西藏西南部拉昂错地幔橄榄岩的地球化学特征及其构造意义

拉昂错蛇绿岩位于西藏西南部雅鲁藏布缝合带(YZSZ)的西段,由地幔橄榄岩和侵入其中的基性岩墙组成.拉昂错地幔橄榄岩普遍发育碎斑结构及熔体注入和交代结构,尖晶石的Cr#值具有较广泛的变化(0.32～0.70),大多数样品富集LREE并伴随HFSE的明显增加,少数亏损LREE,前者部分熔融程度为15%～23%,后者为10%左右,这表明它们并不是地幔单阶段部分熔融的残余物,而是MORB型亏损橄榄岩在俯冲过程中再度部分熔融后熔体与残余地幔相互作用的产物,由于熔体不同程度的混合与交代,形成了各种再饱满程度不同的橄榄岩.对拉昂错地幔橄榄岩岩石学和地球化学特征的研究,为探讨YZSZ蛇绿岩带所代表的特提斯洋盆的形成和演化提供了新的证据.     

Constraining the characters of the upper mantle discontinuities beneath the NE margin of the Tibetan Plateau with a dense broadband seismic array

Based on observations from a dense broadband seismic array located along the northeastern (NE) margin of the Tibetan Plateau in southeastern Gansu Province,we use receiver functions (RFs) to pick the arrival times of P-to-S converted waves and bin the traces in different grids according to the piercing points of the 410 and 660 km discontinuities in the upper mantle.The depths of the two discontinuities are estimated by the ray tracing method with the IASP91 velocity model and a 3-D tomography model.The results indicate the following:(1) The arrival times of the P410s and P660s converted phases are delayed by approximately 1 s than those predicted by the IASP91 model.The mantle transition zone (MTZ) is thicker than that in the global model.(2) The synchronous lags in the P410s and P660s arrival times are consistent with low-velocity anomalies in the upper mantle,which are believed to result mainly from the eastward migration of materials beneath the NE margin of the Tibetan Plateau.(3) Combined with previous tomography results,the depression of the'660'discontinuity and the thickened MTZ are somewhat consistent with the big mantle wedge (BMW) model.However,due to data limitations,more studies are required to explore the BMW in the future.     

张宣幔枝构造的成矿作用研究

地幔热柱具有多级演化特征。河淮亚热柱顶部地幔物质以薄板状斜插至燕山造山带之下，燕山造山带轴部东西向尚义－赤城韧性剪切带与太行山构造岩浆带轴部北北东向乌龙沟－上黄旗韧性剪切带交汇叠加，使深部中下地壳应变软化岩石及深部幔源物质减压释荷产生深熔岩浆并上侵，从而加速了造山带的上隆速度，并可形成地幔热柱的第三级构造单元——幔枝构造。其中隆升较快的地段发育成为典型的变质核杂岩构造。幔枝构造控制着构造变形－岩浆活动－蚀变变质－成矿作用统一体系，并形成与幔枝构造密切相关的内带为金（铜钼）、外带为银（铅锌）多金属的成矿系列。论述了不同构造部位的成矿特征及其相关性、统一性。据此提出了在有利构造部位（参照已有矿点分布）扩大找矿和向深部扩大找矿的认识。     

Shear sense inversion in the Hilti mantle section (Oman ophiolite) and active mantle uprise

Structural analyses in the well-exposed Hilti mantle section in the Oman ophiolite suggest a model of forceful horizontal flow in the uppermost mantle at the edge of a diapir below a oceanic spreading center. Detailed structural mapping, focussed on high-T deformation (i.e., asthenospheric flow), revealed a gently undulated flat structure with a uniform east-west flow direction. When it is related to the N–S to NNW–SSE trending, vertical sheeted dike complex located to the east, this mantle flow is parallel to the spreading direction. Because the Moho is so flat lying, a large dunite occurrence at the south-western region is possibly ascribed to the Moho Transition Zone. Kinematic analysis shows that the shear direction generally changes from top-to-the west in the upper level, to top-to-the east in the lower level with respect to the Moho. This shear sense inversion is explained by a model of forceful flow due to an active mantle uprise and it is not compatible with a passive mantle uprise. In the plan section, the boundary of the shear sense inversion is subparallel to the flow direction and subperpendicular to the spreading axis. In cross section, the boundary appears to occur at various depths in the range of 200 m to 500 m. It shows that the active mantle uprise in the diapir center resulted in a channelled horizontal flow.     

长江中下游庐枞火山岩盆地南侧钾质侵入岩带的成因

庐枞火山岩盆地南侧的钾质侵入岩带由正长岩-石英正长岩-正长花岗岩组成,以石英正长岩为主。它们的形成时间介于123"130 Ma之间,峰值约为126 Ma,其中正长岩和石英正长岩的形成时间稍早,而正长花岗岩的形成时间略晚。整个钾质侵入岩带的侵位时间晚于庐枞盆地内的橄榄玄粗质火山作用约4"7 Ma,也是长江中下游地区除最东段的宁镇地区外中生代最晚的岩浆活动产物之一。地球化学上,该钾质侵入岩带以高钾、富碱、富集Rb、Th、U、K等强不相容元素和轻稀土元素、亏损高场强元素Nb、Ta和Ti为特征。它们的母岩浆主要是由富集型上地幔部分熔融形成的,从正长岩经石英正长岩到正长花岗岩的演化主要受矿物的分离结晶作用控制,地壳物质同化所起的影响不大。但与同样来自富集型上地幔部分熔融的庐枞盆地内火山杂岩的母岩浆相比,前者的母岩浆来源深度可能更大些或其中包含了更多来自软流圈地幔的组分。两者的演化路径也完全不同,钾质侵入岩带的母岩浆除经历过高压下的分离结晶作用外,晚期在低压下还经历过长石为主,可能还有黑云母的分离结晶,甚至上地壳物质一定程度的混染作用;而盆地内火山杂岩的母岩浆低压下矿物的分离结晶作用及上地壳物质的混染都不明显。庐枞盆地南缘的富钾侵入岩与盆地内的火山杂岩一样,地球化学上都具有明显的大陆弧的特征,暗示它们的岩浆源区可能形成于俯冲带环境,意味着扬子地块北缘先前(推测为古元古代晚期)曾发生过俯冲作用,上地幔的交代富集可能就与这次的俯冲作用有关。     

藏北羌塘南部冈玛错地区展金组玄武岩的成因及其构造意义

羌塘南部地区大面积出露玄武岩、基性岩墙等基性岩类,区域上它们沿龙木错—双湖—澜沧江板块缝合带南侧展布。已有较多的研究报道认为基性岩墙的时代为晚石炭世—早二叠世。玄武岩呈夹层状产出于展金组中,随着羌塘地区地质研究程度的不断提高,玄武岩的确切时代、构造属性等已成为探讨研究区构造演化亟待解决的问题。在野外观察的基础上,选择羌塘南部典型的玄武岩进行地球化学研究。玄武岩属于碱性系列岩石,富集Ti和Fe;轻稀土元素富集,轻、重稀土分异明显;在构造环境判别图解上,主要投在板内环境区域;总体表现出与地幔柱成因同类岩石相似的特点。对玄武岩的研究,为羌塘南部晚古生代地幔柱的存在提供了有力的证据。     

东南沿海地区古近纪大陆岩石圈地幔特征及成因

东南沿海地区新生代玄武岩中的橄榄岩包体来自岩石圈地幔 ,上地幔橄榄岩包体的岩石学及地球化学特征都记录了地幔演化的历史。普宁橄榄岩包体斜方辉石含量与太古宙克拉通地幔类似 ,但在矿物学、REE、痕量元素和Sr Nd同位素上又与太古宙岩石圈地幔不同。橄榄岩包体的岩相学、矿物学、REE、痕量元素特征都提供了含H2 O富Si流体交代橄榄岩的证据 ,这种流体可能主要是洋壳物质局部熔融而成。流体交代使橄榄岩富Si,同时富Sr、Pb和强不相容元素等大洋岩石圈物质。这表明普宁大陆岩石圈地幔既保留太古宙岩石圈地幔的特征 ,又具有大洋俯冲地幔的特征 ,它是古老岩石圈地幔向大洋岩石圈地幔转换的一部分 ,这种转换可能是大洋岩石圈与大陆岩石圈地幔相互作用的结果。     

华北上地幔各向异性研究

对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50~1.47 s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性.     

Strain accumulation along an oblique plate boundary: the Reykjanes Peninsula, southwest Iceland

We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion  18⁺⁴₋₃ mm yr⁻¹  and opening of  7⁺³₋₂ mm yr⁻¹  below a locking depth of  7⁺¹₋₂ km  . The resulting deep motion of  20⁺⁴₋₃ mm yr⁻¹  in the direction of  N(100⁺⁸₋₆)°E  agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula.