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Showing papers on "Metamorphism published in 1993"


Journal ArticleDOI
TL;DR: In this article, the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabbie Mountains in the late Jurassic and Cretaceous (at 134 Ma).

987 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a boundary geometry model for the collision of the North China Block and the South China Block (SCB) that predicts the distribution and ages of metamorphism along the suture and the observed local but intense Triassic deformation.
Abstract: Passive continental margins are geometrically irregular as a consequence of either triple-junction evolution or the development of transfer zones in detachment fault systems, whereas active continental margins are smoothly arc-shaped due to subduction of plates on the Earth's spherical surface. We propose that this basic difference in boundary geometry has played an important role in the latest Paleozoic-early Mesozoic collision of North and South China. In particular, we suggest that prior to collision, the active southern margin of the North China Block (NCB) was contiguous across the Qilian Shan, Qinling, Dabie Shan, Shandong peninsula of east central China to the Imjingang area of central Korea. The passive northern margin of the South China Block (SCB), in contrast, had a more irregular shape, such that its northeastern segment in northern Jiangsu and eastern Anhui provinces of China extended some 500 km farther north than its western counterparts in northern Sichuan, southern Shaanxi, and northern Hubei provinces. Collision of the NCB and the SCB began by indentation of the northeastern SCB into the eastern NCB in the late Early Permian and lasted until the Late Triassic-Early Jurassic. The indentation produced the left-slip Tan-Lu fault in northeastern China and the right-slip Honam shear zone in southeastern Korea and caused the northward displacement of the Shandong and the Imjingang metamorphic belts. This model predicts that collision along the Dabie and Qinling metamorphic belt occurred significantly later than along the Shandong belt, which is consistent with radiometric and depositional constraints on the time of collision. The proposed model accounts for the abrupt termination of the Tan-Lu fault at its south end and the drastic decrease in slip along the Tan-Lu fault north of the Shandong metamorphic belt. The model also predicts the distribution and ages of metamorphism along the suture and the observed local but intense Triassic deformation (=Indosinian orogeny) in northeastern China and northern Korea, which was previously an enigmatic feature in this region.

899 citations


Journal ArticleDOI
TL;DR: Baddeleyite is an ideal mineral for U-Pb dating because it has abundant U (up to 3000 ppm), negligible initial common Pb, rarely occurs as xenocrysts, and, in unmetamorphosed samples, experiences negligible Pb loss as discussed by the authors.

416 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2.55-2.51 Ga and was related to accretional processes of juvenile crust.
Abstract: A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.

281 citations


Journal ArticleDOI
TL;DR: In this paper, a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece) is presented, where a major low-angle fault zone separates surface rocks above the contact from an initially deep-seated unit below, showing a metamorphic evolution from high to low pressures.
Abstract: This paper presents a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece). Previous maps and studies of the two islands have shown that a major low-angle fault zone separates surface rocks above the contact from an initially deep-seated unit below, showing a metamorphic evolution from high to low pressures. Structural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault zone is a major normal-sense detachment zone dipping to the north. Rapid denudation of footwall rocks subsequent to high temperature metamorphism, at an estimated rate of 1.8–9.5 mm/yr, attests for tectonic unroofing during regional-scale top-to-the north ductile shearing. The change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment. Mio-Pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone. These sediments are as old as (Aquitanian-25 Ma), or younger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in the Cyclades, which appears significantly older than maximum ages reported up to now (13–5 Ma). Structural data strongly suggest that the detachment fault was initially rather low dipping (≈35°). An evolutionary model is proposed, in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks, during a process of asymmetric boudinage of the crust. This detachment model applies to a previously thickened continental lithosphere, which then suffers thermal relaxation and weakening, allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism. In the Cyclades, crustal-scale extension started after Early Cenozoic thrusting, while the crust was still thick, or less likely, before late underthrusting below the present surface.

240 citations


Journal ArticleDOI
TL;DR: In this paper, a review of the evolution of orogenic belts as interpreted from the P-T-t paths of metamorphic rocks is presented, by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenical belts.
Abstract: Barrow (1893) introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the P–T–t paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts. Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum T and maximum P. Orogenic belts characterized by clockwise P–T paths achieved maximum P before maximum T, the metamorphic peak normally post-dated early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise P–T paths achieved maximum T before maximum P, the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of P–T–t paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual P–T–t paths, and a set of P–T–t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature–low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.

213 citations


Journal ArticleDOI
TL;DR: A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented in this article.
Abstract: A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess 40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.

207 citations


Journal ArticleDOI
01 Apr 1993-Geology
TL;DR: The Nanga Parbat-Haramosh massif of the western Himalaya is a north-trending half-window of Indian crust that provides spectacular exposures of Precambrian basement gneisses that have been overprinted by Himalayan metamorphism.
Abstract: The Nanga Parbat-Haramosh massif of the western Himalaya is a north-trending half-window of Indian crust that provides spectacular exposures of Precambrian basement gneisses that have been overprinted by Himalayan metamorphism. We report here petrologic data and U/Pb dates on zircon and monazite which document that Nanga Parbat gneisses underwent a Pliocene-Pleistocene episode of high-grade metamorphism and anatexis during an interval in which the Nanga Parbat massif was undergoing rapid denudation at mean rates of ∼5 mm/yr. We speculate that by initiating decompression melting, this denudation may be at least partly responsible for the anatexis and high-grade metamorphism.

206 citations


Journal ArticleDOI
TL;DR: Petrological, oxygen isotope and 40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos.
Abstract: Petrological, oxygen isotope and 40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high-pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high-pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some 40Ar/39Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post-peak high-pressure metamorphic conditions. Early stages in the prograde high-pressure evolution may be documented by older apparent ages in the high-temperature steps of some spectra. Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high-pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some greenschists and late-stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism. Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a δ18O-enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ18O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.

203 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used stable isotope data from the Catalina Schist to predict mass changes along different P-T paths, and showed that both mechanisms could be effective at producing the range of observed features, even though the required equilibrium constants are only poorly estimated at the relevant P-Ts conditions.

203 citations


Journal ArticleDOI
TL;DR: In this paper, the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance.

Journal ArticleDOI
TL;DR: In this paper, the authors compared two orogenic domains, Basin and Range province and the Variscan belt in the French Massif Central, and their tectonic significance for extension mechanisms is discussed.
Abstract: From two examples of orogenic domains, some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized. The Basin and Range province and the Variscan belt in the French Massif Central have both suffered important compressional orogenic crustal thickening, and the results of late orogenic processes can be observed in the field. Both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness. Late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed. At the scale of the orogenic domains, the most prominent tectonic features are the metamorphic core complexes (MCC) which expose deformed rocks from the middle crust generally affected by high-temperature, low to medium pressure metamorphism, partial melting, and widespread granite emplacement. In these MCC, large-scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations. They show a complete evolution from early deep-seated ductile deformation (generally achieved under high-temperature, low to medium pressure metamorphism) to a late shallow brittle stage characterized by cataclastic deformation. The late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits. Two main geometries of MCC are defined that are characterized by differing geometry and kinematics of low-angle shear zones. In the first case, two low-angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical MCC (often one system is dominant over the other). The second geometry characterizes asymmetrical MCC bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation. Detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow. Deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain. Strain distribution within the lower crust is less well constrained by field observation; however, analogies between COCORP and ECORS deep seismic reflection profiles give important constraints on crustal structure. Wide zones of highly subhorizontally layered lower crust and a flat high-amplitude reflection Moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension. A kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains. A weak, hot upper mantle allows large-scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation. Heterogeneous strain is accommodated by low-angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust. Combined pure and simple shear occurs along localized shear zones, whereas at the scale of the whole lithosphere, deformation nearly corresponds to a vertical pure shear. Such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics. Thus, as much in the Andean-type west American Cordilleran belt as in the collision-type Variscan belt, late orogenic processes produced similar extensional features.

Journal ArticleDOI
TL;DR: In this paper, inclusions in garnets from a metapelite from the northern Wind River Range have been analysed by SIMS, yielding 2°7pb/Z°6pb age estimates of 2781 + 6 to 2809 + 10 Ma.

Journal ArticleDOI
TL;DR: Petrographic and thermobarometric analysis provided constraints on the path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indochina during the Tertiary as mentioned in this paper.

Journal ArticleDOI
TL;DR: A12O3 trondhjemitic gneisses are found in enclaves within granites at Markampara in the Bastar Craton, Central India.

Journal ArticleDOI
01 Feb 1993-Lithos
TL;DR: In this paper, the authors proposed an alternative to the back-arc model proposed by previous authors, in which the high-grade Marvejols Group and the low-grade Albigeois-Cevennes sedimentary sequence contain bimodal igneous rocks.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the mobility of B and Be in H2O-rich fluids and felsic silicate liquids produced during metamorphism of subducted oceanic slab and sediments through analysis of subduction-zone metamorphic rocks of the Catalina Schist, California.

Journal ArticleDOI
TL;DR: Low-salinity, high-temperature (>200 degrees C), reducing fluids are poorly represented in ore-forming processes, yet appear to account for 90 percent of the primary gold mined to date.
Abstract: Low-salinity, high-temperature (>200 degrees C), reducing fluids are poorly represented in ore-forming processes, yet appear to account for 90 percent of the primary gold mined to date. Such fluids have been implicated in the formation of many Archcan greenstone, Witwatersrand, slate belt, and epithermal gold deposits. These low-salinity auriferous fluids occur in variable time, space, lithological sequence, and tectonic settings and are most easily explained by metamorphic devolatilization of mafic and/or graywacke successions.Metamorphism of chlorite-calcite-albite-quartz assemblages (e.g., mafic or graywacke) at the greenschist-amphibolite facies boundary occurs around 480 degrees + or - 20 degrees C (for 3-5 kbars) and produces large volumes of low-salinity, H 2 O-CO 2 fluid similar in composition to those recorded in many gold deposits. Widespread pyrite in the above assemblage typically leads to elevated levels of dissolved sulfur. Such conditions are ideal for gold transport as a molecular Au-S complex and can lead to deposits rich in gold relative to silver and base metals.Deposition of gold occurs in the temperature range of 250 degrees to 400 degrees C and can be facilitated by lower temperature, lower sulfur activity, and changes in oxygen activity. Lower sulfur activity is most readily achieved by fluid interaction with Fe-rich host rocks with the formation of pyrite or pyrrhotite, whereas lower oxygen activity is readily achieved by interaction with carbonaceous metasediments.Low-salinity fluids produced by metamorphic devolatilization provide a common theme embracing the formation of many Archcan greenstone, slate belt, Witwatersrand, and potentially epithermal gold deposits. It involves the common features of high geothermal gradient, low-salinity fluid, high Au/Ag, high Au/base metals, and broad synchroneity with metamorphism. It also accounts for the considerable variability in hosting structures, detailed timing relations, host rocks, alteration assemblages, and geologic age of different deposits. Interestingly, the similarities are explained by deeper processes, the differences by processes at or near the depositional sites.

Journal ArticleDOI
TL;DR: In this paper, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta- morphism.
Abstract: Based on lithological, structural and geophysi- cal characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones In order to constrain the timing of meta- morphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites In addi- tion, U-Pb ages were obtained for monazites from meta- pelites and for a rutile from an amphibolite These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals: Central Granulite Terrane:

Journal ArticleDOI
TL;DR: In this article, a model is proposed of an initial calc-alkalic enderbitic mass extracted from the mantle at ∼1050 Ma and emplaced in the lower crust; this ender bituminous parent is considered as the source for successive alkali-rich melts which were emplaced synkinematically between 1050 and 850 Ma.

Journal ArticleDOI
05 Aug 1993-Nature
TL;DR: In this paper, the authors reported the discovery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxene and blue amphibole) in clasts from a serpentinite seamount in the forearc of the active Mariana subduction zone.
Abstract: THE high-pressure, low-temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones, where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures. Until now, however, the link between blueschist-facies rocks and subduction zones has been indirect, relying on a spatial association of blueschists with old subduction complexes, and estimates of the geothermal gradients likely to exist in subduction zones. Here we strengthen this link, by reporting the discovery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxene and blue amphibole) in clasts from a serpentinite seamount in the forearc of the active Mariana subduction zone. The metamorphic conditions estimated from the mineral compositions are 150–250 °C and 5–6 kbar (16–20 km depth). The rocks must have been entrained in rising serpentine mud diapirs, and extruded from mud volcanoes onto the sea floor. Further study of these rocks may provide new insight into the tectonics of trench-forearc systems, and in particular, the processes by which blueschist-facies clasts come to be associated with forearc sediments in ancient subduction complexes.

Journal ArticleDOI
TL;DR: In this article, it was shown that the transition from amphibolite to granulite facies is characterized by a decrease in aH2O, as well as a temperature increase.

Journal ArticleDOI
TL;DR: In this paper, U-Pb zircon ages for the Archean southern Abitibi greenstone belt were summarized and integrated with zircons and baddeleyite data for three new samples, into a general framework for the evolution of the region.
Abstract: This paper summarizes U-Pb zircon ages for the Archean southern Abitibi greenstone belt and integrates them, together with zircon and baddeleyite data for three new samples, into a general framework for the evolution of the region. New ages are reported for the Ghost Range gabbroic complex (2713 (super +7) (sub -5) Ma) and for a pyroclastic volcanic rock of the Gauthier Group (2700 + or - 3 Ma). A previously determined age of 2701 (super +3) (sub -2) Ma for the felsic volcanic Skead Group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component (> or =2720 Ma) indicating that the Skead volcano was built on top of an older volcanic sequence. The compiled ages show that the southern Abitibi greenstone belt developed between 2750 and 2670 Ma. The earliest ages date remnants of volcanic complexes that occur at the periphery of the belt. Major preorogenic magmatism was confined mainly to the period 2720 to 2700 Ma. In general, each episode was characterized by the essentially coeval formation of komatiitic, tholeiitic, and calc-alkalic igneous rocks. Taking into consideration the known geologic, geochemical, and isotopic data, this association is consistent with evolution in paired, active are and back-are systems. Older crust was absent from the region, but it may have interacted with these arcs and rift systems farther to the west. The extensional regime was followed by a period of compression that caused folding and thrusting, emplacement of calc-alkalic plutons, and deposition of turbidites between 2700 and 2688 Ma. The magmatism became predominantly alkalic during the main Timiskaming period between 2681 and 2676 Ma. This event was accompanied by the deposition of alluvial-fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting, probably between approximately 2676 and 2670 Ma. Lamprophyre dikes were associated with these final stages of deformation. Postorogenic activity includes faulting and episodes of hydrothermal activity, locally associated with gold mineralization. Ages in the range 2630 to 2580 Ma for hydrothermal minerals (e.g., titanite and rutile; Camflo) suggest a correlation of these events with lower crustal magmatism and metamorphism, as recorded, for example, in the adjacent Kapuskasing zone.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the physical conditions under which a temperature inversion could occur and showed that an inverted temperature gradient cannot form without heat generation in the fault zone unless V × zƒ × sin δ ≳ 100, where V is the rate of underthrusting (in millimeters per year), z is in kilometers, and δ is the dip of the fault.
Abstract: Several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade. This configuration may indicate, but does not require, that temperature gradients were temporarily inverted near the fault. We examine the physical conditions under which such a temperature inversion could occur. The overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth, zƒ, on the fault is less than π times the characteristic time, , for diffusion of heat through the block above the fault, where κ is thermal diffusivity. An inverted gradient cannot form without heat generation in the fault zone unless V × zƒ × sin δ ≳ 100, where V is the rate of underthrusting (in millimeters per year), zƒ is in kilometers, and δ is the dip of the fault. This simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone. Dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone, but whereas the overthrusting of hot upon cold rocks cools the fault zone, dissipation heats it. Thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently. We show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating. Discrepancy between these estimates implies that some additional process must have occurred, such as the post-thrusting disruption of the isograds. If there is such a discrepancy, the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault. We illustrate this analysis by applying it to reported inverted metamorphic zonation in the Pelona Schist, the St. Anthony Complex, the Mt. Everest region of the Main Central Thrust, and the Olympos Thrust. Petrological inferences of maximum temperatures, depths of metamorphism, and magnitudes of apparent inverted gradients, imply that shear stresses of about 100 MPa accompanied thrust faulting in some of these regions, and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured.

Journal ArticleDOI
TL;DR: In this article, the main central thrust (MCT) formed a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments, which is bounded to the north by the Vaikrita (roof) and to the south by the Munsiari (floor) Thrust.
Abstract: Abstract Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south. Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted P-T gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab. Cooling rates for the upper MCT zone determined from 40Ar/39Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende 40Ar/39Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above c. 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.

Journal ArticleDOI
TL;DR: In this article, U-Pb isotopic ages for two subhorizontal granulite gneisses in the Kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m.y. after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust-mantle delamination.

Journal ArticleDOI
Harry Becker1
01 Jul 1993-Geology
TL;DR: In the central Alps the upper Pennine Cima Lunga nappe (Lepontine dome, Swiss Alps) underwent eclogite facies conditions up to 850-900 °C and 3.5-4.2 GPa.
Abstract: In the central Alps the upper Pennine Cima Lunga nappe (Lepontine dome, Swiss Alps) underwent eclogite facies conditions up to 850-900 °C and 3.5-4.2 GPa. Sm-Nd dating of garnet peridotites and an eclogite from the Cima Lunga nappe yields consistent mineral ages of ca. 40 Ma. The mineral ages are probably cooling ages, but petrological arguments indicate that they approximate the time of eclogite facies metamorphism, implying that the latter is related to the collision of the Adriatic plate with Europe. Emplacement of the upper Pennine nappe stack with the hot Cima Lunga nappe at its base onto European basement (lower Pennine units) is suggested to be the primary cause for the middle Tertiary Barrovian-type metamorphism in the central Alps. This process accounts for the different metamorphic evolution of the Cima Lunga nappe compared to underlying lower Pennine units and explains the time difference between Sm-Nd mineral ages from the Cima Lunga nappe and U-Pb monazite ages from the lower Pennine units.

Journal ArticleDOI
TL;DR: In this article, the age of a garnet microstructured from a metapelite in the Acadian metamorphic terrain of eastern Vermont has been analyzed for major elements and segregated into different fractions for isotopic analysis.
Abstract: Garnets, up to 1.2 cm across, from a metapelite in the Acadian metamorphic terrain of eastern Vermont have been analysed for major elements and segregated into different fractions for isotopic analysis. The garnets preserve abundant inclusions of minerals present during garnet growth which allow a nearly complete reaction history to be established. The Sm-Nd and U-Pb isotopic analyses yield concordant ages of ∼380 Ma for the rim of one garnet and this is interpreted as the formation age. The changing mineral assemblages through garnet growth, their evolving compositions and the thermodynamic dataset of Holland and Powell (1990) are used to put constraints on the P-T evolution during growth. These imply growth during heating from 540 to 635° C and decompression from 9.7 to 7.2 kbar, representing a temperature increase of 95° C and an uplift of 7 km during growth of the garnet. While growth during heating and decompression is consistent with both field evidence and analysis of garnet microstructures and is predicted by theoretical models of regional metamorphism, the extent of the temperature increase requires either very slow uplift (≤0.15 mm a-1) or an additional magmatic heat input. Slow uplift is precluded by existing constraints on both the duration of the uplift event and that of garnet growth and it is concluded that an external magmatic heat input is required. Comparison with published data on the timing of metamorphism in other parts of the terrain suggests that the peak occurred earlier in lower grade regions, a conclusion that is again supported by theoretical studies. Following the peak, cooling and uplift occurred at a modest rate consistent with simple isostatic recovery.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the late metamorphic history of these rocks was characterized by high-T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system.
Abstract: Geological 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-T decompression 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 the P–T paths 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 detachment system.

Journal ArticleDOI
TL;DR: In the early Proterozoic Willyama Supergroup, tourmaline-rich rocks are widespread minor lithologies in the Broken Hill district, Australia as discussed by the authors, where tourmalines are associated with Pb-Zn-Ag mineralization and in places with Mn-rich garnet quartzites.
Abstract: Tourmaline-rich rocks are widespread minor lithologies within the Early Proterozoic Willyama Supergroup in the Broken Hill district, Australia. Tourmaline concentrations occur in strata-bound and local stratiform tourmalinites, clastic metasedimentary rocks, quartz-gahnite lode rocks, stratiform Pb-Zn-Ag sulfide ores, garnet quartzites, strata-bound scheelite deposits, quartz-tourmaline nodules, discordant quartz veins, and granitic pegmatites. Most of the tourmaline-rich rocks are within the Broken Hill Group that hosts the main Pb-Zn-Ag ores.At the Globe mine along the northeast end of the main lodes, tourmalinites are closely associated with Pb-Zn-Ag mineralization and in places are interbedded with Mn-rich garnet quartzites. Galena and other ore minerals occur locally in the cores of recrystallized tourmaline grains in these tourmalinites, indicating that tourmaline and sulfides were present together prior to deformation and high-grade metamorphism. Electron microprobe analyses of tourmalines intergrown with Fe sulfides at the Globe mine show Mg-rich compositions relative to tourmalines in sulfide-free assemblages from the same area, suggesting early (premetamorphic) introduction of boron and Mg enrichment of tourmaline by sulfide-silicate reactions during metamorphism.Combined field and geochemical data indicate that the district tourmalinites represent normal clastic sediments that were metasomatically altered by boron-rich hydrothermal fluids at or below the sediment-water interface. Whole-rock chemical analyses of 33 tourmaline-rich rocks show linear trends of data for major and trace elements that closely resemble the trends observed for unmineralized elastic metasedimentary rocks of the district. Average Fe/Al, Mg/Al, Na/Al, and Ti/Al molar ratios of the tourmaline-rich rocks and clastic metasediments are very similar; the average K/Al molar ratio of the tourmaline-rich rocks is significantly lower than that of the clastic metasediments, reflecting the loss of K during tourmalinite formation. Chondrite-normalized patterns of rare earth elements (REE) in the quartz-rich tourmalinites are generally similar to those of the clastic metasediments, except for minor depletions of light REE; local positive and negative Ce anomalies suggest tourmalinite formation in the presence of seawater or a seawater-derived pore fluid. The geochemical data imply relative immobility of Al, Ti, Cr, and heavy REE during hydrothermal alteration and later metamorphism. Boron isotope analyses of 52 tourmaline separates show a total range of delta 11 B values from -26.8 to -17.0 per mil. Fine-grained, euhedral, nonpoikilitic tourmalines from tourmalinites in the andalusite-muscovite zone in the northern part of the district (e.g., Black Prince mine) have delta 11 B values from -21 to -17 per mil, whereas coarse granoblastic and poikilo-blastic tourmalines from the sillimanite and two-pyroxene granulite zones in the southern part of the district (e.g., Globe mine) have delta 11 B values of-24 to -19 per mil. Tourmalines in strongly retrogressed tourmalinites have delta 11 B values from about -27 to -20 per mil. The observed variations in delta 11 B are consistent with prograde and retrograde metamorphic fractionation of boron isotopes, in which the fluid phase is preferentially enriched in the heavier isotope ( 11 B). Premetamorphic hydrothermal fluids that deposited the Black Prince tourmalinites had delta 11 B values of-8 to -5 per mil at 200 degrees to 300 degrees C, suggesting a boron source from nonmarine evaporite borates.Tourmalinites in the Broken Hill district apparently formed by the same submarine hydrothermal processes as the main Pb-Zn-Ag lodes and the siliceous ferromanganese protoliths of the garnet quartzites. In our model, the hydrothermal system(s) acquired abundant boron by leaching evaporitic borates within the Thackaringa Group, the stratigraphic sequence that underlies the Broken Hill Group and most of the tourmaline concentrations. We suggest that evaporites of the Thackaringa Group provided a source of readily extractable boron for formation of the tourmalinites and also the source of the fluoride, sulfur, and perhaps the carbonate in the main lodes; such evaporites may have been critical for increased metal chloride complexing and transport necessary for deposition of the high-grade Pb-Zn-Ag ores. The Broken Hill deposit may have formed contemporaneously with the Mount Isa and McArthur River Pb-Zn-Ag deposits in similar evaporite-bearing sequences during widespread Early Proterozoic continental rifting.