拉萨地块松多榴辉岩的变质演化过程:NCKMnFMASHTO体系中的相平衡关系

拉萨地块松多榴辉岩主要矿物组合为石榴子石、绿辉石、角闪石、多硅白云母、绿帘石、金红石。石榴子石环带不明显,核部成分均一,从核部到边部,镁铝榴石和钙铝榴石含量降低,可能分别记录了榴辉岩峰期及退变质过程信息。绿辉石显示微弱的成分环带,硬玉含量从核部到边部略有升高,部分绿辉石边部发育韭闪石退变质边,反映了在减压过程中外来流体进入体系的过程。多硅白云母具有高的Si含量(3.5~3.6),其中石榴石包体中的多硅白云母相对基质中的白云母有更高的Si值。本文利用Thermocalc变质相平衡模拟软件,结合详细岩相学观察,在NCKMn FMASHTO体系下,模拟松多含多硅白云母榴辉岩的变质演化过程。其中,榴辉岩峰期矿物组合为g+o+law+phn+ru,石榴石核部最大镁铝榴石值和石榴子石包体中多硅白云母最大Si值确定的榴辉岩峰期温压条件约为620℃,32×105Pa,榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明拉萨地块松多榴辉岩经历了超高压变质作用过程,并经历了相对快速的折返过程到中部地壳层次。     

Formation and Evolution of Early Precambrian Continental Crust in Alxa Block

By the analysis of the published zircon U-Pb ages and Hf isotope data, this paper firstly presents a comprehensive review about the staggered growth and reworking of early Precambrian continental crust in Alxa Block. The results show that the ancient crustal remnants of Alxa Block was formed in Meso-Paleo Archean, which was recorded by 3.0~3.6 Ga detrital zircons and Hf model ages. The early crustal growth of Alxa Block could be traced back to early Paleo-archean. Currently, the oldest zircon U-Pb age is about 3.6 Ga. Analogous to the other places of North China Craton, the Alxa Block underwent two-stage crustal growth at 2.7~2.9 Ga and 2.5~2.6 Ga respectively, and the former might be wider. The ~2.5 Ga (TTG) tectono-magmatic event, which represents the North China continent’s cratonization, also existed in Alxa Block. The corresponding zircon Hf isotope data indicate that the TTGs were mainly derived by melting of 2.7~2.9 Ga juvenile crust, possibly by mixing with a certain ancient crust, and a small portion was produced by instant reworking of 2.5~2.6 Ga juvenile crust. Proceeding to Paleo-proterozoic, the Alxa Block underwent multi-stage tectono-magmatic events, approximately peaked at 2.30~2.35 Ga, 2.15~2.17 Ga, 2.00~2.10 Ga, 1.95~1.98 Ga and ~1.90 Ga. The continental crust was mainly formed by reworking of 2.7~2.9 Ga and 2.5~2.6 Ga juvenile crust, simultaneously by a fraction of ~2.1 Ga juvenile crust. In Paleo-proterozoic, not only the Archean crustal reworking but also the juvenile crustal growth existed in Alxa Block.     

钦-杭结合带在中生代构造转折事件以前的板块构造机制

钦州湾—杭州湾结合带是位于扬子与华夏两大古陆块中间的巨型构造结合带,在演化成西太平洋活动大陆边缘之前,经历了多期次的构造-岩浆事件。扬子和华夏板块于新元古代通过四堡造山运动(1 000~880 Ma)沿江山—绍兴断裂,经赣东北、湘中至钦州湾地区发生碰撞-拼合事件,拼合界线大致位于钦州湾—杭州湾结合带内,形成"两陆夹一盆"的主要格局。后碰撞过程经历了造山后岩浆活动和大陆拉张裂解两个过程,在结合带形成广阔的拉张盆地,加里东期(460~410 Ma)以及印支期(250~200 Ma)发生的碰撞-拼合事件导致扬子和华夏地块多次再造,引发强烈的构造-岩浆活动,并形成了华南统一的沉积环境。受西太平洋板块俯冲影响,华南地区中生代构造转折事件(125~140 Ma)使华南地区主要构造背景由碰撞挤压调整为岩石圈减薄,成为华南最重要的岩浆活动和成矿期。根据内部结构的不均一性和演化历史差异,钦州湾—杭州湾结合带可分为北段、中段和南段3段。其中,中段与传统南岭大体一致;北段为南岭以北地区,即绍兴—江山—萍乡一带;南段为南岭以南地区,大致与云开隆起—十万大山盆地相当。年代学和地球化学研究显示,在云开地块西缘的一系列变质基性岩、超基性岩和变质基性火山岩形成于新元古代洋中脊(MORB)或者岛弧(ITA)特征的构造环境,最近在岑溪一带发现形成于加里东期(441 Ma)的变质火山岩同样具有MORB型地球化学特征。在十万大山两侧发现早中生代的酸性火山岩和流纹岩具有典型岛弧型火山岩地球化学特征。可见结合带南段曾经存在古老洋壳,先后经历了新元古代、加里东期和印支期的碰撞造山事件,与北段演化历史具有一致性。     

华北地块南缘花山、五丈山岩体LA-ICP-MS锆石U-Pb年龄、地球化学特征及成因

花山复式岩体位于华北地块南缘,采用LA-ICP-MS技术测得复式岩体中的中细粒似斑状黑云二长花岗岩的锆石U-Pb年龄为128±1Ma,此年龄被解释为成岩年龄。该岩体及邻近的五丈山岩体中花岗岩具有高硅、富碱特征,为准铝-弱过铝质高钾钙碱性系列;在稀土和微量元素上,富集LREE、Rb、Ba、K、Sr等大离子亲石元素,亏损HREE、Zr、Hf、Ta、Nb、P、Ti等高场强元素,总体上Eu异常不明显,显示出I-A过渡型花岗岩的特征。锆石Lu-Hf同位素组成表明,花山花岗岩的ε_Hf(t)主要变化于-25.8~-19.6之间,t_DM2为2.81~2.43Ga,显示源区物质以古老的壳源物质为主,有年轻组分的参与。与岩体外围多金属成矿年龄的对比分析发现,外围金、钼矿床与花山复式岩体的年龄一致,比五丈山岩体晚20~25Ma,表明金、钼矿的形成与花山岩体在时空上联系密切,该岩体可能为成矿提供了热源。     

Metamorphic evolution of granulites in the Minto Block, northern Québec: extraction of peak P-T conditions taking account of late Fe-Mg exchange

In the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene-bearing plutonic suites (granite-granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite-grade mineral assemblages are observed in the paragneiss: garnet (Grt)-orthopyroxene (Opx)-plagioclase-quartz (GOPQ) and garnet (Grt)-cordierite (Crd)-sillimanite-plagioclase-quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d? C, significantly higher than garnet-orthopyroxene Fe-Mg exchange temperatures of 700 ± 50d? C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt-Crd thermometry records temperatures of 750±50d? C. Peak P-T conditions range from 5-6 kbar and 750-800d? C in the Goudalie and eastern Lake Minto domains, to 7-10 kbar and 950-1000d? C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4-5 kbar) across the same area. The GOPQ rocks are inferred to record earlier P-T conditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. Partial P-T paths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe-Mg resetting, document cooling of 100-250d? C from thermal-peak conditions, concomitant with a modest pressure decrease of 2-3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higher P-T conditions recorded by GOPQ assemblages to lower P-T conditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similar P-T conditions of equilibration, the crust may not have been thickened as much as further west, where GOPQ P-T conditions are significantly higher than those of the hornblende granodiorites and the GCSPQ rocks.     

S-wave velocity and Poisson''''s ratio structure of crust in Yunnan and its implication

Receiver function of body wave under the 23 stations in Yunnan was extracted from 3-component broadband digital recording of teleseismic event. Thus, the S-wave velocity structure and distribution characteristics of Poisson's ratio in crust of Yunnan are obtained by inversion. The results show that the crustal thickness is gradually thinned from north to south. The crustal thickness in Zhongdian of northwest reaches as many as 62.0 km and the one in Jinghong of further south end is only 30.2 km. What should be especially noted is that there exists a Moho upheaval running in NS in the Chuxiong region and a Moho concave is generally parallel to it in Dongchuan. In addition, there exists an obvious transversal inhomogeneity for the S-wave velocity structure in upper mantle and crust in the Yunnan region. The low velocity layer exists not only in 10.0-15.0 km in upper crust in some regions, but also in 30.0-40.0 km in lower crust. Generally, the Poisson's ratio is on the high side, however it has a better co     

Transition from platemargin to intraplate environment: Geochemistry of basalts in Paleogene Liaohe basin,northeastern China

Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …     

Syntectonic emplacement of the Middle Jurassic Concón Mafic Dike Swarm, Coastal Range, central Chile (33° S)

The Concón Mafic Dike Swarm (CMDS) consists of basaltic to andesitic dikes emplaced into deformed Late Paleozoic granitoids during the development of the Jurassic arc of central Chile. The dikes are divided into an early group of thick dikes (5–12 m) and a late group of thin dikes (0.5–3 m). Two new amphibole ⁴⁰Ar/³⁹Ar dates obtained from undeformed and deformed dikes, constrain the age of emplacement and deformation of the CMDS between 163 and 157 Ma. Based on radiometric ages, field observations, AMS studies and petrographic data, we conclude that the emplacement of the CMDS was syntectonic with the Jurassic arc extension and associated with sinistral displacements along the NW-trending structures that host the CMDS. The common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously.The early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep-seated coastal rocks in the Early Cretaceous (around 100 Ma). The remanent magnetization in late thin dikes is mainly retained by small amounts of low-Ti magnetite at high temperature and pyrrhotite at low temperature. The magnetization in these dikes appears to be primary in origin. Paleomagnetic results from the thin dikes also indicate that the whole area was tilted  23° to the NNW during cooling of the CMDS.The NNW–SSE extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the Middle–Late Jurassic of northern Chile, pointing to major heterogeneities along the margin of the overriding plate during the Mesozoic or differences in the mechanisms driving extension during such period.     

拉萨紫外辐射特征分析及估算公式的建立

本文利用2005~2010年的辐射观测资料对拉萨地区紫外辐射的时间变化特征及紫外辐射与总辐射比值的变化特征进行了分析,结果表明,紫外辐射与太阳总辐射的变化规律基本一致,日变化表现为正午大、早晚小;季变化特征是夏季6月最大,冬季1月最小.紫外辐射日累积值6年平均为0.87 MJ·m-2·d-1;紫外辐射有逐年递减的趋势.紫外辐射与总辐射比值也存在着明显日变化,表现为正午大、早晚小的规律;其季节变化也是夏季最大,冬季最小.紫外辐射与总辐射比值6年平均为0.0418;紫外辐射与总辐射比值也呈现逐年递减的趋势.利用2010年大气质量数和晴空指数,建立了适合于拉萨紫外辐射估量的公式,估算值的瞬时值与观测值的平均相对误差最大为8.66%,紫外辐射日累积重构值与观测值平均相对误差仅为5.5%.     

Light hydrocarbons geochemistry of surface sediment from petroliferous region of the Mehsana block,North Cambay Basin

A study was carried out to test the usefulness of surface geochemical methods as regional evaluation tools in petroliferous region of the Mehsana block, North Cambay Basin. A suite of 135 soil samples collected from the depth of 2.5 m, were analyzed for adsorbed light gaseous hydrocarbons and carbon isotopes (δ¹³C_methane and δ¹³C_ethane). The light gaseous hydrocarbon analysis show that the concentration ranges 402 ppb, 135 ppb, 70 ppb, 9 ppb and 18 ppb of C₁, C₂, C₃, iC₄ and nC₄, respectively. The value of carbon isotopic ranges of methane −29.5 to −43.0‰ (PDB) and ethane −19.1 to −20.9‰ (PDB). This data, when mapped, indicates patterns coinciding with major known oil and/or gas field of Sobhasan/Linch in this study area. The existence of un-altered petroliferous microseeps of catagenetic origin is observed in the study area. A regional study, such as the one described here, can provide important exploration facts concerning the regional hydrocarbon potential in a block. This method has been confirmed and can be applied successfully in frontier basins.