Showing papers on "Metamorphism published in 1994"

Petrogenesis of metamorphic rocks

Abstract: This revised edition contains numerous diagrams showing the stability relations among minerals found in metamorphic rocks. Part 1 explains basic principles of metamorphism, introduces metamorphic processes and geologic thermobarometry, and defines metamorphic grade. Part 2 presents in a systematic way mineralogical changes and assemblages found in the most common types of rocks. The computation of the diagrams is based on recent advances in quantitative petrology and geochemistry.

Cambrian Orogenic Belt in East Antarctica and Sri Lanka: Implications for Gondwana Assembly

Abstract: Ion microprobe U-Pb dating of zircons from the Lutzow-Holm Complex (LHC) and the Yamato-Belgica Complex (YBC), East Antarctica indicate high-grade regional metamorphism and associated folding of LHC occurred between $$521 \pm 9$$ and $$553 \pm 6 Ma$$. This shows, for the first time, the existence of a Cambrian orogenic belt within the East Antarctic Shield. Many zircons from the LHC contain cores that record inherited ages from ~2900 to ~1500 Ma. Components of ~1000 Ma zircon have been identified in three locations. This may indicate the maximum age of the deposition of LHC. One metasedimentary gneiss from the YBC records a well-defined age at about 600 Ma, whereas another yields a dispersion of ages interpreted as the result of varied radiogenic Pb loss in ~1000 Ma zircons at about 500-600 Ma. These gneisses have inherited ages of up to ~2500 Ma. Our work enables an improved fit to the once contiguous fragments of Gondwana. The Highland/Southwestern Complex (HSWC) of Sri Lanka has remarkable petrological...

U-Pb, Single Zircon Pb-Evaporation, and Sm-Nd Isotopic Study of a Granulite Domain in SE Madagascar

Abstract: Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite...

Post-collision magmatism and tectonics in northwest Anatolia

Abstract: A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavsanli and Sakarya zones. The 40Ar/39Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of ∼3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures <10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (<0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.

Petrogenesis of garnet‐bearing ultramafic rocks and associated eclogites in the Su‐Lu ultrahigh‐P metamorphic terrane, eastern China

Abstract: Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al2O3 (<3.8wt%), CaO (<3.2%) and TiO2 (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al2O3 and TiO2 contents, and a higher CaO/Al2O3 ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle. Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh-pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine-grained garnet after coarse-grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic-hornblende. Stage V is represented by the assemblage Ol + Opx + Prg-Hbl + Spl. The mineral assemblages of stages VIA and VIB are Ol + Tr-Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na2O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz-bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P-T estimates indicate that the ultrahigh-metamorphism (stage II) of ultramafic rocks occurred at 820-900d C and 36-41 kbar and that peak metamorphism of eclogites occurred at 730-900d C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Abstract: Sodic pyroxene (jadeite content X jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X Acm=0.26–0.50) than that in the epidote-free samples (X Acm=0.45–0.47). X Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X Jd from core to rim, the other type by decreasing X Jd from core to rim. Both types of zoned pyroxenes show an increase in X Fe 2+[=Fe2+/(Fe2++Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.

Mesoproterozoic contractional orogeny in western North America: Evidence from ca. 1.4 Ga plutons

Abstract: Kinematic data from cogenetic dikes, magmatic foliation, and high-temperature solid-state deformation observed in 1.4 Ga plutons and their aureoles in the southwestern United States show a synmagmatic strain regime involving regional contraction. We propose that these plutons were preferentially emplaced along Paleoproterozoic shear zones and province boundaries. Thermal softening of the middle crust (8-12 km) during intrusion allowed regional strain and pluton-enhanced metamorphism to be recorded as much as 1500 km inboard of the southern margin of Laurentia. We abandon the term "anorogenic" and instead view these plutons as recording compressional or transpressional plate-margin tectonism from 1.5 to 1.3 Ga along southern Laurentia. Further study of 1.4 Ga plutons may also offer insights into middle-crustal processes that occur in broad zones of transpressional deformation.

40Ar/39Ar geochronology and exhumation of high-pressure to ultrahigh-pressure metamorphic rocks in east-central China

Abstract: New 40 Ar/ 39 Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong9an and Dabie Mountains areas, east- Central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong9an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and hornblende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons. The Triassic through Early Jurassic 40 Ar/ 39 Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences ∼80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode.

Postnappe stacking extension and exhumation of high‐pressure/low‐temperature rocks in the island of Crete, Greece

Abstract: The island of Crete consists largely of nappes of contrasting lithologies and metamorphism that were stacked southward during an Oligocene-early Miocene N-S compression, with the lower nappes undergoing a high-pressure/low-temperature (HP/LT) metamorphism. This was followed by a N-S Miocene crustal extension that caused thinning of the nappes and uplift of the HP/LT metamorphic rocks. Ductile conditions took place in the lower nappes associated with a retrograde greenschist metamorphism, while semiductile to brittle conditions took place in the upper nappes. A major normal detachment fault separates the lower and the upper nappes. The quartz texture analyses and the symmetry of structures indicate bulk coaxial deformation, while the results of strain analysis suggest both constrictional and flattening strains. A younger NE-SW compression affected both the thinned nappe pile and the late Miocene-Pliocene sedimentary basins. Finally, a Pliocene NE-SW extension led to further uplift and exhumation of the HP/LT metamorphic rocks. This cyclic tectonometamorphic process of alternate compression and extension took place during the migration of the Hellenic orogenic belt toward the most external units, including successive tectonic events.

Tectonic evolution of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts from central China

Abstract: In the Triassic suture between the Sino-Korean and Yangtze cratons, the Dabie metamorphic Complex in central China includes three tectonic units: the northern Dabie migmatitic terrane, the central ultrahigh-P coesite- and diamond-bearing eclogite belt, and the southern high-P blueschist-eclogite belt This complex is bounded to the north by a north-dipping normal fault with a Paleozoic accretionary complex and to the south by a north-dipping reverse fault with Yangtze basement plus its foreland fold-and-thrust sequence Great differences in metamorphic pressure suggests that these units reached different depths during metamorphism and their juxtaposition occurred by wedge extrusion of subducted old continental fragments These units were subsequently subjected to (i) Barrovian type regional metamorphism and deformation at shallow depths; (ii) intrusion of Cretaceous granitic plutons; and (iii) doming and segmentation into several blocks by normal and strike-slip faults A new speculative model of tectonic exhumation of UHP rocks is proposed

Precise U-Pb ages for Grenvillian and pre-Grenvillian thrusting of Proterozoic and Archean metamorphic assemblages in the Grenville Front tectonic zone, Canada

Abstract: While the position of the Grenville Front is placed at a structural feature associated with the onset of high-grade rocks when approached from the north, the time of first metamorphism in these rocks is, in general, pre-Grenvillian and highly variable. At Killarney, amphibolite to granulite grade rocks were metamorphosed, lineated, and foliated to their present “front-parallel” configuration mainly by 1453±6 Ma. North of North Bay, amphibolite grade migmatites have been subjected to a pre-2650 Ma metamorphism. Granulite grade gneisses south of the Grenville Front near Val d’Or record a 2640 Ma metamorphic event. South of Chibougamau, mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 Ma. In central Labrador a 1650 Ma foliated granite is present south of the Grenville Front, and at least some of the banding is this old. At the Atlantic coast, granulite facies metamorphism in front-parallel gneisses predates a 1790–1662 Ma deformation. In contrast, each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies NW directed thrusting or crustal loading. Near Killarney on Georgian Bay, titanite records an episode of 977±2 Ma partial resetting, the degree of which increases with distance into the Grenville Province. Here Grenvillian deformation, where present, is manifest as feldspar-porphyroclastic fabric with strong SE plunging lineations. Near North Bay, Archean pegmatites in the Grenville Front zone were similarly deformed at 988±10 Ma, and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987±2 Ma. A 991±4 Ma cooling age for titanite in this melt is analytically identical to the zircon growth age, hence rapid cooling occurred. South of Chibougamau, new zircons grew in high-pressure coronite in response to tectonic loading at 995±6 Ma, and in Central Labrador, melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995±2 Ma. Titanite in the granitic host rock has an analytically identical age of 992±4 Ma, again indicating rapid cooling. This second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651±5 Ma to 988±5 Ma two-age mixing line. Grenville resetting at about 1037±19 Ma in the Grenville Front zone on the Labrador coast is indicated, but major thrusting is pre-1662 Ma. U-Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U-Pb systems, high-pressure assemblages, local melting, and rapid cooling. Crustal loading and shearing, with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling, are required.