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1.
早中生代的扬子大陆向华北大陆的深俯冲碰撞作用以及中、新生代的华北东部岩石圈减薄作用是国际大陆动力学问题研究的两个热点。然而，把它们有机联系起来，探讨深部岩石圈演化动力学过程的研究还很少。报道了中国大陆科学钻探先导孔（CCSD-PP1）橄榄岩的矿物岩石化学分析结果。这些橄榄岩亏损玄武质组分（如低w（CaO＋Al2O3）、高Mg^#等），并经历了中元古代来自软流圈的碳酸岩熔体的交代作用和早中生代的超高压再平衡过程。结合已发表的华北捕虏体橄榄岩资料，对这一橄榄岩的原始地幔属性进行了分析。发现CCSD-PP1橄榄岩与华北古老岩石圈地幔组成相似，是早中生代来自华北岩石圈的构造侵入体。早中生代华北岩石圈的伸展减薄与苏鲁超高压变质岩石的折返提供了早期软流圈物质上涌的构造体制。 相似文献
2.
华北东部橄榄岩与岩石圈减薄中的地幔伸展和侵蚀置换作用 总被引：24，自引：3，他引：24
对比分析了华北不同时代捕虏体橄榄岩及其南部超高压地质体橄榄岩的矿物化学。具古老难熔岩石圈地幔特征的橄榄岩是古生代金伯利岩捕虏体和早中生代苏鲁变质带地质体的主要岩石类型。具这一性质的橄榄岩也构成了河南鹤壁上新世玄武岩捕虏体的主体部分,并可以在辽宁阜新晚中生代玄武岩中被发现。具饱满岩石圈地幔性质的橄榄岩则是阜新晚中生代火山岩、特别是郯庐断裂带(山旺)及其附近地区(栖霞)中新世玄武岩捕虏体的主要类型。从华北东部已有的捕虏体橄榄岩及地质体橄榄岩所表现出的新生饱满与古老难熔地幔的时、空分布特点,即有些地区捕虏体橄榄岩表现出不同性质地幔共存现象(如阜新、鹤壁)或橄榄石Mg#呈渐变关系看:克拉通岩石圈地幔因扬子板块俯冲所引起的早期(如早中生代)地幔伸展、和晚中生代—渐新世因太平洋俯冲所引起的热扰动的软流圈物质上涌对古老地幔产生强烈的侵蚀作用(引起岩石圈的巨大减薄);中新世以来的软流圈热沉降作用出现新生岩石圈地幔并表现为岩石圈的小幅增厚,从而实现地幔置换过程和华北东部岩石圈的整体减薄过程。岩石圈幔内薄弱带及岩石圈深断裂(如郯庐断裂带)起了软流圈物质侵蚀古老岩石圈地幔的通道作用并导引着深部物质运移和不规则减薄作用等。个别地区(如阜新)强烈的软流圈上涌于晚中生代就已经开始,显示地幔置换作用的强烈不均一性。 相似文献
3.
华北东部橄榄岩岩石化学特征及其岩石圈地幔演化意义 总被引：10，自引：3，他引：7
华北东部古生代以来火山岩中捕虏体橄榄岩和苏鲁早中生代构造侵位橄榄岩的岩石化学结果表明: 古生代金伯利岩侵位时仍然存在的难熔、漂浮克拉通地幔在中、新生代时其大部被新生饱满的岩石圈地幔物质取代置换.在100 Ma前(但不早于178 Ma), 新生软流圈物质就已开始沿古老岩石圈内的地幔薄弱带和岩石圈深断裂带对克拉通地幔进行侵蚀、交代和混合作用, 引起岩石圈大幅减薄.这一减薄存在时间、空间的不均匀性, 但在老第三纪达最大.新第三纪以来, 上涌的软流圈物质由于温度下降回落(岩石圈小幅增厚)并转化为新生岩石圈地幔, 实现地幔置换过程.分析的苏鲁造山带橄榄岩是早中生代构造侵位改造过的古老岩石圈地幔物质. 相似文献
4.
华北东部大陆地幔橄榄岩组成、年龄与岩石圈减薄 总被引：16，自引：1，他引：15
对比分析了华北东部地块陆下岩石圈地幔橄榄石Mg#值和单斜辉石的REE配分形式。报道了汉诺坝和鹤壁橄榄岩中不同产状硫化物的激光MC-ICPMS原位Re-Os年龄和信阳橄榄岩中锆石的U-Pb年龄和信阳橄榄岩锆石的U-Pb年龄。在这些资料基础上,进一步讨论了华北东部岩石圈中、新生代时的减薄机制。原位分析在揭示岩石圈深部过程的细节上,有比全岩分析更大的优越性,并揭示出了在华北深部有中元古代(14亿年)和新元古代(7～8亿年)热活动的记录。岩石圈拆沉作用不能很好地解释古老难熔地幔、过渡型地幔和新生饱满地幔并存的事实;同时,单纯的熔体-橄榄岩相互作用也难以解释中、新生代岩石圈的减薄过程和新生地幔单斜辉石中出现强烈LREE亏损现象,即历史复杂的克拉通岩石圈向历史明显简单的“大洋型”地幔的转换。因此,华北东部岩石圈减薄包括地幔伸展、熔-岩作用、侵蚀置换等复杂过程。这些过程可能包括:(1)早中生代时,扬子地块向北俯冲碰撞所引起华北岩石圈的熔/流体交代富集作用、地幔伸展和受扰动软流圈物质上涌并侵蚀被改造了的岩石圈;(2)晚中生代—古近纪,因太平洋俯冲的热扰动致使软流圈物质进一步的强烈侵蚀作用引起岩石圈的巨大减薄;(3)晚第三纪以来的软流圈热沉降作用所带来的小幅度岩石圈增厚过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体的减薄过程。喷发时代为100Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前曾有过地幔置换作用。 相似文献
5.
华北克拉通晚中生代壳-幔拆离作用: 岩石流变学约束 总被引：6，自引：5，他引：1
大陆岩石圈的流变学结构对于岩石圈深部过程(壳/幔过程)有着深刻的影响,直接表现在岩石圈壳-幔结构与浅部构造上.本文注意到华北克拉通晚中生代岩石圈减薄期间地壳的伸展、拆离与减薄在不同地区的宏观、微观构造及地壳岩石流变学等方面的差异表现与区域变化,以及现今和晚中生代时期岩石圈厚度的不均匀性.讨论了以水为主体的地质流体的存在对于岩石圈流变性的影响.综合克拉通东部与西部地壳/地幔厚度变化特点以及下地壳和上地幔含水性特点,阐述了晚中生代时期华北克拉通岩石圈内部壳幔耦合与解耦的规律,提出了华北岩石圈壳-幔拆离作用模型以解释华北克拉通晚中生代岩石圈减薄的基本现象与深部过程.提出区域性伸展作用是岩石圈减薄的主要动力学因素,东部地区在晚中生代伸展作用过程中壳-幔具有典型的解耦性,上部地壳、下部地壳和岩石圈地幔的变形具有显著差异性.而西部区壳幔总体具有耦合性,下地壳与岩石圈地幔共同构成流变学强度很高且难以变形的岩石圈根. 相似文献
6.
中国东部中新生代火山作用的pTtc模型与岩石圈演化 总被引：8，自引：0，他引：8
中国东北和华北地区中新生代火山岩的岩石化学研究表明 ,中新生代火山作用的深部过程均为逆时针的pTt轨迹 ,表现为岩浆源区从逐渐上升到下降的过程。中新生代火山作用的深部过程揭示的岩石圈演化历史为 :早侏罗世至早白垩世早期 ,岩石圈演化主要表现为逐渐减薄的过程 ,直至出现软流圈与地壳直接接触。从早白垩世晚期至中生代末 ,岩石圈演化为一增生过程。第三纪岩石圈演化为减薄过程 ,而到第四纪为岩石圈增生过程。中国东部现代岩石圈地幔是由中生代晚期 (K1末—K3)和第四纪两次增生事件形成的 相似文献
7.
中国东部深部构造特征及其与矿集区关系 总被引：12，自引：3，他引：9
文中以地震层析原始数据为基础,结合大地电磁测深和热流值数据,对老资料重新开发,编制了中国东部岩石圈厚度图、大地热流值图、岩石圈构造与矿集区关系图等,在此基础上对中国东部上地幔岩石圈-软流圈构造及其变异进行了初步研究,认为:(1)中生代燕山期较大型深部构造(如软流圈上涌体)的地震层析影象可存留至今,运用大地热流值及其相应浅表岩浆岩、矿集区等特征差异可以将中、新生代深部构造加以区分;(2)中国东部深部构造总特征:上地幔岩石圈-软流圈构造起伏变化,岩石圈西厚东薄,软流圈上涌发育且形式各异。大体可分为华南(构造线呈NWW)、华北(构造线近SN)、东北(构造线呈NNE)三大块,其基本特征各异。整个中国东部深部构造中,软流圈上涌起主导作用;(3)中生代软流圈上涌是相应浅表燕山期花岗质岩形成的根源,并与金属矿集区密切相关,新生代软流圈上涌则是大陆裂谷、玄武岩喷发及大型油气田形成的基础;(4)岩石圈减薄的主因是软流圈上涌,中生代燕山期主要减薄于中国东部大陆内部,新生代主要减薄于偏东部沿海区域。 相似文献
8.
华北克拉通东部中生代岩石圈减薄的过程与机制:中生代火成岩和深源捕虏体证据 总被引：70，自引：10，他引：60
基于最新的同位素年代学资料 ,华北克拉通东部中生代的岩浆作用可划分成四个阶段 ,即晚三叠世 ( 2 0 5～ 2 2 5Ma)碱性岩浆作用 ;中晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用 ;早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用和晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用。徐淮地区中生代侵入岩中榴辉岩捕虏体的发现及其地质年代学资料 ( 2 1 9Ma)表明 ,华北克拉通东部中生代早期曾发生过一次重要的陆壳加厚过程。俯冲板片的断离以及高压—超高压变质岩的快速折返和晚三叠世 ( 2 0 5～ 2 2 5Ma)的碱性岩浆作用的存在均暗示 ,华北克拉通中生代岩石圈减薄已经开始。拆沉作用则是引起中生代早期岩石圈减薄的主要机制。中、晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用形成于造山期后的伸展环境 ,代表了中生代岩石圈减薄的继续和发展。早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用表明中生代岩石圈减薄达到了峰期。而幔源纯橄岩捕虏体中富硅质熔体的交代作用和玄武岩的高87Sr/ 86Sr值、低ε(Nd ,t)值特征表明 ,软流圈对岩石圈底部的化学侵蚀可能是导致该阶段岩石圈减薄的主导机制。晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用和“海洋型”地幔捕虏体的存在代表了等温面的下降和岩石圈地幔的增生 相似文献
9.
本文综合运用不同时代幔源包体平衡温压对比、玄武岩地球化学性质对岩石圈厚度的反演以及不同时代岩石圈地幔地球化学性质的对比的方法，把华北地块东部岩石圈的减薄时间限定在晚中生代至新生代之间。减薄的机制可能是华北东部地区晚白垩世以来大陆岩石圈的拉张作用。由于机械性拉薄和热、机械和化学侵蚀作用，岩石圈厚度最终减薄到70km以下。但古老的岩石圈地幔并没有完全因减薄而消失，残留部分受到了来自软流圈物质的强烈改造，使其Sr、Nd同位素组成类似于软流圈，但Os同位素没有受到明显的改变。改造后的岩石圈地幔成为华北地块东部新生代岩石圈地幔的主体。在时空上，岩石圈的减薄具有不均一的性质。 相似文献
10.
太行山中生代板内造山作用与华北大陆岩石圈巨大减薄 总被引：13，自引：1，他引：13
近年来,华北大陆岩石圈巨大减薄成为国际地学界关注的焦点之一,但对其减薄的时间、机制仍然知之甚少。约束条件的多解性和表面上相互矛盾的证据导致了对区域构造发展史的模糊认识。笔者认为,华北板内造山过程是理解岩石圈巨大减薄机制的关键,因为华北岩石圈是在造山带而不是在克拉通基础上开始减薄过程的。岩石圈减薄过程可以划分为拆沉减薄、伸展减薄、热减薄和化学侵蚀减薄4种类型。前者依赖于岩石圈重力不稳定性,是一种突变过程;后三者取决于软流圈挤出构造,属于渐变过程。减薄过程主要始于120~110Ma的拆沉减薄,其标志是造山后脉岩组合的形成。亚洲大陆软流圈的多阶段汇聚过程造成软流圈向东挤出,是中国东部中新生代以来岩石圈持续减薄的重要基础。因此,大陆动力学与大洋最重要的区别之一就是大陆岩石圈经常发生减薄作用,特别是拆沉作用,并由此将软流圈系统区分为浅部混染系统和深部纯净系统,火成岩的地球化学属性主要取决于岩浆起源的深度。 相似文献
11.
Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China 总被引：24，自引：0，他引：24
ZHENG JIANPING; GRIFFIN WILLIAM L.; O'REILLY SUZANNE Y.; YANG JINSUI; LI TIANFU; ZHANG MING; ZHANG RUTH Y.; LIOU JOHN G. 《Journal of Petrology》2006,47(11):2233-2256
Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m2 (Paleozoic) and 80 mW/m2 (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al2O3 and TiO2contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement 相似文献
12.
大陆碰撞造山带的两类橄榄岩——以柴北缘超高压变质带为例 总被引：2，自引：0，他引：2
论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。 相似文献
13.
J. P. Zheng H. Y. Tang Q. Xiong W. L. Griffin S. Y. O’Reilly N. Pearson J. H. Zhao Y. B. Wu J. F. Zhang Y. S. Liu 《Contributions to Mineralogy and Petrology》2014,168(1):1-21
The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins. 相似文献
14.
橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素（如Mg和Fe）组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明：地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#（〉92）,而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#（〈91）。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明：该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。 相似文献
15.
Giovanna T. Sapienza Marco Scambelluri Roberto Braga 《Contributions to Mineralogy and Petrology》2009,158(3):401-420
We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants
of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated
with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids
lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing
peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite
and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements
during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite
similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous
CO2-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence),
a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts
and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous
CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from
peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate
between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related
dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’
assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites
like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
In memory of Lauro Morten 1941–2006. 相似文献
16.
《International Geology Review》2012,54(2):182-211
The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (87Sr/86Sr = 0.7046–0.7087; 143Nd/144Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time. 相似文献
17.
Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central China 总被引：44，自引：3，他引：41
Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87 Sr/86 Sr ratios (0.707–0.708), low εNd ( t ) values (−6 to −9) and low δ18 O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1 . 相似文献
18.
为完整了解华北克拉通的破坏程度和机制,加深对其西部陆块岩石圈地幔的研究十分重要,而位于华北克拉通西部集宁新生代碱性玄武岩中的地幔橄榄岩包体,为研究人员认识该地区的岩石圈地幔的性质和演化起到指示作用.运用LA-ICP-MS和LA-MC-ICP-MS对集宁地区橄榄岩矿物进行原位微区测试,获得其主量、微量元素和Sr同位素成分的数据.根据矿物组成,可以将集宁地区的橄榄岩分为两类:第一类为贫单斜辉石橄榄岩 (单斜辉石体积分数小于8%),它们经历了高程度的部分熔融,可能是古老难熔岩石圈地幔的残留;第二类为二辉橄榄岩 (单斜辉石体积分数大于13%),其熔融程度低,代表了新生饱满的岩石圈地幔.第一类橄榄岩中单斜辉石REE含量整体偏低且轻微富集LREE,第二类橄榄岩中单斜辉石具有LREE富集和轻微亏损两种配分模式,大部分样品的核边有一定的强不相容元素及Sr同位素组成变化.这些微量元素和同位素特征都表明集宁橄榄岩包体经历过交代作用.(La/Yb)N和Ti/Eu比值特征表明它们经历过多阶段的交代作用,交代介质有硅酸盐、碳酸盐熔/流体,这些交代介质可能为来源于古亚洲洋板块俯冲时释放的熔/流体. 相似文献
19.
Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps 总被引：1，自引：0，他引：1
M. JANÁK N. FROITZHEIM M. VRABEC E. J. KROGH RAVNA J. C. M. De HOOG 《Journal of Metamorphic Geology》2006,24(1):19-31
New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation. 相似文献