Late Triassic Granites From the Quxu Batholith Shedding a New Light on the Evolution of the Gangdese Belt in Southern Tibet

The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic （-230 Ma）, and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous sub- terranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.

Intracontinental Collisional Orogeny During Late Permian-Middle Triassic in South China: Sedimentary Records of the Shiwandashan Basin

Sedimentary response to an orogenic process is important for determining whether South China had compressional or extensional orogeny during the period from the Late Permian to the Middle Triassic besides the tectonic and magmatologic evidence. An intracontinental collision event took place between the Yangtze and Cathaysia blocks in the Late Permian. Beginning at the Late Triassic, the tectonic movement was completely changed in nature and entered a post-collisional extensional orogenic and basin-making process. This paper presents sedimentological evidence from the Late Permian to the Middle Triassic in the Shiwandashan basin at the southwestern end of the junction zone between the Yangtze and Cathaysia blocks.

Tectonic Evolution of the Tianhuan Depression and the Western Margin of the Late Triassic Ordos

The Ordos Basin is one of the most important oil and gas basins in China. Based on surface outcrop, key exploratory wells and seismic reflection data and by using the technology of ＂prototype basin recovery＂, seismic profile ＂layer flattening＂ and ＂restoration of balanced section＂, and other methods, the sedimentary boundary, structure and the evolution history of the Tianhuan depression on the western margin of the Ordos Basin are reestablished. The following results have been obtained. （1） The west boundary of the Late Triassic Ordos Basin was far beyond the scope of the current basin. The basin is connected with the Late Triassic Hexi Corridor Basin, and its western margin did not have tectonic characteristics of a foreland basin. （2） The Tianhuan depression was first formed in the Late Jurassic. At the late stage it was impacted by the late Yanshanian and Himalayan tectonic movement and the depression axis gradually moved eastwards to the present location with a cumulative migration distance of -30 km. （3） Eastward migration of the depression axis caused adjustment and even destruction of the originally formed oil and gas reservoirs, so that oil and gas remigrated and aggregated, resulting in secondary structural reservoirs formed at high positions on the western flank of the depression.

The Late Triassic Sequence-Stratigraphic Framework of the Upper Yangtze Region,South China

In the transitional period between the Middle and the Late Triassic, the Indochina orogeny caused two tectonic events in South China：（1） the formation and uplift of the Qinling-Dabie orogenic belt along the northern margin of the South China Plate, due to its collision with the North China Plate; and 2） the development of a 1300-km-wide intra-continental orogen in the southeastern part of the South China Plate, which led to a northwestward movement of the foreland thrust-fold zone. These tectonic events resulted in the ending of the Yangtze Platform, and were a stable paleogeographic factor from the Eidacaran to the end of the Middle Triassic. This platform was characterized by the widespread development of shallow-water carbonates. After the end of the Yangtze Platform, the upper Yangtze foreland basin（or Sichuan foreland basin） was formed during the Late Triassic and became a accumulation site of fluvial deposits that are composed of related strata of the Xujiahe Formation. In western Sichuan Province, the Xujiahe Formation overlies the Maantang Formation shallow-water carbonate rocks of the Xiaotangzi Formation siliciclastic rocks（from shelf shales to littoral facies）. The sequence-stratigraphic framework of the Upper Triassic in the upper Yangtze foreland basin indicates a particular alluvial architecture, characterized by sequences composed of（1） successions of low-energy fluvial deposits of high-accommodation phases, including coal seams, and（2） high-energy fluvial deposits of low-accommodation phases, including amalgamated river-channel sandstones. The spatial distribution of these fluvial deposits belonging to the Xujiahe Formation and its relative strata is characterized by gradual thinning-out, overlapping, and pinching-out toward both the east and south. This sedimentary record therefore expresses a particular sequence-stratigraphic succession of fluvial deposits within the filling succession of the foreland basin. The sequence-stratigraphic framework for the Upper Triassic in the Upper Yangtze region provides a record of the end of the Yangtze Platform and the formation of the upper Yangtze foreland basin.

New Insights into the Late Triassic Nadigangri Formation of Northern Qiangtang, Tibet, China： Constraints from U-Pb Ages and Hf Isotopes of Detrital and Magmatic Zircons

We report here U-Pb age and in situ Hf isotopic results for detrital and magmatic zircons from one conglomerate and four tuffite samples from the Late Triassic Nadigangri Formation across the North Qiangtang depression, Tibet. Coupled with previously published data in the region, this paper proposes new insights into the geochronological framework for the Nadigangri Formation. The deposition ages of tuffite from top to bottom in the Woruo Mountain, Quem Co and Dongqu River, are 203 Ma, 226 Ma, 221.5 Ma and 221.1 Ma, respectively. The detrital zircons yield a younger cluster of ages of 201.5-225 Ma from the conglomerate of the Quem Co Formation. The Late Triassic Nadigangri Formation defines a temporal range approximately between 201 and 225 Ma （Norian-Rhaetian）, including three predominant groups of 220-225 Ma, 210-217 Ma and 201-205 Ma, which correspond with the three main rifting episodes of initial rifting, further rifting and final rifting. Positive ~Hf（t） value and low model ages in younger detrital zircons suggests a juvenile character. However, the Hf isotopes of magmatic zircons display the presence of reworked ancient crust with 1.1-1.8 Ga. These results provide strong constraints not only on the temporal range of the Late Triassic Nadigangri Formation, but also on the onset of the Qiangtang Mesozoic rift basin.

The Discovery of a Crinoid Community in the LateTriassic Zhuganpo Formation of Guizhou Provinceand Its Geological Significance

Recently a rich and well—preserved crinoid community including Traumatocrinus and Encrinus has been found in the Late Triassic Zhuganpo Formation in Guanling and Xingyi, Guizhou province, China. Among the fossils Traumatocrinus is the richest, and most of it occurs as clusters, with each cluster containing 3–42 crinoid branches. Study of the stem and calyx of Traumatocrinus shows that the number of stem-joints (columnals) is equal to the total stem length × K (5.85 per cm × stem—diameter). There are about 376 first- and second—order columnals on the whole stem. This number seems to coincide with the number of days in a year at that time. According to the present study of the palaeoecological environment of the crinoid community, it is considered that the reproduction and preservation of the crinoid community were controlled by the Late Triassic regression and the restricted bay of an interior sea behind the S—shaped shoal zone.

On Geodynamic Evolution of Simao Region (Southwestern Yunnan, China) during Late Paleozoic and Triassic

Over the last years, the Simao region, southwestern Yunnan, China, turned out to be a very promising target to elucidate plate tectonic processes around the Permian Triassic boundary within the Tethyan domain of Asia. New data from this area reveal that Upper Paleozoic compressional deformations occurred along the Lancangjiang and in areas to the east. Along Lancangjiang, an angular unconformity is exposed, which separates quartz phyllites—formed during a Carboniferous tectono metamorphic event—from Triassic red beds to roofing rhyolites. The acidic volcanics were often said to be remnants of a volcanic arc that was active during the Triassic subduction and closure of an oceanic realm along the Lancangjiang zone. According to our new data, however, these volcanics indicate most probably an Upper Triassic stage of rifting. In the Yunxian anticline (NW of Simao), an angular unconformity of intra Permian age is exposed where Carboniferous to lower Middle Permian strata, which were deposited in a rather deep basin, are unconformably overlain by a shallow marine sequence of upper Middle to Upper Permian sediments. The angular unconformity is of the same age as the syn orogenous sediments first described and dated from the Phetchabun region in Thailand and hence a convincing argument for a Late Variscan orogeny forming an extensive zone of mountain building that can be traced through the central parts of mainland Southeast Asia.

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

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

Late Triassic Biotite Monzogranite from the Western Litang Area,Yidun Terrane,SW China: Petrogenesis and Tectonic Implications

The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo-Tethys in western China. We present new zircon U-Pb, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic data for the Litang biotite monzogranites, Yidun terrane. The biotite monzogranites have a zircon U-Pb age of 206.1±1.0 Ma(MSWD=1.9,n=30), which indicates Late Triassic magmatism. The biotite monzogranites display I-type affinity, high Na_2O(3.38-3.60 wt%) contente,medii SiO_2(67.12-69.13 wt%), and low P_2 O_5 contents(0.10～0.12 wt%). They enriched in Rb,and Ba and depleted in Nb and Ta, with negative Eu anomalies(Eu/Eu*=0.74—0.81). They have evolved Sr-Nd-Pb-Hf isotopic composition, i.e.,(^(87) Sr/^(86 )Sr)i=0.714225 to 0.714763, negative ?_(Nd(t)) values of -2.0 to-2.6 with two-stage Nd model ages ranging from 1.01 to 1.05 Ga, negative ?_(Ht)(t)) values o f-3.4 to-4.1 with two-stage Hf model ages of 1.85 to1.88 Ga, suggesting a matured crustal sources. Their low Al_2O_3/TiO_2 ratios and medium Cao/Na_2O ratios, medium Mg~# and SiO_2 contents, low [molar Al_2O_3/(MgO+FeO^T)] values, and high [molar Cao/(MgO+FeO^T)] values indicate that the Litang biotite monzogranite was formed by partial melting of metabasaltic rocks. Based on the previous studies, we propose that the Litang biotite monzogranite derived from the westward subduction and closure of the Ganzi-Litang ocean during the Late Triassic-The mantle wedge-derived mafic melts provided sufficient heat for partial melting of ancient metabasalt protolith within the middle-lower crust.

Geochronology and Geochemistry of Late Triassic Intrusive Rocks in the Xiuyan Area,Liaodong Peninsula,Eastern North China Craton:Petrogenesis and Implications for Lithospheric Thinning

The timing and mechanisms of lithospheric thinning and destruction of the North China Craton(NCC)remain controversial,and the overall geodynamics of the process are poorly understood.This paper documents Late Triassic igneous rocks including monzogranite,gabbro,and diorite from the Xiuyan District on the Liaodong Peninsula in the eastern NCC,which have LA-ICP-MS zircon U-Pb ages of 229.0±0.4 Ma,216.2±0.9 Ma,and 210.6±2.0 Ma,respectively.Monzogranite shows high-SiO_(2) adakite affinity,negative ε_(Hf)(t)values(-20.6 to-17.9),and old T_(DM2) ages(3.53-3.29 Ga),suggesting that their parental magma was derived from thickened Paleoarchean mafic lower crust and minor mantle materials that were also involved their generation.Gabbro is ultrapotassic,strongly enriched in LREEs and LILEs,depleted in HFSEs,and has evolved zircon Hf isotopes with negative ε_(Hf) of -10.04 to-5.85 and old T_(DM2) ages(2.59-2.22 Ga).These are diagnostic signatures of a crustal component,but their high contents of Mg O,Cr,Co,Ni indicate that the primary magma originated from enriched mantle.Diorite is enriched in LILEs and LREEs,depleted in HFSEs(with negative Nb,Ta,and Ti anomalies),and contains negative ε_(Hf)(t)values(-13.64 to-11.01).Compared with the gabbro,the diorite is relatively enriched in Nb,Ta and HREEs,and also contains younger T_(DM2) ages(2.11-1.94 Ga),suggesting that the diorite was formed by mixing between ancient lower crust-derived felsic magmas and asthenospheric mantle-derived magmas.Field observations,geochronology,geochemistry,and zircon Lu-Hf isotopes indicate that Late Triassic magmatism and tectonic activity resulted from deep subduction of the Yangtze Craton beneath the NCC in the Xiuyan area.This phase of tectonic activity was completed in the eastern NCC by the Late Triassic(216 Ma),and was subsequently followed by lithospheric thinning that began in the Late Triassic.

New Discovery of the Late Triassic Terrigenous Sediments in the Great Xing＇＇an Range Region,NE China and its Geological Significance

Objective The Great Xing＇an Range is located in the eastern section of Central Asian Orogenic Belt（CAOB）.As a superposed position of multiple tectonic domains,its structural evoIlution has always been a focused issue of geological research.

Geochemistry,petrogenesis and tectonic significance of the late Triassic A-type granite in Fujian,South China

The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of South China. Here, we present the detailed geochronological, geochemical and petrological analyses for the Jinlongyan(JLY) granite in northwest Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yielded a weighted average ^(206)Pb/^(238)U age of 224.1 ±3.3 Ma. The granite is mainly comprised of K-feldspar,plagioclase, quartz, biotite and minor amphibole. It is characterized by enrichments in Rb, Th, REEs(total REE = 295.1–694.3 ppm), and HFSEs(e.g., Zr = 289–520 ppm, Hf = 9.3–15.0 ppm, Y = 36.2–68.2 ppm) but depletions in Ba, Sr, Eu and Ti. The granite is metaluminous to weakly peraluminous and show a clear A-type granite geochemical signature with high SiO_2(70.89 wt%–75.76 wt%), total alkalis(Na_2O + K_2O = 7.51 wt%–8.72 wt%), Ga/Al ratios(10000 Ga/Al = 2.72–3.43). Insitu zircon Hf isotope analysis shows their eHf(t) values ranging from-7.2 to-3.2, with Mesoproterozoic T2DM ages(1308–1525 Ma). Whole-rock Nd isotope data show their eNd(t) values in the range of-9.5 to-9.1 and yield paleoproterozoic TDMages(1606–1985 Ma). These characteristics indicate that the JLY A-type granite magma was formed by the partial melting of Meso-Paleoproterozoic crust rocks in the Cathaysia Block. Our study of the JLY A-type granite, together with other Triassic A-type granitesin South China, defines an extensional environment in the late Triassic which probably was caused by the collision of the South China Block with Indochina Block.

First Late Triassic Record of a Paleoentomofauna from South America(Malargüe Basin,Mendoza Province,Argentina)

Late Middle Triassic to early Late Triassic insects from Argentina have been previously described from the Bermejo and Cuyana Basins where they have been recovered from the Ischichuca-Los Rastros and Potrerillos-Cacheuta Formations, respectively. The insect fauna discussed herein was collected during field studies in 1986/1987 from the Llantenes section （Norian to Rhaetian？ Late Triassic）, which is situated in the Malargiie Basin in southern Mendoza province. The insect remains were found in the upper part of the Llantenes section （Llantenes Formation）, which is built up of two coarsening-upwards cycles reflecting a deltaic progradation of a fluvial into a lacustrine environment （lower part）, succeeded by repeated progradatious into a floodplaindominated environment （upper part; with finds of insects, conchostracans, fish remains, plant fragments, and drifted logs）. The new finds represent the youngest Triassic insect records described from Argentina and even from South America in its entirety. There is only one contemporaneous fossil assemblage in Gondwana： in the Ciarence/Moreton Basin （Aberdare Conglomerate; Late Norian） in Australia. The new Triassic insects include an impression of an isolated Mecopterida-like wing （Mendozachorista volkheimeri gen. et sp. nov.; Mendozachoristidae fam. nov.）, coleopteran elytra of the Permosynidae （Ademosyne rosenfeldi sp. nov. and Ademosyne Uantenesensis sp. nov.） and other isolated body fragments. This new Late Triassic entomofauna from Argentina is of considerable importance in the reconstruction of the biotic recovery of continental environments in Gondwana after the catastrophic mass extinction at the P/T boundary.

Discovery of Fossil Charophytes from the Late Triassic Xujiahe Formation and Early Jurassic Lower Yimen Formation, Sichuan Province, and Its Significance

This paper describes the fossil charophytes collected from the Late Triassic Xujiahe Formation in Dazuand Early Jurassic Lower Yimen Formation in Xichang, Sichuan Province, and their significance. Up to now,papers dealing with fossil charophytes of the Late Triassic, especially of the Early Jurassic, have been rarelypublished. The description of the two new species of Porochara, P. dazuensis and P. xichangensis, and theirfossil assemblage not only provides an important basis for classification and correlation of red beds insouthwestern China but reveals for the first time the features of the charophyte flora of the Late Triassic-EarlyJurassic in the area, thus offering valuable data for the establishment of Triassic and Jurassic charophyte fossilzones in China.

Characteristics and Natural Gas Origin of Middle-Late Triassic Marine Source Rocks of the Western Sichuan Depression, SW China

A scientific exploration well(CK1) was drilled to expand the oil/gas production in the western Sichuan depression, SW, China. Seventy-three core samples and four natural gas samples from the Middle–Late Triassic strata were analyzed to determine the paleo-depositional setting and the abundance of organic matter(OM) and to evaluate the hydrocarbon-generation process and potential. This information was then used to identify the origin of the natural gas. The OM is characterized by medium n-alkanes(n C15–n C19), low pristane/phytane and terrigenous aquatic ratios(TAR), a carbon preference index(CPI) of ~1, regular steranes with C29 > C27 > C28, gammacerane/C30 hopane ratios of 0.15–0.32, and δDorg of-132‰ to-58‰, suggesting a marine algal/phytoplankton source with terrestrial input deposited in a reducing–transitional saline/marine sedimentary environment. Based on the TOC, HI index, and chloroform bitumen "A" the algalrich dolomites of the Leikoupo Formation are fair–good source rocks;the grey limestones of the Maantang Formation are fair source rocks;and the shales of the Xiaotangzi Formation are moderately good source rocks. In addition, maceral and carbon isotopes indicate that the kerogen of the Leikoupo and Maantang formations is type Ⅱ and that of the Xiaotangzi Formation is type Ⅱ–Ⅲ. The maturity parameters and the hopane and sterane isomerization suggest that the OM was advanced mature and produced wet–dry gases. One-dimensional modeling of the thermal-burial history suggests that hydrocarbon-generation occurred at 220–60 Ma. The gas components and C–H–He–Ar–Ne isotopes indicate that the oilassociated gases were generated in the Leikoupo and Maantang formations, and then, they mixed with gases from the Xiaotangzi Formation, which were probably contributed by the underlying Permian marine source rocks. Therefore, the deeply-buried Middle–Late Triassic marine source rocks in the western Sichuan depression and in similar basins have a great significant hydrocarbon potential.

A new member of Sphenopsida,Neolobatannularia gen. nov. from Late Triassic of western Liaoning,China

A new genus Neolobatannularia Sun et Li gen. nov. with the type species Neolobatannularia liaoningensis Sun et Li sp. nov. from the Upper Triassic Yangcaogou Formation of Beipiao, Liaoning, China, is described in this paper. The new taxon is attributed to Equisetales of Sphenopsida based on its branch with internodes, node and leaves borne in whorls on nodes. It is a new member of the Mesozoic Sphenopsida. Although the new taxon shares some morphological characters with the genus Lobatannularia Kawasaki, a genus widely distributed in the Permian Cathaysia flora in East Asia, and the genus Lobatannulariopsis Yang from the Late Triassic of southwestern China, its unique branching pattern is markedly distinguished from the two known genera. Some Triassic known species of Lobatannularia may belong to the present new genus Neolobatannularia.

Late Triassic Intraoceanic Arc Aystem within Neotethys: Evidence from Cumulate Hornblende Gabbro in Gangdese Belt, South Tibet

The Neotethys plays an important role in shaping the Gangdese magmatic belt,southern Tibet.However,the initial time of spreading and subduction of the Neotethys remains contentious.In this study,a suite of late Triassic cumulate hornblende gabbro was identified in the southern margin of the Gangdese magmatic belt.The gabbro exhibits cumulate structure,with hornblende and plagioclase as the primary mineral phases.Isotopic data indicate a hydrous magma source derived from a depleted mantle wedge that has been modified by slab dehydration.Geochemical discriminations suggest that the gabbro was formed in an intraoceanic arc setting,with crystallization ages of ca.220-213 Ma.Hornblende,hornblendelagioclase and ilmenite thermometers reveal that the crystallization temperature of 900-750°C for the gabbro.Hornblende and hornblende-plagioclase geobarometers yield an emplacement depth at ca.14.5-19.5 km.This gabbro constitutes a line of evidence for an intraoceanic arc magmatism that is coeval with the counterparts in the southern Turkey,revealing an intraoceanic subduction system within the Neotethys from west to east in the Late Triassic and that the oceanization of the Neotethys was much earlier than previous expectation.

志丹-甘泉地区晚三叠世储层特征与成岩演化序列

为研究志丹-甘泉地区晚三叠世储层特征以及成岩演化序列,在岩芯、测井分析的基础上,采用普通压汞、恒速压汞等间接测定技术与核磁共振、微米CT等直接观测技术相结合的方式对储层的微观孔隙结构进行研究。结果表明:志丹-甘泉地区晚三叠世储层以次生孔隙为主,发育粒间溶孔、粒内溶孔、填隙物内溶孔及微裂隙,其中常见粒内溶孔,长石溶孔多伴随粒内溶孔发育,填隙物溶孔为少量浊沸石溶蚀形成的孔隙,微裂缝溶孔发育较少;普通压汞确定了志丹甘泉地区储层排驱压力、中值压力的最小值,计算了进汞饱和度值以及退汞效率;恒速压汞试验总结了不同渗透率储层的喉道分布特征,认为喉道是决定渗流率的主要因素;核磁共振T2谱的弛豫时间与储层的孔隙度呈正相关;微米CT技术构建了储层的孔隙网络模型,认为储层孔隙连通性较差。研究结果对志丹-甘泉地区延长组储层特征与成岩演化序列研究提供了理论依据。

鱼粪化石特征对晚三叠世湖泊生态系统的启示——以鄂尔多斯盆地南部长7段为例

粪化石是一类重要的遗迹化石,其内含物常保存被捕食生物的残余及一些微生物、软组织甚至古DNA信息,对解读地质历史时期的湖泊生态信息有重要意义。鄂尔多斯盆地南部铜川地区三叠系延长组长7段下部页岩的鱼粪石,其为略扁平的纺锤形化石,尖而薄,表面有明显的螺旋纹。观察粪化石的内含物情况,发现其主要由未完全消化的动植物残体组成,包括鱼鳞、骨骼碎片,以及以含磷白云岩为生长基的大量矿物聚集体。鱼粪化石形成于开放型淡水湖泊中的半深湖—深湖区,大部分保存完好,属于典型的原地埋葬。该鱼粪的生物源为一种大型的、鱼肠内具有较多螺旋瓣的肉食性鱼类。晚三叠世湖泊中出现了复杂的多层营养系统,主要由生产者、消费者构成,食物链较为复杂。该研究成果表明经历二叠纪末生物大灭绝的晚三叠世多层级营养湖泊生态系统已经基本恢复。另外,微生物的参与及泥质含量较高的厌氧环境使得粪化石保存完好,可为恢复和重建晚三叠世卡尼期湖泊生态系统提供重要依据。

辽东地区赛马碱性杂岩锆石U-Pb年龄、Hf同位素组成及其对构造背景的制约

通过LA-ICP-MS测得辽宁赛马地区浅肉红色霞霓正长岩中的锆石U-Pb年龄为225.8±1.9 Ma,赛马碱性杂岩的侵位时代为晚三叠世。地球化学分析表明,霞霓正长岩SiO_(2)含量为55.87%~60.88%,Na_(2)O为0.41%~5.32%,Al_(2)O_(3)为17.81%~19.53%,K_(2)O为9.46%~11.91%,MgO为0.46%~1.36%,里特曼指数为7.54~17.01;稀土元素总量较高,高于300×10^(-6),强烈富集轻稀土元素,(La/Yb)N值大于10,个别达到100以上;亏损Nb、Ta、P等高场强元素,富集Rb、Th等大离子亲石元素,总体表现出富碱性岩石特征。锆石ε_(Hf)(t)值为-13.37~-9.30,对应的两阶段Hf模式年龄T_(DM2)为2102~1855 Ma。通过岩石成因分析和构造环境判别,赛马碱性杂岩可能形成于由俯冲挤压向陆内伸展、拉张转换的动力背景下的下地壳(或上地幔)部分熔融。赛马碱性杂岩侵位时代(225.8±1.9 Ma)可能代表了华北克拉通北缘岩石圈开始伸展减薄的时间,也是郯庐断裂形成的时间。