The interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust,Fiordland, New Zealand

Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition.     

Petrogenesis of Two Types of Archean TTGs in the North China Craton: A Case Study of Intercalated TTGs in Lushan and Non-intercalated TTGs in Hengshan

TTG(Tonalite-Trondhjemite-Granodiorite) gneisses,a major component of Precambrian continental crust,play a significant role in understanding the process and mechanism of the crustal evolution in the early periods of the Earth. In terms of field occurrence,there are two kinds of Archean TTGs in the NCC(North China Craton): intercalated and non-intercalated TTGs. In this contribution,we make a comprehensive comparison of these two types of TTGs from the typical areas(Lushan and Hengshan) in the NCC with an aim to constrain their petrogenesis. The results suggest that they have similar mineral assemblages of Pl + Qtz + Bt ± Amp ± Kfs but different field appearances and geochemical compositions,thus probably reflecting different source materials and tectonic settings. Differences in the contents of characteristic elements,such as Sr,REE and HFSE,suggest that the nonintercalated TTGs in Hengshan were generated at deeper levels than those of intercalated TTGs in Lushan. Constraints from element contents and geochemical modeling results are consistent with derivation from dual sources involving both garnet amphibolite and rutile-bearing eclogite residues for the non-intercalated TTGs in Hengshan,whereas the compositions of intercalated TTGs in Lushan indicate that they were formed by partial melting with amphibolite to garnet-amphibolite residues. Moreover,accumulation of plagioclase is also required in the petrogenesis of intercalated TTGs in Lushan,at least for part of them. In addition,the non-intercalated TTGs in Hengshan display distinctly higher Mg O,Mg#,Cr and Ni values and lower SiO_2 average contents compared to the intercalated TTGs in Lushan. These features suggest that the former magma,at least a part,might have interacted with the mantle wedge during ascent. Considering all the above factors and in combination with the whole-rock Nd and zircon Hf isotopic data,it is suggested that the non-intercalated TTGs in Hengshan were produced by partial melting of subducted slab contaminated by the overlying mantle wedge at deeper levels and high pressures,whereas the intercalated TTGs in Lushan were generated by melting of the thickened lower crust at lower pressures and shallower depths. The tectonic settings of the two types of TTGs shed new light on the growth of the NCC.     

藏北羌塘中部日湾茶卡地区晚三叠世安山岩与闪长质包体岩石成因及地质意义

龙木错-双湖-澜沧江缝合带将羌塘板块划分为南、北2个次级板块,在缝合带两侧大规模发育晚三叠世岩浆作用,并伴随大量的高压超高压变质岩。在缝合带以南的日湾茶卡地区识别出一套安山岩,其中发育大量的暗色闪长质捕虏体,锆石U-Pb测年结果在安山岩中获得了364.7±1.9Ma和223.9±1.3Ma两组年龄信息,锆石原位Hf同位素测试结果εHf(t)分别为+7.22~+8.69和-5.94~-4.14。结合羌塘中部已有的研究成果,认为闪长质捕虏体形成于晚泥盆世洋壳俯冲背景,是地幔楔部分熔融底侵至地壳的产物;而安山岩形成于晚三叠世碰撞后伸展背景,是深部板片断离机制下深俯冲陆壳边缘一侧熔体混染晚泥盆世新生地壳的产物。安山岩中闪长质捕虏体的发现支持了俯冲带作为大陆生长的重要场所的观点。     

Geodynamic setting,structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO₂ contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO₂ (< 1.7 wt.%) and Fe₂O_3tot and higher contents of Al₂O₃ in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.     

Neotectonics of the Kharaulakh sector of the Laptev Shelf

Seismotectonic deformation and crustal stress pattern have been studied comprehensively in major seismogenic structures of the Kharaulakh sector of the Verkhoyansk fold system and adjacent parts of the Chersky seismotectonic zone. The study focuses on neotectonic structures, deep structure, and systems of active faults, as well as tectonic stress fields inferred by tectonophysical analysis of Late Cenozoic faults and folds. The results, along with geological and geophysical data, reveal main strain directions and structural patterns of crustal stress and strain in the Arctic segment of the Eurasia-North America plate boundary. The area is a junction of mid-ocean and continental structures evolving in a mixed setting of extension, compression, and their various combinations. The rotation pole of the two plates is presumably located near Buor-Khaya Bay. In this case, extension is expected to act currently upon the neotectonic structures north of the bay and compression to control those in the south and southeast. This inference is consistent with the identified zoning of stress and strain in the Kharaulakh sector.     

Early Cretaceous K-rich rhyolites from the Duolong Cu–Au deposit,southern Qiangtang,China: evidence for crustal growth

We report new zircon U–Pb age, Hf isotopic, and major and trace element data for rhyolites from the Duolong Ore Concentration Area of the Southern Qiangtang Terrane. Building on previous studies, we constrain the tectonic setting and propose a model to explain the geodynamics and crustal growth during regional magmatism in the Early Cretaceous. The analysed rhyolites yield laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages of 115 and 118 Ma. The rocks are K-rich (K₂O = 6.66–9.93 wt.%; K₂O/Na₂O = 8.2–19.7 wt.%), alkaline and peraluminous (A/CNK = 1.02–1.46), and are characterized by high SiO₂ contents (72.8–78.8 wt.%) similar to highly fractionated I-type granites. Fractionation of Fe–Ti oxides, plagioclase, hornblende, Ti-bearing phases, apatite, monazite, allanite and zircon contributed to the variations in major and trace element chemistry. High K₂O contents are likely due to partial melting of the continental crust. The samples have positive zircon εHf(t) values ranging from +7.1 to +11.2. These features, together with young zircon Hf crustal model ages of 489–721 Ma, indicate that the K-rich rhyolites were derived from juvenile lower crust with an input of a mantle-derived component. We suggest that the Early Cretaceous K-rich rhyolites formed in a continental arc setting during northward subduction of Bangong Co–Nujiang oceanic lithosphere. Basaltic magma underplating was responsible for vertical crustal growth, triggered by slab roll-back in the Duolong Ore Concentration Area in the Early Cretaceous.     

武夷山中段司前花岗岩体形成时代、地球化学特征及地质意义

本文对武夷山中段的司前岩体进行了系统的岩石学、地球化学、年代学及Lu-Hf同位素研究。结果表明,司前岩体形成于(140±1)Ma,主要由黑云母二长花岗岩组成,富碱(K_2O+Na_2O=6.67 wt%~8.25 wt%),富钾(K_2O/Na_2O=1.16~2.41),A/CNK值介于1.01~1.25之间。岩体具有较高的∑REE(177.73×10~(-6)~427.88×10~(-6))、Zr+Nb+Y+Ce含量(262.6×10~(-6)~581.5×10~(-6))和Zr饱和温度(平均824°C),FeOt/MgO(3.06~3.93)和10 000×Ga/Al(2.64~3.28)比值均较高,属典型的铝质-过铝质A型花岗岩。岩体的锆石εHf(t)值均为负值(–18.6~–7.9),暗示其源于古老的地壳物质重熔。综合上述结果和区域背景推测,司前岩体的源岩为新元古代麻源群变质火山-碎屑岩,源岩可能经历早古生代和白垩纪两期熔融事件,地幔岩浆为花岗岩的形成提供了热源,但并未贡献物质,岩体的形成与古太平洋板块俯冲过程中因俯冲板片后撤诱发的弧后扩张作用有关。     

Cenozoic Crustally-derived Carbonate-rich Magmatic Rocks in West Junggar,North Xinjiang,Western China: Geochronology,Geochemistry and Tectonic Implications

The carbonate-rich magmatic rocks of West Junggar are distributed in the Baijiantan and Darbut ophiolitic mélanges in the forms of extrusive rocks overlying the mélanges and dykes, either along the margins of the mélange or cross-cutting components of mélanges. Chilled margin and flow structures are present. A SHRIMP zircon U-Pb age of 39.7 ± 1.3 Ma indicates that these carbonate-rich rocks in West Junggar were formed during the Eocene. They have low concentrations in REEs, Th, U, Nb, Ta and are characterized by extremely low εN d(t), high(87 Sr/86 Sr)i ratios, relatively high δ18 OV-SMOW values and high δ13 CV-PDB values, which is similar with most sedimentary carbonates. Furthermore, no contemporaneous mantle-derived silicate rocks have yet been found in West Junggar. The carbonate-rich rocks in West Junggar are thus distinct from mantle-derived carbonatites and are interpreted to result from melting of the Carboniferous sedimentary carbonates at crustal levels, these rocks therefore being referred to as 'crustal carbonatites'. The Eocene crustal carbonatites in West Junggar and other Cenozoic magmatic rocks in North Xinjiang are generally situated along regional strike-slip faults or fault intersections. Therefore, we propose that the reactivation of the Darbut and Baijiantan crustal-scale strike-slip fault zones(ophiolitic mélanges), due to the far-field effects of the Indian-Eurasian collision, enables decompression melting of the underlying continental lithospheric mantle. These resulting melts ascended to the lower crust through the strike-slip faults, causing partial melting of the Carboniferous carbonaceous sediments. The crustal carbonatites in West Junggar provide a new piece of evidence for Cenozoic magmatism in North Xinjiang and are also significant for the investigation of tectono-magmatic relations in North Xinjiang and the Central Asian Orogenic Belt.     

基于GPS的青藏块体东北缘近期地壳水平运动与变形研究

以"中国地壳运动观测网络"工程1999-2001年GPS观测数据以及"大陆强震机理与预测"国家基础研究项目青藏块体东北缘GPS网1999,2001,2003年的观测资料为基础,应用GAM IT/GLOBK软件对数据进行精密解算,获得了青藏块体东北缘1999-2001年相对于鼎新基准站以及欧亚板块的水平运动速度场。结合板块构造以及大地动力学理论,对青藏块体进行了块体的划分,分析了块体间的相互作用,并进一步分析了该地区的地壳水平运动特征,得出了以下结论:(1)GPS技术用于地壳运动的监测是可行的;(2)1999-2001年期间,祁连-海原断裂带GPS运动位移与地质结果十分吻合;(3)印度板块与欧亚板块的碰撞以碰撞后印度大陆继续以50 mm/a的速度向北推进是青藏块体东北缘的运动动力;(4)甘青块体西部子块体以北东向挤压运动为主;东部子块体主要表现为以海原断裂带为北边界的顺时针旋转为主;(5)鄂尔多斯块体存在逆时针旋转运动,其旋转速度在块体西南侧的六盘山断裂带一带最大,向北至块体西北部明显减小。     

秭归龙会观5.1级震区及其邻区新生代以来的地壳应力场特征

为了研究三峡坝库区的地壳稳定性,根据奥陶－侏罗纪地层中构造变形的实际观测资料,计算了秭归龙会观５．１级震区及其邻区新生代以来区域应力场的主应力方向。结果表明：主压应力为北东东向,主张应力为北北西向;现代构造应力场与新生代以来应力场基本一致;晚第三纪以来,该区构造应力场相对稳定和统一。