OF NORDFJORD, WEST NORWAY
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Abstract:
hic gneisses of Nordfjord are similar to the oceanic tholeiites in their low content of titanium, ferric iron, potassium, and phosphorus. The chemical composition of the eclogites varies, how ever, within wide limits, and it is probable that alterations of bulk chemical compositions have taken place if the eclogites were indeed formed by transformations from other basic rocks. The garnets contain 36-50% Alm, 0.4-1.9% Spess, 23-43% Py, 0.3-2.1% And, and 13-25% Gross. Clinopyroxenes contain 3-9% Ac, 23-42% Jd, 8-13% Tsch, 3-7% Hd, and 38-56% Di. Titanium, nickel, chromium, and strontium are concentrated in dino pyroxene relative to garnet, while cobalt, manganese, scandium, and zir conium are relatively concentrated in garnet. Eclogites from gneiss of Nordfjord differ from the eclogites endosed in ultrabasites of Sunnmore bothin bulk chemistry, composition of garnet and clinopyroxene, and in the partition of elements between garnet and dinopyroxene. The ultrabashes occur in leetonised zones and might represent rocks which are tectonically emplaced, but the eclogites enclo3ed in gneiss are Iikely to be original crustal rocks. They occur as disrupted layers or boudins in paragneiss in harmony with a formation from dry basic igneous and sedimentary rocks endosed in a thick geosynclinal pile of supracrustals which were rapidly buckled down to great depths and metamorphosed there. Elements that could not easily be accommodatedKeywords:
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The massif is a complex series of metamorphosed sediments and igneous rocks whose lithologic and time relationships are not yet established. The individual mineral phases of the eclogitic rocks have been separated in order to further an investigation begun in 1959. The garnets have been analyzed for major, minor, and trace elements, and the results are here presented. In the lower parts of the gneiss, the garnets are more basic, with traces of Ni, Co, V, and Sc; Sr appears in the upper series. Generally, the composition of the garnets correlates with that of the corresponding eclogitic rocks. The appearance of garnet began with formation of spessartine and almandine phases, in a dry facies and possibly at atmospheric pressures; with increasing P/T conditions pyrope developed. Garnet porphyroblasts in the massif exhibit typical growth, with cores rich in spessartine and almandine and with abundant inclusions, whereas the margins are high in Mg and almost free from inclusions.
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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 Al 2 O 3 (<3.8wt%), CaO (<3.2%) and TiO 2 (<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 Al 2 O 3 and TiO 2 contents, and a higher CaO/Al 2 O 3 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 VI A and VI B are Ol + Tr‐Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na 2 O 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 820‐900d̀ C and 36‐41 kbar and that peak metamorphism of eclogites occurred at 730‐900d̀ 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.
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The Maowu garnet‐bearing ultramafic body (∼ 250 × > 50 m 2 ) in the Dabie ultrahigh‐pressure (UHP) terrane has several distinct petrologic characteristics: (i) most rocks are layers of garnet‐bearing ultramafics including orthopyroxenite and clinopyroxenite with minor harzburgite and omphacite‐rich layers; these compositional layers range in thickness from 5 cm to 1.6 m; (ii) rutile is ubiquitous and is most abundant in clinopyroxenite (up to 1–2 vol%); (iii) monazite is common as inclusions in silicates and as a matrix phase; (iv) exsolved plates of magnetite occur in olivine (Fo 93 ), and monazite in apatite; (v) chromite occurs as fine‐grained inclusions in enstatite and clinohumite; (vi) hydrous phases including talc, clinochlore and amphibole are common as inclusions in coarse‐grained garnet; and (vii) major silicates are high in Mg/(Mg + Fe) values. Most of the ultramafic rocks are high in rare earth elements (REE), P, Cr and TiO 2 , and are significantly different from Mg–Cr or Fe–Ti garnet peridotites which are common in the western Alps, Western Gneiss Region, and the Bohemian Massif. Well‐foliated orthopyroxenite is composed of Grt (Prp 60–74 ) + En (En 92–96 ; 0.05–0.14 wt% Al 2 O 3 ) + Chl (2–3 wt% Cr 2 O 3 ) ± clinohumite ± magnesite ± Di (Di 94–97 ) + chromite. Omphacitic rocks contain mainly elongated omphacite (> 90 vol%) with rare garnet (Prp 45 Alm 41 Gr 13 ), rutile, apatite, and monazite. Omphacites (Jd 63–67 Ac 6–12 Aug 20–31 ) display pronounced compositional zoning and contain inclusions of coesite relics and quartz pseudomorphs after coesite. Minor retrograde phases include talc/tremolite after Px, Chl after Grt, Cpx (Jd 49 Ac 13 Aug 38 ) and Ab after omphacite. Garnet‐bearing ultramafic and omphacitic rocks have been subjected to UHP metamorphism at pressures of ∼ 35–50 kbar and 750 ± 50 °C under extremely low X CO 2 conditions (< 0.001); a minor amphibolite facies overprint took place at P < 15 kbar and 650 °C. The protoliths may be Proterozoic ultramafic crustal cumulates that were subjected to metasomatism prior to Triassic subduction to mantle depths of more than 100 km during the collision of the Sino‐Korean and Yangtze Cratons.
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Eclogites and eclogitized serpentinites are present in a dismembered and metamorphosed ophiolitic association from the Rhodope Massif in South Bulgaria. The ophiolitic association which is widespread throughout the entire massif, marks a stable stratigraphic level in the well stratified Precambrian metamorphic basement. The latter is divided into two supergroups: lower - Prarhodopian - polymetamorphic infracrustal gneiss complex and upper - Rhodopian - a transgressive supracrustal complex, containing the ophiolites. It is considered that the ophiolites represent ocean crust fragments, obducted over the active margin of an ancient continent. They were covered by pelite- carbonate sediments of Riphean age and later folded and metamorphosed together with the gneisses of the basement. Synmetamorphic fold structures are typical for this stage. Linear and dome anticlines are observed, their cores containing gneisses of the Prarhodopian Supergroup. Both the Rhodopian Supergroup and the Ophiolitic Association in it, occupy close to tight subvertical, inclined and recumbent synclines between positive gneiss fold structures. The primary ophiolite components of basic volcanites, gabbros, gabbro-norites and serpentinized peridotites underwent a continuous metamorphic and structural evolution and as result talc-chlorite-actinolite schists, pyroxenites, eclogites, various amphibolites and metasomatic gabbro-diorites were formed. Eclogites are displayed along local shear zones (Kozhoukharova, 1988). They are most often observed in the deepest and most highly compressed synclines and are usually situated at the lithological contacts of rocks with different rheological properties. Eclogitized serpentinites are found only in the intensively folded Avren syncline, Eastern Rhodopes. Its southern part is pinched between two anticlines and is deformed by isoclinal folds, steeply inclined to west. The core of the syncline contains amphibolites and marbles. Numerous small serpentinite bodies are emplaced between the rock layers. Some of them are eclogitized at the contacts and the serpentine is replaced by enstatite, diopside, olivine, dolomite with sporadic presence of small garnet grains. One of the serpentinite bodies is of special interest for the discussion of eclogitization processes. It is a large lenticular body 3 km long and 400-600 m wide, lying between rigid garnet-bearing gneisses. At the periphery of the body serpentinites possess a banded structure, demonstrated by alternation of dark green unaltered serpentine stripes with light beige-rose, thin (1-2 cm) garnet-lherzolite bands. The garnet-lherzolite zones are always in conformity with the boundaries of the body, as well as the general stratification and metamorphic schistosity of the country rocks. The zones gradually disappear towards the central parts of the serpentinite body. At first the garnet disappears and further also - the pyroxenes. The serpentinites have no traces of eclogitization inside of the body.The garnet bearing lherzolite bands consist of: garnet (Prp50-56Alm27- 29Grs16-18Sps1-2), enstatite (En84-86Fs14-15), diopside (Wo49- 51En46-47Fs4-5), olivine (Fo88Fa12) and spinel (Cr pleonaste). Usually the bands have a zonal structure. Their central parts are occupied by garnet, followed by strips consisting mainly of enstatite, diopside, olivine and spinel. A transitional zone of cryptocrystalline talc-chlorite aggregate is formed between the eclogite minerals and serpentinite. The myrmekite-like symplectites are built up of: a. diopside and spinel; b. diopside, enstatite and spinel; c. diopside, spinel and magnetite; d. diopside and actinolite. They are very characteristic reaction products in the transitional zone. Similar layered metaperidotites are also found in North Greece at the Kimi village (Mposkos and Wawrzenitz, 1995).Garnet-free lherzolites from the more internal zones as well as from another small body consist of enstatite (En90Fs10), diopside (En47-50Wo46-50Fs3-4), olivine (Fo89- 90Fa10-11) and spinel-picotite. The Al-content in ortho-and clinopyroxenes from garnet-lherzolites is higher (enstatite 0.12-0.18; diopside 0.11-0.14 apfu) than that from garnetfree lherzolites (enstatite 0.04-0.07; diopside 0.02-0.07 pfu) - evidence for increasing t crystallization temperature in garnet-lherzolite peripheral bands. The P-T conditions of mineral crystallization for garnet-lherzolites estimated by several garnet-pyroxene geothermobarometers vary within the range: T = 640-740oC; P = 12-15 kbar and for garnetfree lherzolites: T = 500-623oC; P = 10-12 kbar. At the same time the background regional metamorphism of the country rocks is typically medium pressure amphibolite facies, constrained by serpentine stability to 580-600oC. A similar strongly expressed spatial anisotropy of the thermodynamic parameters is peculiar to the tribological systems. The genesis of garnet and garnet-free lherzolites is considered as crustal eclogitization of previously serpentinized ultrabasites, due to its deformation during syn-metamorphic folding of the metamorphic basement. Then, narrow shear zones were formed at the periphery of the serpentinite bodies, where interlaminar slip took place. As a result, temperature and pressure increased, the serpentine was dehydrated and replaced by talc, chlorite, ortho-and clinopyroxenes, olivine, spinel and garnet. Some authors (Wintsch, 1985) have proposed an increasing of temperature to 1000oC in such zones. The arguments for the orogenic syn-metamorphic origin of the garnet-lherzolites from the Avren region, Rhodope massif will be sumarised again: 1) The Ophiolitic Association in the Rhodope massif is an integral part of the Precambrian basement with a wide areal distribution and constant stratigraphic position. It is overlain by a well-stratified rock succession with quite well-preserved primary sequence and normal lithological contacts. It does not mark any late Caledonian-Herzynian or Alpine subduction zones; 2) Garnet-lherzolites are found only in the most intensive folded metamorphic terraines in the Rhodope massif; 3) The formations of garnet-lherzolites mostly in the peripheral zones, at lithological contacts of the serpentinite bodies, entirely concordant with general stratification and metamorphic schistosity is evidence for a syn-metamorphic genesis and excludes the probability that these were exotic relict magmatic products; 4) The mineral association of garnetlherzolites is post-serpentinization one. Everywhere the eclogite minerals replace the serpentine, but themselves are not deformed and altered.
This example indicates the possibility to obtain eclogite P-T condition in the confined space of local shear zones, and at the same time for the differential development of the metamorphic processes during orogenesis.
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