Coasts without geology     

ERIC ROBINSON 《Geology Today》1988,4(2):50-50

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Multi‐stage Melting Processes of Ophiolitic Chromitites from Purang Peridotite,the Western Yarlung‐Zangbo Suture Zone,Tibet     

Fahui XIONG  Jingsui YANG  Paul T ROBINSON  Xiangzhe XU 《《地质学报》英文版》2019,93(Z2):35-35

The Purang ophiolite, which crops out over an area of about 600 km2 in the western Yarlung‐Zangbo suture zone, consists chiefly of mantle peridotite, pyroxenite and gabbro. The mantle peridotites are mostly harzburgite and minor lherzolite that locally host small pods of dunite. Some pyroxenite and gabbro veins of variable size occur in the peridotites, and most of them strike NW. On the basis of their mineral chemistry podiform chromitites are divided into high‐alumina (Cr# = 20‐60) (Cr# = 100*Cr/(Cr+Al)) and high‐chromium (Cr# = 60‐80) varieties (Thayer, 1970). Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs which can have different chromitite compositions. However, the Purang massif contains both high chrome and high alumina chromitites within a single mafic‐ultramafic body. Seven small, lenticular bodies of chromitite ore have been found in the harzburgite, with ore textures ranging from massive to disseminated to sparsely disseminated; no nodular ore has been observed. Individual ore bodies are 2‐6 m long, 0.5‐2 m wide and strike NW, parallel to the main structure of the ophiolite. Ore bodies 1 and 6 consist of Al‐rich chromitite (Cr# = 52‐55), whereas orebodies 2, 3, 4 and 5 are Cr‐rich varieties (Cr # = 63 to 89). In addition to magnesiochromite, all of the orebodies contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. On the basis of magnesiochromite and olivine/clinopyroxene compositions two stages of partial melting are identified in the Purang peridotites, an early low‐partial melting event (about 8%), and a later high‐partial melting event (about 40%). We interpret the Al‐rich chromitites as the products of early MORB magmas, whereas the Cr‐rich varieties are thought to have been generated by the later SSZ melts..  相似文献   

Diamond in Oceanic Peridotites and Chromitites: Evidence for Deep Recycled Mantle in the Global Ophiolite Record     

Jingsui YANG  Paul ROBINSON  Xiangzhen XU  Fahui XIONG  Dongyang LIAN 《《地质学报》英文版》2019,93(Z2):42-42

Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km‐long Yarlung‐Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong‐Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in‐situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung‐Zangbo suture (Das et al., 2015, 2017). The above‐mentioned diamond‐bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti‐Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray‐Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in‐situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray‐Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra‐high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro‐diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150‐380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (δ13C ‐18 to ‐28‰) of these ophiolitic diamonds and their Mn‐bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro‐diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in‐situ occurrence of micro‐diamonds has been well‐demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite‐hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in‐situ oceanic mantle. The fundamental scientific question to address here is how and where these micro‐diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.  相似文献   

Melt Events in A Nascent Mantle Wedge: Implication from the Luobusa Ophiolite,Tibet     

TUNG Kuo’an  YANG Houngyi  YANG Huaijen  ZHANG Jianxin  LIU Dunyi  LI Xianhwa 《《地质学报》英文版》2019,93(Z2):49-50

The compositions of minerals and whole rocks of the Luobusa ophiolite in South Tibet, a fragment of Neo‐Tethyan forearc lithosphere, is used to investigate the magmatic evolution of nascent mantle wedges in newly‐initiated subduction zones. Clinopyroxenes in the Luobusa peridotites all have diopsidic compositions, and their Al₂O₃ contents vary from ～ 2% in the dunites and refractory harzburgites to 2‐4% in the cpx‐bearing harzburgites. The REE of clinopyroxenes in the harzburgites have left‐sloping patterns with contents comparable to those in abyssal peridotites that have experienced 5‐15% partial melting. Chromites in the Luobusa chromitites have the highest Cr#s (～ 80) and TiO2 contents (0.1‐0.2%), and those in the cpx‐bearing harzburgites have the lowest Cr#s (20‐60) and TiO2 contents (0‐0.1%), whereas those in refractory harzburgites and dunites have intermediate compositions. Cpx‐bearing and refractory harzburgites show spoon‐and U‐shaped REE patterns, respectively, and their HREE distribution patterns suggest at least 15%‐ 20% partial melting. The REE patterns of dunites and high‐Cr chromitites vary from spoon‐ to U‐shaped and require 15‐30% partial melting in their mantle sources to produce their parental melts. Our dataset reveals that the nascent Luobusa mantle wedge was first infiltrated by slab‐derived fluids and later refertilized by transitional lava‐like melts, resulting in cpx‐bearing harzburgites. Partial melting in the deeper cpx‐bearing mantle generated high‐Ca boninitic to arc picritic melts, which interacted with the peridotites in the uppermost mantle to generate high‐Cr chromitites, dunites and some refractory harzburgites. Lithological variation from cpx‐bearing to refractory harzburgites in forearc ophiolites is the result of multi‐stage melt events rather than increasing degrees of partial melting. Intermittent slab rollback during subduction initiation induces asthenospheric upwelling and high heat flux in nascent mantle wedges. Elevated geothermal gradients play a more important role than slab dehydration in triggering Mg‐rich magmatism in newly‐initiated subduction zones.  相似文献   

Geochronology and Geochemistry of the Magmatic Rocks from Zedong Ophiolite, Eastern Yarlung-Zangbo Suture Zone, Tibet     

XIONG Fahui  YANG Jingsui  Paul T. ROBINSON  GAO Jian  ZHANG Lan  CHEN Yanhong  LAI Shengming 《《地质学报》英文版》2017,91(Z1):45-45

The Yarlung Zangbo suture zone extends more than2000 km along southern Tibet and marks the boundary between the Indian subcontinent and Eurasia.The Zedong terrane has been not suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean,as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic.In this study,we present detailed geochemical and geochronological data of various types of magmatic rocks widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane.We found that the Zedong volcanic rocks belong to high K2O calc-alkaline series,whereas the diabase and gabbro plotted in the low-K calcalkline.The basalt rocks are highly enriched in LREE and LILE,but strongly depleted in HFSE,indicating they were derived from a metasomatized mantle.Both gabbros and diabase have similar N-MORB geochemistry indicates that the cumulates were produced from MOR setting.Zircons from four samples,including the basalt rocks(158-161Ma)are older than the gabbro(131 Ma),certificate the gabbro are as the vein intrude into the basalt rocks.This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedonghave similar positiveεHf(t)values(+2.0 to+15.6)and(+8.6 to+18.4),indicating they were stemmed from similarly depleted mantle sources,same with the gabbro and granitic rocks from the Gangdese arc.Therefore,we proposed that the basalt rocks in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts.A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane,which gave rise to the cumulate and granitic rocks.This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic,although a possible intra-oceanic arc setting cannot be excluded.  相似文献   

Iron Isotopic Fractionation and Origin of Chromitites in the Paleo-Moho Transition Zone of the Kop Ophiolite, NE Turkey     

ZHAGN Pengfei  ZHOU Meifu  SU Benxun  Ibrahim UYSAL  Paul T. ROBINSON  Erdi AVCI  HE Yongsheng 《《地质学报》英文版》2017,91(Z1):53-53

The Kop ophiolite in NE Turkey is a fragment of Neo-Tethyan forearc.It can be mainly divided into a paleo-Moho transition zone(MTZ)in the North and a harzburgitic mantle sequence in the South.Dunites are predominant in the MTZ of the Kop ophiolite,and they are locally interlayered with chromitites and enclose minor bodies of harzburgites near the petrological Moho boundary.Large Fe isotopic variations were observed for magnesiochromite(-0.14‰to 0.06‰)and olivine(-0.12‰to 0.14‰)from the MTZ chromitites,dunites and harzburgites.In individual dunite samples,magnesiochromite usually has lighter Fe isotopic compositions than olivine,which was probably caused by subsolidus Mg-Fe exchange between the two mineral phases.Both magnesiochromite and olivine display an increasing trend ofδ56Fe along a profile from chromitite todunite.This trend reflects continuous fractional crystallization in a magma chamber,which resulted in heavier Fe isotopes concentrated in the evolved magmas.In each cumulative cycle of chromitite and dunite,dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite.Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite,generating chromitite layers in the cumulate pile.Before mixing happened,the primitive melts had reacted with mantle harzburgites during their ascendance;whereas the evolved melts may lie on the olivine-chromite cotectic near the liquidus field of pyroxene.Variable degrees of magma mixing and differentiation are expected to generate melts with differentδ56Fe values,accounting for the Fe isotopic variations of the Kop MTZ.  相似文献   

Determining Crustal Strength from Shoreline Deformation, Central Tibet     

SHI Xuhu  Eric KIRBY  Kevin P. FURLONG  MENG Kai  Erchie WANG  Ruth ROBINSON  LU Haijian 《《地质学报》英文版》2013,87(Z1):151-152

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The dynamics of the zonal index in a simple model of the atmosphere     

WALTER A. ROBINSON 《地球，A辑:动力气象学与海洋学》1991,43(5):295-305

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Platinum-group Mineral (PGM) and Base-metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance   总被引：1，自引：0，他引：1  

GUO Guolin  YANG Jingsui  Paul T. ROBINSON  LIU Xiaodong  XU Xiangzhen  XIONG Fahui 《《地质学报》英文版》2016,90(3):900-912

Voluminous platinum-group mineral(PGM) inclusions including erlichmanite(Os,Ru)S_2, laurite(Ru,Os)S_2, and irarsite(Ir,Os,Ru,Rh)As S, as well as native osmium Os(Ir) and inclusions of base metal sulphides(BMS), including millerite(NiS), heazlewoodite(Ni_3S_2), covellite(CuS) and digenite(Cu_3S_2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite(biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides(such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S_2 and ?O_2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S_2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S_2 increased. The abundance of primary BMS in the chromitites suggests that ?S_2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.  相似文献   

结合有人和无人驾驶船观测以及长期定点观测计算寒潮过境引起的海水输运     

WEEKS Eddie  ROBINSON Mark E  李春雁 《海洋学报(英文版)》2018,37(11):1-7

高分辨率的数据对于理解近海的复杂过程以及制定有效的管理措施日益重要，特别是考虑到恶劣气象的长期效应。这种长期效应的积累可以与潮汐的长期效应一样重要。本文讨论的即是一个大气锋面过境的过程对于Vermilion Bay水输运的影响。我们的研究采用了有人船和自制的无人船作为载具来测量流速剖面。这种自制无人船造价低廉、简单实用、可控性好，可以做比有人船更精确的测量。我们采用安装在这些观测载具上的多普勒流速剖面仪在一个潮周期内反复对流速的横向和垂向断面的水通量做高分辨率的精准测量，然后与一个定点的多普勒流速剖面仪的流速做相关分析得出相关系数。利用所求相关系数把水通量的计算扩展到总共717天的定点观测时间段，以此讨论在这个期间最强的一次大气寒潮过境时产生的水输运并阐述此类过程的重要性。  相似文献