Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The  3–4-km-thick Western Mirdita Ophiolite (WMO) includes lherzolite–harzburgite, plagioclase–lherzolite, plagioclase–dunite in its upper mantle units and a plutonic complex composed of olivine gabbro, troctolite, ferrogabbro, and gabbro. These peridotites and gabbroic rocks are overlain directly by a  600-m-thick extrusive sequence containing basaltic pillow lavas and hyaloclastites. Sheeted dikes are rare in the WMO. The  12-km-thick Eastern Mirdita Ophiolite (EMO) includes tectonized harzburgite and dunite with extensive chromite deposits, as well as ultramafic cumulates including olivine clinopyroxenite, wehrlite, olivine websterite, and dunite forming a transitional Moho with the overlying lower crustal section. The plutonic rocks are made of pyroxenite, gabbronorite, gabbro, amphibole gabbro, diorite, quartz diorite, and plagiogranite. A well-developed sheeted dike complex has mutually intrusive relations with the underlying isotropic gabbros and plagiogranites and feeds into the overlying pillow lavas. Dike compositions change from older basalt to basaltic andesite, andesite, dacite, quartz diorite, to late-stage andesitic and boninitic dikes as constrained by crosscutting relations. The  1.1-km-thick extrusive sequence comprises basaltic and basaltic andesitic pillow lavas in the lower 700 m, and andesitic, dacitic and rhyodacitic massive sheet flows in the upper 400 m. Rare boninitic dikes and lavas occur as the youngest igneous products within the EMO. The basaltic and basaltic andesitic rocks of the WMO extrusive sequence display MORB affinities with Ti and Zr contents decreasing upsection (TiO₂ = 3.5–0.5%, Zr = 300–50 ppm), while _Nd(T) (+ 8 to + 6.5) varies little. These magmas were derived from partial melting of fertile MORB-type mantle. Fractional crystallization was important in the evolution of WMO magmas. The low Ti and HREE abundances and Cs and Ba enrichments in the uppermost basaltic andesites may indicate an increased subduction influence in the evolution of the late-stage WMO magmas. Basaltic andesites in the lower 700 m of the EMO volcanic sequence have lower TiO₂ ( 0.5%) and Zr ( 50 ppm) contents but _Nd(T) values (+ 7 to + 6.5) are similar to those of the WMO lavas. These rocks show variable enrichment in subduction-enriched incompatible elements (Cs, Ba, Th, U, LREE). The basaltic andesites through dacites and boninites within the upper 400 meters of EMO lavas show low TiO₂ ( 0.8–0.3%) and _Nd(T) (+ 6.5 to + 3.0). The mantle source of these rocks was variably enriched in Th by melts derived from subducted sediments as indicated by the large variations in Ba, K, and Pb contents. EMO boninitic dikes and lavas and some gabbroic intrusions with negative _{Nd (T)} values (− 1.4 and − 4.0, respectively) suggest that these magmas were produced from partial melting of previously depleted, ultra-refractory mantle. The MORB to SSZ transition (from west to east and stratigraphically upwards in the Mirdita Ophiolite and the progression of the _Nd(T) values from + 8.0 to − 4.0 towards the east resulted from an eastward shift in protoarc–forearc magmatism, keeping pace with slab rollback in this direction. The mantle flow above the retreating slab and in the arc-wedge corner played a major role in the evolution of the melting column, in which melt generation, aggregation/mixing and differentiation occurred at all levels of the sub-arc/forearc mantle. The SSZ Mirdita Ophiolite evolved during the intra-oceanic collapse and closure of the Pindos marginal basin, which had a protracted tectonic history involving seafloor spreading, protoarc rifting, and trench-continent collision.     

Geological Features of the Eastern Sector of the Bangong Co-Nujiang River Suture Zone: Tethyan Evolution

According to an analysis of the geological features in the eastern sector of the Bangong Co-Nujiang River suture zone, the Tethyan evolution can be divided into three stages. (1) The Embryo-Tethyan stage (Pz1): An immature volcanic arc developed in Taniantaweng (Tanen Taunggyi) Range, indicating the existence of an Embryo-Tethyan ocean. (2) The Palaeo-Tethyan stage (C-T2): During the Carboniferous the northern side of the Taniantaweng Range was the main domain of the Pa-laeo-Tethyan ocean, in which developed flysch sediments intercalated with bimodal volcanic rocks and oceanic tholeiite, and Pemian-Early Triassic arc granites were superimposed on the Taniantaweng magmatic arc; on the southern side the Dengqen-Nujiang zone started secondary extension during the Carboniferous, in which the Nujiang ophiolite developed, and the Palaeo-Tethyan ocean closed before the Middle Triassic. (3) The Neo-Tethyan stage (T3-E): During the Late Triassic the Dengqen zone developed into a relatively matural ocean basin, i     

中国陆壳演化、多块体拼合造山与特色成矿的关系

矿产资源的种类、时空分布、形成演化与成岩作用和大地构造格局密切相关。中国地质构造复杂,成矿条件多样(发育裂谷成矿、碰撞成矿、地幔柱成矿、低温成矿等特色成矿系统),矿床类型比较齐全,如大宗矿产(铁、铝、铜、钾盐)短缺,小宗矿产中盛产稀土元素(REE)、钨、锡、钼矿。中国早前寒武纪矿床相对较少,燕山期成矿集中爆发。这种矿产资源分布格局与中国大陆地壳的性质与演化、多块体拼合造山格局之间的内在联系尚待深入揭示。本文基于对中国陆壳演化、陆块与造山带组成格局和多块体拼合造山的系统分析总结,试图阐明中国成矿特色与其内在联系,从陆壳形成与造山带演化的宏观视角来研究中国大陆成矿特色、成矿物质时空分布规律,其特色包括:(1)中国陆壳的地台区与造山带区质量比约3∶7(全球陆壳地台区占69.6%),太古界面积小且支离破碎,地壳固化时间晚且运动频繁强烈,因此难以形成巨型条带状铁建造(BIF)富铁矿床、太古代火山岩块状硫化物型(VMS)铜锌矿带和元古代内克拉通裂谷有关的扎伊尔-赞比亚巨型铜矿。(2)环绕中朝-塔里木和扬子板块的增生造山带由老到新依次形成,并镶接于古板块边缘,使中国大陆逐渐增生扩展,导致火山岩型、与岩浆岩类和沉积岩系有关的大型矿床空间上向板块边缘推移,时间上越来越新,地壳演化成矿作用和矿床类型越来越多样化。(3)中亚成矿域以古生代多陆块拼合造山、中新生代陆内造山与山盆体系构成独特的地质构造格局。既发育增生造山阶段的弧环境相关矿床(蛇绿岩型铬铁矿、斑岩铜矿、VMS),也发育与碰撞造山有关的矿床(造山型金矿、石棉、滑石、白云母)、地幔柱叠置造山带背景下的岩浆铜镍矿和后碰撞陆内岩石圈伸展相关的大陆环境矿床(斑岩钼矿、热液金矿、伟晶岩型稀有金属矿)。(4)青藏高原(特提斯成矿域)系特提斯洋长期增生演化、印度-欧亚大陆碰撞的产物。其成矿条件优越,具有多期成矿作用、多矿种和多类型的复合成矿系统特点。形成了蛇绿岩套型铬铁矿、密西西比河谷型(MVT)铅锌矿和独具特色的碰撞环境超大型斑岩铜钼矿。(5)我国东部环太平洋成矿域,伴随晚中生代克拉通性质的根本转变及岩石圈明显的减薄过程与破坏,在华北克拉通周缘发生大规模的岩浆活动和强烈的金、铜、钼和轻稀土等成矿作用。不同时期的造山带干涉叠加使得南岭地区盛产花岗岩有关的钨、锡、钼矿,具有叠加改造成矿、大器晚成的鲜明成矿特色。由于中国成矿特色与大陆地壳演化密切相关,中国的找矿勘探部署必须立足于中国大陆演化与多块体拼合造山的基本地质事实,方能取得好的勘查效果。中国大陆小陆块拼合造山成矿还存在诸多未解之谜,文末提出了当前成矿学面临的一系列科学问题,对于今后我国找矿战略选区具有借鉴意义。     

Review of seafloor spreading around Australia. I. synthesis of the patterns of spreading

The existing data on Late Mesozoic and Cenozoic seafloor spreading isochrons (reviewed in the companion paper by Veevers & Li) and fracture zone trends provide the basis for 12 reconstructions of the seafloor around Australia that spread during the dispersal of Argo Land, India, Antarctica, Lord Howe Rise/New Zealand and the Papuan Peninsula. The major changes of plate geometry in the Jurassic, Early Cretaceous, mid‐Cretaceous, early Paleocene and early Eocene reflect global events. The pattern of spreading around Australia was determined by two long‐standing (earlier Phanerozoic) factors that operated in a counter‐clockwise direction: (1) penetration from the northwest by the Tethyan divergent ridge; and (2) rotation from the northeast of the Pacific convergent arc and back‐arc. The only new feature of the modern pattern is the deep penetration by the Indian Ocean ridge into eastern Gondwanaland to fragment it into continents in contrast with the pattern up to 160 Ma ago of breaking off micro‐continents.     

The Mid-Carboniferous to Lowermost Permian succession of Spiti (Po Group and Ganmachidam Formation; Tethys Himalaya,Northern India): Gondwana glaciation and rifting of Neo-Tethys.

Abstract

The upper Lower Carboniferous to lowermost Permian terrigenous succession of the Spiti Valley can be subdivided into five formations (Thabo Formation, Fenestella Shale, Kabjima Quartzarenite, Chichong Formation and Ganmachidam Diamictite), here described in detail and subdivided into members or lithozones.

The Po Group, overlying the platform carbonates and gypsum deposits of the Lipak Formation, records an increase of subarkosic to quartzarenitic terrigenous detritus derived from uplift and erosion of continental blocks in the south during the initial stage of Neo-Tethyan rifting. Increasing mineralogic stability through the Thabo Formation broadly coincides with a climate change from tropical arid to temperate humid conditions during the Visean-Serpukhovian. The Fenestella Shale, containing rich brachiopod associations of Bashkirian age, documents a stage of widespread subsidence and transgression, followed by the regressive Kabjima Quartzarenite, which records recycling of quartzose sedimentary sequences in the south.

The Chichong Formation marks another transgressive event, associated with a significant increase of granitoid detritus, partly from nearby Lesser to High Himalayan source areas. Chaetetid, cephalopod and brachiopod faunas hint at a Moscovian age for the “Chaetetid beds”, at the top of which varve-like lamination and scattered pebbles suggest glacially influenced deposition. The overlying glacio-fluvial (?) cobble conglomerates (“Pebbly beds”) reflect onset of rapid tectonic uplift.

Abundant detritus from sedimentary rocks characterizes the overlying Ganmachidam Diamictite, deposited in glacio-marine environments; cold-water marine faunas of Asselian age occur in its middle part. Erosion of progressively older pre-rift sedimentary successions is ascribed to basin inversion, associated with unroofing of anatectic granitoids of the Lesser and High Himalayas. Basaltic to rhyolitic volcanic detritus documents alkalic magmatism at the climax of continental rifting.     

拉萨地体南缘早石炭世花岗岩类的起源及其对松多特提斯洋开启的意义

近年来在拉萨地体识别出的二叠纪松多大洋型榴辉岩被认为是古特提斯洋(本文称之为松多特提斯洋)的洋壳残余。但由于缺乏相关地质记录,对松多特提斯洋的开启时间,目前并未进行讨论。为探讨这一问题,本文报道了拉萨地体南缘朗县地区片麻状花岗岩类的全岩主量元素和微量元素、锆石U-Pb定年和锆石Hf同位素数据。本文2件样品的锆石U-Pb定年结果为341±2Ma和355±2Ma,表明朗县片麻状花岗岩类侵位于早石炭世。朗县片麻状花岗岩类主要属于高钾钙碱性-钾玄质系列的花岗闪长岩和二长花岗岩,以具有较高的SiO₂含量(69%~76%)和K₂O/Na₂O比值(1.04~3.04)为特征。白云母的出现、高的铝饱和指数(A/CNK=1.04~1.52)和刚玉分子数(>1%),表明朗县片麻状花岗岩类属S型花岗岩。所有样品均富集轻稀土元素和大离子亲石元素,亏损Ba、Eu、Sr和高场强元素。锆饱和温度主要集中在780~800℃之间(最高可达859℃,平均 808℃)。这些早石炭世花岗岩类中发育的闪长质包体、较高的锆饱和温度、低Pb高Ba丰度、小的负锆石ε_Hf(t)值(-7.2~-1.2,平均 -4.4),表明这些花岗岩类为高温S型花岗岩,来源于浅部成熟地壳物质的重熔并伴随着明显的幔源物质注入。朗县片麻状花岗岩类的高温(>780℃)低压(<5kbar)性质指示其很可能形成于伸展背景。结合拉萨地体可能存在的石炭纪早期岩浆弧、泥盆-石炭纪以灰岩和细碎屑岩夹火山岩为特征的沉积地层记录,本文提出朗县片麻状花岗岩类很可能形成于弧后伸展背景。该弧后伸展区进一步发育可能形成弧后盆地并最终发展成以松多榴辉岩为代表的松多特提斯洋,指示松多特提斯洋的持续时间可能不会超过80Myr(即从340Ma到260Ma)。     

Early Cretaceous palynofloral provinces in China: western additions

Two distinct palynofloral provinces have been recognized for the Early Cretaceous of western China, including Xizang (Tibet): one indicated by the presence of the cheirolepidiacean pollen grain Dicheiropollis and the other by abundant representatives of different gymnospermous pollen, especially Araucariacites and Callialasporites. The boundary between them coincides with the Yarlung Zangbo Suture. To the north of the suture, the Xizang–Tarim Dicheiropollis Province occupies a wide region from southern Xinjiang through the central and northern Qinghai–Xizang Plateau to the west of Yunnan. South of the suture, the Southern Xizang Araucariacites‐Callialasporites Province appears to have affinities with the South Gondwanan (floral) Realm. Despite some similarities with the composition of assemblages from other palynofloral provinces, both are distinct and considered to reflect different climates. Hot arid to semi‐arid conditions are indicated by the Dicheiropollis Province, whereas a more humid and somewhat cooler climate is suggested for the Araucariacites‐Callialasporites Province.     

藏南特提斯喜马拉雅带始新世隆子-恰嘎次火山岩区:雅拉香波二云母花岗岩的高位岩浆体系

已有的数据表明,大约在中始新世(44～40Ma),西藏特提斯喜马拉雅带(雅拉香波穹窿地区)经历了一次特别的地壳深熔作用,产生了大量高Sr/Y比值的二云母花岗岩.在雅拉香波穹窿南部的隆子-恰嘎地区,发育一套流纹质次火山岩,以小岩体或岩脉形式侵位于侏罗纪日当组的砂岩和页岩之中.岩相学观察、锆石U-Pb年代学、全岩元素和同住素(Sr、Nd)地球化学数据表明:(1)该次火山岩形成于约43～41Ma,与北部二云母花岗岩体的形成年龄相似;(2)该次火山岩经历了强烈的岩浆演化后期的岩浆-热液作用和钙长石分离结晶作用,导致该套岩浆岩强烈的Eu负异常、明显降低的Sr含量和锆石岩浆增生边的普通Pb和U浓度明显升高;和(3)该套次火山岩和二云母花岗岩属于同一岩浆过程的不同的构造层次,在时间、空间和成岩物质来源上具有一致性,同属于中始新世青藏高原主碰撞时中下地壳部分熔融的产物.     

Palynological Assemblage Age and Palaeoenvironment of the Cretaceous Salt-bearing Strata in Laos and Yunnan,China

正The study of Cretaceous-Palaeogene salt-bearing strata of the Khorat Basin Laos and the Lanping-Simao Basin in Yunnan,China has an great significance not only in explaining the basin evolution and the genesis of potash     

藏北羌塘中部存在志留纪洋盆——来自桃形湖蛇绿岩中斜长花岗岩的锆石U-Pb年龄证据

报道了羌塘西部桃形湖蛇绿岩中斜长花岗岩的锆石U-Pb定年结果和岩石地球化学资料及锆石Hf同位素成分。斜长花岗岩中的锆石具有典型的岩浆生长环带,结合其Th/U值(0.16~0.44),表明为典型的岩浆成因。锆石LA-ICP-MS和Cameca U-Pb定年结果分别为441Ma±2Ma和442Ma±3Ma,相当于早志留世,略年轻于伴生的堆晶辉长岩。锆石的εHf(t)值为+8.13~+11.23,平均值为+9.50,具有明显的幔源成因特征。全岩地球化学分析结果表明,斜长花岗岩是洋壳运移过程辉长岩部分熔融形成的。斜长花岗岩的年龄数据表明,羌塘中部地区存在志留纪洋盆。