Paleo-Tethys Research Articles

Geochemical and Isotopic Compositions of Fluorites from the Yama Fluorite Deposit in the Qilian Orogen in Northwest China, and Their Metallogenic Implications

The Yama area is characterized by numerous large-scale fluorite–quartz veins that are located along faults within the widespread Late Devonian–Late Silurian syenogranites in the Tataleng granitic batholith, Qilian Orogen, Northwest China. These fluorite–quartz veins contribute to an important fluorite reserve, but their ore genesis remains unresolved so far. In this study, trace elements, rare earth elements (REEs), and hydrogen, oxygen, and strontium isotopic compositions of fluorites are analyzed. The studied fluorite samples have similar chondrite-normalized REEs, including Y patterns, with relatively strong enrichment in heavy REEs, negative Eu anomalies, strongly positive Y anomalies, and comparably invariable Y/Ho ratios of 41.43–73.79, suggesting a unique hydrothermal genesis. The relatively variable values of δD and δ18O are −77.4‰ to −102.4‰ and −12.7‰ to −4.3‰, respectively, close to the meteoric water line. These fluorites yield relatively invariable analytical 87Sr/86Sr ratios of 0.749089−0.756628 (except for an anomalously high ratio), and their calculated initial 87Sr/86Sr ratios, based on the ore-forming ages provided, are apparently higher than the calculated initial 87Sr/86Sr ratios of syenogranite wall rocks. Collectively, the geochemistry of trace elements, REEs, and stable isotopes (H, O, and Sr) suggests that the ore-forming fluids were of meteoric origin and that the Sr sources were directly derived from the ore-forming fluids themselves rather than syenogranite wall rocks. Finally, it was considered that the Yama fluorite deposit is a fault-controlled hydrothermal vein-type deposit which was possibly related to the evolution of the Paleo-Tethys Ocean in the Permian–Triassic.

Metallogeny and temporal-spatial distribution of sedimentary phosphorite mineralization in Iran, relation to tethyans oceans evaluation and implications for future exploration

There are several deposits and mineral occurrences of sedimentary phosphorite (SP) mineralization in Iran. The main tectonic-structural zones of Iran that host SP deposits are: (1) Alborz Magmatic Belt (AMB), (2) the Central Iran and (3) Zagros Folded Thrust Belt (ZFTB). These deposits were formed during separate time: (1) Neoproterozoic-Early Cambrian (Soltanieh Formation), (2) Ordovician-(Silurian) (Shirgesht, Mila and Seyahou Formations), (3) Upper Devonian (Jeirud Formation), (4) Permian to Jurassic (Abkhory, Taleqan and Hosseinabad area) and (5) Cretaceous to Paleogene (Gurpi and Pabdeh Formations). The stratigraphic sequences of these deposits are commonly formed from sedimentary rocks, which are similar to other SP districts in the Middle East and North Africa. The main period of SP mineralization in Iran was from the Neoproterozoic-Early Cambrian, Upper Devonian and Late Cretaceous to Paleogene. The Jeirud Formation of the Upper Devonian and the Pabdeh Formation of the Paleocene-Eocene contained the largest sedimentary phosphorites in Iran. The formation and distribution of sedimentary phosphorite deposits in the different structural zones can be explained by divergent and convergent events related to the Proto, Paleo, and Neo-Tethyan oceans. The SP deposits hosted by the Neoproterozoic-Early Cambrian Soltanieh Formation are related to the Proto-Tethys Ocean. Mineralization in the Ordovician Mila and Shirgesht formations and Upper Devonian Jeirud Formation are age related to the Paleo-Tethys Ocean, and finally, the SP deposits formed in the Cretaceous- Paleogene (Gurpi and Pabdeh formations) sequence were formed the evolution of the Neo-Tethys Ocean cycle. Passive margins are the principal plate tectonic settings for Iranian SP deposits. Iranian SP deposits are stratigraphically controlled in which phosphorite mineralization occurs as laminae, disseminated grains, and replacement. SP mineralization is commonly closely related to framboidal pyrite and biologic events. The AMB and ZFTB are the main tectonic zones for Iranian sedimentary phosphorite deposits. These zones are the most favorable metallogenic provinces for the exploration of sedimentary phosphorite deposits, representing in excess of 80% of the as of now known SP deposits and occurrences in Iran. High sea level, low oceanic anoxic events (OAE's), warm climates, latitude and longitude close to the equator and shallow slope environments are among the most important factors controlling Iranian SP deposits.

The origin, conditions and mechanism for the formation of alpine-type hyperbasites of the Lesser Caucasus

The problem of hyperbasites origin is one of the widely discussed topics in geology. This is because they often appear when no expected. Their development does not correspond to the general regularities of the geological complexes’ development. Therefore, when problematic instances of hyperbasites appear, discussion is inevitable. This is due to the imperfections of existing concepts, which are not without flaws. The essence of this concept lies in the fact that during the rotation of the Earth around its axis, geodynamic forces are formed. The hyperbasites complex by its nature belongs to deep igneous formations formed at the initial stage of development of volcano-plutonic processes, where the composition of magmatic products was not subject to decomposition. In general, the origin of igneous rocks is associated with deep anomalous processes, which were formed under the influence of geodynamic forces, where decompression of mantle matter occurs, causing a catastrophic increase in the volume of mantle matter, as well as the associated development of volcanoplutonic processes. Hyperbasites are formed both in divergent and convergent zones of the Earth’s crust. The main factor for their formation is high pressure – deep thermodynamic conditions, where there are no favorable thermodynamic conditions for the complete separation of magmatic melts by composition. The emergence of hypermafic rocks on the surface is associated with geotectonic or denudation processes. Denudation processes can expose only those hypermafic formations that are located at the site of formation. These zones include ancient platforms, sheets, terranes, etc., which were cut by deep erosion processes. As for those hyperbasic formations that are classified as alpinotype hyperbasites, they were moved to the structure of the Alps-Himalayan folded zone from the basement with a collision with subsequent geotectonic processes, where they formed in the bed of the Paleotethys Ocean, both in the process of divergence and convergence. The noted pattern of formation and the mechanism of formation of alpine-type hyperbasites clearly corresponds to the patterns of development of geodynamic forces in the face of the Earth, also with natural laws, which are the main factors in the evolution of the Earth’s crust. From the standpoint of KDEZK, the origin, mechanism of formation, as well as the form of distribution of alpinotype hypermafic rocks of the Lesser Caucasus occurred in the Paleotethys bed, under different thermodynamic conditions and at depths. Further, as a result of the collision, it participated in the formation of folded zones of the Lesser Caucasus. Within the Lesser Caucasus, two genetic types of hypermafic rocks are exposed. Some of them correspond to the convergent zone of the Tethys paleocean, while others correspond to divergent zones. In terms of ore content, the most promising are those hypermafic rocks that are genetically related to convergent zones.

Geochemistry and geochronology of the Neyshabur meta-volcanic rocks, Binalood mountains, NE Iran: witnesses of Paleo-Tethys rifting and closure

Geochemical and geochronologic data are presented for meta-mafic to meta-felsic rocks along the Paleo-Tethys Suture in the Binalood Mountains east of Neyshabur, NE Iran. The rocks have a late Cambrian age (U–Pb zircon, ~ 490 Ma) and were metamorphosed in the Early Jurassic (40Ar/39Ar amphibole and plagioclase, 199–192 Ma). The rocks of this suite are alkaline and sub-alkaline (tholeiitic). The alkaline rocks are enriched in light relative to heavy rare earth elements, and do not show depletion of high-field strength elements on primitive mantle-normalized multi-element diagrams; they are similar to ocean island basalts (OIB). The tholeiitic rocks are depleted in Nb and Ta and have higher MgO and lower TiO2 than the alkaline rocks. Both types have similar, high and variable 87Sr/86Sr(i) isotopic compositions of 0.7044 to 0.7082 and 143Nd/144Nd(i) values of 0.5118 to 0.5122. The alkaline rocks are lower-degree partial melts than the tholeiitic rocks and were generated at greater depths; they likely originated from a garnet pyroxenite-rich source. The spatial, temporal, and geochemical relationships of early Paleozoic meta-mafic to felsic rocks along the Paleo-Tethys Suture (e.g., Shahrud, Jajarm, Binalood, Torbat-e-Jam) substantiate the role of a mantle plume in continental breakup along the northern margin of Gondwana and a late Cambrian-Ordovician onset of rifting that resulted in the opening of the Paleo-Tethys Ocean. The Early Jurassic metamorphism post-dates its closure.Graphical abstract

A Residual Middle Triassic Oceanic Island-Seamount in the Maoershan Area, Northwest Tibet: Implications for the Evolution of the Paleo-Tethys Ocean

The assemblage of oceanic islands and seamounts, arising from the widespread presence of mature oceans, plays a crucial role in reconstructing the evolutionary history of the paleoocean. Oceanic islands or seamounts within the Longmuco-Shuanghu metamorphic complex, a remnant of the Paleo-Tethys Ocean in the central Tibetan Plateau, have seldom been reported due to their remoteness. This study has identified an oceanic island-seamount in the Maoershan area, situated to the west of the Longmuco-Shuanghu metamorphic complex, composed of basalt, diabase, limestone, and siliceous rocks. Based on field observations, petrology, zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopes analyses, we have identified a suite of mafic rocks with OIB affinity. The youngest zircon U-Pb age cluster was concentrated at ~243–241 Ma. The geochemical characteristics of the siliceous rocks indicate a mixture of terrigenous material, suggesting that they formed in a continental margin. In combination with regional geological data, we conclude that the Longmuco-Shuanghu Paleo-Tethys Ocean remained open during the Middle Triassic. Furthermore, a fraction of the oceanic island-seamounts underwent scraping and transformed into a metamorphic complex, while other segments experienced deep subduction, resulting in the formation of high-pressure metamorphic rocks. Collectively, these processes gave rise to the distinctive high-pressure metamorphic complex within the central Qiangtang terrane.

Early Paleozoic extensional tectonics along Gondwana’s northern margin: Insights from Iran

The Paleozoic tectonic evolution of the northern margin of Gondwana is inferred to comprise several episodes of continental breakup and ocean opening. However, the timing and number of rifting events, and the identity and paleogeography of the continental blocks involved in these inferred breakup events remain poorly understood. This study examines the early Paleozoic tectonic evolution of northern Gondwana by analyzing the paleogeography and provenance of Ordovician strata exposed in the Iranian blocks. New detrital zircon U–Pb–Hf measurements from Ordovician siliciclastic strata in Iran define two major age fractions at 600—500 Ma (41 %; ɛHf(t) = −11.9 to +9.6) and 1100—600 Ma (37 %; ɛHf(t) = −21.6 to +10.6), and two minor fractions at > 1100 Ma (13 %; ɛHf(t) = −13.9 to +5.9) and 500—444 Ma (10 %; ɛHf(t) = −9.1 to +4.4). Detrital rutile U–Pb ages from these units similarly include two major age fractions at 1100—600 Ma (49 %) and 600—500 Ma (33 %), and two minor fractions at > 1100 Ma (14 %) and 500—444 Ma (4 %). Zr-in-rutile temperatures and Cr–Nb compositions indicate that most detrital rutiles are sourced from amphibolite facies metapelitic or metafelsic rocks. The new data suggest sediment was ultimately sourced from the Arabian-Nubian Shield (∼54 %) and 600—500 Ma (∼39 %) and Ordovician (∼7%) magmatic rocks in local basement in Iran. The widespread exposure of 600—500 Ma magmatic rocks in Iran indicates significant changes in basin configuration from a stable platform in the Ediacaran—Cambrian to a series of fault-bound extensional basins in the Ordovician. We explore two possible scenarios for the Ordovician extension in Iran. The first involves detachment of a continental block from northern Iran leading to opening of the Paleo-Tethys Ocean. However, because the identity of this hypothesized rifted continental block remains unclear, we interpret that early to middle Paleozoic extension across the region occurred within an already established continental margin.

Partial melting of multiple mantle domains underneath the Paleo-Tethyan cold subduction zone triggered by slab rollback

It is widely believed that the melting of mantle wedge in a subduction zone is triggered by the influx of fluids. However, experimental petrology studies have yielded conflicting results regarding temperatures for the wet solidus of mantle metasomatite, either below or above that of mantle wedge. In the latter case, the influx of fluids cannot trigger mantle melting, particularly in a cold subduction zone. Late Paleozoic magmatic rocks in the Western Yunnan Tethyan belt have been interpreted as being products of the first-stage subduction-related magmatism in the cold subduction zone and are thought to carry crucial information about mantle–wedge melting. Here, we report zircon ages and whole-rock geochemical data for these magmatic rocks to identify both the origin of the magmatism and the mechanism of mantle melting. Rock samples collected from the Nanlianshan complex and the Daxinshan Formation yielded zircon ages ranging from 303 Ma to 297 Ma. The samples can be classified into two groups based on geochemical features. Group I magmatic rocks have higher Ba/Th and lower Th/Nb ratios and depletion in NdHf isotopes [εNd(t) 2.9–5.0; εHf(t) 5.7–14.6], whereas Group II rocks show relatively lower Ba/Th and higher Th/Nb ratios and relative enrichment in NdHf isotopes [εNd(t) 0.7–4.0; εHf(t) 4.1–10.7]. The two groups of rocks may have originated from different mantle domains that were metasomatized by slab-derived aqueous fluids and by sediment-derived melts, respectively. We hypothesize that slab rollback during closure of the Paleo-Tethyan Ocean prompted mixing of different mantle domains; subsequently, the multiple mantle was heated by the laterally flowing asthenosphere, and partial melting occurred.

Geochemistry and Zircon U–Pb Geochronology of the Wugongshan Granites in the Northwestern Jiangxi Area, China: Implications for the Paleozoic Tectonic Development of South China

The properties of the Caledonian orogeny along the transition belt of the Yangtze and Cathaysia blocks have received much attention in recent years. The widespread Early Paleozoic granites provide critical geological clues for unraveling the tectonic evolution and geodynamic processes of the South China Continent (SCC). Here we present new zircon U–Pb chronology, whole-rock major and trace elements, in situ Hf isotopes for Paleozoic granites, i.e., the Wugongshan granites in the northwest Jiangxi province, and aim to explore the magmatism and properties of the Caledonian orogeny involved in their formation. Our new data show that the Wugongshan granites were emplaced during the Early Silurian Period (442–438 Ma). The Paleozoic Wugongshan granites belong to S-type muscovite-bearing peraluminous granites (MPG) and show a single origin. The Wugongshan granites exhibit negative εHf(t) values (−11.56 to −6.19) and TDM2 model ages of 2148–1809 Ma, indicating their derivation from an ancient crustal source, through partial melting of ancient crustal material. The Wugongshan granitic magmatism is probably being generated in an extensional environment related to an intracontinental orogeny setting. It is inferred that the Paleozoic tectonic–magmatic event in the Wugongshan area was associated with the oceanic–continental convergence of the Paleo-Tethys Ocean. The Wugongshan granites highlight the intracontinental magmatism in the Early Paleozoic orogeny in the SCC.

Differences in Geochemical Signatures and Petrogenesis between the Van Canh and Ben Giang-Que Son Granitic Rocks in the Southern Kontum Massif, Vietnam

Permian Ben Giang-Que Son and Triassic Van Canh granitic rocks are widely distributed across the southern Kontum Massif, the basement of which consists mainly of metasedimentary rocks. The Ben Giang-Que Son granitic rocks are classified as I- to S-type and ilmenite-series granitic rocks, while the Van Canh granitic rocks are classified as I-type and magnetite-series granitic rocks. Both granitic rock suites exhibit more or less adakitic properties, suggesting that the subduction of the high-temperature Song Ma Ocean crust, part of the Paleo-Tethys Ocean, beneath the Indochina Block produced adakitic magma. It is hypothesized that the differences between the two granitic rock suites were caused by differences in the quantities of incorporated continental crustal materials and carbon or graphite in clastic sedimentary rocks when their adakitic magma intruded into the continental crust. Based on their high initial Sr isotope ratios, the Ben Giang-Que Son granitic rocks evidently incorporated a higher quantity of continental crustal materials compared to the Van Canh granitic rocks, resulting in the former showing the signatures of ilmenite-series and I- to S-type granitic rocks. Consequently, the Ben Giang-Que Son granitic rocks have relatively high A/CNK ratios and high total Al contents in their biotite, whereas the Van Canh granitic rocks have low A/CNK ratios and low total Al contents in their biotite. The intrusion of the Ben Giang-Que Son granitic rocks caused high-temperature metamorphism, which decomposed some of the carbon or graphite in the surrounding continental crustal materials, such as clastic sedimentary rocks. Meanwhile, the Van Canh granitic rocks, which intruded later than the Ben Giang-Que Son granitic rocks, incorporated smaller quantities of carbon or graphite in continental crustal materials, resulting in them retaining the chemical characteristics of adakitic, magnetite-series, and I-type granitic rocks, different from the Ben Giang-Que Son granitic rocks.

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.

Changes of tectonic regime of the East China Sea Shelf Basin since Mesozoic: Insights from the Tiantai slope belt, East China Sea

Because of its unique tectonic location, the East China Sea shelf basin is an ideal laboratory for studying the transition of the East Asian continent from the Paleo-Tethys tectonic domain to the Pacific tectonic domain. The Tiantai slope belt within the East China Sea shelf basin contains geological information regarding the Mesozoic tectonic system transformation of the basin. Therefore, understanding the structural characteristics and evolution process of the Tiantai slope belt is extremely important. The structural characteristics of the Tiantai slope belt are clarified through the interpretation of seismic data, and its structural model is established. The transition of the East China Sea shelf basin from the Paleo-Tethys tectonic domain to the Pacific tectonic domain is determined. The NWW-trending faults formed in the Jurassic were detachment fault systems. The stress direction shifted during the Early Cretaceous, forming an NE-trending structural feature. The difference in extensional strength between the north and south sides of the Tiantai slope belt allows the NWW-trending basement faults to slip as accommodation faults, regulating deformation and inducing shallow arcuate faults in their hanging wall. The tectonic evolution of the Tiantai slope belt indicates that the East China Sea shelf basin completed the transition from the Paleo-Tethys Ocean tectonic domain to the Pacific tectonic domain in the Early Cretaceous, and the East China Sea shelf basin has entered the back-arc rifting stage of the Pacific tectonic domain after the Jurassic continental marginal depression.

A combined methodology for reconstructing source-to-sink basin evolution, exemplified by the Triassic Songpan–Ganzi basin, central China

Source-to-sink evolution of a basin is a key to understand sedimentary processes, especially in a complex regional orogenic setting. Detrital zircon populations can be traced from their primary sources to their depositional settings. The resulting interpretations are enhanced by calculation of the adjacent orogen's paleoaltimetry, which provides additional insights into paleogeography. In this study, we present a combined methodology which aims to reconstruct source-to-sink evolution by the analysis of detrital zircon age distribution in sandstones, together with the calculation of paleo-elevation of surrounding orogens based on the chemical compositions of coeval magmatic rocks. We test the method using detrital zircon U–Pb geochronological data sets from the Triassic Songpan–Ganzi basin in central China, combined with whole-rock geochemical data from intermediate-composition magmatic rocks in adjacent crustal blocks. Application of the combined methodology supports a syn-collisional basin model for the formation of the Triassic Songpan-Ganzi basin in preference to a continental back-arc basin. The clastic sediments, mainly deep-marine turbidites, accumulated in a remnant Paleotethyan Ocean that was surrounded by the converging North China Block, South China Block, East Kunlun Orogenic Belt and the Qiangtang Block. The North China Block and the North Qaidam Block were major proto-sources of detrital zircons to the basin, contributing on average 12 % and 15 %, respectively. Triassic magmatic rocks in the East Kunlun and Qiangtang regions were major sources of igneous zircons, up to 68 % for the former and up to 56 % for the latter. Despite being located at a calculated elevation of ca. 4000 m, the Qinling Orogenic Belt contributed only ca. <10 % of the zircons, mostly restricted to the eastern depocenter of the basin. In contrast, supply from the North Qiangtang Block, despite its calculated lower elevation (1000–3000 m), accounts for 2–10 % of the detrital zircons in the basin, suggesting high erosion rates of this block. The minimal supply of zircons from the South China Block, restricted to 3–6 % in the central and western depocenters, is inconsistent with the zircon abundances predicted in the alternative back-arc basin model of the Songpan–Ganzi basin.

Geology, geochronology, geochemistry and Hf-Sr-Nd isotopes of the ‘green-bean rocks’ (felsic tuffs) in the western Yangtze Block, China: Implications for genesis and enrichment mechanism of Li

The widespread ‘green-bean rocks’ (GBRs) in the western Yangtze Block (YZB) are traditionally considered to be originally felsic volcanic tuff. These greenish or yellowish green GBRs are characterized by the presence of bean-like grains. Although the GBRs are enriched in Li in addition to K, the enrichment mechanism of Li during the formation process of GBRs remains controversial. Detailed petrography, scanning electron microscopy, X-ray diffraction, whole-rock geochemistry, U-Pb dating and Hf-Sr-Nd isotopes have been conducted on the GBRs from the Longmendong section in the western YZB to shed light on the genesis of GBRs, Li enrichment mechanism, and further exploration.The zircon and apatite from Longmendong GBRs yielded U-Pb ages of 247.2 ± 0.6 Ma and 247.0 ± 11.0 Ma, respectively. Longmendong GBRs show bull-rock (87Sr/86Sr)i ratios ranging from 0.72602 to 0.72948 and εNd(t) values between −9.8 and −10.4. Zircons have low εHf(t) values and old two-stage Hf model ages (TDM2) ranging from −16.1 to −7.4 and 1741 to 2285 Ma, respectively. Apatite grains display high (87Sr/86Sr)i ratios and low εNd(t) values in the range of 0.72225–0.72461 and −9.93–−11.18, respectively. The combined evidence from textures, mineral compositions, whole-rock geochemistry and Sr-Nd-Hf isotopic compositions indicates that the GBRs originated from felsic volcanic ashes that erupted along the Sanjiang Orogen. The magma of volcanic ashes was sourced from the remelting of ancient crust in response to the initial collision of Indochina-Yangtze Blocks during the closure of the eastern Paleo-Tethys Ocean. The apatite grains separated from GBRs show significantly low Sr and Eu contents (136–223 ppm and 4.23–6.53 ppm, respectively) and low (La/Sm)N (0.97–1.11) and (Sm/Yb)N (3.49–4.56) ratios, indicating that apatite crystallized earlier than the plagioclase and the original magma are highly evolved. The magma of GBRs originated from ancient crust with significant differentiation, facilitating the initial enrichment of Li during the magma evolution. On the other hand, the GBRs have elevated levels of K2O, MgO and clay minerals (e.g., illite and illite/smectite mixed layer minerals), which are proportional to the Li concentrations. This suggests that Li in the GBRs is adsorbed by clay minerals in ionic form and experience additional enrichment during the transformation from volcanic ashes into clay minerals. Furthermore, the GBRs in the northeastern Sichuan basin under a limited sedimentary environment have relatively higher Li concentrations, suggesting that continuous evaporation also positively contributed to Li enrichment in GBRs. Geographically, GBRs in the Sichuan basin are closely related to underground Li-rich brines hosted within the adjacent strata. Therefore, the widespread Li-rich GBRs in the western YZB not only hold promise as a potential Li resource, but also serve as a marker layer for the prospecting Li brines in the Sichuan basin.

The gold mineral systems of the West Qinling orogen, central China: New insight from the Baguamiao gold deposit

The Baguamiao gold deposit is the first large deposit discovered in the West Qinling orogen in central China in the 1990 s. However, the occurrence of invisible gold in the deposit has not been reported. There is also no consensus on whether magmatic activity is involved in the gold mineralization, the style of mineralization, and the mechanism of gold precipitation. In this study, the in-situ trace elements, and chemical mapping of pyrite and pyrrhotite in the Baguamiao gold deposit were carried out for the first time. These data combined with the C-H-O isotope data of quartz, ankerite and calcite were used to determine mineralization process. In the Baguamiao gold deposit, five types of pyrite (Py0 to Py4) and pyrrhotite (Po0 to Po4) were identified, corresponding to the sedimentary-diagenetic period and four stages of mineralization (Stage I to Stage IV), respectively. From stage I to stage IV, gold mineralization in pyrite and pyrrhotite occurs as either visible or invisible crystals. The latter occurs dominantly as lattice-bound solid solutions (Au+) and is related to the content of Cu and Te. Co and Ni contents and Co/Ni ratios indicate that pyrite and pyrrhotite are magmatic-hydrothermal origin and affected by fluid-rock interaction. The change of Se and Te concentrations in pyrite and pyrrhotite indicates that oxygen fugacity (ƒO2) was increased from stage I to stage II, and then decreased during stage III and stage IV. In addition, the δ18O values of quartz, calculated δ18OH2O values of the ore-forming fluids and δ13C values range from 16.9 to 20.3 ‰, 7.7 to 12.9 ‰, and − 19.3 to + 0.2 ‰, respectively. Such isotopic signature records typical characteristics of orogenic gold deposit. The ore-forming fluid is a mixed hydrothermal solution involving metamorphic water, magmatic water, and meteoric water, and mixed with marine carbonate and sedimentary organic matter. Gold was precipitated by phase separation, fluid-rock interaction, and fluid mixing. Combined with the tectono-magmatic evolution, metamorphic deformation, and gold mineralization age of the West Qinling orogen, it can be concluded that with the closure of the Mianlue portions of the East Paleo-Tethys Ocean, the Baguamiao gold deposit was formed in the post-collisional extensional environment between the South China Block and the North China Block in the Late Triassic and is an orogenic gold deposit reference to the crustal continuum model.

Early Permian passive-margin magmatism in the Sibumasu Block, northern Thailand: A product of the slab-pull force of the Paleo-Tethys Ocean?

Continental rift-related magmatism provides important clues to continental rifting and breakup, but the petrogenesis and geodynamic processes remain debatable. Here we report a detailed study on zircon and apatite UPb dating, whole-rock geochemistry, and Sr-Nd-Hf isotopes for the hornblende gabbros in northern Thailand. Zircon and apatite UPb age data reveal emplacement of the hornblende gabbros at ca. 290 Ma. The hornblende gabbros are alkaline in composition and characterized by slight negative Nb-Ta-Zr-Hf and marked negative Ti anomalies. They have enriched Sr-Nd-Hf isotopic values [whole-rock (87Sr/86Sr)i = 0.705970–0.706756; εNd(t) = +0.1 to +0.7; zircon εHf(t) = +0.2 to +9.2]. Their geochemical affinities to within-plate basalts suggest a continental rifting setting. These combined features reveal that the hornblende gabbros in northern Thailand originated from a subcontinental lithospheric mantle previously modified by subducted sediments. Our results combined with published data indicate that the continental rift-related magmatism in the Sibumasu Block took place over a long period of ca. 309–282 Ma. We suggest that the slab pull force of the Paleo-Tethys oceanic lithosphere is a likely mechanism for the Late Carboniferous-Early Permian magmatic event in the passive margin of the Sibumasu Block.

Coexisting Nb-enriched basalts and arc volcanic rocks in the northern Qiangtang Terrane, China: Implications for the effects of ambient mantle on subduction zone magmatism

The petrogenesis of Nb-enriched basalts (NEBs) is controversial. NEBs are generally considered to form by melting of either a mixed enriched and depleted mantle source or a mantle wedge metasomatized by adakitic melts. Here we present geochronological, petrological, geochemical, and Sr-Nd-Os isotope data for Early Permian volcanic rocks from the Tuotuohe area in the northern Qiangtang Terrane, China. The studied volcanic rocks can be divided into two groups: arc basalts and basaltic andesites in the lower sequence and NEBs in the upper sequence. Zircon grains from one basaltic andesite sample in the lower sequence yielded a concordia age of 302.0 ± 1.1 Ma. The geochemical and isotopic characteristics suggest that the NEBs were derived from a mixed source consisting of enriched mid-oceanic ridge basalts (MORB)- to oceanic-island basalt-like ambient mantle and slab-derived fluids, whereas the arc basalts and basaltic andesites originated from a source made of the MORB-type ambient mantle modified by slab-derived melts. We propose for the first time that the Nb enrichment of the NEBs was most likely inherited from high-Nb ambient mantle and is independent of the addition of slab-derived components. In addition to slab-derived components, the nature of the ambient mantle is also an important factor controlling the diversity of arc magmatism. Our new findings show that the Late Carboniferous−Early Permian magmatism in the northern Qiangtang Terrane was related to northward subduction of the Paleo-Tethys Ocean, suggesting that subduction might have occurred in at least the Early Permian rather than in the previously thought Middle Permian.

The central Qiangtang Metamorphic Belt in northern Tibet is an in-situ Paleo-Tethys Ocean: Evidence from newly discovered Late Devonian radiolarians

The paleogeographic position of the North Qiangtang Block, as well as the origin of the Central Qiangtang Metamorphic Belt (CQMB) have subjected to considerable debate that hampers the understanding of the early evolution of the Paleo-Tethys Ocean. This study reports a new radiolarian fauna of a Famennian age (Late Devonian) from the ophiolitic mélange south of Gangtang Co, northern Tibet, including Callela parvispinosa Won, Entactinia foveolata Nazarov, and Plenoentactinia pinguis Won. The discovery of Devonian radiolarians in the CQMB strongly supports the model that the Longmu Co–Shuanghu suture zone represents the main branch of the Paleo-Tethys Ocean. A correlation of the Late Devonian radiolarian in Tethys realm reveals that the Longmu Co–Shuanghu suture zone was connected to the Changning–Menglian suture zone in western Yunnan, the Chiang Mai–Inthanon and Chanthaburi suture zones in Thailand, and the Bentong–Raub suture zone in Malay Peninsula. The synchronous advent of Late Devonian radiolarians suggests that the Paleo-Tethys Ocean may have opened during that time.

The Early–Middle Triassic Supervolcano in the Yangtze Block, South China and associated obstacles to biotic recovery

The end–Permian mass extinction was one of the major global crises spanning the entire Early Triassic or longer. Eruptions of volcanos were one of the factors that delayed the biotic recovery after this event. Supervolcano eruptions can cause catastrophic effects on global environment, climate, and life. Here we investigate the tuff layers from Early–Middle Triassic boundary in the Yangtze Block and identify a supervolcano eruption event. The zircon U–Pb ages of the section–Langdai, section–Daijiagou and section–Longmendong tuff samples are 247.1 ± 1.9 Ma, 247.6 ± 2.0 Ma and 247.7 ± 1.7 Ma, respectively. These ages mark the Olenekian–Anisian boundary. The zircon grains from the tuff layers have negative εHf(t) (−15.3 to −0.8), two–stage Hf model (TDM2) ages (1.7 to 2.2 Ga) and display high–δ18O values (mostly > 10‰). Clay minerals and quartz dominate the rock composition. The whole rock compositions show that the tuff layers were derived from magma of intermediate to felsic composition, which formed by the remelting of Paleoproterozoic materials of continental crust. The volcanic eruption site is located in the Jinshajiang–Ailaoshan–Song Ma suture zone in the southwestern margin of the Yangtze Block. A combination of the closure of the Paleo–Tethys Ocean Basin and the collision of the Indochina Block and South China contributed to the eruption, which was a supervolcano eruption under the active continental margin arc settings. We speculate that this supervolcano eruption might have contributed to the delayed biotic recovery after the end–Permian mass extinction.

Genesis of volcanic-hosted Cu vein deposits in the foreland fold-thrust belt, southeastern Tibetan Plateau: Insights from ore geology, C-O-S-Pb-Cu isotopes, and fluid inclusions at Wenyu

Copper deposits hosted by volcanic rocks can have different genetic types. Many copper vein deposits at the margin of the Cenozoic Lanping-Simao foreland fold-thrust belt in southwest China are controlled by thrust faults and hosted by Triassic volcanic rocks formed in association with the evolution of the Paleo-Tethys Ocean. The genesis of these copper deposits is poorly understood. Wenyu is a typical Cu deposit hosted in the Middle−Late Triassic basaltic-andesitic rocks and experienced three major hydrothermal stages characterized by mineral assemblages of calcite, anhydrite, and barite (Stage I); early quartz, chlorite, and epidote (Stage II); and Cu sulfides, galena, pyrite, hematite, and late quartz (Stage III). Consistency of the boiling temperature from fluid inclusions in the calcite and the temperature inferred from the C-O isotopes of the calcite, and the oxygen isotopes of the anhydrite and barite indicate that the stage I fluids probably have a δ13C value of ∼−4.5‰ and δ18O values in the range of ∼−3‰ to +4‰. Oxygen isotopes of the early quartz (δ18O = 15.1‰ to 17.4‰), D-O isotopes of chlorite and epidote, and the fluid inclusion data indicate that the stage II fluids have δ18O values of ∼2.4‰ to 5.0‰ and δ18D values of −68‰ to −53‰. Thus, the stage I and II fluids were inferred to be deep-seated brines with temperatures predominantly between 180 °C and 240 °C and salinities of 16−18 wt% NaCl eq. More significant meteoric water was involved during stage III based on the oxygen isotopes of the late quartz (δ18O = 6.2‰ to 9.5‰). The large variation in δ34S values (−21.3‰ to 1.6‰) in the sulfides, the sample- and grain-scale sulfur isotope fractionation, combined with the ore textural evidence that the sulfates and chlorite were replaced by Cu sulfides accompanied by hematite, indicate that the reduced sulfur responsible for sulfide precipitation was generated by inorganic reduction of the previously formed anhydrite and barite (δ34S = 6.3‰ to 8.6‰) or sulfates in the brines by oxidation of chlorite to hematite. Lead isotopes of galena (206Pb/204Pb = 18.67−18.87; 207Pb/204Pb = 15.68−15.72) and copper isotopes of Cu sulfides (δ65Cu = −0.34‰ to 0.51‰) indicate that the ore-forming metals (e.g., Cu and Pb) were derived from the ore-hosting Triassic volcanic rocks. The Wenyu copper deposit cannot be classified as a porphyry, epithermal, or volcanogenic massive sulfide deposit. It is believed that the deep-seated brines, such as the saline formation water in the Permian marine strata, migrated upward along the thrust faults into the Triassic basaltic-andesitic volcanic rocks, and leached metals from the volcanic rocks and precipitated Cu sulfides via inorganic sulfate reduction at Wenyu during the Cenozoic. Such mineralization processes may explain the genesis of the volcanic-hosted copper vein deposits in the Lanping-Simao foreland fold-thrust belt and other similar belts in the world.

Early Triassic roll-back of subducted Paleo-Tethys oceanic lithosphere: Insights from A2-type silicic igneous rocks in the Pingxiang area, southwest China

Abstract A suite of subduction-related early Triassic igneous rocks crops out in the Pingxiang area of the Dian-Qiong suture (DQS) in southwest China; this suite represents an important geological record of subduction and closure of the Paleo-Tethys Ocean. In this study, we report geochronological, geochemical, and Nd-Hf isotope data for newly discovered rhyolites and biotite granites in the Pingxiang area. We use these data to constrain their emplacement ages, origins, and geodynamic implications. Zircon U-Pb dating indicates that the rhyolites and biotite granites were emplaced at 251–250 Ma and 249 Ma (early Triassic), respectively. The rhyolites and biotite granites have elevated FeOT/(FeOT + MgO) (0.78–0.89) and 10,000 × Ga/Al (2.83–4.11) ratios, with geochemical affinities to A2-type granites. These rocks are enriched in some large-ion lithophile elements (e.g., Rb, Th, and Ba) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti), indicating their formation in a subduction-related arc. The rhyolites and biotite granites have negative whole-rock εNd(t) (−11.5 to −9.7) and zircon εHf(t) (−14.5 to −6.2) values, suggesting that these magmas were derived from an ancient crust-dominated source. Geochemical and Nd isotope data reveal that the peraluminous A2-type rhyolites and biotite granites were derived by partial melting of felsic crustal rocks under low-pressure and high-temperature conditions. By integrating all the available data with the regional tectonic evolution of the southwestern Youjiang Basin and adjacent regions, we attribute the generation of the peraluminous A2-type rhyolites and biotite granites to the extensional setting that existed during oceanic subduction, which was induced by roll-back of thePaleo-Tethys oceanic lithosphere at 251–249 Ma. This study indicates that subduction-related magmatism related to Paleo-Tethys oceanic lithosphere was still active in the early Triassic.