西藏恐弄拉地区中侏罗统桑卡拉佣组的厘定及其地质意义

贵州省地质调查院在开展西藏自治区1∶5万班戈县恐弄拉地区区域地质调查时,在马前乡恐弄拉西侧车尖西南一带前人划分的一套早白垩世桂牙岩组碳酸盐岩地层中采集了腕足类和腹足类化石,据采集的古生物化石时代确定此套碳酸盐岩地层为中侏罗世沉积(晚巴柔期—卡洛期),通过岩石组合特征、基本层序、沉积环境及生物化石时代对比,将该套地层重新厘定为桑卡拉佣组。这一地层的确定为冈底斯带地层区侏罗系地层划分对比、地层格架建立提供了重要的依据,并为中生代盆地的演化历史,古地理轮廓的确定提供了新的基础资料。     

鄂尔多斯盆地马家沟组中组合马五_5亚段白云岩特征及成因机理

通过对鄂尔多斯盆地马家沟组五段5亚段白云岩的岩石学和地球化学研究表明,主要岩性有泥晶白云岩、泥粉晶白云岩和粉晶白云岩等三类。泥晶白云岩类为泥晶结构,阴极发光为暗红光,白云石有序度低;Na、K和Sr值较高;δ~(13)C平均-1.23‰,δ~(18)O平均-7.59‰,锶同位素平均值0.709 81,形成于闭塞局限、盐度相对较高的环境,由准同生白云石化作用所形成,并发育少量晶间微孔。泥粉晶白云岩类为泥晶—细粉晶结构,不发光或发极暗的红光,白云石有序度0.75;常量元素、微量元素大幅降低;δ~(13)C和δ~(18)O的平均值分别是-0.43‰和-7.29‰,锶同位素平均值0.709 36,由准同生后回流渗透白云石化作用所形成,并发育晶间孔。粉晶白云岩不发光或发极暗红光,白云石有序度0.81;常量元素、微量元素有明显的突变;δ~(13)C和δ~(18)O平均值分别为-0.65‰和-6.46‰,锶同位素平均值0.709 67,由埋藏白云石化作用所形成,并发育晶间扩溶孔或溶蚀孔。     

西藏谢通门地区中侏罗世花岗岩的地球化学特征及时代依据

西藏谢通门县一带区域上属于冈底斯花岗岩带南亚带,侵入岩极其发育,目前填绘有白垩纪、古近纪和新近纪岩体。通过同位素年代学研究,获得12个锆石的U-Pb同位素年龄数据,在原划归白垩纪、古近纪和新近纪岩体中解体出侏罗纪侵入体,按其岩石类型特征,归并为早侏罗世东热村序列和中侏罗世东嘎乡序列。其中,在约拉嘎莫岩体中获得锆石U-Pb同位素年龄166.9±2.8Ma,时代为中侏罗世巴柔期。     

新疆哈密地区早泥盆世珊瑚动物群及其地质意义

新疆哈密地区下泥盆统发育,南部图拉尔根地区下泥盆统为大南湖组,北部三道白杨沟地区为卓木巴斯套组。在大南湖组中共发现四射珊瑚2属5种,床板珊瑚7属12种,并建立四射珊瑚组合Syringaxon moriense和床板珊瑚组合Pseudofavosites giganteus;在卓木巴斯套组共发现四射珊瑚5属7种,床板珊瑚2属2种,并建立四射珊瑚组合Orthopaterophyllum junggarense和床板珊瑚组合Pachyfavosites junggarensis。通过珊瑚在地层中的分布及组合的时代对比,确定前者的时代为早泥盆世埃姆斯期早期,后者的时代为埃姆斯期中晚期。通过与国内外典型的早泥盆世埃姆斯期珊瑚动物群的对比,认为哈密地区的珊瑚动物具有典型的早泥盆世温带动物群特征,在生物古地理区系上属于北方大区准噶尔-兴安省。     

Geodynamic setting,structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO₂ contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO₂ (< 1.7 wt.%) and Fe₂O_3tot and higher contents of Al₂O₃ in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.     

羌塘盆地晚侏罗世夏里组磁学参数指示的沉积环境及其成盐意义

羌塘盆地位于青藏高原中部,是一个具有沉积钠盐、钾盐远景的蒸发岩盆地。最近在羌塘盆地上侏罗统夏里组中发现多处具有钾、钠异常的盐泉这一重要的找钾盐线索,然而目前对上侏罗统夏里组的古环境演变仍然不甚了解,限制了对晚侏罗世羌塘盆地的成盐(成钾)条件和可能的层位等重大科学问题的理解。因此本文以热磁分析为主,对羌塘盆地雁石坪地区上侏罗统夏里组沉积物进行高分辨率岩石磁学研究,并讨论其古环境意义。最后,根据成盐(成钾)理论,综合分析羌塘盆地上侏罗统夏里组成盐(成钾)的构造、气候和物源条件。κ-T曲线、磁滞回线和热退磁结果共同表明磁铁矿和赤铁矿是夏里组的主要载磁矿物;磁学参数分析表明夏里组上段赤铁矿含量相对增加,揭示夏里组上段形成于干旱气候环境;综合分析构造、气候和物源条件表明,羌塘盆地上侏罗统夏里组上段最为具备良好的成盐(成钾)条件。     

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area,southern Great Xing’an Range,NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

This study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon ε_Hf(t) values from +4.06 to +11.62 that yield two-stage model ages (T_DM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A₂-type affinities. These volcanics have ε_Hf(t) and T_DM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.     

Early Carboniferous ophiolite in central Qiangtang,northern Tibet: record of an oceanic back-arc system in the Palaeo-Tethys Ocean

Zircon U–Pb dating of two samples of metagabbro from the Riwanchaka ophiolite yielded early Carboniferous ages of 354.4 ± 2.3 Ma and 356.7 ± 1.9 Ma. Their positive zircon εHf(t) values (+7.9 to +9.9) indicate that these rocks were derived from a relatively depleted mantle. The metagabbros can be considered as two types: R1 and R2. Both types are tholeiitic, with depletion of high-field-strength elements (HFSE) and enrichment of large-ion lithophile elements (LILE) similar to those of typical back-arc basin basalts (BABB), such as Mariana BABB and East Scotia Ridge BABB. Geochemical and isotopic characteristics indicate that the R1 metagabbro originated from a back-arc basin spreading ridge with addition of slab-derived fluids, whereas the R2 metagabbro was derived from a back-arc basin mantle source, with involvement of melts and fluids from subducted ocean crust. The Riwanchaka ophiolite exhibits both mid-ocean ridge basalts- and arc-like geochemical affinities, consistent with coeval ophiolites from central Qiangtang. Observations indicate that the Qiangtang ophiolites developed during the Late Devonian–early Carboniferous (D₃–C₁) in a back-arc spreading ridge above an intra-oceanic subduction zone. Based on our data and previous studies, we propose that an oceanic back-arc basin system existed in the Longmuco–Shuanghu–Lancang Palaeo-Tethys Ocean during the D₃–C₁ period.     

伊犁盆地尼勒克地区中二叠世陆相地层碎屑物源分析及其对西天山构造演化过程的约束

西天山在二叠纪时期的大地构造环境有岛弧成因和大陆裂谷成因两种不同的观点,伊犁盆地尼勒克地区中二叠统陆相红层的研究是解决上述争议的关键之一。该段地层自下而上是一套由冲洪积扇相、河流相、浅湖相和半深湖相组成的湖侵序列。本文综合该套地层的古水流、砾岩砾石成分与砂岩碎屑组分、重矿物组合、碎屑岩地球化学以及石榴子石化学特征等方面的研究,认为尼勒克盆地中二叠统陆相红层的物源复杂多样,包括超镁铁质-镁铁质岩、安山岩、英安岩、流纹岩、矽卡岩、片麻岩、灰岩和碎屑岩等不同岩石类型,记录了西天山造山带的构造剥蚀与再沉积过程。结合下二叠统乌郎组双峰式火山岩以及区域性角度不整合等资料的综合分析,本研究认为尼勒克地区中二叠世盆地形成于造山后伸展的大地构造背景。     

The first Surijokocixiidae (Insecta: Hemiptera: Fulgoromorpha) from the Triassic of China

A planthopper originally assigned to the genus Boreocixius in Surijokocixiidae (Surijokocixioidea, Fulgoromorpha) from the Ladinian (Middle Triassic) of Tongchuan in Shaanxi, northwestern China is described as Boreocixius tongchuanensis Zhang et al. sp. nov. As the third species ascribed to the genus, it differs from its congeners in having the much larger tegmen with postnodal veinlets and pigmented bands, providing new morphological information for Boreocixius. The establishment of the new species further increases the diversity of Boreocixius and Surijokocixiidae as well.