大别山中生代钾质岩浆作用与超高压变质地体的剥露机理

在已知的超高压变质地体中, 大别山是碰撞后花岗岩类侵入作用最为强烈的唯一地区。这里, 中生代的钾玄岩系列和高钾钙碱性系列的侵入岩, 可以依其年龄和组成特征划分为３ 组。第 Ｉ组, 主要由晚三叠世 （约２１０ Ｍａ） 二长闪长岩 辉长岩体组成, 它可能是在板片断离过程中富集的大陆岩石圈地幔部分熔融的产物 （板片断离型）; 第 Ｉ Ｉ组, 由中侏罗世－ 早白垩世 （１６０～１２０ Ｍａ） 的角闪石英二长岩、黑云母二长花岗岩和正长花岗岩组成, 主要是由幔源岩浆的分离结晶与地壳混染共同作用的产物 （岩浆底垫型）; 第 Ｉ Ｉ Ｉ组, 以白垩纪 （１２５～９５ Ｍａ） 的花岗岩和花岗斑岩为代表, 是在热穹窿作用过程中大别杂岩深熔作用和高度演化的产物 （穹窿型）。大别山超高压变质岩通过地幔的剥露可能与晚三叠世—早侏罗世的板片断离有关, 而大规模的岩浆侵位与超高压变质岩的快速剥露相伴出现说明, 岩浆底垫和岩浆漂浮作用可能在大别山中侏罗世—早白垩世的快速剥露中发挥了重要作用。     

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.

The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.

North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the ⁴⁰Ar/³⁹Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.

The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the ⁴⁰Ar/³⁹Ar dates.

A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented.     

Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps

Pelagic metasediments and MORB-type metabasalts of the former Tethyan oceanic crust at Cignana, Valtournanche, Italy, experienced UHP metamorphism and subsequent exhumation during the Early to Late Tertiary. Maximum PT conditions attained during UHP metamorphism were 600–630 °C, 2.7–2.9 GPa, which resulted in the formation of coesite-glaucophane-eclogites in the basaltic layer and of garnet-dolomite-aragonite-lawsonite-coesite-phengite-bearing calc-schists and garnet-phengite-coesite-schists with variable amounts of epidote, talc, dolomite, Na-pyroxene and Na-amphibole in the overlying metasediments. During subduction the rocks followed a prograde HP/UHP path which in correspondance with the Jurassic age of the Tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating. Inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle-plastic transition in the quartz-aragonite-dominated rocks, induced convection in the asthenospheric mantle wedge and/or heat consumption by endothermic reactions over a restricted PT segment during subduction. After detachment from the downgoing slab some 50–70 Ma before present, the Cignana crustal slice was first exhumed to ca. 60 km and concomitantly cooled to ca. 550 °C, tracing back the UHP/HP prograde path displaced by 50–80 °C to higher temperatures. Exhumation at this stage is likely to have occurred in the Benioff zone, while the subduction of cool lithosphere was going on. Subsequently, the rocks were near-isothermally exhumed to ca. 30 km, followed by concomitant decompression and cooling to surface conditions (at < 500 °C, < 1 GPa). During this last stage the UHPM slice arrived at its present tectonic position with respect to the overlying greenschist-facies Combin zone. In contrast to the well-preserved HP/UHPM record of the coesite-glaucophane eclogites, the HP/UHP assemblages of the metasediments have been largely obliterated during exhumation. Relics from which the metamorphic evolution of the rocks during prograde HP metamorphism and the UHP stage can be retrieved are restricted to rigid low-diffusion minerals like garnet, dolomite, tourmaline and apatite.     

桐柏高压变质地体:对桐柏-大别-苏鲁高压/超高压变质带构造框架和俯冲/折返机制的制约

桐柏造山带位于秦岭造山带和大别-苏鲁造山带之间,是揭示秦岭-桐柏-大别-苏鲁巨型造山带中各地质体之间构造关系及地质演化差异的关键地区。桐柏高压变质地体主要由两个高压岩片(I和II)及其北侧的构造混杂岩带和南侧的蓝片岩-绿片岩带构成。高压岩片I以北、南两条榴辉岩带为代表,构成桐柏山背形构造的两翼,其峰期变质条件分别为530~610℃、1.7~2.0GPa和460~560℃、1.3~1.9GPa。高压岩片II以桐柏杂岩中的变质岩包体为代表,其峰期变质条件推测在<700℃、>1.2GPa的榴辉岩相范围内,而退变质条件为660~700℃、0.80~1.03GPa。U-Pb、Lu-Hf、Rb-Sr和Ar-Ar同位素年代学研究表明,高压岩片I的峰期变质时代为255Ma,冷却至白云母封闭温度的时代为238Ma;而高压岩片II的主期变质作用发生在232~220Ma,作为桐柏杂岩主体的片麻状花岗岩则侵位于140Ma。这说明,高压岩片I和II分属于两个时代不同的俯冲/折返岩片,当高压岩片II被俯冲到地壳深处并经受高压变质时,其上覆的高压岩片I已经折返到中上地壳的水平。这一结果验证了在西大别、东大别和苏鲁地区提出的高压/超高压岩石的穿时(或差异)俯冲/折返模型,同时说明华南大陆地壳最早的俯冲发生在晚二叠世,这也代表华北与华南陆块之间从洋壳俯冲转化为陆壳俯冲的时间。基于桐柏杂岩与北大别杂岩的可比性,认为桐柏高压变质地体相对低温低压的变质环境以及超高压岩石的缺乏缘于华南陆块的俯冲深度向西逐渐变浅,而早白垩世的构造挤出造成了桐柏-大别高压/超高压变质带东宽西窄的构造格局。     

中国主要高压-超高压变质带的大地构造背景及俯冲/折返机制的探讨

造山带中发现超高压矿物柯石英和金刚石,被认为与洋壳或陆壳岩片的深俯冲(>100km)有关。但探讨这些岩片是如何俯冲和折返的？却是一个极具挑战的难题。目前,中国境内含榴辉岩的高压超高压（HP/UHP）变质带已经发现11条,此外,世界各地发现的高压超高压变质带还有至少20条。高压超高压变质带,特别是中国众多的HP-UHP变质带,在什么特定的大地构造条件中形成？又是在怎样的构造背景下折返而剥露地表？中国大陆上为什么出现众多规模可观的HP-UHP变质带？为什么出现洋壳（深）俯冲与陆壳（深）俯冲不同类型的HP-UHP带？这是本文试探讨的问题。根据中国境内的11条高压/超高压变质带形成时代和区域构造背景,将其分为4类：Ⅰ.始特提斯（早古生代）高压/超高压变质带,包括（1）柴北缘-南阿尔金超高压变质带,（2）北祁连-北阿尔金高压变质带,（3）东秦岭超高压变质带;Ⅱ.古特提斯高压/超高压变质带,包括（4）大别高压/超高压变质带,（5）苏鲁高压/超高压变质带,（6）西藏羌塘高压变质带;（7）西藏松多（超）高压变质带;Ⅲ.新特提斯高压/超高压变质带,包括（8）雅鲁藏布江东构造结南迦巴瓦（超）高压变质带;Ⅳ.古亚洲域南缘高压/超高压变质带,包括（9）新疆西南天山超高压变质带,（10）甘肃北山高压变质带,和（11）冀北高压变质带。中国高压/超高压变质带形成的大地构造背景有洋壳（深）俯冲和陆壳（深）俯冲两大成因类型,认为前者大都与始-古特提斯洋盆中微陆块之间的汇聚碰撞有关;后者为大陆块之间剪式碰撞和撕裂式岩石圈舌形板片深俯冲的产物。由于中国（邻区）大陆是三大陆块与许多小陆块聚集构成的巨大拼合体,小陆块在特提斯洋盆（特别是始、古特提斯洋盆）中的独特位置,使陆块之间的刚性洋盆岩石圈得以（深）俯冲插入小陆块之下。而大陆块之间特殊部位的碰撞为陆壳（深）俯冲创造条件。研究表明,高压/超高压变质岩石和蛇绿岩、混杂堆积、俯冲增生楔一起构成俯冲/折返杂岩带;认为代表印支造山带山根物质的大别-苏鲁高压/超高压俯冲/折返杂岩带,呈面形推覆岩片的构造样式叠置在扬子陆块之上,提出汇聚陆块边缘深部地幔物质折返的“斜向挤出”和“沿岩石圈板片的多层隧道的多重/分片挤出”的两种模式;认为走滑断裂在高压/超高压变质岩石的快速折返中起重要作用,即阿尔金走滑断裂、郯庐走滑断裂和喀喇昆仑走滑断裂,分别制约了阿尔金和祁连山中的南北两条早古生代高压/超高压变质带、大别-苏鲁印支期超高压变质带和喜马拉雅西构造结的喜山期超高压变质带的快速折返。     

苏鲁高压-超高压变质带折返抬升过程中构造界面和应力场的变化

作为地质历史时期深俯冲作用产物的超高压变质岩,尽管有不同的形成演化历史,但其最终的折返作用无不是在逆冲扩展过程中完成的。因此,逆冲扩展作用及其数值模拟的研究对完整认识苏鲁高压-超高压变质带的折返机制和折返过程具有重要意义。逆冲扩展作用最显观的构造效应是不同层次构造界面和应力场的变化,对于地壳尺度的逆冲扩展作用而言,陆表面和Moho是最重要的活动性构造界面,陆表面的变化导致山体抬升和滞后伸展盆地的形成,出现盆-山相间的构造格局。初步的模拟计算表明,山体的抬升量、滞后伸展盆地的坳陷量和Moho的上拱量与逆冲地块的平移速度和逆冲扩展速度成正相关关系,而且随着时间的推移,山体的抬升速度、滞后伸展盆地的坳陷速度和Moho的上拱速度都有逐渐增大趋势。逆冲扩展过程中,构造应力场的变化总的表现为随着平移速度和逆冲扩展速度的增大和逆冲扩展作用的持续进行,逆冲块体内部由挤压应力状态逐渐向拉张应力状态转化。地块的平移速度是构造强度的一个重要标量,当高压-超高压变质带以仰冲块体为运动载体,沿断裂带向陆壳浅部折返时,构造界面的移动规律基本反映了高压-超高压变质带的折返过程,可见,构造作用的强度和性质应该是制约高压-超高压变质带折返的重要因素之一,在岩石密度差相同的条件下,拉张构造应力场更有利于折返作用的进行。苏鲁高压-超高压变质带的折返是通过多期构造作用完成的,根据数值模拟结果可以推测,每期构造作用都伴随有折返速度由慢到快的变化,在整个折返过程中,构造运动性质和强度的差异导致了折返速度的不均一,总体上,折返速度将随着逆冲地块由挤压向拉张状态的转化和拉张强度（构造作用强度）的不断增强而逐渐增大,最终在以拉张为主导的构造应力场中完成了高压-超高压变质带折返的全过程。     

苏鲁—大别超高压变质带地壳速度结构及其俯冲、折返机制

三维P波速度解析研究结果表明，苏鲁-大别超高压变质带岩石圈地壳速度结构均具有上地壳明显高速且上凸、中地壳增厚、下地壳埋藏较深且莫霍面下凹等基本特征。与大别地区相比较，苏鲁超高压变质带存在着上地壳波速更高，且地表高速区面积与上地壳高速体体积大于大别；而莫霍面下凹程度不如大别地区，地壳山根已逐步趋向消失等独特的区域特征。显示了苏鲁地区曾发生过更激烈的俯冲与折返构造运动，与大别地区相比，有更多高速、高密度的超高压变质岩折返到上地壳与地表；然而在造山运动过程中比大别更早进入了造山运动后期等特征。对比研究结果表明，苏鲁-大别地段的造山、演化过程中，在构造运动基本相似的背景下，存在着区域性特征。苏鲁地区的造山运动以及超高压变质作用，有起始略晚、发生时期较短但相对激烈、结束早、比大别更早进入了造山运动的后期等特征。笔者分析了苏鲁区域性特征形成的主要构造原因是，郯庐断裂带的大规模左旋走滑运动以及通过中国华北区域的大范围NXV—SE向扩张应力场的影响。其中，中生代以来大华北地区的大区域扩张应力场的影响可能是该区俯冲到地幔内的超高压变质岩能够大量折返回地表或上地壳的重要构造原因。     

Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

苏鲁-大别造山带岩石圈应力场、构造运动特征以及超高压变质带折返机制的研究

苏鲁-大别造山带是华北、华南地块之间的大地构造交界带.本文分析研究了我国东部地区的3000余年的地震活动性,并根据1918～2006年问苏鲁-大别及其周围地区发生的1000余个地震的震源机制解,系统研究了地壳应力场和构造运动的区域特征,探讨了超高压变质带的折返机制.结果表明,苏鲁带以及华北地区受到太平洋板块向欧亚板块俯冲挤压的同时,受到从贝加尔湖经过大华北直到琉球海沟的广阔地域里存在着的统一的、方位为170°的引张应力场的控制.右旋走滑运动是苏鲁地区和郯庐断裂带的现代构造运动的主要特征.地震发生类型多为右旋走滑型或右旋走滑正断层型地震.华南地区构造应力场主要表现为,受北西西向运动的菲律宾板块向欧亚板块碰撞挤压运动所产生的方位为110～120°挤压应力的控制.华北、华南地块之间现代地壳应力场的分界线,西部与秦岭带大致相符,在大别东开始则逐渐偏离大地构造,到黄山附近向东南偏转,在温州附近转为向东延伸,最终穿过东海直至琉球海沟.研究结果还表明,苏鲁-大别超高压变质带的折返运动机制,即致使大量超高压变质岩折返到上地壳或地表的岩石圈应力场背景原因为,中生代以来大华北地区存在着较强扩张应力场的主控作用.此外,岩石圈地幔的蘑菇云构造增强了华北地区扩张应力场及扩张构造运动,导致形成大量深裂谷、裂隙,深部物质上涌.苏鲁及东大别地带处于或接近蘑菇云构造运动发育地区的中心,深部地幔物质的上涌导致并加剧了超高压变质岩折返到上地壳或地表,形成了世界著名的苏鲁-别超高压变质带.     

Petrological and geochronological constraints on the formation, subduction and exhumation of the continental crust in the southern Sulu orogen, eastern-central China

Detailed petrological, geochemical and geochronological studies were carried out for the core samples from the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) with a final depth of 5158 m. This borehole has penetrated into an ultrahigh-pressure (UHP) metamorphic slice consisting mainly of eclogites, gneisses, garnet-pyroxenites and garnet-peridotites. Geochemical characteristics indicate that their protoliths are igneous rocks, and occur in a continental rifting tectonic setting. Quartz-, rutile- and ilmenite-rich eclogites from 0 to 710 m occur as alternating layers; the eclogites, together with interlayers of peridotites and gneisses form a layered ultramafic-mafic-acidic intrusion, which was formed by extensive fractional crystallization of basaltic magma in continental environments. The granitic gneisses from 1190 to 1505 m and 3460 to 5118 m show affinity to within-plate granite, whereas the granitic gneisses from 710 to 1190 m and 1505 to 3460 m exhibit characteristics of volcanic-arc granite. Zircon U-Pb dating demonstrates that the magmatic zircon cores, which have relatively high Th/U ratios (mostly > 0.4), from both eclogites and gneisses, yield the same age at c. 788.8 Ma, suggesting that the protoliths of UHP rocks were formed by bimodal magmatism in Neoproterozoic rifting tectonic zones along the northern margin of the Yangtze Plate, in response to the breakup of the supercontinent Rodinia. U-Pb dating of metamorphic zircons with coesite and other eclogite-facies mineral inclusions and with relatively low Th/U ratios (mostly < 0.14) gives similar Triassic ages, which define two main zircon-forming events at 221.1 Ma and 216.7 Ma. We suggest that the older weighted mean age represents the peak-UHP metamorphic event at a pressure of 5.0 GPa (corresponds to ∼ 165 km depth), whereas the younger mean age reflects the UHP/HP retrograde event at a pressure 2.8 GPa (∼ 92 km depth). Therefore, a maximum rate of vertical movement during early exhumation of the UHP rocks from the Sulu orogen would be 17 mm/year, which is quite similar to initial exhumation rates (16 to 35 mm/year) of many UHP terranes in the world.     

柴北缘锡铁山两类榴辉岩的退变质过程及其对俯冲带折返机制的制约

榴辉岩作为俯冲带中重要的岩石类型保存有丰富的地球动力学信息。对榴辉岩及其退变质岩石的研究有助于建立俯冲带演化的p-T轨迹,了解俯冲岩石在折返过程中温压条件及矿物相的变化,从而对俯冲带折返的动力学机制进行限定。对柴北缘锡铁山双矿物榴辉岩及含多硅白云母榴辉岩进行了详细的岩石学研究。在NC（K）FMASH体系中对两类榴辉岩进行变质相平衡模拟,得到双矿物榴辉岩的峰期温压条件为745～790℃,大于2．8～3．0GPa（M1）,后经历等温降压过程达到角闪石榴辉岩岩相（670～770℃,1．6～2．2GPa,M2）,与含多硅白云母榴辉岩经历了相同的折返过程。锡铁山双矿物榴辉岩的原岩具有N-MORB的地球化学特征,而含多硅白云母榴辉岩则显示E-MORB或者OIB特征,二者原岩成分存在明显差异。两类榴辉岩的p-T演化过程和地球化学特征表明．锡铁山双矿物榴辉岩与含多硅白云母榴辉岩矿物学特征的差异是其原岩的多源性造成的,而与俯冲后折返过程中的退变质作用无必然联系。     

柴北缘锡铁山两类榴辉岩的退变质过程 及其对俯冲带折返机制的制约

榴辉岩作为俯冲带中重要的岩石类型保存有丰富的地球动力学信息。对榴辉岩及其退变质岩石的研究有助于建立俯冲带演化的p-T轨迹,了解俯冲岩石在折返过程中温压条件及矿物相的变化,从而对俯冲带折返的动力学机制进行限定。对柴北缘锡铁山双矿物榴辉岩及含多硅白云母榴辉岩进行了详细的岩石学研究。在NC（K）FMASH体系中对两类榴辉岩进行变质相平衡模拟,得到双矿物榴辉岩的峰期温压条件为745~790℃,大于2.8~3.0GPa（M1）,后经历等温降压过程达到角闪石榴辉岩岩相（670~770℃,1.6~2.2GPa,M2）,与含多硅白云母榴辉岩经历了相同的折返过程。锡铁山双矿物榴辉岩的原岩具有N-MORB的地球化学特征,而含多硅白云母榴辉岩则显示E-MORB或者OIB特征,二者原岩成分存在明显差异。两类榴辉岩的p-T演化过程和地球化学特征表明,锡铁山双矿物榴辉岩与含多硅白云母榴辉岩矿物学特征的差异是其原岩的多源性造成的,而与俯冲后折返过程中的退变质作用无必然联系。     

北大别的高温超高压变质作用与多阶段折返

近期的变质岩石学、地球化学及同位素年代学研究表明,北大别整体经历了高温超高压变质作用和多阶段折返历史,因而表现为广泛发育的多期减压结构和极少保留早期的超高压变质记录。北大别榴辉岩以高温变质作用以及折返期间因麻粒岩相和角闪岩相退变质变质作用而形成的多期后成合晶为显著特征。石榴子石中伴有放射状胀裂纹的单晶和多晶石英包体指示早期柯石英的转变结果,这已被锆石中发现的柯石英残晶所证实。结合北大别北部榴辉岩和片麻岩中发现的金刚石等超高压证据以及三叠纪变质记录,由此证明北大别整体经历过深俯冲和印支期超高压变质作用。北大别榴辉岩的多阶段高温条件主要来自石榴子石-绿辉石矿物对温度计、单斜辉石中紫苏辉石+石英针状矿物出熔体以及金红石中较高的Zr含量和变质锆石中较高的Ti含量等得出的温度证据。此外,多期后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个多阶段、快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩经历了长时间的高温变质演化与缓慢冷却过程。长时间的高温变质作用与缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压记录的重要原因。因此,这些成果为大别山三个不同超高压岩片的差异折返模型的建立提供了新的证据。     

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835-1280 m), DFb (2010-2280 m) and DFc (2920-3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. ⁴⁰Ar/³⁹Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223-202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m}<a> and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N-S-striking foliation and a subhorizontal N-S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001)<a> is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.     

Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from Ar/Ar geochronology

New ⁴⁰Ar/³⁹Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.     

显生宙碰撞造山带超高温变质作用的加热机制：来自二维数值模拟的约束

大量的岩石学证据表明: 碰撞造山带中常发育900~1100℃的超高温变质作用。然而, 碰撞造山带中如何出现如此极端的超高温条件仍然存在争议。为了更好地理解超高温变质作用加热机制和碰撞造山带中主要热源的相对贡献, 我们建立了一系列高分辨率二维热-动力学模型, 借此探讨了俯冲大陆岩石圈密度亏损程度、大陆地壳放射性生热率和大陆汇聚速率等因素对碰撞造山过程中超高温变质主要热源的影响。当大陆岩石圈密度亏损(Δρ=ρ软流圈地幔-ρ岩石圈地幔)大于50kg/m3时, 有利于发生大陆平板俯冲, 软流圈地幔无法上涌为地壳物质提供热源; 此时, 具有较高放射性生热率(>3μW/m3)的地壳可以发生"浅俯冲-折返"型超高温变质作用。而当大陆岩石圈密度亏损小于10kg/m3时, 大陆上地壳在深俯冲阶段首先发生超高压榴辉岩相变质作用, 随后伴随着大陆岩石圈地幔后撤和软流圈上涌, 进而出现以异常高的地幔热流加热为主的"深俯冲-折返"型超高温变质作用。此外, 较低的大陆汇聚速率(< 1cm/yr)更有利于"深俯冲-折返"型超高温变质作用的产生。将数值模拟结果与特提斯构造域的变质岩石数据和地球物理观测进行对比, 我们认为在现今板块构造体制下, 由具有密度亏损程度较高的大陆岩石圈平俯冲有利于"浅俯冲-折返"型超高温变质作用的发生, 而由密度亏损程度较低的大陆岩石圈俯冲可能导致"深俯冲-折返"型超高温变质作用的产生。     

喜马拉雅东构造结——南迦巴瓦构造及组构运动学

喜马拉雅东端-南迦巴瓦构造结的构造格架总体呈现由叠置构造岩片构成的复式背形构造.自NW到SE由比鲁构造岩片、直白构造岩片、南派乡构造岩片和多雄拉变质穹隆组成,它们之间的界限分别是直白-丹娘-南伊沟韧性拆离断裂、直白-丹娘韧性逆冲断裂和多雄拉韧性逆冲断裂.由高压麻粒岩相组成的直白构造岩片被直白-丹娘-南伊沟韧性拆离断裂和直白-丹娘韧性逆冲断裂所夹持,为挤出构造岩片.根据印度斯-雅鲁藏布江大拐弯缝合带西侧和北侧的变形特征及石英组构运动学的EBSD测量结果,表明大拐弯缝合带存在各段的差异,并具有逐渐演化的特征.大拐弯缝合带的北端为拉月-迫隆乡韧性逆冲剪切带;西段为鲁朗-拉月左行走滑剪切带,西南段为嘎马-米林左行伸展转换剪切带,指示南迦巴瓦变质体相对拉萨地体的运动转为水平走滑运动.根据大拐弯缝合带东侧右行走滑和西侧左行走滑特征,推测在印度-亚洲碰撞之后,南迦巴瓦变质体受制于这两条走滑断裂,而相对喜马拉雅地体向北推移,并深深插入拉萨地体之下,形成东构造结.由于南迦巴瓦变质体的强烈上隆,其上部原存的特提斯喜马拉雅的古生代-中生代盖层沉积被俯冲和被剥蚀贻尽.南迦巴瓦变质体中直白组高压麻粒岩相中石榴石辉石岩形成的温压条件(T=800～900℃,P=2.6～2.8GPa)表明,岩石经历了相当于80km～100km深度的峰期榴辉岩变质作用的条件,印度板片深俯冲于拉萨地体之下又折返挤出到由派乡组和多雄拉组角闪岩相(混合岩化)组成的南迦巴瓦变质基底之中.     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.     

U–Pb zircon geochronology of coesite‐bearing eclogites from the southern Dulan area of the North Qaidam UHP terrane,northwestern China: spatially and temporally extensive UHP metamorphism during continental subduction

Coesite‐bearing eclogites from >100 km² in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean ²⁰⁶Pb/²³⁸U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus, c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic.     

Differential Evolution of High-Pressure and Ultrahigh-Pressure Metapelites from Habutengsu,Chinese Western Tianshan: Phase Equilibria Modelling and 40Ar/39Ar Geochronology

Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure(HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40Ar/39 Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O(Mn NCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70(a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite(Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40Ar/39 Ar age spectra of white mica(dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.