1997), indicate that the oceanic basin was not closed in early‐middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid‐ocean ridge basalt (N‐MORB), enriched mid‐ocean ridge basalts (E‐MORB), oceanic island basalt (OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo‐Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

Squeezing Water From a Stone: H2O In Nominally Anhydrous Minerals From Granulite Xenoliths and Deep, Hydrous Fractional Crystallization

AbstractWater is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H2O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H2O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H2O contents range from ~60 to ~650 ppm. Intermineral H2O ratios overlap experimental mineral/melt D values and are used to calculate H2O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H2O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.

Late Silurian to early Devonian development of the Chingiz accretion arc, West Junggar: insights into accretion arc evolution in the Central Asia Orogenic Belt

ABSTRACT Early Palaeozoic evolution of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB) is key to reconstructing the Palaeozoic tectonic framework in Central Asia. This study presents field mapping, geochemistry, and geochronological data from the Wulasitai area of northern West Junggar to constrain the subduction polarity and major tectonic events in the Chingiz arc of the Kazakhstan orocline during the Early Palaeozoic. The mapping area outcrops imbricated coherent turbidite slices and mélanges that consist of chert, limestone, silt, sandstone, and conglomerate blocks in mudstone matrix, representing an OPS (Ocean Plate Stratigraphy) mélange in an accretionary complex. Existing fossil ages from these blocks range from the Ordovician to Silurian, while our new detrital zircon U-Pb samples from the turbiditic matrix yield maximum depositional ages (MDTs) from 428 ± 2 Ma to 450 ± 1 Ma. Detrital zircon ages of ~510 to 430 Ma in the mélange suggest the Chingiz arc to the south of the study area as a major sediment source, providing an indirect constraint on the polarity of subduction. We suggest that the northern part of the Chingiz arc may be underlain by southward subduction, with the accretionary complex located on the Wulasitai area of the northern West Junggar. The Wulasitai mélange is overlain depostionally by volcanic rocks carrying depleted HFSE and enriched LILE, LREE. The andesite and tuff are dated to 415 ± 5 Ma, 414 ± 3 Ma, and 402 ± 6 Ma, which we interpret as reflecting the development of the volcanic arc onto the previous subduction complex. This model implies the generation of the arc (accretion arc) as the slab rolling back and trench retreating, which may play an important role in the evolutionary history of CAOB.