A 100 m.y. record of volcanic arc evolution in Nicaragua

The processes that result in arc magmas are critical to understanding element recycling in subduction zones, yet little is known about how these systems evolve with time. Nicaragua provides an opportunity to reconstruct the history of a volcanic arc since the Cretaceous. Here we present the stratigraphy of the Cretaceous–Eocene volcanic units in Nicaragua and their relationship to the different tectonic units where the arc developed. We discovered an evolution from an arc‐dominated by calc‐alkaline compositions in the Cretaceous–Eocene, to transitional compositions in the Oligocene–Miocene, to finally tholeiitic magmas common in the modern volcanic front. Our petrographic studies confirm that in the Cretaceous–Eocene the olivine + clinopyroxene cotectic was followed by clinopyroxene + plagioclase ± amphibole. Given the abundance of amphibole and the lack of this mineral in the modern volcanic front, the Cretaceous–Eocene Arc melts were likely more water‐rich than modern Nicaragua, suppressing the crystallization of plagioclase after olivine. We also found temporal changes in element ratios that are sensitive to variations in sediment input. The Cretaceous–Eocene Arc is characterized by a lower Ba/Th compared to the Oligocene–Miocene and modern volcanic front samples, suggesting that the sediment input was lower in Ba, possibly analogous to old deep siliceous sediment subducting in the western Pacific. Both U/Th and U/La are higher in the modern volcanics, reflecting higher U/Th in the subducting sediments following the 'Carbonate Crash'. Finally, we found that the orientation of the arc axis also changed, from northeast‐southwest in the Cretaceous–Eocene to northwest‐southeast after the Oligocene. This change probably records variations in the location of the subduction zone as this region shaped into its current geographic configuration.     

Estimating the thickness of a faulted mechanical layer based on fault spacing: An example from Miyako‐jima Island,southern Ryukyu Arc,Japan

The thickness of the mechanical layer that hosts a group of faults can be estimated from the spacing of saturated faults (i.e. the constant spacing between faults when the fault system is fully developed and has attained its final fault density). We measured fault spacing for a group of saturated active normal faults on Miyako‐jima Island (southern Ryukyu Arc, Japan) and estimated the thickness of the faulted mechanical layer. The measured fault spacing is 1.30 ±0.14 km, and the thickness of the mechanical layer is < 3 km, which is the upper limit of the seismogenic zone. This faulted mechanical layer corresponds to a sedimentary layer in which earthquakes cannot occur. Results indicate that the shallow (< 3 km depth) normal faults on Miyako‐jima Island do not have the potential to cause medium‐size earthquakes as individual faults. The origin of the shallow normal faults might be related to the presence of a larger‐scale, deeper fault. The results indicate that fault spacing provides important information on the potential magnitude of earthquakes associated with active faults.     

山东黄县弧形断裂带断层泥铁元素化学种分布特征及其地震地质意义

本文利用德国Wissel公司生产的新型穆斯堡尔谱仪(Bench-MB500)对山东龙口常庄子村北黄县弧形断裂断层剖面上的多种断层泥样品进行了穆斯堡尔谱测试分析,讨论了该断层剖面上铁元素地球化学分布特征及其地震地质意义。结果显示,该断裂中央部分比较年轻的断层泥相对富集para-Fe~(2+),而两侧较老的断层泥基本由para-Fe~(3+)组成;剖面断层泥样品中铁元素的地球化学特征与断层的活动演化历史密切相关,显示了断裂带内存在复杂的水岩相互作用。     

Early to mid-Paleozoic magmatic and sedimentary records in the Bainaimiao Arc: An advancing subduction-induced terrane accretion along the northern margin of the North China Craton

The early to mid-Paleozoic subduction-induced terrane accretion along the northern margin of the North China Craton is not well understood. To address this issue, we investigate the magmatic and sedimentary records, including both new and previously published geochemical, Sr–Nd isotopic, and zircon U–Pb–Hf isotopic data from the Bainaimiao Arc. The collected gabbro–diorites and granitoids have been dated to 431–453 Ma. The gabbro–diorites have high Mg/(Mg + Fe) molar ratios (44.41–73.39); depleted Nb, Ta and Ti; and negative ε_Nd(t) values (-9.43–-6.80). They were derived from a mantle wedge metasomatized by subduction-derived fluids with crustal contamination. The granitoids are characterized by high silica, low to high K, low Fe and Mg contents, strong fractionation of rare earth elements, and positive ε_Hf(t) values (+1.42–+8.19). They were derived from crustal melts with juvenile additions. The clastic rocks from the Baoerhantu Group and Xibiehe Formation are dominated by early Paleozoic zircons, whereas those from the Bainaimiao Group are dominated by early Paleozoic and Precambrian zircons. Detrital zircon geochronology and field geology confirm their deposition in early to mid-Paleozoic. The U–Pb ages and petrographic and geochemical analyses indicate that the clastic rocks were deposited in arc-related basins with felsic sources from the Bainaimiao Arc. The xenocrystic and detrital zircons in the magmatic and clastic rocks, respectively, imply a Precambrian basement for the Bainaimiao Arc. The early Paleozoic magmatic rocks of the Bainaimiao Arc show secular changes with decreasing age: increasing K₂O contents and Sr/Y ratios and decreasing Fe₂O₃^T + MgO contents and ε_Hf(t) and ε_Nd(t) values. This is likely in response to advancing subduction and related crustal thickening. Accordingly, the following tectono-paleogeographic model was proposed for the Bainaimiao Arc: (a) ∼500–455 Ma initial subduction and juvenile arc development, (b) ∼455–415 Ma continuous subduction with mature arc development, and (c) ∼415–400 Ma accretion to the North China Craton.     

Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane,southeastern Central Asian Orogenic Belt

Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT) is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma) diabases display low SiO_2(46.52-49.24 wt.%) with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm) and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zircon ε_(Hf)(t)(-12.0 to-24.7) and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma) arc intrusions in western BAT can be divided into Periods Ⅰ and Ⅱ at approximately 450 Ma.The Period Ⅰ(450 Ma) intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zircon ε_(Hf)(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the Period Ⅱ(≤450 Ma) intrusions are characterized by variable zircon e_(Hf)(t)(-15.0 to+11.5) with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(45) of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from Period Ⅰ to Ⅱ can be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.     

动态分段思想在公路地理信息系统中的实现

动态分段是一种全新的线性特征的动态分析技术,可以有效地解决公路线性特征的动态分析问题,实现公路图形和属性数据间的双向动态可视化查询。文章以Arc/Info为代表,分析了动态分段技术的原理,结合VB6.0、MapObjects2.2和Access,初步介绍了公路GIS中动态分段的数据结构与算法,并探讨了如何实现公路GIS的过程。     

Origin of high field strength element enrichment in volcanic arcs: Geochemical evidence from the Sulu Arc, southern Philippines

Lavas from the Sulu Arc, southern Philippines, exhibit an enrichment in high field strength elements (HFSE) that represents a departure from the typical volcanic arc geochemical signature. It has been postulated that this relative enrichment arises from metasomatism of mantle wedge peridotites by melts derived from the subducting oceanic lithosphere, through formation of amphibole which subsequently breaks down and enriches the mantle source of parental arc magmas in HFSE. Divergent chemical and isotopic characteristics between Sulu Arc HFSE-enriched lavas and the Sulu Sea crust being subducted—the presumed source of slab-derived melts—render it unlikely, however, that HFSE enrichment arises from the influence of such melts. New geochemical data suggest that the varying degrees of HFSE enrichment in Sulu Arc lavas are instead the result of variable amounts of mixing between enriched and depleted mantle end-components—the sources of South China Sea intraplate lavas and Sulu seafloor basalts, respectively—within a compositionally heterogeneous mantle wedge.     

A rapid method to map the crustal and lithospheric thickness using elevation, geoid anomaly and thermal analysis. Application to the Gibraltar Arc System, Atlas Mountains and adjacent zones

We present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness. The main assumptions are local isostasy and a four-layered model composed of crust, lithospheric mantle, sea water and the asthenosphere. We consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle. We perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of RMS error of elevation and geoid anomaly databases. The application of this method to the Gibraltar Arc System, Atlas Mountains and adjacent zones reveals the presence of a lithospheric thinning zone, SW–NE oriented. This zone affects the High and Middle Atlas and extends from the Canary Islands to the eastern Alboran Basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western Betics, eastern Rif, Rharb Basin, and Gulf of Cadiz. A number of different, even mutually opposite, geodynamic models have been proposed to explain the origin and evolution of the study area. Our results suggest that a plausible slab-retreating model should incorporate tear and asymmetric roll-back of the subducting slab to fit the present-day observed lithosphere geometry. In this context, the lithospheric thinning would be caused by lateral asthenospheric flow. An alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the Canary Island, and extending as far as Central Europe.     

Volatile (S,Cl and F) and fluid mobile trace element compositions in melt inclusions: implications for variable fluid sources across the Kamchatka arc

Volatile element, major and trace element compositions were measured in glass inclusions in olivine from samples across the Kamchatka arc. Glasses were analyzed in reheated melt inclusions by electron microprobe for major elements, S and Cl, trace elements and F were determined by SIMS. Volatile element–trace element ratios correlated with fluid-mobile elements (B, Li) suggesting successive changes and three distinct fluid compositions with increasing slab depth. The Eastern Volcanic arc Front (EVF) was dominated by fluid highly enriched in B, Cl and chalcophile elements and also LILE (U, Th, Ba, Pb), F, S and LREE (La, Ce). This arc-front fluid contributed less to magmas from the central volcanic zone and was not involved in back arc magmatism. The Central Kamchatka Depression (CKD) was dominated by a second fluid enriched in S and U, showing the highest S/K₂O and U/Th ratios. Additionally this fluid was unusually enriched in ⁸⁷Sr and ¹⁸O. In the back arc Sredinny Ridge (SR) a third fluid was observed, highly enriched in F, Li, and Be as well as LILE and LREE. We argue from the decoupling of B and Li that dehydration of different water-rich minerals at different depths explains the presence of different fluids across the Kamchatka arc. In the arc front, fluids were derived from amphibole and serpentine dehydration and probably were water-rich, low in silica and high in B, LILE, sulfur and chlorine. Large amounts of water produced high degrees of melting below the EVF and CKD. Fluids below the CKD were released at a depth between 100 and 200 km due to dehydration of lawsonite and phengite and probably were poorer in water and richer in silica. Fluids released at high pressure conditions below the back arc (SR) probably were much denser and dissolved significant amounts of silicate minerals, and potentially carried high amounts of LILE and HFSE. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

云南中甸地区两个斑岩铜矿容矿斑岩的地球化学特征——以雪鸡坪和普朗斑岩铜矿床为例

位于义敦岛弧南端的中甸岛弧已成为我国又一重要的斑岩铜矿产地,其中雪鸡坪中型斑岩铜矿床产于西斑岩带,普朗超大型斑岩铜矿床产于东斑岩带。详细对比研究了这两个斑岩铜矿床矿床地质,含矿斑岩的岩石化学、地球化学以及年代学等特征,认为:①两者的元素组成非常相似,如稀土配分形式均为轻稀土元素富集型,亏损重稀土元素和Y,富集Sr、K、Rb、Ba、Th等大离子亲石元素,亏损Nb、Ta、Zr、Hf、Ti等高场强元素,显示了埃达克岩的地球化学特征,表明它们可能是俯冲的甘孜—理塘洋壳部分熔融的产物,并受到地壳物质的混染;②中甸岛弧的结构及时空演化保持了义敦岛弧的共性,发育完整的沟-弧-盆体系,其中东斑岩带属于外弧,而西斑岩带属于内弧,中斑岩带及零星分布的属都蛇绿混杂岩产于弧间盆地。