中国东部早白垩世岩浆作用的伸展构造性质——以辽东半岛南部饮马湾山岩体为例

饮马湾山岩体位于辽东半岛南部,侵位于辽南变质核杂岩之中。岩体主要由三种岩石组成,由外向里分别是片麻状花岗岩、斑状花岗岩和中细粒二长花岗岩类。其中片麻状花岗岩类主要岩石类型为石英二长闪长岩和花岗闪长岩,具有明显的片麻理构造,部分岩石具有明显的变形组构,其片麻理方向与变质核杂岩中拆离断层方向一致。似斑状花岗岩仅显示微弱片麻理,而细粒二长花岗岩则为明显的块状构造,表明该岩体为同伸展变形就位产物。锆石U-Pb同位素测定显示,该岩体不同岩相岩石具有大体一致的年龄,其形成于120～130Ma的早白垩世,而不是以前认为的三叠纪。该岩体岩石具有较大的地球化学成分变化范围,其中片麻状和似斑状岩石具有高Sr含量(>600ppm)、低Y、Yb含量、轻重稀土分馏强烈的特点;而细粒二长花岗岩则相对低Sr高Rb、低~(87)Rb/~(86)Sr比值。Sr-Nd同位素综合分析表明,饮马湾山花岗岩主要来源于下地壳,但较大的范围Mg~#、Sr、Nd同位素组成则表明可能有其它物质(如岩石圈地幔、新生地壳)参与成岩作用。上述研究充分地显示,我国东部早白垩世岩浆活动发育在伸展构造背景,并可能与岩石圈减薄存在密切的联系。     

一种新的分层冻胀量观测方法及其在季节冻土区的应用

Observation of delamination frost-heave is of significance in the study of foundation frost damage. The three main methods of observing delamination frost-heave, which are mono-frost-heave-ruler method, multi-frost-heave-ruler method and magnetic-loop method, are briefly introduced in this paper at first. Their respective advantages and disadvantages are analyzed. And then, according to the mechanism of frost-heave and the law of water migration and ice-forming in the freezing soil, a new approximate indirect method of observing the delamination frost-heave, which is antitheses, is brought forward by comparing surface frost-heave and frost-depth. On the basis of observation result in Chengyu Expressway and Daqing, its applicability is analyzed from three aspects under different groundwater level and soil property in seasonal frozen area. The result shows that this method is applicable for these two areas.     

Crystalline inclusions and C isotope ratios in diamonds from the Snap Lake/King Lake kimberlite dyke system: evidence of ultradeep and enriched lithospheric mantle

U-type paragenesis inclusions predominate (94.7%) among the crystalline inclusion suite of 115 diamonds (−4+2 mm) obtained from the recently discovered Snap Lake/King Lake (SKL) kimberlite dyke system, Southern Slave, Canada. The most common inclusions are olivine (90) and enstatite (22). Sulfide, Cr-pyrope, chromite and Cr-diopside inclusion are less abundant (15, 10, 5 and 1, respectively). Results of the inclusion composition study demonstrate the following. (a) The relatively enriched character of the mantle parent rocks of the U-type diamonds. The average Mg# of olivine inclusions is 92.1, and of enstatite inclusions average 93.3. CaO content in Cr-pyrope inclusions is relatively high (3.73–5.75 wt.%). (b) Four of ten U-type Cr-rich pyrope inclusions contain a majoritic component up to 16.8 mol.% which requires pressures of 110 kbar. Carbon isotopes compositions for 34 diamonds with U-type inclusions have a δ¹³C range from −3.2‰ to −9‰ with a strong peak around −3.5‰. This is much heavier than the ratios of U-type diamonds from Siberia and South Africa (4.5‰). Diamonds with olivine inclusions can be divided into two groups based on their δ¹³C values as well as the Mg# and Ni/Fe ratio in the olivines. Most show a narrow range of δ¹³C values from −3.2‰ to −4.8‰ (average −3.72‰) and have olivine inclusions with Mg# less than 92.3 and relatively high Fe/Ni ratios. A second group is characterized by a much wider variation of C isotope composition (δ¹³C varies from −3.8‰ to −9.0‰, average −5.97‰), and the olivine inclusions having a higher Mg# (up to 93.6) and relatively low Fe/Ni ratios. This difference in the C isotope composition may have several explanations: (a) peculiarities of asthenosphere degassing coupled with an abnormal thickness of lithosphere; (b) the abnormal thickness and enriched character of lithospheric mantle; (c) involvement of subducted C of crustal origin in the processes of the diamond formation. The presence of subcalcic Cr-rich majorite (up to 17 mol.%) pyropes of low-Ca harzburgite paragenesis among the crystalline inclusion suite of SKL diamonds is strong evidence for the existence of diamondiferous depleted peridotite in lithospheric mantle at depth near 300 km beneath Southern Slave area and is postulated to be one of the main reasons for the much heavier C isotope composition of SKL U-type diamonds in comparison with those from Siberian and South African kimberlites.     

吉林至珲春地区晚二叠世-早三叠世花岗岩成因机制

吉林——蛟河——敦化——珲春地区的晚二叠世——早三叠世花岗闪长质岩石中暗色微粒闪长质包体广泛发育，其形成与基性岩浆同酸性岩浆混合作用有关。该岩浆岩带形成于晚古生代末期陆——陆碰撞造山的动力背景下，碰撞过程中俯冲岩石圈的拆沉导致幔源岩浆的底侵作用是其形成重要机制。     

用多层变密度直接反演法研究下扬子地区的岩石圈构造

本文用多层变密度直接反演方法研究下扬子地区的岩石圈结构,并讨论其大地构造涵义及演化问题。文中给出了三次样条密度-深度函数的重力正演公式,采用非线性最小二乘优化算法,得到岩石圈内G_5、G_6、G_7三个主要密度界面,它们分别相应于中地壳低密度层底面、Moho面和上地幔顶部低密度层上界面。通过岩石圈结构分析,得到以下结果:杭州湾—海盐—嘉兴—长兴—高淳—芜湖隐伏深断裂是一条北西向的一级构造边界,它对下扬子区的大地构造发展和地壳演化有重要影响。     

太行山地区中、新生代玄武质岩浆的源区特征与时空演化

本文总结了太行山地区中生代辉长岩和新生代玄武岩的元素和同位素组成，其组成显示它们的地幔源区存在着明显的差别。新生代玄武岩以碱性玄武岩为主，含有少量橄榄拉斑玄武岩，它们来自软流圈的部分熔融，并伴有少量古老岩石圈地幔组分的加入。相比之下，中生代辉长岩具有明显不同的地球化学特征：SiO2含量高，LREE和LILE（Ba，Th，U）富集以及HFSE（Nb，Ta，Zr和Ti）亏损，Sr-Nd同位素富集（大多数^87Sr／^86Sri〉0．705，^143Nd／^144Nd；〈0．512）、Pb同位素亏损（^206Ph／^204Pbi〈17．5，^207Ph／^204Pbi〈15．5，^208Pb／^204Pbi〈38．0）。这些特征表明中生代辉长岩来源于经过富硅熔体强烈改造的古老岩石圈地幔，这种富集改造过程很可能与华北克拉通内部元古代的俯冲／碰撞事件有关。古老且同位素富集的岩石圈地幔残存至新生代，并被新生代玄武岩携带的地幔橄榄岩捕虏体所记录。     

Os, Nd, and Sr isotopic and chemical compositions of ultramafic xenoliths from Kurose, SW Japan: Implications for contribution of slab-derived material to wedge mantle

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan.

The Kurose xenoliths have less radiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.     

Lithospheric deformation under pre- and post-seismic stress fields along the Nicobar–Sumatra subduction margin during 2004 Sumatra mega-event and its tectonic implications

A high-resolution study was carried out under pre-seismic (i.e., static) and post-seismic stress fields (i.e., dynamic) in a space–time frame along the Nicobar–Sumatra margin. The study reveals that the descending lithosphere records minimum stress obliquity, predominant thrust movement, and down-dip least compressive stress axis under static stress field in northwest Sumatra (sector III). The imbalance between down-dip component of slab pull force and viscous resistive force possibly caused cyclic stress loading in compressive field around the flexing zone ( 25 km), and that undergone brittle failure through generation of mega-thrust event on 26th December' 2004. A sharp decrease in stress obliquity towards north (sector II), redressing of least compressive stress axes from horizontal to down-dip direction, and increasing thrust movements under dynamic stress field account for continued upward shortening of the lithosphere. The weak thinner zone (i.e., between  159 and  217 km depth), an age-discontinuity portion, possibly was collapsed through rapid enhancement of stress-induced weakening and strain localization following the 2004 Sumatra mega-shock. It is also well appreciated in the literature that such shallow disruption of the lithosphere is inevitable in the upper mantle, if the slab is weakened or broken there, and this phenomenon is not uncommon below the Sumatra.     

Water partitioning between mantle minerals from peridotite xenoliths

The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H₂O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H₂O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D _water^cpx/opx = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H₂O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D _water^cpx/oliv(gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D _water^cpx/oliv(sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units.     

The Jiashan Syenite in northern Hebei: A record of lithospheric thinning in the Yanshan Intracontinental Orogenic Belt

The Early Cretaceous Jiashan Syenite is located in a region of late Jurassic crustal thickening. The Jiashan Syenite can be divided into three concentrically arranged units, the Jiangjiawan, Longtangou and Longtannangou units, which were intruded sequentially. Geochemically, the Jiashan Syenite has a high Ga/Al ratio (>3), is enriched in silica, alkalis, Fe, REE, Th, Ga, Nb, Zr and Hf, is depleted in Mg, Ba, Sr and Ti and in transition elements such as Cr, Co, Ni and V. The three units of the Jiashan Syenite have Ce/Pb ratios ranging from 6.12 to 13.41 and are enriched in light REE (LREE) with a moderate Eu negative anomaly. The ⁸⁷Sr/⁸⁶Sr initial ratios range from 0.701409 to 0.707405, with a mean of 0.70379. The ε_Nd (t) values of −2.27 to −5.58 indicate that the magma was probably derived from enriched mantle. The Jiashan Syenite is a post-orogenic intrusion, and can be considered to be an A-type granite. It was emplaced in an environment of lithospheric extension during asthenospheric mantle upwelling. This suggests that the regional compressional shortening and crustal thickening tectonic regime in the Yanshan Orogenic Belt during the Late Jurassic (ca. 135 Ma) changed to lithospheric extension and thinning in the Early Cretaceous.