Proterozoic mafic magmatism in Siberian craton: An overview and implications for paleocontinental reconstruction

We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton, including recently published U–Pb and ⁴⁰Ar–³⁹Ar dates. These new precise ages suggest that at least some of the previously published K–Ar ages of Siberian mafic bodies should be ignored. The time–space geochronological chart, or the ‘barcode’ of mafic magmatic events shows significant differences between northern and southern Siberia. Both are characterized by ∼1900–1700 Ma magmatic events, but then there was an almost 1 Ga mafic magmatic ‘pause’ in south Siberia until ∼800 Ma. Meanwhile there are indications of multiple mafic magmatic events in North Siberia (Anabar shield and Olenek uplift) between ∼1600 and 1000 Ma. A series of magmatic events probably related to the breakup of Rodinia occurred in southern Siberia after ∼800 Ma. So far, there are no indications of late Neoproterozoic mafic magmatism in North Siberia. Ca. 1000–950 Ma mafic sills were reported from Meso- to Neo-Proterozoic sedimentary successions in the Sette-Daban area on the east side of the Siberian craton, but their tectonic setting is debated. Recent Ar–Ar dates of ∼1750 Ma for NW-trending dykes in the Aldan and Anabar shields, together with similar-age NNE-trending Baikal uplift dykes in south-eastern Siberia suggest the existence of a giant radial dyke swarm possibly related to a mantle plume centred in the Vilyui River area.     

西藏拉萨地块北部晚白垩世晚期基性岩墙的成因:来自锆石U-Pb年代学及地球化学的制约

拉萨地块北部阿索地区新发现的晚白垩世晚期基性岩墙对于讨论班公湖-怒江洋盆闭合后的碰撞过程具有重要研究意义。本文报道了阿索地区目思基性岩墙的全岩地球化学组成和LA-ICP-MS锆石U-Pb年龄。研究表明,目思基性岩墙中一组最年轻锆石的U-Pb加权平均年龄为74Ma。岩墙内的辉绿玢岩SiO2含量介于51.90%~53.55%之间,MgO含量介于3.98%~4.97%之间,Mg^#为50.0~57.5,低Cr(51.30×10^-6~79.48×10^-6)和Ni(55.94×10^-6~74.17×10^-6)含量。岩墙具有轻稀土元素富集的特征,明显的负Eu异常。在微量元素方面,富集Ba、Th、U、K等大离子亲石元素富集,亏损Nb、Ta、Ti等高场强元素,并具有Pb的正异常。La/Sm-Sm/Yb投图结果显示基性岩墙来自于尖晶石+石榴石二辉橄榄岩地幔源区1%~5%的部分熔融,并在演化过程中发生了橄榄石、单斜辉石及斜长石的结晶分异,同时在岩浆上升过程中受到地壳混染。结合区域地质背景,拉萨地块与羌塘地块碰撞后下地壳增厚并发生榴辉岩化,导致拆沉作用。目思基性岩墙的锆石U-Pb年龄表明拆沉作用持续到晚白垩世晚期。     

A slab break-off model for mafic–ultramafic igneous complexes in the East Kunlun Orogenic Belt,northern Tibet: insights from early Palaeozoic accretion related to post-collisional magmatism

ABSTRACT

The East Kunlun Orogenic Belt (EKOB) in northern Tibet provides an important record of the amalgamation of the Wanbaogou oceanic basalt plateau and the Qaidam Block. Here we report geochemical, geochronological, and Hf isotopic data for newly identified late Silurian–Early Devonian mafic–ultramafic igneous complexes from the EKOB at the northern margin of the Tibetan Plateau. These complexes are dominantly composed of gabbro and pyroxenite rocks. Three complexes yield zircon U–Pb ages of 398.8 ± 1.8, 420.2 ± 1.2, and 413.4 ± 0.78 Ma. The ε_Hf(t) values of zircons range from +0.8 to +3.3 with T_DM1 ages of 897 to 998 Ma. Modelling of the geochemical data indicates that these igneous complexes have a hybrid origin, involving depleted mantle fluids derived from a previous subduction event and crustal materials. The geochemical and geochronological data suggest that these complexes formed in a post-collisional setting linked to break-off of a subducted oceanic slab, which occurred after the Wanbaogou oceanic basalt plateau amalgamated with the Qaidam Block in the late Silurian–Early Devonian.     

新疆磁海铁(钴)矿区镁铁质岩锆石U-Pb年龄、Hf同位素特征及岩石成因

磁海铁(钴)矿床是北山地区一大型富钴铁矿床,矿体产于辉绿岩中。矿区出露有大量的辉长岩。本文利用锆石LA-ICP-MS定年法,获得磁海矿区角闪辉长岩和磁南矿区粗晶辉长岩的206Pb/238U年龄分别为294.8±1.3Ma和276.1±0.63Ma,表明它们属早二叠世岩浆活动的产物。地球化学特征上,角闪辉长岩与辉绿岩为同源岩浆演化的产物,它们具有中等偏低的Mg#值(49～66)和相对高的TiO2含量(2.27%～2.57%),属钙碱性和拉斑玄武岩系列(FeOT/MgO=0.93～1.84),其微量元素显示富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损Nb、Ta和Ti,并具明显的Eu负异常。粗晶辉长岩与前者具有不同特征,它们有高的Mg#值(72～81)和低的TiO2含量(0.24%～0.39%),属钙碱性玄武岩系列,具有富集大离子亲石元素Ba、Th、U、Pb、Sr和轻稀土元素(LREE),亏损Nb、Zr、Hf和P,并具明显的Eu正异常特征。角闪辉长岩和粗晶辉长岩具有正的εHf(t)值(分别为4.6～11.7和1.5～5.1)。与北山地区同时代的具铜镍矿化的镁铁-超镁铁岩具有相同的特征。因此,认为磁海铁矿区基性岩形成于后碰撞伸展背景,由亏损的软流圈地幔熔体与俯冲板片物质的混合物。不同类型的岩石系不同成分的原始岩浆经不同演化过程的产物。     

陕西商县秦王山镁铁质岩体的特征及成因

秦王山镁铁质岩体是一在动荡的造山环境里形成的同构造异地岩块。岩体的普通铅模式年龄为407±3Ma,锶同位素初始比为0.7045,δ~(18)O=7.92,稀土配分式样为比球粒陨石富集的右倾型。岩石是在T=939℃,P=2.5×10~8Pa下结晶的。     

Record of Early Tonian mafic magmatism in the central Espinhaço (Brazil): New insights for break-up of the Neoproterozoic landmass ancestor of São Francisco-Congo paleocontinent

Petrological characterization, U–Pb geochronology, Lu–Hf analyses and major and trace element data from mafic intrusions in the Central Espinhaço (central portion of the Brazilian shield) are used here to investigate the geological significance of the Early Neoproterozoic magmatism in the context of the São Francisco-Congo paleocontinent. These mafic bodies are represented by medium to coarse-grained metagabbros with plagioclase, amphibole and clinopyroxene. Zircon U–Pb isotopic data from two samples yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 895 ​± ​3.4 ​Ma (MSWD ​= ​1.7) and 896 ​± ​2.4 ​Ma (MSWD ​= ​0.64), regarded as the best estimates for the crystallization age of these mafic rocks. Major and trace element data (including REEs) show that the gabbros originated from a subalkaline tholeiitic magma, typical of intraplate magmatism. Such rocks are slightly enriched in LREEs and LILEs and depleted in HFSEs. Our new isotope and geochemical data, along with regional knowledge, indicate that these metagabbros mark the beginning of an important Tonian-age extensional tectonic event of the landmass of which the São Francisco-Congo paleocontinent was part (Rodinia supercontinent or Central African block?). We furthermore suggest that these rocks belong to a prominent suite of Tonian-age mafic rocks that mark a diachronic breakup attempt of this landmass which may have occurred from south to north along the Espinhaço mountain range.     

Phanerozoic magmatism in the Proterozoic Cuddapah Basin and its connection with the Pangean supercontinent

Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin.The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe.Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation,Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma(Late Carboniferous) and 227 Ma(Triassic).Further,these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian,Panjal Traps,Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature.During the Phanerozoic Eon,the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.     

The Paleoproterozoic North Hebei Orogen: North China craton's collisional suture with the Columbia supercontinent

Understanding the geologic history and position of the North China craton in the Paleoproterozoic Columbia supercontinent has proven elusive. Paleoproterozoic orogenic episodes (2.00–1.85 Ga) are temporally associated with ultimate stabilization of the North China craton (NCC), followed by the development of extensive craton-wide rift systems at 1.85–1.80 Ga. The age difference between the sedimentary cover and the metamorphic basement is up to 500–700 Ma, suggesting that uplift and doming of cratonic basement occurred in the latest Paleoproterozoic. Mafic dike swarms (1.80–1.77 Ga) and anorogenic magmatism (1.80–1.70 Ga) record the extensional breakup and dispersal of the North China craton during this stage. The late Paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the NCC within the Late Paleoproterozoic supercontinent of Columbia.An east-west striking thousand kilometer long belt of khondalites (granulite facies metapelites) stretches along the northern margin of the North China craton, on the cratonward side of the Northern Hebei orogenic belt. This granulite belt includes Mg–Al (sapphirine bearing) granulites that reached ultrahigh-temperature “peak” metamorphic conditions of  1000 °C at 10 kbars at 1927 ± 11 Ma. Following peak ultrahigh-temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and these trajectories are interpreted to be part of an overall anti-clockwise P-T evolution indicating that the northern margin of the craton experienced continental collision at 1.93–1.92 Ga. The position of the khondalite belt south of the Northern Hebei orogenic belt makes it analogous to Tibet, a continental collision-related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth. We suggest that the tectonic evolution of the NCC during this period was closely related to the assembly and break-up of the Columbia supercontinent, and that the NCC was adjacent to the Baltic and Amazonian cratons in the period 2.00–1.70 Ga. Craton-wide extension occurred within 100–150 Ma of collision along the northern margin of the craton at 1.93–1.92 Ga. It is concluded that mantle upwellings are chiefly responsible for the breakup of the NCC from the Paleoproterozoic supercontinent.     

青藏东缘六合—香多镁铁质深源包体的平衡 P-T条件及其指示意义

依据青藏东缘六合-香多镁铁质深源包体中石榴石、辉石、角闪石和斜长石的矿物电子探针成分,采用Grt Cpx温度计（T EG79、T P85、T K88、和 T R2000）、Grt Hbl温度计（T GP84和 T RV2000）和Hbl Pl温度计（T BH90）以及角闪石Al压力计（P Al in Hb l）估算了包体的平衡温压条件。结果表明,石榴透辉岩、石榴斜长角闪岩、石榴角闪岩、斜长角闪岩和角闪石岩包体的平衡温压分别是770～920℃和 0．83～1．50GPa（相当于27～50km深）、730～950℃和 0．85～1．04GPa（相当于28～34km深）、 720～900℃和 0．74～1．32GPa（相当于24～44km深）、512～753℃和 0．71～0．98GPa（相当于23～32km深）以及 0．60～0．84GPa（相当于20～28km深）。前 3类包体主要来自下地壳,平衡温压较高（720～950℃、0．74～1．50GPa）;后 2类包体主要来自中地壳,平衡温压较低（510～735℃、0．60～0．98GPa）。包体的平衡温压条件还暗示了高钾岩浆起源的最小深度在50km以上。     

Chronological and geochemical studies of the Middle Neoproterozoic mafic rock and turbidite in the Dahongshan orogenic belt on the northern margin of the Yangtze Block: Implications for the evolution of the back-arc basinSCIEI北大核心CSCD

扬子北缘~820Ma可能记录了从挤压环境向伸展环境的重要转换,是解决扬子新元古代演化争议的关键时间节点。本研究在扬子北缘大洪山造山带圆潭地区识别出一套基性岩和浊积岩组合。其中基性岩全岩Sm-Nd等时线年龄为845Ma±81Ma(MSWD=1.5),浊积岩碎屑锆石年龄谱包括四个主要峰值年龄(~835Ma、~1597Ma、~2002Ma、~2482Ma)和两个次要峰值年龄(~2680Ma、~2934Ma),综合前人研究成果将浊积岩的形成时代约束在835~820Ma。浊积岩与花山群的形成时代一致,应代表花山群的北向延伸。浊积岩的物源来自扬子北缘桐柏-大别地区,不同于前人对花山群的分析结果,指示花山群可能具有多个物源。以上地层碎屑锆石年龄谱和地层层序特征与弧后盆地相似。本研究基性岩样品具有低的SiO_(2)(45.41%~50.29%)、K 2O(0.03%~1.49%),高的TiO_(2)(1.42%~3.03%),和低的U/Th(0.15~0.29)、Th/Zr(0.003~0.011)和Nb/Y(0.059~0.147)以及正的εNd(t)(+1.18~+1.57),类似N-MORB地球化学特征。基性岩来自于软流圈,经历<3%部分熔融而生成,并在上升过程中受到了一定程度的地壳混染。综合以上分析,本文认为这些基性岩和浊积岩共同构成一套弧后背景环境岩石组合。