Late Triassic Volcanic Rocks Research Articles

New Rock Magnetism and Magnetic Fabrics Studies on the Late Triassic Volcanic Rocks from Qaidam Block, Northern Tibetan Plateau

The Qaidam Block, located at the northern Qinghai–Tibet Plateau, is a pivotal area in unraveling the closure time of the Kunlun Ocean basin which might have recorded the transformation process between the Proto-Tethys and Paleo-Tethys Ocean basins. However, the late Triassic position of the Qaidam Block remains enigmatic, largely due to the scarcity of paleomagnetic data essential for quantitatively determining its paleolatitude. The widespread presence of the Elashan formation, particularly along the southern periphery of the Qaidam block, presents good material for conducting paleomagnetic work. Nevertheless, the primary magnetic carriers preserved within the Elashan formation might be influenced by multiple tectonic thermal events, particularly those associated with collisions between southern blocks and the Qaidam Block. Here we present rock magnetism and magnetic fabrics studies to identify the content and composition of magnetic minerals within the Elashan formation. The rock magnetic and petrologic results show that the magnetic carriers in the samples from the Elashan formation are dominated by magnetite with a small amount of goethite, pyrrhotite, and hematite. The results of Anisotropy in Magnetic Susceptibility indicate that the south of the Longwalangku section might not be obviously influenced by the tectonic events. Our results also provided guidance for future paleomagnetic research, emphasizing the importance of conducting further sampling away from adjacent faults, particularly in the southern Longwalangku area.

Paleomagnetism of Late Triassic Volcanic Rocks From the South Qiangtang Block, Tibet: Constraints on Longmuco‐Shuanghu Ocean Closure in the Paleo‐Tethys Realm

AbstractThe South Qiangtang block of the Qinghai‐Tibet Plateau represents an area critical to understanding the late Paleozoic and early Mesozoic history of the Tethyan realm, but its drift history remains poorly constrained. Here we report a new quantitative paleogeographic constraint for the South Qiangtang block from a paleomagnetic study of Late Triassic volcanic rocks of the Xiaoqiebao Formation. A characteristic remanent magnetization isolated from 25 sites passes both fold‐ and reversal tests, and likely represents a primary magnetization. On the basis of these data, we estimate that the South Qiangtang block occupied a paleolatitude of 30.1 ± 4.6°N at ca. 222 Ma. When combined with existing paleomagnetic constraints, these new results indicate that the South Qiangtang block (and other “Cimmerian” blocks) moved rapidly northward (in true latitude) between the middle Permian and Late Triassic. Our new data further suggest that the southern branch of the Paleo‐Tethys (Longmuco‐Shuanghu Ocean) likely closed by the mid‐Late Triassic.

Formation and evolution of a magmatic system in the Indian passive margin: Insights from the Triassic Yumai Complex from the eastern Tethyan Himalaya

Abstract The Indian passive margin has preserved several pulses of magmatism during and after the disassembly of Gondwana since the late Paleozoic, providing valuable insights into the long-term magmatic evolution of various passive margins, including the Indian passive margin. In the Yumai Complex, eastern Tethyan Himalaya, a pulse of Late Triassic alkaline volcanism (ca. 227–216 Ma) is evident. The Late Triassic volcanic rocks are mildly alkaline to tholeiitic basalts with minor ultrabasic rocks, similar in geochemistry to within-plate flood basalts. The TiO2 contents (1.46–3.38 wt%, mainly >2 wt%), (La/Yb)N values (4.05–7.50), εNd (t) values (+4.86 to +6.98), and results from partial-melt modeling suggest that the basalts likely originated from garnet peridotite. Elemental and Sr-Nd systematics of magmatic rocks emplaced during the Triassic indicate oceanic island basalt (OIB) components in the magma source, interpreted as enriched mantle components rather than crustal contamination products. Spatiotemporal and geochemical patterns of magmatism reveal that the bulk compositions of the basalts changed from enriched OIB-like to depleted mid-ocean-ridge basalt (MORB)–like compositions. This phenomenon likely resulted from the evolving nature of the rifting basin, changing from a nascent continental setting to a mature ocean basin. The Triassic magmatism in the Tethyan Himalaya can be attributed to remnant lithospheric instability arising from the prolonged rifting of eastern Gondwana, leading to the formation of a magmatically passive margin.

Constraints of Late Triassic mafic-felsic volcanic rocks in northwestern Laos on the Eastern Paleotethyan post-collisional setting

Early Mesozoic volcanic rocks in northwestern Laos are rarely reported and important for investigating the Paleotethyan tectonic evolution and regional correlation in SE Asia. In this study, New zircon ages and whole-rock geochemical data are presented for the newly identified volcanic rocks in northwestern Laos. Twelve volcanic rocks yield zircon U–Pb ages of 228–234 Ma. The volcanic rocks in northwestern Laos contain basalt, basaltic andesite, andesite, and rhyolite and can be geochemically subdivided into three groups. Both Group 1 and 2 samples belong to the calc-alkaline mafic-intermediate volcanic rocks. Group 1 has εNd (t) values of −0.52 to +1.20, εHf (t) values of +0.9 to +3.6 and δ18O values of 6.3–7.0‰. In contrast, Group 2 has lower εNd (t) values of −2.82 to −5.89 and εHf (t) values of −4.0 to +1.4 and higher δ18O values of 6.6–8.1‰ than Group 1. Group 3 samples are high-K calc-alkaline rhyolites with high εNd (t) (−0.08 to +0.74) and positive εHf (t) (+4.6 to +13.2) values. Geochemical characteristics suggest that the Group 1 and 2 samples were derived from a metasomatized lithospheric mantle modified by sediment-derived components, and Group 3 rhyolites were derived from partial melting of juvenile mafic rocks with a crustal component. Our results, along with the geological observations and reported ages suggest that the Late Triassic volcanic rocks in northwestern Laos were formed in a post-collisional setting, and provide the key studies for the connection between the Lancangjiang and Chiang Khong-Lampang-Tak zones.

Petrology of the late Triassic mafic volcanic rocks from the Antalya Complex, southern Turkey: evidence for mantle source characteristics during the Neotethyan rifting

The Antalya Complex in southern Turkey comprises a number of autochthonous and allochthonous units that originated from the Southern Neotethys. Late Triassic volcanic rocks are widespread in the Antalya Complex and are important for the onset of the rifting stage of the southern Neotethys. The studied Late Triassic volcanic rocks within the Antalya Complex are exposed in the southern part of Saklıkent (Antalya) region. They are represented by pillow, massive, and columnar-jointed lava flows with volcaniclastic breccias and pelagic limestone intercalations. Spilitic basalts exhibit intersertal, microlithic porphyritic, and ophitic textures and are represented by plagioclase, pyroxene, and olivine. Secondary phases are characterized by serpentine, calcite, chlorite, epidote, zeolite, and quartz. Based on Zr/Ti vs. Nb/Y ratios, the volcanic rocks are represented by alkaline basalts (Nb/Y = 1.54–2.82). A chondrite normalized REE diagram for the volcanic rocks displays significant LREE enrichment with respect to HREE ([La/Yb]N = 15.14–19.77). Trace element geochemistry of the studied rocks suggests that these rocks are more akin to ocean island basalt (OIB) and were formed by small degrees (~2–4%) of partial melting of an enriched mantle source (spinel + garnet-bearing lherzolite). The volcanic rocks of the Saklıkent region exhibit similarities to the Late Triassic volcanics of the Koçali Complex in SE Anatolia and the Mamonia Complex (Cyprus) in terms of their geochemical features. All evidence suggests that the Late Triassic alkaline volcanics in Antalya, Mamonia (Cyprus), and the Koçali (Adıyaman) Complexes were formed in an extensional environment at the continent-ocean transition zone during the rifting of the southern Neotethyan Ocean.

滇西保山地块晚三叠世火山岩特征及其构造环境

滇西保山地块晚三叠世火山岩特征及其构造环境

Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau

The origin of the central Qiangtang metamorphic belt (CQMB) has long been in debate, which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block, or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks. Here we report field observations, zircon U-Pb ages and Lu-Hf isotopes, as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB. The ca. 229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount. Their geochemical and isotopic compositions are similar to OIB-type lavas. Unlike other metabasalts (eclogites and blueschists) in the CQMB, the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism; this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab. This work provides new solid evidences for an in situ origin of the CQMB.

Palaeomagnetism of Late Triassic volcanic rocks from the western margin of Khorat Basin, Thailand and its implication for ambiguous inclination shallowing in Mesozoic sediments of Indochina

SUMMARY Palaeomagnetic constraints are essential factors in the reconstruction of the Mesozoic convergence of Eastern Asia blocks. As one of the key blocks, Indochina was constrained only by sedimentary-rocks-derived palaeomagnetic data. To evaluate whether the palaeomagnetic data used to restore the Late Triassic position of Indochina suffered inclination shallowing effects, we conducted a palaeomagnetic and geochronologic study on a coeval volcanic clastic rocks sequence in the western margin of the Khorat Basin, Thailand. The U-Pb SIMS dating on zircons indicates the age of the sampling section is between 205.1 ± 1.5 and 204.7 ± 1.4 Ma. Site mean directions are Dg/Ig = 217.2°/−39.4° (κg = 45.1, α95g = 10.1°) before and Ds/Is = 209.2°/−44.5° (κs = 43.8, α95s = 10.2°) after tilt correction. The new data set indicates a positive reversal test result at ‘Category C’ level. The characteristic remanent magnetization recorded by the coexistent magnetite and hematite is interpreted to be primary remanence acquired during the initial cooling of the volcanic clastic rocks. The consistence of the corresponding palaeolatitudes derived from the volcanic clastic rocks and the former reported sedimentary rocks suggests that there is probably no significant inclination shallowing bias in the sedimentary-rocks-derived palaeomagnetic data. Therefore, the estimates of the Late Triassic position of Indochina are confirmed to be reliable. The Indochina Block had collided to the southern margin of Eurasia by the Late Triassic and played an important role in the Mesozoic convergence of the Eastern Asia blocks.

Geochemical characteristics of Early Cretaceous marine oil shale from the Changshe Mountain area in the northern Qiangtang Basin, Tibet: Implications for palaeoweathering, provenance, tectonic setting, and organic matter accumulation

The Early Cretaceous Shengli River–Changshe Mountain marine oil shale belt, located in the northern Qiangtang Basin, Tibet, is estimated to be the largest marine oil shale resource in China. However, its palaeoweathering, provenance, and tectonic setting during the depositional history have received little interest. In this paper, 19 outcrop marine oil shale samples from the ChangsheMountain (CSM) area were studied to investigate the provenance, palaeoclimate, palaeoweathering, and tectonic setting during the marine oil shale deposition. The total organic carbon and organic sulphur contents of the CSM oil shales range from 2.2% to 13.44% and 0.19% to 1.08%, respectively. The CSM oil shale samples contain abundant carbonate (e.g., calcite and dolomite), quartz, and clay minerals. The clay minerals of the CSM oil shale samples is dominated by illite and then by smectite. Other minerals, including haematite, apatite, pyrite, baryte, and celestite, were also identified by scanning electron microscope–energy‐dispersive X‐ray spectrometer. The total rare earth element (∑REE) contents of the oil shales range from 9.01 to 105.96 ppm (average 57.56 ppm), close to the average REE contents of U.S. coals. The REE contents in oil shale samples are mainly controlled by terrigenous input. The CSM oil shales show LREE‐enriched, HREE‐depleted, with the LREE/HREE ratios ranging from 5.12 to 10.18, negative Eu anomalies (0.56–0.84) and slightly negative Ce anomalies (0.87–1.05). The chemical index of alteration values of the CSM oil shales range from 67 to 76 (average 73), indicating a moderate chemical weathering condition. Sr/Cu and Rb/Sr ratios suggest the CSM oil shales were mainly deposited under a warm and humid climate regime. Sr/Ba ratios, ranging from 0.54 to 7.84, suggest a palaeoenvironment with fluctuant salinity. Major element, trace element, and REE relationships indicate that the CSM oil shales are mainly from the felsic and intermediate igneous rocks. The REE distribution patterns of the CSM oil shales are similar to the Late Triassic Nadi Kangri felsic volcanic rocks in the Qiangtang Basin, indicating that the CSM oil shale may be derived from the Late Triassic volcanic rocks and the oil shales were mainly formed in rift tectonic setting. Favourable preservation condition (e.g., warmer palaeoclimate, higher palaeosalinity, and dysoxic to anoxic water column environment) could be the main controlling factor for organic matter accumulation.

Geochronology, petrogenesis, and tectonic setting of Late Triassic volcanic rocks of the Hadataolegai Formation, central Great Xing'an Range, Northeast China

AbstractIn this study, new geochemical, zircon U–Pb, and Lu–Hf isotopic data are presented for volcanics from the Hadataolegai Formation of the central Great Xing'an Range (GXR) in Northeast China. These new data offer insights into the petrogenesis of the volcanics of the Hadataolegai Formation and the tectonic evolution of the Paleo–Asian Ocean (PAO) and Mongol–Okhotsk Ocean (MOO). These volcanics of the Hadataolegai Formation are divided into andesite‐trachyandesites and dacite‐trachydacites. Zircon U–Pb ages show that the volcanics of the Hadataolegai Formation erupted between 230 Ma and 228 Ma during the Late Triassic, which agrees with recently obtained data. The volcanic rocks in this study have low Y (9.9–21.1 ppm) and Yb (0.78–2.02 ppm) contents, high Sr (444–954 ppm) contents, and slight Eu anomalies (δEu = 0.82 to 0.94), similar to ‘adakite‐like’ rocks. The dacites were formed by fractional crystallization of coeval andesitic magmas. The zircons within the andesite and trachyandesite yield higher positive εHf(t) values (+6.3 to +12.0) and model ages (TDM2) between 860 Ma and 453 Ma, which indicates that the magmas were generated by a newly accreted continental crustal source. Moreover, some of the volcanics are relatively high in MgO contents. These characteristics indicate that the volcanic magmas were derived from the partial melting of delaminated lower crust and mixing with mantle materials. Combining these data with previous studies, we suggest that the magmatism in the central GXR was governed by extension due to the closure of the PAO and the back‐arc extension associated with the southward subduction of the MOO plate (western GXR, near the Erguna Block).

A Keleti-Bakony triász időszaki vulkanogén képződményeinek petrográfiai vizsgálata és képződési körülményeik értelmezése

A Tethys közép-európai triász időszaki fejlődéstörténetének nyomozása az egymástól távolabbi területeken található nem vulkanogén üledékes kőzetek összehasonlító vizsgálatával régóta eredményesen zajlik. A térben és időben is sokkal változatosabb kifejlődésű vulkanogén képződmények ezirányú vizsgálatában rejlő lehetőségek még messze nincsenek kihasználva. Ennek leginkább az lehet az oka, hogy ezeknek a hazai kőzeteknek a részletes kőzettani leírása még nem történt meg. Ladin vulkanogén törmelékes képződmények a Dunántúli-középhegységen belül a Keleti-Bakony területéről ismertek. Az itt található középső–késő-triász vulkanogén kőzetek részletes petrográfiai vizsgálata lehetőséget teremtett arra, hogy kőzettani alapon is összevessük más hasonló korú, képződésük idején egymáshoz sokkal közelebb elhelyezkedett Dunántúli-középhegységben és Déli-Alpokban megtalálható vulkáni képződményekkel.A vizsgált keleti-bakonyi összletek az inotai Hideg-völgy útbevágásában lévő egykori felszíni feltárásban, valamint a Várpalota Vpt–3 és a Bakonykúti Bút–2 fúrások anyagában tanulmányozhatók. A vulkanogén összlet két részre tagolható: az alsó részben uralkodóan vulkanit törmelékszemcsékből álló, szenesedett növénymaradványt és elszórtan crinoidea váztöredékeket tartalmazó finomszemcsés üledékes kőzetet, fölül vulkanit- és mészkőkavicsokat nagy mennyiségben tartalmazó, Daonella vázelemekben gazdag durvaszemcsés rétegcsoportot találhatunk. A vulkáni törmelék szemcsék között döntően intermedier (andezit), kisebb mennyiségben bázisos (bazaltsalak, bazalt és mikrodolerit) és savanyú (riolit, aplit) vulkanitok és ezekhez kapcsolódó kristálytörmelékek jelennek meg.A vulkanogén összlet forrását olyan középső- és felső-triász vulkáni kőzetek (bázisos-intermedier lávakőzetek és freatomagmás kitörések termékei) szolgáltatták, melyeknek mai rokon képviselői a Dunántúli-középhegységben (legközelebb a balatonfői területen), de legnagyobb kiterjedésben a vele ezen időszakban szomszédos Déli-Alpokban fordulnak elő. A felső vulkanogén konglomerátum / kavicsos homokkő képződésére jó analógia a déli-alpi Wengeni Formáció és Marmoladai Konglomerátum keletkezése. E formációk nagy vastagságú törmelékes összlete szárazulatra került, döntően mészkő és bázisos-intermedier kőzetek lepusztulásával, majd tenger alatti tömegmozgásokkal üledékgyűjtő medencékbe történő felhalmozódásával keletkezett. Az alsó vulkanogén homokkő keletkezése ettől némiképp eltér. Bázisos-intermedier összetételű törmelékanyaga első lépésben sekélytengeri/szárazföldi környezetben freatomagmás kitörés eredményeként halmozódott fel, majd közvetlen ezután tenger alatti lejtő mentén történő áthalmozódás után ülepedett le véglegesen.

Petrogenesis of Middle–Late Triassic volcanic rocks from the Gangdese belt, southern Lhasa terrane: Implications for early subduction of Neo-Tethyan oceanic lithosphere

The Gangdese belt is dominantly composed of igneous rocks that formed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane and has played a crucial role in understanding the pre-collisional evolution of southern Tibet. This paper presents new geochronological and geochemical (whole-rock major and trace element and Sr–Nd and zircon Hf isotope) data for recently identified volcanic rocks exposed in Changguo area, southernmost part of the Lhasa terrane. Zircon U–Pb dating from six samples yields consistent ages of 237.1±1.1Ma to 211.7±1.5Ma for magma emplacement through volcanic eruption, showing the Middle–Late Triassic magmatic activity in the southernmost Gangdese Belt. The Changguo volcanic rocks are mainly composed of basaltic and andesitic rocks and exhibit LILE enrichment and HFSE depletion. They also exhibit relatively uniform Nd–Hf isotopic compositions (εNd(t)=+5.20 to +7.74 and εHf(t)zircon=+10.2 to +15.9). The basaltic magmas were likely sourced from partial melting of sub-arc mantle wedge that was metasomatized by not only the aqueous fluid derived from subducting altered oceanic crust but also hydrous melt derived from subducting seafloor sediments, and subsequently experienced fractional crystallization and juvenile crustal contamination during ascent. The andesitic magmas were generated by partial melting of mafic–ultramafic metasomes through melt/fluid–peridotite reaction at slab–mantle interface. Taking into account the temporal and spatial distribution of the Early Mesozoic magmatic rocks and regional detrital zircon data, we further propose that the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane commenced by Middle Triassic.

Petrogenesis of the Late Triassic volcanic rocks in the Southern Yidun arc, SW China: Constraints from the geochronology, geochemistry, and Sr–Nd–Pb–Hf isotopes

Studies on zircon ages, petrology, major and trace element geochemistry, and Sr–Nd–Hf–Pb isotopic geochemistry of intermediate volcanic rocks from the Southern Yidun arc, Sanjiang-Tethyan Orogenic Belt, SW China have been undertaken in this paper. They are used to discuss the petrogenesis of these rocks and to constrain the tectonic setting and evolution of the Yidun arc. These intermediate volcanic rocks were erupted at ca. 220Ma (U–Pb zircon ages). Trachyandesite is the dominant lithology among these volcanic rocks, and is mainly composed of hornblende and plagioclase, with minor clinopyroxene and biotite. A hornblende geobarometer suggests that the stagnation of magma in the lower crust, where plagioclase crystallization was suppressed while hornblende crystallized, giving rise to high Sr/Y ratios that are one of the distinguishing features of adakites, after the primary magma originated from the lithospheric mantle wedge. Steeply right-inclined Rare Earth Element (REE) pattern combined with high La/Yb ratios suggests adakitic affinity of these volcanic rocks, implying that slab-melt from the subducting oceanic crust is a necessary component in the primary magma. Besides, trace element geochemistry and isotopic geochemistry also indicate that partial melting of pelagic sediments in the subduction zone and noticeable contamination with the lower crust were involved in the evolution of parental magma of these volcanic rocks. Based on previous work on the Northern Yidun arc and this study, we propose that the subduction was initiated in the Northern Yidun arc and extended to the southern part and that the Northern Yidun arc is an island arc while the Southern Yidun arc represents a continental arc, probably caused by the existence of the Zhongza Massif, that was invoked to be derived from Yangtze Block, as a possible basement of the Southern Yidun arc.

Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes

LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201Ma), Early–Middle Jurassic (190–173Ma), Middle–Late Jurassic (166–155Ma), early Early Cretaceous (145–138Ma), late Early Cretaceous (133–106Ma), and Late Cretaceous (97–88Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166Ma) and early Early Cretaceous (145–138Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Geology, geochemistry and geodynamic implications of the mafic–ultramafic rocks from the northern part of the Antalya Complex, SW Turkey

The Antalya Complex in the Isparta Angle area (SW Turkey) includes allochthonous Mesozoic rock assemblages occuring as nappe stacks or disrupted tectonic units formed in response to different mélange forming processes. The allochthonous rock assemblages in the northern part of the Antalya Complex are mainly characterized by slope basin deposits associated with the late Triassic volcanic rocks varying in composition from alkali basalt to fractionated trachyandesite, the late Jurassic–early Cretaceous tholeiitic volcanic rocks in composition from basaltic to rhyolitic, and MORB-type volcanic rocks most likely in Cretaceous age that spatially associated with olistostromal and subophiolitic tectonic mélange. Whole rock chemistry of the Eğridir Kızıl Dağ peridotites and the isolated diabasic dykes intruding the peridotites, the mineral chemistry of the harzburgitic spinels points to depleted residual mantle peridotite character and two tectonic settings: supra-subduction zone and ocean ridge environments, a characteristic feature of forearc setting.Extensive geological field investigations and previous studies from the Antalya Complex and the Mamonia Complex (SW Cyprus) have led to a conclusion that the late Triassic alkaline volcanic rocks are associated with continental rift-related deposits in the apex of the Isparta Angle, interbedded with thin pelagic limestone layers to the south, and overlain by the late Triassic reefal limestones and pelagic deposits in the Mamonia Complex, suggesting progressive deepening and widening in ocean basin towards the south during the late Triassic time. Moreover, the Triassic lavas of the southern part of the Isparta Angle and the Mamonia Complex range from WPB-type to transitional and MORB-type, suggesting rapid rifting and drifting in the southern part of the Antalya ocean giving rise to evolution of northward narrowing wedge-shaped ocean basin. Constrains on the geological and geochemical characteristics of the Mesozoic volcanic rocks associated with the allochthonous assemblages between the apex of the Isparta Angle and the Mamonia Complex suggest a continental rifting in the Isparta Angle area that was followed by nearly northward propagated opening and wedge-shaped ocean basin evolution with MORB-type oceanic crust and evolution of supra-subduction type ophiolite with forearc affinity within the same ocean basin.

Geochemistry and geochronology of Late Triassic volcanic rocks in the Chiang Khong region, northern Thailand

AbstractThe Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub‐belts of mafic to felsic volcanic, volcaniclastic, and associated intrusive rocks. Associated sedimentary rocks are largely non‐marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass‐spectroscopy U–Pb zircon ages. Felsic‐dominated sequences in the Chiang Khong Western and Central Sub‐belts are high‐K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. The Western Sub‐belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub‐belt. In contrast, the Eastern Sub‐belt is dominated by mafic lavas and dykes with compositions transitional between E‐mid‐oceanic ridge basalt and back‐arc basin basalts. The Eastern Sub‐belt rocks have higher FeO* and TiO2 and less light rare earth element enrichment than basalts in the high‐K sequences. Basaltic and doleritic dykes in the Western and Central sub‐belts match the composition of the Eastern Sub‐belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non‐marine associated sediments, we propose an alternative origin, in a post‐collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc‐like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks, and strong compositional analogies with other post‐collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post‐collisional, basin‐and range‐type extensional setting.

Paleomagnetic Differentiation and Correlation of the Late Triassic Volcanic Rocks in the Central Appalachians (with Special Reference to the Connecticut Valley)

During the late Triassic and early Jurassic, the location of the earth9s magnetic pole changed rapidly. As a result of these changes the inclination of the magnetic field in North America increased considerably in a relatively short time. The igneous rocks emplaced during the late Triassic and early Jurassic, show thermo-remanent directions (T.R.M.) of magnetization with progressively higher positive inclinations. In Connecticut four periods of late Triassic-early Jurassic volcanic activity have been dis­tinguished geologically. They are from old to young: the Talcott, Holyoke, Hampden, and Higganum volcanic events. The mean inclination of the T.R.M. is + 12° for the Talcott, + 25° for the Holyoke, + 42° for the Hampden, and + 35° for the Higganum volcanic units. This paleomagnetic stratigraphy enables one to differentiate between the volcanic units and to correlate the units of one basin, with contemporaneous outflows or intrusions in other basins. By means of their distinctive paleomagnetic data it is possible to correlate the volcanics of the Newark and Gettysburg basins (Pennsylvania) with the Holyoke volcanic event, and the vol­canics of the Bay of Fundy (Nova Scotia) with the Hampden volcanic event of Connecticut. Such correlations suggest the possibility of a northeastward displacement of the volcanic activity in late Triassic time. This would indicate a progressive northeastward expansion of the broad geanticlinal arching of the Appalachians in the early Mesozoic time. The origin of the rapid shift is unknown. It appears, however, that the shift is related to a high order reversal of the magnetic field. The shift occurred in an interval of relatively low field strength separating the Permian positive magnetic era from the Cretaceous negative magnetic era. The same interval was characterized by an increase in the frequency of lower order reversals (magnetic epochs and events).