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1.
The Mesozoïc sediments in the Eastern North Pyrenean Zone have suffered a high temperature-low pressure metamorphism which reached its climax before the major deformation event. The mineral associations in pelitic rocks are consistent with temperatures of 500°–600°C and a maximum pressure of 3–4 kb. Post-metamorphic brittle deformation has disturbed the initial thermal pattern. The Albo-Cenomanian (98–87 Ma) metamorphism is related to thermal anomalies contemporaneous with the crustal thinning in the North Pyrenean Zone. The distribution of paleotemperatures suggests that the intensity of metamorphism may have been related to the magnitude of crustal attenuation. High-grade rocks are associated with lherzolites and granulites, whereas low-grade rocks are associated with higher-level crustal material (gneisses and micaschists). Crustal thinning and metamorphism developed during sinistral transcurrent displacement of Iberia with respect to Europe.  相似文献   

2.
Rare but widespread relics of sodic amphibole occur in metabasites and metacherts of the Haast Schists and related Caples Terrane rocks. Present main-stage metamorphic assemblages are frequently chemically equivalent to earlier sodic amphibole bearing assemblages, indicating that these rocks underwent an earlier, higher P/Tmetamorphism prior to formation of the present pumpellyite-actinolite and greenschist facies assemblages. The earlier assemblages were stable during and after early isoclinal folding, but were replaced by the present moderate P/T assemblages prior to the last major fabric-forming deformation. The change in conditions was due to thermal relaxation, probably accompanied by uplift and erosion, and peak metamorphic temperatures were about 350–370° C in the pumpellyite-actinolite zone of the Caples Terrane and near 390° C in the greenschist facies chlorite zone near Queenstown. According to Henley (1975) these greenschist facies rocks attained a pressure of at least 6.4±0.4 kb during their history, but a pressure of 4.6±0.6 kb has been estimated for a chlorite zone rock from Middlemarch, and so the 6.4 kb estimate probably refers to the maximum pressure attained during the earlier, higher P/T metamorphism. Similar changes in metamorphic facies series with time occur in some older and more complex metamorphic belts such as the Caledonides, and this study suggests that it may be possible to interpret particular elements in the metamorphic development of such belts in terms of specific circum-Pacific analogues.  相似文献   

3.
K–Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.  相似文献   

4.
Garnet-peridotite stability and occurrence in crust and mantle   总被引：10，自引：0，他引：10  
The boundary between the spinel- and garnet-lherzolite mineral facies is strongly curved between 1300° C and 1450° C; below 1200° C it lies almost parallel to the temperature axis of the T-P diagram.Pressure of at least 12 kb is required to stabilise garnet-peridotite on the geothermal gradient and the depth at which the boundary is encountered is not sensitive to variations in the geotherm.Garnet-peridotite is metastable with respect to spinel-peridotite in normal continental crust. Natural occurrences are mantle derived and have either suffered rapid upward transit in diatremes or, in orogenic zones, either they were emplaced after upward tectonic transport through tens of kilometres, or have originated by in situ metamorphism of pre-existing crustal peridotite in an orogenic root downfolded to depths of at least 40 km.  相似文献   

5.
Neoproterozoic rocks constitute the Kenticha, Alghe and Bulbul litho-tectonic domains in the Negele area of southern Ethiopia. Structural features and fabrics in these rocks were developed during north-south folding (D1), thrusting (D2) and shearing (D3) deformation. From micro-structural inferences and fabric relationships in semi-pelitic schists/gneisses of the Kenticha and Alghe domains, three episodes of metamorphic mineral growths (M1, M2 and M3) are inferred to have accompanied the deformational events. Pressure-Temperature estimates on equilibrium garnet-plagioclase-biotite and garnet-biotite assemblages from semi-pelitic schists/gneisses of the two domains indicate metamorphic recrystallization at temperatures of 520–580°C and 590–640°C, and pressures of 4–5 kb and 6–7 kb in the Kenticha and Alghe domains, respectively. These results correspond to regional metamorphism at a depth of 16–20 km for the Kenticha and 22–25 km for the Alghe domains. The P-T results suggest that the protoliths to the rocks of the Kenticha and Alghe domains were subjected to metamorphism at different crustal depths. This implies exhumation of the Alghe gneissic rocks from intermediate crustal level (ca. 25 km) before juxtaposition with the Kenticha sequence along a north-south trending thrust at the present crustal level during the Neoproterozoic. The combined deformation, fabric and mineral growth data suggest that rocks in the Kenticha and Alghe domains evolved under similar tectono-metamorphic conditions, which resulted from crustal thickening and uplift followed by extension and orogenic collapse, exhumation and cooling before litho-tectonic domains coalesced and cratonized in the Neoproterozoic southern Ethiopian segment of the East African Orogen.  相似文献   

6.
Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P tot to the assemblage enstatite+forsterite+spinel+H2O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H2O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.  相似文献   

7.
Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.  相似文献   

8.
The Moldanubian basement of the Schwarzwald contains basic to ultrabasic rocks of both crustal and mantle origin which display high-pressure mineral assemblages or relics of such. In order to constrain the P-T-t evolution of the crustal high-pressure rocks, petrological and geochronological studies have been carried out on three eclogite samples. Geothermobarometric estimations indicate minimum metamorphic pressures of 1.6 GPa and equilibration temperatures of 670 750°C. Reaction textures document various metamorphic stages during exhumation of the high-pressure rocks. The age of high-pressure metamorphism is constrained by Sm-Nd isochrons of 332±13 Ma, 334±11 Ma, and 337±6 Ma defined by garnet, whole rock and clinopyroxene. For one sample, large garnets show prominent growth zoning in terms of major elements, Sm, Nd, and inclusions, dividing the grains into two growth stages. Sm-Nd isotope analyses on these garnets indicate that the time span between the two growth stages is too small to be resolved, reflecting a rather rapid metamorphic evolution. This result is further constrained by a Rb-Sr isochron age of 325±6 Ma on retrograde biotite and whole rock on the same sample. For one of the studied eclogites, formation of the magmatic precursor rocks is possibly approximated by the Ordovician U-Pb upper intercept age of a discordia from zircons.  相似文献   

9.
High-pressure conditions of 11–13 kbar/500–540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm–Nd mineral-whole rock isochron at 465 ± 24 Ma dates fluid influx at peak temperatures of ∼600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12–13 °C/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Marañon Complex. Here, low PT conditions at 2.4–2.6 kbar, 300–330 °C were estimated for a phyllite–greenschist assemblage representing a contrasting metamorphic geotherm of 32–40 °C/km characteristic for a magmatic arc environment.  相似文献   

10.
Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).  相似文献   

11.
Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite.All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions.The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.  相似文献   

12.
吴宗絮 《现代地质》1994,8(2):133-138
本文根据变质岩矿物对的平衡热力学研究结果，提出冀东地区早前寒武纪陆壳平均地温分布曲线，并讨论现代陆亮地温曲线的两种可能形态．早前寒武纪在相当于中、下地壳的部分，地温的平均梯度约为７．３℃／ｋｍ，它预示了对流热传输占有重要地位（约占６５％）．基于传导热传输的假设，现代地温分布是一条平滑的曲线，而基于中地壳是一个含水的低速层以及对流热传输的假设，现代地温分布则可能是一条折线．退变质作用引起的放热与扩容可能是诱发地震的能量来源之一．  相似文献   

13.
The Mogok metamorphic belt (MMB), over 1450 km long and up to 40 km wide, consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the Western margin of the Shan Plateau in central Myanmar and continuing northwards to the eastern Himalayan syntaxis. Exposures in quarries allow correlation of Palaeozoic meta-sedimentary, early Mesozoic meta-igneous and late Mesozoic intrusive rocks within a 230 km long northerly-trending segment of the MMB, from Tatkon to Kyanigan north of Mandalay, and with the Mogok gemstone district 100 km to the northeast. Relationships among the metamorphic and intrusive rocks, with sparse published radiometric age controls, indicate at least two metamorphic events, one before and one after the intrusion of Late Jurassic to early Cretaceous calc-alkaline rocks. These relationships can be explained by either of two possible tectonic histories. One, constrained by correlation of mid-Permian limestones across Myanmar, requires early Permian and early Jurassic regional metamorphic events, prior to an early Tertiary metamorphism, in the western part of but within a Shan-Thai – western Myanmar block. The second, not compatible with a single laterally continuous Permian limestone, requires pre-Upper Jurassic regional metamorphism and orogenic gold mineralization in the Mergui Group and western Myanmar, early Cretaceous collision of an east-facing Mergui-western Myanmar island arc with the Shan Plateau, and early Tertiary metamorphism in the MMB related to reversal in tectonic polarity following the arc-Plateau collision.  相似文献   

14.
Summary Precambrian amphibolites and quartz-mica schists in the Saidapuram-Podalakuru area fall within the almandine-amphibolite facies of regional metamorphism. The analysed rocks represent metamorphosed basic igneous rocks. It is suggested that the quartz-muscovite-staurolite schists underwent metamorphism between 550° and 700° C at an average pressure of 7.2 kb; the quartz-muscovite (±biotite), hornblende-biotite (±garnet), and hornblende-garnet schists between 600°–700° C/7.5 kb; and the quartz-biotitekyanite schists between 650°–700° C/8 kb.
Petrologie der metamorphen Gesteine in Almandin-Amphibolit-Fazies im Gebiet von Saidapuram-Podalakuru, Distrikt Nellore, Andhra Pradesh, Indien
Zusammenfassung Präkambrische Amphibolite und Quarz-Glimmer-Schiefer im Gebiet von Saidapuram-Podalakuru gehören der Almandin-Amphibolit-Fazies an. Die analysierten Gesteine stellen metamorphe basische Erstarrungsgesteine dar. Die Quarz-Muskovit-Staurolith-Schiefer wurden bei 550°–700°C und einem durchschnittlichen Druck von 7,2 kb metamorph, die Quarz-Muskovit-(±Biotit-), die Hornblende-Biotit- (±Granat-) und die Hornblende-Granat-Schiefer bei 600°–700° C/7,5 kb, die Quarz-Cyanit-Schiefer bei 650°–700° C/8 kb.

With 6 Figures  相似文献   

15.
《Precambrian Research》2001,105(2-4):143-164
New fieldwork, map interpretation, petrography and single zircon U–Pb geochronology has allowed the identification of different crustal blocks in the Paamiut region, in the southern portion of the West Greenland Archaean Craton. Changes of metamorphic grade from only amphibolite facies to granulite facies (some subsequently retrogressed) corresponds with zones of Archaean high strain ductile deformation ± mylonites. U–Pb zircon dates are presented for the TTG (tonalite, trondhjemite, granodiorite) protoliths from each block in the Paamiut region, and the southern portion of the previously identified Tasiusarsuaq terrane lying to the north. The southern part of the Tasiusarsuaq terrane contains 2880–2860 Ma TTG rocks and underwent amphibolite facies metamorphism. Structurally underneath the Tasiusarsuaq terrane to the south is the Sioraq block containing 2870–2830 Ma TTG rocks partly retrogressed from granulite facies. Structurally underneath and to the south is the Paamiut block, dominated by 2850–2770 Ma granodioritic rocks that have only undergone amphibolite facies metamorphism. Also structurally overlying the Paamiut block, but cropping out separately from the Sioraq block, is the Neria block. This appears to be dominated by 2940–2920 Ma gneisses that have been totally retrogressed from granulite facies and strongly deformed. In the southernmost part of the region the Neria block overlies the greenschist to lowermost amphibolite facies Sermiligaarsuk block that contains the ⩾2945 Ma Tartoq Group. Rocks from all the blocks record ancient loss of Pb from zircons and some new zircon growth at 2820 Ma, interpreted to indicate a high grade metamorphic event at that time, including granulite facies metamorphism in the Sioraq and Neria blocks. The blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 Ma metamorphism, explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures. The juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions. The contacts are cut by undeformed Palaeoproterozoic dolerite dykes which post-date amphibolite facies metamorphism. These results, together with previously published data from the Godthåbsfjord region (north of Paamiut) shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north (∼550 km) consists of a mosaic of ductile fault-bounded packages that attained their present relative positions in the late Archaean.  相似文献   

16.
The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.  相似文献   

17.
Coalification and graphitization in high-pressure schists in New Caledonia   总被引：1，自引：0，他引：1  
The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.  相似文献   

18.
Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1–3.3 Ga) and Dharwar Supergroup (2.6–2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0–2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0–2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.  相似文献   

19.
Ultramafic and mafic granulites from Archaean gneisses in N.W. Scotland (the Scourian) show evidence of two periods of granulite facies mineral growth. The first produced a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12–15 kb and 1,000° C. Uplift of the complex caused partial breakdown of the garnet by reaction with clinopyroxene to produce orthopyroxene +plagioclase ±spinel±amphibole symplectites, at an estimatedP-T of 10–14 kb and 800°–900° C. Garnet stability is shown to depend on both whole-rock Fe/Mg ratios and onP-T conditions. The pressures imply crustal thicknesses in the Archaean of least 35–45 km.  相似文献   

20.
40Ar/39Ar dating and estimates of regional metamorphic P–T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.  相似文献   

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