云南高黎贡山典型斜长角闪岩地震波速、各向异性及其晶格优选定向的研究

斜长角闪岩和角闪岩相变质岩石是大陆中—下地、岛弧深部地壳以及俯冲大洋地壳中最重要的组成岩石之一,查明目前地壳中斜长角闪岩的体积含量、空间分布及其应变状态对于深入研究大陆地壳的形成与演化过程极其重要.笔者等实验测量了云南高黎贡韧性剪切带典型变形斜长角闪岩7个关键性方向上的地震(P和S波)波速随静水围压(0～600 MPa)的变化规律,并利用电子背散射衍射(EBSD)技术测定了主要造岩矿物角闪石、斜长石和石英的品格优选方位,查清斜长角闪岩中地震波速各向异性与剪切波分裂的成因,确定斜长角闪岩中有限应变椭球与波速椭球的对应关系,为今后利用原地地震波速的各向异性调查地壳深部区域构造应变场提供必要的实验和理论基础.     

榴辉岩组构运动学与大陆深俯冲——中国大陆科学钻探主孔榴辉岩的EBSD研究

中国大陆科学钻探主孔位于苏鲁超高压带南部的东海县毛北榴辉岩体之上。主孔0-600米深度的榴辉岩的塑性变形以具中等倾角的东倾面理,近南北向的水平拉伸线理、“A”型剪切褶皱和一系列平行面理的微型韧性剪切带为特征。使用电子背散射（EBSD）技术测量的主孔7个榴辉岩样品的石榴石和绿辉石的晶格优选定向（LPO）表明：石榴石基本上无序排列,而绿辉石表现出强烈的LPO。绿辉石的[001]轴近平行于拉伸线理方向,（010）面的法线和[100]轴垂直面理分布,{110}的法线形成垂直面理的环带,反映绿辉石的位错蠕变由[001]（100）和1／2〈^-110〉滑移系控制,其不对称的LPO指示了由北向南的剪切指向。根据单斜辉石的高温实验结果,毛北榴辉岩经历了800-900℃的超高压变质作用。通过构造重塑,揭示毛北榴辉岩体为剪切流变褶皱,形成于扬子板块深俯冲时的超高压变质过程。因此榴辉岩中保留的早期岩石组构特征可以为板块的深俯冲运动学和俯冲极性提供重要信息。     

大洋板块俯冲带地震波各向异性及剪切波分裂的成因机制

大洋板块俯冲带是许多重要地质作用(例如脱水、部分熔融、岩浆和地震活动)发生的场所.对位于俯冲带之上的地震台站所检测到的不同剪切波的数据解析,可以获得源于上覆板块、地幔楔、俯冲板块和板下地幔的地震波各向异性的关键信息.本文系统总结了世界各地大洋俯冲带的剪切波分裂样式,对目前国际上流行的大洋俯冲带的地震波各向异性的主要成因...     

红河-哀牢山剪切带角闪岩中角闪石变形特征及地震波各向异性

角闪岩作为中下地壳的重要物质组成,其岩石和矿物的变形行为及力学强度表现直接制约着中下地壳力学属性与状态,因此开展对其中重要组成矿物角闪石的变形行为和地震波各向异性研究,具有重要地质意义.以红河-哀牢山剪切带中出露的变形角闪岩中角闪石为研究对象,其中显微构造分析表明,变形角闪岩分别呈现出粗、中粒条带状糜棱岩和细粒条带状超糜棱岩.分别对这3种变形岩石中的角闪石矿物颗粒进行了EBSD晶格优选定向分析和地震波各向异性计算,结果表明3种变形角闪岩中的角闪石呈现出不同取向及典型晶质塑性变形特征,(100)[001]主要滑移系发育,同时发育不同程度的(010)[001]和(110)[001]次级滑移系.我们认为在剪切变形过程中,角闪石双晶滑移和解理面滑移共同作用致使角闪石细粒化.从粗粒到细粒条带状角闪石,随着角闪石颗粒粒度减小,角闪石中AV_p也有逐渐变小的趋势,表明角闪石变形行为、形态优选定向及晶格优选定向共同影响着地震波各向异性.     

地震波各向异性：窥测地球深部构造的“探针”

地震波各向异性日益成为不可忽视的地质地球物理现象。地球内部不同圈层（地壳、地幔和地核）都存在着地震波各向异性，并表现为不同的规模（小到单矿物和岩石，大到地体甚至上地幔）和强度。通过地震波各向异性可以间接获取岩石圈厚度、地球深部结构与构造变形、地球动力学和地幔对流等信息。主要从地震波各向异性的表现形式、原因及地质地球物理意义等方面对近年来大洋俯冲带、大陆裂谷、地幔转换带和大陆碰撞造山带（青藏高原）等构造环境中的研究成果进行了评述，讨论了各向异性[JP2]研究中需要重视的几个问题：①剪切波分辨率；②矿物组构研究；③其它各向异性成因机制。还强调了各向异性研究与流变学、高温高压岩石物理实验相结合的新方向。     

大陆俯冲带热结构的数值模拟

俯冲带热结构是控制俯冲板块演化的最主要因素之一.前人通过建立解析模型和数值模型对大洋俯冲带热结构进行了一系列研究,发现俯冲板块年龄和俯冲速度是影响俯冲带热结构的关键因素.为了认识大陆俯冲带热结构,特别是理解数值模型结果与岩石学结果之间的差异,我们建立了二维大陆俯冲带运动学和动力学数值模型研究其热结构演化.模型结果显示,如果大陆俯冲板块的俯冲速度与角度和大洋板块一致的话,较低的大陆俯冲带初始温度导致其板块温度比大洋俯冲带低.但是,当大陆俯冲板块的初始温度较高,俯冲速度超慢并且考虑大陆地壳中的放射性元素生热时,模型得到的大陆俯冲带热结构能够解释通过高压和超高压变质岩得到的较热的俯冲板块温度.另一方面,如果俯冲板块与上覆板块存在动力学解耦作用,也能够得到较热的俯冲温压数据.      

岩石地震波性质的实验与理论研究

岩石的地震波性质是解释地震观测资料的基础。本文首先以中国大陆超深钻主孔的榴辉岩为例,将榴辉岩在高温高压(最大为500MPa和700℃)下观测矿物组成、晶格优选定向(LPO)、温度和压力对其地震波性质的影响。然后对岩石地震波性质数据库进行统计分析,指出长英质岩石的波速与石英或长石的含量没有相关性;从酸性岩、基性岩到超基性岩,随着SiO2含量的降低和铁镁矿物含量的增高,岩石波速逐渐增高,云母和角闪石的LPO是陆壳地震波各项异性的主要来源;波速与孔隙率的负相关受流体与压力的影响。     

松辽盆地南部岩石圈地幔变形特征及各向异性分析——来自橄榄岩包体的约束

SKS测量结果显示松辽盆地快波方向分布较为复杂, 由于缺乏深部岩石变形资料约束, 制约了地震波各向异性成因解释。本文通过对松辽盆地南部双辽地区橄榄岩包体进行详细的岩石学、地球化学、显微结构、各向异性研究, 结果显示双辽地区橄榄岩包体的平衡温度为893~1152℃, 来源于岩石圈地幔。橄榄岩中橄榄石的晶格优选方位(CPO)类型主要为A型、D型和AG型, 其中, AG型和D型CPO可能形成于西太平洋板块向欧亚大陆俯冲回撤导致的岩石圈变形, AG型橄榄石CPO也可能形成于熔体存在下的橄榄石变形。基于CPO计算获得的橄榄岩包体全岩V_P各向异性(AV_P)为4.79%~11.80%, 最大剪切波各向异性(AV_Smax)为3.13%~7.93%。结合地球物理测量结果, 推断松辽盆地南部复杂的SKS各向异性的主要贡献可能来源于面理陡倾或直立的岩石圈地幔。

     

深俯冲和折返动力学:来自中国大陆科学钻探主孔及苏鲁超高压变质带的制约

中国大陆科学钻探工程和苏鲁高压-超高压变质带为大陆岩石圈的深俯冲与折返动力学的研究提供了以下制约(1)苏鲁高压/超高压变质地体迭置于南、北苏鲁两个不同时代及属性的基底之上;(2)苏鲁巨量表壳岩石深俯冲至200km以下的上地幔深度,并经历超高压变质作用;(3)根据不同类型超高压变质岩石锆石的SHRIMP-U/Pb原位精确定年,获得超高压变质岩石的深俯冲-折返全过程(240～252Ma→230～237Ma→207～218Ma)时限.并建立了新的深俯冲-折返全过程的P-T-t轨迹;(4)富钛铁的辉长岩在大陆地壳的深俯冲过程中,经历了超高压变质作用并转变成了富含金红石的榴辉岩,形成了超高压变质的钛矿床;(5)通过榴辉岩和石榴石橄榄岩的显微构造分析及石榴石、绿辉石和橄榄石EBSD测量,确定深俯冲过程中绿辉石和橄榄石的组构运动学和流变学特征;(6)在大陆的深俯冲过程中,强烈水化的陆壳岩石经历了进变质脱水过程,巨量的地表水带入到＞100～200Km的地幔深处,在超高压变质峰期的极端条件下,通过含水超高压变质矿物的分解形成超临界的含水熔体,导致有效的壳-幔物质交换和岩石圈物质分异;(7)苏鲁超高压变质地体在折返阶段形成挤出纳布构造,与岩石圈深俯冲管道流的折返挤出机制有关;(8)提出新的深俯冲-折返动力学模式陆.陆碰撞的深俯冲剥蚀模式及大陆地壳多重性、分层型和穿时性的俯冲和折返模式.     

岫玉的地震波速、各向异性、弹性力学性质及其构造地质意义

为了研究水化岩石圈地幔和地幔楔的地震波速、各向异性及其与蛇纹石化程度及应变状态的关系,作者实验测量了8块叶蛇纹岩(采自辽宁的岫玉)标本在不同构造主方向(X,Y和Z)上的纵、横波速度随围压(0~600 MPa)的变化规律,详细研究了叶蛇纹石在塑性变形过程中通过(001)[010]位错滑移形成很强的晶格优选定向和高达21%的纵波速度各向异性与24%的剪切波分裂的特征,发现高温叶蛇纹岩与低温利蛇纹岩具有截然不同的地震波性质,例如,在围压600 MPa下,高温叶蛇纹岩的Vp=6.73 km/s、Vs=3.74 km/s,Vp/Vs=1.80,而低温利蛇纹岩的Vp=5.10 km/s、Vs=2.32 km/s,Vp/Vs=2.20。前人利用低温蛇纹石化橄榄岩的波速数据解释温度高于300℃的水化地幔楔和岩石圈地幔的地震波速不可避免地要低估研究区域内蛇纹石化的程度和水含量。本研究查明叶蛇纹岩的地震波性质及其各向异性特征亦为解释全球大洋板块俯冲带乃至青藏高原地区的剪切波分裂资料提供了新的思路。     

Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch,South Africa and implications for seismic anisotropy

Seismic anisotropy in the upper mantle provides important constraints on mantle dynamics, continental evolution and global tectonics and is believed to be produced by the flow-induced lattice-preferred orientation (LPO) of olivine. Recent experimental studies at high pressure and temperature have suggested that the LPO of olivine is affected by pressure in addition to water and stress. However, there has been no report yet for the pressure-induced LPO of natural olivine because samples from the deep upper mantle are rare and often unsuitable for study due to ambiguous foliation and lineation. Here we show evidence of the pressure-induced LPO of natural olivine in diamond-bearing garnet peridotites from Finsch, South Africa. We found that the [010] axes of olivine are aligned subnormal to foliation and that the [001] axes are aligned subparallel to lineation, which is known as B-type LPO of olivine. The equilibrium pressure of the samples, as estimated using geobarometer, was greater than 4 GPa, indicating that the samples originated from a depth greater than ∼120 km. In addition, FTIR spectroscopy of the olivine showed that the samples are dry, with a water content of less than 90 ± 20 ppm H/Si (5.5 ± 1.2 ppm wt. H₂O). These data suggest that the samples are the first natural examples of olivine displaying B-type LPOs produced due to high pressure under dry condition. Our data indicate that the trench-parallel seismic anisotropy observed in many subduction zones in and below subducting slabs at depths greater than ∼90 km under dry condition may be attributed to the pressure-induced olivine fabrics (B-type LPO) and may be interpreted as the entrainment of the sub-lithospheric mantle in the direction of subduction rather than anomalous trench-parallel flow.     

Effect of water and stress on the lattice-preferred orientation of olivine

The influence of water and stress on the lattice-preferred orientation (LPO) of olivine aggregates was investigated through large strain, shear deformation experiments at high pressures and temperatures (P = 0.5–2.1 GPa, T = 1470–1570 K) under both water-poor and water-rich conditions. The specimens are hot-pressed synthetic olivine aggregates or single crystals of olivine. Water was supplied to the sample by decomposition of a mixture of talc and brucite. Deformation experiments were conducted up to γ (shear strain)  6 using the Griggs apparatus where water fugacity was up to  13 GPa at the pressure of 2 GPa. The water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at P = 0.5–2 GPa was  400–1200 ppm H/Si. Several new types of LPO in olivine are found depending on water content and stress. Samples deformed in water-poor conditions show a conventional LPO of olivine where the olivine [100] axis is subparallel to the shear direction, the (010) plane subparallel to the shear plane (type-A). However, we identified three new types (type-B, C, and E) of LPO of olivine depending on the water content and stress. The type-B LPO of olivine which was found at relatively high stress and/or under moderate to high water content conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (010) plane subparallel to the shear plane. The type-C LPO which was found at low stress and under water-rich conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (100) plane subparallel to the shear plane. The type-E LPO which was found under low stress and moderate water content is characterized by the olivine [100] axis subparallel to the shear direction, the (001) plane subparallel to the shear plane. Observations by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) show that the dislocations in water-poor samples (type-A) are curved and both b =  [100] and b = [001] dislocations have a similar population. Numerous subgrains are seen in water-poor samples in backscattered electron images. In contrast, water-rich samples (both type-B and type-C) contain mostly b = [001] dislocations and dislocations are straight and sub-grain boundaries are rare compared to those in water-poor samples. These observations suggest that (1) dominant slip systems in olivine change with water fugacity (and stress) and (2) grain boundary migration is enhanced in the presence of water. Seismic anisotropy corresponding to the fabrics under water-rich condition is significantly different from that under water-poor condition. Consequently, the relationship between seismic anisotropy and flow geometry in water-rich regions is expected to be different from that in water-poor regions in which type-A fabric dominates (i.e., the lithosphere). A few cases are discussed including anisotropy in the subduction zone and in the deep upper mantle.     

Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab

Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8% in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/10⁶ Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.     

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.     

Anomalous elasticity of talc at high pressures: Implications for subduction systems

Talc is a layered hydrous silicate mineral that plays a vital role in transporting water into Earth’s interior and is crucial for explaining geophysical observations in subduction zone settings. In this study, we explored the structure, equation of state, and elasticity of both triclinic and monoclinic talc under high pressures up to 18 GPa using first principles simulations based on density functional theory corrected for dispersive forces. Our results indicate that principal components of the full elastic constant tensor C₁₁ and C₂₂, shear components C₆₆, and several off-diagonal components show anomalous pressure dependence. This non-monotonic pressure dependence of elastic constant components is likely related to the structural changes and is often manifested in a polytypic transition from a low-pressure polytype talc-I to a high-pressure polytype talc-II. The polytypic transition of talc occurs at pressures within its thermodynamic stability. However, the bulk and shear elastic moduli show no anomalous softening. Our study also shows that talc has low velocity, extremely high anisotropy, and anomalously high V_P/V_S ratio, thus making it a potential candidate mineral phase that could readily explain unusually high V_P/V_S ratio and large shear wave splitting delays as observed from seismological studies in many subduction systems.     

深俯冲陆壳部分熔融初始熔体的厘定:来自苏鲁超高压地体混合岩中浅色体证据

深俯冲陆壳物质部分熔融产生的熔体,实验岩石学方面已有广泛报道,而天然初始熔体的组分却难以厘定。对此,本文从苏鲁超高压地体荣成混合岩中识别出了深俯冲花岗质陆壳部分熔融产生的天然初始熔体组成。野外露头显示,混合岩中主要矿物组成为钾长石+斜长石+石英的浅色熔体呈不连续的条带状与残余体互层产出,指示了原位或近源区的部分熔融特征。混合岩浅色体锆石CL图像呈明显的核-边结构,继承核部为扬子板块来源的岩浆锆石,形成时代为721±24Ma;新生边部CL图像具震荡环带结构,微量元素上REE呈明显左倾,具有Eu的负异常及Ce的正异常,低的Hf/Y和Th/U比值,具深熔锆石特征,指示形成于花岗质陆壳物质的部分熔融。边部U-Pb谐和年龄为225.9±2Ma,略晚于苏鲁超高压地体超高压峰期变质年龄,表明初始熔融发生在超高压地体折返早期。浅色熔体的全岩地球化学特征表明,主量元素上具有高SiO_2、K_2O及Na_2O含量,低的Fe_2O_3~T、MgO及CaO含量,A/CNK=1.02~1.04,呈弱过铝质亚碱性花岗岩的特征,这与实验岩石学中富硅陆壳物质部分熔融产生的熔体组分极为相近;微量元素上富集大离子亲石元素(如Rb、Ba、Pb等),亏损Nb、Ta、Ti等高场强元素,REE呈较为平坦的配分模式,具弱的Eu负异常并亏损Sr。本文通过上述对天然样品研究,厘定了深俯冲花岗质陆壳部分熔融及其初始熔体的组成,为理解大陆俯冲带壳幔相互作用提供了关键依据。     

从洋壳俯冲到陆壳俯冲和碰撞：来自羌塘中西部地区榴辉岩和蓝片岩地球化学的证据

羌塘中部晚三叠世低温/高压变质带是目前青藏高原内部延伸规模最大的高压变质带,但大量关键高压变质岩石出露地区地球化学资料匮乏,严重制约了对高压变质带原岩建造以及构造演化的全面认识。本文以羌塘中西部地区尚无地球化学资料的果干加年山榴辉岩和红脊山蓝片岩为研究对象,进行了系统的地球化学以及原岩恢复工作。研究表明,果干加年山榴辉岩呈透镜状产于围岩石榴石多硅白云母片岩和少量大理岩中,其原岩为亚碱性玄武岩,具有较低的稀土总量(∑REE=51.19×10-6~59.43×10-6)和轻稀土亏损的特征[(La/Yb)N=0.59~0.70],不具有Nb、Ta、Ti的亏损,与典型的N-MORB特征一致,暗示其原岩可能来源于亏损的地幔源区,形成于洋中脊环境。红脊山地区基性蓝片岩的原岩为碱性玄武岩-亚碱性玄武岩,具有高的TiO2(2.97%~4.14%)和P2O5(0.29%~0.48%)含量,富集轻稀土元素[(La/Yb)N=6.10~11.6]和高场强元素,地球化学特征类似于OIB。但是这些基性蓝片岩与大量的陆源碎屑岩伴生产出,且具有明显的硅铝质上地壳物质混染的特征,与南羌塘地区二叠纪大陆板内基性岩墙的产出特征以及地壳混染特征一致,可能是其俯冲消减的产物。通过本文研究结果并结合区域内已识别出的E-MORB型洋壳和洋岛/海山物质深俯冲的证据,我们认为羌塘中部晚三叠世高压变质带以洋壳物质深俯冲为主,同时亦保留了部分陆壳物质俯冲的证据,暗示大洋向北俯冲消减结束之后,又牵引至少一部分南羌塘北缘陆壳物质经历了随后的俯冲过程。     

大陆造山运动：从大洋俯冲到大陆俯冲、碰撞、折返的时限——以北祁连山、柴北缘为例

北祁连山和柴北缘是典型的早古生代大陆造山带,分别发育有北祁连山大洋型俯冲缝合带和柴北缘大陆型俯冲碰撞带.作为早古生代大洋冷俯冲的典型代表,北祁连山经历了从新元古代-寒武纪大洋扩张、奥陶纪俯冲和闭合及早泥盆世隆升造山的过程.高压变质岩变质年龄为490～440Ma,证明古祁连洋经历了至少50m．y．的俯冲过程.柴北缘超高压变质带是大陆深俯冲的结果,岩石学、地球化学和同位素年代学表明,柴北缘超高压变质带中榴辉岩的原岩分别来自洋壳和陆壳两种环境.高压/超高压变质的蛇绿岩原岩的年龄为517±11Ma,与祁连山蛇绿岩年龄一致.榴辉岩早期的变质年龄为443～473Ma,与祁连山高压变质年龄一致,代表大洋地壳俯冲的时代,而柯石英片麻岩和石榴橄榄岩所限定的超高压变质时代为420～426Ma,代表大陆俯冲的年龄.从大洋俯冲结束到大陆俯冲最大深度的转换时间最少需要20m．y．.自420Ma起,俯冲的大洋岩石圈与跟随俯冲的大陆岩石圈断离,大陆地壳开始折返,发生隆升和造山.北祁连山和柴北缘两个不同类型的高压-超高压变质带反映了早古生代从大洋俯冲到大陆俯冲、隆升折返的造山过程.     

The solubility of TiO2 in olivine: implications for the mantle wedge environment

One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite and/or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO₃ rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO₂ in olivine was >0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO₂ in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO₂ was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to >1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P=10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones.