Geochemical element mobility during alteration/mineralization in the Sungun porphyry copper deposit,Azerbaijan–Iran

The Sungun porphyry copper deposit of northwestern Iran is associated with Miocene diorite/granodiorite to quartz-monzonite intrusive into Eocene volcanic–sedimentary and Cretaceous carbonate rocks. Three main mineralization-related alteration episodes (I, potassic; II, transition; and III, phyllic alterations) were studied in terms of mass transfer during hydrothermal evolution of the Sungun deposit. Isocon plots (Grant, J.A. 1986 Grant, J.A. 1986. The isocon diagram – a simple solution to Gresens’ equation for metasomatic alteration. Economic Geology, 81: 1976–1982. [Crossref], [Web of Science ®] , [Google Scholar], The isocon diagram – a simple solution to Gresens’ equation for metasomatic alteration: Economic Geology, v. 81, p. 1976–1982) were employed to illustrate these changes quantitatively. These plots illustrate that Al, Ti, and Ga were relatively immobile during alteration, and that the alteration was essentially mass-conservative. At all stages in the evolution of the hydrothermal system, computed volume changes were close to zero. In the potassic alteration zone, an obvious enrichment of K and depletions of Na, Ca, Mn, and Fe took place. These changes were due to replacement of plagioclase and amphibole by K-feldspar and biotite, respectively. Potassic alteration was associated with significant addition of Cu, as might be expected from the occurrence of disseminated chalcopyrite and bornite in this zone. In the transition alteration zone, Ca was added, Na, Fe, and Mg were relatively unchanged, and K, Ba, and Cu were depleted. The loss of K and Ba relative to Na reflects replacement of K-feldspar by albite. Phyllic alteration was accompanied by the depletion of Na, K, Fe, and Ba and enrichment of Si and Cu. The losses of Na, K, and Fe reflect the sericitization of alkali feldspar and destruction of ferromagnesian minerals. The addition of Si is consistent with the widespread silicification, which is a major feature of phyllic alteration and the addition of Cu with mobilization from the transition zone, which is depleted in this element.     

Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin,Northwest China

Siliceous rocks were widely deposited in many continents during the Ediacaran–Cambrian (E–C) transition. Based on detailed field investigations in the Aksu area of the Tarim Basin in Northwest China, this study presents evidence of a submarine silica-rich hydrothermal system preserved in the E–C boundary successions. This system consists of a lower stockwork silica-dominant vein swarm zone in the karstified dolostone of the uppermost Ediacaran Qigebulake Formation, which terminates directly under the overlying bedded chert and black shale succession of the lowermost Cambrian Yurtus Formation. The stockwork vein swarms were filled dominantly by a wide spectrum of silica precipitates (amorphous silica, chalcedony, spherulite, fine to coarse quartz) with subordinate pyrite, Fe-(oxyhydr)oxide, and barite. The host dolostones that were dissected by the vein swarms also suffered extensive silicification and recrystallization. The vertical stacking relationship of silica-dominant vein swarms and overlying bedded chert suggests they were formed by an identical low-temperature, silica-rich diffusive submarine hydrothermal system in the earliest Cambrian. This suggestion is further supported by fluid inclusion microthermometry (T_h 40–200°C) of the quartz-barite vein fills. In this case, silica-rich hydrothermal fluids were channelled and precipitated partially along the stockwork veins in the antecedent karstified dolostone and vented mostly into seawater, promoting widespread deposition of bedded chert on the seafloor of Tarim Basin in the earliest Cambrian. This study provides a useful clue and analogue to understand the widespread silica deposition and coeval vast oceanic and geochemical changes during the E–C transition in the Tarim Basin and elsewhere.     

Asia: a frontier for a future supercontinent Amasia

Asia is the world’s largest but youngest continent, in which Pacific-type (P-type) and collision-type (C-type) orogenic belts coexist with numerous amalgamated continental blocks. P-type orogens represent major sites of continental growth through tonalite-trondhjemite-granodiorite type (TTG-type) juvenile granitoid magmatism and accretion of oceanic crust and intra-oceanic arcs. The Asian continent includes several P-type orogenic belts, of which the largest are the Central Asian and Western Pacific. The Central Asian Orogenic Belt is dominated by P-type fossil orogens arranged with a regular northward subduction polarity. The Western Pacific is characterized by ongoing P-type orogeny related to the westward subduction of the Pacific plate. Asia has a multi-cratonic structure and its post-Palaeozoic history has witnessed amalgamation of the Laurasia composite continent and Pangaea supercontinent. Nowadays, Asia is surrounded by double-sided subduction zones, which generate new TTG-type crust and supply oceanic crust and microcontinents to its active margins. The TTG-crust can be tectonically eroded and subducted down to the mantle transition zone to form a ‘second’ continent, which may generate mantle upwelling, plumes, and extensive intra-plate volcanism. Moreover, recent plate movements around Asia are dominated by northward directions, which resulted in the India–Eurasia and Arabia–Eurasia collisions beginning at 50–45 and 23–20 Ma, respectively, and will result in Africa–Eurasia collision in the near future. Therefore, Asia is the best candidate to serve as the nucleus for a future supercontinent ‘Amasia’, likely to form 200–250 Ma in the future. In this paper we unravel a puzzle of continental growth in Asia through P-type orogeny by discussing its tectonic history and geological structure, subduction polarity in P-type orogens, tectonic erosion of TTG-type crust and arc subduction at convergent margins, generation of mantle plumes, and prospects of Asia growth and overgrowth.     

Effect of oil and gas deposits on dynamic characteristics of reflected waves

ABSTRACT

The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО₃ (1550°С) and К₂СО₃ (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450–1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid.     

Early Mesozoic tectonic transition of the eastern South China Block: constraints from Late Triassic Dashuang complex in eastern Zhejiang Province

ABSTRACT

The Mesozoic tectonic transition from the Palaeo-Tethys tectonic regime to the Palaeo-Pacific tectonic regime in the eastern South China Block has long been debated. Geochemical and zircon U–Pb–Hf isotopic studies were conducted on the Dashuang complex in the eastern Zhejiang Province. The Dashuang complex consists mainly of quartz syenite in the northwestern part and quartz monzonite in the southeastern part. New laser ablation inductively coupled plasma mass spectrometry zircon U–Pb data show that the quartz syenite, the quartz monzonite, and its chilled margin (fine-grained granite) crystallized at 235 ± 4 Ma, 232 ± 3 Ma, and 230 ± 1 Ma, respectively. The Dashuang complex intrudes into the Chencai Group gneiss that postdated ~646 Ma and underwent anatexis at 443 ± 14 Ma. The quartz monzonite shows A-type granite affinity, characterized by high K₂O + Na₂O and Zr + Nb + Ce + Y, high FeO_T/(MgO + FeO_T) and Ga/Al ratios, an enrichment in light rare earth elements, and depletions in Ba, Sr, and Eu. The quartz monzonite has zircon ε_Hf(t) values of ?14.2 to –11.9 and two-stage model ages of 1788–1922 Ma. Zircon ε_Hf(t) values of the chilled margin (fine-grained granite) and wall rock (gneiss) are scattered (?18.2 to –6.3 and ?19.5 to 10.2). The corresponding two-stage model ages are 1482–2133 Ma and 1184–2471 Ma, respectively. The Dashuang complex was derived mainly from partial melting of Neoproterozoic clastic rocks in the Cathaysia Block. Geochemical data indicate that the quartz monzonite formed in a post-collision extensional environment. These results, considered with previous data, indicate that the transition from the Palaeo-Tethys to the Palaeo-Pacific tectonic regimes of the eastern South China Block occurred during the Late Triassic (225–215 Ma).     

Unique authigenic mineral assemblages reveal different diagenetic histories in two neighbouring cold‐water coral mounds on Pen Duick Escarpment,Gulf of Cadiz

Alpha Mound and Beta Mound are two cold‐water coral mounds, located on the Pen Duick Escarpment in the Gulf of Cadiz amidst the El Arraiche mud volcano field where focused fluid seepage occurs. Despite the proximity of Alpha Mound and Beta Mound, both mounds differ in their assemblage of authigenic minerals. Alpha Mound features dolomite, framboidal pyrite and gypsum, whereas Beta Mound contains a barite layer and predominantly euhedral pyrite. The diagenetic alteration of the sedimentary record of both mounds is strongly influenced by biogeochemical processes occurring at shallow sulphate methane transition zones. The combined sedimentological, petrographic and isotopic analyses of early diagenetic features in gravity cores from Alpha Mound and Beta Mound indicate that the contrast in mineral assemblages between these mounds is caused by differences in fluid and methane fluxes. Alpha Mound appears to be affected by strong fluctuations in the fluid flow, causing shifts in redox boundaries, whereas Beta Mound seems to be a less dynamic system. To a large extent, the diagenetic regimes within cold‐water coral mounds on the Pen Duick Escarpment appear to be controlled by fluid and methane fluxes deriving from layers underlying the mounds and forcings like pressure gradients caused by bottom current. However, it also becomes evident that authigenic mineral assemblages are not only very sensitive recorders of the diagenetic history of specific cold‐water coral mounds, but also affect diagenetic processes in turn. Dissolution of aragonite, lithification by precipitation of authigenic minerals and subsequent brecciation of these lithified layers may also exert a control on the advective and diffusive fluid flow within these mounds, providing a feedback mechanism on subsequent diagenetic processes.     

始新世—渐新世气候转变研究进展

始新世—渐新世（E—O）气候转变（34 Ma前后）是新生代气候演化过程中最显著的变冷事件之一,标志着地球气候由“温室”进入“冰室”。这一转变伴随着地球环境的一系列重大变化,对于研究新生代气候变冷的驱动机制、区域气候对全球重大气候事件的响应方式、重大气候事件对生态环境及生物演替的影响等具有重要意义。近年来地质记录和气候模拟在E—O转变研究上取得了一系列重要进展：①不同纬度区的地质记录揭示出这一转变伴随着全球性的显著降温,指示其触发因素是全球性的;②气候模拟研究揭示出大气CO2浓度降低及与之联系的全球碳循环变化是导致这一转变的主因,否定了传统认为的环南极流形成导致E—O气候变冷的假说;③深海沉积记录揭示出这一转变过程持续400～500 ka,表现为全球降温和南极冰盖形成先后两阶段变化;④海—陆气候记录的对比初步揭示出陆地区域的干旱化可能主要与全球降温（对应于E—O转变的第一阶段）相关。当前对E—O气候转变的研究还存在地质记录分辨率不够高、模拟结果与地质记录不完全吻合、陆地记录相对较少等方面的不足。更精确的大气CO2浓度和温度的重建、更多高分辨率海—陆气候记录的研究以及古气候数值模拟的改进有望进一步揭示出E—O转变过程中气候系统各要素的变化特征和相互关系,为更深入认识这一转变的驱动机制提供依据。     

Tectono-sedimentary evolution of the External Liguride units (Northern Apennines,Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract

In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions.

The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust. © 2001 Éditions scientifiques et médicales Elsevier SAS     

断层带流体对断层强度和强震孕育的影响

大量研究表明,流体在断层弱化中起着非常重要的作用.在地壳浅部脆性域,自由水通过流体孔隙压力减小断层有效正压力,从而降低断层摩擦强度;在地壳深部,矿物中的微量结构水弱化岩石流变强度.另外,流体-岩石相互作用等化学过程,如长石水解反应,对断层强度的影响也非常显著.断层深部流体通过物理作用与化学作用影响着岩石的变形机制,从而影响断层力学性质与地震孕育和发生.断层内部流体孔隙压力周期性变化是断层带脆-塑性转化、裂缝张开与愈合等的直接体现,这种变化控制着断层强度与强震周期性发生现象.