北祁连山高压变质带的变形和变质特征及其对俯冲带折返的意义

北祁连造山带是早古生代大洋板块"冷"俯冲的典型地区,形成了一系列高压低温变质的岩石组合.基性榴辉岩是俯冲洋壳变质的产物,根据其结构构造特征可以划分为(1)粒状榴辉岩,峰期变质矿物组合为Grt+Omp+Phn+Rut±Lws,硬柱石呈包裹体和假象分布于石榴石内,变质温压条件T=465～508℃,P=2.30～2.60GPa.(2)片状榴辉岩,岩石具片状构造,其共生矿物组合Omp+Czo/Ep+Phn+Gln构成透入性面理S1和线理L1,以及宏观和显微同斜褶皱F1.其峰期变质温压条件为T=466～510℃,P=1.9～2.2GPa.(3)蓝片岩相退化变质榴辉岩,蓝片岩相变质矿物Gln+Ep+Phn±Ab强烈交代榴辉岩相矿物,并发生强烈D2期变形作用,岩石形成明显的透入性面理S2和线理L2,以及同斜剪切褶皱F2.蓝闪石和多硅白云母线理的统计揭示D2期变形以斜向走滑剪切为主,与D1期变形的榴辉岩的运动学特征大体相似.D2同变质的温压条件为T=422～487℃,P=1.15～1.37GPa.高压变质带晚期的绿片岩相叠加和D3期变形,形成透入性面理S3和线理L3,其运动特征为近于垂直构造线的逆冲剪切.结合榴辉岩变形特征,变质温压条件和同位素年代学资料,我们认为退变质的强弱与变形程度有密切关系.峰期变质后的榴辉岩经过早期绿帘石榴辉岩相到蓝片岩相斜向走滑剪切,和晚期绿片岩相逆冲,在泥盆纪早期快速折返的出露地表.     

桐柏地区高压变质地体在地壳中的抬升机制

桐柏杂岩位于秦岭与大别造山带之间,南北两侧为NWW-SEE向延伸的含榴辉岩高压变质地体,其构造就位过程及其与高压变质地体的构造关系为地质学家们所关注.本文研究表明,所谓的桐柏杂岩实际上是一个早白垩世花岗质片麻岩杂岩体,其内包裹一定数量的三叠纪中高级变质岩石.桐柏杂岩南北两条边界韧性剪切带具有相反的运动学指向,分别为大型右行和左行韧性剪切带,其形成时代为131Ma.这两条韧性剪切带中的剪切面理分别朝SSW和NNE向倾斜,韧性剪切带及杂岩体内部发育的拉伸线理均稳定地朝SEE方向缓倾伏.而在杂岩体的东端发育低角度近南北向韧性剪切变形带,其糜棱面理朝SEE方向低角度缓倾斜,矿物拉伸线理也朝SEE方向低角度缓倾伏,运动学标志指示东侧地质体朝295°～310°的方向逆冲.因此,桐柏杂岩实际上被一个顺造山带的韧性剪切带所围限,这个韧性剪切带及杂岩体内部韧性变形运动学指示了一个早白垩世由东向西的低角度抬升过程,而这一过程直接导致了含榴辉岩高压变质地体沿着造山带的方向从地壳深部抬升到近地表.这一研究结果对理解和认识桐柏-大别-苏鲁地区高压一超高压变质地体的晚期抬升过程具有重要意义.     

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.     

中亚巴尔喀什成矿带晚古生代最晚期岩浆侵入事件及其热演化历史

哈萨克斯坦巴尔喀什成矿带是中亚成矿域重要的晚古生代斑岩铜钼和云英岩-石英脉型钨钼成矿带,是受走滑断裂边界控制的中亚多核成矿系统的核心之一。本文根据锆石SHRIMP U-Pb定年、40Ar/39Ar热年代学、磷灰石裂变径迹定年和热历史模拟,厘定了巴尔喀什成矿带西部地区晚古生代最晚期深成岩浆侵入事件。研究表明,原来被认为是属于三叠纪的后碰撞花岗岩类侵入岩体,给出锆石SHRIMP U-Pb年龄为289.7±2.3Ma,为早二叠世。结合前人研究,本文探讨了巴尔喀什成矿带西部从深成岩浆侵入、钨钼成矿作用、区域冷却到剥露作用热历史的全过程。晚古生代最晚期花岗岩类侵入体具有与钨钼矿床相同的晚中生代剥露作用年龄(92.4±5.9Ma)。     

A kinematic evolutionary model for the Penninic sector of the central Ligurian Alps

The nappe pile presently cropping out in the central sector of the Ligurian Alps, is represented by some principal groups of tectonic units. Starting from the foreland, the outer and lower, weakly metamorphic (up to 0.3 GPa) Briançonnais units support the high-pressure (up to 1.3 GPa) ensemble of inner Briançonnais nappes, in turn overridden by the Prepiedmont units, sourced from the European continental margin. Prepiedmont units form two superposed groups. The lower is composed only of a pre-Namurian basement (Alpine metamorphism up to 0.6 GPa); and the upper is mainly composed of a slightly metamorphic (greenschist facies) post-Namurian cover. At the top lie the high-pressure metamorphosed (up to 0.8 GPa in the sector here considered) ophiolitic units. The group of the non-metamorphic Helminthoid Flysch nappes (original stratigraphic cover of the ophiolitic units) has travelled the greatest distance and is presently mainly set onto the outer part of the chain. Only events up to the stacking of the nappe pile are discussed, disregarding late-stage deformation. As the examined sector is located at a considerable distance from the collisional zone, late processes did not change the overall order of superposition formerly acquired. The model proposes the development of two major, subhorizontal detachment surfaces. The first, shallower one confines at the base a very thin-skinned set of nappes, nearly totally made up of Prepiedmont sedimentary covers that are bounded at their top by the Helminthoid Flysch units. Both these groups underwent a mainly horizontal outwards transport. In contrast, the underlying Prepiedmont crust and the adjoining Briançonnais inner sector (separated by the second, deeper major detachment surface) were progressively dragged into the subduction zone under the ophiolitic units and duplexes were generated. Exhumation of the metamorphic units occurred along the subduction channel, as did stacking of the nappe pile.     

Review of human decomposition processes in soil

In-soil human decomposition is comprehensively described in terms of the physico-chemical and bacterial environmental conditions. Much of the understanding comes from considerations of cemetery studies and experimentation with adipocere. The understandings are relevant for further studies in cemetery management, exhumations, forensic investigations and anthropology. In the soil, cadavers are subject to various sets of decomposition processes principally resulting from aerobic (usually the initial) or anaerobic (usually the sustaining) conditions. The presence of percolating groundwater and microorganisms further affects the rate of breakdown and fate of the products. The major human tissue components—protein, carbohydrate, fat and bone, are discussed; and the likely pathways of decomposition products enumerated. The effects of liquefaction, availability of oxygen and other in-grave processes are considered.     

天山北缘侏罗系地层热历史演化及其地质意义：磷灰石裂变径迹和镜质体反射率证据

对天山北缘石场-玛纳斯、安集海河和四棵树河地区18个样品进行了磷灰石裂变径迹年龄测定,同时测定了中生界地层10件裂变径迹样品相应煤层的镜质体反射率。结果表明地层由老到新镜质体反射率逐渐增加,磷灰石裂变径迹中值年龄逐渐降低。石场-玛纳斯地区,下部三叠系煤层镜质体反射率 R_o 值较低,为O.56%,磷灰石裂变径迹中值年龄较大,为125.3±9.1Ma;八道湾组煤层 R_o 为0.53%～0.64%,磷灰石裂变径迹中值年龄介于81.3±4.7～87.8±5.9Ma;上部西山窑组煤层 R_o 最高,达到0.81%,磷灰石裂变径迹中年龄较低,为44.0±5.4～11.8±1.8Ma;相同层位,东部石场-玛纳斯一带 R_o比西部四棵树地区高,磷灰石裂变径迹年龄刚好相反。磷灰石裂变径迹模拟结果表明中生界三叠系、下侏罗统地层的埋藏深度相对较浅,上覆沉积持续的时间到晚侏罗世到早白垩世基本上已经结束,然后保持在基本不变的深度,直至中新世,不整合在三叠系之下的花岗岩的模拟结果也支持这样的认识。晚侏罗世—早白垩世的冷却降温事件可能是地温梯度变化和隆升作用的共同结果;中侏罗统地层埋藏增温过程持续时间较长,在玛纳斯地区直至渐新世末期。所有样品中磷灰石裂变径迹模拟都记录了10Ma 左右的快速冷却过程,近4～5km 的地壳表层物质被剥蚀,平均剥蚀速率400～500m/Ma。这一剥蚀过程应该与天山地区的快速隆升,以及向北的冲断推覆作用相对应。天山地区山前带的变形应不早于10Ma,这一认识与野外地质证据一致。     

对中国中部超高压榴辉岩的P—T轨迹及回返机制的新认识

中国中部超高压变质带中的榴辉岩一般发育４～５个变质演化阶段：前榴辉岩阶段、榴辉岩峰期阶段、早期高压退变阶段、晚期退变阶段和后期叠加变质阶段。本文依据这一变质演化的研究，得出了一条与以往不同的Ｐ－Ｔ轨迹。它表明榴辉岩回返时首先沿俯冲带缓慢上升到较浅部位，出现与进变质过程相似的Ｐ－Ｔ轨迹；然后主要涉及地壳范围的，与碰撞造山有关的推覆作用使其进一步迅速回返，出现绝热抬升轨迹。这种双阶段双机制的回返模式不仅与榴辉岩中发育的变质作用过程相符，而且可以解释榴辉岩研究中一系列难以理解的问题。     

Structural development of the Tso Morari ultra-high pressure nappe of the Ladakh Himalaya

A continental subduction-related and multistage exhumation process for the Tso Morari ultra-high pressure nappe is proposed. The model is constrained by published thermo-barometry and age data, combined with new geological and tectonic maps. Additionally, observations on the structural and metamorphic evolution of the Tso Morari area and the North Himalayan nappes are presented. The northern margin of the Indian continental crust was subducted to a depth of > 90 km below Asia after continental collision some 55 Ma ago. The underthrusting was accompanied by the detachment and accretion of Late Proterozoic to Early Eocene sediments, creating the North Himalayan accretionary wedge, in front of the active Asian margin and the 103–50 Ma Ladakh arc batholith. The basic dikes in the Ordovician Tso Morari granite were transformed to eclogites with crystallization of coesite, some 53 Ma ago at a depth of > 90 km (> 27 kbar) and temperatures of 500 to 600 °C. The detachment and extrusion of the low density Tso Morari nappe, composed of 70% of the Tso Morari granite and 30% of graywackes with some eclogitic dikes, occurred by ductile pure and simple shear deformation. It was pushed by buoyancy forces and by squeezing between the underthrusted Indian lithosphere and the Asian mantle wedge. The extruding Tso Morari nappe reached a depth of 35 km at the base of the North Himalayan accretionary wedge some 48 Ma ago. There the whole nappe stack recrystallized under amphibolite facies conditions of a Barrovian regional metamorphism with a metamorphic field gradient of 20 °C/km. An intense schistosity with a W–E oriented stretching lineation L₁ and top-to-the E shear criteria and crystallization of oriented sillimanite needles after kyanite, testify to the Tso Morari nappe extrusion and pressure drop. The whole nappe stack, comprising from the base to top the Tso Morari, Tetraogal, Karzok and Mata–Nyimaling-Tsarap nappes, was overprinted by new schistosities with a first N-directed and a second NE-directed stretching lineation L₂ and L₃ reaching the base of the North Himalayan accretionary wedge. They are characterized by top-to-the S and SW shear criteria. This structural overprint was related to an early N- and a younger NE-directed underthrusting of the Indian plate below Asia that was accompanied by anticlockwise rotation of India. The warping of the Tso Morari dome started already some 48 Ma ago with the formation of an extruding nappe at depth. The Tso Morari dome reached a depth of 15 km about 40 Ma ago in the eastern Kiagar La region and 30 Ma ago in the western Nuruchan region. The extrusion rate was of about 3 cm/yr between 53 and 48 Ma, followed by an uplift rate of 1.2 mm/yr between 48 and 30 Ma and of only 0.5 mm/yr after 30 Ma. Geomorphology observations show that the Tso Morari dome is still affected by faults, open regional dome, and basin and pull-apart structures, in a zone of active dextral transpression parallel to the Indus Suture zone.