Three-dimensional seismic modelling of crustal structure in the TESZ region based on POLONAISE''97 data

This paper reports the results of 3-D tomographic modelling of crustal structure in the Trans European Suture Zone region (TESZ) of Poland, eastern Germany and Lithuania. The data are the product of a large-scale seismic experiment POLONAISE'97, which was carried out in 1997. This experiment was designed to provide some 3-D coverage. The TESZ forms the boundary between the Precambrian crustal terranes of the East European Craton (EEC) and the younger Phanerozoic terranes to the southwest. The 3-D results generally confirm the earth models derived by earlier 2-D analyses, but also add some important details as well as a 3-D perspective on the structure. The velocity model obtained shows substantial horizontal variations of crustal structure across the study area. Seismic modelling shows low (<6.1 km/s) velocities suggesting the presence of sedimentary rocks down to a depth of about 20 km in the Polish basin. The shape of the basin in the vicinity of the profile P4 shows significant asymmetry. Three-dimensional modelling also allowed tracing of horizontal irregularities of the basin shape as well as variations of the Moho depth not only along profiles, but also between them. The slice between P2 and P4 profiles shows about 10-km variations of the Moho over a 100-km interval. The crustal thickness varies from about 30 km in SW, beneath the Palaeozoic platform, to about 42 km beneath East European Craton in NE. High seismic velocities of about 6.6 km/s were found in the depth range 2–10 km, which coincides with K trzyn anorthosite massif. The results of this 3-D seismic modelling of the POLONAISE'97 data will ultimately be supplemented by inversion of seismic data from previous experiments.     

华南地区地壳速度结构分析

笔者利用地球物理综合剖面上获取的P波速度值,对岩石圈地壳进行分层研究,划分出上、中、下地壳。按照大地构造单元进行数据的归类总结,计算出地壳各层平均速度和厚度值,以及地壳的平均速度和厚度值,最终通过所取得的数据资料的分析研究,探讨地球内部结构的动力学过程。     

Must magmatic intrusion in the lower crust produce reflectivity?

The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin.     

Possible orogeny-parallel lower crustal flow and thickening in the Central Andes

The relatively low elevation and thick crust in the Altiplano, in comparison to the higher elevation, but thinner crust in the Puna plateau, together with geophysical data, suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the Altiplano. Excess density in the Altiplano mantle could create differential horizontal stress in the order of 25 MPa between both lithospheric columns. Numerical models accounting for pressure and temperature-dependent rheology show that such stress can induce horizontal ductile flow in the lower crust, from the Puna towards the Altiplano. With a minimum viscosity of 10¹⁹ Pa s, this flow reaches 1 cm/year, displacing more than 50 km of material within 5 Ma. If the lower crust viscosity is smaller, the amount of orogeny-parallel lower crustal flow can be even greater. Such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the Altiplano and in the Puna. Because of the strength of the elastic-brittle upper crust, this channel flow does not necessitate large amounts of surface deformation (except vertical uplift), making it difficult to detect from the geology.     

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.     

中国东南部下地壳物质与花岗岩成因探索

在中国东南部中－新生代玄武质岩石中陆续找到了来自下地壳的辉长岩质麻粒岩／辉长岩深源捕掳体。本文着重研究了沿海地区广东麒麟新生代玄武质角砾岩筒中的辉长岩质麻粒岩捕掳体,并与内陆湖南道县早中生代玄武质角砾岩筒中的辉长岩、花岗片麻岩捕掳体作了比较。研究表明,辉长岩质麻粒岩是底侵作用形成的基性岩浆底垫于下地壳,并经变质作用的产物。底侵作用是重要的壳－幔作用过程,也是中国东南大陆边缘陆壳演化的重要过程。Ｓｍ－Ｎｄ同位素定年显示,中国东南大陆边缘底侵作用物质在地壳底部的结晶年龄约为１１２．３±１７．８Ｍａ,属晚中生代时期。此外,底侵作用在不同大地构造域的发育程度是不一致的,底侵作用发育强弱的标志是基性火山岩是否发育、壳幔型花岗岩是否广泛产出     

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is 15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km² footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.     

鄂东南铜山口、殷祖埃达克质（adakitic）侵入岩的地球化学特征对比：（拆沉）下地壳熔融与斑岩铜矿的成因

鄂东南地区铜山口花岗闪长斑岩体是与斑岩铜钼矿床共生的岩体,但殷祖花岗闪长岩体是与金属成矿无关的岩体。铜山口和殷祖侵入岩的元素地球化学特征与埃达克岩的地球化学特征非常类似,如高Al2O3、Sr含量与La/Yb、Sr/Y比值,富Na2O(Na2O/K2O>1.0),亏损Y与Yb,极弱负Eu异常-正Eu异常以及正Sr异常等。但是铜山口和殷祖侵入岩也存在 明显的差别:前者比后者更偏酸性,但具有较高的K2O,MgO,Cr,Ni和Sr含量,较低的Y和Yb含量,轻重稀土元素分异更明显,并主要显示出正铕异常,区别于后者的极弱负Eu异常-不明显Eu异常。这表明铜山口埃达克质侵入岩的岩浆来源可能比殷祖埃达克质侵入岩的岩浆来源更深:前者可能由拆沉的下地壳熔融形成,残留物主要含石榴子石;而后者可能由增厚的下地壳熔融形成,残留物可能为石榴子石±斜长石±角闪石。另外,热的地幔上涌,底辟(diapir)进入下地壳,导致含角闪石的榴辉岩发生熔融也可形成铜山口埃达克质岩浆。铜山口埃达克质岩浆在穿过地幔的过程中,将会与地幔橄榄岩发生交换反应:一方面由于受橄榄岩的混染而使得岩浆的MgO,Cr和Ni增高;另一方面岩浆中的Fe2O3不断加入到地幔中,导致地幔的氧逸度(fo2)增高,地幔中金属硫化物被氧化并进入岩浆中,富含Cu-Mo等成矿物质的岩浆上升很容易形成斑岩铜钼矿     

A composite geologic and seismic profile beneath the southern Rio Grande rift, New Mexico, based on xenolith mineralogy, temperature, and pressure

A complete understanding of the processes of crustal growth and recycling in the earth remains elusive, in part because data on rock composition at depth is scarce. Seismic velocities can provide additional information about lithospheric composition and structure, however, the relationship between velocity and rock type is not unique. The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from reflection and refraction data in the area, offers an opportunity to place constraints on the composition of the crust and upper mantle from the surface to depths of  60 km. In this work, we calculate seismic velocities of crustal and mantle xenoliths using modal mineralogy, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are then combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle. Lower crustal xenoliths include garnet ± sillimanite granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust–mantle boundary is not present beneath the southern Rio Grande rift. Abundant mid-crustal felsic to mafic igneous xenoliths, however, suggest that plutonic rocks are common in the middle crust and were intraplated rather than underplated during the Cenozoic. Calculated velocities for garnet granulite are between  6.9 and 8.0 km/s, depending on garnet content. Granulites are strongly foliated and lineated and should be seismically anisotropic. These results suggest that velocities > 7.0 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion. Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with refraction and reflection studies that report P_n velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low P_n velocities compared to average uppermost mantle are the result of relatively high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed P_n velocities.     

南海东北陆缘的地壳速度结构及其构造意义:来自广角地震剖面的约束

南海东北部深部地壳结构蕴含着南海陆缘伸展张裂过程的重要信息。在南海东北陆缘布设的一条广角地震测线(DP13)沿NW-SE方向依次穿过东沙隆起和台西南盆地。本文利用射线追踪和正演走时拟合软件RayInvr构建地壳纵波速度结构,模型表明：沉积层速度1.6～4.6 km/s,厚度0.5～3.8 km,横向分布不均匀,沉积基底起伏剧烈;莫霍面埋藏深度由陆架区的25.5 km急剧减小到陆坡下方的13 km,随后向下陆坡远端增深至16 km;陆架处东沙隆起下方地壳厚度从～25 km减薄到～21 km,下陆坡远端地壳厚约10～13 km,地壳拉张因子分别为1.3～1.5和2.6～3.1,表现为轻微和中等减薄;陆坡区台西南盆地内地壳厚度从17 km急剧减薄至7～8 km,地壳拉张因子高达4.6,呈超伸展减薄;地壳厚度由陆向海非单调减薄,地壳伸展具有明显的空间差异性;陆架-上陆坡和下陆坡下地壳底部发现两个相对孤立的不连续高速体,速度分别为7.0～7.5 km/s和7.0～7.3 km/s,厚度分别3～5 km和1～3 km,前者位于古太平洋俯冲带前缘,几乎与南海东北部高磁异常重叠,推测由中生代古太平洋板...     

发育大洋斜长花岗岩的南海管事平顶海山：深部地壳和莫霍面钻探的优选区？

本文紧密围绕IODP“面向2050年大洋钻探科学框架”中的“地球深部探测”旗舰计划和“莫霍面”梦想，研究并总结全球现代洋壳发现的大洋斜长花岗岩的分布规律、岩石组合特征和成因模式，探讨发育大洋斜长花岗岩的南海管事平顶海山是否为深部地壳和莫霍面钻探潜在优选区。统计结果表明大洋斜长花岗岩在多种不同构造背景形成的洋壳上均有发现，包括西南印度洋超慢速扩张构造环境，大西洋、西北太平洋、西印度洋和中印度洋慢速扩张构造环境，东太平洋快速扩张构造环境，南海等边缘海构造环境，伊豆- 小笠原- 马里亚纳(IBM)岛弧、九州- 帕劳海脊、Amami海底高原等洋内俯冲构造环境。多数大洋斜长花岗岩呈脉状零散分布于辉长岩中，意味大洋斜长花岗岩可能形成于洋壳深部，在后期断裂等地质作用下被剥蚀而更容易被发现，成为了解洋壳深部岩浆过程和洋壳结构的一个窗口。管事平顶海山位于南海东部次海盆古扩张脊附近，拖网获得MORB型大洋斜长花岗岩，前人基于地球化学数据认为其可能为辉长岩部分熔融形成。本文推测管事平顶海山大洋斜长花岗岩很可能为洋壳深部物质剥露海底，是南海的一个重要构造窗口，有望成为南海深部地壳和莫霍面钻探的潜在优选区，但需要开展进一步调查研究以验证推测：① 海山大洋斜长花岗岩为拖网所得，位置误差较大，需开展可精确定位的电视抓斗、浅钻或有缆遥控水下机器人(ROV)调查；② 海山目前只发现大洋斜长花岗岩，需调查海山是否发育辉长岩等深部地壳岩石组合；③ 需开展重磁、深反射地震、海底地震仪(OBS)、大地电磁等调查研究，建立管事平顶海山洋壳和上地幔结构模型，查明断裂带分布，揭示莫霍面深度。     

Laboratory measurement of P-wave velocity in crustal and upper mantle xenoliths from Ichino-megata, NE Japan: ultrabasic hydrous lower crust beneath the NE Honshu arc

Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25–400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7–7.2 km/s for the hornblende gabbros (38.6–46.9 wt.% SiO₂), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO₂), 6.9–7.3 km/s for the amphibolites (36.1–44.3 wt.% SiO₂), 8.0–8.1 km/s for the spinel lherzolites (46.2–47.2 wt.% SiO₂) and 6.30 km/s for the biotite granite (72.1 wt.% SiO₂). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329–2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6–7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1–7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.     

Crustal lineaments, distribution of lower crustal intrusives and structural evolution of the Vøring Margin, NE Atlantic; new insight from wide-angle seismic models

Five lineaments on the volcanic Vøring Margin, NE Atlantic, have been identified in crustal scale models derived from Ocean Bottom Seismograph (OBS) data. It is suggested that the Vøring Basin can be divided in four compartments bounded by the Jan Mayen Fracture Zone/Lineament, a new lineament defined from this study, the Gleipne Lineament, the Surt Lineament and the Bivrost Lineament. The NW–SE trending Jan Mayen-, Gleipne- and Bivrost lineaments probably represent old zones of weakness controlling the onset of the early Eocene seafloor spreading, whereas the Surt- and New lineaments, rotated ca. 30° symmetrically from the azimuth of the Gleipne Lineament, may represent adjustment features related to the early Cretaceous/early Tertiary rifting. The longest landward extent of a lower crustal high-velocity body, assumed to represent intrusions related to the last phase of rifting, is found between the New Lineament and the Gleipne Lineament, where the body extends across the Helland Hansen Arch. Northeastwards in the Vøring Basin, the landward limit of the body steps gradually seawards, closely related to the interpreted lineaments. Northeast of the Gleipne Lineament, the body terminates close to the Fles Fault Complex, north of the Surt Lineament, it extends across the Nyk High, and northeast of the Bivrost Lineament the intrusions terminate around the Vøring Escarpment. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Icelandic plume, and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement. The increased thickness of the continental crust on the seaward side of the Vøring Escarpment, the upwarping of Moho and thinning of the lower crustal high-velocity layer in the western part of the Vøring Basin, as well as a strong shallowing of the Moho observed in parts of the area between the Jan Mayen Fracture Zone/Lineament and the New Lineament, can be explained by lithospheric delamination models.     

华北克拉通南缘下地壳麻粒岩地体的形成与演化

研究前寒武纪地体的构造-岩浆事件和变质作用过程是揭示早期陆壳的生长与演化等热点问题的重要途径。目前,尽管已经开展了大量的相关研究,但关于华北克拉通南缘前寒武纪变质地体的成因、构造背景仍存在争议,限定了我们对于华北克拉通南缘前寒武纪形成和演化的认识。本文根据前人研究成果,包括华北南缘太华杂岩的岩相学、地球化学、年代学以及变质P-T-t轨迹等,结合对鲁山地区太华杂岩的全岩主微量、长石矿物原位Pb同位素组成特征以及基性麻粒岩的年代学等研究成果,系统梳理并探讨了太华杂岩代表的新太古代-古元古代下地壳物质组成、岩石成因以及多阶段构造-岩浆事件。结果显示华北南缘记录了2.92~2.71Ga和2.53~2.45Ga两期重要的地壳生长事件,2.35~2.30Ga和2.19~2.06Ga的陆壳再造,以及1.96~1.84Ga古元古代的变质事件。再结合华北克拉通中部造山带中、北部的变质程度、变质P-T轨迹和变质年龄,我们认为华北南缘前寒武纪基底与中部造山带的中、北部相似,构造归属于中部造山带,发生了一个较长的俯冲碰撞造山过程(1.96~1.86Ga)。

     

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.     

Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in Siberia

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.     

Petrology of high-grade crustal xenoliths in the Chalcatzingo Miocene subvolcanic field,southern Mexico: buried basement of the Guerrero-Morelos platform and tectonostratigraphic implications

The Miocene Chalcatzingo trondhjemitic volcanic field, sited along the southern margin of the Trans-Mexican Volcanic Belt, is a newly discovered locality with deep-seated crustal xenoliths that provide fundamental petrologic information on the nature of the unexposed metamorphic basement. The volcanic field lies along the eastern edge of the Cretaceous Guerrero-Morelos platform, which juxtaposes the Guerrero and Mixteco terranes of southern Mexico. Xenoliths consist of high temperature to ultra-high temperature metapelites as well as mafic and quartzofeldspathic gneisses, all of which show evidence of multiple granulite to amphibolite facies metamorphism and ductile deformation. A detailed petrologic study of representative xenoliths indicates a metamorphic evolution that apparently followed a clockwise pressure–temperature path leading from biotite-sillimanite₁/kyanite(?)-quartz assemblages (M₁) to the assemblage plagioclase-garnet-sillimanite₂-rutile/ilmenite (M₂) with a peak at ～9–11 kbar and >870°C. These conditions were followed by rapid uplift to <6 kbar and >800°C, which produced the decompression assemblage spinel-cordierite-sillimanite₃-corundum ± orthopyroxene ± quartz (M₃) before shallow emplacement of the xenolith-bearing trondhjemitic magma. Three possible sources for the xenoliths are considered: (1) early Mesozoic metasediments buried in the middle crust; (2) Precambrian lower crust; and (3) subducted Cenozoic sediments trapped in the mantle wedge. Based on the deep-seated, polymetamorphic nature of the xenoliths, the Nd depleted mantle model age of an orthogneissic xenolith, and on regional tectonostratigraphic considerations, we suggest that the xenolith source was Proterozoic continental crust. Although old zircon inheritance in the host trondhjemite is minimal, it may be explained by a lack of interaction of the magma with the traversed lithosphere. Studies of Palaeogene shallow intrusions exposed 140 km west of Chalcatzingo in the Guerrero terrane (Pepechuca plug) and 80 km southeast of that place in the Mixteco terrane (Puente Negro dikes) reveal the presence of similar very high-grade aluminous xenoliths. However, these magmas were probably generated by partial melting of Triassic–Jurassic metasediments of the Guerrero terrane underplated by basaltic magmas in Jurassic–earliest Cretaceous times or from Precambrian crust assimilated by underplated mafic magmas of Oligocene age, respectively.     

Petrology and mineralogy of granulite-facies mafic xenoliths (Sardinia, Italy): Evidence for KCl metasomatism in the lower crust

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T = 850–900 °C, P = 0.8–1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56–72 mol%) + Fe–Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO₂ = 4.7–6.5 wt.%; F = 0.24–1.19 wt.%; Cl = 0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6–50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks.     

Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re-equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near-isobaric trajectories. Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA-type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in X_gr, X_jd (increase) and X_an (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km). An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA-type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.     

藏北羌塘地块新生代火山岩中麻粒岩捕虏体的岩石学和地球化学研究:对青藏高原新生代火山岩成因及下地壳性质的约束

青藏高原北部羌塘地块的新生代高钾钙碱性火山岩中含有很多下地壳捕虏体,这些捕虏体的主要岩石类型为二辉石麻粒岩和单斜辉石麻粒岩.本文对乌兰乌拉湖南侧枕头崖地区新生代高钾钙碱性火山岩中的9个麻粒岩样(6个基性麻粒岩和3个酸性麻粒岩)进行了系统的岩石学,矿物化学,地球化学和Sr-Nd-Pb同位素地球化学研究.其中,紫苏辉石具有高Mgo FeO,低Al2O3的特征,单斜辉石具有低Al2O3和TiO2的特征,黑云母具有高TiO2的特征,这些特征都表明这些矿物为变质成因.矿物温压计算表明二辉石麻粒岩形成的平衡温度为783℃～818℃.单斜辉石麻粒岩形成压力在0.845～0.858GPa之间,来源深度约28km.表明它们可能是来自青藏高原加厚陆壳中部的岩石样品.基性麻粒岩的SiO2=48.76%～58.61%,Al2O3=18.34%～24.50%,Na2O=3.16%～5.41%,K2O=1.58%～3.01%,低Mg#(30～67),富集轻稀土(LREE)和大离子亲石元素(LILE),其具有较高的Rb/Sr(0.09～0.21)和(La/Yb)N(17.32～49.35),具有较低的Nb/Ta(9.76～14.92),其Eu异常变化于0.19～0.89之间.基性麻粒岩的Sr-Nd-Pb同位素地球化学表现为87Sr/86Sr=0.710812～0.713241,εSr=169.13～203.88,143Nd/144 Nd=0.512113～0.512397;εNd=-4.70～-10.05,206Ph/204Pb=18.7000～-18.9565,207Pb/204Pb=15.7135～-15.7662,208pb/204Pb=39.1090～39.4733.和基性麻粒岩类似,酸性麻粒岩也表现出富集轻稀土(LREE)和大离子亲石元素(LILE)的特征,它们的87Sr/86Sr=0.712041～0.729088,εNd=180.71～430.59,143Nd/144Nd=0.512230～-0.512388;εNd=-4.74～-7.96,206Ph/204Pb=18.9250～-19.1717,207Pb/204Pb:15.7662～-15.7720,208Pb/204Pb=39.2109～39.6467.上述地球化学特征表明这些基性麻粒岩的源岩是下地壳岩石,而非地幔岩或玄武质堆晶岩,而酸性麻粒岩的源岩极有可能是准铝质的花岗质岩石.这就表明青藏高原新生代下地壳的地温梯度很高,并含有部分沉积岩,而非典型的辉长质下地壳.而且,详细的研究表明,这种特殊的下地壳可能对青藏高原新生代高钾钙碱性和橄榄粗安质岩浆的起源有重要作用.因此目前所认为的超钾质-橄榄粗-安质岩浆源于富集岩石圈地幔在对流减薄作用下发生部分熔融的观点值得重新考虑.