Metamorphic evolution of the Central Metamorphic Belt, Klamath Province, California: an inverted metamorphic gradient beneath the Trinity peridotite     

S. M. PEACOCK  P. J. NORRIS 《Journal of Metamorphic Geology》1989,7(2):191-209

ABSTRACT Metabasalts and metasedimentary rocks of the Devonian Central Metamorphic Belt comprise the lower plate of the east-dipping Trinity thrust system in the Klamath province. An inverted metamorphic gradient is preserved in the Central Metamorphic Belt; metamorphic conditions decrease from amphibolite facies adjacent to the Trinity thrust, through albite-epidote amphibolite facies, to upper greenschist facies at the base of the Central Metamorphic Belt. Mineral chemistry, mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 ± 50° C at the Trinity thrust to 500 ± 50° C at the base of the Central Metamorphic Belt, under pressures of 5 ± 3 kbar. Synmetamorphic Ab + Qtz veins, up to 1 m thick, increase in abundance towards the Trinity thrust. Infiltration of H₂O-CO₂ fluids derived from prograde devolatilization reactions in the Central Metamorphic Belt caused extensive hydration and metasomatism of the Trinity peridotite; the hanging wall block of the Trinity thrust zone. Geological relationships and the preserved inverted metamorphic gradient suggest that the Central Metamorphic Belt formed in an east-dipping Devonian subduction zone in an oceanic environment. The Central Metamorphic Belt appears to represent a discrete slice of accreted oceanic crust several km thick, rather than progressively accreted material. Metamorphic pressures recorded by the Central Metamorphic Belt are intermediate between the ∼2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7–10 kbar preserved in subduction-related inverted metamorphic gradients. The lack of blueschist facies mineral assemblages in the Central Metamorphic Belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth.  相似文献   

Convective Mixing In the Mantle: the Role of Viscosity Differences     

D. A. Spence  J. R. Ockendon†  P. Wilmott†  D. L. Turcotte†  L. H. Kellogg† 《Geophysical Journal International》1988,95(1):79-86

  相似文献   

西太平洋板块向我国东北地区深部俯冲的数值模拟   总被引：6，自引：3，他引：3       下载免费PDF全文

朱桂芝  石耀霖  陈石  张怀 《地球物理学报》2009,52(4):950-957

本文采用依赖温度的黏度结构以及考虑海洋板块和大陆板块厚度差异等特征,以太平洋板块向欧亚板块会聚速率作为板块速度的主要约束,通过变化海沟后撤速度模型,数值模拟西太平洋板块向中国东北的俯冲过程.结果表明,要产生类似于中国东北之下低角度的板片俯冲,海沟后撤是重要条件;而上下地幔黏度的较大差异是决定俯冲板片不穿透660 km相变面的决定因素;西太平洋板块向欧亚板块的俯冲应早于70 Ma B.P.,海沟后撤速度可能小于一些地质学家估计的45 mm/a, 而且可能是分阶段变化的;速度场表明运动学模型的反过程：大陆岩石圈之下物质的不断水平向东的流动和推挤可能成为海沟后撤的力源之一,地幔物质的这种东向流动可能与印度板块挤压碰撞欧亚板块有关,沿欧亚板块东缘的扩张构造可能是太平洋-欧亚板块运动和印度-欧亚板块运动的综合效应.  相似文献   

Petrologic evolution of Palau,a nascent island arc     

James W. Hawkins  Osamu Ishizuka 《Island Arc》2009,18(4):599-641

Palau Islands, 7°30′N, are the only emergent feature on the more than 2500‐km‐long Kyushu–Palau Ridge. Small islands are mainly uplifted reef carbonate. Larger islands are volcanic with basalt to dacite and rare boninite. Polymict breccia is abundant: sills, flows, and dykes are common but pillows are rare. Palau Trench samples include all types found on the islands as well as high‐Mg basalt. Volcanism began in the late Eocene and ended by early Miocene. All igneous rocks comprise a low‐K primitive island arc‐tholeiite series. None are mid‐ocean ridge basalts. Rare earth elements and high field‐strength elements indicate a depleted mantle source. Elevated large ion lithophile elements and light rare earth elements indicate influx of ‘dehydration fluid’. Ce/Ce* and Eu/Eu* ratios show no evidence for recycling of arc‐derived clastics. Plate reconstructions and paleomagnetic data suggest that the arc probably formed on the trace of a transform fault that migrated northward and rotated clockwise up to 90°. Episodes of transtension caused upwelling of hot mantle into depleted mantle and sheared altered rocks of the transform. Episodes of transpression may have initiated subduction of old seafloor with a thin cover of pelagic sediments deposited far from terrigenous sediment sources.  相似文献   

Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system: Insights from marine observations and sandbox experiments     

Dominguez  S.  Lallemand  S.  Malavieille  J.  Schnürle  P. 《Marine Geophysical Researches》1998,20(5):383-402

The Gagua Ridge, carried by the Philippine Sea Plate, is subducting obliquely beneath the southernmost Ryukyu Margin. Bathymetric swath-mapping, performed during the ACT survey (Active Collision in Taiwan), indicates that, due to the high obliquity of plate convergence, slip partitioning occurs within the Ryukyu accretionary wedge. A transcurrent fault, trending N95° E, is observed at the rear of the accretionary wedge. Evidence of right lateral motion along this shear zone, called the Yaeyama Fault, suggests that it accommodates part of the lateral component of the oblique convergence. The subduction of the ridge disturbs this tectonic setting and significantly deforms the Ryukyu Margin. The ridge strongly indents the front of the accretionary wedge and uplifts part of the forearc basin. In the frontal part of the margin, directly in the axis of the ridge, localized transpressive and transtensional structures can be observed superimposed on the uplifted accretionary complex. As shown by sandbox experiments, these N330° E to N30° E trending fractures result from the increasing compressional stress induced by the subduction of the ridge. Analog experiments have also shown that the reentrant associated with oblique ridge subduction exhibits a specific shape that can be correlated with the relative plate motion azimuth.These data, together with the study of the margin deformation, the uplift of the forearc basin and geodetic data, show that the subduction of the Gagua Ridge beneath the accretionary wedge occurs along an azimuth which is about 20° less oblique than the convergence between the PSP and the Ryukyu Arc. Taking into account the opening of the Okinawa backarc basin and partitioning at the rear of the accretionary wedge, convergence between the ridge and the overriding accretionary wedge appears to be close to N345° E and thus, occurs at a rate close to 9 cm yr^–1. As a result, we estimate that a motion of 3.7 cm yr^–1±0.7 cm should be absorbed along the transcurrent fault. Based on these assumptions, the plate tectonic reconstruction reveals that the subducted segment of the Gagua Ridge, associated with the observable margin deformations, could have started subducting less than 1 m.y. ago.  相似文献   

The metamorphic history of rocks buried,accreted and exhumed in an accretionary prism: an example from the Otago Schist,New Zealand     

Å. FAGERENG  A. F. COOPER 《Journal of Metamorphic Geology》2010,28(9):935-954

Although subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low‐grade metamorphic assemblages. In this study a multi‐technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion‐collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench‐fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo‐Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b₀ spacing analyses of white mica. Rare lawsonite occurs in a post‐foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher‐T than experienced along the prograde path. Observed subduction‐related subgreenschist assemblages are therefore likely to indicate lower‐P than experienced during subduction, as higher‐P mineral compositions re‐equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.  相似文献   

Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis,south Tibet: petrology,zircon geochronology and implications for the subduction of Neo‐Tethys and the Indian continent beneath Asia     

Z. M. ZHANG  G. C. ZHAO  M. SANTOSH  J. L. WANG  X. DONG  J. G. LIOU 《Journal of Metamorphic Geology》2010,28(7):719-733

The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus‐Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium‐P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36–33 Ma. However, the High Himalayan rocks experienced only a single high‐P granulite facies metamorphic event at 37–32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium‐P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo‐Tethyan ocean, and that the Oligocene (37–32 Ma) high‐P (1.8–1.4 GPa) rocks of the High Himalayan and coeval medium‐P (0.8–1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet.  相似文献   

Trace element composition of continentally subducted slab‐derived melt: insight from multiphase solid inclusions in ultrahigh‐pressure eclogite in the Dabie orogen     

X.‐Y. Gao  Y.‐F. Zheng  Y.‐X. Chen  Z. Hu 《Journal of Metamorphic Geology》2013,31(4):453-468

The ultrahigh‐pressure (UHP) eclogite in the Dabie orogen preserves petrological evidence for the existence of hydrous silicate melts that formed during continental subduction‐zone metamorphism. This is indicated by occurrence of multiphase solid (MS) inclusions in garnet that primarily consist of K‐feldspar + quartz ± epidote/allanite. All the MS inclusions are euhedral to subhedral in morphology and surrounded with radial cracks in the host garnet. Their trace element compositions were analysed by two different approaches of laser sampling. The mass budget method was used to estimate the trace element abundances of MS inclusions from their mixtures with the host garnet. The results are compared with the direct sampling of MS inclusions, providing a first‐order approximation to the trace element composition of MS inclusions. The MS inclusions exhibit consistent enrichment of LILE, Sr and Pb, but depletion of HFSE in the primitive mantle‐normalized spidergram. Such arc‐like patterns of trace element distribution are common for continental crustal rocks. The melts have variably high K, Rb and Sr abundances, suggesting that breakdown of phengite is a basic cause for partial melting of the UHP eclogite. These MS inclusions also exhibit consistently low HFSE and Y contents, suggesting partial melting of the eclogite in the stability fields of rutile and garnet. Consequently, the trace element composition of MS inclusions provides a proxy for that of hydrous silicate melts derived from dehydration melting of the UHP eclogite during continental collision.  相似文献   

Isostatic constraints on the central Victorian lower crust: Implications for the tectonic evolution of the Lachlan Fold Belt     

G. J. O'Halloran  P. Rey 《Australian Journal of Earth Sciences》2013,60(4):633-639

Interpretation and 2‐D forward modelling of aeromagnetic datasets from the Olary Domain to the north of the outcropping Kalabity Inlier, South Australia, is consistent with a buried structural architecture characterised by isolated anticlines (also referred to as growth anticlines) bounded by steeply dipping reverse faults. The isolated anticlines are interpreted to have formed by half‐graben inversion during crustal shortening associated with the ca 1600–1580 Ma Olarian Orogeny. We interpret the bounding reverse faults as reactivated high‐angle normal faults, originating from a listric extensional fault architecture. As shortening increased, ‘break‐back bypass’ and ‘short‐cut‘ thrusts developed because of buttressing of the hangingwall successions against the footwall. The resulting architecture resembles a combination of a thrust‐related imbricate fan and an accumulation of inverted basins. Using this structural architecture, synrift sediments proximal to interpreted normal faults were identified as prospective for sediment‐hosted massive sulfide Pb–Zn–Ag mineralisation.  相似文献   

40Ar/39Ar and K–Ar age constraints on the timing of regional deformation,south coast of New South Wales,Lachlan Fold Belt: Problems and implications     

C. L. Fergusson  D. Phillips 《Australian Journal of Earth Sciences》2013,60(3):395-408

Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and ⁴⁰Ar/³⁹Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar ⁴⁰Ar/³⁹Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of ³⁹Ar, leading to anomalously old ⁴⁰Ar/³⁹Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average ⁴⁰Ar/³⁹Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of ³⁹Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock ⁴⁰Ar/³⁹Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock ⁴⁰Ar/³⁹Ar data, should be treated with caution.  相似文献