东昆仑东段三叠纪岩浆岩Nd–Hf同位素组分特征、物源演变规律及其构造背景

作为中央造山系西段的重要组成部分,东昆仑造山带以大面积区域性展布的三叠纪岩浆岩为鲜明特色。依据收集的东昆仑东段三叠纪岩浆岩96件锆石U–Pb年代学数据,限定三叠纪岩浆作用时间为252～212 Ma。结合岩石组合,将其进一步划分为早（252～238 Ma）、中（238～226 Ma）、晚（226～212 Ma）3期,其中岩浆活动峰值为早期（252～238 Ma）。Nd同位素数据（106件）统计结果表明：东昆仑东段三叠纪岩浆岩εNd(t)值为–9.4～–1.7,大多为–6.5～–3.0;Nd模式年龄TDM(Nd)为0.72～1.88 Ga,大多为1.00～1.80 Ga。Hf同位素数据（全岩、锆石;41件）统计结果表明：东昆仑东段三叠纪岩浆岩εHf(t)值变化较大（–8.4～+12.4）,主要为–4.5～+2.0;地壳模式年龄TDMC(Hf)值为0.49～ 1.80 Ga,大多为1.15～1.55 Ga。整体而言,三叠纪岩浆岩物源以中元古代壳源物质的再造为主,新生地壳（＜1.0 Ga）和古元古代地壳有所参与,但比例小。从岩浆活动早期（252～238 Ma）到中期（238～226 Ma）再到晚期（226～212 Ma）,εNd(t)值在岩浆活动早期（特别是在早三叠世）较高,正的εHf(t)值占据很大比例,物源中存在较多的新生物质;中期以较低的εNd(t)值和负的εHf(t)值为主,Hf模式年龄显示出现古元古代物质;晚期Nd–Hf模式年龄揭示较古老的壳源组分增多。这种岩浆岩物源演变趋势,与东昆仑东段三叠纪具俯冲到同碰撞再到碰撞后的构造演化背景一致。     

An anomalous high-velocity layer at shallow crustal depths in the Narmada zone, India

The Narmada zone in central India is a zone of weakness that separates the region of Vindhyan (Meso-Neoproterozoic) deposition to the north from Gondwana (Permo-Carboniferous–lower Cretaceous) deposits to the south. The reinterpretation of analogue seismic refraction data, acquired during the early 1980s, using 2-D ray-tracing techniques reveals a basement (velocity 5.8–6.0 km s⁻¹ ) topography suggesting that the Narmada zone, bounded by the Narmada North and Narmada South faults is a region of basement uplift. A layer of anomalously high velocity (6.5–6.7 km s⁻¹ ) at depths between 1.5 and 9.0 km appears to be present in the entire region. Within the Narmada zone this layer occurs at shallower depths than outside the Narmada zone. At two places within the Narmada zone this layer is at a depth of about 1.5 km. This layer cannot be considered as the top of the lower crust because in this case it should have produced large positive gravity anomalies at the shallowest parts. Instead, these parts correspond to Bouguer gravity lows. Furthermore, lower crust at such shallow depths has not been reported from any other part of the Indian shield. Therefore, this layer is likely to represent the top of a high-velocity mafic body that has different thicknesses in different places.     

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.     

Crustal structure of Iraq from receiver functions and surface wave dispersion: implications for understanding the deformation history of the Arabian–Eurasian collision

We report the crustal structure for two locations in Iraq estimated by joint inversion of P -wave receiver functions (RFs) and surface (Rayleigh) wave group velocity dispersion. RFs were computed from teleseismic recordings at two temporary broad-band seismic stations located in Mosul (MSL) in the Zagros Fold Belt and Baghdad (BHD) in the Mesopotamian Foredeep. Group velocity dispersion curves at the sites were derived from continental-scale tomography. The inversion results show that the crustal thicknesses are 39 km at MSL and 43 km at BHD. We observe a strong Ps _Moho at BHD consistent with a sharp Moho discontinuity. However, at MSL we observe a weak Ps _Moho suggesting a transitional Moho where crustal thickening is likely to be occurring in the deep crust. Both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station MSL and 7 km at BHD and agreeing well with the previous reports. Ignoring the sediments, the crystalline crustal velocities and thicknesses are remarkably similar at both stations. The similarity of crustal structure suggests that the crust of the northeastern proto-Arabian Platform was uniform before subsidence and deposition of the sediments in the Cenozoic. If crystalline crustal structure is uniform across the northern Arabian Platform then crustal thickness variations in the Zagros Fold Belt and Thrust Zone should reveal the history of deformation and crustal shortening in the Arabian–Eurasian collision zone and not reflect pre-existing crustal thickness variations in the Arabian Plate.     

Mid-Late Neoproterozoic rift-related volcanic rocks in China: Geological records of rifting and break-up of Rodinia

Early Cambrian and Mid-Late Neoproterozoic volcanic rocks in China are widespread on several Precambrian continental blocks,which had aggregated to form part of the Rodinia supercontinent by ca.900 Ma....     

中国海城地震区地壳与上地幔构造特征的研究

研究表明,海城地震区地壳与上地幔构造特征可以归纳为如下四点:1.震区所处的辽南地区,其地壳是由高速与低速相间的成层介质组成,地壳与上地幔结构不论纵向和横向都是不均匀的,且被深大断裂切割为若干块体;2.本区地壳按其界面分布特征可分为上层地壳、中层地壳和下层地壳;3.本区地壳介质在纵向及横向上均存在明显的不均一性;4.海城地震震源区位于耿庄——海城上地幔局部隆起东侧的中层地壳上部。     

新疆及其毗邻地区地震层析成像

利用新疆地震台网及其毗邻地区的国外地震台站提供的地震Ｐ波到时数据,重建了新疆及其毗邻地区地壳上地幔的三维速度图像．主要结果是：１．研究区地壳上地幔的地震波速度存在明显的横向不均匀性．这种不均匀性出现在不同构造单元之间,也出现在各构造单元之内．２．一般来说,研究区域内盆地是上地幔的隆起区,褶皱系是上地幔的坳陷区．３．岩石层厚度和软流层的深度与全球的平均结果有较大不同．４．海洋岩石层或大陆岩石层板块在兴都库什和克什米尔与中国交界地区的消减明显存在．５．地壳上地幔物质运动比较剧烈,有部分熔融物质上涌到地壳中部或更浅的深度．     

新疆尼勒克县圆头山后碰撞花岗斑岩的同位素年代学及地球化学特征

圆头山黑云母花岗斑岩位于尼勒克县城南,岩体普遍发育的铜矿化,以中部碎裂带中矿化最强。岩石具有典型的斑状结构,斑晶由碱性长石和黑云母组成,基质以长石、云母和石英为主,并含有少量的硫化物。圆头山黑云母花岗斑岩的轻重稀土分馏明显,并且明显富集大离子亲石元素(LILE) (除Sr外)和Pb,同时亏损高场强元素(HFSE)和Nb、Ta、P、Ti,显示出弧火山岩的地球化学特征。圆头山黑云母花岗形成年龄为269±3Ma,其源区以具有弧火山岩特征的下地壳为主,同时含有少量的幔源物质,而且在岩体的侵位过程中有高(⁸⁷Sr/⁸⁶Sr)_i (0.706054~0.709228)和高Pb含量(5.05×10^-6~32.5×10^-6)的陆壳物质混染。在下地壳物质熔融过程中,位于下地壳底部的富水和成矿元素的MASH带物质也被圈入,使原始岩浆富水和成矿物质。圆头山黑云母花岗斑形成于碰撞后阶段,岩石的形成主要受岩石圈地幔拆沉作用的控制,而岩石圈地幔的拆沉可能与中亚造山带的垮塌或塔里木地幔柱活动有关。     

西藏错那洞电气石花岗岩中电气石化学组成、硼同位素特征及意义

为了对西藏错那洞电气石花岗岩源区进一步约束,利用显微镜、电子探针和激光剥蚀多接收等离子质谱仪,对错那洞电气石花岗岩中电气石的形态、成分及硼同位素组成进行了研究.结果表明,错那洞电气石花岗岩中的电气石为碱族黑/铁电气石,直接结晶自富硼熔体,与熔体之间未发生明显的硼同位素分馏.电气石δ11B值主要在-6.91‰^-9.17‰之间,与大陆地壳平均δ11B值(-10‰±3‰)相近,表明错那洞电气石花岗岩主要源自变质沉积岩的部分熔融.然而,与起源于变质沉积岩的花岗岩相比,样品的δ11B值明显偏高,而与前人报道的雅拉香波淡色花岗岩(源自石榴石角闪岩部分熔融)的δ11B值相似.因此,错那洞电气石花岗岩源区中,除了变质沉积岩外,可能还混入了少量石榴石角闪岩.