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1.
The geochemistry of metamorphic quartz vein formation in Barrow’s index mineral zones north of Stonehaven, Scotland, was investigated in order to assess regional fluid flow and mass transfer. Metamorphic grade in the Dalradian metasediments increases to the north–northwest away from the Highland Boundary Fault (HBF) and associated ophiolitic rocks of the Highland Border Complex (HBC), passing through the Chlorite (Chl), Biotite (Bt), Garnet (Grt), Chloritoid (Cld), and Staurolite (St) zones. Syn-metamorphic fluid infiltration at 462±8.8 Ma (Breeding et al. in Am Mineral 89:1067, 2004) produced considerable quartz veining. Vein abundance varies from about 5 to 15 volume percent of the outcrops; veins tend to be more abundant in metapelitic layers than in metapsammitic ones. Metamorphic veins are surrounded by centimeter- to decimeter-wide zones of chemical and mineralogical alteration (selvages). Porphyroblasts, particularly Bt, Grt, Cld, and St, are typically larger in selvages than in wallrocks distal to veins. The altered selvages underwent fluid-driven addition of Na, Ca, and Sr, and loss of K, Rb, and Ba. Alteration is most intense within ∼750 m of the HBF, but is still very significant at the northern end of the field area some 2 km away. Mg/FeT (FeT=total iron) was either unchanged or increased due to alteration. Silica was added at some Chl and Bt zone localities near the HBF. Pb mass transfer was variable although Pb was added at a number of locations. Rare Earth elements (REE) were generally immobile, but light REE and possibly heavy REE were lost at one field site. The gain of Na and Ca and loss of K promoted the growth of plagioclase at the expense of micas (particularly muscovite) in selvages and wallrock inclusions throughout the field area and, probably, some calcite and/or dolomite growth directly adjacent to the HBF. The Ca gains were also critical for epidote production in the Bt zone. Gains of Ca and increases in Mg/FeT helped to stabilize Grt at the expense of Cld and St in some selvages. Hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one Cld zone locality. The metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow. In the first model, fluid flow in a direction of increasing temperature downward along the HBF added Na and Ca, and removed K from the Dalradian. In the second model, fluid flow upward from the HBC transported Na and Ca into the overlying Dalradian and, at the same time, stripped out K. The latter model is favored because it can most readily account for silica addition near the HBF, but neither model can be ruled out at present. In either case, the veins represent fractures that transmitted very large time-integrated fluid fluxes of at least ∼104 m3 (fluid)/m2 (rock). Consequently, the veins were conduits for regional fluid flow that caused considerable open-system chemical and mineralogical alteration during metamorphism. Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

2.
Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI1, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H2O that took place in psammitic rocks, and the second, BI2, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI1, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.  相似文献   

3.
In central Scotland, the Moy Intrusive Complex consists of (1) the Main Phase — syntectonic peraluminous granodiorite to granite emplaced at c. 455 Ma, intruded by (2) the Finglack Alaskite — post-tectonic leucocratic granite emplaced at 407+/-5 Ma. The Main Phase was emplaced into country rocks at amphibolite facies temperatures. Rb-Sr dates and a compositional spectrum of decreasing celadonite content in Main Phase muscovite suggest the persistence of c. 550° C temperatures for c. 30 Ma but with a declining pressure regime, i.e. isothermal uplift. The Finglack Alaskite was intruded at high structural level, leading to the development of a contact metamorphic aureole in the Main Phase. The thermal effects of contact metamorphism include intergrowths of andalusite, biotite and feldspar in pseudomorphs after muscovite. This is associated with recrystallized granoblastic quartz. Muscovite breakdown and reaction with adjacent biotite, quartz and feldspar, i.e. a function of local mineral assemblage rather than bulk rock composition, is postulated to explain the occurrence of metamorphic andalusite in a granitoid rock.The Main Phase pluton of the Moy Intrusive Complex lies within a NNE trending belt of c. 450 Ma Caledonian tectonic and magmatic activity paralleling the Moine Thrust, and extending from northern Scotland to the Highland Boundary Fault. Syntectonic S-type magmatism with upper crustal source areas implies crustal thickening and suggests an intracratonic orogeny.  相似文献   

4.
A number of gneiss-cored domes and antiforms are exposed along the regional strike-slip Najd fault system in the Arabian Shield and the eastern desert of Egypt. The mode of origin is still controversial, although plausible comparisons with modern metamorphic core complexes were made in some well-studied areas. The Kirsh dome is located within the major Ar Rika shear zone and consists of a core of orthogneiss/migmatite and an envelope of paragneisses with locally abundant kyanite-bearing quartzites. The dome is surrounded by the low-grade metasediments of the Murdama Group and is bound from the south by a low-angle dip-slip fault. Beyond the southern strand of the Ar Rika Fault is the Kibdi Basin which hosts unmetamorphosed sediments belonging to the Jibalah Group; this group occupies scattered pull-apart basins closely associated with releasing bends along the Najd fault system. Little dating has been done on the gneiss domes of the Arabian Shield; however, recent dates from similar structures in the eastern desert and Sinai range from 580 to 620?Ma. A similar, albeit younger 40Ar/39Ar age of 557?±?15?Ma was obtained from a biotite paragneiss south of Jabal Kirsh; this age difference probably represent the time interval it took the Kirsh rocks to cool below the biotite closure temperature and would place a lower age limit for the dome. The Kirsh dome occupies an extensional zone between left-stepping faults; movement within this zone might have caused enough decompression to trigger fluid-absent melting in the middle crust especially as the rocks cross the biotite dehydration solidus. Diapiric ascent aided by strike-slip dilatancy pumping led to the emplacement of the Kirsh rocks in their present position within the Murdama Group metasediments.  相似文献   

5.
The Leannan Fault of north-west Ireland is a sinistral strike-slip fault system which juxtaposes Dalradian metasediments of differing structural trends and metamorphic grades. It probably represents a south-west splay of the Great Glen Fault of Scotland. The recognition and tracing of the Foyle Synform across the fault zone, together with the correlation of regional Dalradian strike swings, lateral sedimentary facies variation and metamorphic grades, suggest a sinistral displacement of 34 km across the fault. Members of the Leannan Fault system displace a Lower Devonian (about 397 Ma) granite, but are overlain by Viséan (about 352 Ma) sandstones, thus constraining major late Caledonian sinistral motions to the Middle to Upper Devonian.  相似文献   

6.
Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass.

The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco2<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole.  相似文献   

7.
Cleaved metasedimentary clasts are present in stratigraphically and geographically distinct conglomerates in the Argyll and Southern Highland Groups of the Neoproterozoic Dalradian succession in the SW Scottish Highlands and NW Ireland. The significance and relationships of these clasts are that: (1) they were unequivocally reworked and deposited by sedimentary processes; (2) their internal foliation is probably due to contractional deformation that pre‐dates regional Caledonian fabrics; and (3) most of the cleaved clasts are only moderately deformed psammites and pelites and thus cannot be construed as having been derived from extensional mylonites. These conclusions, coupled with the generally accepted inferences that the Dalradian succession post‐dates Grenville deformation (c. 1100–1000 Ma) and pre‐dates early Palaeozoic Caledonian deformation (c. 470 Ma) and that the lowermost exposed Dalradian rocks, the Grampian Group, are truncated by a c. 806 Ma shear zone, imply that the clasts must have been foliated during an episode of mid‐Neoproterozoic contractional deformation. The clasts may thus represent further evidence in support of the contentious c. 870–800 Ma Knoydartian orogeny and thereby further render as equivocal interpretations that the Neoproterozoic tectonostratigraphic evolution of the Scottish Highlands and NW Ireland is a record of long‐lived ‘episodic’ extensional tectonism. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

8.
This study has examined the ~300 MPa partial melting behaviour of four metapelites collected from the highest grade rocks occurring below the anatectic zone of the Mt. Stafford area, Arunta Inlier, central Australia. In this area, metasediments are interpreted to have undergone partial melting within the andalusite stability field; possibly as a result of a lowering of the metapelite solidus by the presence of boron in the rocks. Two of the samples were two mica metapelites (MTS70 and MTS71). These both contained significant quantities of tourmaline and were thus boron enriched. The other two samples are biotite metapelites. One of these rocks contains only a trace of tourmaline (MTS8) and the other is tourmaline free (MTS7). Despite expectations that muscovite in the two mica samples would break down via a subsolidus reaction, muscovite was stable to above 750°C due to the incorporation of Ti, phengitic and possibly F components into its structure. Between 750 and 800°C, muscovite melted out completely via a coupled muscovite + biotite fluid-absent incongruent reaction. Tourmaline was partially consumed in this reaction, with the elbaitic component being preferentially consumed. In the most mica-rich sample this reaction produced ~60% melt at 800°C. In the biotite metapelites, biotite melting began at a temperature below 800°C and was accompanied by very modest melt production at this low temperature. In contrast to the two mica metapelites, the main pulse of melt production in these samples occurred at a temperature between 850 and 950°C. In both these samples biotite + melt coexisted over a temperature range in excess of 150°C, and in MTS8, biotite was still in the run products at 950°C. The very refractory nature of these evolved biotite compositions is most likely a consequence of both the presence of a Ti buffering phase in the assemblage (ilmenite) and the essentially plagioclase-free nature of the starting compositions. Under the fluid-absent conditions of this study, tourmaline is clearly a reactant in the partial melting process, but does not appear to shift the fluid-absent incongruent melting reactions markedly. In the tourmaline-rich two mica metapelites, tourmaline only disappears from the run products at a temperature above 850°C, where it coexisted with a substantial melt proportion. This appears to coincide with the point of maximum boron concentration in the melts.
Esmé M. SpicerEmail:
  相似文献   

9.
The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-XH2O relations among chlorite, biotite,garnet, staurolite, and kyanite at Ptotal above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H2O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H2O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H2O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.  相似文献   

10.
Phase relations of biotite and stilpnomelane in the greenschist facies   总被引：1，自引：0，他引：1  
Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified:
  1. muscovite+stilpnomelane+actinolite→ biotite+chlorite+epidote
  2. chlorite+microcline→ biotite+muscovite. Biotite produced by the first of these reactions has a limited range of variation in Fe/Mg. As grade advances within the biotite zone more magnesian and ferruginous biotites become stable in consequence of the two continuous reactions:
  3. muscovite+actinolite+chlorite→ biotite (Mg-rich)+epidote
  4. muscovite+stilpnomelane→ biotite (Fe-rich)+chlorite.
Stilpnomelane is stable in muscovite-free rocks throughout the biotite zone, and even up to the grade at which hornblende becomes stable. Phengitic muscovite is stable throughout the biotite zone in New Zealand and thus apparently does not contribute to the formation of biotite until a higher grade is reached.  相似文献   

11.
Regional metamorphic zones, based on mineral assemblages in pelites, are presented for the Dalradian rocks of Aberdeenshire and Banffshire, in the type area of Buchan metamorphism; electron microprobe analyses of cordierite (C), staurolite (S), chlorite (Chl), biotite (B) and white mica (Ms) are reported for rocks from the classic sections of the Banffshire coast and the valley of the Ythan.A low grade biotite zone, separates two NE-SW trending sets of higher grade zones, in which the sequence s defined by the entry of cordierite, andalusite (A) and (in the west only) staurolite. The zones are characterised by the assemblages (with quartz and muscovite) B-Chl, C-B-Chl, A-C-B and S-A-B.The western sequence contains a transition towards higher pressure, Barrovian type, metamorphism. The isograds arise from continuous reactions affecting rocks of restricted bulk composition (M/FM). With increasing grade, there is a regular trend towards more magnesian ranges of composition for the assemblages C-B-Chl, A-C-B and finally (as P increases in the west), S-A-B. The isograds form when these assemblages intersect the most Fe++-rich rock compositions present which occurs in each case when the biotite M/FM=40. A complex of divariant equilibria, derived for the system KFMASH, is used to model the natural reactions.  相似文献   

12.

Metamorphosed turbidites from the Omeo Metamorphic Complex show only minor changes in δ18O values with increasing metamorphic grade from 13.4 ± 1.7% in the chlorite and biotite zones to 12.3 ± 1.0% in the sillimanite + K‐feldspar zone. Rocks within 5 km of the S‐type granite at Hume Dam have δ18O values of 6.8–8.1% that probably reflect interaction with heated meteoric‐igneous fluids. Interaction with igneous fluids has also occurred close to other I‐ and S‐type granites in this region. However, pervasive metamorphic fluid‐rock interaction in this terrain did not occur, which limits the region's potential for hydrothermal mineralisation. Anatexis at high grades was probably via dehydration‐melting reactions that consumed muscovite and biotite, which is consistent with there being little fluid present during metamorphism. Small (kilometre scale or less) S‐type granites in the sillimanite + K‐feldspar zone have δ18O values similar to those of the surrounding metasediments and probably formed by melting of those rocks. By contrast, larger (tens of kilometres scale) Ca‐rich, peraluminous, S‐type granites have lower δ18O values than the surrounding metasediments, and may represent melts of underlying middle to lower crust.  相似文献   

13.
In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al2O3 plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al2 ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature.  相似文献   

14.
The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.  相似文献   

15.
18O/16O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The 18O/16O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ 18O of quartz of 12 to 14‰ (SMOW) and δ 18O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in 18O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high 18O/16O ratios of granitic rocks of this district were discussed in relation to the 18O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive 18O-depletion or 18O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.  相似文献   

16.
We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.  相似文献   

17.
In the year 1787, while visiting the Isle of Arran in southwestern Scotland, James Hutton made the initial discovery of what is now called an angular unconformity. In his example, Dalradian (Neoproterozoic - Cambrian) metasedimentary rocks are overlain by Early Carboniferous (Dinantian) sedimentary rocks of fluvial origin. The exact position of the plane of unconformity has been uncertain because of the presence of a resistant carbonate-rich zone (cornstone) on the Dalradian paleosurface. Whereas the unconformity was previously placed in an erosional re-entrant beneath the prominent carbonate-rich layer, it should be located at the contact between the Dalradian metasedimentary rocks, including those affected by cornstone formation, and overlying Dinantian pebbly sandstones. A prominent set of joints in the Dalradian rocks is sub-parallel to bedding in the overlying Carboniferous rocks and has contributed to ambiguity concerning placement of the plane of unconformity. A second exposure of Hutton's unconformity, at the SW end of the same small Carboniferous outlier on the Arran coast, displays the contact relationships unambiguously. Geochemical investigation of samples from the new locality shows that the cornstones are enriched in CaO, volatiles (LOI), MgO, and MnO. Formation of the cornstone involved replacement of siliciclastic Dalradian metasediments by dolomite. Relative to the fresh Dalradian rocks, the cornstones are depleted in most trace elements, but display enrichment in Sr, Y, Cu, and REE. The cornstone is enriched in REE in a systematic fashion, with a particularly strong concentration of MREE. A mirror-image pattern in the associated Dalradian rocks suggests that the REE enrichment observed in the cornstone is a result of leaching from the country rocks by groundwater, followed by evaporative concentration.  相似文献   

18.
Effects of H2O on the Disequilibrium Breakdown of Muscovite+Quartz   总被引：1，自引：0，他引：1  
We have examined in detail the effect of H2O on the texturesand mechanisms produced during the breakdown of muscovite+quartzunder experimental disequilibrium conditions using low-porosityrock samples. Under H2O conditions (1 wt.% H2O added) muscovitereacts completely in <5 months at 757?C to a metastable assemblageof peraluminous meltmullite+biotite, a reaction which delaysthe formation of the stable equilibrium assemblage. In contrast,muscovite in the H2O undersaturated experiments breaks downby both the stable dehydration reaction producing K feldspar+sillimanite+biotiteand by metastable melting reactions in the same sample. Thecom position of the metastable melt is controlled by a numberof kinetic factors which change as a function of time and reactionprogress. Early melts are highly siliceous and sodic due tothe rapid dissolution of quartz relative to muscovite, coupledwith the incongruent melting of the paragonite component ofmuscovite and crystallization of biotite. The delayed nucleationof mullite results in Al supersaturation of the melt, whichinhibits the rate of the melting reaction. Once mullite nucleatesthe degree of Al melt supersaturation is decreased and the rateof muscovite dissolution increases. After complete reactionof muscovite the melt chemistry continues to change as Si diffusesinto the pseudomorphs from adjacent quartz. Even after 5 monthsat 757?C large compositional gradients in Si and Al still persistwithin the melt. In the H2O experiments metastable melting occursinitially, catalysed by traces of grain boundary fluid or byfluid released from the dehydration of muscovite. However, oncemelting starts any fluid is strongly fractionated into the meltphase, reducing µH2O Under these conditions further meltingis inhibited and the stable dehydration reaction will continue.Examination of examples of natural muscovite reacted under disequilibriumconditions in xenolithic and contact metamorphic rocks at lowpressures suggests that metastable melting is an important processunder certain geological conditions. It is possible that itis widespread in contact metamorphic rocks, but the texturesindicative of metastable melting reactions are obscured duringthe extended cooling histories of such rocks.  相似文献   

19.
Abstract In the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event (D2) synchronous with the emplacement of muscovite-bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse-grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth. Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non-graphitic layers, were preferentially replaced by quartz. This resulted in muscovite-depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation of D2 crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite. Lack of deflection of the pre-existing mylonitic foliation and anastomosing of the axial planes of D2 crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existing foliations.  相似文献   

20.
The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An20–An29). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification.  相似文献   

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