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1.
Ivano Gasco Marco Gattiglio Alessandro Borghi 《International Journal of Earth Sciences》2011,100(5):1065-1085
A new structural and petrological study, associated with a detailed geological mapping, allowed to better understand the tectonic
relationships between the Dora Maira Massif and the Piedmont Zone in the middle Susa Valley. In the study area, a thick sequence
of calcschists, previously attributed to the stratigraphic cover of the Dora Maira, is interpreted as belonging to the Piedmont
Zone. Four deformation phases were identified: D1 represents the eclogite facies stage, D2 developed under greenschist facies conditions, transposed the early foliation and was responsible for the development of
the regional schistosity S2. D3 is characterized by close to open folds with N dipping axial surfaces and finally, D4 developed macro-scale folds with E-dipping axial planes. Structural analyses allow to infer a relative timing for the tectonic
contact between the two nappes which were coupled after the eclogite facies metamorphism but before the development of the
S2 foliation under greenschist facies conditions. Petrographic investigation into metapelite samples permitted to identify two
main metamorphic assemblages within the Dora Maira polymetamorphic basement: M1 (Phe + Pg + Cld + Grt + ChlI + Qtz + Rt) assemblage defines the S1 relict foliation, while M2 (Ms + Pg + Ab + ChlII + Qtz + Ilm ± Bt) assemblage is related to the regional foliation S2. PERPLE_X pseudosection modelling allowed to reconstruct a P-T path for a garnet–chloritoid-bearing micaschist from the northern
part of the Dora Maira Massif, which reached eclogite facies conditions at 18–20 kbar and 515–525°C (M1/S1 event) and then was exhumed during increasing T (10–11 kbar and 555–565°C). The M2 assemblage defining the S2 regional foliation developed at P < 7 kbar and T < 575°C. According to the structural evolution, the tectonic coupling between Dora Maira and Piedmont Zone took place during
exhumation along the subduction channel. 相似文献
2.
Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones.
The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly
foliated (S1) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration
assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of
gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished
from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally
associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite,
tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite,
chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase
alteration assemblages are developed in microlithons of the S2–S3 crenulation cleavage and are replaced along S3 by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D2 shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper
greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D3 dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C),
which formed the auriferous laminated quartz veins. Later D4 crosscutting veins and D5 faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points
to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the
Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during
two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of
various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and
a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage
mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than
in the western Dharwar craton.
Editorial handling: G. Beaudoin 相似文献
3.
Rajib Kar 《Journal of Earth System Science》2007,116(1):21-35
The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation
(D1 and D2) attended with foliation (fabric) development (S1 and S2). Two distinct metamorphic events at P-T conditions of ∼900°C at ∼9 kbar and ∼600°C at ∼6 kbar are correlated with D1 and D2 respectively. The reaction textures in S1-microdomains are interpreted to be the product of near isobaric cooling at ∼9 kbar from 950°C to 600°C, whereas those in
the S2-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D1-M1 high P-T granulite event is interpreted to be Archean in age (ca. 3 Ga) on the basis of the isotopic data obtained from the charnockite
suite of the area. The later relatively low P-T granulite facies event, attendant to D2-S2 is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt. 相似文献
4.
This paper examines the metamorphic evolution of three juxtaposed units of the Maures massif (France) with respect to the tectonic processes related to the Variscan orogeny. All sampled rocks are metabasalts or metagabbros metamorphosed during Palaeozoic tectonic events. The inferred metamorphic evolution takes into account the relative chronology of mineral parageneses with respect to microstructures, the mineral chemistry of zoned amphiboles, and calculated P–T–t–d paths derived from each unit. Three successive and contrasting tectono-metamorphic events are clearly identified. The D1 event is associated with coarse-grained amphiboles of an early S1–L1 fabric that recorded prograde/retrograde anticlockwise paths at high-grade amphibolite facies conditions (7–8 kbar/700–750 °C). The D2 event is related to fine-grained amphiboles of the main S2 foliation that recorded prograde/retrograde clockwise paths at MP–MT conditions (4–6 kbar/550–650 °C). The D3 event corresponds to late post-S2 amphiboles crosscutting the main foliation and recording retrograde clockwise paths at lower grade conditions (4–2 kbar/500–350 °C). The D1 event results from Silurian–Devonian continental subduction and subsequent thrust tectonics during an early stage of the Variscan evolution, before the Carboniferous. The D2 event is connected to the Visean continental collision, marked by nappe stacking (burial) then crustal folding and sinistral strike-slip shearing (exhumation). The D3 event is an effect of the Namurian late-orogenic extension (late exhumation) that mostly affected the previously thickened whole central block. This paper demonstrates that the whole metamorphic history of the Maures massif consists of two successive stages of burial/exhumation at different metamorphic conditions during the Variscan mountain building. Similar stages of subduction-uplift then collision-uplift processes have also been recognised in others parts of the Variscan belt and in the Alpine orogen. 相似文献
5.
Contrasting metamorphic evolution of metasedimentary rocks from the Çine and Selimiye nappes in the Anatolide belt, western Turkey 总被引:1,自引:0,他引:1
J. L. Régnier U. Ring C. W. Passchier K. Gessner T. Güngör 《Journal of Metamorphic Geology》2003,21(7):699-721
Abstract P–T conditions, mineral isograds, the relation of the latter to foliation planes and kinematic indicators are used to elucidate the tectonic nature and evolution of a shear zone in an orogen exhumed from mid‐crustal depths in western Turkey. Furthermore, we discuss whether simple monometamorphic fabrics of rock units from different nappes result from one single orogeny or are related to different orogenies. Metasedimentary rocks from the Çine and Selimiye nappes at the southern rim of the Anatolide belt of western Turkey record different metamorphic evolutions. The Eocene Selimiye shear zone separates both nappes. Metasedimentary rocks from the Çine nappe underneath the Selimiye shear zone record maximum P–T conditions of about 7 kbar and >550 °C. Metasedimentary rocks from the overlying Selimiye nappe have maximum P–T conditions of 4 kbar and c. 525 °C near the base of the nappe. Kinematic indicators in both nappes are related to movement on the Selimiye shear zone and consistently show a top‐S shear sense. Metamorphic grade in the Selimiye nappe decreases structurally upwards as indicated by mineral isograds defining the garnet‐chlorite zone at the base, the chloritoid‐biotite zone and the biotite‐chlorite zone at the top of the nappe. The mineral isograds in the Selimiye nappe run parallel to the regional SR foliation, parallel the Selimiye shear zone and indicate that the Selimiye shear zone formed during this prograde greenschist to lower amphibolite facies metamorphic event but remained active after the peak of metamorphism. 40Ar/39Ar mica ages and the tectonometamorphic relationship with the Eocene Cyclades–Menderes thrust, which occurs above the Selimiye nappe in the study area, suggests an Eocene age of metamorphism in the Selimiye nappe. Metasedimentary rocks of the Çine nappe 20–30 km north of the Selimiye shear zone record maximum P–T conditions of 8–11 kbar and 600–650 °C. An age of about 550 Ma is indicated for amphibolite facies metamorphism and associated top‐N shear in the orthogneiss of the Çine nappe. Our study shows that simple monophase tectonometamorphic fabrics do not always indicate a simple orogenic development of a nappe stack. Preservation in some areas and complete overprinting of those fabrics in other areas apparently occur very heterogeneously. 相似文献
6.
Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics.Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M2) reaching upto greenschist facies (~450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M1) reaching upper amphibolite facies (~4.0–7.9 kbar and ~500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F2 – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S2 foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age. 相似文献
7.
A. L. Läufer W. Frisch G. Steinitz J. Loeschke 《International Journal of Earth Sciences》1997,86(3):612-626
The Eder unit in the Carnic Alps, which is situated immediately south of the Periadriatic lineament (PL), represents a fault-bounded block consisting of a low-grade (up to 400?°C, indicated by epizonal illite “crystallinity” values, recrystallized quartz, and non-recrystallized white mica) metamorphic Paleozoic metasedimentary sequence. Until now, it has been assumed to represent a separate Variscan nappe. The rocks of the Eder unit show a strong E- to W-oriented stretching lineation on steep foliation planes (D1) subparallel to the PL. D1 structures originated near the temperature peak of metamorphism, and shear sense indicators show dextral ductile shear parallel to the PL. Tight mesoscale D2 folds formed on the cooling path. K–Ar and Ar–Ar ages from newly formed white mica cluster around 32–28 and 18–13 Ma and suggest a two-stage Tertiary history of the Eder unit. We interpret the Eder unit as a fault-bounded block formed during Oligocene large-scale dextral shearing along the PL (near Tmax) and exhumed in mid-Miocene times during another phase of activity along the PL. Its nature as a separate Variscan nappe is questioned. 相似文献
8.
A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile
Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after
ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after
gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types
were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies
at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is
unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all,
pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes,
which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with
the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their
host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels. 相似文献
9.
《Geodinamica Acta》1999,12(1):25-42
The Early Eocene to Early Oligocene tectonic history of the Menderes Massif involves a major regional Barrovian-type metamorphism (M1, Main Menderes Metamorphism, MMM), present only in the Palaeozoic-Cenozoic metasediments (the so-called “cover” of the massif), which reached upper amphibolite faciès with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels. It is not present in the augen gneisses (the so called “core” of the massif), which are interpreted as a peraluminous granite deformed within a Tertiary extensional shear zone, and lie structurally below the metasediments. A pronounced regional (S1) foliation and approximately N-S trending mineral lineation (L1) associated with first-order folding (F1) were produced during D1 deformation coeval with the MMM. The S1 foliation was later refolded during D2 by approximately WNW-ESE trending F2 folds associated with S2 crenulation cleavage. It is now commonly believed that the MMM is the product of latest Palaeogene collision across Neo-Tethys and the consequent internal imbrication of the Menderes Massif area within a broad zone along the base of the Lycian Nappes during the Early Eocene-Early Oligocene time interval. However, the meso- and micro-structures produced during D1 deformation, the asymmetry and change in the intensity and geometry of the F2 folds towards the Lycian thrust front all indicate an unambiguous non-coaxial deformation and a shear sense of upper levels moving north. This shear sense is incompatible with a long-standing assumption that the Lycian Nappes were transported southwards over the massif causing its metamorphism. It is suggested here that the MMM results from burial related to the initial collision across the Neo-Tethys and Tefenni nappe emplacement, whereas associated D1 deformation and later D2 deformation are probably related to the northward backthrusting of the Lycian nappes. 相似文献
10.
Tibor Gasparik 《Contributions to Mineralogy and Petrology》1996,124(2):139-153
Melting relations on the enstatite−diopside (En, Mg2Si2O6−Di, CaMgSi2O6) join, including the compositions of crystalline phases and melts coexisting along the solidi, were experimentally determined
in the pressure range 70–224 kbar with a split-sphere anvil apparatus (USSA-2000). Melting is peritectic in enstatite-rich
compositions at 70–124 kbar (1840–2100° C) and eutectic at higher pressures, while the diopside-rich clinopyroxene melts azeotropically
at 70–165 kbar and up to 300° C lower temperatures than the eutectic. Orthopyroxene is replaced with enstatite-rich clinopyroxene
at 120 kbar and 2090°C. First garnet with 17 mol% Di forms on the solidus at 158 kbar and 2100° C. Two garnets coexist on
the solidus at 165–183 kbar and 2100° C, garnet coexists with CaSiO3 perovskite at 183–224 kbar (2100–2230° C) and two coexisting perovskites are stable at higher pressures. The melting curve
of diopside was determined at 80–170 kbar; the slope becomes negative at 140 kbar and 2155° C. At 170 kbar and 2100° C, diopside
with 96% Di breaks down to garnet with 89% Di and CaSiO3 perovskite. The new data were used to calculate an improved temperature-pressure phase diagram for the CMAS system, which
can be useful for estimating the mineralogy of the Earth's upper mantle.
Received: 15 October 1994 / Accepted: 15 October 1995 相似文献
11.
Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France 总被引:1,自引:1,他引:1
Mineral assemblages in the blueschist-facies metapelites fromthe Ile de Groix (Armorican Massif, France) permit the distinctionof two main units. The Upper Unit is characterized by: (1) highmodal proportions of garnet; (2) larger grain size; (3) therarity of graphite-bearing layers; (4) a single, although composite,foliation S1. A Lower Unit is defined by: (1) low modal proportionsof garnet; (2) smaller grain size; (3) an abundance of graphite-bearinglayers; (4) a pervasive crenulation cleavage S2. In the UpperUnit, coexisting garnet and chloritoid are more magnesian andless manganiferous than in the Lower Unit. The differences inmodal proportions and chemistry of coexisting minerals reflectdifferent PT conditions. The PT history of theblueschist-facies metapelites is estimated using a simplifiedpetrogenetic grid in the NFMASH system and thermodynamic calculations,which suggest peak PT conditions at about P = 1618kbar, T = 450500°C and P = 1416 kbar, T =400450°C in the Upper and Lower Units, respectively.Peak PT conditions were followed by a nearly isothermaldecompression for both units at slightly different temperatures(of the order of 50°C). The contact between the two units,i.e. the garnet isograd, is interpreted as a greenschist-faciesductile thrust. Thrusting of the higher-grade unit, i.e. theUpper Unit, over the Lower Unit occurred after the high-pressureevent, i.e. during the exhumation of both units. The observedsuperposition of higher-grade rocks over lower-grade rocks arguesagainst models where the exhumation history is entirely controlledby crustal-scale vertical shortening (i.e. extension). KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; PT path; garnet isograd 相似文献
12.
The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized
orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to,
the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary
rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during
ductile D1 imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate
stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the
greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper
greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C,
6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar
represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural
sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage
characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht
Group. This early mineralization may correlate with the local D1 deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D2. The D2 deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear
zone system, the Shires Shear Zone, which separates two regional SE plunging D1 synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated
at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of
gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520
to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade,
ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite +
loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic
with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally
underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As
the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence
can be ruled out as the source of the mineralizing fluids. 相似文献
13.
At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D1M and D2M) are reoriented along N–S‐trending and E‐dipping shear planes (S3M), with (S1M–S3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D3M (≥900 °C, 9.5 kbar), as inferred from syn‐D3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D1B & D2B), with the L2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S3B shears to the west, and post‐D2B neosome segregation along D3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D3P–T –deformation histories. The shared D3 deformation in the two units, the syn‐D3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton. 相似文献
14.
Y. Vapnik V. V. Sharygin V. Samoilov Z. Yudalevich 《International Journal of Earth Sciences》2007,96(4):663-684
The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic
bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine
nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in
olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study.
The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the
data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of
rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions
of melts are usually enriched in SiO2, Al2O3, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO2 inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred
in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree
of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon. 相似文献
15.
Jean-Clair Duchesne Jean-Paul Liègeois Viorica Iancu Tudor Berza Dmitry I. Matukov Mihai Tatu Sergei A. Sergeev 《International Journal of Earth Sciences》2008,97(4):705-723
The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic
Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite
diorite and geochemical analyses (major and trace elements, Sr–Nd isotopes) of representative rock types from the two intrusions
grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for the intrusion
of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline.
Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks
corresponding to several magma batches: the Poniasca biotite diorite (P1) shows a clear crustal character while the Poniasca granite (P2) is more juvenile. Conversely, Sichevita biotite diorite (S1), and a granite (S2*) are more juvenile than the other Sichevita granites (S2). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within
P1 and S1 groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a
subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita–Poniasca plutons
took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago,
especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous—early Permian times) being known in the
Getic nappe. The geodynamical environment of Sichevita–Poniasca was typically post-collisional of the Variscan orogenic phase. 相似文献
16.
B. SCHULZ 《Journal of Metamorphic Geology》1990,8(6):629-643
A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet-bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path. Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D1–D2, which produced sheath folds as well as the main foliation S2. Spessartine-rich first-generation garnets, situated in microlithons enclosed by S2, record the onset of shearing under increasing high-pressure–low-temperature conditions (7 kbar/380°C). Geothermobarometry on second-generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting Pmax of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high-pressure–high-temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions. Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D3 and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D4 (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension. Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line. 相似文献
17.
Zakaria Hamimi Mohamed El-Shafei Ghazi Kattu Mohammed Matsah 《Mineralogy and Petrology》2013,107(5):849-860
Detailed field-structural mapping of Neoproterozoic basement rocks exposed in the Wadi Yiba area, southern Arabian Shield, Saudi Arabia illustrates an important episode of late Neoproterozoic transpression in the southern part of the Arabian-Nubian Shield (ANS). This area is dominated by five main basement lithologies: gneisses, metavolcanics, Ablah Group (meta-clastic and marble units) and syn- and post-tectonic granitoids. These rocks were affected by three phases of deformation (D1–D3). D1 formed tight to isoclinal and intrafolial folds (F1), penetrative foliation (S1), and mineral lineation (L1), which resulted from early E-W (to ENE-WSW) shortening. D2 deformation overprinted D1 structures and was dominated by transpression and top-to-the-W (?WSW) thrusting as shortening progressed. Stretching lineation trajectories, S-C foliations, asymmetric shear fabrics and related mylonitic foliation, and flat-ramp and duplex geometries further indicate the inferred transport direction. The N- to NNW-orientation of both “in-sequence piggy-back thrusts” and axial planes of minor and major F2 thrust-related overturned folds also indicates the same D2 compressional stress trajectories. The Wadi Yiba Shear Zone (WYSZ) formed during D2 deformation. It is one of several N-S trending brittle-ductile Late Neoproterozoic shear zones in the southern part of the ANS. Shear sense indicators reveal that shearing during D2 regional-scale transpression was dextral and is consistent with the mega-scale sigmoidal patterns recognized on Landsat images. The shearing led to the formation of the WYSZ and consequent F2 shear zone-related folds, as well as other unmappable shear zones in the deformed rocks. Emplacement of the syn-tectonic granitoids is likely to have occurred during D2 transpression and occupied space created during thrust propagation. D1 and D2 structures are locally overprinted by mesoscopic- to macroscopic-scale D3 structures (F3 folds, and L3 crenulation lineations and kink bands). F3 folds are frequently open and have steep to subvertical axial planes and axes that plunge ENE to ESE. This deformation may reflect progressive convergence between East and West Gondwana. 相似文献
18.
The results of investigation of the Bikkulovskoe manganese deposit confined to volcanosedimentary piles of the Magnitogorsk
paleovolcanic belt are presented. The paper characterizes the geological setting of the deposit and mineral-chemical compositions
of ores and enclosing rocks (volcanomictic sandstones; ferruginous, manganiferous, and ferruginous-siliceous tuffites; and
jasperites). Analysis of the data obtained made it possible to identify four sequential stages of deposit formation: (1) accumulation
and diagenesis of ore-bearing sequences (D2–3); (2) burial and low-grade (T = 200–250°C, P = 2 to 3 kbar) regional metamorphism of rocks (D2–3-C1); (3) tectonic deformations of volcanosedimentary piles (C2-P); and (4) hypergenesis and partial denudation of rocks (MZ-CZ). According to the model proposed for the accumulation of
ore-bearing rocks, the productive member was formed in a zone of hydrothermal solution outflow to the seafloor surface. Discharge
of solutions and precipitation of Fe and Si began below the seafloor surface (rather than above the surface) in the near-bottom
sequence of volcanomictic sediments. Upon reaching the seafloor, the impoverished solutions mixed with seawater and gave up
metals completely: Fe and Mn were transferred to sediments to make up the ore-bearing bed. Thus, zonal sediments with ferruginous
tuffites at the base and manganese ores at the top were formed. 相似文献
19.
A nappe of amphibolite-facies metamorphic rocks of pre-Permian age in the southern Vanoise massif (the Arpont schist) has been affected by an Alpine HP/LT metamorphism. The first mesoscopically recognizable deformation (D1) post-dated the high-pressure peak (jadeitic pyroxene + quartz, glaucophane + ?lawsonite), and was associated with glaucophane + epidote. D1 produced a flat-lying schistosity and a NW-trending glaucophane lineation, and was probably associated with nappe displacement involving NW-directed subhorizontal shear. D2 formed small-scale folds and a foliation associated with chlorite + albite. The changing parageneses during the period pre-D1 to D1 to D2 suggest decreasing pressure, so that the deformation appears to have been related to the uplift history, rather than to the process of tectonic burial. D2 was followed by a static metamorphism (green biotite + chlorite + albite), possibly of Lepontine age. SE-directed backthrusting and folding (D3), and later differential uplift along steep faults, took place under low-grade conditions. 相似文献
20.
Y. J. Bhaskar Rao T. R. K. Chetty A. S. Janardhan K. Gopalan 《Contributions to Mineralogy and Petrology》1996,125(2-3):237-250
Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone
(CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of
tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D1-M1) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation
(D2-MCSZ). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60
Ma, ɛNd + 1.85±0.16 and Bhavani – 2899±28 Ma, ɛNd + 2.18±0.14 (2σ errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered
complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M1 metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the MCSZ as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830°C (minimum). The MCSZ is associated with significant isothermal decompression of the order of 4.5–3.5 kbar followed by static high-temperature
rehydration and retrogression around 600°C. The timing of MCSZ is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for
a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic
plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking
of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in
a model Proterozoic Gondwana supercontinent.
Received: 5 December 1995 / Accepted: 3 May 1996 相似文献