Weathering of volcanic tuff rocks caused by moisture expansion

Moisture expansion in natural building stones is considered one of the most important factors affecting their weathering and deterioration. The processes that may be responsible for the expansion under determinate relative humidity (hygric dilatation) and water-saturated conditions (hydric dilatation) are generally attributed to the presence of swellable clay minerals. In contrast to this assumption, our investigations show that moisture expansion also takes place in volcanic tuff building stones almost free from clay minerals. To provide a deeper understanding of the processes, swelling and deterioration were performed on 14 volcanic tuffs used as important building stones of different ages, compositions and weathering stages from Mexico, Germany and Hungary. The investigations undertaken include extensive chemical, petrophysical and fabric analyses. The samples show a wide range of effective porosity, microporosity, capillary water absorption, moisture expansion, and CEC values. High moisture expansion does not seem to depend on clay mineral content alone. We also observed that there is no significant effect on dilatation if clay minerals are present but only form a thin coat on the outer shell of bigger pores. Moreover, we identified a correlation between microporosity, average pore radius and moisture expansion. The investigations highlight the fact that moisture expansion cannot only be attributed to swellable clay minerals, and suggest that the presence and accumulation of micropores and their average radius and distribution play an important role for non-clay associated swelling intensity, which can most probably be attributed to the disjoining pressure.     

Complementary trace-element fractionation in volcanic and plutonic rocks: imperfect examples from ocean-floor basalts and gabbros

A simple trace-element model that is generalized from one devised by Albarede (1976) can be used to treat plutonic rocks as mixtures of cumulate phases and trapped liquid. The model is based on the Rayleigh fractionation law, mass balance, and modal data and, with certain assumptions, can give estimates of the mass fraction of the liquid component (X ^l) in a cumulate rock and of some of the characteristics of that liquid, including trace-element concentrations and the mass fraction of the original liquid (F) that remained in the magma chamber when some of it was trapped. Application of the model to Cr and Zr data for ten dredged gabbros from the Oceanographer Fracture Zone (OFZ) in the North Atlantic yields results in the range X ^l=0.014–0.17 and F=0.04–0.22. A more complex model extends the equations developed by Greenland (1970) and combines them with constraints from 1-atm experiments on ocean-floor basalts to effectively map phase-saturation boundaries onto trace-element variation diagrams. On a Zr-Sr plot, the model is capable of mimicking mixing chords and differences in liquid lines of descent (or parental compositions) inferred from major-element chemistry for several OFZ basalts. Qualitatively, the associated gabbros can also be rationalized as cumulate/melt mixtures on this diagram, although the allowable range of liquid compositions is large. In addition, there is some suggestion that the Zr-Sr liquid line of descent fitted to the OFZ basalts may have more general relevance to other ocean-floor rocks (e.g., Galapagos and FAMOUS glasses).     

Thermal simulation of basic volcanic fluid influence on different source rocks

Based on thermal simulation experiment, interactions between volcanic fluids and source rocks were studied. Gas generations in the dry system and fluid system under different temperatures were analyzed. The results showed that the various types of source rocks are similar in composition, containing gaseous C1-C5 hydrocarbons, H2 and CO2 whose gas yields increase with increasing temperature. The gas yield of source rocks of type I is the high- est, followed by type II, and that of source rocks of type III is the lowest, indicating that the yield of hydrocarbon gases is related to their hydrocarbon generating potential. Although the generating potential of type III is the lowest, it can still be regarded as a useful gas source when it is buried deeply enough. The basic volcanic fluid restrains the generation of gaseous hydrocarbons in different types of source rocks, but promotes the generation of inorganic gases.     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

大陆亚碱性火山岩的成因多样性：以敦化—密山和东宁火山岩带为例

对东北牡丹江海浪、鸡西鸡林、东宁老黑山三处亚碱性玄武岩类进行了柏A卜”Ar定年和元素与Sr-Nd-Pb同位素组成研究,结果显示,始新世海浪（玄武）安山岩属钙碱性系列,相对富硅碱,贫铁钙,高度富集Rb、Ba、Sr,亏损Th、U、Nb、Ta,富集LREE及极低的HREE含量,与五大连池钾质火山岩相近的同位素组成（^206Pb／^204Pb=16．56～16．66,^207Pb／^204Pb=15．44—15．47,^208Pb／^204Pb=36．80—36．95;ISr=0．704882～0．705564;εNd=-4．05～2．29）,表明来源于较厚的、受交代作用影响的含石榴石富集（LoMu）岩石圈地幔;中中新世鸡林拉斑玄武岩分布极为局限,辉石斑晶发育骸晶结构,富铁、钙、钛,不亏损Nb、Ta,富集Ba、Sr,REE相对平坦,HREE高于OIB,Sr、Nd同位素组成相似于Samoa岛玄武岩,显示源区除软流圈成分外,还有EMII富集组分的加入;晚中新世老黑山拉斑玄武岩,低碱低钾,LREE轻度富集,Nb、Ta不明显亏损,同位素比值与镜泊湖一带中新世碱性玄武岩范围一致,主要来源于软流圈并与富集岩石圈（EMI）发生过相互作用。地幔源区经历了古近纪富集地幔源到中新世软流圈组分增多的演化。东北新生代拉斑玄武岩不同的地球化学特征为认识大陆拉斑玄武岩成因的多样性提供了有益启示。     

Obtaining equilibrium oxygen isotope fractionations from rocks: theory and examples

A generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope closure temperatures. Examples using literature data and new analyses from 32 natural amphibolite-facies schists demonstrate both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks. By screening samples according to the theoretical models, natural data are shown to have highly consistent mineral fractionations (±2σ reproducibilities of ±0.16 to 0.54‰) that within uncertainty reproduce experimental determinations among the minerals quartz, biotite, muscovite, and calcic amphibole. This correspondence indicates that the proposed theoretically-based selection criteria improve the likelihood of measuring equilibrium fractionations. The new data further corroborate the expected progressive enrichment of δ¹⁸O in the orthosilicates with increasing Al+Si relative to Fe+Mg: Δ(Ky-Grt) ∼1.05‰, Δ(St-Grt) ∼0.6‰, and Δ(St-Cld) ∼0.3‰ at 525–575 °C. In contrast, typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz, biotite, and amphibole that are strongly disequilibrium because of exchange during cooling. Theoretical screening of samples prior to isotope analysis allows robust, independent assessment of theoretical and experimental determinations of equilibrium isotope fractionations. Received: 14 January 1997 / Accepted: 9 March 1998     

伊宁地块石炭纪火山岩及其对构造演化的约束

西天山伊宁地块的构造格局及其演化之认识久存争议,倍受关注。分歧焦点有三:(1)石炭纪有无洋盆存在?(2)若有洋盆,何时闭合?(3)"沟-弧-盆"消亡时究竟是单向俯冲还是双向俯冲?若是单向俯冲,俯冲方向是由北向南或是相反(即俯冲极性)?因此,这些重大地质问题均聚焦于本区的火山岩。石炭纪火山岩是伊宁地块中的最主要建造和指示构造演化的关键层位,争论长久而激烈。本文认为,伊宁地块早石炭世发育弧前-岛弧-弧后盆地钙碱性火山-沉积建造,是塔里木板块北缘的主要组成部分;而晚石炭世碱性系列双峰式火山岩及其沉积组合则是大陆裂谷火山-沉积建造,形成于区域性伸展构造环境,是准噶尔板块与塔里木板块缝合后的陆内建造,因而古洋盆的关闭时限是早石炭世晚期(鄯善运动)。依据区内早石炭世建造的空间分布和变化规律,作者们认为古天山洋盆与当今地学界作为典型的日本沟-弧-盆体系有着极好的相似性和空间分布可对比性。石炭纪火山岩为本区的构造演化研究提供了重要的约束信息。     

Strontium isotope studies on young volcanic rocks from Germany and Italy

⁸⁷Sr/⁸⁶Sr ratios of Tertiary tholeiitic, basalts alkali olivine basalts and olivine nephelinites from Lower Saxony and Hessia and Quaternary leucite-nepheline tephrites from the Laacher See area are similar to those obtained from Hawaii and range from 0.7031 to 0.7054. Three trachytes and one phonolite from the Westerwald and one phonolite from the Laacher See area have higher values (0.7063 to 0.7093). Three Vesuvian lavas, three Somma lavas and two trachytes of the Phlegraic Fields show substantially higher ratios than the comparable basaltic rocks from N.W. Germany (0.7071 to 0.7102). Three peridotite nodules vary between 0.7048 and 0.7081. Three limestone composites of Paleozoic and Mesozoic age show values between 0.7129 and 0.7174.The ⁸⁷Sr/⁸⁶Sr ratios of the trachytes and phonolites from the Westerwald and Laacher See area are probably influenced by crustal material. Assimilation processes of limestones producing the olivine-nephelinites from N.W. Germany seem to be unlikely. A discussion of the origin of the Sr ratios in Vesuvian rocks and of mantle homogeneity is included.     

Effects of desferrioxamine-B on the release of arsenic from volcanic rocks

Microorganisms play an important role in As mobilization into groundwater by directly influencing As speciation or indirectly inducing solubilisation from As-bearing phases, such as Fe, Mn and Al oxides. Iron oxide dissolution could also be induced by siderophores, small-molecule compounds produced by microorganisms to favour Fe uptake. Well waters exceeding the potable water limit of 10 μg As L⁻¹ (0.133 μM) have been widely reported in geothermal areas. Mechanisms responsible for these high As concentrations have not yet been thoroughly elucidated and the complexity of As mobilization in volcanic aquifers is still open to multiple interpretations. The present study was based on batch release experiments aimed at verifying and quantifying the effect of siderophores on As mobilization from volcanic rocks (lava, tuff, peperino and fallout deposit) at different pH and ligand concentration. In the experiments the siderophore trihydroxamate desferroxamine B (Dfob) was used and its effect on As release from volcanic rocks was manifest after the first days. The most favourable pH for As release was pH 6 while concentrations above 250 μM Dfob considerably enhanced As and Fe concentrations in solution. The As release from rocks was between 2.0–10% at pH 6 and 2.4–8.8% at pH 8. The As/Fe ratio in solution changed with time suggesting different release mechanisms and higher mobility of As compared to Fe during the first phase of the experiment. The presence of siderophore increased Fe dissolution rates up to 10 orders of magnitude. The As release correlated with Al, Mn, Fe, Si, V, Ga and Sb and the release of all these elements increased with increasing Dfob concentration. In alkaline environments also Cu, Zn and Pb were mobilized. The presence of siderophores represents a possible trigger for As mobilization from iron binding minerals to the water phase, with interesting implications for groundwater quality, plant uptake and bacterial communities.     

大陆碰撞过程的火山岩响应：以西藏林周林子宗火山岩为例

印度-亚洲大陆碰撞伴生有大量火山活动,其中,林子宗火山岩发育最广,遍布碰撞带北侧的冈底斯带,形成长逾1200 km的火山岩带。林周地区作为林子宗火山岩的命名地,该套火山岩发育相对齐全,为安山岩、流纹岩及相应的火山碎屑岩夹沉积碎屑岩组合,顶部发育巨厚流纹质凝灰岩,可以划分出三个火山旋回,其生成时代介于63.89~48.73 Ma。岩石学和地球化学资料显示,林子宗火山岩自下而上SiO₂和K₂O含量以及Al₂O₃饱和度增加,其岩浆从早到晚由中性、中钾和准铝质变化到酸性、高钾和过铝质,晚期喷发巨厚的火山灰流,反映区域地壳明显的加厚,由早期的30~40 km变化到晚期的50~60 km。火山岩相对富集Cs、Rb、K、U,亏损Ta、Nb、Ti、Sr、Ba、P,早期与桑日组安山岩地球化学特征相近,而中—晚期与乌郁、扎嘎等地渐新世高钾火山岩相似,表明早期岩浆具有新特提斯洋俯冲板片印迹,而中—晚期具有后碰撞作用特点。林子宗火山岩作为印度-亚洲大陆碰撞过程的响应,记录着古新世至始新世（64~48 Ma）印度-亚洲大陆之间的碰撞向碰撞后演化过程。     

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.     

冈底斯中北部晚侏罗世-早白垩世地球动力学环境：火山岩约束

利用新近完成的1：25万区域地质调查资料和笔者分析数据,初步研究了西藏冈底斯带晚侏罗世-早白垩世火山岩的分布特点及地层古生物、同位素年代学约束的火山活动时代以及冈底斯中北部火山岩的性质、构造环境、岩石成因和俯冲极性.冈底斯带晚侏罗世-早白垩世火山岩主要分布于南冈底斯、中冈底斯、北冈底斯和班公湖-怒江缝合带南缘,基本同期开始于早白垩世（大约120～130 Ma）并可能延续到晚白垩世,以冈底斯中北部地区规模最大为特征.已有数据显示中冈底斯则弄群火山岩包括部分中钾钙碱性系列的中基性岩石和占优势的高钾钙碱性系列的中酸性火山岩,不同于传统岛弧火山岩,但与中安第斯厚地壳背景下的岛弧火山岩相似,北冈底斯主要为与传统岛弧火山岩相似的中钾钙碱性系列火山岩.中冈底斯则弄群中基性火山岩很可能与来自消减沉积物和/或蚀变玄武质洋壳的含水流体引起上覆地幔楔物质的部分熔融有关,并在岩浆上升过程中经历了明显的分离结晶作用和中上部地壳物质的同化混染（即AFC过程）,长英质火山岩很可能主要与地壳重熔有关,但并不能完全排除镁铁质岩浆的分离结晶作用.目前冈底斯中北部晚侏罗世-早白垩世火山岩地球化学数据揭示的成分变化、地壳厚度、与俯冲带的距离以及冈底斯带同期火山岩的分布特点暗示冈底斯中北部火山岩很可能与班公湖-怒江特提斯洋壳向南的俯冲作用有关.     

Geochemistry of volcanic rocks from transform margins: Evidence from the Alchan basin,northwestern Primorie

Geological, petrochemical, and geochemical data are reported for volcanic rocks of a Cretaceous pull-apart basin in the Tan Lu strike-slip system, Asian continental margin. A comparison of these volcanic rocks with magmatic rocks from typical Cenozoic transform margins in western North America and rift zones of Korea made it possible to distinguish some indicator features of transform-margin volcanic rocks. Magmatic rocks from strike-slip extension zones bear island-arc, within-plate, and, occasionally, depleted MORB geochemical signatures. In addition to calc-alkaline rocks, there are bimodal volcanic series. The rocks are characterized by high K₂O, MgO, and TiO₂ contents. They show variable enrichment in LILE relative to HFSE, which is typical of island-arc magmas. At the same time, they are rich in compatible transition elements, which is a characteristic of within-plate magmas. The trace-element distribution patterns normalized to MORB or primitive mantle usually display a negative Ta-Nb anomaly typical of suprasubduction settings. Their Ta/Nb ratio is lower, whereas Ba/Nb, Ba/La, and La/Yb are higher than those of some MORB and OIB. In terms of trace-element systematics, for example, Ta-Th-Hf, Ba/La-(Ba/La)_n, (La/Sm)_n-La/Hf, and others, they fall within the area of mixing of magmas from several sources (island arc, within plate, and depleted reservoirs). The magmatic rocks of transform settings display a sigmoidal chondrite-normalized REE distribution pattern, with a negative slope of LREE, depletion in MREE, and an enriched or flat HREE pattern. The magmas with mixed geochemical characteristics presumably originated in a transform margin setting in local extension zones under the influence of mantle diapirs, which caused metasomatism and melting of the lithosphere at different levels, and mixing of melts from different sources in variable proportions. Original Russian Text ? V.P. Simanenko, V.V. Golozubov, V.G. Sakhno, 2006, published in Geokhimiya, 2006, No. 12, pp. 1251–1265.     

火山构造组合研究和填图方法应用——以西藏措麦—邦多地区火山岩为例

以125万邦多区幅、措麦区幅火山岩区野外地质调查为基础,运用岩石地层-火山岩相-火山构造法一体化研究思路,吸收20世纪90年代以来国内外对火山学研究的新成果,并借鉴15万区域地质调查中大比例火山地质图编制的经验,从单个火山机构调查扩展到火山机构组合研究,从而填制出火山构造面貌清楚的古火山地质图,全面地反映了不同时期(旋回)火山作用特点、演化规律及火山岩相的分布特征,合理地建立起火山地层层序.     

Two-stage contamination during crustal assimilation: isotopic evidence from volcanic rocks in eastern Nevada

Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclaserich andesites and dacites have greater Sr and ⁸⁷Sr/⁸⁶Sr, but less Nd and lower _Nd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two bites. The separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of ⁸⁷Sr/ ⁸⁶Sr versus _Nd that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.     

U-series disequilibria in volcanic rocks from the Canary Islands: Plume versus lithospheric melting

U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (²³⁸U)/(²³²Th), (²³⁰Th)/(²³²Th), (²³⁰Th)/(²³⁸U) and (²³⁴U)/(²³⁸U) for 43 samples, most of which are minimally differentiated, along with (²²⁶Ra)/(²³⁰Th) and (²³¹Pa)/(²³⁵U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (²³⁰Th)/(²³⁸U) ranges from 1.1 to 1.6. (²²⁶Ra)/(²³⁰Th)_o ranges between 0.9 and 1.8 while (²³¹Pa)/(²³⁵U)_o ranges between 1.0 and 2.0.Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume.Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.     

新疆于田县阿羌火山岩的时代再讨论

阿羌火山岩作为一个独立的地质构造单元在西昆仑的地质演化中扮演着重要的角色。为了调查阿羌火山岩的年龄,在阿羌火山岩的不同部位中采集了5件样品,进行LA-MC-ICP-MS锆石U-Pb定年。其中2件样品没有获得有效年龄,另外3件样品得到4个年龄数据:485Ma、461Ma、432Ma、145Ma,反映该带经历了加里东、华力西、燕山等运动。该结果对重新认识和研究西昆仑的地质演化历史具有重要的地质意义。     

Geotectonic evolution of the Western Cordillera of Colombia: New aspects from geochemical data on volcanic rocks

The Western Cordillera of Colombia (WCC) is part of the Basic Igneous Complex (BIC), which is one of the world's largest ophiolitic complexes, extending from Costa Rica through Panama and Colombia to Ecuador. Major and trace element data on 32 volcanic rocks from the central and northern parts of the Western Cordillera are presented; no data have been available to date for volcanic rocks from the northern parts of the Western Cordillera. Petrographical and geochemical investigations show that the rocks are altered and have undergone low-grade metamorphism. The subalkaline rocks are represented by tholeiitic basalts, calc-alkaline basic andesites, andesites, and one dacite. It is concluded that a mature oceanic island arc existed in the Cretaceous, in what is now the northern part of the Western Cordillera. The tectonics of the region, particularly the intensive imbrication of the chain, indicates the presence of a paleo-subduction zone with an oceanic island arc that accreted on the old continental margin. These new data, combined with new and previous data from the central part of the BIC of Colombia, suggest that volcanic rocks of the Western Cordillera can be interpreted as allochthonous slabs. These slabs were imbricated with back-arc and fore-arc sediments and tonalitic bodies during the closing of a back-arc basin in northwestern South America and accretion of an oceanic island arc. Oblique subduction accreted these different areas to the continental margin during Late Cretaceous and early Tertiary times. Two plate-tectonic models are proposed: a) development of the calc-alkaline volcanic rocks in the northern parts of the Western Cordillera, separated by tholeiitic rocks, formed along a transform fault represented by the tholeiitic basalts of the central and southern parts of the Western Cordillera; or b) development of an oceanic island arc along the Cretaceous continental margin of northwestern South America. In the central and southern parts of this island arc, accretion took place early and therefore only an island-arc tholeiitic suite was formed.     

Carbonate tufas as archives of climate and sedimentary dynamic in volcanic settings,examples from Gran Canaria (Spain)

Three Holocene tufas from Gran Canaria volcanic island were studied with the aim of deciphering their sedimentary evolution through space and time. Las Temisas tufa (south-eastern arid part of the island) is dominantly composed of oncoids, intraclasts, phytoclasts, coated stems, minor thin stromatolites, and a high amount of siliciclastics. It was deposited in a fluvial system with variable flow velocities and palustrine conditions areas, which alternated with high energy events. Azuaje tufa (northern humid part of the island) is composed of coated stems, stromatolites, oncoids and phytoclasts, with relatively low amounts of siliciclastics, suggesting slow-flowing and palustrine conditions and a relatively low incidence of (high energy) floodings. Los Berrazales tufa (north-west of Gran Ganaria, the most humid one), is mainly composed of coated stems and crystalline crusts, formed in a laminar flow regime. Dominant clastic sedimentation in Las Temisas and high calcite growth rates in Los Berrazales led to a poor development of stromatolites in comparison with Azuaje. Las Temisas and Azuaje deposits have similar upward evolution with decreasing trend in siliciclastics and increasing trend in carbonates. However, Las Temisas has higher siliciclastic and lower phytoclastic contents suggesting a less vegetated area and more arid climate than in the other deposits. Additionally, tufas record local events common in volcanic terrains. Azuaje presents three units bounded by erosive discontinuities, which reveal significant erosion by enhanced runoff that could be caused by loss of vegetation due to wildfires related to volcanic eruptions at headwaters. Las Temisas record a possible interruption in sedimentation represented by aligned boulders due to rockfalls from the hillsides. These deposits formed from waters with similar chemistry providing to the carbonates their similar signals in δ¹³C–δ¹⁸O stable isotopes and ⁸⁷Sr/⁸⁶Sr ratios like that of the volcanic rocks. This work shows how, in volcanic areas, tufas are unique archives of the climate, vegetation and volcanic-related processes, because all imprint the sedimentary regime of tufa deposition.