Home About us Contact
|
Home About us Contact | ||
|
|
||
Metamorphic History (metamorphic + history)
Selected AbstractsStructural position of the Seba eclogite unit in the Sambagawa Belt: Supporting evidence for an eclogite nappeISLAND ARC, Issue 2 2002Article first published online: 4 JUL 200, Mutsuki Aoya Abstract Eclogite-bearing units in the Sambagawa Metamorphic Belt have long been considered tectonic blocks that have disparate tectonic and metamorphic histories that are distinct from each other and from the major non-eclogitic Sambagawa schists. However, recent studies have shown that eclogite facies metamorphism of the Seba eclogite unit is related to the subduction event that caused the metamorphism of the non-eclogitic Sambagawa schist. New structural data further show that the Seba eclogite unit, which appears to be isolated from the other eclogite units, is in fact in structural continuity with them, occupying the highest structural levels in the Sambagawa Belt. This suggests that eclogitic metamorphism of the other eclogite units is also related to the Sambagawa subduction event. It is, therefore, possible that all eclogite units in the Sambagawa Belt constitute a single coherent unit, the eclogite nappe, members of which underwent the same eclogitic metamorphism related to the Sambagawa subduction event. [source] Contrasting metamorphic histories of lenses of high-pressure rocks and host migmatites with a flat orogenic fabric (Bohemian Massif, Czech Republic): a result of tectonic mixing within horizontal crustal flow?JOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2008TÍPSKÁ Abstract Migmatites with sub-horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite-K-feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane-wide exhumation of lower orogenic crust, or whether smaller portions of higher-pressure lower crust were combined with a lower-pressure matrix. Kyanite-K-feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7,8 kbar to around 700,840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite-K-feldspar granulite at 7,10 kbar and 750,850 °C. The contrasting prograde metamorphic histories indicate that kilometre-scale portions of high-pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P,T paths with different pressure peaks show that tectonic models explaining high-pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P,T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub-horizontal flow. [source] Separate or shared metamorphic histories of eclogites and surrounding rocks?JOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2006An example from the Bohemian Massif Abstract Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite-dominated volcano-sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite-bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet,staurolite,kyanite,biotite,plagioclase,muscovite,quartz,ilmenite ± rutile ± silli-manite and prograde-zoned garnet includes chloritoid,chlorite,paragonite,margarite, staurolite,chlorite,paragonite,margarite and kyanite,chlorite,rutile. In pseudosection modelling in the system Na2O,CaO,K2O,FeO,MgO,Al2O3,SiO2,H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite,+,paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet,staurolite,biotite,muscovite,quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet,amphibole,plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610,660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase,clinopyroxene symplectites, epidote and late amphibole,plagioclase domains. Garnet commonly includes rutile,quartz,epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O-undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote,amphibole,plagioclase indicating that decompression occurred under decreasing temperature into garnet-free epidote,amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km,1, respectively, to metamorphic pressure peaks that are 6,7 kbar apart. The eclogite,orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared. [source] Two contrasted P,T,time paths of coronitic metanorites of the French Massif Central: are reaction textures reliable guides to metamorphic histories?JOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2005C. NICOLLET Abstract Metanorites from two eclogitized metagabbros of the Hercynian French Massif Central preserve coronitic textures of hornblende, garnet, quartz and/or kyanite produced at the expense of the primary magmatic assemblage orthopyroxene and plagioclase. Using a petrogenetic grid in the CFMASH system, two possible P,T evolutions for the origin of the coronas are evaluated. The sequence of reactions involving the formation of Hbl (,Ky) ± Grt and Qtz coronitic assemblages is consistent with an isobaric cooling at high pressure (c. 1,2 GPa) under hydrated conditions. However, this P,T path, inferred by using only petrographical observations, is inconsistent with the geochronological constraints: emplacement of the gabbro at 490 Ma and high-pressure metamorphism at 410 Ma. In order to reconcile petrographical observations with geochronological constraints, we propose a discontinuous two-stage evolution involving a change in water activity with time. (1) Emplacement and cooling of the norite at low pressure under anhydrous conditions, at 490 Ma. (2) During the Hercynian orogeny, the norite experienced an increase in pressure and temperature under fluid-present conditions. Adding water to the system implies a dramatic change in the petrogenetic grid topology, restricting the orthopyroxene,plagioclase assemblage only to high temperatures. Therefore, the breakdown of the unstable magmatic assemblage, through apparent retrograde reactions, occurred along the prograde P,T path which never crossed the equilibrium boundaries of these reactions. [source] Zeolites in fissures of granites and gneisses of the Central AlpsJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2010T. WEISENBERGER Abstract Six different Ca-zeolite minerals are widespread in various assemblages in late fissures and fractures in granites and gneisses of the Swiss Alps. The zeolites formed as a result of water,rock interaction at relatively low temperatures (<250 °C) in the continental upper crust. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as monomineralic fissure fillings. They represent the youngest fissure minerals formed during uplift and exhumation of the Alpine orogen. A systematic study of zeolite samples showed that the majority of finds originate from three regions particularity rich in zeolite-bearing fissures: (i) in the central and eastern part of the Aar- and Gotthard Massifs; (2) Gibelsbach/Fiesch, in a fissure breccia located at the boundary of Aar Massif and Permian sedimentary rocks; and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca). Rail and road tunnel construction across the Aar- and Gotthard Massif provided excellent data on zeolite frequency in Alpine fissures. It was found that 32% (Gotthard NEAT rail base tunnel, Amsteg section) and 18% (Gotthard road tunnel) of all studied fissures are filled with zeolites. The number of different zeolites is limited to six species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Calcium is the dominant extra-framework cation, with minor K and Na. Heulandite and chabazite contain Sr up to 29 and 10 mol.% extra-framework cations respectively. Na and K contents in zeolites tend to increase during growth as a result of changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compared to subsurface samples may indicate late stage cation exchange with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations show that the Ca-dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite. The necessary components for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite fissures occur in areas of H2O-dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature. [source] Age and early metamorphic history of the Sanbagawa belt: Lu,Hf and P,T constraints from the Western Iratsu eclogiteJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2009S. ENDO Abstract Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c. 120,110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c. 88,89 Ma based on a garnet,omphacite Lu,Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units,all ,20 kbar and 600,650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu,Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote,amphibolite facies parageneses equilibrated at 550,650 °C and ,10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu,Hf age of c. 116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap (c. 27 Myr) between the two Lu,Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone. [source] Constraints on the early metamorphic evolution of Broken Hill, Australia, from in situ U-Pb dating and REE geochemistry of monaziteJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2009C. R. M. McFARLANE Abstract The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c.1630 Ma and c.1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c.1657 Ma. [source] Monazite geochronology in central New England: evidence for a fundamental terrane boundaryJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2008F. S. SPEAR Abstract Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west-central New Hampshire, each major tectonic unit (nappe) displays a distinctive P,T path and metamorphic history that requires significant post-metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low-pressure, high-temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. 40Ar/39Ar ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high-level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages (c. 430,340 Ma). Both the metamorphic style and monazite ages suggest that the low-grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257,300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian. [source] Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern AlpsJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2006M. JANÁK Abstract New evidence for ultrahigh-pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet-bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase-peridotite or the spinel-peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low-Al orthopyroxene + clinopyroxene + Cr-spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr-spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high-Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet-olivine and garnet-orthopyroxene Fe-Mg exchange thermometers and (ii) the Al-in-orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation. [source] The formation of eclogite facies metatroctolites and a general petrogenetic grid in Na2O,CaO,FeO,MgO,Al2O3,SiO2,H2O (NCFMASH)JOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2002G. Rebay Abstract Eclogite facies metatroctolites from a variety of Western Alps localities (Voltri, Monviso, Lanzo, Allalin, Zermat,Saas, etc.) that preserve textural evidence of their original form as bimineralic olivine-plagioclase rocks are considered in terms of calculated mineral equilibria in the system Na2O-CaO-FeO-MgO-Al2O3 -SiO2 -H2O (NCFMASH). Pseudosections, based on a new petrogenetic grid for NCFMASH presented here, are used to unravel the metamorphic history of the metatroctolites, considering the rocks to consist of different composition microdomains corresponding to the original olivine and plagioclase grains. On the basis that the preservation of the mineral assemblage in each microdomain will tend to be from where on a rock's P,T path the metamorphic fluid phase is used up via rehydration reactions, P,T pseudosections contoured for water content, and P,T path-MH2O (amount of water) pseudosections, are used to examine fluid behaviour in each microdomain. We show that the different microdomains are likely to preserve their mineral assemblages from different places on the P,T path. For the olivine microdomain, the diagnostic mineral assemblage is chloritoid + talc (+ garnet + omphacite). The preservation of this assemblage, in the light of the closed system P,T path-MH2O relationships, implies that the microdomain loses its metamorphic fluid as it starts to decompress, and, in the absence of subsequent hydration, the high pressure mineral assemblage is then preserved. In the plagioclase microdomain, the diagnostic assemblage is epidote (or zoisite) + kyanite + quartz suggesting a lower pressure (of about 2 GPa) than for the olivine microdomain. In the light of P,T path-MH2O relationships, development of this assemblage implies breakdown of lawsonite across the lawsonite breakdown reaction, regardless of the maximum pressure reached. It is likely that the plagioclase microdomain was mainly fluid-absent prior to lawsonite breakdown, only becoming fluid-present across the reaction, then immediately becoming fluid-absent again. [source] High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P,T paths and geotectonic significanceJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2002J. H. Guo Abstract High-pressure basic granulites are widely distributed as enclaves and sheet-like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P,T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low-Ca core of growth-zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750,870 °C and 11,14.5 kbar (M2) is defined by high-Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770,830 °C and 8.5,10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500,650 °C and 5.5,8 kbar (M4). These results help define a sequential P,T path containing prograde, near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P,T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high-pressures (6,14.5 kbar) of the four metamorphic stages and the geometry of the P,T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper-plate might be the tectonically overlying Khondalite series, which was subjected to medium- to low-pressure (MP/LP: 7,4 kbar) granulite facies metamorphism with a clockwise P,T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90,1.85 Ga in a collisional environment created by the assembly process of the North China craton. [source] Cretaceous high- P granulites at Milford Sound, New Zealand: metamorphic history and emplacement in a convergent margin settingJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2000G. L. Clarke Granulite facies orthogneiss of the Arthur River Complex (ARC) at Milford Sound, western Fiordland records a complex Early Cretaceous magmatic and orogenic history for the Pacific Gondwana margin that culminated in the emplacement and burial of a dioritic batholith, the Western Fiordland Orthogneiss (WFO). Enstatite-bearing mafic to intermediate protoliths of the ARC and WFO intruded the middle to upper crust. The early deformation history of the ARC is preserved in the Pembroke Granulite, where two-pyroxene S1 assemblages that reflect P<8 kbar and T >750 °C were only patchily recrystallized during later deformation. S1 is cut by garnet-bearing, leucogabbroic to dioritic veins, which are cut by distinctive D2 fractures involving anorthositic veins and garnet,diopside,plagioclase-bearing reaction zones. These zones are widespread in the ARC and WFO and record conditions of P,14 kbar and T >750 °C. Garnet,clinopyroxene-bearing corona reaction textures that mantle enstatite in both the ARC and WFO reflect Early Cretaceous burial by approximately 25 km of continental crust. Most of the ARC is formed from the Milford and Harrison Gneisses, which contain steeply dipping S4 assemblages that envelop the Pembroke Granulite and involve garnet, hornblende, diopside, clinozoisite, rutile and plagioclase, with or without kyanite. The P,T history of rocks in western Fiordland reflects pronounced Early Cretaceous convergence-related tectonism and burial, possibly related to the collision and accretion of island arc material onto the Pacific Gondwana margin. [source] | ||||||||||