Evidence for a seismic activity mainly constituted of hybrid events at Cayambe volcano,Ecuador. Interpretation in a iced-domes volcano context

The high activity level of Hybrid Events (HE) detected beneath the Cayambe volcano since 1989 has been more thoroughly investigated with data from a temporary array. The unusual HE spectral content allows separating a high-frequency signal riding on a low-frequency one, with a probable single source. HEs are interpreted as high frequency VT events, produced by the interaction between magmatic heat and an underground water system fed by thaw water from the summital glacier, which trigger simultaneous low-frequency fluid resonance in the highly fractured adjacent medium. Pure VTs are interpreted as ‘aborted’ HEs occurring probably in the oldest and coldest part of the volcano complex. To cite this article: B. Guillier, J.-L. Chatelain, C. R. Geoscience 338 (2006).     

Geochronology and geochemistry of the Hekoulinchang Sn-Pb-Zn-Ag polymetallic deposit (73240 t) in Heilongjiang Province,China

The Hekoulinchang Sn-Pb-Zn-Ag polymetallic deposit(20000 t Sn at 0.27%,236 t Ag at 122.89 g/t,15000 t Pb at 0.84%,and 38000 t Zn at 1.43%)is located in the Wandashan Terrane of the easternmost segment of the Central Asian Orogenic Belt.The timing of Sn-Pb-Zn-Ag polymetallic mineralization remains unclear due to a lack of precise isotope dating directly conducted on ore minerals.The authors herein report that the LA-ICP-MS U-Pb ages of cassiterite and zircon from the granite porphyry in the Hekoulinchang Sn-Pb-Zn-Ag polymetallic deposit are 101.4±7.9 Ma and 115.4±1.0 Ma,respectively,indicating that Sn mineralization and magmatism occurred during the Early Cretaceous.The granite porphyry belongs to the subalkaline series peraluminous I-type granites that are depleted in Nb,Ta,and Ti and enriched in Rb,Th,U,and Pb.TheεHf(t)values of the granite porphyry range from 0.9 to 7.4,with an average of about 5.6 and two-stage model ages(T_DM2)of 705–1116 Ma,with an average age of 819 Ma.The ε_Nd(t)values of the apatites are–1.60–0.45,with an average of–0.9,and two-stage model ages(T_DM2)of 872–1040 Ma,with an average age of 983 Ma.The Nd-Hf isotope data indicate that the magma may have been derived from the partial melting of juvenile crustal material.     

The Pindos Fold-and-thrust belt (Greece): inversion kinematics of a passive continental margin

Continuous exposure in the Pindos mountain chain (Greece) and the detailed stratigraphic measurements in the area enable us to construct eight balanced cross sections across the Pindos Fold-and-thrust belt (PFTB) and to approach quantitatively some parameters which controlled foreland evolution. The 160-km-wide passive continental margin of the Apulian continent in Greece was progressively shortened from east to west at rates of 6 mm/year between the Early Oligocene and Late Eocene. From the rear to the frontal part of the wedge, fault-bend folds, duplexes and imbricates were formed, while strain was partitioned into faulting (~34%), layer parallel shortening (~23%) and buckling (~9%). Foreland subsidence and internal deformation of the orogenic wedge are strongly affected by two parameters of equal importance: the thrust load of the overthrusted microcontinents and the rigidity of the underthrusted Apulian passive margin. Changes in the thickness of the pre-orogenic sediments and reactivated transform faults induced salients. During the Lower Miocene, the orogenic wedge in the Peloponnese suffered additional uplift and westward gravitational gliding induced by the intracontinental subduction of the Palaeozoic rift zone of the Phyllite-Quartzite Series, which was reactivated and returned to the earths surface during the Hellenic orogeny.     

High‐T and low‐P metamorphism in the Xilingol Complex of central Inner Mongolia,China: An indicator of extension in a previous orogeny

The Xilingol Complex comprises biotite gneisses and amphibolite interlayers with extensive migmatization. Four representative samples were documented and found to record either two or three metamorphic stages. Phase modelling using thermocalc suggests that the observed assemblages represent the final stages that underwent cooling from temperature peaks, and are consistent with a fluid‐absent solidus in P–T pseudosections. Their P–T conditions are further constrained to be 5–6 kbar/680–725°C and 4–5 kbar/650–680°C for two garnet‐bearing gneiss samples, 4–5 kbar/660–730°C for a cordierite‐bearing gneiss sample, and 4–5 kbar/680–710°C for an amphibolite sample based on mineral composition isopleths, involving measured Mg content in biotite, anorthite in plagioclase, grossular and pyrope in garnet and Ti content in amphibole. The peak temperature conditions recovered are 760–790°C or >760°C at 5–6 kbar based on the composition isopleths of plagioclase, biotite, garnet and especially the comparison of melt contents between the calculated and observed. A pre‐peak heating process with slight decompression can be suggested for some samples on the basis of the core–rim increase in the plagioclase anorthite, and the stability of ilmenite. Zircon U–Pb dating using the LA‐ICP‐MS method provides systemic constraints on the metamorphic ages of the Xilingol Complex to be 348–305 Ma, interpreted to represent the post‐peak cooling stages. Moreover, metagabbroic dykes that intruded into the Xilingol Complex yield 317 ± 3 Ma from magmatic zircon, and are considered to have played a significant role for heat advection triggering the high‐T and low‐P metamorphism. Thus, the clockwise P–T paths involving pre‐peak heating, peak and post‐peak cooling recovered for the Xilingol Complex are consistent with an extensional setting in the Carboniferous that developed on a previous orogen in response to addition of mantle‐derived materials probably together with upwelling of the asthenospheric mantle.     

http://www.sciencedirect.com/science/article/pii/S1674987112000357

Field and remote sensing studies reveal that Au-bearing quartz±carbonate lodes in Romite deposit,in the extreme South Eastern Desert of Egypt,are controlled by NNE-striking shear zones that splay from the ca.660—550 Ma Hamisana Zone.Quartz in releasing bends with sinistral shear geometry and abundant boudinaged quartz-carbonate lodes with serrate ribboned fabrics suggest vein formation throughout a transpressive wrench system.Ubiquitous hydrothermal quartz,carbonate,and subordinate chlorite and sericite within the shear zones and as slivers in veins,indicate that gold deposition and hydrothermal alteration occurred under greenschist fades conditions.The Al（Ⅳ） in chlorite indicates a formation temperature of～300℃.comparable with temperatures estimated from arsenopyrite composition for grains intimately associated with gold in quartz veins. The new geological and geochemical data indicate that splays off the Hamisana Zone are potential gold exploration targets.Quartz veins along the high order（2nd or 3rd） structures of this crustal-scale shear zone are favorable targets.In the Romite deposit and in surrounding areas,a Au-As-Cu-Sb-Co-Zn geochemical signature characterizes mineralized zones,and particularly rock chips with>1000 ppm As and high contents of Cu,Zn,and Co target the better mineralized areas. The carbonateδ¹³C_pdb andδ¹⁸O_Smow isotope signatures preclude an organic source of the ore fluid,but metamorphic and magmatic sources are still valid candidates.The intense deformation and lack of magmatism in the deposit area argue for metamorphic dewatering of greenstone rocks as the most likely fluid source.The narrow ranges ofδ¹³C（-4.6‰to -3.1‰） andδ¹⁸O（11.9‰-13.7‰） in carbonate minerals in lodes imply a corresponding uniformity to the ambient temperature andδ¹³C_CO2(δ¹³C_∑C) of the ore fluids. The calculatedδ¹⁸Oh₂o values of 6.9‰—7.9‰for ore fluids,based onδ¹⁸O values of vein quartz further suggest a likely metamorphic origin.     

辽宁营口虎皮峪地区古元古代岩浆事件及地质意义

辽宁营口虎皮峪地区存在3期古元古代侵入岩浆活动:早期虎皮峪辽吉花岗岩成岩时代为2119±16Ma,富硅、富碱、低铝、低钙及负Eu异常显示其具有类似A型花岗岩的地球化学特征,非常低Sr高Yb的特点指示其形成压力很低,与减薄的地壳有关;第二期后松树沟奥长花岗岩成岩时代为1892±16Ma,富硅、富钠、高铝,Na2O/K2O1.35及无Eu异常或正Eu异常显示其具有埃达克质岩石的特征,中—高Sr低Yb的特点指示其形成压力较高、深度较大,与碰撞后加厚地壳底侵玄武岩的部分熔融有关;晚期南台子石英二长岩成岩时代为1850±11Ma,富碱(Na2O+K2O=11.32%~11.93%),里特曼指数δ=4.78~7.20,属碱性岩,低Sr低Yb的特点指示其形成压力中等,深度大概40~50km,是造山作用完成后板内伸展背景下的产物。上述3期岩浆事件对整个辽吉古元古代活动带的构造演化具有重要指示意义:早期辽吉花岗岩可能形成于辽吉古元古代活动带早期弧后伸展阶段;后松树沟奥长花岗岩则代表古元古代晚期碰撞后地壳加厚阶段的产物;南台子石英二长岩属后造山花岗岩,标志着辽东地区克拉通化的完成,开始进入后造山伸展演化阶段。     

Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S₁) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S₂–S₃ crenulation cleavage and are replaced along S₃ by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D₂ shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D₃ dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D₄ crosscutting veins and D₅ faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton. Editorial handling: G. Beaudoin     

永安盆地晚古生代—中—新生代沉积构造环境演化特征

永安盆地发育在前泥盆纪基底之上,位于华夏块体南缘,沿NE向政和—大埔断裂带展布。利用层序地层学方法,辅之以野外调查,研究了永安盆地的地层序列和岩石组合：① 对晚古生代至早三叠世地层格架及柱状剖面的分析表明,伴随海平面的升降,海相沉积序列具有阶段性,不同沉积相在盆地东、中、西区段分布不均,盆地沉积沉降中心大致位于龙岩和梅县地区;② 对横贯盆区的地质剖面及盆内辅助剖面的研究表明,中—新生代,盆内以断块作用为主,改造了晚古生代地层,代之以断褶复合地层;③ 有机碳质层为盆地的海陆变迁作了时域界定,并且其赋存部位和形态也在空间上表征了盆地构造演化的力学机制。通过分析构造事件、古地理及物源区、深部构造、边界构造等盆地要素,研究了盆地的沉积构造环境演化及其应力机制,结果表明：① 晚古生代,海进海退的快慢受控于不同时期构造事件的强弱变化,由此产生的沿岸隆起区的剥蚀以及拉张背景的山体风化剥蚀为盆地提供了物源,存在北、南两个物源区;中—新生代,盆地总体为火山活动背景下的山前及河湖相沉积环境。② 盆地具有隆起—伸展构造发育的特点,主要经历了华力西期海陆交互相巨厚沉积阶段,印支期稳定的台地向活动大陆边缘转变阶段并伴随挤压隆升,以及印支期后中—新生代由挤压向拉张机制转换的构造改造阶段。以上认识为中国东南部盆地的薄弱基础研究提供了新的基础参考信息。