摘要:

AbstractSeventy‐seven spatially orientated, serial thin sections cut from a single rock reveal changes in the geometry of spiral‐shaped inclusion trails (SSITs) in garnet porphyroblasts. The observed SSITs are doubly curved, non‐cylindrical surfaces, with total inclusion‐trail curvature decreasing systematically from the cores to the rims of porphyroblasts. The three‐dimensional geometry of the SSITs, reconstructed with the aid of computer graphics, shows that the orientations of spiral axes defined by the SSITs are not related in any expected nor predictable way to the main foliation in the matrix. This suggests continued deformation after or during the latest stages of porphyroblast growth, which has important implications for the use of SSITs as shear‐sense indicators. Whether the formation of SSITs involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the ava

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00177.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe formation of spiral‐shaped inclusion trails (SSITs) is problematical, and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing. One model argues for porphyroblast rotation, with respect to a geographically fixed reference frame, whereas the other argues for no such porphyroblast rotation, but instead rotation of the matrix foliation around the porphyroblast. Thus, porphyroblasts with SSITs cannot be used as shear‐sense indicators until it is conclusively determined which model best explains them.Any successful model must explain features associated with SSITs, including: (1) foliation truncation zones, (2) smoothly curving SSITs, (3) millipede microstructure, (4) total inclusion‐trail curvature in median sections, (5) porphyroblasts with SSITs that have grown together, (6) evidence for relative porphyroblast displacements, (7) shear‐sense indicators inside and outside porphyroblasts; (8) crenulations associated with porphyroblasts and (9) geometries in sections subparallel to spiral axes (axes of rotation). A detailed study of these features suggests that most, if not all, can be explained by both the rotational and non‐rotational models, in spite of these models involving diametrically opposed movement senses. Therefore, geometrical analysis of individual porphyroblast microstructures may not determine which model best explains SSITs until the kinematics required to form these microstructures are better understood, in particular the sense of shear along a developing crenulation cleavage. Specific tests for determining the shear sense along crenulation cleavages are proposed, and results of such tests may conclusively resolve the debate over how S

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00178.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMuscovite‐poor pelitic schists in the wallrocks of the Proterozoic Annex sulphide deposit, near Prieska, South Africa, contain peak metamorphic assemblages including Crd + Bt + Sil, St + Sil + Bt, Crd + St + Bt and, rarely, Ky + St ° Crd. All rocks include oligoclase, quartz and commonly Fe–Mn garnet, with or without muscovite. Peak assemblages, assigned to M2 regional metamorphism in the Gordonia Belt (Namaqua Province), are syn‐ to post‐kinematic with respect to the main S2 fabric although larger staurolite grains contain S1 inclusion trails. Garnet–biotite thermometry, utilizing corrections for Fe3+, Mn, AlVIand Ti, yields peak temperatures of 571–624°C at pressures of 4.5–6.0 kbar. Consideration of the sympathetic variation ofXMnin garnet withXMgin biotite and the preserved zoning patterns in prograde garnets, together with the inferred prograde transition from kyanite to sillimanite, indicates that heating occurred during mild decompression to the M2 metamorphic peak. Sillimanite and cordierite grew last in the prograde sequence, possibly related to a pulse of thermal metamorphism (M3) that is found along the margin of the Keimoes Suite batholith to the north.Retrograde assemblages, including Ms + Ky + Chl + Qtz (after Crd + Bt), Ky + Ms (after Sil) and Chl + Ms (after St) indicate a period of isobaric cooling (M4a) terminated by rehydration in the kyanite stability field at about 500°C.The size difference between prograde (1–2‐mm) and retrograde (0.05–0.1‐mm) mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration, and explains why cordierite that grew during M2 is almost completely destroyed. Post‐M4a regrowth of staurolite and garnet (M4b) is spatially linked to sites of M4a rehydration. It reached temperatures of 510–530°C, remaining within the stability field of kyanite.A best fit of the observed textural history to the Namaqua orogenic cycle involves collision and heating (M2/D2) followed by granite intrusion (M3), rifting (M4a) and renewed heating due to crustal loading during volcanism (M4b). TheP–Tpath for the Annex region is consistent with those derived from elsewhere in the Gordonia Belt and, with modification, to that published already for

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00179.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe 6‐km‐thick Karmutsen metabasites, exposed over much of Vancouver Island, were thermally metamorphosed by intrusions of Jurassic granodiorite and granite. Observations of about 800 thin sections from the Campbell River and Buttle Lake area show that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion. The most important observations are as follows. (1) The compositional change of Ca‐amphiboles with increasing metamorphic grade is not straightforward. The tremolite component decreases from the prehnite–actinolite facies to the greenschist facies with a compensating tschermak component increase, but the tendency is not clear thereafter. Instead, the edenite component increases from the amphibolite facies to the pyroxene hornfels facies. (2) The most pargasitic Ca‐amphibole occurs in high‐Fe2+/Mg metabasite from the greenschist/amphibolite transition zone. (3) The reasons for such irregular compositional trends, even in the rather uniform MORB‐like composition of the Karmutsen metabasites, are non‐ideal solid solutions of Ca‐amphibole at low temperature and the effective control by bulk rock composition in the amphibolite facies. (4) The data from this study support, but do not prove, a transition loop for the actinolite–hornblende compositional gap rather than a solvus. If the gap is a solvus, its shape is asymmetric, and is highly dependent on the other compositional parameters such as Fe3+/Al and Fe2+/Mg. (5) TheXNaA/XA±XAb) ratios between Ca‐amphibole and plagioclase are most useful as an indicator of metamorphic grade even within the amphibolite facies, and these change systematically from 0.2 to 0.5 from the greenschist to pyroxene hornfels facies. (6) The compositional trend of Ca‐amphibole from the Karmutsen metabasites indicates a typical low‐P/Tmetamorphic facies series

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00180.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe central sector of Mühlig‐Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene‐bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite‐bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K‐feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K‐feldspar combined with interaction with an H2O‐rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high‐grade K‐feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re‐equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in c

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00181.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractLarge calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre‐scale fluid transport during amphibolite facies metamorphism. These 10‐ to 100‐m‐scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10‐km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcitevalues of –2 to –7% and δ18Ocalcitevalues of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole‐rockvalues of 3.5 to 7%. and unaltered banded calc‐silicate and marble wallrocks have δ13Ccalciteof –1.6 to –0.6%, and δ18Ocalciteof 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc‐silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc‐silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100‐ to 500‐m‐scale isotopic exchange with the surrounding metadolerites and calc‐silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc‐silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc‐silicates, or thermal fluctuations attending deformation and metamorphism. The overall L‐shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc‐silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc‐silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geother

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00182.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractGeological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge‐shaped body bounded below by the N‐dipping Main Central thrust system and above the N‐dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high‐Tdecompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between theP–Tpaths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan de

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00183.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractPortions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C.The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low‐angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates aP–Tpath dominated by decompression and cooling. The low‐angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low‐grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right‐slip movement along the steeply dipping Fowler Pass shear zone.An argon isotope‐correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a40Ar/39Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope‐correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low‐grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post‐dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages ofc.1100 Ma. Similar ages are not obtained north‐east of the Fowler Pass shear zone, sugges

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00184.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMetre‐scale amphibolite boudins in the Cheyenne Belt of south‐eastern Wyoming are cut and deformed by shear zones which preserve a full strain transition across 7 cm, from relatively undeformed amphibolite with a relict igneous texture to mylonitic amphibolite with an L‐S tectonic fabric. The strain transition is marked by the progressive rotation of amphibole + plagioclase aggregates into parallelism with the shear‐zone boundary. An increase in strain magnitude is indicated by development of the tectonic fabric and progressive reduction of amphibole and plagioclase grain size as a result of cataclasis. Bulk chemistry of five samples across a single strain transition shows no significant or systematic variation in major element chemistry except for a minor loss of SiO2, which indicates that the shear zone was a system essentially closed to non‐volatile components during metamorphism and deformation. Amphibolites throughout the shear zone consist of amphibole and plagioclase with only minor amounts of quartz, chlorite, epidote, titanite and ilmenite. Within the relatively undeformed amphibolite, amphibole and plagioclase have wide compositional ranges in single thin sections. Amphibole compositions vary from actinolitic hornblende to magnesio‐hornblende with increases in Al, Fe, Na and K contents and decreases in Si and Mg that can be modelled as progress along tschermakite, edenite and FeMg‐1exchange vectors from tremolite. Plagioclase ranges from An60in cores to An30within grain‐boundary domains. With increasing strain magnitude, local variation of amphibole composition decreases as amphibole becomes predominantly magnesio‐hornblende. Plagioclase composition range also decreases, although grain‐boundary domains still have higher albite content. These petrological data indicate that shear‐zone metamorphism was controlled by the magnitude of strain during synmetamorphic deformation. SEM and microprobe imaging indicate that chemical reactions occurred by a dissolution and reprecipitation process during or after cataclastic deformation. This suggests that grain‐boundary formation was an important process in the petrological evolution of the shear zone, possibly by providing zones for fluid ingress to facilitate metamorphic reactions. These results highlight the necessity for conducting detailed microstructural evaluation of rocks in order to interpret petrological, isotopic an

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00185.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Quantitative Methods in Petrology. Edited by T. M. Gordon.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00186.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY