56090452
Neoproterozoic to early Paleozoic tectonic evolution of the Zavkhan terrane of Mongolia: Implications for continental growth in the Central Asian orogenic belt
The Zavkhan terrane is a Proterozoic cratonic fragment in southwestern Mongolia that forms the core of the Central Asian orogenic belt. We provide new geologic and U-Pb zircon geochronologic constraints on the Neoproterozoic and early Paleozoic tectonic evolution of the terrane. Orthogneisses dated as ca. 1967 and ca. 839 Ma form the basement and are intruded and overlain by ca. 811–787 Ma arc-volcanic and volcaniclastic rocks that lack a gneissic fabric, suggestive of a mid-Neoproterozoic metamorphic event. Rifting and formation of the Zavkhan ribbon continent occurred from ca. 770–717 Ma and was followed by passive margin sedimentation between 717 and 580 Ma. During the latest Ediacaran to Cambrian, the southern margin of the Zavkhan terrane was reactivated with the obduction of the Lake terrane, slab break-off and reversal, and ca. 509–507 Ma magmatism. Metamorphosed Proterozoic and Cambrian units are cut by undeformed ca. 496 Ma gabbro, providing a tight constraint on the age of Cambrian metamorphism. Late Ordovician to Silurian rifting is marked by bimodal magmatism and deposition in narrow fault-bound basins. Our data indicate that the Zavkhan terrane traveled alone in the Neoproterozoic, collided with the Lake terrane in the late Ediacaran to Cambrian, accreted an unknown crustal block during Cambrian Epoch 2–Epoch 3, and then rifted away in the Ordovician. We suggest the majority of continental growth in Mongolia occurred through the trapping and oroclinal bending of ribbon continents rather than long-lived accretion on the margin of a major craton.
129479722
Tectonic models for accretion of the Central Asian Orogenic Belt
The Central Asian Orogenic Belt (c. 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.
130758529
Latest Jurassic–earliest Cretaceous closure of the Mongol-Okhotsk Ocean: A paleomagnetic and seismological-tomographic analysis
The Mongol-Okhotsk Ocean closed when the Amuria block, normally considered to have been part of the North China block since the early Mesozoic, and the southern margin of Siberia collided in Late Jurassic to Early Cretaceous times. The resulting suture runs WSW-ENE and is reasonably well defi ned to the east of longitude 100°E. Because no evidence exists for any westward prolongation of the MongolOkhotsk Ocean suture toward the Tarim block, the cryptic termination of the suture is an enigma, compounded by the fact that a tomographically identifi ed slab in the lower 1000 km of the mantle, interpreted as a remnant of Mongol-Okhotsk oceanic lithosphere, has a clear N-S trend, at almost right angles to the surface suture. No sensible explanation can be constructed for a rotation of some 90° of this slab. There is a solution, however, to both these enigmas if we consider that the Triassic MongolOkhotsk Ocean existed east of an initially meridian-parallel, but later progressively *Corresponding author; e-mail: voo@umich.edu Van der Voo, R., van Hinsbergen, D.J.J., Domeier, M., Spakman, W., and Torsvik, T.H., 2015, Latest Jurassic–earliest Cretaceous closure of the Mongol-Okhotsk Ocean: A paleomagnetic and seismological-tomographic analysis, in Anderson, T.H., Didenko, A.N., Johnson, C.L., Khanchuk, A.I., and MacDonald, J.H., Jr., eds., Late Jurassic Margin of Laurasia—A Record of Faulting Accommodating Plate Rotation: Geological Society of America Special Paper 513, p. 589–606, doi:10.1130/2015.2513(19). For permission to copy, contact editing@geosociety.org. © 2015 The Geological Society of America. All rights reserved. 590 Van der Voo et al.
131321505
Lithostratigraphic and geochronological constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia: Early Paleozoic rifting followed by late Paleozoic accretion
New SHRIMP U-Pb and evaporation Pb-Pb zircon ages, together with a revision of the lithostratigraphy of “suspect” terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian–early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. The zircon ages from early Ordovician volcaniclastic rocks and syntectonic felsic dikes reveal an heterogeneous stretching of the Precambrian Dzabkhan microcontinent (Lake Zone basement) during the Ordovician, followed by the development of a carbonate platform on a proximal margin (Gobi-Altai Zone), serpentinite breccias and Silurian chert sequences on a distal margin and possibly also the formation of oceanic crust. The assumed early Neoproterozoic South Gobi continental zone may either represent an allochthonous block detached from Dzabkhan or, less likely, the conjugate margin of a Paleo-Asian continental rift. Early Devonian volcanism subsequently affected both types of margins with back-arc spreading centers and arcs located in the core of the future Trans-Altai Zone. During the late Devonian to early Carboniferous a Japan-type magmatic arc developed on the previously stretched continental crust of the Gobi-Altai Zone. This event was associated with shortening of the entire domain, exhumation of the deep arc core and formation of intramontane basins with Devonian and Carboniferous detrital zircons of the adjacent Lake Zone continent. Clastic, flysch-type sedimentation occurred on the former distal margin and in oceanic areas. During this early Carboniferous contraction event the continental and oceanic units were imbricated and accreted to the continent in the north. Subsequently, late Carboniferous volcanic arc sequences and a Japan-type magmatic arc developed on the Trans-Altai oceanic crust and the southern South Gobi Zone, respectively. Finally, a Permian thermal event was localized in the Gobi-Altai–Lake Zone contact domain and was responsible for formation of Permian grabens, bimodal volcanism and substantial melting of the accreted crust.
130569573
Structural constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia
We provide a detailed description of the structures along a 300 km long and 50 km wide transect across the Central Asian Orogenic Belt (CAOB) in southwestern Mongolia, covering the Precambrian Dzabkhan continental domain with overthrust Neoproterozoic ophiolites in the north (Lake Zone), a Silurian-Devonian passive margin association (Gobi-Altai Zone) and oceanic domain (Trans-Altai Zone) in the center, and a continental area (South Gobi Zone) in the south. Structural analysis suggests late Cambrian collapse of the thickened Lake Zone continental crust, leading to stretching of the lithosphere and followed by Silurian-Devonian formation of oceanic crust in the Trans-Altai domain. Subsequent emplacement of Devonian-Carboniferous and late Carboniferous magmatic arcs occurred on the Gobi-Altai and South Gobi Zone crusts, respectively, during E-W shortening. Finally, the entire system was affected by N-S convergence from the Permian to Jurassic, leading to heterogeneous shortening of the orogenic domain. The model best fitting these observations is one of generalized westward drift of the Tuva-Mongol-Dzabkhan-Baydrag ribbon continents during the Silurian-Devonian, associated with westward-subduction of the Mongol-Okhotsk Ocean and sequential growth of syn-convergent magmatic arcs. Back-arc basins opened during this period in the area of the western Paleoasian Ocean. The present-day shape of the CAOB in southern Mongolia was probably formed during Permian to Mesozoic anticlockwise rotation and folding of the Tuva-Mongol-Dzabkhan-Baydrag continental ribbons, combined with a strike-slip (transpressional) reactivation of ancient transform boundaries in the Paleoasian oceanic domain. All continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous Permian magmatic-thermal input. The sequence of tectonic events is tested against published paleomagnetic data, paleogeographic reconstructions and tectonic models, leading to a revised model for the accretion of juvenile crust to a continental margin in the CAOB of southern Mongolia.
129031022
Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model
Abstract The Altaids are an orogenic collage of Neoproterozoic–Paleozoic rocks located in the center of Eurasia. This collage consists of only three oroclinally bent Neoproterozoic–Early Paleozoic magmatic arcs (Kipchak, Tuva–Mongol, and Mugodzhar–Rudny Altai), separated by sutures of their former backarc basins, which were stitched by new generations of overlapping magmatic arcs. In addition, the Altaids host accreted fragments of the Neoproterozoic to Early Paleozoic oceanic island chains and Neoproterozoic to Cenozoic plume-related magmatic rocks superimposed on the accreted fragments. All these assemblages host important, many world-class, Late Proterozoic to Early Mesozoic gold, copper–molybdenum, lead–zinc, nickel and other deposits of various types. In the Late Proterozoic, during breakup of the supercontinent Rodinia, the Kipchak and Tuva–Mongol magmatic arcs were rifted off Eastern Europe–Siberia and Laurentia to produce oceanic backarc basins. In the Late Ordovician, the Siberian craton began its clockwise rotation with respect to Eastern Europe and this coincides with the beginning of formation of the Mugodzhar–Rudny Altai arc behind the Kipchak arc. These earlier arcs produced mostly Cu–Pb–Zn VMS deposits, although some important intrusion-related orogenic Au deposits formed during arc–arc collision events in the Middle Cambrian and Late Ordovician. The clockwise rotation of Siberia continued through the Paleozoic until the Early Permian producing several episodes of oroclinal bending, strike–slip duplication and reorganization of the magmatic arcs to produce the overlapping Kazakh–Mongol and Zharma-Saur–Valerianov–Beltau-Kurama arcs that welded the extinct Kipchak and Tuva–Mongol arcs. This resulted in amalgamation of the western portion of the Altaid orogenic collage in the Late Paleozoic. Its eastern portion amalgamated only in the early Mesozoic and was overlapped by the Transbaikal magmatic arc, which developed in response to subduction of the oceanic crust of the Paleo-Pacific Ocean. Several world-class Cu–(Mo)-porphyry, Cu–Pb–Zn VMS and intrusion-related Au mineral camps, which formed in the Altaids at this stage, coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs. Major Pb–Zn and Cu sedimentary rock-hosted deposits of Kazakhstan and Central Asia formed in backarc rifts, which developed on the earlier amalgamated fragments. Major orogenic gold deposits are intrusion-related deposits, often occurring within black shale-bearing sutured backarc basins with oceanic crust. After amalgamation of the western Altaids, this part of the collage and adjacent cratons were affected by the Siberian superplume, which ascended at the Permian–Triassic transition. This plume-related magmatism produced various deposits, such as famous Ni–Cu–PGE deposits of Norilsk in the northwest of the Siberian craton. In the early Mesozoic, the eastern Altaids were oroclinally bent together with the overlapping Transbaikal magmatic arc in response to the northward migration and anti-clockwise rotation of the North China craton. The following collision of the eastern portion of the Altaid collage with the Siberian craton formed the Mongol–Okhotsk suture zone, which still links the accretionary wedges of central Mongolia and Circum-Pacific belts. In the late Mesozoic, a system of continent-scale conjugate northwest-trending and northeast-trending strike–slip faults developed in response to the southward propagation of the Siberian craton with subsequent post-mineral offset of some metallogenic belts for as much as 70–400 km, possibly in response to spreading in the Canadian basin. India–Asia collision rejuvenated some of these faults and generated a system of impact rifts.
128612510
The Early Baikalian crystalline complex in the basement of the Dzabkhan microcontinent of the Early Caledonian orogenic area, Central Asia
Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.
128930150
Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent
Abstract The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units of the TMM, occurred in the short time interval of 497±4 to 489±3 Ma. We propose that there is no simple basement-cover relationship in the Tuvino-Mongolian Massif. Instead, the massif consists of tectonic domains, composed of heterogeneous metasedimentary successions with distinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite facies conditions. We suggest that this thrusting event was associated with early Palaeozoic collisional processes related to amalgamation of the Central Asian Mobile Belt. Our new model for the evolution of the TMM implies derivation of Neoproterozoic terrigeneous metasediments from ∼800–900 Ma granitoids formed in an Andean-type active continental margin setting.
129037514
Late Riphean alkali granites of the Zabhan microcontinent: Evidence for the timing of Rodinia breakup and formation of microcontinents in the Central Asian Fold belt
The estimation of chronological boundaries in the geological history of the Rodinia supercontinent, in particular, the age of its breakup is far from a final solution. The enormous size of the supercontinent rules out synchronization of geological events throughout its territory. In addition, the estimation is complicated by unreliable reconstructions of positions of particular cratons within the supercontinent and a shortage of geochronological data on substantiation of the timing of breakup in separate parts of Rodinia. Most likely, this was a long-term process similar to that of the breakup of Pangea, which lasted for almost 150 Ma from the Early Jurassic to the Early Cenozoic [1]. The long-term character of these events is evidenced by the available geochronological data on the processes of rifting that initiated the breakup in various parts of Rodinia. For example, according to the reconstruction [2], two age levels of rifting are established beyond the Laurasian part of the supercontinent. The older event occurred from 830 to 795 Ma ago. The younger event (780‐ 745 Ma ago) completed the breakup of the continental lithosphere. The Laurasian part of Rodinia was broken into the Siberian and Laurentian continents 720‐ 630 Ma ago [3]. The breakup of Rodinia promoted the origin of the Paleoasian ocean, the evolution of which produced the Central Asian Fold belt (CAFB). The terranes (microcontinents) of the Precambrian crust within the fold belt are regarded as fragments of supercontinent margins [3]. Such an interpretation is supported by structural and historical similarities of the terranes with some continental massifs in Rodinia and by wide occurrence of shelf complexes therein. However, the timing of separation of these terranes from the supercontinent and their initial location remain uncertain. In this communication, new data on the isotopic age and composition of the Late Riphean alkali granites of the Zabhan Terrane established in the CAFB are reported for the first time and the timing of the breakup and approximate position of this microcontinent in Rodinia is outlined. Geological characteristics. The Zabhan microcontinent (Fig. 1) represents terranes with an Early Precambrian basement, which are rare in the CAFB. The oldest metamorphic rocks of the Baidarik Block are subdivided into the Upper Archean Baidarag and the Lower Proterozoic Bumbuger crystalline complexes [4]. The stages of the microcontinent evolution are broadly correlated with those of the North Chinese and Siberian cratons [4]. The collisional processes responsible for the formation of the main tectonic units of these cratons and the microcontinent occurred almost synchronously 1.90‐1.85 Ga ago. In the northeast, the basement rocks are unconformably overlapped by primarily greenschist-facies rocks of the Ul’dzit-Gol Complex (metasandstones, black shales, and marmorized dolomites) of presumably Middle‐lower Upper Riphean age. Based on the K‐Ar actinolite dating, the age of greenschist-facies metamorphism of rocks of this complex is estimated at ~840 Ma [5]. In the western part of the microcontinent, the basement rocks are overlain by gently dipping subaerial volcanics of the Zabhan Group [6]. They are composed of virtually unmetamorphosed violet, black, and redbrown subaerial volcanic glasses, vitreous rhyodacites and trachyrhyolites, as well as ignimbrites with rare small feldspar and quartz phenocrysts. The subordinate basic and intermediate volcanic rocks are usually confined to the base of the group and to its roof in some places [6]. Their share increases toward the western
129471846
Proterozoic supercontinent, its latest Precambrian rifting, breakup, dispersal into smaller continents, and subsidence of their margins: Evidence from Asia
Several large cratons and numerous smaller blocks, all with Precambrian sialic crust, are seen clearly in the present-day structure of Asia. The outer parts of larger Precambrian terranes and entire smaller ones are covered with Vendian-early Paleozoic shallow-water carbonate sedimentary rocks coupled with the so-called "old" (late Vendian-Early Cambrian) phosphorites in Siberia, southern China, India, Kazakhstan, and Mongolia. These sedimentary rocks are interpreted to represent a global belt of near-equatorial sedimentation on passive continental margins of numerous Vendian-early Paleozoic continents with free access to the world ocean. Later Precambrian rifting is recorded clearly in many Precambrian terranes of Asia; rifting was repeated during Cenozoic time, with the same pattern and in the same places. It was preceded and accompanied by various magmatic activity from 750 to 600 Ma. Tillites filling rifts are about 650 Ma, and are believed to be a feature of a single Proterozoic super-continent. Rift-to-drift transition, as attested by the beginning of thick and laterally continuous carbonate sedimentation on passive margins, seems to be diachronous; it started in early Vendian time or even earlier in some places, but not earlier than Cambrian in others.
128963385
A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia
Abstract We present a new terrane synthesis for Mongolia that incorporates geological, geochemical and geochronological data from more than 60 years of Mongolian, Russian and joint international studies. Forty-four terranes are distinguished and classified into cratonal, metamorphic, passive margin, island arc, forearc/backarc, accretionary complex, or ophiolitic types. New detailed stratigraphic columns for all terranes are presented which summarize the geological evolution of each terrane. Our analysis reveals that small Precambrian cratonic blocks in the Hangay region acted as a central nucleus around which Paleozoic arcs, backarc/forearc basin assemblages, associated subduction complexes and continental slivers were accreted. The temporal and spatial order of accretion and amalgamation was complex and probably not simply from north to south with time. The timing of terrane accretion is partly constrained by sedimentary overlap assemblages and post-amalgamation intrusive complexes. The main stages of amalgamation occurred during the Neoproterozoic, Cambrian–Ordovician, Devonian, Pennsylvanian–Permian and Triassic. The arcuate trends of terranes around the central Hangay region provide the first-order structural grain for Mongolia. This crustal anisotropy has played a major role in controlling the geometry and kinematics of all subsequent Phanerozoic deformation and reactivation of structures in the region, including the Cenozoic development of the Altai and Gobi Altai. Our results provide the most detailed synthesis to date of the basement geology of Mongolia which should provide an important crustal framework for interpreting the Phanerozoic tectonic evolution of a large part of Central Asia. In addition, our synthesis allows the economic resources of Mongolia to be placed in a modern tectonic context.
127629985
Recognition, Character and Analysis of Tectonostratigraphic Terranes In Western North America.
The pacific margin of North America is composed of many allochthonous terranes that accreted to the cratonal nucleus during Mesozoic and early Cenozoic time. More than 150 terranes have now been recognized, each of which is characterized by distinctive rock assemblages and geologic histories that differ markedly from those of their neighbors. These terranes are composed of scraps of island arcs, disrupted oceanic basins with abundant radiolarian chert, parts of seamounts, continental-rise prisms, and continental fragments, some of which seem to be of North American affinity. Combined paleomagnetic, paleobiogeographic, and lithologic data substantiate that some terranes have moved northward as much as several thousand kilometers; some terranes now at high latitudes may have been situated in the Southern Hemisphere during Triassic time. Tectonic effects during accretion include polyphase isoclinal folding, large-scale thrust and strike-slip faulting, development of penetrative metamorphic fabrics on a regio...
132340147
Paleomagnetism of the Teel basalts from the Zavkhan terrane: Implications for Paleozoic paleogeography in Mongolia and the growth of continental crust
A narrow extensional basin on the Zavkhan terrane of Mongolia exposes a >1.8-km-thick succession of basalt flows within the Teel Formation, along with rhyolites and interflow sediments. We present new U-Pb zircon ages of 446.03 ± 0.21 Ma (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) on a rhyolite in the Teel Formation and 286 ± 5 Ma (laser ablation–inductively coupled plasma–mass spectrometry) on a nearby granitic intrusion (Tonkhil Complex). New paleomagnetic data yield a magnetite remanence that is likely primary, acquired during cooling of flows. The mean direction is statistically improved after tilt corrections; however, the tilt test significance is limited given the low variation in tilt between flows. We interpret a second remanence, held by hematite, as an overprint that was likely acquired later in the Paleozoic Era. The tilt-corrected magnetite direction implies a paleolatitude of ∼20°, while the hematite overprint is equatorial in both geographic and tilt-corrected coordinates. The ca. 446 Ma Teel remanence is consistent with an Ordovician paleogeographic position near Siberia; however, the hematite direction requires subsequent drift to the equator, indicating that these Mongolian terranes were not continuously connected to Siberia, which moved away from the tropics during the Paleozoic Era. This result is consistent with biogeographic constraints and a previously proposed model wherein Amuria traveled with North China during the Permian Period and collided with Siberia during the Jurassic to Triassic closure of the Mongol-Okhotsk Ocean. In this model, continental growth occurred through the collision and oroclinal buckling of a ribbon continent rather than long-lived accretion on the margin of a major craton.
54041105
Phlogopite- and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia
We present field relationships, petrography, and mineral major and trace element data for the Neoproterozoic Dariv Igneous Complex of the Altaids of Western Mongolia. This unique complex of high-K plutonic rocks is composed of well-exposed, km-scale igneous intrusions of wehrlites, phlogopite wehrlites, apatite-bearing phlogopite clinopyroxenites, monzogabbros, monzodiorites, and clinopyroxene-bearing monzonites, all of which are intruded by late stage lamprophyric and aplitic dikes. The biotite-dominated igneous complex intrudes depleted harzburgitic serpentinite. The observed lithological variability and petrographic observations suggest that the plutonic rocks can be ascribed to a fractionation sequence defined by olivine + clinopyroxene ± Fe–Ti oxides → phlogopite + apatite → K-feldspar + plagioclase → amphibole + quartz. Notably, phlogopite is the dominant hydrous mafic mineral. Petrogenesis of the observed lithologies through a common fractionation sequence is supported by a gradual decrease in the Mg# [molar Mg/(Fetotal + Mg) × 100] of mafic minerals. Crystallization conditions are derived from experimental phase petrology and mineral chemistry. The most primitive ultramafic cumulates crystallized at ≤0.5 GPa and 1,210–1,100 °C and oxygen fugacity (fO2) of +2–3 ∆FMQ (log units above the fayalite–quartz–magnetite buffer). Trace element modeling using clinopyroxene and apatite rare earth element compositions indicates that the dominant mechanism of differentiation was fractional crystallization. The trace element composition of a parental melt was calculated from primitive clinopyroxene compositions and compares favorably with the compositions of syn-magmatic lamprophyres that crosscut the fractionation sequence. The parental melt composition is highly enriched in Th, U, large ion lithophile elements, and light rare earth elements and has a pronounced negative Nb–Ta depletion, suggestive of an alkaline primitive melt originating from a subduction-imprinted mantle. Comparison with a global compilation of primitive arc melts demonstrates that Dariv primitive melts are similar in composition to high-K primitive melts found in some continental arcs. Thus, the high-K fractionation sequence exposed in the Dariv Igneous Complex may be a previously unrecognized important fractionation sequence resulting in alkali-rich upper crustal granitoids in continental arc settings.
40164971
Neoproterozoic stratigraphy of the Zavkhan terrane of Mongolia: The backbone for Cryogenian and early Ediacaran chemostratigraphic records
The Neoproterozoic Tsagaan-Olom Group is exposed in the Zavkhan Terrane of southwestern Mongolia and hosts unique geochemical, paleoclimate, and paleontological records that have become central to our understanding of this pivotal interval of Earth history. New sedimentological, stratigraphic, geochronological, and geochemical data provide context for and further develop these records. Detrital zircon provenance indicates that Neoproterozoic strata of the Zavkhan Terrane were derived from basement with age peaks between 1950 to 2100 and 2400 to 2600 Ma. At ∼800 Ma, the Zavkhan Terrane transformed from an active arc and back-arc complex to a rifted ribbon continent with passive margins on both sides. Deposition was accommodated by extension, which is recorded with syn-sedimentary normal faulting and alluvial fan deposition in the Zavkhan and Khasagt formations. Passive margin sedimentation in the overlying Tsagaan-Olom Group begins with the glacigenic Maikhan-Uul Formation, which consists of two massive diamictite units separated by clast-poor graded beds of the middle member. Detrital zircon at the base of the middle member of the Maikhan-Uul Formation were dated with U-Pb chemical abrasion isotope-dilution thermal ionization mass spectrometry and constrained its age to <729.8 ± 1.4 Ma. This, along with chemostratigraphy and Re/Os geochronological constraints from the overlying Taishir Formation, supports our correlation of the Maikhan-Uul Formation with the ∼717 to 660 Ma Sturtian glaciation. The Taishir Formation was deposited on a carbonate ramp in four large-scale sequence tracts that thin to the southwest. The Taishir Formation preserves a large negative δ13C excursion referred to as the Taishir excursion that covaries in carbonate and organic carbon isotopes in limestone sections. A dolomitization front at the top of the Taishir Formation also results in depleted δ13C values, however, these are related to local processes and do not represent a global Trezona excursion. Although δ13C values in the Ol Formation are highly variable along strike, 0.70756 initial strontium isotope values in limestone of the upper Ol Formation are consistent with earliest Ediacaran values. A sandstone-filled karst surface at the top of the Shuurgat Formation that overlies the Ol Formation defines the top of the Tsagaan-Olom Group and is interpreted to mark a major unconformity. Carbon and strontium isotope values in the uppermost Shuurgat Formation are also consistent with early Ediacaran values and suggest that most of the late Ediacaran Period is missing in the Zavkhan Terrane of Mongolia. Carbon isotope profiles from sections preserved as limestone and dolostone display large differences and indicate that isotopic data from dolomites should be used with caution. With our new data and correlations, we construct composite Cryogenian and Ediacaran carbon and strontium isotope curves from limestone-dominated successions in Mongolia, and then integrate additional geochronological and geochemical data sets from around the globe.
39426821
A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations
The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.
20745149
Integrated stratigraphic, geochemical, and paleontological late Ediacaran to early Cambrian records from southwestern Mongolia
In their comment, Landing and Kruse disagree with some of our discussion and, at times, use blanket statements to incorrectly summarize from the Smith et al. (2016) paper. This is not the venue for a discussion and rehashing of the details discussed at length in the text of the publication. Here, we respond only to what we were able to identify as the key questions and issues raised in the comment.
130407647
Stratigraphic importance of the Devonian Flora of the USSR
Abstract In many regions of the USSR, Devonian deposits are represented by continental formations. The remains of terrestrial plants are used for their stratigraphic subdivision and correlation. Occurrences of plant remains are known from the Lower Devonian (Siberia, northeastern USSR, Kazakhstan), the Middle Devonian (in the regions mentioned above, and also in the Urals in middle Asia, Donbass, Timan and the Kola Peninsula) and the Upper Devonian (in most of the regions mentioned above). The Devonian flora includes representatives of Psilopsida, Lycopsida and Pteropsida; types of Arthropsida occur in small quantity. In addition, vast numbers of plants of unestablished systematic position are nevertheless important in stratigraphic subdivision and correlation. The plant assemblages typical of different intervals of Devonian time may be very similar or very different, depending upon ecological conditions. As a result of the recognition of distinct floral assemblages relevant to definite intervals of the stratigraphic section, phytostratigraphic successions for the territories of Donbass, Timan, Kazakhstan and West Siberia have been worked out. The study of the Devonian flora in different regions of the USSR shows that, beginning in the Early Devonian, the first distinctions in geographical distribution of separate forms appear. This permits the recognition of phytogeographic regions, in some cases provinces, which are distinguished from each other by groups of genera and proportions within these groups. The evolution of the Devonian flora in each phytogeographic region has its own peculiarities, the recognition of which is important for correlation of coeval deposits of remote districts.
56215598
Zircon Ages from the Baydrag Block and the Bayankhongor Ophiolite Zone: Time Constraints on Late Neoproterozoic to Cambrian Subduction‐ and Accretion‐Related Magmatism in Central Mongolia
Central Mongolia represents a heterogeneous crustal domain of the Central Asian Orogenic Belt and is composed of contrasting lithotectonic units with distinct preorogenic histories. We report single‐zircon evaporation and SHRIMP ages for high‐grade rocks of the Neoarchean‐Paleoproterozoic Baydrag block and for metaigneous rocks of the junction between the late Neoproterozoic Bayankhongor ophiolite zone (BOZ) and the Baydrag block. Zircon ages for metamorphic rocks of the Baydrag block indicate a major tectonothermal event between 1840 and 1826 Ma, coeval with the emplacement of granitic rocks at middle‐crustal level dated at 1839 Ma. A granite‐gneiss yielded a much younger crystallization age of 1051 Ma, the first Grenvillian age reported for this region. Together with predominantly Mesoproterozoic detrital zircon ages for a quartzite lens from the Burd Gol accretionary complex, these data attest to the heterogeneity and long Precambrian history of the Baydrag block. Crystallization ages for granite‐gneisses from the northeastern margin of the Baydrag block indicate prolonged plutonic activity between 579 and 537 Ma, probably related to southward subduction of the Bayankhongor oceanic crust. A syntectonic granite vein yielded a crystallization age of 519 Ma, probably linked to accretion of the BOZ onto the northeastern active margin of the Baydrag block. Lastly, a felsic metavolcanic rock from the southeastern termination of the BOZ yielded a crystallization age of 472 Ma and suggests that punctuated volcanic centers developed during the early Ordovician in response to protracted convergence.
130660169
Geochronological evidence for existence of South Mongolian microcontinent—A zircon U-Pb age of grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex
A zircon U-Pb age of (916±16) Ma is measured for grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex and represents the crystallization age of the grantoid magma. This age provides evidence for the existence of the South Mongolian microcontinent, which is consistent with the analysis of the regional geology.
129559761
Precambrian crystalline basement in southern Mongolia as revealed by SHRIMP zircon dating
Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai, southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian. The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.
128236141
Paleozoic accretionary orogenesis in the Paleo‐Asian Ocean: Insights from detrital zircons from Silurian to Carboniferous strata at the northwestern margin of the Tarim Craton
A detrital zircon U-Pb and Lu-Hf isotopic study was carried out in the Middle Silurian to Late Carboniferous sedimentary strata of the northwestern Tarim Craton in order to understand accretionary processes in the southern part of the Central Asian Orogenic Belt. Detrital zircons from these strata yielded U-Pb ages clustering around 2.8–2.3 Ga, 2.0–1.7 Ga, 1.3–0.9 Ga, 880–600 Ma, and 500–400 Ma, with age populations and Hf isotopic signatures matching those of magmatic rocks in the Tarim Craton and the Central Tianshan Block. Abundant 500–400 Ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an Andean-type magmatic arc to the north, supporting the northern Tarim-Central Tianshan connection during early Paleozoic time. The absence of 380–310 Ma zircon population in the Carboniferous siliciclastic rocks suggests that the Central Tianshan Block may have been separated from the Tarim Craton in the Early Devonian, caused by the interarc/back-arc opening of the South Tianshan Ocean. We propose an accretionary orogenic model switching from advancing to retreating mode during Paleozoic time in the southwestern part of the Paleo-Asian Ocean. This transition most likely occurred coevally with the rifting of Southeast Asian blocks from the northeastern margin of Gondwana.
140647112
Neoproterozoic tectonic evolution of the Precambrian Aksu blueschist terrane, northwestern Tarim, China: Insights from LA-ICP-MS zircon U–Pb ages and geochemical data
Abstract The Precambrian Aksu blueschist terrane (ABT) located in the northwestern Tarim Craton was formerly regarded as a Mesoproterozoic or an early Neoproterozoic complex. Yet, its tectonic significance remains poorly understood due to the lack of reliable age. We have conducted a detrital zircon U–Pb geochronological study of both the metasedimentary rocks from the ABT and the unmetamorphosed sandstones from the overlying Sinian succession to better constrain the age of the ABT. In addition, geochemical analyses were performed on the metasedimentary rocks to establish the broad tectonic setting of the source region of sediments. Our first U–Pb dating results suggest a maximum deposition age of ca. 730 Ma for the protolith of the metasedimentary rocks in the ABT and a maximum deposition age of 602 Ma for the unmetamorphased Sinian sandstones immediately overlying the ABT. Therefore, the blueschist-facies metamorphism in the ABT must have taken place after ca. 730 Ma, but prior to 602 Ma. This metamorphism may manifest the Pan-African orogeny (ca. 700–500 Ma), which is related to the assemblage of Gondwana, in the northern Tarim. Furthermore, the age range of 1.3–0.9 Ga was not recorded in the detrital zircons from both the metasedimentary rocks and the Sinian sandstones, suggesting that the northern Tarim Craton may not be significantly affected by the Grenville-age orogeny. A major age population at Paleopreoterozoic (ca. 2.0–1.8 Ga) was found in all samples, implying a Paleoproterozoic orogeny in the northern Tarim, which is coincident with the timing of the orogeny associated with the assembly of the Columbia supercontinent. Taking together the geochemical and chronological data, we propose that the northern margin of the Tarim Craton was probably a late Neoproterozoic active continental margin and a major source for the sedimentary rocks of the ABT, which provided a mixture of both old recycled sedimentary material from the basement rocks and juvenile material from the igneous rocks.
132299569
Activation of northern margin of the North China Craton in Late Paleozoic: Evidence from U-Pb dating and Hf isotopes of detrital zircons from the Upper Carboniferous Taiyuan Formation in the Ningwu-Jingle basin
LA-MC-ICPMS U-Pb dating has been performed on detrital zircons from the Upper Carboniferous Tai-yuan Formation (N-8) in the Ningwu-Jingle Basin, west of the North China Craton (NCC). The ages of 72 detrital zircon grains are divided into three groups: 303–320 Ma (6 grains), 1631–2194 Ma (37 grains, peaked at 1850 Ma), 2318–2646 Ma (29 grains, peaked at 2500 Ma). Detrital zircons of Group 2 and Group 3 were likely derived from the basement of the NCC. Group 1 zircons exhibit 176Hf/177Hf ratios ranging from 0.281725 to 0.282239, with corresponding negative ɛHf(t) values of −12.4–−30.3 and old Hf model ages of 1.4–2.2 Ga. These characteristics show a strong resemblance to those of Carboniferous igneous zircons from the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCC, but differ significantly from those of the Xing-Meng Orogenic Belt, suggesting that the source of the Taiyuan Formation partly came from the IMPU. All detrital zircons of Group 1 have relatively high Th/U ratios (> 0.67), indicating a magmatic origin. The mean age (304 ± 6 Ma) of the two youngest grains is close to the depositional age of the Taiyuan Formation, suggesting a strong tectonic uplift and magmatism in the IMPU during the Late Carboniferous. This paper provides important geological evidence for the activation of the northern margin of the NCC in the Late Paleozoic.
140190729
Detrital zircon provenance of early Palaeozoic sediments at the southwestern margin of the Siberian Craton: Insights from U–Pb geochronology
Abstract Detrital zircons from the Ordovician and Devonian sedimentary cover of the Siberian Craton were analyzed for U/Pb geochronology to understand their sediment provenances. Five main age-peaks were identified in the zircon U/Pb age-spectra: (1) Neoarchaean – early Palaeoproterozoic (2.7–2.4 Ga); (2) late Palaeoproterozoic (2.0–1.65 Ga); (3) minor early Neoproterozoic (1.0–0.75 Ga); (4) Ediacaran (0.65–0.60 Ga) and (5) Cambrian – Early Ordovician (0.54–0.47 Ga), reflecting the main magmatic events in the sediment source regions. The oldest zircons (groups 1 and 2) are derived from the Siberian Craton which amalgamated during the Neoarchean – Palaeoproterozoic. The Neoproterozoic zircons (groups 3 and 4) likely sourced from southwestern basement uplifts and Neoproterozoic belts of the Siberian margin such as the Yenisey Ridge and Baikal-Muya region. The provenance of the youngest zircons (group 5) can be traced to the Altai–Sayan fold-belt, where peri-Gondwanan microcontinents and island-arcs accreted to Siberia during late Neoproterozoic – early Palaeozoic progressive consumption of the Palaeo-Asian Ocean.
128886472
Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: Significance for the origin of crustal fragments in the Central Asian Orogenic Belt
Abstract The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments. The Mongolian pattern is dominated by zircons in the age range ca. 350–600 and 700–1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event. The 700–1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India. Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt.
55819135
Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite
An unusual late Neoproterozoic (c. 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.
128900695
Composition, sources, and mechanisms of formation of the continental crust of the Lake zone of the Central Asian Caledonides. II. Geochemical and Nd isotope data
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.
140593503
Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data
Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments. The volcanic rocks of the basalt complex are noted for high concentrations of TiO2 and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO2 concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane was formed.
73611684
Mid-Ordovician and Late Devonian magmatism in the Togtokhinshil Complex: new insight into the formation and accretionary evolution of the Lake Zone (western Mongolia)
New field, petrographic, geochemical and geochronological data from the Togtokhinshil Complex (western margin of the Lake Zone, Central Asian Orogenic Belt) reveal the presence of two separate Early Palaeozoic magmatic pulses: Mid-Ordovician and Late Devonian. The Mid-Ordovician (c. 460 Ma) magmatism produced various types of mafic plutonic rocks (gabbro–diorite suite) emplaced into the volcanosedimentary sequence of the Baatar Fm. Gabbros to gabbrodiorites have metaluminous and mostly low to normal-K calc-alkaline or possibly tholeiitic character. The majorand trace-element as well as Nd isotopic signatures suggest its magmatic-arc geotectonic setting and imply the derivation either exclusively from the mantle or in part from the juvenile mafic crust. The Late Devonian (c. 376 Ma) granite suite intruded the rocks of both the gabbro–diorite suite and the Baatar Fm. Granites are subaluminous and have a high-K calc-alkaline to shoshonitic character. Their magmatic arc-like geochemical signature is interpreted as inherited from the recycled arc-related crustal source. The granite suite seems a product of extensive mantle heat-induced crustal anatexis. The Depleted Mantle Nd model ages (c. 0.75 Ga) indicate derivation of the magmatic rocks of the both suites from relatively juvenile, and geochemically immature, sources. These results provide a clear evidence of the Mid-Ordovician arc-related magmatism in the western part of the Lake Zone. They thus prolong the assumed period of magmatic-arc activity and constrain onset of accretionary processes in this part of the Central Asian Orogenic Belt. The gabbro–diorite suite represents either part of a long-lived magmatic-arc or, more likely, a member of multiple island-arc system along the western margin of the Lake Zone. Newly described Late Devonian magmatism in the Togtokhinshil Complex provides further evidence of tectono-thermal event that was widespread in the Central Asian Orogenic Belt. The geodynamic cause for this event remains uncertain, but effects of lithospheric thinning and/or asthenospheric mantle upwelling are considered the most likely.
128445148
Devonian arc-related magmatism in the Tseel terrane of SW Mongolia: chronological and geochemical evidence
Abstract: The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Palaeozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 ± 3 Ma (2σ) and 397 ± 3 Ma (2σ) for two calc-alkaline metarhyolite samples indicate arc volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 ± 2 Ma (2σ). Geochemical and Nd isotopic data for low-grade mafic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc–back-arc setting. In addition, there are high- and low-grade felsic metaigneous rocks with subduction-related chemical signatures and heterogeneous Nd isotopic compositions indicating an origin from mixed sources, including juvenile and Precambrian crustal components. The field observations combined with the geochemical and isotopic data suggest that the Tseel terrane represents a segment of a mature Japan-type arc associated with a juvenile back-arc basin. Supplementary material: Latitudes and longitudes of sample localities, major- and trace-element abundances for rock samples, data for reference materials obtained by LA-ICP-MS and Sm–Nd isotopic data for rock samples are available at http://www.geolsoc.org.uk/SUP18338.
129379343
Geophysical and geochemical nature of relaminated arc‐derived lower crust underneath oceanic domain in southern Mongolia
The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.
128584002
Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia
Lower Paleozoic successions of the Gobi-Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarse- and fine-grained marine siliciclastic and carbonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyperpycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some component of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pragian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bounding faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proximal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history.
56087353
Astrophyllite-alkali amphibole rhyolite, an evidence of early Permian A-type alkaline volcanism in the western Mongolian Altai
A dyke of alkali rhyolite intrudes the Tsetseg and Zuun Nuruu volcanosedimentary sequence of Ordovician–Silurian age (Hovd Zone, Central Asian Orogenic Belt) at the Botgon bag, Mankhan Soum, Hovd District in Western Mongolia. The rock consists of quartz and K-feldspar phenocrysts set in fine-grained groundmass composed of quartz, K-feldspar, albite, blue alkali amphibole (riebeckite–arfvedsonite containing up to 1.94 wt. % ZrO2), tiny brown radial astrophyllite, annite and accessory zircon, ilmenite, fluorite, monazite, hematite, chevkinite and bastnäsite. Astrophyllite has unusual, highly ferroan composition and occurs as two sharply bound zones of astrophyllite I and II with the average empirical formulae: (K1.71 Na0.01Rb0.08Cs0.01) (Na0.93Ca0.07) (Fe 2+ 6.52Mn0.31Zn0.06) (Ti0.84Zr0.50Nb0.55) Si7.68Al0.32 O26 (OH)3.78 F0.66 (astrophyllite I, Zr–Nb-rich); (K1.52Rb0.07) (Na0.81Ca0.19) (Fe 2+ 6.31 Mn0.28Zn0.06) (Ti1.28Nb0.30Zr0.28) Si7.68Al0.32 O26 (OH)2.85 F0.67 (astrophyllite II). Geochemically, the rhyolite corresponds to strongly fractionated silicic alkaline A-type (ferroan) magmatic rock with 75.5–75.9 wt. % SiO2, 4.4 wt. % K2O, 3.9–4.3 wt. % Na2O and 1.98–2.23 wt. % Fe2O3 t, poor in CaO (0.26–0.37 wt. %), MgO (0.01–0.11 wt. %), and P2O5 (0.01 wt. %). The rock is enriched in Zr, Nb, Ta, Ga, Sn, Y, Rb, Cs, U and Th, depleted in V, Sr, Ba, Sc, and exhibits a pronounced negative Eu anomaly (Eu/Eu* = 0.03–0.05). The conventional whole-rock K–Ar geochronology yielded an age of 299.9 ± 9.1 Ma (1σ), which indicates latest Carboniferous or early Permian extension associated with the A-type alkaline volcanic activity.