Deformation partitioning and porphyroblast rotation in meta-morphic rocks: a radical reinterpretation   总被引：1，自引：0，他引：1  

T. H. BELL 《Journal of Metamorphic Geology》1985,3(2):109-118

Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.  相似文献   

Saline lake carbonates within an Upper Jurassic-Lower Cretaceous continental red bed sequence in the Atacama region of northern Chile     

C. M. BELL 《Sedimentology》1989,36(4):651-663

The Codocedo Limestone Member is a thin but laterally persistent lacustrine sequence within the red beds of the Upper Jurassic-Lower Cretaceous Quebrada Monardes Formation, in the Atacama region of northern Chile. The thick succession of clastic terrigenous sediments of the Quebrada Monardes Formation was deposited in an arid to semi-arid environment. Sedimentary facies are indicative of deposition of aeolian dunes, alluvial fans and braided streams, playa-lake mudflats, and saline lakes and coastal lagoons. The strata accumulated in a N-S elongated extensional back-arc basin on the landward side of an active volcanic arc. The 3 m thick Codocedo Limestone Member marks striking facies changes within the Quebrada Monardes Formation. It is underlain by a thick sequence of conglomerates and sandstones, deposited on alluvial fans. The limestone itself is characterized by evaporite minerals and laterally continuous laminations, indicative of deposition by vertical accretion in a perennial saline lake. The overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa-lake mudflats. The limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession. The lake was possibly formed quite suddenly, for example by damming of the basin by a lava flow. Sedimentation in the perennial lake was predominantly cyclical. Seasonal planktonic algal blooms produced millimetre-scale laminations. Interbedded with these laminites are centimetre-scale beds of evaporitic gypsum, anhydrite and minor halite. The evaporite minerals have been largely replaced by calcite, chalcedony and quartz. The centimetre-scale cycles may have resulted from periodic freshwater input into the lake. After a period of about 3000 yr the lake dried up, to be replaced by extensive playa-lake mudflats. The Codocedo Limestone Member possibly formed a plane of detachment during an early Tertiary phase of E-W directed regional compression. The limestones and evaporites were folded and extensively brecciated. This deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement by calcite and quartz.  相似文献   

Dissolution, solution transfer, diffusion versus fluid flow and volume loss during deformation/metamorphism   总被引：2，自引：0，他引：2  

T. H. BELL  C. CUFF 《Journal of Metamorphic Geology》1989,7(4):425-447

ABSTRACT Dissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries. Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins. Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation. Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large-scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one-way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure. Deformation partitioning-controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well preserved.  相似文献   

Strontium Isotopic Studies of Alkalic Rocks: The Potassium-rich Lavas of the Birunga and Toro--Ankole Regions, East and Central Equatorial Africa     

BELL  KEITH; POWELL  J. L. 《Journal of Petrology》1969,10(3):536-572

The present-day Sr⁸⁷/Sr⁸⁶ ratios of 117 representative samplesfrom the Birunga and Toro-Ankole regions vary significantlyand range between 0.7036 and 0.7111. The feldspar-bearing lavashave higher ratios (average = 0.707) than the melilite- andnepheline-bearing varieties (average = 0.705). Samples of carbonatedlavas have slightly lower Sr⁸⁷/Sr⁸⁶ ratios. The Sr⁸⁷/Sr⁸⁶ ratios show a highly significant, positive, linearcorrelation with Rb/Sr ratio, and a negative correlation withSr, Nb, and Zr abundances. Graphs of Sr⁸⁷/Sr⁸⁶ ratios againstelemental abundances in some cases give hyperbolic patterns.Such relationships are true, not only for the volcanic fieldas a whole, but also for lava flows from one extensively sampledvolcano. Hypotheses involving simple fractional crystallization or limestonesyntexis are inconsistent with the isotopic data. The elementaland isotopic abundance patterns are most easily explained bythe mixing of two end members of quite different Sr isotopicand chemical compositions. If mixing is assumed, approximatelimits can be set for the compositions of the two end members.These limits are consistent with the hypothesis of assimilationof sialic material by either a carbonatitic or nepheliniticparent magma.  相似文献   

PUNCTUATED DRAINAGE OF AN ICE-DAMMED QUATERNARY LAKE IN SOUTHERN SOUTH AMERICA     

C.M. BELL 《Geografiska Annaler: Series A, Physical Geography》2008,90(1):1-17

A chain of east‐west elongated lakes fringes the eastern flank of the Andes Mountains between 39 and 52°S in southern Chile and Argentina. These deep lakes fill valleys left by the retreat of outlet glaciers of the Patagonian Icefield about 16000 years ago. Two of the lakes, Lago General Carrera/Buenos Aires and Lago Cochrane, represent the shrunken remnants of a much larger lake (called here the predecessor lake) that developed in a moat alongside the icefield. A series of seven stepped lacustrine braid deltas on the southeastern shore of Lago General Carrera/Buenos Aires mark the progressive, punctuated drainage of the predecessor lake. The deltas were formed by the build‐out of fans of sand and gravel into the lake. They have the form of Gilbert deltas, with virtually flat subaerial topset beds and steeply inclined subaqueous foreset beds. The exposed delta fronts are marked by a series of small terraces produced by wave erosion during falling lake levels. On either side of the fans are embayments with beaches formed by deposition in the lee of the pro‐grading fans. These embayments are filled with ridges of well‐sorted gravel deposited by wave‐driven long‐shore drift. About 13 000 years ago the regional drainage underwent a remarkable change. Rivers flowing eastwards into the Atlantic reversed their direction to flow westwards through the Andes Mountains and into the Pacific Ocean. The watershed moved some 200 km to the east. The cause of this change, and the falling lake levels, was melting of the Patagonian Icefield. As the ice retreated the icefield split into north and south components. This broke the ice dam that impounded the western end of the predecessor lake. The stepped, progressive fall of the lake level suggests that as the ice melted a series of lower overflow sills of hard rock were exposed. Each of the major episodes of sudden drop in lake level was followed by a long period of up to thousands of years when lake level remained constant. Terraces on the delta fronts indicate that each major episode of fall in the lake level was punctuated by up to ten minor episodes possibly representing major storm events or annual cycles of freezing and melting, blocking the overflow sills.  相似文献   

An Unusual, Three-Dimensionally Preserved, Large Hadrosauriform Pes Track from “Mid”-Cretaceous Jiaguan Formation of Chongqing, China     

Xing Lid  Phil R. Bell  Jerald D. Harris  Philip J. Currie 《《地质学报》英文版》2012,86(2):304-312

Three-dimensional tracks provide unique insights into the locomotor mechanics of their track makers. An isolated, large hadrosauriform print attributable to Caririchnium lotus from the “mid”-Cretaceous Lotus track site (Jiaguan Formation) in China permits reconstruction of the footfall, weight-bearing, and kick-off phases of the step cycle. Large-scale modifications of the pes during the step cycle indicate C. lotus trackmakers were capable of locomotory modifications in response to substrate consistency beyond the “expected” shift between bipedal and quadrupedal postures. An unusual curvature to the trace of one of the outer digits indicates substantial transverse mobility. The remaining digits demonstrate lesser degrees of transverse movement accompanied by extension of the digits during footfall. The absence of overprinted scale-scratch marks and toe drags are consistent with a vertical kick-off of the pes and concomitant flexion of the digits. This track suggests that pedal mobility in C. lotus track makers was greater than previously suspected and has implications for reconstructions of hadrosauriform locomotion.  相似文献   

The inter‐relationships between long‐lived metamorphism,pluton emplacement and changes in the direction of bulk shortening during orogenesis     

I. V. SANISLAV  T. H. BELL 《Journal of Metamorphic Geology》2011,29(5):513-536

Foliation intersection/inflexion axes combined with pseudosections and garnet‐core isopleths reveal only 1.5 kbar variation in P–T conditions while plutons were emplaced regionally and deformation and metamorphism continued during orogenesis lasting 70 Myr. Tectonism ended with slight decompression into the cordierite stability field. Garnet growth was always overstepped by up to 100 °C occurring at conditions that staurolite growth was also possible. Episodic start, stop, start growth behaviour of both of these phases throughout this period did not result from the effects of bulk composition on their stability fields. Different porphyroblast growth patterns in same bulk composition and outcrop samples reveals reaction start/stop behaviour was controlled by the manner in which deformation partitioned through an outcrop. The regional isograds were established during the first period of bulk shortening near orthogonal to the orogen trend. They did not migrate across lower grade rocks during each of the subsequent periods of metamorphism in spite of dramatic changes in the direction of bulk shortening; rather they contracted slightly. During the youngest periods of orogenesis directed at a high angle to the current orogen trend the isograds were folded about axial planes parallel to the fold belt. The regional distribution of these isograds directly reflects the oldest period of pluton emplacement, with both controlled by orogen‐scale partitioning of bulk shortening at a high angle to the current orogen trend relative to intervening zones of transform‐like shear.  相似文献   

Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa   总被引：9，自引：0，他引：9  

BELL  DAVID R.; ROSSMAN  GEORGE R.; MOORE  RORY O. 《Journal of Petrology》2004,45(8):1539-1564

Concentrations of OH, and major and trace elements were determinedin a suite of mantle-derived megacrysts that represent the crystallizationproducts of a kimberlite-like magma at  相似文献   

A COMPARATIVE STUDY OF GLAUCONITE AND THE ASSOCIATED CLAY FRACTION IN MODERN MARINE SEDIMENTS   总被引：1，自引：0，他引：1  

DAVID L. BELL  H. GRANT GOODELL 《Sedimentology》1967,9(3):169-202

The crystallinity and mineralogy of both the glauconite and the clay fraction of samples from six contemporary marine environments were investigated by X-ray diffraction. In those areas where glauconite is now forming, the mineralogy and the degree of crystallinity of both the glauconite pellets and the associated clay fraction are similar. In contrast, detrital and relic glauconites are observed to have mineralogies that are different from their clay fractions. No consistent relationship was observed between degree of crystallinity and color of the pellets. Further, only two classes of glauconite as defined by BURST (1958) were common: expandable, interlayered clays and two or more clay minerals in a mixed assemblage. Based on the clay fraction-glauconite similarities and other supporting evidence, glauconite on the Scotia Ridge is concluded to be authigenic. Glauconite from Santa Monica Bay, California and from the continental shelf off Morocco appear to be detrital. Glauconite pellets in the shelf sediments off Guinea and the southeastern Atlantic Shelf of the United States are both detrital and authigenic. The poor crystallinity exhibited by the Chatham Rise glauconite is in contrast to the well-crystallized associated clay fraction and indicates that they are not genetically related. However, the origin of this glauconite remains in doubt.  相似文献   

The relationships between sedimentary structures, transport directions and dune types in Mesozoic aeolian sandstones, Atacama Region, Chile     

C. M. BELL 《Sedimentology》1991,38(2):289-300

Extensive occurrences of Upper Jurassic to Lower Cretaceous aeolian sandstones form part of a thick sequence of terrigenous red beds in northern Chile. These sediments accumulated in a N-S-elongated intermontane back-arc basin on the landward side of an active volcanic chain. This volcanic chain, which was produced by subduction beneath the continental margin, provided the source of almost all the clastic sediments. The aeolian sands formed isolated dune fields covering only part of the depositional basin at any one time. Cosets of cross-bedded sandstone up to 40 m thick represent the deposits of linear draas. Cross-bedded sets, which average between 0.5 and 1 m in thickness, were produced by the migration of small dunes down and along the gentle lee slopes of the draas. The majority of foreset laminations in the cross-bedded sets are thin, parallel and persistent. They are the product of the lateral migration of dunes which developed oblique to the prevailing winds. Foreset laminations dip at low angles (averaging 19°), well below the angle of repose of dry sand. Most of the laminations originated by tractional processes, rather than by grainflow (avalanching) or grainfall. This conclusion indicates that the dunes were gently undulating with no slip faces. The orientation of the direction of maximum dip of foreset laminations does not provide a reliable indicator of the palaeowind direction. Although individual sets or cosets commonly show a unidirectional pattern, significant variations have been recorded between sets of the same age at nearby locations and vertically between one coset and the next. Variations in vertical sections were produced by the superimposition of draas with different orientations.  相似文献