Late Triassic Volcanism Research Articles

Late Triassic paleolatitude of the Southern Qiangtang terrane, central Tibet: Implications for the closure of the Longmu Co-Shuanghu Paleo-Tethyan Ocean

Studies on the paleogeographic evolution of the Southern Qiangtang terrane, as an integral component of the Cimmerian continent, are essential for understanding the geodynamic evolution of the Tethys domain. Here, we present a paleomagnetic study of Late Triassic volcanics (∼222–206 Ma) from the Southern Qiangtang terrane, with the aim of enhancing our understanding of the evolutionary processes of the Longmu Co-Shuanghu Paleo-Tethyan Ocean. The characteristic remanent magnetizations (ChRMs) carried by magnetite and hematite, which formed during the high-temperature oxidation in the initial cooling phase of the volcanic rocks, are effectively isolated through successive thermal demanetizations. The ChRMs pass the fold test, indicating a primary magnetization. It appears to average out paleosecular variation well enough to yield a reliable paleolatitude estimate. Following tilt correction, the ChRMs yield an overall mean direction of Ds = 185.4°, Is = 53.8°, ks = 94.3, α95 = 3.8°, N = 16, indicating a paleolatitude of 34.3°N and a paleopole located at –22.5°N, 82.9°E with A95 = 4.4°. Comparing the Triassic latitudinal history of the Southern Qiangtang with that of the Northern Qiangtang suggests that the western part of Longmu Co-Shuanghu Paleo-Tethyan Ocean likely closed during the early Late Triassic. Moreover, integration with other geological evidence from the Southern Qiangtang terrane suggests that the Longmu Co-Shuanghu Paleo-Tethyan Ocean closed diachronously from east to west.

Crystal mush rejuvenation in the formation of Triassic dacites in Mengku, SE Tibetan Plateau

The tectono-magmatic event of the Changning-Menglian Paleo-Tethys orogen is an important process for understanding the tectonic evolution of the SE Tibetan Plateau. This orogen is composed of Changning-Menglian suture, Lancang group, Lincang batholith, and Yunxian volcanics. The Lincang batholith and its volcanic counterparts are part of Triassic magmatism in this orogen. The botholith is composed of biotite monzogranite, granodiorite and diorite. The Middle to Late Triassic volcanics include Manghuai rhyolite, Xiaodingxi basalt and Mengku dacite. The Mengku dacite is volcanic dome in west of the batholith. Lincang biotite monzogranite and Mengku dacite have similar geochemistry and are characterized by negative εHf (t) values (–13.2 to –5.2) and εNd (t) values (–14.7 to –8.8), whereas diorite dikes within the Lincang batholith have slightly high εHf (t) values (–8.3 to –4.9) and εNd (t) values (–6.9 to –6.7). Manifestations of resorption textures in quartz and reverse compositional zoning in feldspars are recognized from Mengku dacite. We consider that Mengku dacites resemble the erupted Lincang biotite monzogranite as they occur in the same tectonic settings and have similar geochemical features and mineral assemblages dominated by plagioclase, alkali-feldspar, quartz, and biotite. The evolution of the Mengku dacite magma is characterized by the rejuvenation of crystal mush represented by the Lincang batholith. The required thermal input was supplied by mafic magma, represented by diorite dikes and enclaves within the Lincang batholith. The Middle to Late Triassic magmatic event in this orogen likely resulted from lithospheric delamination and upwelling of the asthenosphere in a post-collisional environment.

Triassic felsic magmatism in SE Peninsular Malaysia: Petrogenesis and geodynamic implications for the Eastern Paleotethyan tectonic transition

Middle–Late Triassic volcanism and contemporary granitoids in East Malaya provide significant insights into the magmatic evolution and geodynamic processes of the Paleotethyan Ocean in SE Asia. This paper presents new zircon ages and whole-rock geochemical data including Sr-Nd-Pb-Hf isotopic analyses for newly identified felsic volcanic rocks and granites in SE Peninsular Malaysia. Zircon geochronological data constrain their formation time at 238–235 Ma. All samples have relatively high SiO2 contents with enrichment in light rare earth elements but have negative anomalies at Nb, Ta, Ti, and Eu on spidergrams. Calculated zircon saturation temperatures are between 840 °C and 957 °C. Samples have (87Sr/86Sr)i ratios between 0.70501 and 0.71014 and εNd (t) values ranging from −6.6 to −5.9, with (206Pb/204Pb)i = 18.45–18.77, (207Pb/204Pb)i = 15.68–15.71, and (208Pb/204Pb)i = 38.88–38.97. The zircon in-situ εHf (t) values and TDM2 ages range from −5.6 to +4.5 and 1.62 Ga to 0.98 Ga, respectively. The primary melting source which produced these felsic volcanic rocks and granites was Proterozoic crustal rocks with a juvenile middle-lower crustal component. Triassic volcanism in SE Peninsular Malaysia northerly correlates with synchronous volcanic rocks in the Lancangjiang and Chiang Khong-Lampang-Tak volcanic zones. Our data, along with regional observations, indicate that these volcanic rocks and granites formed in a localized extensional setting in response to the Paleotethyan slab breakoff. While the initial collision of Sibumasu with East Malaya may have occurred at ∼237 Ma.

Paleogeographic and tectonic evolution of Mesozoic Qiangtang basins (Tibet)

Reconstructing the Mesozoic paleotectonic and paleogeographic evolution of marine Qiangtang basins is essential for understanding the evolution of diverse Tethyan oceanic seaways and pre-Cenozoic mountain building in the Tibetan Plateau, with significant implications for hydrocarbon exploration. The geological knowledge of these basins is, however, largely incomplete. Here we illustrate new geological mapping, stratigraphic, structural, sedimentological, and provenance data collected along a >40-km-long, well exposed cross-section of the Biluoco fold-thrust belt in South Qiangtang, together with a compilation of regional data from the Qiangtang, Amdo, and Hoh-Xil terranes. We also provide refined biostratigraphy and chronostratigraphy of the exposed successions in the Biluoco type area. Newly achieved information indicates that the North Qiangtang and South Qiangtang basins evolved independently during the Mesozoic. The South Qiangtang basin started to subside by the Late Triassic and closed by the Late Jurassic. In contrast, sedimentation in the North Qiangtang basin commenced in the latest Early Jurassic and ended in the latest Jurassic. The Qiangtang basins received sediments from volcanic rocks exposed in the central Qiangtang and Hoh-Xil terranes since the Late Triassic, whereas Paleozoic strata in the Central Qiangtang mountain range were the primary source of detritus for the North Qiangtang basin in the early Middle Jurassic and for South Qiangtang basin in the Late Triassic and Middle Jurassic. Middle to Late Triassic volcanism and convergence associated with the closure of Paleo-Tethys within the Hoh-Xil and central Qiangtang terranes resulted in the topographic uplift of the central and northern Qiangtang regions. The Jurassic development of Qiangtang basins and their final inversion were controlled by the dynamics of the northward-subducting Bangong-Nujiang oceanic slab, microcontinent accretion to South Qiangtang, and Lhasa/Qiangtang collision.

In situ geochemistry and Fe–O isotopic composition of iron oxides from the Pha Lek Fe deposit, northwest Truong Son orogenic belt, Laos: implications for ore-forming processes

The Pha Lek Fe-polymetallic deposit in the southwest of the Truong Son orogenic belt records the evolution and metallogenesis of the southeastern Asia Tethys. Debates remain on the roles the Late Triassic volcanism played in the formation of the iron oxides of the deposit. Petrographic and SEM studies led to the recognition of two dominant iron oxides (magnetite and hematite) and three stages of mineralization (pre-ore, syn-ore, and post-ore stages). In situ LA–ICP–MS analyses for trace elements and Fe and O isotopic analyses were applied to understand the origin of the iron oxides. Generally, iron oxide of the syn-ore stage has higher total trace elements than post-ore stage, and magnetite has higher lithophile elements, whereas hematite has higher chalocophile elements. The trace-element abundance in magnetite is most comparable to porphyry-skarn-hydrothermal magnetite, and hematite is more comparable to supergene and secondary, non-magmatic hydrothermal chemistry. All δ56Fe values of the iron-oxide separates are above 0‰ and have an igneous signature, and syn-ore stage magnetite has relatively higher δ56Fe values than the post-ore stage hematite. All δ18O values of the iron-oxide separates are below 1‰ and plot within the non-igneous field, and show a similar decreasing trend from syn-ore stage magnetite to post-ore stage hematite. We propose that the Triassic volcanic activities played a limited role during the mineralization process; the deposit was formed because of the Late Carboniferous–Early Permian, porphyry-skarn-related, magmatic hydrothermal alteration; and the mixing of non-magmatic fluid and/or the supergene weathering caused the wide reprecipitation of Fe oxides. An evolutionary mineralization model related to the low-angle subduction for the deposit is also proposed.

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.

Sequence and petrogenesis of the volcanic rocks from the middle Sanjiang Tethys Orogen, SW China: Implications for the Sanjiang Paleo‐Tethyan evolution

The volcano‐sedimentary sequences from the Weixi belt provide significance for the Sanjiang Paleo‐Tethys evolution. This manuscript constrains the ages of volcano‐sedimentary sequences by LA‐ICPMS U–Pb, including bimodal volcanics (245.0–244.0 Ma) from Pantiange Formation, trachyandesite–basaltic andesite (234 ± 2.0 Ma) from Cuiyibi Formation, high silica subalkaline rhyolite (221 ± 1.4 Ma) from Waigucun Formation and Early Jurassic high silica subalkaline rhyolite (184 ± 2.2 Ma). The volcanics show various Hf isotopic composition, which includes Middle Triassic dacite (ε Hf[t] = −15.7 to −8.5, TDM2 = 2,006–1,613 Ma), Late Triassic rhyolite (ε Hf(t) = −9.8 to +1.9, TDM2 = 1,667–1,021 Ma), Late Triassic trachyandesite (ε Hf(t) = +2.6 to +5.7, TDM2 = 990–816 Ma) and Early Jurassic rhyolite (ε Hf(t) = −2.7 to +1.1, TDM2 = 1,250–1,145 Ma). The Middle–Late Triassic volcanics exhibit similar characteristics, the content of SiO2 exerts positive correlation with K2O and negative correlation with oxide of Ti, Fe, Al, Mg, Ca, Mn and P. The Hf isotopic and geochemical features indicate that the Middle Triassic basalt and rhyolite derived from mantle and crust by crystal fractionation, respectively. The absence of Lower Jurassic strata and existence of Triassic bimodal volcanic sequences suggest that the middle Sanjiang Tethys Orogen experienced collision during the Middle Triassic with local extension resulting from oblique convergence. Late Triassic–Early Jurassic rhyolite exhibit high SiO2 (77.12–77.81 wt%), K2O (4.35–6.10 wt%), Na2O (1.91–3.15 wt%) content, and a negative Eu anomaly. Together with the sedimentary sequence, the middle Sanjiang Tethys Orogen evolved into post‐collision phase during Late Triassic – Early Jurassic.

Timing and evolution of Mesozoic volcanism in the central Great Xing'an Range, northeastern China

In this paper, we report on LA‐ICP‐MS zircon U–Pb dates of 14 Mesozoic volcanic rocks from the central Great Xing'an Range (CGXR). These data are integrated with the temporal and spatial distribution of local magmatism‐related Mesozoic mineralization, in an effort to develop an understanding of the geodynamic evolution of the region. Periodic magmatic events in the GXR date from ~1,703 through ~145 Ma. Mesozoic volcanism in the CGXR can be subdivided into three episodes: Triassic (250–205 Ma), Early–Middle Jurassic (182–165 Ma), and Late Jurassic–Early Cretaceous (155–115 Ma). A revision of the previously defined volcanic sequence is offered using a combination of our zircon U–Pb ages and published Mesozoic geological and geophysical evidence from northeast China. We propose that (a) Early–Middle Triassic volcanism in the CGXR resulted from subduction of the Paleo‐Asian oceanic plate; (b) Late Triassic volcanism was related to subduction of the Mongol–Okhotsk oceanic plate (especially near the Erguna Block) and subsequent extension after the closure of the Paleo‐Asian Ocean; (c) Early–Middle Jurassic volcanism was mainly controlled by the southward subduction of the Mongol–Okhotsk oceanic plate; (d) Late Jurassic–initial Early Cretaceous magmatism was initiated by closure of the Mongol–Okhotsk Ocean; and (e) late Early Cretaceous volcanism was induced following the final closure of the Mongol–Okhotsk Ocean and rollback of the Paleo‐Pacific oceanic plate.

Sandstone provenance and U–Pb ages of detrital zircons from Permian–Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic-arc evolution in response to Paleo-Tethys subduction

Provenance analysis and U–Pb dating of detrital zircons in Permian–Triassic forearc sediments from the Sukhothai Arc in northern Thailand clarify the evolution of a missing arc system associated with Paleo-Tethys subduction. The turbidite-dominant formations within the forearc sediments include the Permian Ngao Group (Kiu Lom, Pha Huat, and Huai Thak formations), the Early to earliest Late Triassic Lampang Group (Phra That and Hong Hoi formations), and the Late Triassic Song Group (Pha Daeng and Wang Chin formations). The sandstones are quartzose in the Pha Huat, Huai Thak, and Wang Chin formations, and lithic wacke in the Kiu Lom, Phra That, Hong Hoi and Pha Daeng formations. The quartzose sandstones contain abundant quartz, felsic volcanic and plutonic fragments, whereas the lithic sandstones contain mainly basaltic to felsic volcanic fragments. The youngest single-grain (YSG) zircon U–Pb age generally approximates the depositional age in the study area, but in the case of the limestone-dominant Pha Huat Formation the YSG age is clearly older. On the other hand, the youngest cluster U–Pb age (YC1σ) represents the peak of igneous activity in the source area. Geological evidence, geochemical signatures, and the YC1σ ages of the sandstones have allowed us to reconstruct the Sukhothai arc evolution. The initial Sukhothai Arc (Late Carboniferous–Early Permian) developed as a continental island arc. Subsequently, there was general magmatic quiescence with minor I-type granitic activity during the Middle to early Late Permian. In the latest Permian to early Late Triassic, the Sukhothai Arc developed in tandem with Early to Middle Triassic I-type granitic activity, Middle to Late Triassic volcanism, evolution of an accretionary complex, and an abundant supply of sediments from the volcanic rocks to the trench through a forearc basin. Subsequently, the Sukhothai Arc became quiescent as the Paleo-Tethys closed after the Late Triassic. In addition, parts of sediments of supposed Devonian–Carboniferous age within the Sukhothai Arc were revised as the Triassic Lampang Group, and the Early Cretaceous Khorat Group.

The age of predictable primary diamond sources in the northeastern Siberian Platform

The U–Pb (SHRIMP) age was determined for zircons collected from 26 observation and sampling sites of diamonds and index minerals in the northeastern Siberian Platform. This part of the region hosts 15 low-diamondiferous Paleozoic and Mesozoic kimberlite fields, excluding the near economic Triassic Malokuonapskaya pipe in the Kuranakh field. Four epochs of kimberlite formation (Silurian, Late Devonian to Early Carboniferous, Middle to Late Triassic, and Middle to Late Jurassic) of the Siberian Platform, including its northeastern part, are confirmed as a result of our studies. Most observation points, including economic Quaternary diamond placers, contain Middle to Late Triassic zircons, which confirms the abundant Late Triassic volcanism in this region. The positive correlation of diamonds and major index minerals of kimberlites (mostly, garnets) at some observation sites indicates the possible Triassic age of the predictable diamondiferous kimberlites.

Petrogenesis and tectonics of late Permian felsic volcanic rocks, eastern Qiangtang block, north-central Tibet: Sr and Nd isotopic evidence

The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO2 concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (87Sr/86Sr) i of 0.70319–0.70547, and ϵNd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean.

GEOCHEMICAL AND PETROLOGICAL CHARACTERISTICS OF LATE TRIASSIC BASIC VOLCANIC ROCKS FROM THE KOCALI COMPLEX; SE TURKEY: IMPLICATIONS FOR THE TRIASSIC EVOLUTION OF SOUTHERN TETHYS

The Kocali Complex in SE Anatolia is mainly composed of tectonically imbricated slices of pelagic rock suits (pelagic limestones; cherts; etc); platform carbonates; clastics; serpentinites and basic volcanics. It can be subdivided into four main parts as the Tarasa Unit; the Konak Unit; the Cilo Limestones and the Kale Ophiolite. In this paper; geochemical characteristics of basic volcanic rocks associated with pelagic sediments (radiolarian cherts; pelagic limestones; etc) have been studied along four stratigraphical sections (the Tarasa; the Bulam-2; the Korun-1 and the Korun-2) in the Konak and the Tarasa Units. The age of the volcanic sequences have been assigned to Late Triassic (middle Carnian to Rhaetian) based on the radiolarian data from the associated pelagic rock units. Geochemical data indicates the presence of two different types of Late Triassic volcanic rock groups characterized by E-MORB-type (Group-1 volcanites from the Tarasa Unit formed by mixing of OIB and MORB-type melts) and OIB-type (Group-2 volcanites from the Konak Unit) mantle sources. None of the samples from the Kocali Complex has characteristics of N-MORB type mantle source and was affected by crustal contamination. Based on these data; it can be suggested that all these volcanic rocks formed in a marginal oceanic basin away from the ocean ridge. The presence of Radiolaria and Bivalvia-bearing pelagic limestones and cherts associated to the basic volcanic rocks in the Kocali Complex also emphasize that they all formed in deep marine basin. Geochemical characteristics of Late Triassic volcanics in the Kocali Complex have great similarities and well-correlative to the coeval volcanics previously studied in the Southern Tethyan Belt (e.g. the Baer-Bassit region in NW Syria; the Mamonia Complex in SW Cyprus and the Antalya Nappes in SW Turkey). Taking into the consideration of the geochemical data obtained from Late Triassic basic volcanics in the Kocali Complex and geochemical characteristics of the coeval volcanics in the adjacent units in the Southern Tethyan Belt; it can be suggested that rifting age of the Southern Tethyan Oceanic Basin in SE Anatolia is probably earlier than early Late Triassic (middle Carnian time).

Nature and origin of the Triassic volcanism in Albania and Othrys: a key to understanding the Neotethys opening?

AbstractTriassic volcanic rocks, stratigraphically associated with pelagic or reef limestones, are tectonically juxtaposed with Mesozoic ophiolites in the Tethyan realm. From the central (Dinarides, Hellenides) and eastern Mediterranean (Antalya, Troodos, Baër Bassit) to the Semail nappes (Oman), they occur either associated to the tectonic sole of the ophiolitic nappes or as a distinct tectonic pile intercalated between the ophiolites and other underthrust units. In the Dinaro-Hellenic belt, the Pelagonian units represent the lower plate, which is underthrust beneath the ophiolites. Middle to Late Triassic volcanic sequences are interpreted as the eastern flank of the Pelagonian platform and are therefore considered as a distal, deep-water part of the Pelagonian margin.The Triassic volcanics from Albania and Othrys are made up of basaltic pillowed and massive flows, associated locally with dolerites and trachytes. New elemental, Nd and Pb isotopic data allow to recognize four types of volcanic suites: (1) intra-oceanic alkaline and tholeiitic basalts, (2) intra-oceanic arc-tholeiites, (3) back-arc basin basalts, (4) calc-alkaline mafic to felsic rocks. Nd and Pb isotopic initial ratios suggest that the within-plate volcanic rocks were derived from an enriched oceanic island basalt type mantle source, devoid of any continental crustal component. The lower εNd value of the trachyte could be due to assimilation of oceanic altered crust or sediments in a shallow magma chamber. Island arc tholeiites and back-arc basin basalts have a similar wide range of εNd. The absence of Nb negative anomalies in the back-arc basin basalts suggests that the basin floored by these basalts was wide and mature. The high Th contents of the island arc tholeiites suggest that the arc volcanoes were located not far away from the continental margin.Albania and Othrys volcanics contrast with the Late Triassic volcanism from eastern Mediterranean (SW Cyprus, SW Turkey), which displays solely features of oceanic within plate suites. The presence of back-arc basin basalts associated with arc-related volcanics in Central Mediterranean indicates that they were close to a still active subduction during the Upper Triassic, while back-arc basins developed, associated with within-plate volcanism, leading to the NeoTethys opening.

Petrology, geochemistry and geochronology of late Triassic volcanics, Kunlun orogenic belt, western China: Implications for tectonic setting and petrogenesis

The Qimantage volcanic suite (QVS) of late Triassic age, dominated by dacitic to rhyolitic lavas and pyroclastic rocks with minor basaltic andesites, outcrops mainly in the middle segment of Qimantage Mt. region, Kunlun orogenic belt, which extends along the northern margin of Tibetan plateau, western China. Petrological and geochemical studies indicate that the volcanic rocks are of typical calc-alkaline affinity with silica contents ranging from ∼53 to ∼80 wt%, exhibiting a medium- to high-K characteristics. Major element variations define relatively smooth compositional trends, while large ion lithophile (LIL) trace elements correlate positively with increasing SiO2. Chondrite-normalized incompatible trace element spidergram shows conspicuous sparks at Rb and Th, but pronounced Nb and Ti depletion. The chondrite-normalized REE patterns of QVS are generally paralleled to each other, exhibiting highly fractionated LREE/HREE spectra with small to pronounced negative Eu anomalies. Two whole-rock Rb-Sr isochrons yield ages of ca. 209 and ca. 225 Ma, respectively, indicating the volcanic suite was emplaced during the late Triassic. The initial 87Sr/86Sr ratios (0.708-0.709) and initial eNd(t) values (-2.05∼-2.35) imply that the magmas were derived from a slightly enriched mantle source with considerable crustal contamination. Considering its petrological, geochemical and isotopic characteristics in a regional geological background, it is proposed that QVS was generated upon an active continental margin during the late Triassic north-directed subduction event. Both the volcanics and the closely-related coeval granitoid plutons collectively make up the late Triassic arc magmatic belt on the paleogeographically southern margin of the Tarim continent.

The Triassic unconformity of south-central British Columbia

At five localities investigated in south-central British Columbia, Upper Triassic rocks are observed or inferred to unconformably overlie upper Paleozoic and older rocks. Paleozoic rocks beneath the unconformity show polyphase deformation and low-grade regional metamorphism which are absent in overlying rocks. Data from these and other localities define a regional angular unconformity of Late Permian or Early Triassic age on the western and southern margins of the Shuswap Metamorphic Complex. Permian and Triassic rocks preserve evidence of structural, sedimentary, and metamorphic events which permits separation of Triassic rocks into three fault-bounded tectonostratigraphic belts. The Eastern Belt contains the transition from miogeoclinal sedimentation throughout Triassic time in the Canadian Rockies to island arc volcanism in the Late Triassic to the west. Basal beds of the Triassic sequence become younger southwest-ward from the axis of the Early to Middle Triassic depocentre lying west of the Rockies. Rocks preserving Early Triassic deformation and metamorphism are restricted to the southwest corner of the belt and are truncated by the Pasayten Fault. The Central Belt, dominated by the products of Late Triassic volcanism in northern and central British Columbia, consists mainly of Middle (?) and Upper Triassic sediments in the south. Meagre evidence indicates that widespread deformation and low-grade regional metamorphism occurred just prior to the Late Triassic. Evidence for these events is not found beyond the faulted margins of the Central Belt. In the Western Belt, an Upper Triassic sequence of tholeiitic basalt and overlying calcareous sediments disconformably overlies Permian rocks. In the western Cordillera, low-grade regional metamorphism and minor plutonism characterize Triassic orogenies. Early Triassic orogenesis in the southwestern corner of the Eastern Belt is coeval with the Sonoma Orogeny and the Middle–Late Triassic orogenesis of the Central Belt represents the Tahltanian Orogeny.

Le volcanisme alcalin du sud-ouest de chypre et le probleme de l'ouverture des regions tethysiennes au trias

An important volcanism of Late Triassic age is known from SW Cyprus. It occurs in the Mamonia nappe system emplaced during the Late Maastrichtian. Three main volcanic episodes interbedded with detrital and pelagic sediments can be seen from the base to the top: 1. (1) pyroclastic rocks (breccias, tuffs) associated with coarse-grained sandstone, suggesting explosive eruptions in grabens 2. (2) basaltic or andesitic pillowed flows, interbedded first with fine-grained sandstone and small Halobia limestone strata, then with pelagic limestones and radiolarian red cherts 3. (3) columnar trachyte flows. The whole volcanic series belongs to a very differenciated sodic suite with high titanium contents. The Mamonia lavas are very similar to the Afar volcanics and can be considered as belonging to an interplate volcanism in a rift system. This alkaline basaltic suite is found in many places of the East Mediterranean Alpine orogenic domain, especially in the Antalya nappes (South Turkey) and in the Baer-Bassit (Syria). In Greece, a similar volcanism has been noticed (Othrys—Pindos). In Italy there exists a Middle or Late Triassic volcanism with alkaline affinities. Therefore, this Late Triassic magmatism, which is widespread in the whole Mediterranean Alpine region and always in tectonic association with ophiolites, has a very great paleogeographic significance. We thus propose the existence of a rift system associated with an alkaline basaltic suite along the northern edge of the African plate during Norian—Carnian times. Afterwards a mid-oceanic ridge would have been formed during the Jurassic and Cretaceous. To explain this evolution two hypotheses can be proposed: 1. (1) A single mid-Tethysian ridge existed and all the ophiolites (Greece, Turkey, Cyprus, the ‘croissant ophiolitique peri-arabe’) have been thrust from the same area. 2. (2) A marginal sea existed along the mid-Tethysian ridge north of the African plate but separated from the Tethys by a carbonate shelf, where, after the Triassic events, oceanization began with slightly different ophiolites (a large sheeted complex, low-K tholeiites with some calc-alkaline affinities). Therefore, Troodos, Hatay, Zagros and Oman would not have come from the main ophiolite zone present further north, but from a marginal ocean, now obducted on the African plate. We think that the second hypothesis is more reasonable because the Upper Cretaceous sedimentary cover (Kannaviou Formation) of the Troodos is very similar to the detritic formation present in South Turkey (Kastel Formation) which is known to grade to shelf carbonates belonging to the Arabian plate towards the south.