Geochronology and Geochemistry of the Triassic Mafic Complexes from the Western Garzê-Litang Ophiolitic Mélange and Implications for the Melt Evolution of a Continental Margin

<正>There is a general consensus that most ophiolites on the earth formed above a subduction zone and they often display a characteristic,sequential evolution of MORB to island arc tholeiities(IAT)to bonnites(Dilek et al.,2010,2009;Pearce et al.,2003).However,ophiolites occurred in     

The Petrology, Geochemistry, and Petrogenesis of E-MORB-type Mafic Rocks from the Guomangco Ophiolitic Mélange, Tibet

The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic melange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic melange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic melange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E- MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%-30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.     

The Oldest Paleo-Tethyan Ophiolitic Mélange in the Xizang (Tibetan) Plateau

正An Early Paleozoic ophiolitic mélange has recently been documented in the W.Gangma Co area,north-central Tibetan Plateau.It is composed of serpentinite,isotropic and cumulate gabbros,basalt and plagiogranite.Whole-     

Petrogenesis and Tectonic Implications of Middle Ordovician Ocean Island Basalts from the Chagantaolegai Ophiolitic Mélange in Junggar, NW China

Seamount accretion is one of the most significant accretionary orogenic processes in the Central Asian Orogenic Belt, but there are few paleo-seamounts reported from and debate on the tectonic evolution of the Junggar Ocean still exists. In this study, we present geochronological, mineralogical, geochemical and isotopic data for basalts from the Chagantaolegai ophiolitic mélanges in Junggar. Zircon U-Pb dating on one basalt yielded a weighted mean 206 Pb/238 U age of 469 ± 7 Ma, which suggests that it formed in the Middle Ordovician. All rock samples belong to alkaline basalt and show similar geochemical characteristics, displaying high TiO2(～3 wt%),(La/Yb)N(17.6–19.0), ΣREE(232–289 ppm) and enrichment in Nb and Ta, which implies an ocean island basalt(OIB) affinity. Based upon positive εN d(t)(+4.16 to +4.23), ΔNb(0.20–0.22) and low initial 87 Sr/86 Sr(0.70425 to 0.70452) and Zr/Nb(3.35–3.57), we suggest that the Chagantaolegai OIB samples were likely derived from a fertile mantle source related to plume. The OIB rock assemblage, chert and marble in the southern part of the Chagantaolegai ophiolitic mélange indicates that a Middle Ordovician seamount was accreted to the Boshchekul-Chingiz arc due to the northward subduction of the Junggar-Balkhash Ocean.     

Geochemistry of Basalt from the Barleik Ophiolitic Mélange in West Junggar and its Tectonic Implications

西准噶尔地区巴尔雷克蛇绿混杂岩中的玄武岩与蛇纹岩、放射虫硅质岩和晚泥盆世铁列克提组的泥质粉砂岩与沉凝灰岩形成混杂堆积.对玄武岩进行详细的岩石地球化学研究表明,SiQ含量为42.15％～44.71％,高TiO2 (3.17％～3.77％)、Na2O(1.73％～2.28％),低Al2O3 (13.54％～14.31％)、K2O(1％～1.82％),MgO含量相对稳定(6.75％～8.14％),Mg#为43～46,属于碱性玄武岩系列.稀土总量∑REE=186×10-6～219.06×10-6,轻、重稀土分馏较为明显((La/Yb)N=11.37～12.62),无明显Eu异常(Eu/Eu* =0.96～1),稀土配分模式类似于OIB.相对富集LILE(如Rb、Ba、Th),亏损HFSE(如Zr、Hf),没有明显的Nb和Ta异常,具有高的Ti/Yb(7395～8724)和Zr/Yb(120～136)比值,为典型的OIB地球化学特征.综合研究认为玄武岩形成于弧后盆地的海山环境,其岩浆源区可能为EMI型富集地幔,即软流圈的上涌导致尖晶石相二辉橄榄岩地幔源区大比例部分熔融形成的玄武岩.在区域上,蛇绿混杂岩中的玄武岩所代表的泥盆纪古洋盆是西准噶尔古洋盆向北收缩的残余洋盆.     

Petrogenesis and Geodynamic Implications of Late Jurassic Diorite Porphyry in the Neoproterozoic Ophiolitic Mélange of NE Jiangxi (South China)

Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U-Pb dating, Hf isotopes and whole-rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U-Pb dating by SIMS and LA-ICP-MS methods yielded an emplacement age of ～160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low Si02(58.08 wt%-59.15 wt%), and high Na_2 O(5.00 wt%-5.21 wt%) and MgO(4.60 wt%-5.24 wt%) contents with low TFeO/MgO ratios(1.02-1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios(24.8-31.1) and high Y contents(14.6-18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions(ε_(Hf)(t)=-6.1--0.1; T_(DM2)=1597-1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust(i.e., early Neoproterozoic mafic arc-related rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal-spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat-slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat-slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.     

Geodynamic significance of the Cretaceous pillow basalts from North Anatolian Ophiolitic Mélange Belt (Central Anatolia,Turkey): geochemical and paleontological constraints

The most widespread blocks within the Cretaceous ophiolitic mélange (North Anatolian ophiolitic mélange) in Central Anatolia (Turkey) are pillow basalts, radiolarites, other ophiolitic fragments and Jurassic-Cretaceous carbonate blocks. The pillow basalts crop out as discrete blocks in close relation to radiolarites and ophiolitic units in Cretaceous ophiolitic mélange.

The geochemical results suggest that analyzed pillow basalts are within-plate ocean island alkali basalts. The enrichment of incompatible elements (Nb, Ta, Light REE, Th, U, Cs, Rb, Ba, K) demonstrates the ocean island environment (both tholeiites and alkali basalts) and enriched MORB. Dated calcareous intrafills and biodetrital carbonates reveal an age span of Callovian—Early Aptian. The thin-shelled protoglobigerinids, belonging to the genus Globuligerina, in the calcareous intrafills between pillow basalt lobes indicates a Callovian—Barremian age interval, most probably, Valanginian to Late Barremian. The volcanic and radiolarite detritus-bearing orbitolinid—Baccinella biodetrital carbonates dated as Late Barremian-Early Aptian in age, were probably deposited around atolls and have a close relationship with the ocean island pillow basalts.

The results collectively support the presence of a seamount on the Neo-Tethyan oceanic crust during the Valanginian—Late Barremian and atolls during the Late Barremian-Early Aptian interval. The presence of an oceanic crust older than that seamount along the Northern Branch of Neo-Tethys is conformable with the geodynamic evolution of the Tethys.     

Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo-Tethyan Ocean

The Changning–Menglian Belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana- and Eurasia-derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low-Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies, but negligible Eu and Zr-Hf anomalies. The volcanics have similar Sr-Nd-Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)-like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB-related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source (i.e., asthenospheric mantle) containing garnet-spinel lherzolite material. Combing this new data with that from oceanic seamount volcano-sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward-migrating microcontinent of Lanping–Simao.     

The Zunhua-Zanhuang Ophiolitic Mélange: Traces of an Archean Suture in the North China Craton

<正>Sutures mark places where oceans have closed and two once widely separated terranes have collided.Sutures are defined on the basis of geological relationships,and differences in the geologic,structural,magmatic,sedimentary,and metamorphic histories of the terranes on either side of the suture.Sutures are characterized by     

An Early Paleozoic Tectonic Mélange at the Western Margin of West Cathaysia: Constraints from Organic-walled Microfossils

The Jiangshan-Shaoxing-Pingxiang Fault(JSP Fault) is traditionally considered as the boundary between the Yangtze and Cathaysia blocks in South China. Whether the previously defined Shenshan and Kuli formations located along the JSP fault and near the Xinyu City, Jiangxi Province, are continuous strata or parts of a tectonic mélange is important for understanding the geological history of South China. A carbonaceous phyllite from the area, previously considered as part of the Neoproterozoic Shen...     

The Late Triassic I–Type Granites from the Longmu Co–Shuanghu Suture Zone in the interior of Tibetan Plateau, China: Petrogenesis and Implication for Slab Break–Off

The Jiangaidarina granitic mass(JM) is an important part of the magmatic belt in Longmu CoShuanghu Suture Zone(LSSZ) in the central Tibetan Plateau. An integrated research involving wholerock geochemistry, zircon LA-ICP-MS U-Pb ages and Hf isotopic compositions was carried out to define the timing, genesis and tectonic setting of the JM. Zircon LA-ICP-MS U-Pb ages have been obtained ranging from 210 to 215 Ma, rather than the Early Jurassic as previously thought. Fifteen granite samples contain hornblendes and show a negative correlation between P_2 O_5 and SiO_2, indicating that the JM is an I-type granite. All the granites are enriched in LREE relative to HREE, with negative Eu anomalies(Eu/Eu*=0.56-0.81), and have similar trace elements patterns, with depletion of Ba, Nb, Sr and P. These suggest that the JM was fractionated, and this is also proved by the characteristic of negative correlations between oxide elements(TiO_2, MgO, FeOt, MnO, CaO) and SiO_2. Almost all ε_(Hf)(t) values of the granites are between-10.3 and-5.8, implying that the JM has a crustal source intimately related with the South Qiangtang Block(SQB), except for one(+10.2), showing a minor contribution from mantle source.Moreover, relatively low Na_2 O/K_2 O ratios(0.42-0.93) and high A/CNK values(0.91-1.50) reflect that the JM was predominately derived from the medium-high potassium basaltic crust, interacted with greywacke. Our new geochemical data and geochronological results imply that the Late Triassic magmas were generated in a post-collisional tectonic setting, probably caused by slab break-off of the Longmu Co-Shuanghu Tethyan Ocean(LSTO). This mechanism caused the asthenosphere upwelling, formed extension setting, offered an enormous amount of heat, and provided favorable conditions for emplacement of voluminous felsic magmas. Furthermore, the LSTO could be completely closed during the Middle Triassic, succeed by continental collision and later the slab broke off in the Late Triassic.     

High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone,southeast Tibetan Plateau: P-T-t path and tectonic implication

The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone (CMSZ) provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys. An integrated petrologic, phase equilibria modeling and geochronological study of the garnet amphibolites, southeast Tibetan Plateau, shows that the garnet amphibolites have a peak mineral assemblage of garnet, glaucophane, lawsonite, chlorite, rutile, phengite and quartz, and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495 –515°C, indicating a cold geothermal gradient of about 240 –260°C/GPa. Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies. The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies, and therefore the obtained zircon U-Pb age of 231 ± 1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence. The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded high-pressure eclogite-facies blueschist, instead of the previously proposed eclogites, and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.©2021 China Geology Editorial Office.     

Extrusion tectonics inferred from fabric study of the Guanzizhen ophiolitic mélange belt in the West Qinling orogen,Central China

The recently identified Guanzizhen ophiolitic mélange belt (GOMB) in the West Qinling orogen is regarded, by many geochemical and geochronological works, as the westward extension of the Shangdan belt in the East Qinling that separated the North and South China blocks. The GOMB trends generally NW–SE and comprises ophiolites and forearc volcanic–sedimentary complex (the Liziyuan Group). Ductile shear zones with various widths are a striking structural feature of the GOMB and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area. Here we characterize the deformation style of these shear zones by combining the low-field anisotropy of magnetic susceptibility (AMS) data, microstructural and field observations. The new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition. Combining the dextral transpression of the GOMB and the coeval sinistral transpression of the Shangdan belt in the East Qinling, we propose a Late Triassic bilateral extrusion tectonics of the Qinling orogen in response to the collision between the Ordos and Sichuan blocks.     

Revision of the Genus Conophyllia d'Orbigny 1849(Anthozoa,Scleractinia, Triassic) and Conophyllia from the Renacuo area of Gêrzê, Tibet

There have been divergent opinions on whether Conophyllia develops synapticulae and dissepiments since its erection in 1849, with different views existing on the morphology of columella in Conophyllia.This paper presents a comprehensive review of Conophyllia, and revises the diagnosis according to new material from the Renacuo area of Gêrzê, Tibet.The revised diagnosis emphasises the following features: solitary coral; abundant radial elements subequal in thickness, lateral faces with granules and synapticulae; parietal, papillar or incompact columella, a few species lacking in columella; developed dissepiments.Individuals of Conophyllia have laminar septa of Triadophyllum type in earlier ontogenetic stages, while their septa in later ontogenetic stages are more fenestrate.The geological and geographical distribution of this genus is discussed, with five species of Conophylliafrom the Renacuo area of Gêrzê, Tibet being described and illustrated, including one new species and two in open nomenclatue.     

Triassic and Jurassic radiolarians from sedimentary blocks of ophiolite mélange in the Avala Gora area (Belgrade surroundings, Serbia)

Blocks of cherty rocks and Aptychus Limestone embedded into ophiolite mélange south of Avala Gora (Serbia) contain radiolarians of different ages. We distinguished here Late Jurassic (middle Oxfordianearly Tithonian), Middle-Late Jurassic (Bathonian-early Tithonian), and Middle Triassic (early Ladinian) radiolarian assemblages. The respective stratigraphic data suggest that the ophiolite mélange was formed after the early Tithonian.     

The Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China — Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton; the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.