Albite-epidote Amphibolite Facies Research Articles

Metamorphic Evolution of Garnet-bearing Epidote-Barroisite Schist from the Meratus Complex in South Kalimantan, Indonesia

DOI:10.17014/ijog.2.3.139-156This paper presents metamorphic evolution of metamorphic rocks from the Meratus Complex in South Kalimantan, Indonesia. Eight varieties of metamorphic rocks samples from this location, which are garnet-bearing epidote-barroisite schist, epidote-barroisite schist, glaucophane-quartz schist, garnet-muscovite schist, actinolite-talc schist, epidote schist, muscovite schist, and serpentinite, were investigated in detail its petrological and mineralogical characteristics by using polarization microscope and electron probe micro analyzer (EPMA). Furthermore, the pressure-temperature path of garnet-bearing epidote-barroisite schist was estimated by using mineral parageneses, reaction textures, and mineral chemistries to assess the metamorphic history. The primary stage of this rock might be represented by the assemblage of glaucophane + epidote + titanite ± paragonite. The assemblage yields 1.7 - 1.0 GPa in assumed temperature of 300 - 550 °C, which is interpreted as maximum pressure limit of prograde stage. The peak P-T condition estimated on the basis of the equilibrium of garnet rim, barroisite, phengite, epidote, and quartz, yields 547 - 690 °C and 1.1 - 1.5 GPa on the albite epidote amphibolite-facies that correspond to the depth of 38 - 50 km. The retrograde stage was presented by changing mineral compositions of amphiboles from the Si-rich barroisite to the actinolite, which lies near 0.5 GPa at 350 °C. It could be concluded that metamorphic rocks from the Meratus Complex experienced low-temperature and high-pressure conditions (blueschist-facies) prior to the peak metamorphism of the epidote amphibolite-facies. The subduction environments in Meratus Complex during Cretaceous should be responsible for this metamorphic condition.

Late Paleozoic onset of subduction and exhumation at the western margin of Gondwana (Chilenia Terrane): Counterclockwise P–T paths and timing of metamorphism of deep-seated garnet–mica schist and amphibolite of Punta Sirena, Coastal Accretionary Complex, central Chile (34° S)

In this study, the Paleozoic albite–epidote–amphibolite occurring as meter-sized intercalations within garnet–mica schist at Punta Sirena beach (Pichilemu region, central Chile) is characterized for the first time. These rocks constitute an unusual exposure of subduction-related rocks within the Paleozoic Coastal Accretionary Complex of central Chile. Whereas high pressure (HP) greenschist and cofacial metasediments are the predominant rocks forming the regional metamorphic basement, the garnet–mica schist and amphibolite yield higher P–T conditions (albite–epidote amphibolite facies) and an older metamorphic age. Combining detailed mineral chemistry and textural information, P–T calculations and Ar–Ar ages, including previously published material from the Paleozoic Accretionary Complex of central Chile, we show that the garnet–mica schist and associated amphibolite (locally retrograded to greenschist) are vestiges of the earliest subducted material now forming exotic bodies within the younger HP units of the paleo-accretionary wedge. These rocks are interpreted as having been formed during the onset of subduction at the southwestern margin of Gondwana. However, we show that the garnet–mica schist formed at a slightly greater depth (ca. 40 km) than the amphibolite (ca. 30 km) along the same hot-subduction gradient developed during the onset of subduction. Both lithotypes reached their peak-P conditions at ca. 335–330 Ma and underwent near-isobaric cooling followed by cooling and decompression (i.e., counterclockwise P–T paths). The forced return flow of the garnet–mica schist from the subduction channel started at ca. 320 Ma and triggered the exhumation of fragments of shallower accreted oceanic crust (amphibolite). Cores of phengite (garnet–mica schist) and amphibole (amphibolite) grains have similar chemical compositions in both the S1 and S2 domains, indicating rotation of these grains during the transposition of the burial-related (prograde peak-T) foliation S1 by the non-coaxial exhumation-related foliation S2. During exhumation and retrograde D2 deformation, the garnet–mica schist and amphibolite were tectonically mingled at a depth of ca. 30 km at ca. 315 Ma. We propose that the Punta Sirena unit comprises a “pseudo”-coherent sequence formed by heterogeneous lithologies that followed non-chaotic exhumation mingling, now representing the remnants of the fossil subduction channel developed at the onset of the Late Paleozoic subduction at central Chile.

A counterclockwise PTt path of high-pressure/low-temperature rocks from the Coastal Cordillera accretionary complex of south-central Chile: constraints for the earliest stage of subduction mass flow

Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of ∼15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to ∼300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist. The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic.

Low-grade metamorphic rocks from the Pulur complex, NE Turkey: implications for the pre-Liassic evolution of the Eastern Pontides

In the Pulur complex (Sakarya Zone, Eastern Pontides, Turkey) a low-grade tectonometamorphic unit (Dogankavak) is exposed in three tectonic windows beneath a complex medium-pressure high-temperature metamorphic unit of late Carboniferous age. The thrust plane between both units is transgressively covered by Liassic conglomerates. The Dogankavak unit comprises a sequence of metabasites with MORB-type chemical compositions and phyllites, with subordinate calcareous phyllites, marbles, quarzofeldspathic schists and metacherts. This sequence is interpreted as a former accretionary complex related to the consumption of the Palaeotethys. Mineral parageneses in the metabasites allow for the distinction of two domains with slightly different peak metamorphic conditions, i.e. 375–425 °C/0.5–0.8 GPa (greenschist facies) and 400–470 °C/0.6–1.1 GPa (albite-epidote amphibolite facies). The age of metamorphism is constrained at ~ 260 Ma (early Late Permian) by two Rb-Sr mineral-whole rock ages (hornblende, phengite) and one 40Ar/39Ar single step total fusion age (phengite). In conjunction with previous data on other accretionary complexes in the Sakarya zone in Northern Turkey, the data presented in this study suggest a continuous subduction of the Palaeotethys at least from Early/Late Permian to Late Triassic and a discontinuous preservation of accretion complexes in both space and time.

Oscillatory zoning in eclogitic garnet and amphibole, Northern Serpentinite Melange, Cuba: a record of tectonic instability during subduction?

AbstractExotic blocks of eclogite from distant localities along the Northern Serpentinite Melange of Cuba have comparable P–T histories that include high‐pressure prograde sections (450–600 °C, >15 kbar) associated with subduction of oceanic lithosphere, and retrograde sections within the albite–epidote amphibolite facies (<500 °C, <10 kbar) related to melange uplift. 40Ar/39Ar and Rb/Sr cooling ages (118–103 Ma) of one of the blocks indicate pre‐Aptian subduction and Aptian–Albian uplift. Detailed X‐ray imaging and profiling further reveals that minerals in these eclogite blocks (notably garnet and amphibole) display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks, previous to attainment of peak eclogitic conditions. The chemistry (e.g. coupled changes of Mg# and Mn in garnet, and of Si, Ti, Al and Na in amphibole) and geometry (euhedral to anhedral shapes) of the oscillations can be interpreted in terms of subtle fluctuations in P–T during the general prograde subduction‐related metamorphic path. A (near‐) equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution‐growth reaction processes. This model for near‐peak conditions, and the chemical signatures of earlier oscillations (notably in amphibole), suggest that episodes of retrogression (upward movement?) affected parts of the subducting slab. It is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and, probably, of the upper plate mantle, either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate‐tectonic rearrangement of the Caribbean region during the Early Cretaceous.

Geochronological Evidence for the In situ Tectonic Relationship in the Dabie UHP Metamorphic Terrane, Central China

The Dabie Mountains and the Su-Lu region in central China are known to contain the largest distribution of ultrahigh pressure metamorphic (UHPM) rocks in the world. They are parts of the Qinling-Dabie orogen formed by collision between the Sino-Korean and Yangtse cratons. The preservation of UHP minerals has inspired different models to explain the deep subduction of continental crust and the mechanisms for subsequent unroofing of the UHPM rocks. In the absence of good constraints from geochemical, isotopic and age data for different lithotectonic units, numerous tectonic models have been proposed. It appears that the greatest controversy to date concerns the primary age and the nature of eclogite protoliths, their tectonic relation with the associated ultramafic rocks and enclosing granitic gneisses ('in situ' vs 'foreign' hypothesis), the areal extent of the UHPM rocks, and their mode of exhumation. Here we report new geochemical and Sr-Nd for the host gneisses of the largest coesite-bearing eclogite body in this terranne the Bixiling Complex. These data will be used to tackle the problem of eclogitegneiss tectonic relationship. The Bixiling Complex has well preserved its UHP paragenesis of which coesite and quartz pseudomorph occur as inclusions within garnet or omphacite. In contrast, the host gneisses exhibit a typical phase assemblage of the albite-epidote amphibolite facies (plagioclase + biotite + zoisite + K-feldspar + quartz + muscovite + apatite) corresponding to the retrograde metamorphism of the Bixiling Complex. However, the presence of garnet, though very minor in amount, may suggest a previous higher metamorphic condition. The country rocks are paragneisses or orthogneisses of TTG composition (tonalite-trondhjemite-granodiorite). They are all characterized by negative Ti, P, Ta and Nb anomalies in their spidergrams, hence suggesting their protolith formation in ancient arc environments. Much of the protoliths of the gneisses from the Dabie UHP terrane was probably formed in late Proterozoic (750-800 Ma; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 1998). However, there is indication of older protoliths from Sm-Nd model age (TDM) data for gneisses from Shuanhe (Chen and Jahn, 1998). The Bixiling gneisses have TDM of 1.3 to 1.5 Ga, and and(800 Ma) = +4.2 _+ 0.4, indicating their late Proterozoic formation and juvenile character. Geochronological constraints are obviously essential for understanding the gneiss-eclogite tectonic relationship. Rb-Sr isotope analyses of whole-rock and constituent phases for five gneisses yielded a tight range of Ms-WR-Zo isochron ages from 194 to 198 Ma, and Bt-Zo isochron ages from 170 to 184 Ma (Fig. l and 2). On face values, the Ms Rb-Sr ages are only slightly younger than the Phg-WR Rb-Sr ages obtained on the coesite-bearing eclogite from the Bixling Complex (Fig. 3; 198 to 223 Ma; Chavagnac and Jahn, 1996). Moreover, the Bt RbSr ages are also in agreement with that of an amphibolite near the Bixiling Complex. Thus, when combined the new age information with the published zircon age data (Ames et al., 1996;

A mechanism for Na incorporation in garnet; an example from garnet in orthogneiss from the Su-Lu Terrane, eastern China

Sodian and yttrian (Na-Y) garnet occurs in orthogneisses of the Su-Lu terrane of eastern China with major quartz, microcline, and albite and subordinate biotite, titanite, hematite, and zircon. Acmite-rich pyroxene, muscovite, epidote, allanite, and hastingsitic amphibole occur in a few of the samples. The Na- Y garnet can be divided into almandine-spessartinegrossular [AI > 1.6 per formula unit (pfu) for 0 = 12, hereafter denoted as AI-garnet], and andradite-spessartine (FeH > 1.3 pfu, hereafter denoted as FeH-garnet) groups. Average composition of the AI-garnet in each sample ranges from Alm3sSps13Prp,Grs49Adr2 to AIm2lSps67PrpoGrssAdr7' The FeH-garnet contains a calderite component (Cdr) and its composition is Alm4-7SpslO_2,Prp,_2Adrs9_8,Cdr3_12' Na, Y, and Yb contents of the AI-rich garnet reach 0.37, 1.46, and 0.42 oxide wt%, respectively, and Na/(Y + Yb) = 0.99 ::t 0.16. The FeH-garnet contains up to 0.11,2.1, and 0.48 wt% of Na20, Y203, and Yb203, respectively, and its Na/(Y + Yb) value (0.17 ::t 0.1) is lower than that of the AI-garnet. We conclude that Na, Y, and Yb are incorporated in the AI-rich garnet by the [8JNa[8J(Y,Yb)(Ca,Mn)_2substitution, whereas in the FeH-garnet, Y and Yb are dominantly incorporated by the [8J(Y,Yb)l4J(AI,FeH)(Ca,Mn)_,SL, substitution with up to 3 mol% Y + Yb for Ca + Mn. Comparison ofNa/(Y + Yb) of garnets in the orthogneisses and in some pegmatites implies that increasing pressure favors incorporation of Na with Y and Yb, and that under the albite-epidote amphibolite facies or higher-pressure conditions, Y and Yb probably substitute into the garnet structure by [8JNa[8J(Y,Yb)(Ca,Mn)_2'instead of the [8J(Y,Yb)[4J(Al,FeH)(Ca,Mn)_,SL]-type substitution effective under lower-pressure conditions.

Structural and kinematic evolution of the teslin suture zone, Yukon: record of an ancient transpressional margin

The Teslin suture zone (TSZ), Yukon, which forms the fundamental boundary between rocks deposited along the ancient margin of North America, and allochthonous terranes to the west, preserves a complex history of pre-mid-Jurassic convergence and dextral strike-slip translation which is overprinted locally by late Cretaceous dextral strike-slip shear. The TSZ, comprised of a 15–20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata, basalt, peridotite and granitoids, is divisible into distinct elongate structural domains based on the distribution of two well-defined but differently oriented stretching lineations, L ds and L ss, L ds and L ss both formed during non-coaxial ductile deformation; L ds trends E-W and plunges down-dip, whereas L ss trends NNW-SSE and plunges shallowly. Two 1–2 km wide, NNW-trending anastomosing shear zones of L ss cross-cut L ds structures, indicating L ss is younger. Other field relations indicate further that L ds began forming earlier than L ss, followed by a period of coeval L ds and L ss formation; latest motion was dominantly parallel to L ss. Synkinematic mineral assemblages associated with both L ds- and L ss-related structures record greenschist to albite-epidote amphibolite facies conditions. L ds tectonites record a complex movement history: to the west textural asymmetries indicate west-side-down, or normal shear, whereas in eastern domains top-to-the-east thrust-style asymmetries dominate. L ss tectonites consistently record dextral, or top-to-the-north, shear parallel to L ss. Tectonic motion began as chiefly penetrative dip-slip shear ( L ds) at a high angle to the present trend of the TSZ, and evolved to dominantly dextral strike-slip shear ( L ss) parallel to the trend of the zone. The geometry, kinematics and sequence of deformation in TSZ tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin-parallel translation.

九頭竜川上流地域における飛だ外縁帯の低度変成岩類

Low-grade metamorphic rocks occur in small areas of Ise and Hakogase, Izumi-mura, Fukui Prefecture. Geologically, they lie in the Hida marginal belt. The rocks are divided into two major units based on their geographical distribution: the Ise and Hakogase masses. The Ise mass consists of metagabbro and basic schist with subordinate pelitic schist. Actinolite, epidote and chlorite are common metamorphic minerals in basic metamorphic rocks throughout the major part of the mass. Only in the limited southern part of the mass, hornblende and biotite are found in basic and pelitic schists, respectively. Garnet is also found in the southern part of the mass. The Hakogase mass consists of basic and pelitic schists. Characteristic metamorphic minerals are soda amphibole, epidote, chlorite, actinolite, lawsonite and pumpellyite in basic schists and muscovite, chlorite and stilpnomelane in pelitic schists. Mineral associations indicate that the Ise mass belongs to the greenschist facies up to the albite-epidote amphibolite facies probably with an increase in metamorphic grade southwards and the Hakogase mass belongs to the glaucophane schist facies throughout the mass. Both the masses differ from each other in metamorphic grade but are products of the same glaucophanitic metamorphism.

Petrologic comparison of the franciscan and sanbagawa metamorphic terranes

Mafic schists in California carry the typical blueschist facies assemblage of glaucophane—crossite + lawsonite 4 sphene + chlorite ± aragonite or calcite ± pumpellyite. Albite, epidote and actinolite are not common in “in situ” metabasalts. In contrast, typical Sanbagawa schists contain crossite or actinolite + albite + epidote + chlorite ± calcite; the highest-grade rocks generally contain garnet and blue—green hornblende, lower grade rocks may contain pumpellyite and/or stilpnomelane, rarely lawsonite. Franciscan metaclastic rocks show three principle assemblages, all of which contain the phases quartz 4 white mica + chlorite + sphene ±lawsonite ± stilpnomelane: albite 4 calcite, albite 4 aragonite; and jadeitic pyroxene ± aragonite ± glaucophane. Quartzose Sanbagawa metaclastic rocks represent the albite ± calcite type compatibilities; lawsonite, aragonite and jadeitic pyroxene are either absent or are very rare. Many Franciscan siliceous metasediments contain, in addition to quartz, the phases riebeckite, stilpnomelane and/or spessartine. In contrast, many Sanbagawa siliceous schists carry abundant piemontite ± magnesioriebeckite in addition to spessartine. Correlation of the above mineral assemblages with experimental data suggests that blueschist-facies metamorphism took place in the Franciscan terrane at high fluid pressures, and involved temperatures of 200–300°C and lithostatic pressures of 6.5–8 kbar. Apparently Sanbagawa metamorphism reflected physical conditions transitional between the blueschist, greenschist and albite—epidote—amphibolite facies, approximating 250–400°C at 5–7 kbar lithostatic pressure.

FORMATION OF EPIDOTE IN TANGANYIKA

Minerals of the epidote group in Tanganyika are commonly produced during cataclasis of lime-bearing rocks; invariable accession of the hydroxyl radical during dislocation assists their formation. Epidote minerals are not produced from plagioclase by the action of water, even at high temperatures and pressures; shearing stress or the presence of other material is required. Saussuritization follows, and is often a result of, uralitization. Metasomatism, progressive metamorphism, and granitization of lime-bearing rocks can give rise to epidote minerals. Epidote and clinozoisite are stable in the greenschist facies and the albite-epidote amphibolite facies, but are unstable in the almandine-diopside amphibolite subfacies of the amphibolite facies. Zoisite is unstable in the greenschist facies, but is stable in the albite-epidote amphibolite facies and throughout the amphibolite facies. Lime metasomatism, which is responsible for epidotization, is largely a result of displacement by the alkalies of lime from lime-bearing minerals. Epidotization often accompanies ferruginization and sulfide mineralization. Much epidote in the anatectic granites is the result of contamination; primary magmatic epidote is unlikely to occur. The epidote minerals form between 300°C and 500°C. High hydrostatic pressures favor the formation of zoisite. Under conditions of shearing stress monoclinic epidotes are formed. The epidotes are not stress minerals; under metasomatic conditions they are frequently formed without stress.