新疆富蕴县苏普特一带双峰式火山岩的发现及其地质意义

新疆阿尔泰苏普特一带中泥盆统阿勒泰组为一套以玄武质岩石为主的双峰式火山岩组合,下部以变质玄武岩为主,夹少量流纹岩,中部以变质玄武岩、玄武质凝灰岩、沉凝灰岩为主,夹少量酸性火山岩及其碎屑岩,上部流纹质岩石有所增加。玄武质岩石往往变质为角闪片岩、绿帘阳起石片岩等,属拉斑玄武岩系列,低SiO2、K2O,高Ti,富Na、Rb、Ba、Th、U等,轻稀土略富集,无明显铕异常,Nb负异常明显,相似于MORB和岛弧型玄武岩的某些特征,与弧后盆地玄武岩的特征基本一致。流纹质岩石属钙碱性系列,高SiO2,低K2O,为钠质型;大离子亲石元素富集,具明显的Nb、P、Ti负异常和弱的Th正异常,Sr含量较低;稀土元素含量较高,∑REE为135.67×10-6～407.71×10-6,轻稀土略富集,(La/Yb)N为2.55～3.73,铕负异常较明显,δEu值为0.34～0.54。流纹质岩石地球化学特征也与典型弧后盆地双峰式火山岩中的流纹岩相似。结合区域资料分析,苏普特双峰式火山岩是阿尔泰南缘俯冲撕裂型裂谷盆地演化晚期局部形成的类似于弧后环境的产物。     

Geochronology,geochemistry and geological significance of Early Cretaceous volcanic rocks from Niangniangshan Formation,Ningwu Basin,middle and lower reaches of the Yangtze River

Ningwu Basin is one of the Mesozoic continental volcanic basins in the middle and lower reaches of the Yangtze River.The volcanic rocks of the Longwangshan,dawangshan,Gushan and Niangniangshan formations,as well as the homologous subvolcanic rocks or small intrusions,are developed from old to new in the Ningwu Basin.Zircon U-Pb dating results show the latialite phonolite of Niangniangshan Formation was erupted at 128±1 Ma(i.e.,Early Cretaceous).The latialite phonolite contains moderate SiO2 contents(57.28%-60.96%)with high Na 2O+K 2O contents,belonging to shoshonite series.The samples have high REE contents,and display right-inclined REE distribution pattern.They are characterized by enrichment in some large ion lithophile elements(e.g.,LILEs,Rb,K),and depletion in some high field strength elements(e.g.,HFSEs,Nb,Ta,Ti).All volcanic samples have relatively depleted Nd isotopic compositions(ISr=0.707197--0.707878;εNd(t)=-0.5--0.9),indicating no genetic relationship with the lower crust of Yangtze plate,but a drift trend towards the EMII.The geochemical data suggest that the Early Cretaceous latialite phonolite was derived from the partial melting of an enriched lithospheric mantle metasomatized by subduction-related fluids in an arc-related setting.Based on the temporal and spatial distribution and geochemical variation characteristics of the regional volcanic rocks,it is suggested that the tectonic system within the study area changed from a subduction-related compression to an extensional environment in the early Early Cretaceous,which was caused by the ridge subduction of the Paleo-Pacific Ocean.     

Assessment of the Enhanced Geothermal System Resource Base of the United States

This paper describes an assessment of the enhanced geothermal system (EGS) resource base of the conterminous United States, using constructed temperature at depth maps. The temperature at depth maps were computed from 3 to 10 km, for every km. The methodology is described. Factors included are sediment thickness, thermal conductivity variations, distribution of the radioactive heat generation and surface temperature based on several geologic models of the upper 10 km of the crust. EGS systems are extended in this paper to include coproduced geothermal energy, and geopressured resources. A table is provided that summarizes the resource base estimates for all components of the EGS geothermal resource. By far, the conduction-dominated components of EGS represent the largest component of the U.S. resource. Nonetheless, the coproduced resources and geopressured resources are large and significant targets for short and intermediate term development. There is a huge resource base between the depths of 3 and 8 km, where the temperature reaches 150–250°C. Even if only 2% of the conventional EGS resource is developed, the energy recovered would be equivalent to roughly 2,500 times the annual consumption of primary energy in the U.S. in 2006. Temperatures above 150°C at those depths are more common in the active tectonic regions of the western conterminous U.S., but are not confined to those areas. In the central and eastern U.S. there are identified areas of moderate size that are of reasonable grade and probably small areas of much higher grade than predicted by this analyses. However because of the regional (the grid size is 5′ × 5′) scale of this study such potentially promising sites remain to be identified. Several possible scenarios for EGS development are discussed. The most promising and least costly may to be developments in abandoned or shut-in oil and gas fields, where the temperatures are high enough. Because thousands of wells are already drilled in those locations, the cost of producing energy from such fields could be significantly lowered. In addition many hydrocarbon fields are producing large amounts of co-produced water, which is necessary for geothermal development. Although sustainability is not addressed in this study, the resource is so large that in at least some scenarios of development the geothermal resource is sustainable for long periods of time.     

Seismicity of Garfagnana-Lunigiana (Tuscany, Italy) as recorded by a network of semi-broad-band instruments

Eight seismic stations equipped with5-second sensors have recently beeninstalled in the Lunigiana-Garfagnana areato monitor activity in this sector of theNorthern Apennines shaken by the 5.0 Mdearthquake of October 1995. The stations(RSLG network) represent an eastern branchof the RSNI (Regional Seismic network ofNorthwestern Italy) managed by Dipteris,Universita' di Genova.The installation of this dense networkallowed the operators to improve themagnitude detection level, providinginformation about the seismicity of thearea down to magnitude 1.0. Severalanalyses have been carried out to evaluatethe performance of the network and thereliability of the computed locations. Itseems that the epicentral location forevents occurring inside the network isalready constrained using 8–10 P+S phases,but usage of more readings ensuresreliability of depth and the reduction oflocation errors.Though not conclusive, as the network hasonly been fully operating for two years, apreliminary study carried out on a selecteddataset of high-quality locations confirmed that the seismicity of theGarfagnana-Lunigiana is characterised by alow-to-medium magnitude level and it issubdivided into two bands, a superficialone about 30–35 km thick and the secondbelow 50 km. This distribution, confirmedby other studies carried out in the past,reflects the complex structure of this areawhere two tectonic plates (European andAdriatic) meet.     

Synthesis of Fluid Inclusion Reference Materials in the H2O–NaCl–CH4 System

In this study, we detail procedures to synthesise inclusions of fluids of known composition under diagenetic conditions – low temperatures (50–180 °C) and pressures (2–25 MPa) – hosted by quartz. Pre‐existing microcavities in polished quartz chips were used, decrepitated and rinsed before being healed in an autoclave. Each synthesis required around 4 weeks. A range of biphase liquid–vapour inclusions in the H₂O–CH₄–NaCl system, similar to natural ones, were synthesised in quartz, varying the content of all three components. The quartz chips were checked before and after synthesis by optical microscopy, microthermometry and Raman spectroscopy. The method was used to build a series of calibration curves relating methane‐to‐water count ratios to the methane content of aqueous fluid inclusions. Good correlation was obtained between Raman band area ratios and the expected CH₄ solubilities from equations of state. The chosen procedure's main advantages, compared with other published methods, are as follows: quicker healing of the microfractures at lower temperatures, larger inclusions, simple quartz sample preparation, control of fluid inclusion composition and the use of commercial reactors. Such reference material inclusions allow the calibration of Raman microspectrometers with the objective of PTX reconstruction of palaeofluid migration in petroleum reservoirs.     

The 1986 Kermadec earthquake and its relation to plate segmentation

To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (M _w =7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (=275°, =61°, =156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×10²⁰ N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes.We performed a simultaneous inversion ofP andSH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy.     

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex,western California

ABSTRACT

The Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U?Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.     

Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites

Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix mélange. Peak lawsonite-eclogite facies mineral assemblages (garnet + omphacite + lawsonite + rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field.

Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths.