浅谈山西省古元古代末期火山岩的构造背景

对山西古元古代末期火山岩的构造属性进行了研究,旨在为解决现今国内外关于古/中元古界(代)分界问题的分岐提供依据。该期火山岩角度不整合于具活动大陆边缘特征的结晶基底之上。无论是汉高山群(或小两岭组)还是西洋河群(—熊耳群),均呈SEE向展布,熔岩占绝对优势,偶见碎屑岩;以裂隙(串珠状)宁静式溢流喷发为主,同期的基性侵入岩墙群为其岩浆上升的通道。岩性以安山岩-玄武安山岩为主,含少量玄武岩和流纹岩。岩石斑晶为辉石和斜长石,偶见角闪石和黑云母。岩石地球化学富大离子亲石元素和轻稀土元素,亏损高场强元素,w(Th)>w(Ta)。全岩εNd(t)值和εHf(t)值均为负值,在SiO₂-K₂O图解中以SH+HKCA系列为主,为钾质火成岩系列。该期火山岩是古元古代中晚期华北克拉通结晶基底的西部陆块与东部陆块碰撞拼合的延续,为后碰撞火山弧岩浆。     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.     

Delaunay三角网生长法的一种改进方法

为了提高Delaunay三角网的构建速度,本文对三角网生长法进行了改进,提出了一种基于生长法的"弧带搜索排除"方法,通过一系列的圆弧将离散点区域划分成几个弧带,构网时,规定只可在当前弧带搜索第三点,该弧带离散点搜索完毕后进入下一弧带,如此循环。采用VC++6.0加以实现。测试表明,该方法能够加快构网速度,生成的三角形形状良好,具有一定的实际效用。     

天津市新建的具有独特风格的桥梁

天津市近几年来新建了多座具有独特风格和景观效果的新型桥梁,本文主要介绍其中的三座斜拉桥、三座拱桥、一座自锚式悬索桥和一座集交通、观光、游览为一体的巨型摩天轮桥梁,以及三座经装修改建、使旧桥换新颜的实例。     

The Xiong'er volcanic belt at the southern margin of the North China Craton: Petrographic and geochemical evidence for its outboard position in the Paleo-Mesoproterozoic Columbia Supercontinent

The Xiong'er volcanic belt, covering an area of more than 60,000 km² along the southern margin of the North China Craton, has long been considered an intra-continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the Paleo-Mesoproterozoic supercontinent Columbia. However, such interpretations cannot be accommodated by lithology, mineralogy, geochemistry and geochronology of the volcanic rocks in the belt. Lithologically, the Xiong'er volcanic belt is dominated by basaltic andesite and andesite, with minor dacite and rhyolite, different from rock associations related to continental rifts or mantle plumes, which are generally bimodal and dominated by mafic components. However, they are remarkably similar to those rock associations in modern continental margin arcs. In some of the basaltic andesites and andesites, amphibole is a common phenocryst phase, suggesting the involvement of H₂O-rich fluids in the petrogenesis of the Xiong'er volcanic rocks. Geochemically, the Xiong'er volcanic rocks fall in the calc-alkaline series, and in most tectono-magmatic discrimination diagrams, the majority of the Xiong'er volcanic rocks show affinities to magmatic arcs. In the primitive mantle normalized trace-element diagrams, the Xiong'er volcanic rocks show enrichments in the LILE and LREE, and negative Nb–Ta–Ti anomalies, similar to arc-related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge. Nd-isotope compositions of the Xiong'er volcanic rocks suggest that 5–15% older crust has been transferred into the upper lithospheric mantle by subduction-related recycling during Archean to Paleoproterozoic time. Available SHRIMP and LA-ICP-MS U–Pb zircon age data indicate that the Xiong'er volcanic rocks erupted intermittently over a protracted interval from 1.78 Ga, through 1.76–1.75 Ga and 1.65 Ga, to 1.45 Ga, though the major phase of the volcanism occurred at 1.78–1.75 Ga. Such multiple and intermittent volcanism is inconsistent with a mantle plume-driven rifting event, but is not uncommon in ancient and existing continental margin arcs. Taken together, the Xiong'er volcanic belt was most likely a Paleo-Mesoproterozoic continental magmatic arc that formed at the southern margin of the North China Craton. Similar Paleo-Mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of Laurentia, the southern margin of Baltica, the northwestern margin of Amonzonia, and the southern and eastern margins of the North Australia Craton, which are considered to represent subduction-related episodic outbuilding on the continental margins of the Paleo-Mesoproterozoic supercontinent Columbia. Therefore, in any configuration of the supercontinent Columbia, the southern margin of the North China Craton could not have been connected to any other continental block as proposed in a recent configuration, but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the North China Craton.     

New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec

O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ¹⁸O does not increase northwards as would be expected from the ~ 4 fold increase in subduction rate if slab-derived fluids had high ¹⁸O/¹⁶O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust.     

Mafic plutonic rocks in a continental‐arc setting: geochemistry of 1.87–1.78 Ga rocks from south‐central Sweden and models of their palaeotectonic setting

Mafic intrusive rocks (1.79–1.78 Ga) of the Transscandinavian Igneous Belt (TIB) and the c. 1.87 Ga Hedesunda Igneous Complex in the Fennoscandian Shield of south‐central Sweden were studied using whole‐rock and isotope geochemistry. Rock types vary from gabbros/norites (and leucogabbros) to quartz diorites, with Mg# between 76 and 49, and wt% SiO₂ between 43.6 and 59.7, indicating some variation in evolutionary levels and variable cumulus components. Geochemical signatures are calc‐alkaline to shoshonitic, large ion lithophile elements and light rare earth elements enriched and high‐field strength elements depleted of continental‐arc type. ε_Nd(t) ranges between +1.0 and +2.7, and ⁸⁷Sr/⁸⁶Sr(t) between 0.7020 and 0.7038. There is no systematic correlation between chemical parameters and isotope ratios. These isotopic data overlap with other mafic plutonic TIB rocks; samples from the Dala Province (DP) tend to overlap with the c. 1.7 Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios. With two exceptions, the ε_Nd(t) of +1 to +2 conform to an isotopically ‘mildly depleted’ source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above ε_Nd(t) +2.0 are scarce in the TIB. Mantle sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (T_DM c. 2.0 Ga), that is during subduction in the preceding Svecofennian (2.0–1.87 Ga) and/or during the TIB‐0&1 event (1.85–1.78 Ga). The palaeotectonic settings inferred are active continental margins; N–S‐directed convergence at 1.87 Ga and E–W‐directed at 1.79–1.78 Ga. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical modeling of intraoceanic arc growth

We investigate crustal growth processes on the basis of a 2D coupled geochemical—petrological-thermomechanical numerical model of retreating intraoceanic subduction. The model includes spontaneous slab retreat and bending, subducted crust dehydration, aqueous fluid transport, mantle wedge melting, and melt extraction resulting in crustal growth. Our numerical experiments show that (1) the rate of plate retreat influences both the rate of crust formation and composition of newly formed crust; (2) both the intensity of melt extraction and the age of subducted plate affect the volume of newly formed crust. In the present paper we describe possible scenarios of magmatic and geodynamic evolution of the arc system and then compare results with magmatic addition rates of Western Pacific arcs. Published in Russian in Vestnik Moskovskogo Universiteta. Geologiya, 2009, No. 4, pp. 30–43. The text was submitted by the authors in English.     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

Modern-style Subduction Processes in the Archean: Evidence from the Shangyi Complex in North China Craton

Abstract: Three fragments of the Archean oceanic crust have been found between the Archean granulite belt and the Paleo-Proterozoic Hongqiyingzi group in North China craton, which spread along the Shangyi-Chicheng ancient fault. This paper presents integrated field, petrology, geochemistry and geochronology evidence of the ancient oceanic fragments. The magma crystallizing age of the tonalite in the Shangyi complex is 2512±19?Ma and the geochemical characteristics suggest that the Nb-enriched basalts may be related to crustal contamination and formed in the intra-oceanic arc of the supra subduction zone setting.