scholarly journals Magmatic record of India-Asia collision

2015 ◽  
Vol 5 (1) ◽  
Author(s):  
Di-Cheng Zhu ◽  
Qing Wang ◽  
Zhi-Dan Zhao ◽  
Sun-Lin Chung ◽  
Peter A. Cawood ◽  
...  
Keyword(s):  
Mafic Rock ◽  
Oceanic Lithosphere ◽  
Indian Plate ◽  
Geochemical Data ◽  
Southern Tibet ◽  
Collision Zone ◽  
Slab Rollback ◽  
Continental Arc ◽  
Slab Breakoff ◽  
Sudden Decrease

Abstract New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80–40 Ma migrated from south to north and then back to south with significant mantle input at 70–43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52–51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma.

Gangdese magmatism in southern Tibet and India–Asia convergence since 120 Ma

2018 ◽  
Vol 483 (1) ◽  
pp. 583-604 ◽  
Author(s):  
Di-Cheng Zhu ◽  
Qing Wang ◽  
Sun-Lin Chung ◽  
Peter A. Cawood ◽  
Zhi-Dan Zhao
Keyword(s):  
Oceanic Lithosphere ◽  
Southern Tibet ◽  
Ridge Subduction ◽  
Slab Rollback ◽  
Rate Acceleration ◽  
Slab Breakoff ◽  
Magmatic Temperature ◽  
Intrusive Rocks ◽  
Rate Deceleration ◽  
Compositional Diversity

AbstractA compilation of 290 zircon U–Pb ages of intrusive rocks indicates that the Gangdese Batholith in southern Tibet was emplaced from c. 210 Ma to c. 10 Ma. Two intense magmatic pulses within the batholith occur at: (1) 90 ± 5 Ma, which is restricted to 89–94° E in the eastern segment of the southern Lhasa subterrane; and (2) 50 ± 3 Ma, which is widespread across the entire southern Lhasa subterrane. The latter pulse was followed by a phase of widespread but volumetrically small, dominantly felsic adakitic intrusive rocks at 16 ± 2 Ma. The Linzizong volcanism in the Linzhou Basin was active from 60.2 to 52.3 Ma, rather than 69–44 Ma as previously estimated. During 120–75 Ma, Gangdese Batholith magmatism migrated from south to north, arguing against rollback of the downgoing, north-dipping Neo-Tethyan oceanic lithosphere for the generation of the 90 ± 5 Ma magmatic pulse. Petrological, geochemical and metamorphic data indicate that this pulse was likely to have been generated through subduction of the Neo-Tethyan oceanic ridge lithosphere. Subsequent Gangdese Batholith magmatism propagated both south and north during 70–45 Ma, and finally concentrated at the southern margin of the Lhasa Terrane at 45–30 Ma. The enhanced mafic magmatism since c. 70 Ma, magmatic flare-up with compositional diversity at c. 51 Ma and increased magmatic temperature at 52–50 Ma are interpreted as the consequences of slab rollback from c. 70 Ma and slab breakoff of the Neo-Tethyan oceanic lithosphere that began at c. 53 Ma. The India–Asia convergence was driven by Neo-Tethyan subduction with a normal rate of convergence at 120–95 Ma, ridge subduction at 95–85 Ma, then subduction of a young and buoyant oceanic lithosphere after ridge subduction with rate deceleration at 84–67 Ma, Deccan plume activity and slab rollback with rate acceleration at 67–51 Ma, slab breakoff for sudden drop of the convergence rate at c. 51 Ma, and finally the descent of the high-density Indian continental lithosphere beneath Asia since c. 50 Ma.

Eocene post-collisional magmatism in Himalaya orogenic belt: evidence from petrography, zircon U-Pb age and Sr-Nd-Hf isotope of the Mayum alkaline complex, southern Lhasa subterrane

2020 ◽  
Author(s):  
Xiaoshuang Chen ◽  
Haijin Xu
Keyword(s):  
Oceanic Lithosphere ◽  
Hf Isotope ◽  
Alkaline Magmatism ◽  
Early Eocene ◽  
Alkaline Complex ◽  
Southern Tibet ◽  
Quartz Syenite ◽  
Alkaline Rocks ◽  
Slab Breakoff ◽  
T Values

<p>Alkaline magmatism is commonly generated in extensional settings, playing an important role in constraining the timing of slab breakoff. Eocene post-collisional magmatism is widely distributed along the Gangdese belt of southern Tibet. However, few Eocene post-collisional alkaline magmatism has been identified. Here, we present a comprehensive study of whole-rock geochemistry, zircon U-Pb ages and Sr-Nd-Hf isotopes of the Mayum alkaline complex from the Southern Lhasa Subterrane, providing an insight into the timing of breakoff of the Neo-Tethyan slab. The alkaline complex is composed of amphibolite syenite, quartz syenite and alkaline granite. The mafic microgranular enclaves are ubiquitous in the syenites. Zircon U-Pb analyses indicates that the alkaline rocks were generated in Early Eocene (ca. 53-50 Ma). These ages suggest that the alkaline rocks emplaced shortly (10-15Ma) after the continental collision between the Indian and Eurasian plates. The alkaline rocks have high SiO<sub>2 </sub>(64.32-77.36 wt.%), Na<sub>2</sub>O + K<sub>2</sub>O (6.63-9.03 wt.%) contents, low MgO (0.14-2.52 wt.%) contents. These rocks show obvious arc-like geochemical features in trace elements, i.e., enrichment in LILEs (e.g., Rb, K), LREEs, Th and U, and depletion in HFSEs (e.g., Nb, Ta, Ti), HREEs with strongly to moderately negative Eu anomalies (δEu=0.28–0.72). These features together with high FeO<sup>T</sup>/MgO, Ga/Al, Ce/Nb and Y/Nb values, and low Ba, Sr contents, suggesting that the Mayum alkaline rocks belong to an A2-type granitoids. Besides, the alkaline rocks have homogeneous initial <sup>87</sup>Sr/<sup>86</sup>Sr ratios (0.7052-0.7059) and negative ε<sub>Nd</sub>(t) values (-2.1 to -0.9) for whole-rock, and positive zircon ε<sub>Hf</sub>(t) values (+0.73 to +11.16). Nd-Hf isotope decoupling suggests that the alkaline was likely produced by mixing of mantle- and crust-derived magmas under a post-collisional extensional setting. Combined with previous published results, we propose that the slab breakoff of the subducting Neo-Tethyan oceanic lithosphere at least prior to Early Eocene (ca. 53Ma). The Eocene Mayum alkaline complex might be related to asthenosphere upwelling trigged by the slab breakoff.</p>

Late Cretaceous magmatism in the NW Lhasa Terrane, southern Tibet: Implications for crustal thickening and initial surface uplift

2019 ◽  
Vol 132 (1-2) ◽  
pp. 334-352 ◽  
Author(s):  
Ming Lei ◽  
Jian-Lin Chen ◽  
Ji-Feng Xu ◽  
Yun-Chuan Zeng ◽  
Qiu-Wei Xiong
Keyword(s):  
Late Cretaceous ◽  
Oceanic Lithosphere ◽  
Crustal Thickening ◽  
Geochemical Data ◽  
Initial Surface ◽  
Southern Tibet ◽  
Late Mesozoic ◽  
Lhasa Terrane ◽  
Surface Uplift ◽  
Cretaceous Magmatism

Abstract Crustal thickening and uplift of southern Tibet have been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies indicated that the crust of the northwestern (NW) Lhasa Terrane was thickened during the late Mesozoic. Here we report geochronological and geochemical data for the Gaerqiong diorite porphyries (GPs) and Xiongma plutons (XPs) in the NW Lhasa terrane, southern Tibet. Zircon U-Pb dating suggests that these intrusive rocks were generated at ca. 85 and ca. 88 Ma, respectively. The GPs are characterized by high MgO, Cr, and Ni contents, and they have adakitic affinities. These geochemical features, combined with their depleted εNd(t) (+1.7 to +2.0), 87Sr/86Sr(i) (0.705103–0.705259), and zircon εHf(t) (+5.2 to +10.2) isotopic compositions, indicate that the GPs were produced by partial melting of the delaminated juvenile continental crust. In contrast, the XPs are composed of host granites and mafic microgranular enclaves (MMEs). The MMEs have low SiO2 and high MgO contents, and low εHf(t) (–14.0 to –5.8) values, indicating that their parental magmas were derived from an enriched mantle. The host granites have high SiO2 and low MgO contents, and variable εNd(t) (–7.4 to –6.3) and zircon εHf(t) (–11 to –4.1) values. These observations, combined with the presence of MMEs in the Xiongma granites, suggest that the host granites were the result of mixing of crust- and mantle-derived magmas. Detailed study of these two plutons, combined with the previous researches, suggests that Late Cretaceous (ca. 90 Ma) magmatism in the NW Lhasa Terrane occurred in a post-collisional extensional setting related to delamination of the regionally thickened lithosphere after collision of the Lhasa-Qiangtang Terranes. We propose that the crust of the NW Lhasa Terrane reached a maximum thickness (average of >50 km) before the Late Cretaceous (ca. 90 Ma). This crustal thickening was caused by underplating of mafic magmas during slab roll-back and break-off of the southward-subducting Bangong-Nujiang oceanic lithosphere and subsequent tectonic thrusting during Qiangtang-Lhasa Terrane collision, respectively. Given that crustal thickening generally results in elevated terrain, the regional uplift (driven by isostasy due to crustal thickening) probably commenced before the Late Cretaceous (ca. 90 Ma).

Plate kinematic and mantle seismic tomography constraints for the India-Asia collision, Part II: Insights from Myanmar to Sulawesi

2021 ◽  
Author(s):  
Andrew Parsons ◽  
Karin Sigloch ◽  
Kasra Hosseini
Keyword(s):  
Plate Boundary ◽  
Oceanic Lithosphere ◽  
Continental Collision ◽  
Indian Plate ◽  
Spreading Ridge ◽  
Lateral Migration ◽  
Southern Tibet ◽  
Cross Sectional ◽  
Kinematic Constraints ◽  
Australian Continent

<p>The India-Asia collision is one of the most globally significant tectonic events of the Cenozoic era. It is widely cited as providing a unique natural laboratory for studying collisional tectonics, offering invaluable insights of processes associated with continental collision across a multitude of scales. Yet despite its importance, significant debate continues to surround the validity of three mutually exclusive models to explain the India-Asia collision. These include: (1) the single subduction model; (2) the double subduction model; and (3) the Greater India Basin hypothesis. In our recent review (Parsons et al. 2020, Earth-Science Reviews) we demonstrated that available constraints from the Himalayan orogen and Tibetan plateau, including tomographic analysis of subducted slabs beneath these regions, are unable to robustly define the relative likeliness of each model. In this contribution, we expand upon the work of Parsons et al. (2020), with geological, geophysical, and plate kinematic constraints from the southern Eurasian margin between Myanmar and Sulawesi.</p><p>Our analysis focuses on the interpretation of subducted oceanic lithosphere beneath Myanmar to Sulawesi and includes a cross sectional area-based restoration of actively subducting India-Australia plate oceanic lithosphere. Our results provide a new restoration for the southern Eurasian margin and the India-Australia plate boundary (the Wharton ridge) during the India-Asia collision. Our integration of plate kinematic constraints with tomographic interpretation of subducted slabs suggests that the plate boundary between the Indian continent and southern Tibet migrated ~1000-2000 km northwards during collision. This includes ~1000 km lateral migration of subducted Indian plate oceanic lithosphere, now imaged beneath northern India and southern Tibet.</p><p>Our reconstruction proposes that northward migration of the India-Tibet suture and subducted Indian plate oceanic lithosphere initiated at ~43 Ma and reflects a major plate network reorganisation event. At this time, “hard collision” of the Indian continent with southern Eurasia occurred synchronously with (1) reduction in Indian plate velocity; (2) cessation of the Wharton ridge and coupling of the Indian and Australian plates; (3) subduction initiation of Australian plate oceanic lithosphere beneath southeast Eurasia, and onset of northeast migration of the Australian continent; (4) accelerated ocean spreading between Australia and Antarctica; and (5) southwest ridge jump of the Central India spreading ridge. Buoyancy of the Indian continent kept it afloat, whilst oceanic lithosphere to the east continued to drive wholesale motion of the coupled India-Australia plate. This forced the Indian continent to migrate northwards, dragging the subducted Indian plate oceanic slab with it, which effectively unzipped the coupled India-Australia plate along the extinct Wharton ridge, during subduction.</p><p>Our findings are most consistent with models (2) and (3), which are characterised by two collisions, the latter of which occurred between India and the Eurasian margin at ~45-40 Ma. More generally, our study demonstrates how changes in the balance of forces within a plate network, caused by events such as continental collision, can lead to significant plate network reorganisations. Such events can have dramatic effects on the position and geometry of subducted slabs and should be considered when interpreting plate restorations from deep-mantle structure.</p>

Constraints on the formation of the giant Daheishan porphyry Mo deposit (NE China) from whole-rock and accessory mineral geochemistry

2021 ◽  
Author(s):  
Kai Xing ◽  
Qihai Shu ◽  
David R Lentz
Keyword(s):  
High Water ◽  
Accessory Mineral ◽  
Ne China ◽  
Accessory Minerals ◽  
Slab Rollback ◽  
Continental Arc ◽  
Arc Setting ◽  
Porphyry Mo Deposit ◽  
T Values ◽  
Water Contents

Abstract There are more than 90 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1.09 Mt Mo with an average Mo grade of 0.07%. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from -1.1 to 1.4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of + 1.8 to + 4.1 (+2.7 in average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1.3–26.4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0.7–1.1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g., >5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1.09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit like Daheishan, whereas a specific magma composition (e.g., with unusually high Mo and/or S concentrations), might be less critical.

Three stages of arc migration in the Carboniferous-Triassic in northern Qiangtang, central Tibet, China: Ridge subduction and asynchronous slab rollback of the Jinsha Paleotethys

2021 ◽  
Author(s):  
Yin Liu ◽  
Wenjiao Xiao ◽  
Brian F. Windley ◽  
Kefa Zhou ◽  
Rongshe Li ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Volcanic Rocks ◽  
Late Triassic ◽  
Geochemical Data ◽  
The Tibetan Plateau ◽  
Ridge Subduction ◽  
Temporal Relationships ◽  
Slab Rollback ◽  
Geodynamic Processes ◽  
Central Tibet

Carboniferous-Triassic magmatism in northern Qiangtang, central Tibet, China, played a key role in the evolution of the Tibetan Plateau yet remains a subject of intense debate. New geochronological and geochemical data from adakitic, Nb-enriched, and normal arc magmatic rocks, integrated with results from previous studies, enable us to determine the Carboniferous-Triassic (312−205 Ma), arc-related, plutonic-volcanic rocks in northern Qiangtang. Spatial-temporal relationships reveal three periods of younging including southward (312−252 Ma), rapid northward (249−237 Ma), and normal northward (234−205 Ma) migrations that correspond to distinct slab geodynamic processes including continentward slab shallowing, rapid trenchward slab rollback, and normal trenchward rollback of the Jinsha Paleotethys rather than the Longmuco-Shuanghu Paleotethys, respectively. Moreover, varying degrees of coexistence of adakites/High-Mg andesites (HMAs)/Nb-enriched basalt-andesites (NEBs) and intraplate basalts in the above-mentioned stages is consistent with the magmatic effects of slab window triggered by ridge subduction, which probably started since the Late Carboniferous and continued into the Late Triassic. The Carboniferous-Triassic multiple magmatic migrations and ridge-subduction scenarios provide new insight into the geodynamic processes of the Jinsha Paleotethys and the growth mechanism of the Tibetan Plateau.

scholarly journals Supplemental Material: Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

2020 ◽  
Author(s):  
H.J. Blatchford ◽  
et al.
Keyword(s):  
New Zealand ◽  
Cross Sections ◽  
Geochemical Data ◽  
Arc Magmatism ◽  
Sample Collection ◽  
Pb Isotope ◽  
Continental Arc ◽  
Geologic Map ◽  
Backscattered Electron ◽  
Continental Arc Magmatism

Analytical methods used are described in Supplemental Text. Sample collection locations shown in Figure 3 are detailed in Table S1. Isotopic and geochemical data are presented in Tables S2–S4. U-Pb isotope data and geochemistry data for the sample are presented in Tables S5 and S6, respectively. A series of cross sections and a geologic map of the study area including field stations and associated structural measurements are presented in Figure S1. Cathodoluminescence images of zircon grains are shown in Figures S2 and S3. Backscattered electron images of analyzed titanite grains are presented in Figure S4.<br>

Composition, structure and tectonic analysis of the Shangrimuce arc-continent collision zone on the northern margin of the Central Qilian, NW China

2021 ◽  
Author(s):  
Hongyuan Zhang ◽  
Zhibin Lei ◽  
Bo Yang ◽  
Qing Liu ◽  
Haijun Zhang ◽  
...  
Keyword(s):  
Shear Zones ◽  
Oceanic Lithosphere ◽  
Late Ordovician ◽  
Island Arc ◽  
Qilian Mountain ◽  
Northern Margin ◽  
Early Ordovician ◽  
Collision Zone ◽  
Two Stages ◽  
Back Arc

&lt;p&gt;A 1:50000 regional survey, covering an area of about 2000 km&lt;sup&gt;2&lt;/sup&gt;, was carried out in the Shangrimuce area of Qilian Mountain in Northwest China. The results show that during Caledonian, the northern margin of the Central Qilian block experienced collision with mature island arcs and subsequently northward expansion. In the Shangrimuce study area, five geological units have been identified; they are, form south to north, back-arc basin, early Ordovician island arc, inter arc basin, middle Late Ordovician island arc, and fore-arc and oceanic lithosphere amalgamation zone.&amp;#160;&lt;/p&gt;&lt;p&gt;(1) back-arc basin. In the Yangyuchi- Shule River- Cuorigang- Wawusi area, there may be a back-arc spreading basin, and there should be spreading basins in this area. It is speculated that there was a northward reverse subduction in the late Ordovician, accompanied by a syenite body, a broad spectrum dyke swarms and an accretionary wedge zone in the whole area.&lt;/p&gt;&lt;p&gt;(2) early Ordovician island arc. In the Shangrimuce-Dander area, the Proterozoic basement granitic gneiss, the early Ordovician island arc block and the high-pressure geological body all occur in the form of thrust horses, forming a double metamorphic belt, which reveals the existence of ocean subduction to south in the early Ordovician.&amp;#160;&lt;/p&gt;&lt;p&gt;(3) inter arc basin. On both banks of Tuolai River to the east of Yanglong Township, there are early Middle Ordovician inter-arc basins with oceanic crust.&amp;#160;&lt;/p&gt;&lt;p&gt;(4) middle Late Ordovician island arc. To the north of Tuolai River, there is a middle Late Ordovician island arc belt. Both sides of the island arc zone experienced strong ductile shear deformation, which recorded a complex arc-continent collision.&amp;#160;&lt;/p&gt;&lt;p&gt;(5) fore-arc and oceanic lithosphere amalgamation zone (Fig.1). The Yushigou area has developed a fore-arc and oceanic lithospheric amalgamation zone, with weakly deformed fore-arc flysch basin, strongly deformed siliceous rocks, pillow Basalt, diabase, gabbro, peridotite and other rock assemblages.&lt;/p&gt;&lt;p&gt;Combined with the characteristics of arc-continent collision zone in the Western Pacific, there are two stages of shear zone series (Fig.2). One is ductile shear zones formed by the South dipping gneissic belt, revealing the existence of oceanic subduction accretion wedge and emplacement of high-pressure rocks. Another superimposed one is north dipping. This indicates that the arc-continent collision caused by back-arc reverse subduction, which ultimately controls the overall geometric and kinematic characteristics of the shear zones in the region.&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.8219836ca50067454890161/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=40b3389c641f2d0ca723e1527c32927e&amp;ct=x&amp;pn=gepj.elif&amp;d=1&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&lt;p&gt;Figure 1 United sections showing a Caledonian trench-arc system in the Qilian Mountain, NW China.&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.8def566da50066084890161/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=e82258ecc235c4e618abd6c035b58232&amp;ct=x&amp;pn=gepj.elif&amp;d=1&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&lt;p&gt;Figure 2 Structural analysis at Hongyahuo, indicating two stages of deformation.&lt;/p&gt;&lt;p&gt;The research has been supported by projects from the Ministry of Land and Resources (No.201211024-04; 1212011121188) and the 2020 undergraduate class construction project from China University of Geosciences (Beijing) (No. HHSKE202003).&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Magmatic evidence for Late Carboniferous-Early Permian slab breakoff and extension of the southern Mongolia collage system in Central Asia

2021 ◽  
Author(s):  
Hai Zhou ◽  
Guochun Zhao ◽  
Donghai Zhang
Keyword(s):  
Subduction Zones ◽  
Volcanic Rocks ◽  
Early Carboniferous ◽  
Ocean Basin ◽  
Magma Source ◽  
Central Asian ◽  
Slab Rollback ◽  
Slab Breakoff ◽  
Subduction Fluids ◽  
Arc Setting

&lt;p&gt;Oceanic subduction and its last underthrusted part can both triggers arc-like magmatism. As the existence of multi-subduction zones in the Central Asian Orogenic Belt, controversy still surrounds on when and especially how the subduction of the (Paleo-Asian Ocean) PAO terminated. We present geochronological, geochemical, and Lu-Hf isotopic data for a suite of basalt-andesites, dacite-rhyolites and later trachyandesite-mugearitic dykes from the Khan-Bogd area in the Gobi Tianshan Zone (GTZ) of the southern Mongolia. U-Pb dating of zircons indicate the basalt-andesites and dacite-rhyolites were formed at ~334-338 Ma, and the dykes at ~300 Ma. These Early Carboniferous volcanic rocks display high U/Th, Ba/Th, low La/Sm and variable Zr/Nb ratios, implying the involvement of subduction fluids or sediment melt. They display arc geochemical features such as calc-alkaline and metaluminous nature and positive Ba and U and negative Nb, Ta and Ti anomalies. Moreover, their continental geochemical signals (e.g. positive Pb, K anomalies) and some old captured zircons implying a continental arc setting. Comparatively, the ~300 Ma dykes are characterized by high alkaline contents, which are common for coeval (~320-290 Ma) and widespread post-subductional granites there. Given a mainly crust-derived magma source for those granites, these dykes likely reflect a mantle disturbance due to: (1) their relative low SiO&lt;sub&gt;2 &lt;/sub&gt;(51.71-55.85 wt. %) and high Mg# (40.3-67.3) values, and (2) positive zircon &amp;#400;&lt;sub&gt;Hf&lt;/sub&gt;(t) (most &gt; 12). Considering a slab rollback model during the Carboniferous and Triassic, the mantle disturbance was possibly induced by the oceanic slab breakoff. Combined with previous work, this ~320-290 Ma slab breakoff-induced extension marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) north of the main ocean basin of the PAO. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).&lt;/p&gt;

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