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1.
New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development.The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sri ca. 0.703; model μ1 values ca. 8.0; initial εNd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Strømfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon UPb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion.The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada.  相似文献   

2.
Ion probe UPb age determinations on zircons from two samples of metasediment belonging to the Malene supracrustals of southern West Greenland closely constrain the age of sedimentation, between the youngest age obtained from detrital material and the age of metamorphic overgrowth. For both samples, older and younger limits of ca. 2900 Ma and ca. 2650 Ma, respectively, are indicated. Some of the detrital zircons are best interpreted as derived from their source rock after the regional high-grade metamorphism at ca. 2800 Ma: if so, the older limit of the age of sedimentation is younger than 2800 Ma. The hypothesis that all Malene supracrustal rocks pre-date the middle to late Archaean Nuˆk gneisses is no longer valid. This has major implications for interpretations of the late Archaean crustal evolution of western Greenland: the period between 2800 and 2500 Ma was characterised by major tectonic activity and metamorphism.  相似文献   

3.
Abstract The chemical Th-U-total Pb isochron method (CHIME) was applied to determine the age of monazite and thorite in five gneisses and zircon in an ultra high-pressure (UHP) phengite schist from the Su-Lu region, eastern China. The CHIME ages and isotopic ages reported in the literature show that gneisses in the Su-Lu region are divided into middle Proterozoic (1500–1720 Ma) and Mesozoic (100–250 Ma) groups. The Proterozoic group includes paragneiss and orthogneiss of the amphibolite-granulite facies, and their protolith age is late Archean-early Proterozoic. The Mesozoic group is mainly composed of orthogneiss of the greenschist-epidote amphibolite facies, and the protolith age is Middle Paleozoic-Early Proterozoic. The Proterozoic and Mesozoic gneisses occupy northern and southern areas of the Su-Lu region, respectively, which are divided by a major Wulian-Qingdao-Yantai fault. Ultra high-pressure metamorphic rocks occur as blocks in the Mesozoic gneisses, and form a UHP complex.
The UHP phengite schist in the Mesozoic orthogneiss contains detrital zircons with late Proterozoic CHIME age ( ca 860 Ma). Age of the UHP metamorphism is late Proterozoic or younger, and protolith age of the UHP metamorphic rocks is probably different from that of the surrounding Mesozoic gneisses.  相似文献   

4.
UPb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma UPb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A UPb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma UPb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma UPb zircon age.Regional greenschist metamorphism has been dated directly, with a UPb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma UPb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.  相似文献   

5.
The exposed elements of the Lower Proterozoic orogenic belts of the Halls Creek sub-province, Northern Australia, lie in fault zones which have suffered repeated tectonic activity at various times through the Proterozoic and Phanerozoic. The Halls Creek and King Leopold orogenic domains subtend an angle of 80° and are characterized by linear late tectonic batholithic complexes several hundred kilometres long but only a few tens of kilometres wide, reminiscent of those in Phanerozoic Cordilleran orogenies. The associated superposed folding and high temperature metamorphism are more akin to those in Phanerozoic collision orogenies.The sub-province is analyzed in the wider context of the North Australian orogenic province which was deformed, metamorphosed and intruded by granitic plutons approximately 1900-1800 Ma ago. In this province the Archaen basement was extended and broken into a mosaic of blocks, some of which (now largely concealed by younger Kimberley and McArthur basin sediments) retained a more positive character and fed sediment to intervening regions (such as the Pine Creek Geosyncline) which suffered greater extension and subsidence, but which retained a thinned Archaean basement.The Halls Creek Group was deposited in a trough to the south-east of the Kimberley island continent, and deposition was probably broadly contemporaneous with, and continuous with, that in the Pine Creek geosyncline. A volcanic—fine grained clastic—carbonate phase of marine deposition, following basin formation, is represented by the Biscay Formation. During the later phase of basin evolution widespread flysch facies (Olympio Formation), partly derived from the island continent, was deposited and is now preserved in low grade zones on both sides of the main belt of high strain and upper amphibolite to lower granulite facies metamorphism which displays recumbent folding and nappe tectonics with fold axes oblique to the major faults.No island arc compex or paired metamorphic belts are present in the orogenic belts, and it is concluded that the lithospheric extension and subsequent convergence did not involve the generation of oceanic crust or B-subduction.In the Halls Creek domain vergence is south-easterly across all zones and is related to oblique convergence leading to limited A-subduction of the basinal area in the south-east beneath the island continent to the north-west, accompanied by left-lateral strike-slip or transform fault movements on the north-trending major faults. The convergence generated the associated high temperature metamorphism and plutonism on the leading edge of the lower plate.A phase of upright folding (with trends varying continuously form E-W in the King Leopold belt to NNE-SSW in the Halls Creek belt) intervenes between the main recumbent deformation and metamorphism (ca 1920 Ma ago) and the emplacement of the late tectonic granite batholiths (ca 1840 Ma ago) which are fault controlled.The province represents a distinctive type of linear Proterozoic ensialic orogeny, not explicitly identified previously, and it needs to be distinguished both from true collision orogenies of the Phanerozoic, involving a Wilson Cycle, and from the areally extensive Proterozoic orogenies with which it is associated. Its essential characteristics are due to convergence between a small continent and an ‘oceanic’ area underlain by thin continental crust, resulting in limited A-subduction of the latter prior to crustal shortening.  相似文献   

6.
Mahshar  Raza  MohdShamim  Khan  MohdSafdare  Azam 《Island Arc》2007,16(4):536-552
Abstract   The northern part of the Aravalli mountain belt of northwestern Indian shield is broadly composed of three Proterozoic volcano-sedimentary domains, i.e. the Bayana, the Alwar and the Khetri basins, comprising collectively the north Delhi fold belt. Major, trace and rare earth element concentrations of mafic volcanic rocks of the three basins exhibit considerable diversity. Bayana and Alwar volcanics are typical tholeiites showing close similarity with low Ti–continental flood basalts (CFB) with the difference that the former shows enriched and the latter flat incompatible trace element and rare earth element (REE) patterns. However, the Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. It appears that the melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field possibly in the presence of mantle plume. During ascent to the surface the Bayana tholeiites suffered crustal contamination but the Alwar tholeiites erupted unaffected. Geochemically, the Khetri volcanics are arc-like basalts which were generated in a segment of mantle overlying a Proterozoic subduction zone. It is suggested that at about 1800 Ma the continental lithosphere in northeastern Rajasthan stretched, attenuated and fractured in response to a rising plume. The produced rifts have undergone variable degrees of crustal extension. The extension and attenuation of the crust facilitated shallowing of the asthenosphere which suffered variable degree of melting to produce tholeiitic melts – different batches of which underwent different degrees of lithospheric contamination depending upon the thickness of the crust in different rifted basins. The occurrence of subduction-related basaltic rocks of Khetri Belt suggests that a basin on the western margin of the craton developed into a mature oceanic basin.  相似文献   

7.
The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.  相似文献   

8.
This study presents the chronological evolution of the upper amphibolite facies Orue Unit in NW Namibia. Metasedimentary and meta-igneous rocks of the Orue Unit were investigated using the Pb–Pb stepwise leaching technique on garnet and rutile, U–Pb multi-grain analysis on rutile, Sm–Nd–Lu–Hf leaching technique on garnet, SHRIMP analysis on zircon and Ar–Ar dating on amphibole. Each of these techniques pertains to different processes that occurred before, during, or after the metamorphic peak. Our age data can be integrated with petrological constraints to provide a more complete understanding of the metamorphic cycle. Our pre-peak metamorphic zircon ages, peak metamorphic garnet ages and peak to late peak metamorphic amphibole 39Ar–40Ar ages bracket the upper amphibolite facies metamorphic event including hydration or dehydration processes into a time span of only ca. 20 Ma. The age data obtained by peak metamorphic mineral analyses cluster around 1340–1320 Ma. Based on age data and field observation, we interpret the upper amphibolite facies metamorphism as a large-scale regional mid-crustal event. Spot analyses of inherited zircon cores obtained by SHRIMP reflect the sedimentary origin of the respective rocks of the Orue Unit and derivation from Palaeoproterozoic protoliths. The metamorphic rocks south of the anorthositic Kunene Intrusive Complex (KIC) have previously been ascribed to the Palaeoproterozoic Epupa Complex at the SW margin of the Congo craton and were thus thought to be older than the Mesoproterozoic KIC. Our data show that the high-grade metamorphic overprint took place 30–50 Ma after emplacement of the KIC. Rutile growth ages of 1248 Ma in one sample reflect fluid activity which seems to be a local phenomenon since there is no other evidence of geological activity throughout the Orue Unit at that time. The rutile ages predate the emplacement of satellite intrusions in that area by 30 Ma and there is no causal relation between these two events.  相似文献   

9.
Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm3. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.  相似文献   

10.
In the north Tyrrhenian region and adjoining shores of Alpine Corsica and central Italy, four magmatic phases (15–14; 8–6; 5–2; and 1.3–0.1 Ma) are present which consist prevailingly of either old acidic and lamproitic rocks or young (1.3–0.1 Ma) basic volcanics with potassic–ultrapotassic affinity (the rocks of the K and HK series of the Roman province, and the kamafugitic rocks). Composition, age and location of the igneous phases reveal a geochemical polarity oriented from west to east. The episodes of magmatic activity reflect the evolution of the geodynamic environment (ensialic plate convergence): backarc stretching and attenuation of an Alpine orogenic wedge were accompanied by the eastward roll-back and progressive steepening of the descending Adria plate. The latter process led to enhanced metasomatic modifications of the mantle wedge composition and to eruption of the large volumes of KS and HKS volcanic rocks in central Italy (0.6–0.1 Ma).  相似文献   

11.
A model involving buoyancy, wedging and thermal doming is postulated to explain the differential exhumation of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie Mountains, China, with an emphasis on the exhumation of the UHP rocks from the base of the crust to the upper crust by opposite wedging of the North China Block (NCB). The Yangtze Block was subducted northward under the NCB and Northern Dabie microblock, forming UHP metamorphic rocks in the Triassic (240–220 Ma). After delamination of the subduction wedge, the UHP rocks were exhumed rapidly to the base of the crust by buoyancy (220–200 Ma). Subsequently, when the left-lateral Tan–Lu transform fault began to be activated, continuous north–south compression and uplifting of the orogen forced the NCB to be subducted southward under the Dabie Orogen (`opposite subduction'). Opposite subduction and wedging of the North China continental crust is responsible for the rapid exhumation of the UHP and South Dabie Block units during the Early Jurassic, at ca 200–180 Ma at a rate of ∼ 3.0 mm/year. The UHP eclogite suffered retrograde metamorphism to greenschist facies. Rapid exhumation of the North Dabie Block (NDB) occurred during 135–120 Ma because of thermal doming and granitoid formation during extension of continental margin of the Eurasia. Amphibolite facies rocks from NDB suffered retrograde metamorphism to greenschist facies. Different unit(s) and terrane(s) were welded together by granites and the wedging ceased. Since 120–110 Ma, slow uplift of the entire Dabie terrane is caused by gravitational equilibrium.  相似文献   

12.
Granitic gneisses have been widely found in crystalline rocks in the Dinggye area of the Higher Himalaya (HHM) and the LhagoiKangri area of the North Himalaya (NHM), Tibet. In the HHM, the gneisses intruded in the granulite-amphibolite facies metamorphosed sedimentary rocks, known as Nyalam group. In the NHM, the gneisses intruded in the amphibolite facies metamorphosed ones, known as LhagoiKangri group. These granitic gneisses are peraluminous monzonitic granites in terms of their mineral assemblage, and are considered as being derived from metamorphosed sedimentary rocks by anatexis based on the transitional relationship of the gneisses with their migmatitized wall rocks. Zircons are similar in crystal shape and interior structure from both gneisses. Most of them are euhedral or subhedral elongated prism-shaped transparent crystals, with fine oscillatory zoning, showing the magmatic genesis. Some of them are short prism-shaped and with relict core inherited from magma source and oscillatory zoning mantle crystallized from magma. SHRIMP U-Pb dating of zicons shows that both the granitic gneisses in the HHM and NHM are Paleoproterozoic (1811.6±2.9 Ma and 1811.7±7.2 Ma, respectively). These ages are similar to those (1815 to 2120 Ma) from granitic gneiss which is widely distributed in the Lesser Himalaya (LHM). The ages of inherited zircons (>2493.9±7.0 Ma, 2095.8± 8.8 Ma, 1874±29 Ma) exhibit the possible presence of several thermal events in Paleoproterozoic. All of the ages suggest the same India basement beneath the different units in Himalaya area, and do not support the idea that the HHM and NHM are accretionary terranes in Pan-Africa orogenic event. The fact that the basement in HHM is as old as or even younger than LHM is inconsistent with the presently prevalent orogenic models such as either extrusion of low-viscosity mid-crust or orogenic channel.  相似文献   

13.
Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province(WSP) is located at the eastern part of the North China Craton(NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.  相似文献   

14.
We present 39Ar–40Ar dating of phengite, muscovite and paragonite from a set of mafic and metasedimentary rocks sampled from the high-pressure (HP) metaophiolites of the Voltri Group (Western Alps) and from clasts in the basal layer conglomerates from the Tertiary molasse which overlie the high-pressure basement. The white mica-bearing rocks display peak eclogitic and blueschist-facies parageneses, locally showing complex greenschist-facies replacement textures. The internal discordance of age spectra is proportional to the chemical complexity of the micas. High-Si phengites from eclogite clasts record a 39Ar–40Ar age of ca. 49 Ma for the eclogite stage and ca. 43 Ma for the blueschist retrogression; phengites from a blueschist basement sample yield an age of ca. 40 Ma; low-Si muscovite from a metasediment dates the formation of the greenschist paragenesis at ca. 33 Ma. Our data indicate that the analyzed samples reached high-pressure conditions at different times over a time-span of c.a. 10 Ma. Subduction was continuing during exhumation and blueschist retrograde re-equilibration of higher-pressure, eclogite-facies rocks. This process kept the isotherms depressed, allowing the older HP-rocks to escape thermal re-equilibration. Our results, added to literature data, fit a tectonic model of a subduction–exhumation cycle, with different tectonic slices subducted at different times from Early Eocene until the Eocene–Oligocene boundary.  相似文献   

15.
The tectonic activities during late Archaean-earlyProterozoic is the crisis during the process of crustevolution. The tectonic kinematical mode and dynamicprocess of metamorphic complexes formed is the keyproblem in geosciences[1—7], related to many importantgeological events, such as the substitute of dynamicalsystems and the corresponding relationship betweendeep crust-mantle structure and upper regional stressfield. The predecessors have made a great deal ofstudy on this topic and achieved…  相似文献   

16.
The Hidaka Metamorphic Belt is a well-known example of island-arc crustal section, in which metamorphic grade increases westwards from unmetamorphosed sediment up to granulite facies. It is divided into lower (granulite to amphibolite facies) and upper (amphibolite to greenschist facies) metamorphic sequences. The metamorphic age of the belt was considered to be ~55 Ma, based on Rb – Sr whole-rock isochron ages for granulites and related S-type tonalities. However, zircons from the granulites in the lower sequence yield U – Pb ages of ~21 – 19 Ma, and a preliminary report on zircons from pelitic gneiss in the upper sequence gives a U – Pb age of ~40 Ma. In this paper we provide new zircon U – Pb ages from two pelitic gneisses in the upper sequence to assess the metamorphic age and also the maximum depositional age of the sedimentary protolith. The weighted mean 206Pb/238U ages from a biotite gneiss in the central area of the belt yield 39.6 ± 0.9 Ma for newly grown metamorphic rims and 53.1 ± 0.9 Ma for the youngest detrital cores. The ages of zircons from a cordierite–biotite gneiss in the southern area are 35.9 ± 0.7 Ma for metamorphic rims and 46.5 ± 2.8 Ma for the youngest detrital cores. These results indicate that metamorphism of the upper sequence took place at ~40 – 36 Ma, and that the sedimentary protolith was deposited after ~53 – 47 Ma. These metamorphic ages are consistent with the reported ages of ~37–36 Ma plutonic rocks in the upper sequence, but contrast with the ~21–19 Ma ages of metamorphic and plutonic rocks in the lower sequence. Therefore, we conclude that the upper and lower metamorphic sequences developed independently but coupled with each other before ~19 Ma as a result of dextral reverse tectonic movement.  相似文献   

17.
C. W. Oh  & J. G. Liou 《Island Arc》1998,7(1-2):36-51
The petrogenetic grid between the eclogite and other high-pressure/temperature (P/T) metamorphic facies in a basaltic system is constructed by considering barroisite as one of the important phases in high-P/T metamorphism and by using previous petrological data combined with Schreinemakers' analysis and slope calculation. In the constructed petrogenetic grid, the eclogite facies is bounded by the blueschist and epidote–amphibolite facies with negative-slope reactions at lower temperatures (450–550 °C) and by the epidote–amphibolite, amphibolite and granulite facies with positive-slope reactions at higher temperatures (> 550–600 °C). The eclogite facies does not contact the greenschist facies, and the lowest P condition for the eclogite facies exists at the boundary between the eclogite and epidote–amphibolite facies. The temperature range of the epidote–amphibolite facies increases with increasing pressure until 8–11 kbar and then decreases up to 13–15 kbar. Compared to boundaries of other facies, boundaries of the eclogite facies may have wider P–T ranges. The boundary between the blueschist and eclogite facies occurs over a large temperature range from 450 to 620 ± 30 °C, and the transitions between the eclogite and amphibolite or high-pressure granulite facies occur over a pressure range in excess of 6–10 kbar.  相似文献   

18.
Measurements on thermal conductivity and diffusivity as functions of temperature (up to 1150 K) and pressure (up to 1000 MPa) are presented for Archaean and Proterozoic mafic high-grade rocks metamorphosed in middle and lower crustal pressures, and situated in eastern Finland, central Fennoscandian Shield. Decrease of 12–20% in conductivity and 40–55% in diffusivity was recorded between room temperature and 1150 K, which can be considered as typical of phonon conductivity. Radiative heat transfer effects were not detected in these samples. Pressure dependencies of the samples are weak if compared to crystalline rocks in general, but relatively typical for mafic rocks.The temperature and pressure dependencies of thermal transport properties (data from literature and the present study) were applied in an uncertainty analysis of lithospheric conductive thermal modellings with random (Monte Carlo) simulations using a 4-layer model representative of shield lithosphere. Model parameters were varied according to predetermined probability functions and standard deviations were calculated for lithospheric temperature and heat flow density after 1500 independent simulations. The results suggest that the variations (uncertainties) in calculated temperature and heat flow density values due to variations in the temperature and pressure dependencies of conductivity are minor in comparison to the effects produced by typical variations in the room temperature value of conductivity, heat production rate or lower boundary condition values.  相似文献   

19.
Two volcanic zones (Bukavu and Kamituga) south of Lake Kivu (southeastern Zaire) are part of the western branch of the Eastern African rift. They were formed during three volcanic cycles, one pre-rift (70-7 Ma old) and the other two syn-rift (7.8-1.9 Ma old and 14,000 y.-sub-Recent, respectively), and evolved from quartz tholeiites of the pre-rift period to alkali basalts of the rift stage. The basaltic rocks, which strongly predominate, are compositionally similar to other rift-related basalts and also to oceanic-island rocks. Most of the basalts have undergone only limited fractional crystallization (5–10%) dominated by olivine and clinopyroxene. The distinct variations of incompatible elements even in rocks of very similar major-element composition imply that the basaltic rocks were derived from a heterogeneous source by variable degrees of melting. The inferred source composition closely resembles that of metasomatized peridotite xenoliths from alkali basalts.  相似文献   

20.
Within the Tethyan realm, data for the subduction history of the Permo–Triassic Tethys in the form of accretionary complexes are scarce, coming mainly from northwest Turkey and Tibet. Herein we present field geological, petrological and geochronological data on a Triassic accretionary complex, the A?vanis metamorphic rocks, from northeast Turkey. The A?vanis metamorphic rocks form a SSE–NNW trending lozenge‐shaped horst, ～20 km long and ～6 km across, bounded by the strands of the active North Anatolian Fault close to the collision zone between the Eastern Pontides and the Menderes–Taurus Block. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes of Early Eocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°C and ～0.60–0.90 GPa. Stepwise 40Ar/39Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ～180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, the A?vanis metamorphic rocks represent the vestiges of the Late Triassic or slightly older subduction in northeast Turkey. Estimated P–T conditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐day Cascadia. Contact metamorphic mineral assemblages around an Early Eocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ～14 km during the Early Eocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐mica Ar–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision between Eastern Pontides and the Menderes–Taurus Block.  相似文献   

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