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51.
Abstract In the Fleur de Lys Supergroup, western Newfoundland, inclusion trails in garnet and albite porphyroblasts indicate that porphyroblasts overgrew a crenulation foliation, without rotation, probably during the deformation event that produced the crenulations. Further deformation of the matrix resulted in strong re-orientation and retrograde metamorphism of the matrix foliation, which is consequently highly oblique to the crenulation foliation preserved in the porphyroblasts. The resulting matrix foliation locally preserves relics of the early crenulations, and also has itself been crenulated later in places. Thus the porphyroblasts grew before the later stages of deformation, rather than during the final stage, as had been suggested previously. The new interpretation is consistent with available 40 Ar/39 Ar cooling ages which indicate a late Ordovician-early Silurian metamorphic peak, rather than the Devonian peak suggested by previous workers. The inclusion patterns and microprobe data indicate normal outward growth of garnet porphyroblasts from a central nucleus, rather than as a series of veins as proposed by de Wit (1976a, b). However, the observations presented here support growth of porphyroblasts without rotation, which is implied by the de Wit model. 相似文献
52.
The lithostratigraphy, depositional environment and age of the Marwar Supergroup have been reviewed in the light of report
of δ13C depletion recorded in the carbonates of the Bilara Group (middle part of Marwar Supergroup) and discovery of trilobite-like
trace fossils from the ·Red bedsŽ of Nagaur Group (upper part of Marwar Supergroup). The δ13C depletion observed in Bilara carbonates is not a result of glaciation rather due to rapid burial and poor water circulation
in the low energy water of the protected basin. Secondly, the trace fossils are, in fact, traces of notostracan crustaceans
found in shallow fluvial and shallow lacustrine environment. The present paper also records a spiral, burrowing trace-fossil,
possibly Gyrolithes, from a cross-bedded sandstone of the Jodhpur Group. 相似文献
53.
The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U–Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U–Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex). 相似文献
54.
Mukund Sharma 《Journal of Earth System Science》2006,115(1):67-98
Mesoproterozoic (∼ 1600 Ma old) Salkhan Limestone (Semri Group) of the Vindhyan Supergroup, exposed in Rohtas district of
Bihar, India, preserves an abundant and varied ancient microbial assemblage. These microfossils are recorded in three distinctly
occurring cherts viz., bedded chert, stromatolitic chert and cherty stromatolites. 27 morphoforms belonging to 14 genera and
21 species have been recognized. Six unnamed forms are also described.
The microbial assemblage, almost exclusively composed of the remnants of cyanobacteria, is dominated by entophysalidacean
members and short trichomes and can be termed as ‘typical Mesoproterozoic microbiotas’. The assemblage includes characteristic
mat-forming scytonematacean and entophysalidacean cyanobacteria.Eoentophysalis is the dominant organism in the assemblage. Ellipsoidal akinetes of nostocalean cyanobacteria(Archaeollipsoides) and spherical unicells also occur; both are distinct from mat forming assemblage, allochthonous and possibly planktic. Co-occurrence
of the microbiotas and precipitates is related to the depositional environment of the Mesoproterozoic tidal flats with high
carbonate saturation. 相似文献
55.
Abstract In metapelitic schists of the north-eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn-S1 and syn-S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn-S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred:
staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H2 O.
The amount of retrogression is controlled primarily by the amount of H2 O added by infiltration. As the syn-S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H2 O was added, the retrogression is focused in and around the porphyroblasts. With enough H2 O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred:
staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H2 O.
This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage. 相似文献
staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H
The amount of retrogression is controlled primarily by the amount of H
staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H
This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage. 相似文献
56.
Crystal size distributions (CSD) and shapes of accessory apatite,titanite and allanite are investigated in three texturally distinctgarnet zones (Z1Z3) and in the matrix (Z4) of a garnetepidotebiotitegneiss from the Moine Supergroup. Additionally, textural relationshipsand interactions between the accessory minerals and surroundingrock-forming minerals are considered, results of numerical CSDmodelling are presented, and geochronological and geologicalconsequences of the inferred CSD evolutions are discussed. Texturesand CSDs indicate that the accessory minerals were in, or near,a stage of nucleation and initial growth immediately prior togarnet Z1 overgrowth, and formed within less than 20 000 years,either by a size-independent or size-dependent growth mechanism.Subsequently, the CSDs were modified by different growth mechanisms,as supported by several parameters including CSDs, grain numbers,grain sizes, specific volumes and others. The apatite CSD evolutionfrom Z1 to Z4 is consistent with open-system LPE (Law of ProportionateEffects) growth accompanied and followed by supply controlledrandom ripening, whereas transformation of the original titaniteCSD is more consistent with Ostwald ripening, temporarily accompaniedby positive or negative McCabe growth. The allanite CSDs alsopoint to Ostwald ripening between Z3 and Z4. The textural observationsindicate that the growth evolution of the accessory phases wasinfluenced by mineral reactions with surrounding rock-formingminerals, as well as by deformation and matrix coarsening, ina manner similar to that found in more simple ceramic systems.The observed textures require a successive temperature risethroughout the tectono-metamorphic evolution of the investigatedrock, in agreement with existing PT data. Fast nucleationand initial growth of the accessory minerals during Z1 was perhapsinitiated by contact metamorphism, whereas subsequent growthand annealing (Z2Z4) result from regional metamorphicevents. KEY WORDS: apatite; titanite; allanite; CSD; Moine Supergroup; textures; kinetics 相似文献
57.
A. STALLARD 《Journal of Metamorphic Geology》1998,16(6):711-728
Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure–porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1–S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1–G4) and preserve a succession of five fabrics (S1–S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.
Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix. 相似文献
Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix. 相似文献
58.
59.
60.
A well preserved assemblage of compressed, straight, circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the Early Mesoproterozoic Rohtas Formation, Semri Group within Vindhyan Supergroup exposed in Katni district of central India. These megascopic remains are preserved as impressions, compressions, partially mineralized remains, and/or epi-relief. Some of the forms are typical filamentous empty sheaths and others are trichomes, with cell like entities under various stages of degradation. This study, based on fresh collections and also of the topotype material of the helically coiled megascopic fossils, straight forms and related fossilized remains occurring as epi-relief from Katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes. Grypania spiralis and Katnia singhii are most likely of cyanobacterial origin. Spirally coiled and circular fossils, with epi-relief, and which probably represents a tissue grade organism, are considered as Spiroichnus beerii Mathur, 1983. Linear sheet-like carbonaceous solitary form has been placed in the morphotaxon Proterotainia and described as P. katniensis n. sp. Certain rare circular, carbonaceous forms are considered as Chuaria sp. A few circular disc-like forms found in the assemblage are treated as dubiofossils. 相似文献