云南腾冲全新世火山岩岩浆演化和岩石成因

腾冲火山岩区是我国全新世以来记载火山喷发的少数地区之一,该地区岩浆作用的性质与成因是揭示青藏高原东缘的现今侧向生长过程与深部作用的重要依据。本文对腾冲火山岩区的马鞍山、黑空山、打鹰山全新世火山岩开展了矿物化学和岩石地球化学研究,以期揭示岩石成因和深部动力学过程。腾冲全新世火山岩主体岩性为高钾钙碱性系列的玄武粗安岩和粗安岩。岩石的Ca O、Fe_2O_3~T、Ti O_2与Si O_2负相关,而K_2O与Si O_2正相关,表明岩浆演化过程中可能存在橄榄石、辉石和斜长石的分离结晶作用。岩石中存在酸性斜长石(更长石,An=28)大颗粒捕掳晶,其边部发育了基性斜长石(拉长石,An=65)增生边;在大颗粒石英捕掳晶的边部发育了辉石的反应边,这些结构表明在岩浆上升到地壳浅部时,曾受到了花岗岩围岩的混染,但岩石的Th/Nb值均小于1.16,表明地壳混染总体不显著。腾冲全新世火山岩大离子亲石元素富集、高场强元素相对亏损,高Th/U、低Ba/La,富集Sr-Nd同位素,其岩浆源区应为经历过洋壳沉积物交代后的富集地幔。腾冲火山岩属于大陆板内环境,是印度与亚洲大陆碰撞后岩浆作用的产物。火山岩是沿着腾冲盆地南北向展布,且熔岩分布面积有限。由于高原侧向生长过程中的区域性走滑断裂会引起局部的伸展,腾冲火山岩产出可能与富集岩石圈地幔的减压熔融有关。     

青藏高原申扎-双湖剖面岩石圈电性结构特征及其含义

为了研究班公湖-怒江缝合带的壳幔电性结构及构造特征,并为其俯冲极性提供电性约束,对青藏高原中部申扎-双湖大地电磁测深剖面进行全面数据处理分析,获得了可靠的二维电性结构模型,研究表明:沿剖面上地壳分布的是规模不等的高阻体,底面埋深在10~25 km变化,高阻层之下发现由不连续的高导体构成的中下地壳高导层.通过对电性结构的分析,认为班公湖-怒江特提斯洋的俯冲消亡极性可能是双向的,随后拉萨-羌塘地体碰撞带处的上地壳高阻体发生拆沉,以上两次动力学事件可能共同作用于缝合带处的壳幔高导体,同时北拉萨地体的壳幔高导体还可能体现了构造作用、岩浆活动和成矿作用之间的关系.     

Crystallization Conditions and Mineral Chemistry in the East of Tafresh,Central Iran,with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K-feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I-type, metaluminous and belong to the calc-alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas. Most crystal size distribution(CSD) line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.     

板块俯冲起始与成熟阶段的差异变质响应

板块俯冲起始是俯冲带最不为人知的关键过程之一,对理解全球板块构造如何启动有重要意义。本文综合了最新的数值模拟、变质岩石学和年代学研究结果,探讨板块俯冲起始到成熟阶段的热结构和变质作用差异。从俯冲起始到成熟阶段,俯冲带热结构由"热"变"冷",初始俯冲更可能形成的是角闪岩或麻粒岩,形成于热俯冲环境,以逆时针p-t轨迹为主,通常伴随板块部分熔融过程;而成熟阶段形成的主要是低温蓝片岩和榴辉岩,形成于冷俯冲环境,具有典型的顺时针p-t轨迹,以板块变质脱水为主。对俯冲带高t/p变质岩(如变质底板)的进变质过程、时间以及构造背景等方面需进一步加强研究,以获取更详实的现代板块俯冲起始阶段的地质过程。     

大别-苏鲁造山带橄榄岩记录的碰撞造山过程中地幔楔的地壳交代作用

地壳交代作用是洋壳俯冲带板片-地幔界面的普遍现象,由于地幔楔样品的缺乏,其识别存在困难。而碰撞造山带广泛出露的地幔楔来源的造山带橄榄岩则是理想的研究对象。本文对大别-苏鲁造山带橄榄岩的已有研究成果进行了系统总结。这些成果表明这些橄榄岩在大洋俯冲向大陆碰撞转换的不同阶段经历了多期地壳来源流体的交代作用。地壳交代作用不仅改变了地幔楔橄榄岩的地球化学成分,而且导致了交代矿物的生长以及超镁铁质交代体的形成。这些交代体或作为同碰撞和碰撞后镁铁质火成岩的地幔源区,或将地壳组分传输到深部地幔,或释放流体交代大陆俯冲隧道中的俯冲陆壳。本文对地壳交代作用研究中存在的重要问题和解决思路提出了建议。     

Single subduction zone for the generation of Devonian ophiolites and high‐P metamorphic belts of the Variscan Orogen (NW Iberia)

Within the Variscan Orogen, Early Devonian and Late Devonian high‐P belts separated by mid‐Devonian ophiolites can be interpreted as having formed in a single subduction zone. Early Devonian convergence nucleated a Laurussia‐dipping subduction zone from an inherited lithospheric neck (peri‐Gondwanan Cambrian back‐arc). Slab‐retreat induced upper plate extension, mantle incursion and lower plate thermal softening, favouring slab‐detachment within the lower plate and diapiric exhumation of deep‐seated rocks through the overlying mantle up to relaminate the upper plate. Upper plate extension produced mid‐Devonian suprasubduction ocean floor spreading (Devonian ophiolites), while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication. Accretion of the remaining Cambrian ocean heralded Late Devonian subduction of inner sections of Gondwana across the same subduction zone and the underthrusting of mainland Gondwana (culmination of NW Iberian allochthonous pile). Oblique convergence favoured lateral plate sliding, and explained the different lateral positions along Gondwana of terranes separated by Palaeozoic ophiolites.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Stratigraphic architecture of Solander Basin records Southern Ocean currents and subduction initiation beneath southwest New Zealand

Solander Basin is characterized by subduction initiation at the Pacific‐Australia plate boundary, where high biological productivity is found at the northern edge of the Antarctic Circumpolar Current. Sedimentary architecture results from tectonic influences on accommodation space, sediment supply and ocean currents (via physiography); and climate influence on ocean currents and biological productivity. We present the first seismic‐stratigraphic analysis of Solander Basin based on high‐fold seismic‐reflection data (voyage MGL1803, SISIE). Solander Trough physiography formed by Eocene rifting, but basinal strata are mostly younger than ca. 17 Ma, when we infer Puysegur Ridge formed and sheltered Solander Basin from bottom currents, and mountain growth onshore increased sediment supply. Initial inversion on the Tauru Fault started at ca. 15 Ma, but reverse faulting from 12 to ca. 8 Ma on both the Tauru and Parara Faults was likely associated with reorganization and formation of the subduction thrust. The new seabed topography forced sediment pathways to become channelized at low points or antecedent gorges. Since 5 Ma, southern Puysegur Ridge and Fiordland mountains spread out towards the east and Solander Anticline grew in response to ongoing subduction and growth of a slab. Solander Basin had high sedimentation rates because (1) it is sheltered from bottom currents by Puysegur Ridge; and (2) it has a mountainous land area that supplies sediment to its northern end. Sedimentary architecture is asymmetric due to the Subtropical Front, which moves pelagic and hemi‐pelagic sediment, including dilute parts of gravity flows, eastward and accretes contourites to the shelf south of Stewart Island. Levees, scours, drifts and ridges of folded sediment characterize western Solander Basin, whereas hemi‐pelagic drape and secondary gravity flows are found east of the meandering axial Solander Channel. The high‐resolution record of climate and tectonics that Solander Basin contains may yield excellent sites for future scientific ocean drilling.     

Extensional tectonics during the Tyrrhenian back‐arc basin formation and a new morpho‐tectonic map

We present a new tectonic map focused upon the extensional style accompanying the formation of the Tyrrhenian back‐arc basin. Our basin‐wide analysis synthetizes the interpretation of vintage multichannel and single‐channel seismic profiles, integrated with modern seismic images, P‐wave velocity models, and high‐resolution morpho‐bathymetric data. Four distinct evolutionary phases of the Tyrrhenian back‐arc basin opening are further constrained, redefining the initial opening to Langhian/Serravallian time. Listric and planar normal faults and their conjugates bound a series of horst and graben, half‐graben and triangular basins. Distribution of extensional faults, active throughout the basin since Middle Miocene, allows us to define an arrangement of faults in the northern/central Tyrrhenian mainly related to a pure shear which evolved to a simple shear opening. At depth, faults accommodate over a Ductile‐Brittle Transitional zone cut by a low‐angle detachment fault. In the southern Tyrrhenian, normal, inverse and transcurrent faults appear to be related to a large shear zone located along the continental margin of the northern Sicily. Extensional style variation throughout the back‐arc basin combined with wide‐angle seismic velocity models allows to explore the relationships between shallow deformation, faults distribution throughout the basin, and crustal‐scale processes as thinning and exhumation.     

吴尚时对中国近代地理学的贡献与学术思想探讨

吴尚时（1904-1947）是中国近代地理学重要开创者之一。1928年吴尚时获金牌毕业于中山大学,次年考取公费前往法国学习地理学。他先后就读于里昂大学和格日诺布尔大学,授业于A. 阿历克斯,M.巴尔台和R.布朗夏尔等,布氏是维达尔·布拉什知名弟子。1934年吴氏学成归国,任中山大学地理系教授,1939-1945年任地理系主任。时逢日军侵华,学校不断搬迁逃避战火,华南、西南千里迁徙之途成为先生的野外考察路线,这需要非同一般的坚毅意志和对科学的献身精神。先生学术文稿泰半完成于此艰危时期,过度的劳累和极度困苦的环境严重损害了他的健康。1947年,先生去世,年仅43岁。