Isotopic constraints on crustal architecture and Permo‐Triassic tectonics in New Guinea: Possible links with eastern Australia

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high‐grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle‐like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian‐type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post‐breakup margin comprised promontories of extended Proterozoic‐Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south‐southwest beneath northeast Australia generating arc‐related magmatism in New Guinea and the New England Fold Belt.     

The Grenvillian and Pan-African orogens: World's largest orogenies through geologic time, and their implications on the origin of superplume

The Neoproterozoic Earth was shaped largely by the Grenvillian and Pan-African orogenies. Out of these, the Grenvillian orogeny has long been regarded to be of minor nature in terms of global-scale orogenic episodes, whereas the Pan-African orogeny has been widely recognized in many continental fragments, although not in major parts of Asia. Based on chronological information in zircons from major river mouths across several important terrains of the globe, we show here that the Grenvillian orogeny contributed significantly to the formation of the continental crust. The time period between 0.6 Ga and 0.8 Ga marked the climax at the dawn of the Pan-African orogeny. Continental crust formed in this period is concentrated in the Pan-African orogenic belts widely across the globe. These regions were widespread over the half hemisphere of the globe, and were subsequently reduced in size after they moved to form Laurasia. The normalized frequency distribution of zircon ages from river-mouth sand over the world clearly demonstrates that Neoproterozoic and (0.9–0.6 Ga) and Grenvillian (1.3–1.0 Ga) peaks define the largest population. This means that extensive subduction, and hence active plate tectonics, might have operated through these periods. The zircon study has also brought to light new regions of the Grenvillian orogenic belts, particularly in the continents which are now covered by thick Phanerozoic sedimentary basins. Based on the new locations of Grenvillian orogens identified in this study, and using the distribution patterns as a marker bed, we propose revised paleogeographic configurations of the Rodinia and Gondwana supercontinents.Our results demonstrate that the Neoproterozoic was the most active period of crust formation in the Earth. The cold basins, formed right after the assembly of Rodinia, exhibit a basin chain fringing the northern periphery of Rodinia, which turned into sites of mantle upwellings and led to the rifting and separation of the supercontinental assembly. The continents then moved northwards after the formation of Gondwana at ca. 540 Ma, and enlarged the northern half of the supercontinent Pangea since 250 Ma.Based on the results, we also evaluate the role of supercontinents in the mechanism of generation of superplumes addressing the enigma that the coldest mantle right above the Core–Mantle Boundary turns to the hottest one over a period of several hundreds of million years. Slab graveyard formed by the Pan-African subduction can be imaged through P-wave tomography. We postulate that the high-velocity anomaly in the D” layer underneath Gondwana has now transformed to the low-V regions to generate the African superplume. The tectonic history of solid Earth in the Phanerozoic seems to be controlled by the slab graveyards formed by the Grenvillian orogeny ca. 1.0 Ga.     

云南特提斯带保山-腾冲地块早古生代岩浆岩

云南滇西地区作为东特提斯构造带的重要组成部分之一，其大地构造格局和对全球特提斯演化意义长期以来是众多研究者的关注热点。滇西特提斯构造带（包括川西南部分）是由多个地块和地块相间的造山带组成，如腾冲地体、保山地体、思茅地体及它们之间的属新特提斯带的高黎贡碰撞带和属古特提斯带的昌宁-孟连缝合带等。经受晚古生代—早中生代古特提斯和中生代—新生代新特提斯造山作用的影响，该地区经历了复杂的变质变形。由于地块内部基底出露局限和多期构造作用的叠加影响，特提斯构造带内的变质基底地体的属性和演化历史难于恢复。 位于云南西南部的腾冲-保山地块，包含保山地块，腾冲地块和其间的潞西地槽，属于缅泰马微大陆的北部。在三叠纪期间，该地块处于东部古特提斯主洋盆即昌宁-孟连古特提斯洋封闭时地前陆部位。部分学者认为，在新特提斯洋扩张时期，腾冲地块和保山地块分离，形成属于班公湖-怒江洋盆的东延分支海槽。该海槽在早侏罗世闭合，并导致腾冲地块和保山地块的碰撞，形成高黎贡碰撞构造带。以高黎贡群为典型代表的一套中—新元古代陆源沉积、大陆玄武岩、壳源花岗岩组成的角闪岩相变质岩系，类似于喜马拉雅结晶杂岩、念青唐古拉群和聂拉木群相当。基底岩石在岩石建造、构造热事件和沉积盖层等方面与冈瓦纳大陆有亲缘关系。变质基底组成部分大部分被元古代晚期—早古生代低级变质陆源沉积物所覆盖，如南部公养河群。单颗粒错石U-Pb同位素稀释法定年结果表明，在保山地块南部出露有早古生代花岗岩，年龄范围大致在490~470 Ma之间。这些花岗岩初始。εNd值为-8.4~-9.3之间，亏损地幔钕同位素模式年龄为1.9~-1.7 Ga。锆石铬同位素模式年龄集中在1.4 Ga，其初始εHf值变化范围为0~-10之间，平均-6左右。一些侵入于早古生代花岗岩的晚中生代花岗岩也发现早古生代年龄的继承锆石。早古生代岩浆岩也曾在特提斯带内有报道，如Sibumasu和Indochina地块之间的Nan-Uttaradit缝合带内晚古生代混杂岩内的花岗岩滚石。这一早古生代岩浆作用可能暗示特提斯构造带东段和西段，如Alpine造山带甚至包括欧洲华力西造山带基底，在早古生代期间有相似的演化历史和属性，均源于冈瓦纳大陆北缘，可能先后从冈瓦纳大陆漂离，最终与位于北部的泛大陆拼合。     

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist,New Zealand

The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.     

三江地区两类古陆成分的铅同位素组成——Ⅰ.碳酸盐岩类

根据铅同位素组成,西南三江地区前中生代的碳酸盐岩可分成具高放射性和低放射性成因的铅同位素特征的两种类型,其数值范围和样品的空间分布都可分别与已知的冈瓦纳和劳亚古陆群的铅同位素组成对比。运用不同古陆群显示的块体铅同位素组成差异作为标志,判别兰坪盆地中以灰岩和细碎屑岩组成的上三叠统可能不是中生代兰坪盆的沉积产物,而是新生代早期从盆地西部冈瓦古陆群中逆冲推推覆过来的构造岩片。     

中国东北地区泛非造山岩浆活动的记录:来自扎兰屯地区铜山组碎屑锆石年代学和Hf同位素的证据

本文报道了内蒙古扎兰屯地区铜山组的碎屑锆石U-Pb年代学和Hf同位素分析结果,首次发现中国东北地区记录了泛非造山岩浆事件,并探讨了中国东北微陆块的构造归属.年代学研究表明:(1)扎兰屯地区铜山组碎屑岩最年轻锆石年龄峰值为569 Ma,与泛非造山岩浆活动(东、西冈瓦纳大陆碰撞-拼贴事件)的时代一致;其他3个峰期年龄为87...     

滇西保山地块早古生代碎屑锆石U-Pb-Hf同位素研究及其对冈瓦纳大陆重建的制约

滇西地区出露的古生界公养河群和孟定街群分别位于保山地块的西部和东部,其沉积时代、构造归属、碎屑物质来源及在早古生代冈瓦纳大陆重建中的古地理位置具有重要研究意义。本文对公养河群2个砂岩样品和孟定街群的4个变质砂岩样品的碎屑锆石进行U-Pb定年和Lu-Hf同位素分析。其中,公养河群的碎屑锆石年龄谱范围是3862～533Ma,ε_(Hf)(t)值的范围是-37. 8～+11. 0,最小年龄峰期为549～533Ma,表明该群的最大沉积年龄为早寒武世早期,该群可能为寒武系地层。孟定街群的碎屑锆石年龄谱范围是3097～542Ma,ε_(Hf)(t)值的范围是-39. 5～+10. 5,其最小年龄峰期(576～458Ma)将该群的沉积下限时代限制在晚奥陶世。结合保山地块的其他古生代地层的碎屑锆石研究,我们认为保山地块在早古生代位于冈瓦纳大陆北缘,物源主要来自印度大陆,但～1. 17Ga年龄峰的出现说明其东部可能接受了一定比例来自西澳大利亚的沉积物质。这一结果表明保山地块在冈瓦纳大陆北缘重建中的位置应该靠近印度大陆,而其东缘接近澳大利亚大陆。     

东阿尔卑斯原-古特提斯构造演化

新生代阿尔卑斯是非洲和欧洲之间的陆陆碰撞造山带。强烈的造山作用使大量前中生代基底出露地表,尽管这些基底被强烈逆冲推覆和走滑叠置,但是仍保留较丰富的前中生代基底演化信息。结合近几年对东阿尔卑斯原-古特提斯的研究,本文梳理和重建了阿尔卑斯前中生代基底的构造格局,认为前阿尔卑斯基底受原特提斯、南华力西洋、古特提斯洋构造体系影响而经历了多期造山过程。新元古代-早古生代的原阿尔卑斯作为环冈瓦纳地块群的组成部分,受原特提斯洋俯冲的制约,是新元古-早古生代环冈瓦纳活动陆缘的组成部分,其中,海尔微-彭尼内基底组成外缘增生系统,包括卡多米期地壳碎片在内的陆缘弧/岛弧以及大量增生楔组成内缘增生系统。早奥陶世瑞亚克洋打开,随后原阿尔卑斯从冈瓦纳陆缘裂离,在泥盆纪-石炭纪受南华力西洋控制,海尔微-彭尼内-中、下奥地利阿尔卑斯从冈瓦纳分离。在早石炭世(维宪期)南阿尔卑斯(或与之相当的冈瓦纳源地块)与北部阿莫里卡地块群拼贴增生于古欧洲大陆南缘,共同组成华力西造山带(广义),华力西期缝合带保留在绍山-科尔山南侧。晚石炭世-早二叠世,阿尔卑斯受古特提斯洋的俯冲影响,在华力西造山带南侧形成安第斯山型活动大陆边缘,古特提斯洋在阿尔卑斯的演化至少持续到早三叠世,消亡遗迹保留在中奥地利阿尔卑斯基底的Plankogel杂岩中。     

First record of microsporophyll genus Caytonanthus Thomas from Early Cretaceous beds of South Rewa Gondwana Basin,India: Its evolutionary and palaeogeographical significance

Pteridosperms (seed ferns) are an extinct group of large and diverse plants. They were dominant elements during Palaeozoic and were also an important component in the Mesozoic vegetation but declined globally in the Cretaceous time with the radiation of angiosperms. The order Caytoniales of Mesozoic pteridosperm is an extinct group of seed ferns in the vegetation. They thrived for about 100 million years from the Late Triassic to the Late Cretaceous. Although they never attained dominant status, their rare occurrence in the vegetation is quite important and fascinating. For this extinct group of seed fern, Thomas ( 1925 ) erected a new order Caytoniales which includes three organ genera Caytonia (megasporophyll), Caytonanthus (microsporophyll) and Sagenopteris (leaves). This paper is aimed to document first record of microsporophyll genus Caytonanthus, obtained from the Lower Cretaceous beds of the South Rewa Gondwana Basin of the Indian Peninsula. Its morpho‐characters, palaeogeographic distribution, along with its plausible southern origin through ancestral group like Glossopteridales, are surmised here.     

西藏冈底斯B型山链南缘松多群的构成及其变质变形特征

松多群是冈底斯B型山链变质基底的组成部分之一 ,并于晚三叠世—晚第三纪雅江缝合带俯冲超碰撞阶段演化为该B型山链活化根带逆冲推覆超岩片 ,系冈瓦纳大陆北缘于震旦—寒武纪小洋盆及陆缘火山沉积的产物。松多群岔萨岗组、马布库组、雷龙库组及罗马岭超镁铁质岩带 ,彼此间均为构造接触 ,实为一套构造岩石组合体 ,由时代相近而来源、性质不同的地质体构成 ,并经历了低绿片岩相—低角闪岩相变质作用和 4期主要构造变形。岔萨岗组主体为变质基性火山岩 ,化学成分相当TMORB ;马布库组、雷龙库组片岩、石英岩相当于被动陆缘泥砂质复理石建造及石英质复理石建造。罗马岭超镁铁质岩经综合分析判别则形成于伸展裂陷小洋盆构造环境。因而得出松多群为一较完整有限小洋盆火山沉积产物的结论。