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1.
New data on the composition of tagamites (impact melt rocks) of the Popigai atsrobleme (Siberia) are presented. The preliminary results support the following major conclusions: (1) the matrix of the studied tagamites is crystallized and is composed of identified minerals; (2) the degree of the matrix crystallization and the amount of inclusions in tagamites can vary significantly; (3) tagamites are identical to the intracrater gneisses of the Popigai astrobleme by the REE contents and are distinct from them by significantly lower SiO2 and higher Al2O3, FeO, MgO, and CaO contents; (4) the Popigai tagamites and intracrater gneisses are enriched in Ni (on average) in comparison with gneisses of the Khapchan Group; and (5) the various (often sufficient) degrees of crystallization of tagamites indicate the duration of the cooling of the melt, when diamonds could possibly have been dissolved.  相似文献   

2.
Rare-earth elements abundance in black shales of the Upper Jurassic (Tithonian Stage)–Lower Cretaceous (Berriassian Stage) Bazhenov Formation is discussed. This formation is the principal oil source rocks of West Siberia. The deposits within the formation can be subdivided into two main marine groups: (a) moderately hemipelagic deposits (clayey-siliceous, including phosphatic and carbonate rocks) and low-density distal clayey turbidites (argillites), both are considered as normal and (b) silty argillites and clayey-silt rocks, which are channel deposits and considered as anomalous. The hemipelagic rocks of normal sections, which are enriched in the rare-earth elements (REE), accumulated under both slow rates of sedimentation (clayey-siliceous rocks) and faster rates of sedimentation (argillites). The channel deposits of anomalous sections, which are impoverished in the REE, accumulated exclusively under fast rates of sedimentation.Within the hemipelagic group the rate of sedimentation of the argillites was faster than of the clayey-siliceous rocks, but the REE concentration in the former rocks (140.4 ppm) is higher than in the latter group (97.4 ppm). The argillites are more than twice enriched in clayey material than clayey-siliceous rocks. It is likely that the clay fraction was the main carrier of REE in these rocks. In the channel group of rocks, the REE abundance in clayey-silt rocks (21.2 ppm) is lower than in the silty argillite (84.6 ppm), in which the clay content is elevated.With respect to redox potential the Bazhenov Formation rocks can be subdivided further into three groups, based on the degree of pyritization index (DOP): (1) the highly reducing clayey-siliceous rocks of normal sections, with high DOP; (2) the substantially reducing argillites and carbonate rocks of normal sections, with intermediate DOP; (3) the moderately reducing rocks of anomalous sections with low DOP. The rocks with the high DOP (group 1) are characterized by ΣLREE/Σ(M+H)REE ratios between 7.37 and 7.5, whereas the rocks with the lower DOP (group 2 and 3) are characterized by ΣLREE/Σ(M+H)REE ratios between 12.8 and 13.5. Negative Ce anomalies are either small or absent in all deposits, which is typical for reducing conditions.Thus, the Bazhenov Formation exemplifies the complex depositional conditions that influence the REE concentration in black shale. However, it is this very complexity that has contributed to the development of six separate depositional models (REE contents in ppm are given in brackets). (1) Phosphatic clayey-siliceous rocks of normal sections (367.95); (2) argillites of normal sections (130.73); (3) clayey-siliceous rocks of normal sections (85.97); (4) carbonate rocks, largely dolomites of normal sections (23.23); (5) silty argillites of anomalous sections (78.7) and (6) clayey-silt rocks of anomalous sections (19.66).  相似文献   

3.
The paper presents data on the structure, composition, and age of granitoid associations (Tokhtogeshil’skii Complex) composing the Kharanur and Sharatologoi polychronous plutons in the northern part of the Ozernala zone in western Mongolia. The Tokhtogeshil’skii Complex was determined to consist of a number of independent magmatic associations, which were formed at 540–450 Ma, within three age intervals (540–520, 510–485, and 475–450 Ma), have different composition, were derived from different sources, and were emplaced in different geodynamic environments. During the first, island-arc stage (540–520 Ma), high-Al plagiogranites were produced, which belong to tonalite-plagiogranite (531 ± 10 Ma) and diorite (529 ±6 Ma) associations in the Kharanur pluton, low-Al plagiogranites of the tonalite-plagiogranite association (519 ± 8 Ma) in the Sharatologoi pluton, and rocks of the Khirgisnur peridotite-pyroxenite-gabbronorite complex (Kharachulu and Dzabkhan massifs). The rocks of the diorite and plagiogranite associations of the Kharanur pluton have ɛNd(T) from +7.9 to +7.4, TNd(DM) = 0.65 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039. The plagiogranites of the Sharatologoi pluton (tonalite-plagiogranite association) are characterized by ɛNd(T) from +6.5 to +6.6, TNd(DM) = 0.73–0.70 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039, which suggest predominantly juvenile subduction sources of the parental melts at a subordinate role of ancient crustal material. During the second, accretionary stage (510–485 Ma), low-Al plagiogranites of the diorite-tonalite-plagiogranite association of the Sharatologoi pluton (494 ± 10 Ma, M type) were formed. The Sr-Nd isotopic characteristics of these rocks ɛNd(T) = +6.6, (87Sr/86Sr)0 = 0.7039 are analogous to those of the plagiogranitoids of the early type. This suggests that the melted sources were similar in composition. During the third, postcollisional stage (475–450 Ma), rocks of the diorite-granodiorite-granite association were formed (459 ± 10 Ma, type I) in the Kharanur pluton. These rocks have ɛNd(T) = +5.1, TNd(DM) = 0.74 Ga, and (87Sr/86Sr)0 = 0.7096. The parental melts were supposedly derived by means of partial melting of “the Caledonian” juvenile crust with the addition of more ancient crustal material.  相似文献   

4.
Crustal structure across the passive continental margin of the northeastern South China Sea (SCS) is presented based on a deep seismic survey cooperated between Taiwan and China in August 2001. Reflection data collected from a 48-hydrophone streamer and the vertical component of refraction/reflection data recorded at 11 ocean-bottom seismometers along a NW–SE profile are integrated to image the upper (1.6–2.4 km/s), lower (2.5–2.9 km/s), and compacted (3–4.5 km/s) sediment, the upper (4.5–5.5 km/s), middle (5.5–6.5 km/s) and lower (6.5–7.5 km/s) crystalline crust successively. The velocity model shows that the thickness (0.5–3 km) and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the SCS. Furthermore, several volcanoes and igneous rocks in the upper/middle crust (7–10 km thick) and a high velocity layer (0–5 km thick) in the lower crust of the model are identified as the ocean–continent transition (OCT) below the lower slope in the northeastern margin of the SCS. A thin continent NW of the OCT and a thick oceanic crust SE of the OCT in the continental margin of the northeastern SCS are also imaged, but these transitional crusts cannot be classified as the OCT due to their crustal thickness and the limited amount of the volcano, the magma and the high velocity layer. The extended continent, next to the gravity low and a sag zone extended from the SW Taiwan Basin, may have resulted from subduction of the Eurasian Plate beneath the Manila Trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the SCS.  相似文献   

5.
The aliphatic hydrocarbon composition (acyclic isoprenoids, hopanoids and steroids) of oils from the most productive fields in the southern geological Province of Cuba have been studied. This province is defined by its position with respect to the Cretaceous overthrust belt generated during the formation of oceanic crust along the axis of the proto-Caribbean Basin. The relative abundances of 18α(H)-22,29,30-trisnorneohopane, gammacerane and diasteranes suggest that Pina oils are related to the carbonate oils from the Placetas Unit in the northern province (low Ts/(Ts+Tm) and C27,29 rr/(rr+sd) ratios). The Cristales and Jatibonico oils exhibit some differentiating features such as higher Ts/(Ts+Tm) and absence of gammacerane. The oils from this province do not exhibit significant differences in either hopane, C32 22S/(S+R) and C30 αβ/(αβ+βα), or sterane, C29 αα 20S/(S+R), maturity ratios. However, the relative content of 5α(H),14β(H),17β(H)-cholestanes (C29 ββ/(ββ+αα) ratio) indicates that Pina oils are more mature than Cristales and Jatibonico oils. Several of these oils (Cristales, Jatibonico and Pina 26) are heavily biodegraded, lacking n-alkanes, norpristane, pristane and phytane (the two former oils do not contain acyclic isoprenoid hydrocarbons). Other biodegradation products, the 25-norhopanes, are found in all the oils. Their occurrence is probably due to mixing of severely biodegraded oil residues with undegraded crude oils during accumulation in the reservoir.  相似文献   

6.
The possibility to use some widely known standard discrimination diagrams such as the K2O/Na2O-SiO2/Al2O3, SiO2-K2O/Na2O, (Fe2O3* + MgO)-TiO2, F1-F2, Th-La-Sc, Sc-Th-Zr/10, and Sc/Cr-La/Y for deciphering the paleogeodynamic settings of sedimentary sequences is considered with reference to the Lower and Middle Riphean (Mesoproterozoic) deposits of the Uchur-Maya region (Far East) and the Bashkir meganticlinorium (South Urals). It was shown that only some of them can be used with a certain degree of confidence for reconstructing the settings of the platform sedimentary sequences made up of both sandstones and fine-grained terrigenous rocks.  相似文献   

7.
Based on generalization of available geochronological data, Late Mesozoic magmatic associations in the northeastern part of the Amurian microcontinent are divided into three groups: 142–125, 124–115, and <110 Ma. The age of these associations decreases with approaching the Pacific margin of Asia. In the same direction, they show a change in sources of their parental melts: continental crust (142–125 Ma) → continental crust + PREMA (DM) (124–115 Ma) → continental crust + PREMA (DM) + EMII (<110 Ma). Isotope-geochemical (Sr-Nd) study indicates that intrusive and volcanic rocks of the Late Mesozoic magmatic associations in the northeastern part of the Amurian microcontinent were originated in geodynamic settings that provided access of enriched mantle sources to magma formation. The most probable of these settings are as follows: (1) plate sliding accompanying by the formation of slab window beneath continental margin; (2) passage of the Asian margin over the East Asian mantle hot field in the Late Mesozoic; (3) asthenospheric upwelling due to delamination of the lower crust during closure of the Mongolian-Okhotsk ocean caused by collision between the Amurian microcontinent, Dzhugdzhur-Stanovoy, and Selenga-Stanovoy superterranes in the Central Asian fold belt.  相似文献   

8.
This paper addresses the geochemistry of intrusive (Osinitsk complex) and volcanic (Klesov Group) rocks of the Osnitsk-Mikashevichy volcanoplutonic belt (OMVPB) of the Ukrainian shield, which is an active continental margin existing approximately 1980–2000 Ma ago. The Osnitsk complex comprises a wide range of rocks, from ultrabasics to granitoids, and the Klesov Group is dominated by extrusive rocks of basaltic and rhyolitic compositions metamorphosed under epidote-amphibolite facies conditions. The Sr-Nd-Hf isotopic systematics (?Sr1990 from ?4 to +10, ?Nd1990 from ?0.6 to +2.3, and ?Hf1990 from 0.1 to 1.4) indicates a juvenile source for the OMVPB rocks. Geochemical data suggest independent origin of the rocks of gabbroid (SiO2 < 60 wt %) and granitoid (SiO2 > 60 wt %) series. The gabbroids are subdivided into pre- and post-granite groups on the basis of the higher contents of incompatible trace elements and lower contents of compatible elements in the post-granite rocks. The geochemical characteristics of the two groups of basic rocks indicate their formation in a convergent tectonic setting. The origin of the granitoid melts is attributed to the low-degree (eutectoid) melting of basic rocks at relatively low temperatures.  相似文献   

9.
Aragonite occurrences from two areas of the Western Alps are described. It is shown that aragonite has been formed under blueschist metamorphic conditions in the Western Vanoise, while it has been precipitated under sub-surface conditions in the Queyras region. An uplift (P, T) path of the aragonite-bearing rocks of the Western Vanoise is constructed using two independent methods: (1) temperature estimates using the Sr++ content of aragonite in the successive veins and (2) the kinetics of the aragonitecalcite solid-state transition. The uplift (P, T) path has an unusual shape with an important temperature decrease (100° to 150° C for a pressure decrease of 0.2 GPa) following blueschists metamorphism (P=0.7 GPa, T=300° C). Thermal models show that this unusual (P, T) path of the Western Vanoise can be explained if one maintains a low temperature (between 125° and 175° C) at the base of the tectonic unit containing the aragonite-bearing rocks during part of its burial history, followed by the whole of its exhumation. A tectonic scenario is proposed to account for the observed and modelled (P, T) path.  相似文献   

10.
叶大年  董麒 《沉积学报》1992,10(4):1-10
液态的碳氢化合物分子体积等于官能团拓扑体积之和,即服从于加和性原则。在不同的烃类中相同的官能团有十分接近的体积贡献。官能团的拓扑体积,V（CH2）=27.24Å3,V（CH3）=55.11Å3,V（-CH=CH2）=70.12Å3,V（-CH=CH-）=40.98Å3,V（-C≡CH）=51.58Å3,V（-C≡C-）=21.18Å3,V（H）端部=29.32Å3。V（C6H5-）=120.08Å3,V（-C6H4-）=89.92Å3,V（-C6H4=）=61.51Å3。液态碳氢化合物的分子体积还可以用如下公式来表示。V=k+13.89·C+6.67HÅ3）K为液体常数,烷、烯和炔K=40,苯同系物K=25,环烷烃K=15。C、H分别是分子中C和H的原子数。  相似文献   

11.
An extensive passive margin was formed in the Triassic along the periphery of Arabia, including the Tauric carbonate platform. This event is related to the opening of the Mesozoic Tethys when a number of microcontinents split off from Gondwana. Triassic extension and continental rifting resulted in the formation of a structural pattern which is uniform from the Dinarides to Oman. It includes the following elements:
1. (1) shelf,
2. (2) continental slope,
3. (3) deep basin probably with a floor of attenuated sialic crust,
4. (4) inner carbonate platform. In the Jurassic-Cretaceous stable conditions prevailed, influenced only by eustatic oscillations of the sea level. Turbidites accumulated on the continental rise while cherts and radiolarites were deposited in the deep basins (Hawasina, Pichakun, Antalya, Pindus) below the CCD level. Sedimentation on the shelf was controlled by north-northeast transverse tectonic elements which also continued across the passive margin, dividing it into a number of segments. Collision with an island arc led to obduction of the oceanic crust, deformation of the passive margin and overthrusting of its sedimentary cover onto the Arabian shelf. Obduction and deformation lasted for about 10 m.y. and created a new tectonic pattern with concentric structural zones surrounding the Arabian promontory.
These zones include:
1. (1) the flysch basin—a remnant of the closing Tethys;
2. (2) an uplift—a site of periodical emergence and erosion, corresponding to the frontal part of the ophiolitic nappes;
3. (3) the Border furrow—a depocenter of low-energy calcareous marls,
4. (4) the Arabian shield constantly emerged during the Tertiary. Tectonic deformation of these zones caused by the collision of Arabia with Eurasia began prior to the Early Miocene and it is still going on.
Data on Afghanistan demonstrate that its central part (the Gelmend-Argandab and Kabul blocks) belonged during the Paleozoic and Early Mesozoic to the continental shelf of India.  相似文献   

12.
Previous research has shown that speleothems from the northern rim of the European Alps captured submillennial-scale climate change during the last glacial period with exceptional sensitivity and resolution, mimicking Greenland ice-core records. Here we extend this so-called NALPS19 record across the Late Glacial using two stalagmites which grew continuously into the Holocene. Both specimens show the same high-amplitude δ18O signal as Greenland ice cores down to decadal resolution. The start of the warming at the onset of the equivalent of Greenland Interstadial (GI) GI-1e at 14.66 ± 0.18 ka agrees with the North Greenland Ice Core Project (NGRIP) (14.64 ± 0.28 ka) and comprised a temperature rise of about 5–6 °C. The transition from the equivalent of GI-1a into the equivalent of Greenland Stadial (GS) GS-1 (broadly equivalent to the Younger Dryas) commenced at 13.02 ± 0.13 ka which is consistent with NGRIP (12.80 ± 0.26 ka) within errors. The onset of the Holocene started at 11.78 ± 0.14 ka (11.65 ± 0.10 ka at NGRIP) and involved a warming of about 4–5 °C. In contrast to δ18O, δ13C values show no response to (sub)millennial climate shifts due to strong rock-buffering and only record a long-term trend of soil development starting with the rapid warming at 14.7 ka.  相似文献   

13.
Australian Proterozoic gold-producing deposits, emplaced mainly at 1.55–2.00 Ga, are divided into the following categories: (1) iron oxide-dominated, brecciahosted, Cu-U±Au replacement deposits spatially associated with felsic intrusions (273t Au); (2) stratabound Au±Cu-bearing iron formations (152.4t Au); (3) unconformity-style U ±Cu/PGM/Au deposits (53t Au); (4) Iron oxide-dominated Au±Cu mineralisation hosted within elements of ductile deformation (146.7t Au); (5) Broken Hill and volcanic-hosted massive sulphides (150t Au); (6) iron-sulphide-dominated veins and replacement zones spatially related to felsic intrusions (150.7t Au), and (7) iron-sulphide-dominated veins and replacement zones spatially related to elements of regional deformation (159.9t Au). Categories (1) to (4) are mainly confined to Proterozoic rocks, constituting an association in which Au and Cu are commonly present together, with variable amounts of U, Bi, Co, W, Se, Te and REE. Most examples in categories 1–4 fall into either of two groups: Cu-Aumagnetite ±hematite types formed at relatively high temperature (300–450 °C), and Cu-U±Au-hematite types formed at 150–300 °C. We postulate that these ores formed from a common high salinity (15–35 wt. % NaCl equiv.), low total sulphur (aS = 10–3 to 10–2), high fO2 fluid-type, in which metal transport was dominated by chloride-complexing. The most effective method of metal deposition was fluid mixing, achieving a synchronous decrease in fO2 and temperature. This unusual oxidised fluid association was favoured in high heat-flow extensional settings containing oxidised and/or oxidised-evaporitic sedimentary sequences. The intrusion of oxidised fractionated granites, which are commonly temporally associated with metal emplacement, acted in some places to heat and focus basinal fluids, and in others was the ultimate source of metals.  相似文献   

14.
Age determinations, mostly by Rb---Sr analyses, of the Precambrian rocks of Rajasthan by us and by others are summarized and discussed. Broad periods of acid magmatism at (1) 3000−2900 m.y., (2) 2600−2500 m.y., (3) 2000−1900 m.y., (4) 1700−1500 m.y. and (5) 850−750 m.y. were identified. The oldest rocks in the area are the yet undated banded gneisses (BGC) east of Udaipur, intruded by the Untala granite dated at 2950 m.y. and hence of mid-Archean age. The basal status hitherto attributed to the Berach granite dated at 2600 m.y. is no longer tenable. The radiometric control on the beginning and duration of the overlying Aravalli Supergroup is not yet satisfactory, though a lower limit at 2000 m.y. is indicated. Heron's original Delhi rocks have recorded two magmatic events widely separate in space and time. While the earliest granitic activity at 1600 m.y. is recorded only in the Alwar basin in the northeast, the younger activity between 850-750 m.y. is widespread, as shown by the nearly concordant ages of “Erinpura-type” granites along the Aravalli mountain Range and the Malani rhyolites in the western plains of the Aravalli Range.  相似文献   

15.
The North China Craton (NCC) is one of the largest blocks composing the continent. Different types of continental margins well developed around the NCC, along with lots of metallogenic systems of different metals and different times. Based on the study on the structural evolution of the NCC, the authors made a new division of tectonic units of the NCC. Through an analysis of the data of 1:25000 geochemical survey on stream sediments, regional geochemical features of main ore-forming elements including Au, Ag, Cu, Pb, Zn, W, Ni, Co and Mo of the NCC are discussed in the paper. Then different metallogenic systems and their forming processes and geodynamics are discussed in detail. At last, temporal and spatial distribution regularities are summarized and ten favorable ore-control factors on the paleocontinental margins are put forward, including (1) abundance of ore sources; (2) rendezvous of ore-forming fluids; (3) high thermo-dynamic anomaly; (4) remarkable Earth crust-mantle interaction; (5) cluster of  相似文献   

16.
U-Pb zircon isotopic data on rocks from the Kandalaksha-Umba zone of the Lapland granulite belt in the Por’ya Bay area constrain the age of the protolith of the apodacite (apotonalite) Opx-Bt granulite gneisses at 2799 ± 4 Ma, and the age of the apogabbronorite Grt-Opx-Cpx-Hbl crystalline schists at 2315 ± 23 Ma. The U-Pb sphene age of the magmatic crystallization of the postmetamorphic granodiorites is 1901 ± 5 Ma. The zircon yields the U-Pb age of the contamination of xenogenic zircons, which were captured during the dissolution of xenoliths of the host Grt-Opx-Cpx-Hbl crystalline schists in granodiorite melt. The comparison of the most important attributes of the endogenic histories of the adjacent Lapland Granulite and Belomorian Mobile belts testifies to their similar evolutionary histories: (1) the protolith age of the acid Opx-Bt granulites of the Lapland Belt (2799 ± 4 Ma) coincides with the protolith age of acid gneisses in the Belomorian Belt (2890-2690 Ma); (2) the ages of the gabbronorite protolith of Grt-Opx-Cpx-Hbl granulites in the Lapland Belt (2315 ± 23 Ma) and gabbro-anorthosite in the Kolvitsa Massif (2462-2423 Ma) are close to the protolith age of eclogitized gabbronorites in the Belomorian coronite suite (2.46–2.36 Ga); (3) the age of granulite metamorphism of acid and mafic rocks in the Lapland Belt is 1912–1925 Ma, and the age of eclogite metamorphism of gneisses and metabasites in the Belomorian Belt is approximately 1.9 Ga, i.e., their metamorphism took place in Svecofennian time; (4) the peak pressure of granulite metamorphism in the Lapland Belt was 9–11 kbar at a temperature of 800–850°C, whereas the peak metamorphic parameters of eclogite metamorphism in the Belomorian Belt were 10–12 kbar and 640–700°C. This means that the metamorphic complexes of the Lapland and Belomorian belts had the same Mezo- and Neoarchean protoliths hosting bodies of Paleoproterozoic gabbroids and were completely formed largely by a single cycle of Svecofennian high-pressure zonal metamorphism within a temperature range from the lowest grade of the eclogite to the granulite facies.  相似文献   

17.
The Middle Cenozoic lava sequence of the Lake Kizi region was studied. It characterizes the activity of sources in the Northern zone of the eastern Sikhote Alin: a Middle Eocene pulse of slab-related magmatism and prolonged injection of magmas from the sublithospheric convecting mantle in the Late Oligocene. Low contents of high field strength elements (Nb and Ta) with low Nb/Ta, Ce/Pb, and Nb/La and high K/Nb ratios and a low (87Sr/86Sr)0 of 0.703399 were determined in a Middle Eocene dacite with an age of ∼43.5 Ma. Three phases of Late Oligocene volcanic eruptions were distinguished: (1) basaltic andesites (29–27 Ma), (2) basaltic trachyandesites and trachyandesites (27–24 Ma), and (3) andesites (∼23 Ma). The lavas of the first and third phases showed low Ce/Pb, Nb/La, and Ba/La and high K/Nb ratios, which are also characteristic of supraslab processes. The lavas of the second phase are shifted with respect to these ratios toward ocean island basalt compositions. The entire Late Oligocene volcanic sequence falls within a narrow range of the initial strontium isotope ratios, (87Sr/86Sr)0, from 0.703661 to 0.703853. Such ratios are characteristic of volcanic and subvolcanic rocks with ages of ∼37, 31–23, and ∼16 Ma over the whole region of the Tatar Strait coast.  相似文献   

18.
The Yinggehai basin is located on the northwestern shelf of the South China Sea. It is the seaward elongation of the Red River Fault Zone (RRFZ). The orientation and rift shape of the Yinggehai basin are mainly controlled by NW-, NNW- and nearly NS-trending basal faults. The depocenter migrated southeastward when the basin developed. The depocenter trended northwest before about 36 Ma, then jumped southward and became nearly N–S trending and migrated toward the southeast up to 21 Ma; thereafter, the depocenter trended northwest again. Based on above and structural evolution in neighbor areas, it is believed that the Yinggehai basin formation was mainly controlled by the extrusion accompanied by clockwise rotation of Indochina. We set up analogue models (thin basal plate model and thick basal plate model) to investigate the evolution of Yinggehai basin. From the experiments, we consider that the basin evolution was related to the extrusion and clockwise rotation of the Indochina block, which was caused by the collision of the Indian plate and Tibet. This process took place in four main stages: (1) Slow rifting stage (before 36 Ma) with a NW-trending depocenter; (2) rifting stage formed by sinistral slip of the Indochina block accompanied by rapid clockwise rotation between 36 and 21 Ma; (3) rifting-thermal subsidence stage affected by sinistral slip of the Indochina (21–5 Ma) block and (4) dextral strike–slip (5–0 Ma).  相似文献   

19.
The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.  相似文献   

20.
The composition of the continental crust of the North China Craton (NCC) is more felsic than that of the average bulk crust, which is regarded to be the result of the delamination of the thickened lower crust during Mesozoic. However, whether the thickened continental crust existed and when the delamination event happened along the southern margin of the NCC are still debated. Here, we report geochronological, geochemical and Sr-Nd-Hf-Pb isotopic evidence that granitoids from the Late Jurassic Wuzhangshan pluton and the Early Cretaceous Huashani complex were derived by partial melting of the lower crust with different thickness. Our new data shows that the two lithofacies of the Wuzhangshan pluton were mainly formed between ca. 157 and 156 Ma, whereas the five lithofacies of the Huashani complex were mainly emplaced between ca. 132 and 125 Ma. The Wuzhangshan pluton and the earlier four lithofacies granitoids of the Huashani complex (ca. 160–125 Ma) both display adakitic geochemical features, which are characterized by as high SiO2 (63.26–72.71 wt%), Al2O3 (13.97–16.89 wt%) and Sr (413–1218 ppm) contents, and low Y (6.30–14.98 ppm) and YbN (1.55–4.43), and high Sr/Y (33−112) and (La/Yb)N (11.53–29.72) ratios. They also have high (87Sr/86Sr)i (0.7066–0.7086), and low εNd(t) (−9.9 to −18.8) and εHf(t) (−11 to −26) values, and two-stage Nd and Hf model ages ranging from 2.4 to 1.7 Ga and 2.7 to 1.7 Ga, respectively. In contrast, the late Early Cretaceous (ca. 125–110 Ma) granitoids have higher SiO2 (71.30–76.78 wt%) and lower Sr (64–333 ppm) contents, and lower Sr/Y (17–29) and (La/Yb)N (13.25–18.36) ratios, and similar εNd(t) (−10 to −16) and relatively higher εHf(t) (−10 to −14) values. These geochemical variations suggest that the ca.160–125 Ma granitoids were most probably produced by partial melting of thickened crust (>45 km) with eclogite, garnet amphibolite or amphibolite residues, whereas that the ca. 125–110 Ma granitoids were formed by partial melting of the thinner crust (<33 km). We thus suggest that the NCC likely underwent a synchronous tectonic transformation at ca. 125 Ma from a compressional setting with thickened crust to an intensive extensional setting with thinner crust at ca. 125 Ma, which demonstrates that the lower crust was most likely delaminated.  相似文献   

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