Source rock potential assessment of the Huai Hin Lat Formation,Sap Phlu Basin, Nakhon Ratchasima Province, northeastern Thailand

The Huai Hin Lat Formation has a high-potential resource, and the Ban Nong Sai part was researched and sampled. To achieve this goal, petrographic analysis(kerogen types), geochemical analysis(total organic carbon content, TOC), vitrinite reflectance(Ro), and Rock–Eval(RE) pyrolysis were carried out in this study. According to the findings, types Ⅱ, Ⅲ, and Ⅳ were identified using a modified Van-Krevelen diagram because the higher mature source rock showing hydrogen index(HI) and oxygen index(OI) are continuously depleted and raised. However,microscopic observation describes macerals as primarily sapropelic amorphinite, therefore, type I is important. The TOC was determined to be between 1.90% and 7.06%,which is considered very good to excellent. The original total organic carbon(TOCo) was decided to use its maceral components to determine how to convert extremely mature TOC to TOCo. It varies between 5.13% and 10.74% and reaches a maximum of 57.21% which is comparable to TOC. At 0.82%–1.04%, 443–451 ℃, 0.50%–38.10%, and69.00%–99.59% are the vitrinite reflectance(Ro), maximum temperature(Tmax), production index(PI), and transformation ratio(TR), respectively. Late peak maturity refers to a mixture of oil and gas, whereas most TR ratios refer to the main gas phase. Similarly, the petroleum residual shows no indication of gas trapped at a volume of6309.50 mcf/ac-ft. In summary, source rock potential was assessed within a suitable risk range defined by Tmax(445.70 ℃), Ro(0.91%), TR(90.63%), TOC(8.15%),shale thickness(46 m), and kerogen type(type I).

Proto-Tethys ophiolitic mélange in SW Yunnan: Constraints from zircon U-Pb geochronology and geochemistry

An early Paleozoic Proto-Tethys ocean in western Yunnan has long been postulated although no robust geological evidence has been identified.Here we investigated the recently-identified Mayidui and Wanhe ophiolitic mélanges in SW Yunnan,which occurs in a N-S trending belt east of the late Paleozoic Changning-Menglian suture zone.The ophiolites consist mainly of meta-basalts(amphibole schists),meta-(cumulate)gabbros and gabbroic diorites,and meta-chert-shale,representing ancient oceanic crust and pelagic and hemipelagic sediments,respectively.Six samples of gabbros and gabbroic diorites from 3 profiles(Mayidui,Kongjiao and Yinchanghe)yielded zircon U-Pb ages between 462±6 Ma and 447±9 Ma,constraining the formation of the Mayidui and Wanhe ophiolites to Middle Ordovician.Gabbros from the Mayidui and Kongjiao profiles share similar geochemical characteristics with affinities to tholeiitic series,and are characterized by depleted to slightly enriched LREEs relative to HREEs with(La/Sm)N=0.69-1.87,(La/Yb)N=0.66-4.72.These,along with their predominantly positive wholerock eNd(t)and zircon eHf(t)values,indicate a MORB-like magma source.By contrast,the meta-mafic rocks from the Yinchanghe profile show significantly enriched LREEs((La/Sm)N=0.97-3.33,(La/Yb)N=1.19-14.93),as well as positive whole-rock eNd(t)and positive to negative zircon eHf(t)values,indicating an E-MORB-type mantle source.These geochemical features are consistent with an intra-oceanic setting for the formation of the Mayidui-Wanhe ophiolites.Our data,integrated with available geological evidence,provide robust constraints on the timing and nature of the Mayidui-Wanhe ophiolitic mélange,and suggest that the ophiolites represent remnants of the Proto-Tethys Ocean,which opened through separation of the Indochina and Simao blocks from the northern margin of Gondwana before the Early Cambrian,and evolved through to the Silurian.

Geochronological and geochemical constraints on the petrogenesis of late Mesoproterozoic mafic and granitic rocks in the southwestern Yangtze Block

Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent.In the present study,we report new geochronological,geochemical,and Nd-Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block.Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U-Pb ages of 1168-1162 Ma,constituting a bimodal igneous assemblage.The plagioclase amphibolites have high and variable TiO2 contents(1.15-4.30 wt.%)and Mg#(34-66)values,similar to the tholeiitic series.They are characterized by enrichment in LREEs and LILEs,and have OIB-like affinities with positive Nb and Ta anomalies.The plagioclase amphibolites have positive whole-rockεNd(t)(+3.2 to+4.3)and zirconεHf(t)(+4.3 to+10.7)values,indicating that they were derived from an OIB-like asthenospheric mantle source.The granites belong to the reduced peralkaline A-type series and have negativeεNd(t)value of-6.0 andεHf(t)values of-5.8 to-13.8,indicating a derivation from the partial melting of ancient mafic lower crust.In combination with the~1.05-1.02 Ga bimodal igneous assemblage in the SW Yangtze Block,we propose that the Cuoke 1168-1162 Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.

Study of Rock Mechanic Property and Mineralogy Relationship of the Huai Hin Lat Formation, Sap Phlu Basin, Northeastern Thailand: Implications for Understanding of Shale Gas Reservoir Rock

Thailand is lacked of gas that more information of probable (P2) and possible (P3) reserve data including shale gas can be acceptable to prove (P1) reserve data for new gas field. This research had implicated for understanding of unconventional reservoir rock by rock mechanical, micro-CT, and geochemistry analysis of the Huai Hin Lat Formation. The rock mechanical analysis is composed of average young’s modulus, Poisson’s ratio, and compressive strength of 1933.79 MPa, 0.1472, and 52.56 MPa. The average porosity of 6.89% consists of 5.41% and 1.48% of closed and open porosities. The average mineralogical results consist mainly of 57.60% and 42.40% of brittle and ductile minerals indicating more elasticity except Bed 6. The Bed 6 is significantly higher quartz (15%) and brittle minerals (64%) indicating to easier fracture are, therefore, lower compressive strength (25.93 MPa), young’s modulus (1729.10 MPa) and Poisson’s ratio (0.0705). The Beds 3B is slightly higher clay containing slightly higher closed porosity (5.46%) but the Bed 14 is slightly higher brittle mineral indicating to slightly higher open porosity.

Detrital Zircon of Devonian Sandstones in ChangningMenglian Suture Zone, Yunnan, SW China: Implications for the Early Evolution of Paleo-Tethys

The Changning-Menglian suture zone is a critical tectonic belt pivotal to the evolution of the Paleo-Tethys.The Wenquan Formation,characterized as turbidite deposited on the western side of the Changning-Menglian suture zone as well as the eastern edge of the Baoshan Block.Analysis of detrital zircons from the Wenquan Formation reveals two significant age peaks approximately at 440 and 980 Ma,with additional age populations around 600,780,and 2500 Ma.The major age peak at about 440 Ma is come from the andesitic-dominant volcanic arc within the Lancang Block and the western Simao Block,corroborated by the presence of andesite fragments in thin section studies.Meanwhile,the zircons that form the secondary age peak at~980 Ma and other older age groups probably originate from the Baoshan Block.Deposited on the western side of the Paleo-Tethyan ophiolites,the Wenquan Formation received detrital materials from the continental margin on the opposite side.Thus,the main Paleo-Tethyan Ocean basin was not sufficiently broad enough to cut off the transference of detrital materials.It was a relatively narrow basin in the Early Devonian.

Tethyan evolution from early Paleozoic to early Mesozoic in southwest Yunnan

The Tethys orogenic belt in SW Yunnan constitutes a critical part of the expansive Tethys-Himalayan tectonic domain.The abundant,well-preserved geologic records make it an ideal area for studying the tectonic evolution of Proto-and Paleo-Tethys.In this paper,we focus on several major tectonic units in SW Yunnan and reconstruct the Tethyan evolution from the early Paleozoic to the early Mesozoic,based on stratigraphic,sedimentologic,and magmatic evidences.The recently discovered early Paleozoic Yunxian-Menghai ophiolitic belt in the Lincang Terrane situated east of the Changning-Menglian Belt represents the suture zone of the Proto-Tethys.The oceanic basin of Proto-Tehtys opened in the latest Neoproterozoic and subsequently began subducting in the late Miaolingian of the Cambrian(about 505 Ma).From the late Late Ordovician to the ealiest Silurian(about 450–442 Ma),the Proto-Tethys basin gradually closed resulting in the collision of the continental plates on both sides of the Proto-Tethyan ocean.The main collision stage occurred in the early Silurian(about 442–430 Ma)and the postcollision stage lasted from the mid-Silurian to the early Carboniferous(430–355 Ma).The earliest record of Paleo-Tethyan oceanic crust was generated in the late Devonian,and the ocean was then subducted in an eastward direction in the middle Late Carboniferous(about 310 Ma).The initial collision stage in the Paleo-Tethys took place at the end of the Permian(about 253Ma),and the main stage of the collision persisted into the early Ladinian(about 253–238 Ma).This was followed by postcollision extension from the late Ladinian to the early Jurassic(ca.238–196 Ma).We suggest that the opening of Paleo-Tethyan Ocean in SW Yunnan was a result of the extensional rift basin of the Proto-Tethys.Additionally,the activity of the Manxin mantle plume was likely a crucial factor in the rapid expansion of the Paleo-Tethyan Ocean.

早三叠世-晚白垩世古太平洋俯冲:沙巴的弧前岩浆记录

厘定东亚大陆边缘中生代弧前岩浆记录以限定其俯冲边界及其时序,是解码古太平洋俯冲体系的关键.基于婆罗洲所处特殊大地构造位置且其具有弧前指示意义的基性-超基性岩石报道较少,本文选择菲律宾巴拉望构造带南延之婆罗洲沙巴地区开展了系统研究.对昔加麦火成岩、古达、特鲁比和达弗尔湾蛇绿岩基性岩及相关岩石的全岩Ar-Ar和锆石U-Pb年代学、全岩元素和Sr-Nd-Pb同位素地球化学的研究表明:沙巴基底发育有前三叠系岩石,构造上亲缘于华夏陆块或地处巽他古陆东缘.原定义为“沙巴非蛇绿岩的昔加麦基底岩石”的安山岩和英云闪长岩等形成于251~179 Ma,具埃达克质岩石地球化学属性,87Sr/86Sr(i)=0.70285~0.70307,εNd(t)=+6.5~+7.7,具太平洋洋中脊玄武岩(Mid-Ocean Ridge Basalt,MORB)型Pb同位素组成,源自俯冲板片派生组分交代的地幔源区,是古太平洋俯冲体系的重要组成部分.限定出沙巴地区蛇绿岩的形成时代介于早-中侏罗世至晚白垩世(约185~85 Ma),其中特鲁比蛇绿岩可能较早发育(约185~140 Ma)、古达蛇绿岩主体形成于早白垩世(约135~112 Ma)、而达弗尔湾蛇绿岩形成持续至晚白垩世(约85 Ma).沙巴蛇绿岩中镁质岩石发育具相似Sr-Nd-Pb同位素组成的MORB型、高铌和富铌基性岩,其87Sr/86Sr(i)=0.70291~0.70567,εNd(t)=+6.6~+10.2,(206Pb/204Pb)i=17.91~19.00、(207Pb/204Pb)i=15.43~15.58和(208Pb/204Pb)i=37.61~38.47,其源区受板片派生熔体的交代改造,形成于古太平洋俯冲弧前背景.该研究限定出东亚大陆边缘的古太平洋西向俯冲至少自早三叠世(约251 Ma)即已启动、并持续至晚白垩世(约85 Ma),具长寿命(>150 Ma)“多阶段幕式进撤”的安第斯型俯冲特征,其白垩纪俯冲前锋位于现今中国台湾东部、菲律宾西部,经沙巴、延入古晋带、甚至帕朗卡拉亚一线.该地区构造格局直至晚白垩世末期-新生代早期才转换为现今西太平洋俯冲体系.

A Diverse Microfossil Assemblage from the Ediacaran–Cambrian Deep-Water Chert of the Liuchapo Formation in Guizhou Province,South China

The Ediacaran to Cambrian transition witnessed great biological and environmental turnovers from the Precambrian to Phanerozoic.These changes are also reflected by the fossil records during that time.However,the fossil distribution was highly heterogenous.While diverse fossils were constantly found from the shallow shelf including restricted basins,reports from deep-water areas,which are characterized by chert-dominated formations straddling the Ediacaran-Cambrian boundary,were scarce.Among them was the Liuchapo Formation that has been considered barren.In this study,a fossiliferous assemblage was found from the Liuchapo Formation in east Guizhou Province,South China.This assemblage comprises Palaeopascichnus jiumenensis,Horodyskia minor,multicellular algae that possibly related with red algae,cyanobacteria,sponge spicules,small shelly fossils,membrane structures of uncertain affinity,and fossils with structures resemble the Ediacaran Megasphaera.Palaeopascichnus and Horodyskia have been widely reported and abundant in the Liuchapo Formation and equivalents,while the others have seldomly,or never,been found from this formation or,more widely,the deep-water chert of transitional interval.The discovery not only contributes to the rare paleontological records preserved in deep-water sediments,but also expanded the geographic distribution of the fossils,providing new materials of biological diversity during this critical interval.

Geochemistry,Zircon U-Pb Age and Hf Isotopic Constraints on the Petrogenesis of the Silurian Rhyolites in the Loei Fold Belt and Their Tectonic Implications

Zircon U-Pb dating, Lu-Hf isotopic and geochemical data for the Silurian rhyolites from the Loei fold belt are presented to constrain their petrogenesis and tectonic settings. The rhyolites give a weighted mean 206Pb/238 U age of 423.7±2.7 Ma, and are characterized by high SiO2, Al2O3, K2 O and low MnO, MgO and P2O5. All samples are enriched in LILEs（e.g., Ba, K, Pb） and LREEs and depleted in HFSEs（e.g., Nb, Ta, Ti） with obvious negative Eu-anomalies（δEu=0.56–0.63）. The calc-alkaline rhyolites are typical arc-related rocks. The Loei rhyolites have high A/CNK ratios（1.19–1.34） and positive εHf（t）（4.03–5.38）, which can be interpreted as partial melting of juvenile crustal materials followed by multistage melting and differentiation, similar to highly fractional I-type rocks. Combined with regional geological surveys, the Loei rhyolites should be formed in a volcanic arc environment and may be in contact with the Truong Son fold belt during the Early Paleozoic. Moreover, the Simao Block might be in contiguity with the Indochina Block during Silurian.

Paleoproductivity and Paleoredox Condition of the Huai Hin Lat Formation in Northeastern Thailand

The petroleum exploration has been conducted in the Khorat Plateau since 1962 and two gas fields have been discovered and commercially produced. The lacustrine facies of the Huai Hin Lat Formation is believed to be one of the main source rocks of the gas. Therefore, investigation and analysis of the Huai Hin Lat shale for understanding the paleoenvironment and petroleum source rock are carried out in this study. Petrographical study and geochemical analysis of shale samples were performed to explain the paleoproductivity and past redox condition. The palynofacies assemblage comprises abundant AOM, acritarchs, phytoclasts, and very small amount of spores and pollen. Geochemical analysis was used to determine the total organic carbon（TOC） and the concentration of major, trace, and rare earth elements. The paleoproductivity proxies are composed of palynofacies, TOC, excess SiO2, Ba/Al, and P/Al. They reflect a high paleoproductivity except the middle of the lower part（bed 3） and the lower bed 13 of the upper part. Bed 3 shows the highest peak of TOC and the lower bed 13 exhibits a relatively lower TOC, which can be explained by the excellent and the poorer preservation condition, respectively. The paleoredox proxies consist of U/Th, V/Cr, Ni Co,（Cu＋Mo）/Zn, Ni/V, and Ce anomaly. They are used to establish the depositional environments, to characterize the organic matter content, and to assess the source rock potential. They reflect many high peaks and predominantly high values of paleoredox proxies except the middle part and the lower bed（lower bed 13） of the upper part. They indicate that the section was mainly under anoxic or reducing condition, which is supported by the high Ce/Ce＊（〉0.8） and V/Cr（〉2.0） values. The middle of the lower part（bed 3） shows lower productivity but it contains the highest peak of TOC, which is conformed to be the excellent preservation of organic matters in the strong reducing condition. The middle part, which shows high productivity, contains relatively lower TOC as it possesses a less reducing condition compared to the more reducing intervals. The lower bed 13 of the upper part shows a less reducing condition and a lower TOC, which conforms to a lower productivity. The organic matters of the Huai Hin Lat Formation consist mainly of AOM and acritarchs and possess good to excellent TOC（2%–7%）. They belong mainly to type I and type II kerogens with some mixture of type III as indicated by the presence of phytoclasts, spores, and pollen. The organic matters of the Huai Hin Lat Formation, based on the kerogen type and the thermal history, have already generated significant amount of oil and some gas to the Sap Phlu Basin.

Geochemical and geochronological constrains on the Chiang Khong volcanic rocks （northwestern Thailand） and its tectonic implications

在在 Chiang Khong 区域主导地暴露的西北的泰国的暴烈的岩石，通常被认为遗传上被连接到 Paleo-Tethyan 海洋的构造进化。暴烈的岩石主要由 andesitic 组成到 rhyolitic 岩石并且传统地作为二叠三叠纪的序列被印射。我们二 andesitic 取样的锆石 U-Pb geochronological 结果表演(TL-1-B 和 TL-31-B ) ，姚尧的代表是暴烈的地区，并且给一个平均数 241.2 的加权的年龄？？

Petrochemistry and Tectonic Setting of the Middle Triassic Arc-Like Volcanic Rocks in the Sayabouli Area,NW Laos

The volcanic rocks from the Sayabouli area in northwestern Laos have been poorly studied. These volcanic rocks are traditionally mapped as the Permian–Early Triassic sequences on the geological map. One basaltic-andesite from the Sayabouli area yields a zircon U-Pb age of 237.7±1.7 Ma, suggesting a Middle Triassic origin. All basalt and basaltic-andesite samples from the Sayabouli area show depletions in HFSEs（e.g., Nb, Ta, Ti） and have high LILE/HFSE ratios, and exhibit the geochemical affinity to the continental arc volcanic rocks and are geochemically similar to the continental arc volcanic rocks from the Phetchabun belt in northeastern Thailand, suggesting a Late Permian–Middle Triassic continental margin in the Sayabouli area of northwestern Laos and Phetchabun area of northeastern Thailand. Our data indicate that the Phetchabun arc volcanic belt through the western Loei sub-belt can be linked to the Sayabouli area in northwestern Laos.

Geochemistry of Triassic Siliceous Rocks of the Muyinhe Formation in the Changning-Menglian Belt of Southwest China

The Changning-Menglian belt, distributed over southwestern Yunnan Province in Southwest China, contains oceanic rocks that are considered to be remnants of the Paleotethys. This study observed Triassic siliceous rocks of the Muyinhe Formation in the Changning-Menglian belt and analyzed their geochemistry. The samples have high concentrations of SiO2（81.65 wt.%–88.38 wt.%; average： 84.99 wt.%±2.14 wt.%）. Most of the samples were plotted in the non-hydrothermal field on the Al-Fe-Mn diagram. Most of the samples were plotted in the continental margin field on the Fe2O3/TiO2-Al2O3/（Al2O3＋Fe2O3） and（La/Ce）N-Al2O3/（Al2O3＋Fe2O3） diagrams. Moreover, the samples show a flat REE（rare earth element） pattern normalized to NASC（North America shale composite）. These geochemical results, in addition to the lack of rhythmical bedding of the siliceous rocks, strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits. Although previous studies have suggested that the siliceous rocks are pelagic deposits, the present results indicate that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.

New Siliceous Microfossils from the Terreneuvian Yanjiahe Formation,South China:The Possible Earliest Radiolarian Fossil Record

Radiolarians form an important part of the planktonic realm in the ocean of Early Paleozoic, but their origin and evolutionary processes has long been enigmatic. The ancestral representatives of radiolarians have been considered to belong to the order Archaeospicularia, whose unquestionable fossil records were dated back to the Middle Cambrian. Here we report ？Blastulospongia and unnamed spherical radiolarians in the Terreneuvian from the Yanjiabe Formation in Hubei Province, South China. Blastulospongia is an enigmatic siliceous microfossil genus, with affinities proposed amongst the radiolarian, sphinetozoan-grade sponges and uncertain protists. As for the newly discovered unnamed radiolarians, morphologically they possess latticed shell, spherical shape and are all small in size. Our discoveries support the idea that spherical radiolarians is an ancient representative, whose origin and diversification was probably much earlier than generally accepted. The hypothesis that the oldest radiolarians belong to the order Archaeospicularia needs to be re-examined.

The Upper Ordovician Microfossil Assemblages from the Pagoda Formation in Zigui, Hubei Province

The Pagoda Formation is a lithologically and biologically distinctive unit among the Upper Ordovician. The strata are characterized by a nodular limestone, which yields fossil assemblages of high diversity. Conodonts of 14 genera 20 species （6 undetermined species） and ostracods of 14 genera 26 species （16 undetermined species） are identified. In addition, spherical radiolarians and minute-walled foraminifers were first reported from this formation in South China. The conodont assemblage is attributed to the Hamarous europaeus Zone, which indicates the Early Katian Age. The conodont HDS biofacies, along with thin-shelled ostracods, spherical radiolarians and foraminifers, suggest relatively deep and quiet water depositional environment. Accordingly, these fossil materials not only enrich the diversity of the Pagoda biota, but also provide evidence for discussions of deposi- tional environments and stratigraphic correlations for the Pagoda Formation.

Petrography, Geochemistry and U-Pb Detrital Zircon Dating of the Clastic Phu Khat Formation in the Nakhon Thai Region, Thailand: Implications for Provenance and Geotectonic Setting

The purpose of this paper is to determine the provenance and tectonic setting of the Phu Khat Formation and get a better understanding of the tectonic evolution of the Nakhon Thai region using the petrography and whole-rock geochemistry integrated with the U-Pb detrital zircon dating. The sandstone of the Late Cretaceous to Early Tertiary Phu Khat Formation is chiefly characterized by unsorted texture and highly unstable volcanic lithic fragments. The formation overlies unconformably on a high textural and mineral maturity of clastic sandstone of the Late Cretaceous Khao Ya Puk Formation. Geochemically, the tectonic setting discrimination（K2O/Na2O-SiO2, Al2O3/SiO2-Fe2O3＋Mg O, and Th-Sc-Zr/10） and the petrography indicate that the Phu Khat Formation was accumulated in a passive margin tectonic setting which is the same as the Khao Ya Puk Formation but with a different depositional environment. The plots of geochemical provenance discrimination（La/Th-Hf, Th/Sc-Zr/Sc, Eu anomaly Eu/Eu＊ 0.42 to 0.74） and the petrography reveal that the provenance of the Khao Ya Puk Formation is mainly recycled sedimentary rocks while the Phu Khat Formation consists primarily of recycled sedimentary rocks associated with minor felsic volcanic rocks from the old continental island arc of the uplifted either western or eastern continental terranes or both. However, the U-Pb detrital zircon dating indicates a unique provenance of the Phu Khat Formation from the terrane west of the Nakhon Thai region where the volcanic continental arc is active predominantly in the Middle to Late Triassic. The results indicate that while the Phu Khat Formation was accumulated in Nakhon Thai region, the western terrane was uplifted by reactivation of the preexisting structure probably since the Maastrichtian time to be the source area of sediments. Meanwhile, the eastern terrane（mainly Loei-Phetchabun fold belt） had not been uplifted probably until, the accumulation of the Phu Khat Formation terminated. Subsequently, the whole region began to uplift forming a high mountainous area since the Ypresian time when the Greater India collided with the Eurasia.

Geochronological and Geochemical Constraints on the Petrogenesis of Early Paleoproterozoic(2.40–2.32 Ga) Nb-Enriched Mafic Rocks in Southwestern Yangtze Block and Its Tectonic Implications

Recent geological survey has identified the Early Paleoproterozoic meta-mafic intrusions in the southwestern Yangtze Block.We present geochronological,whole-rock geochemical and Nd isotopic data for these meta-mafic rocks to better address the tectonic evolution of the Yangtze Block during the Early Paleoproterozoic Period.Geochronological data show that the meta-mafic rocks have zircon ages of 2 395-2 316 Ma.They have high TiO_2 contents of 1.40 wt.%-3.66 wt.% and Nb concentrations of 13.7 ppm-45.5 ppm,thus aregrouped as Nb-enriched mafic rocks.These mafic rocks are characterized by tholeiitic compositions with enrichment of LREEs and LILEs,and can be divided into two groups.Group 1 samples display E-MORB-like geochemical characteristics.Group 2 samples have positive ENd(t) values of 4.0-5.0.Geochemical data indicate that all meta-mafic rocks were likely derived from a depleted asthenospheric mantle.REE modeling indicates lower degree of partial melting for Group 2 samples(3%-10%) relative to Group 1 samples(15%-20%).Taking into account contemporaneous post-collisional granitoids in southwestern Yangtze Block,we propose that these meta-mafic rocks were formed in a post-collisional extension setting.These meta-mafic rocks can be compared with those in Africa,South America and Europe,and might be linked with the Arrowsmith orogenic belt.

再循环碳酸岩火山作用对新元古代碳酸盐碳同位素显著负偏的贡献

埃迪卡拉纪晚期Shuram碳酸盐记录了地球历史上最显著的碳同位素负偏(δ^(13)C=-12‰)现象(简称SE).前人认为SE事件是溶解有机质氧化、成岩作用或者自生沉淀作用的结果.上述机制面临着巨量的大气氧以及大量δ^(13)C极负物质的持续输入等多方面挑战.本文研究表明记录SE的典型地层华南九龙湾剖面中的陡山陀组具有与高温过程相关的矿物学和地球化学特征.碳同位素负偏样品中铁白云石、长石、碳硅石和自形石英的出现以及碳酸盐Ce异常和石英O同位素组成稳定的特征均表明有高温火成作用的参与.此外,碳同位素负偏样品的微量元素和碳同位素组成与沉积碳酸盐岩通过脱碳熔融作用形成的再循环碳酸岩一致.这些特征表明与罗迪尼亚超大陆裂解有关的古老碳酸盐岩在俯冲过程中经过脱碳作用后形成的碳酸岩岩浆喷发可能是引起SE的原因.这种火成模型可能提供了地球深部和浅部碳循环之间的联系.

Zircon U-Pb Geochronological Evidence for the Evolution of the Nan-Uttaradit Suture in Northern Thailand

The Nan-Uttaradit suture is marked by a narrow N-S trending and discontinuous ophiolite belt in northern Thailand. This suture zone is a mélange composed of gabbro, tholeiitic meta- basalt, andesite and radiolarian chert. Samples of gabbro and meta-basalt in the Nan-Uttaradit suture yield zircon U-Pb ages of 311±10 and 316±3 Ma, respectively, interpreted as the crystallization ages of the rocks, suggesting the Nan-Uttaradit Ocean existed in the Late Carboniferous. Our results indicate that the Nan-Uttaradit Ocean co-existed with the Ailaoshan-Jinshajiang Ocean to the north and was probably an along-strike extension of the latter.

Geochemistry of Radiolarian Cherts from a Late Devonian Continental Margin Basin,Loei Fold Belt,Indo-China Terrane

More than 42 species of well-preserved Upper Devonian radiolarians have been obtained from cherts and siliceous shales in four sections（Chom Noi, Phu Kham Phe（P）, Phu Kham Phe（C） and Pha Samyod） from the NE Thailand sector of the Loei fold belt, Indochina terrane. They include Ceratoikiscum sp. cf. planistellare Foreman, Helenifore laticlavium Nazarov ＆ Ormiston, Astroentactinia sp. cf. paronae（Hinde）, Stigmosphaerostylus herculea（Foreman）, S. pusilla（Hinde）, S. cf. vulgaris（Won）, Trilonche davidi（Hinde）, T. echinata（Hinde）, T. elegans Hinde, T. hindea（Hinde）, T. palimbola（Foreman）, T. vetusta Hinde, Polyentactinia polygonia Foreman, Spongentactinella sp. cf. windjanensis Nazarov, Archocyrtium sp.？, Palaeoscenidium sp., Triaenosphaera sp., and others. Radiolarian faunas from these sections indicate a range from lower Frasnian to Famennian. Analyses of rare earth, trace and major elements suggest that the Upper Devonian chert and siliceous successions from Loei were deposited in a continental margin environment near to a volcanic arc. These successions contain distinctive weak negative Ce anomalies with strong positive Eu anomalies which differ from Upper Devonian cherts reported from northern Thailand, the Truong Son fold belt（Laos） and from South China. Negative Ce anomalies and low to moderate Lan/Cen ratios imply proximity between the Chom Noi and Phu Kham Phe（C）, which were deposited in more distal parts of the depositional basin than the Pha Samyod and Phu Kham Phe（P） samples. Most of the Pha Samyod and Phu Kham Phe（P） samples exhibit high continental-derived trace element levels including Cr, Zr, Hf, Rb, Th and in part V, which supports close proximity to terrigenous sources. Paleozoic stratigraphic sequences of the Loei fold belt and geochemical results suggest that the Upper Devonian deep marine sequences in the Loei fold belt were deposited within a rifted, continental-margin basin, possibly a back-arc basin and not in a large oceanic basin as has been previously suggested.