Ultra-depleted melt product preserved in the Ladong ophiolitic peridotites of the North Qilian Orogenic Belt, Northern Tibet
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Pyroxene
Peridotite
Ophiolites are conventionally regarded as fragments of former oceanic lithosphere. Mineralogical and field evidence indicates that peridotite of the Neyriz ophiolite was intruded at high temperature into folded crystalline limestones, forming skarns. This excludes the formation of the ophiolite at a mid-ocean ridge but is consistent with its origin by intrusion during continental rifting.
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华北克拉通中部带是一条形成于古元古代之前的构造碰撞拼合带。在中部带的北段赤城地区，出露一套特殊的角闪斜长片麻岩、退变榴辉岩以及蛇纹石化橄榄岩组合，其成因众说纷纭。这套岩石组合归属于红旗营子群，是前寒武纪结晶基底，其中蛇纹石化橄榄岩和退变榴辉岩以构造团块的形式包裹在片麻岩之中。赤城蛇纹石化橄榄岩由纯橄岩、方辉橄榄岩和超镁铁质堆晶岩组成。其中纯橄岩和方辉橄榄岩的全岩主量元素都具有高Mg低Si的特征，大部分方辉橄榄岩中的橄榄石具有很高的Fo值（91.0~92.2），尖晶石具有高的Cr#值（53.9~85.9）。这些特征表明方辉橄榄岩和纯橄岩都经历了高程度部分熔融，它们很可能代表残余的大洋岩石圈地幔，并且可与典型的蛇绿岩套橄榄岩剖面相对比。超镁铁质堆晶岩表现出饱满的岩石特征，代表残余地幔之上的岩浆早期堆晶。橄榄岩Re亏损模式年龄表示，赤城橄榄岩的部分熔融作用发生在~2.5Ga之前。赤城红旗营子群角闪斜长片麻岩和退变榴辉岩的锆石U-Pb年代学揭示其原岩形成于~2.5Ga，经历了~1.85Ga的峰期变质作用和300Ma的退变质作用叠加。角闪斜长片麻岩和退变榴辉岩的原岩分别为大陆火山岩和大洋拉斑质岩石。因此，红旗营子群的橄榄岩、退变榴辉岩和角闪斜长片麻岩这一套岩石组合属于典型的蛇绿岩剖面，它很可能代表了华北克拉通东部和西部陆块之间曾经存在的大洋岩石圈。赤城蛇纹石化橄榄岩属于新太古-古元古代蛇绿岩残片，它可能记录了古大洋的消亡和中部造山带的形成，指示早期板块构造在新太古代已经启动。
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Platinum group
Chromitite
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Summary Lodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi 5 O 12 . Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs 13.8 with little variation. Olivine composition averages Fa 12.6 , but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found. A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene. Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies. The Harvard University meteorite is a mesosiderite and not closely related to Lodran.
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Chromite
Parent body
Diopside
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مجموعه افیولیتی نورآباد- هرسین به عنوان بخش جنوبی افیولیت کرمانشاه در پهنه زاگرس مرتفع قرار دارد. این مجموعه افیولیتی از بالا به پایین شامل پریدوتیت های سرپانتینی، گابروهای ایزوتروپ، گابروهای لایه ای، پلاژیوگرانیت، مجموعه دایک های صفحه ای، گدازه های بازالتی، آندزیت و سنگ های رسوبی هستند. ساختارهایی همچون جهت یافتگی، کشیدگی و انحنا در بلورها نشان می دهد که پریدوتیت های نورآباد- هرسین در شرایط گوشته بالایی تشکیل و سپس در محیط پوسته ای قرار گرفته اند. داده های حاصل از آنالیز نقطه ای بیانگر ترکیب فورستریتی الیوین با میزان (Fo 91-93) می‌باشد. پیروکسن ها از نوع انستاتیت و دیوپسید بوده و مقادیر ناچیز TiO2 و Al2O3 دارند. اسپینل ها نیز دارای مقادیر بالای ( 61-89)Mg# و (57-80) Cr# و مقادیر پایین TiO2 هستند که ماهیت افیولیتی و تهی شده دارند. در سرپانتین ها مقدار Al2O3 0.01 الی 10.42 درصد وزنی و مقدار SiO2 36.75 الی 41.08 درصد وزنی است که از نوع کریزوتیل و لیزاردیت هستند. مودال پایین کلینوپیروکسن، الیوین غنی از منیزیم و مقدار بالای Cr# و Cr/Al در اسپینل های منطقه مورد بررسی همگی نشان دهنده نرخ ذوب بخشی بالا برای هارزبورژیت ها و دونیت ها است. ترکیب شیمی کانی های مختلف، نشان از ژنز متفاوت سنگ های الترامافیک کمپلکس نورآباد- هرسین دارد و ویژگی هایی از محیط آبیسال تا پهنه فرافرورانش را نشان می دهند. بنظر می رسد این پریدوتیت ها از درجات بالای ذوب بخشی یک گوشته تهی شده زیر اقیانوسی در یک محیط فرافرورانش -پیشانی قوس تشکیل شده اند.
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The northeast Indian ophiolite belt is exposed in parts of Nagaland and Manipur states. We present the results of an investigation into the petrology and constituent mineral chemistry of peridotite from the ophiolite belt of Phek District, Nagaland. Based on relict primary mineral compositions, the studied rocks are identified as abyssal peridotite. Spinel composition shows that the rocks have undergone low degree of partial melting (5 11%). Equilibration of the ophiolite mantle sequence peridotite was calculated at a temperature range of 850–1165oC, pressure ranging between 19 to 25 kbar, and an oxygen fugacity between 0.437 to 0.657 log units above the FMQ buffer, whereas for the xenolith peridotite the temperature was estimated to be between 850 to 1100 oC, pressure ranging from 12 to 17 kbar and oxygen fugacity between 0.577 to 0.706 log units above the FMQ buffer. Equilibrium condition suggests that the rocks were equilibrated in an upper mantle environment.
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As far the ophiolites sequence study has concerned, the characteristics of chromian spinel (Cr-spinel) from mantle rocks (peridotite) and associated chromitite has been a ubiquitous petrogenetic indicator with regard to three tectonic settings (mid-oceanic ridges, subduction zone and arc) in terms of Cr# (= Cr/Cr+Al atomic ratio), Mg# and Ti content. In this study, we attempt to understand the tectonic environment and genetic relationship of mantle peridotites associated chromitites using chromian spinel characteristics from the southern Manipur Ophiolite Complex (MOC) which occupied an integral part of the Indo-Myanmar Orogenic Belt (IMOB), India. The study area mainly consists of peridotites (clinopyroxene-bearing harzburgite and lherzolite) with small pods of podiform chromitite, mafic intrusive - extrusive rocks and oceanic pelagic sediments. The Cr-spinel in peridotites are characterised by high Al2O3 (45.59-50.85), Mg# (89.25-90.48) and low Cr# (19.66 - 23.56) collectively suggest a typical abyssal peridotite derived from fertile mantle at mid-oceanic ridges (MOR) tectonic setting. Whereas, chromitites are low Al2O3 (14.35 to 19.68) and melt compositions (12.01 – 13.69 wt.%) with high Cr# (65.5 to 71.83) suggests that they were crystallized from boninitic magma series in the fore-arc subducting environment. Our petrological and geochemical evidences reveal that two stages of magmatism evolved during the formation of MOC. In the initial stage, magma was generated at the mid - oceanic spreading centre (MOR) by a small degree of partial melting (8–10 %) resulting high Al-rich and low Cr-rich chromian spinel bearing peridotites essentially of lherzolitic composition. At the second stage, high Cr-rich and low Al-rich magma evolved by high degree of partial melting intruded the earlier formed lherzolitic mantle exhibit harzburgite with high Cr# chromitite that formed at the fore-arc related setting above supra-subduction zone. It has been concluded from the present study that the magmatism in the mid-oceanic ridges environment followed by subduction tectonic process were responsible for the evolution of the Nagaland-Manipur Ophiolites that were emplaced along the Eastern plate margin of Indian subcontinent.
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Chromitite
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Peridotite
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Brucite
Forsterite
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