ISSN:0072-1050    

DOI:10.1002/gj.3350300302

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractQuantitative textural analysis of garnetiferous rocks from diverse metamorphic environments has demonstrated that diffusionally influenced nucleation and thermally accelerated diffusion‐controlled growth are commonly among the dominant controls on porphyroblast crystallization. A new numerical model of diffusion‐controlled nucleation and growth that incorporates time as an explicit variable is capable of replicating the essential features of many of these natural porphyroblastic textures using geologically reasonable values for the kinetic parameters governing crystallization. This model has been used to interpret quantitative textural data from four rocks in which garnet grew during prograde regional metamorphism, by assuming constant heating rates and by accepting a previously published estimate for the activation energy for intergranular diffusion. From fits of the model to natural textures, estimates have been extiacted for three previously undetermined kinetic quanities that control the nucleation and growth of garnet porphyroblasts, namely the activation energy for nucleation and the pre‐exponential rate constants for nucleation and intergranular diffusion. The uncertainty in derived values for the pre‐exponential rate constant for intergranular diffusion is probably not much larger than a factor of two because it is proportional to the uncertainty in prograde heating rates, given a defined temperature interval over which crystallization occurs. The uncertainty in estimates of the activation energy and pre‐exponential rate constant for nucleation is larger, perhaps a factor of ten, because values for these quantities are derived from fits of the model to natural crystal size distributions; the precision of these estimates appears to be limited by the fact that the crystal size distributions are affected by factors not encompassed within the model (probably variable heating rates and inhomogeneity in the distribution of nutrients in the rock's p

ISSN:0072-1050    

DOI:10.1002/gj.3350300303

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe deformability of rocks, i.e. the resistance to deformation and the mode of failure, may be strongly influenced by syn‐tectonic metamorphic reactions. Such mechanistic interactions can often be inferred from studies on naturally deformed tectonites or from suitably designed experiments. The localized nature of retrograde reactions sometimes means that the particular interaction can be inferred as a result of the preservation of both the deformed rock and its protolith, but in prograde situations recrystallization at the metamorphic peak tends to destroy microstructural evidence of the deformation mechanism.Syn‐metamorphic mechanistic interaction can be classified according to the effects of: (a) elevated fluid pressure produced during dehydration or decarbonation reactions; (b) transient grain size changes with a possible transition to grain size sensitive flow; (c) effects of changes in pore fluid chemistry and recrystallization on the resistance to plastic flow of silicates (e.g. quartz); (d) enhancement of chemical potential gradients driving diffusive creep; and (e) enhancement of plasticity through the effects of solid phase volume changes. The first three of these effects are expected to be the most important. A soil mechanics approach to the description of the flow of porous rocks provides a basis for understanding deformation during regional prograde reactions, but simple, steady‐state constitutive laws of the type used to describe plastic flow of monomineralic rocks are unlikely to be able to be applied to syn‐metamorphic defo

ISSN:0072-1050    

DOI:10.1002/gj.3350300304

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAmong the most important controls on metamorphism is the rate at which tectonic processes operate. The purpose of this paper is to review briefly a couple of recent advances in the field of geochronology and metamorphic petrology which have helped to elucidate these processes. Re‐equilibration at high temperatures often eradicates information that might have been recorded by rocks during their prograde and high‐temperature evolution. One of the major advances of recent years has been the recognition of the utility of garnet in preserving information on prograde thermal histories. A case study involving garnet‐bearing rocks from New England is described: In this area, garnet grew during the latter stages of prograde metamorphism and records heating through 100°C during decompression of around 2.5 kbar. Combined with chronological data on garnet cores and rims, which yield a time interval for garnet growth of 10 Ma, these data yield a heating rate of ∼ 10°C Ma‐1 and an exhumation rate of about 1 mm a‐1. Comparison with simple thermal models suggests that the heating rate is too high to be accounted for by simple conductive relaxation. At temperatures appropriate to crustal melting, even garnet re‐equilibrates such that any chronological information is lost. At these grades one approach is to use the extent of diffusional modification of growth zonation profiles to provide time constraints on the process operating. Mn distributions in garnets from the Zanskar Himalaya are used to limit the time available for high‐grade metamorphism. The times obtained of 1–2 Ma are extremely short and require cooling by emplacement of cold upper crustal rocks adiacent

ISSN:0072-1050    

DOI:10.1002/gj.3350300305

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractReaction textures are often used to derive pressure‐temperature‐time (P‐T‐t) paths for metamorphic rocks. However, because dry rocks in particular react very little unless P‐T changes are accompanied by deformation and/or fluid influx, it is possible that mineral assemblages and reaction textures in polycyclic rocks record segments of P‐T paths that are unrelated in time. Such a case is documented, by garnet Sm‐Nd chronology, for a mafic granulite with well‐developed two‐stage garnet breakdown textures from SøHstrene Island, Prydz Bay, Antarctica. The combined geothermobaro‐metric‐geochronological evidence shows that the garnet formed during M1 at ca. 1000 Ma, ca. 850–900°C and 10 kbar. The garnet breakdown reactions producing symplectites of (1) orthopyroxene and plagioclase and (2) orthopyroxene, plagioclase and spinel occurred, respectively, during post‐M1 decompression and during a second granulite facies metamorphic episode (M2), in response to heating and decompression at ca. 500 Ma, ca. 750°C and 6 kbar. The date for the latter event derives from a nearby garnet‐sillimanite gneiss and corresponds to garnet ages which provide evidence for a regional granulite episode extending further north‐east along Prydz Bay. These data demonstrate that, in the absence of reliable criteria for heating and cooling and without age constraints, reaction textures cannot be assumed to record pressure or temperature changes in a single thermo‐tectonic event. Preservation of the ca. 1000 Ma age of garnet in rocks that have been reheated to ca. 700–750°C indicates that the closure temperature for Sm and Nd in garnet is at least 100°C higher tha

ISSN:0072-1050    

DOI:10.1002/gj.3350300306

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe results of SHRIMP U‐Pb zircon dating are presented for igneous and metamorphic rocks of enigmatic age in northern Japan. The SHRIMP analyses for the Tsubonosawa metamorphic rocks included in the Hikami Granite reveals (1) Ordovician sedimentation of the original rocks, including Archaean‐Proterozoic detrital zircons, and (2) thermal metamorphism with partial melting or assimilation followed by intrusion of the Hikami Granite at ca. 440 Ma (end of Ordovician). This study allows the reconstruction of Siluro‐Ordovician palaeogeography and magmatism‐associated metamorphism in East Asia. The SHRIMP results are compared with those of CHIME (chemical U‐Pb total Th isochron method) chronology, which is evaluated as providing significant reconnaissance ages in some

ISSN:0072-1050    

DOI:10.1002/gj.3350300307

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMica separates from 44 metamorphic rocks (mostly pelitic) from the southern area of Kamuikotan Gorge, central block of the Kamuikotan zone were dated using the K‐Ar method. The ages of white micas, separated from pelitic schists, range from 51 to 74 Ma. The Palaeogene K‐Ar ages are supported by 40Ar‐39Ar dating. Judging from mineral paragenesis and X‐ray diffraction studies (RM values) of white micas, most of the pelitic schists belong to the intermediate pressure type. In the Orowen River district, the northern part of the study area, biotite from metavolcanic rocks of a lower pressure assemblage yields the youngest K‐Ar ages (45–50 Ma). Although the Kamuikotan zone was considered to have been a subduction‐related metamorphic belt along a Mesozoic consuming margin, the time span of the metamorphism and the lowering of metamorphic pressure in younger rocks suggest that: (1) exhumation of the younger Kamuikotan rocks accompanied the erosion of forearc basin sediments (the Yezo Supergroup); (2) the increase in the thermal gradient was probably related to the approach of the Kula‐Pacific ridge to the subduction zone; and (3) the main stage of exhumation of the Kamuikotan rocks ceased before westward movement of the Pacific plate, which induced the collision of the Hidaka‐Nemuro belts (palaeo‐Kurile arc‐trench system) wit

ISSN:0072-1050    

DOI:10.1002/gj.3350300308

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractContinuing element exchange between minerals during cooling from the metamorphic peak modifies mineral compositions from those existing at the peak. After closure, when the minerals stop changing composition, the compositional relationships hold important information regarding, for example, cooling rates. Whereas the classic result of Dodson (1973) regarding closure provides an important limit, the situation involving element exchange with a slow diffusing mineral equilibrating with afinitefaster diffusing mineral is more valuable in considering metamorphic systems. An analytical extension to Dodson's equation for this situation in one dimension is presented here, and some of the consequences of departing from Dodson's limiting case are illustrated.

ISSN:0072-1050    

DOI:10.1002/gj.3350300309

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractHigh Mg‐Al granulites and calc‐silicate granulites provide evidence for ultra‐high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9–10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg‐Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile‐ilmenite or titanohaematite‐ferrianilmenite. These high Mg‐AI rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra‐high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinel‐cordierite‐quartz and sapphirine‐quartz and stabilization of the assemblages orthopyroxene‐sillimanite‐cordierite and spinel‐quartz‐sapphirine‐garnet and (ii) the high Al2O3 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P‐T path of evolution for the granulites. In calc‐silicate granulites stabilization of nearly pure meionite and of the wollastonite‐plagioclase‐andradite‐rich garnet, wollastonite‐scapolite‐grandite garnet‐calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9–10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near‐isobaric cooling followed by near‐isothermal decompression. On the basis of the available evidence it appears that non‐extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra‐high temperature me

ISSN:0072-1050    

DOI:10.1002/gj.3350300310

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractPelitic metasediments of the Witwatersrand Supergroup in the Vredefort Dome contain textural evidence for a two‐stage metamorphic history. The pressure‐temperature (P‐T) path derived for the rocks indicates that they were subjected to a high geothermal gradient (≈40°C km‐1), mid‐amphibolite facies, Mla metamorphic event concomitant with thickening of the overlying upper crust. Peak temperatures of 570–600°C were reached during Mla at depths of 14–16 km. After initial isobaric cooling following Mla, the rocks experienced rapid exhumation during a high strain rate deformation event associated with the formation of the dome. This event led to the development of abundant pseudotachylite and a brittle cleavage in these rocks. These features are overgrown. together with the Mla assemblages, by a low P M1b paragenesis comprising microporphyroblastic cordierite+biotite. Estimated P‐T conditions during Mlb were<3.5 kbar, ≈500–;530°C.The two‐stage P‐T path is incompatible with existing contact metamorphic and diapiric models that have attempted to link the mid‐amphibolite facies metamorphism with the formation of the Vredefort Dome. Instead, it indicates that the doming event occurred some time after the peak of a regional low P‐high T metamorphic event, during cooling of the terrain. Given an age of 2.02 Ga for the doming event, the Mla event is attributed to a widespread magmatic‐thermal event on the Kaapvaal Craton that accompanied the formation of the Bushveld Complex at 2.05–2.06 Ga. The Mlb event developed in response to the localized exhumation of deep parts of this terrain while the crustal geotherm was still elevated (≈30°C km‐1). The association, elsewhere in the dome, of pseudotachylite with shock metamorphic features generated at pressures in excess of 20–120 kbar indicates that exhumation was accomplished by the i

ISSN:0072-1050    

DOI:10.1002/gj.3350300311

出版商:John Wiley&Sons Ltd    

年代:1995

数据来源: WILEY