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1.
Mafic dikes, which transect the Mesoarchaean Singhbhum Granitoid Complex, are the most abundant members of the Newer Dolerite dikes of the Singhbhum Orissa craton. These dikes are subalkaline and exhibit a tholeiitic differentiation trend. Studied dikes underwent fractional crystallization of clinopyroxene and plagioclase. They show enriched patterns for the light rare earth elements (LREE) and large ion lithophile elements (LILE). On primitive mantle-normalized multi-element patterns, they possess Ba, Nb, Sr, P, and Ti depletions similar to subduction-related basaltic rocks. The high (La/Yb) n and (Gd/Yb) n ratios suggest that the studied mafic dikes were derived by low degrees of partial melting of a garnet-bearing source. Judging by trace elemental ratios (e.g. Ba/Y, Nb/Y, Ba/Th and Th/Nb), the studied dikes were derived from a mantle source metasomatized by a subduction component (e.g. fluids derived by dehydration of the subducting slab). We conclude that interaction between these fluids and the overlying mantle was the main cause of (LREE and LILE) enrichment and Nb (high field strength elements) depletion in the mafic dikes.  相似文献   

2.
为确定云南省个旧地区晚中生代大规模岩浆活动过程中基性端元的时限、地幔源区特点及大地构造环境,选取贾沙辉长．二长岩体为对象进行年代学和地球化学研究。贾沙辉长．二长岩体位于个旧西区,岩性主要为辉长岩和二长岩。锆石U．Pb同位素测年结果表明,岩石侵位时代为（84．0＋0．6）Ma,属于晚白垩世,与个旧地区花岗岩、碱性岩和煌斑岩形成年代范围一致（76～85Ma）。贾沙岩体的辉长岩和二长岩Si02为47．3％～60．O％,K20＋Na20为7．31％～10．1％。稀土含量较高,轻稀土富集重稀土亏损,Eu异常不明显。相对于原始地幔,贾沙岩体富集轻稀土和大离子亲石元素Rb、K、Pb,亏损高场强元素Nb、Ta、Ti和P。地球化学研究显示贾沙岩体母岩浆起源于与俯冲有关的交代地幔,由石榴子石二辉橄榄岩经历了较低程度的（〈5％）部分熔融形成。原始岩浆在就位过程中经历了广泛的地壳混染和橄榄石、辉石的分离结晶作用。二长岩由辉长岩浆结晶分异作用形成。研究显示,贾沙辉长．二长岩体是晚白垩世滇东南．桂西地区大规模岩浆活动的产物,表明这些岩浆岩形成于统一的岩石圈伸展的动力学背景下。  相似文献   

3.
This work considers geochemical and isotopic characteristics of the source of the Archean Panozero pluton derived from LILEand LREE-enriched lithospheric mantle. Sr and Nd isotopic data on clinopyroxenes and augites define a source with Sri = 0.7017 and ɛNd(t) varying within a narrow range from + 0.7 to + 1.4 (averaging + 1.1), which is close to previously obtained whole-rock isotopic data. Similar ɛNd(t) were obtained for the Archean alkaline rocks of Canada, whereas the Archean mafic rocks of the Baltic and Canadian Shields formed from depleted mantle have ɛNd(t) ∼2. Lead isotope measurements on K-feldspars (KFsp) and monzonite showed that the source of the pluton has μ = 8.98 for the Stacey-Kramers two-stage model, at low U/Pb and high Th/U ratios. Different lead isotope composition corresponding to μ = 10.43 was determined in KFsp from quartz monzonites. Diverse interpretations of obtained data have been proposed. It was noted that the Pb-Pb isotopic system was disturbed by a later (∼ 1.9 Ga) thermal event. The ratios of elements of similar compatibility were used to determine the geochemical specifics of source of the Panozero pluton. Their comparison with numerous literature data on metasomatized mantle xenoliths and minerals in them showed that the mantle source strongly differed from primitive mantle in ratios of elements, whose mineralmelt partitioning coefficients considerably differs from mineral-fluid partitioning, for instance, Nb/La. Mantle source that was responsible for geochemical peculiarities of the Panozero pluton was made up of Phl, CPx, and Ap.  相似文献   

4.
The Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic-monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg#（55.0–59.0）, and enrichments in Na2 O＋K2 O（7.28–8.94 %）, Ni（21-2312 ppm）, Cr（56-4167 ppm）, Sr（553-923 ppm）, Ba（912-1355 ppm） and LREE（（La/Yb）N =9.47–15.3）, from negative to slightly positive Eu anomalies（δEu=＋0.61 to ＋1.10）, but also depletion in Nb, Ta and Ti. The granodioritic-monzogranitic rocks diaplay various Mg#of 6.00-53.0, high Na2 O＋K2 O（7.20– 8.30%）, Sr（455–1081 ppm） and（La/Yb）N（27.6–47.8）, with positive Eu anomalies（δEu=1.03–1.57） and depleted Nb, Ta and Ti. Laser ablation inductively coupled plasma mass spectrometry（LA-ICPMS） zircon U-Pb isotopic dating reveals that the sanukitoid-like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid–like rocks have their εHf（t） values ranging from ＋0.3 to ＋15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its εHf（t） values ranging from 21.6 to ＋10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu –Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic-monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc.  相似文献   

5.
The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.  相似文献   

6.
《International Geology Review》2012,54(13):1522-1558
The Melrose Stock in the Dolly Varden Mountains of east-central Nevada is one of the many Mesozoic intrusion s in the Basin and Range Province. It consists of monzonites, quartz monzonites, granodiorites, and granites sharply intruding Mississippian to Triassic units. Phenocrysts of plagioclase (An38–An24) with oscillatory zoning and albitic rims, hornblende ± diopside, and biotite are common. Coexisting phases include orthoclase, quartz and accessory magnetite, apatite, titanite, ilmenite, and allanite. Mineral compositions suggest that the intrusion was emplaced at ～720 ± 40°C and 1.8–2.3 kbar.

All rocks are metaluminous to slightly peraluminous, defining a calcalkalic trend in which the monzonites and syenites are shoshonitic. Rare earth element patterns indicate that all studied rock types are comagmatic. Harker plots show curvilinear trends with some kinks consistent with fractionation, and mixing/assimilation. Major-element modelling and petrographic evidence suggest three stages of fractionation/mixing: Stage 1 marked by the fractionation of diopside and plagioclase; Stage 2 by fractionation of plagioclase, hornblende ± orthoclase ± biotite, accompanied by mixing through convection; and Stage 3 by fractionation of biotite, hornblende, plagioclase, and orthoclase.

Mineralogic, petrographic, and major- and trace-element data demonstrate that all rocks are I-type granitoids, suggesting a significant mantle contribution. Spider diagrams show troughs for Ti, P, and Nb, indicating magma genesis in a subduction-zone setting. Discrimination diagrams classify all rocks as late orogenic. Magma was therefore generated from mantle metasomatized by subduction, differentiated to a monzonitic magma, and emplaced in the thinned continental crust during a period of extension late in the cycle of Elko orogeny.  相似文献   

7.
《International Geology Review》2012,54(18):2249-2275
ABSTRACT

The Piqiang intrusion is one of the two important ma?c-ultrama?c layered intrusions that host giant Fe-Ti-V oxide deposits in the Permian Tarim Large Igneous Province, NW China. The intrusion mainly consists of gabbro, anorthosite and minor plagioclase-bearing clinopyroxenite in the marginal zone. Disseminated to massive Fe-Ti oxide ores occur as layers and lenses within the gabbro. SHRIMP zircon U-Pb results from both a gabbro from the Piqiang intrusion and a granite from the surrounding granitic dyke yield ages of ~270 Ma. Geochemically, the Piqiang silicate rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high ?eld strength elements (HFSE), and have a limited range of Sr-Nd-Hf isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the layered units suggest that all the rocks are co-magmatic. The parental magma is Fe-Ti-rich and is akin to the most primitive diabasic dyke which is associated with the Piqiang intrusion. Partial melting of the Tarim mantle plume with involvement of a subduction-metasomatized lithospheric mantle source best explains the geochemistry and petrogenesis of the parental magmas of the Piqiang intrusion. We propose that the lithospheric mantle source may have been metasomatized by subduction-related materials and the metasomatic enrichment of this source region which may be correlated with oceanic sediment recycling during southward subduction of the South Tianshan oceanic slab during the Early-Middle Paleozoic. Crystal settling and mechanical sorting is the predominant process responsible for the formation of the massive Fe-Ti oxide ores in the Piqiang intrusion. Central to ore formation is a combination of the protracted differentiation history of a Fe-Ti-enriched parental magma and the later addition of external H2O from the country rocks to the slowly cooling magma chamber.  相似文献   

8.
The Itapetim Complex is a multiple facies intrusion of porphyritic monzogranite hosting isolated, or swarms of, dioritic enclaves and cut by late dikes of biotite granodiorite. It is a syn-tectonic intrusion in relation to the D3 regional deformation phase of the Brasiliano Orogeny. The complex has contacts with metagreywackes and gneissic granites associated with the Mesoproterozoic Cariris Velhos event and, to the northwest, with a belt of gneisses of Paleoproterozoic ages. At least two different sources seem to be involved in the evolution of the Itapetim Complex. The diorite source appears to be a depleted mantle that underwent some small degree of mixture with a Paleoproterozoic enriched mafic crust. The monzogranite source is probably metagreywacke, a mixture of enriched crust of Paleoproterozoic age and volcanic rocks related to the Cariris Velhos event. The granodiorites were generated by partial melting of a source compositionally similar to those suggested for the monzogranite. U–Pb in zircon gave a crystallization age of 638±4.9 Ma for the porphyritic monzogranites and associated mafic rocks. The granodiorite dikes represent a late magmatic event, under intermediate fO2 conditions. The dikes, intruded into brittle fractures, are probably related to 570 Ma intrusions within the Pajeú Paraı́ba Terrane.  相似文献   

9.
The late Archaean Panozero pluton in Central Karelia (BalticShield) is a multi-phase high-Mg, high-K intrusion with sanukitoidaffinities, emplaced at 2·74 Ga. The magmatic historyof the intrusion may be subdivided into three cycles and includesmonzonitic and lamprophyric magmas. Compositional variationsare most extreme in the monzonite series and these are interpretedas the result of fractional crystallization. Estimates of thecomposition of the parental magmas to the monzonites and lamprophyresshow that they are enriched in light rare earth elements, Sr,Ba, Cr, Ni and P but have low contents of high field strengthelements. Radiogenic isotope data indicate a low U/Pb, highTh/U, high Rb/Sr, low Sm/Nd source. The magmatic rocks of thePanozero intrusion are also enriched in H2O and CO2; carbonisotope data are consistent with mantle values, indicating afluid-enriched mantle source. The similarity in trace elementcharacter of all the Panozero parental magmas indicates thatall the magmas were derived from a similar mantle source. Thepattern of trace element enrichment is consistent with a mantlesource enriched by fluids released from a subducting slab. Nd-isotopedata suggest that this enrichment took place at c. 2·8Ga, during the main episode of greenstone belt and tonalite–trondhjemite–granodioriteformation in Central Karelia. Sixty million years later, at2·74 Ga, the subcontinental mantle melted to form thePanozero magmas. Experimental studies suggest that the monzoniticmagmas originated by the melting of pargasite–phlogopitelherzolite in the subcontinental mantle lithosphere at 1–1·5GPa. The precise cause of the melting event at 2·74 Gais not known, although a model involving upwelling of asthenosphericmantle following slab break-off is consistent with the geochemicalevidence for the enrichment of the Karelian subcontinental mantlelithosphere by subduction fluids. KEY WORDS: Archaean; sanukitoid; monzonite; Karelia; mantle metasomatism  相似文献   

10.
The study of interaction between mantle melts and crustal rocks is of great importance for deciphering the evolution of the Earth’s crust and for better understanding the composition of mantle sources, in particular, the degree of their compositional heterogeneity. This work presents the results of Rb-Sr and Sm-Nd isotopic studies of 37 samples taken from the Kivakka layered intrusion, host rocks, and rocks at the contact. The studies were aimed at verifying the hypothesis of possible crustal contamination of mafic melt during magma chamber crystallization. It was found that the section of the Kivakka layered massif is characterized by initial Sr and Nd isotopic heterogeneity, with negative correlation between initial Nd isotopic ratio and its content. The rocks of the massif have low ɛNd(T) values.  相似文献   

11.
In order to better understand the origin and character of late-Archean mantle beneath the Baltic Shield, we have analyzed mafic-ultramafic rocks from one of the best-preserved, least-metamorphosed regions of Karelia, Russia. Trace-element data for samples from the ultramafic and gabbronorite zones of the large (700 km2) Burakovsky layered intrusion (BLI) are presented. Samples from the ultramafic zone are LREE enriched, indicating that they formed from a LREE-enriched parental magma. Indeed, a calculated parental magma for the ultramafic zone has a (Ce/Yb)n ratio of 2.6, a (Nd/Sm)n ratio of 1.1, and a (Dy/Yb)n ratio of 1.6. The LREE enrichment in the parental magma suggests either that the source region was LREE enriched or that the melt was contaminated by crust en route to the BLI magma chamber. Samples from the gabbronorite zone also are LREE enriched and indicate two distinct parental magmas. Group-I magmas, from the lower part of the gabbronorite zone, have (Ce/Yb)n ratios of 6.9 to 13.9, whereas Group-II magmas, from the upper portion, have (Ce/Yb)n ratios of 15.8 to 27.3. Volcanic rocks in Karelia that are coeval to the Burakovsky layered intrusion, as well as volcanic rocks of a similar age in other parts of the Baltic Shield, also are LREE enriched. Furthermore, the BLI has an initial εNd value of ?2.0, and other layered intrusions in the Baltic Shield of similar age also have negative initial εNd values (e.g., ?1.8 to ?2.2). The consistency of these εNd values for layered intrusions throughout Karelia precludes contamination as a controlling factor in their isotopic compositions. All of these data are most consistent with the development of LREE-enriched mantle beneath the eastern Baltic Shield, prior to the earliest Proterozoic.  相似文献   

12.
Geochemical, isotopic-geochemical, and geochronological information was obtained on magmatic rocks from the Saltychan anticlinorium in the Azov domain of the Ukrainian Shield. The rocks affiliate with the calc-alkaline series and a high-Mg series. The rocks of these series notably differ in concentrations of trace elements and REE and range from gabbro to granodiorite-quartz diorite in composition. The NORDSIM ionprobe U-Pb zircons ages of rocks belonging to the Obitochnen Complex and having both elevated and normal mg# correspond to 2908–2940 Ma. The Osipenkovskaya intrusion has an age of 2855 ± 19 Ma. The most alkaline North Obitochnen intrusion was emplaced in the Proterozoic, at 2074 ± 11 Ma. The age of the amphibolite metamorphism of the host gneisses is reliably dated at 3120–3000 Ma. The model Sm-Nd ages of the intrusive rocks do not exceed 3150 Ma. According to geochemical evidence, the parental melts of the magmatic rocks were derived from mantle domains variably enriched in lithophile elements. The results obtained by studying the Sm-Nd isotopic system corroborate the conclusion drawn from geochemical evidence that most of the melts were derived from the mildly enriched mantle, practically without involvement of ancient crustal material. The mantle became enriched in LREE at approximately 3000 Ma, which corresponds to the age of metamorphism of the supracrustal rocks. This process was separated from the derivation of the melts by a time span of 70–80 Ma. The relative age of the intrusive rocks and their variable composition can be most adequately explained by a contribution of heat and material from a plume to the derivation of the parental melts of these rocks.  相似文献   

13.
胡朝斌  李猛  查显锋  高晓峰  李婷 《地球科学》2018,43(12):4334-4349
幔源岩浆活动的成因研究,对约束区域构造演化历史具有重要意义.东昆仑祁漫塔格鹰爪沟镁铁-超镁铁质层状岩体由橄榄辉长苏长岩、含长橄榄二辉岩及橄榄二辉岩组成.获得橄榄辉长苏长岩的LA-ICP-MS锆石U-Pb年龄为263±4 Ma,指示岩体形成于中二叠世晚期.岩石地球化学特征显示,鹰爪沟岩体具有低SiO2、高MgO、FeOt含量,富集LREE、LILE,亏损HREE以及Th、U、Nb、Ta、Ti等不相容元素等特征.全岩εNd（t）=0.73~0.92,锆石εHf（t）=8.33~13.50.综合区域地质背景资料,认为其形成于古特提斯洋俯冲作用下的活动大陆边缘裂谷,源自于受软流圈熔体和俯冲流体交代的岩石圈地幔.结合同期具有壳幔混源特征的花岗岩资料,认为东昆仑地区在中二叠世已有幔源岩浆底侵活动,主要形成于古特提斯洋俯冲体制下局部的伸展背景.   相似文献   

14.
The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.  相似文献   

15.
华南大陆内部燕山期的岩浆事件可分为2期:燕山早期180~150 Ma,燕山晚期140~80 Ma。而处于扬子板块西南缘的滇黔贵等地,仅见有第二期晚白垩世中酸性侵入岩与相关成矿作用,缺乏第一期岩浆活动的时间记录。本文在滇东建水地区首次发现了麦地村煌斑岩,对其进行高精度LA-ICP-MS锆石U-Pb定年和主微量元素测试分析。结果显示煌斑岩的锆石U-Pb年龄为(158.4±2.4)Ma(MSWD=4.2),为晚侏罗世的产物,补充了该区较为空白的第一期岩浆热事件。麦地村煌斑岩不相容元素明显高于原始地幔,表现出不同程度的富集,大离子亲石元素Rb、Ba、Ce等富集程度不明显,高场强元素中U、Ta、Pb等表现出一定程度富集,LREE相对富集,HREE相对亏损,不具有Nb-Ta-Ti负异常的俯冲型配分曲线特征,具有明显的OIB型洋岛玄武岩稀土元素配分特征,为板内伸展环境下的产物。滇东地区发育中生代时期的NNE构造,早期NNE向褶皱叠加有后期同方向正断层。早期褶皱表现为SEE-NWW挤压应力作用下的构造特征,后期断层表现出同方向张应力作用下的特征。煌斑岩正是由于该区发生挤压之后伴随着一期应力松弛调整,伸展减压作用诱发地幔物质部分融熔,来自于深处的软流圈地幔流体与岩石圈地幔发生交代作用,形成富集型地幔,交代富集的岩石圈地幔局部受热、部分熔融,发生上升侵位。煌斑岩的侵位时间即是滇东地区中生代从挤压向伸展转换的起始时间。  相似文献   

16.
New geological, isotopic-geochronological, petrochemical, and isotopic geochemical data were obtained on the Porosozero and Kolmozero sanukitoid intrusions in the Kola region. The Porosozero differentiated intrusion was formed in four phases successively emplaced during approximately 60 Ma. Phase 1 consists of a gabbrodiorite-quartz monzodiorite-granodiorite-granite series. The zircon ages of granodiorite and quartz monzodiorite from the Porosozero are 2733 ± 6 and 2734 ± 4 Ma, respectively. Phase 2 of the intrusion comprises biotite leucogranites and aplite and leucoplagiogranite veins. The zircon age of the leucogranite is 2712 ± 6 Ma. Phase 3 consists of lamprophyre dikes of odinite-spessartite-vogesite composition. The emplacement age of the lamprophyres is constrained by the age of magmatic zircon from an odinite dike: 2680 ± 10 Ma. The age of the metasomatic zircon is 2629 ± 8 Ma. Phase 4 is composed of the youngest pegmatite veins. The Rb-Sr isochron age of the phase-1 rocks is 2724 ± 74 Ma. The zircon age of granitoids from the Kolmozero is 2736 ± 4 Ma. The rocks of the sanukitoid intrusions affiliate with the calc-alkaline series, have Mg# = 0.45?0.60, are enriched in Ba, Sr, K, P, and LREE, and contain elevated concentrations of Cr and Ni. Sm-Nd isotopic data on sanukitoids from both intrusions suggest that they were derived from a mantle source enriched in LILE and LREE and having ?Nd(2740) from +1.02 to +0.36. It was melted approximately 140 Ma after its origin [T(DM) = 2.9?2.8 Ga]. The rocks of the Porosozero and Kolmozero are proved to be similar to magmatic sanukitoid series of Archean and Phanerozoic age whose genesis was controlled by mantle-crustal interaction in suprasubduction environments at active continental margins. Elevated concentrations of Ag and Au in rocks from the Porosozero make it metallogenically promising in terms of precious metals.  相似文献   

17.
The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO2 (an average of 0.89 vs. 1.94 wt.%), Fe2O31 (9.54 vs. 14.71 wt.%), and P2O5 (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, εNd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.  相似文献   

18.
The timing and extent of cratonic destruction are crucial to understanding the crustal evolution of the North China Craton (NCC). Zircon U–Pb–Hf isotope data and the whole-rock major and trace element characteristics of the Huyu igneous rocks in northwestern Beijing, China, provide possible new evidence for the initial destruction of the NCC. The igneous rocks occur as several sills and dikes, including lamprophyre, monzonite porphyry, and aplite. The lamprophyres have high Mg# and K2O contents. The monzonite porphyries have high Mg#, high K2O contents, and negative εHf(t) values with zircon U–Pb ages of 225.5–227.7 Ma. These two types of rocks are both enriched in large ion lithosphere elements (LILEs) and light rare earth elements (LREEs) but are depleted in high field strength elements (HFSEs) and high rare earth elements (HREEs) and have almost no Eu anomalies and relatively high total rare earth element (ΣREE) contents. In contrast, the aplites exhibit high silica and K2O contents, low MgO contents, and more negative εHf(t) values with a zircon U–Pb age of 206.2 Ma. The aplites are also enriched in LILEs and LREEs but are depleted in HFSEs and HREEs, with strongly negative Eu, Ti, P, La, Ce, and Sr anomalies and relatively low ΣREE contents. These results indicate that the lamprophyres and monzonite porphyries represent a continuous cogenetic magma evolution series after melt derived from an enriched metasomatized lithospheric mantle experienced crust assimilation and fractional crystallization. The aplites were produced by the fractional crystallization of low-Mg parental magma derived from melting of the ancient Archaean crust. The occurrence of the Huyu intrusive rocks with many other plutons of similar ages on the northern margin of the NCC suggests that the northern NCC entered an intraplate extensional tectonic environment in the Late Triassic.  相似文献   

19.
The Roshtkhar area is located in the Khaf-Kashmar-Bardaskan volcano-plutonic belt to the northeastern Iran along the regional E–W trending Dorouneh Fault, northeastern of the Lut Block. There are several outcrops of subvolcanic rocks occurring mainly as dikes in the area, which intruded into Cenozoic intrusive rocks. We present U–Pb dating of zircons from a diabase dike and syenite rock using LA-ICP-MS that yielded an age of 1778 ± 10 Ma for the dike, indicating this Cenozoic dike has zircon xenocrysts inherited from deeper sources; and 38.0 ± 0.5 Ma, indicating an Late Eocene crystallization age for the syenite. Geochemically, the dikes typical of high-K calc-alkaline to shoshonitic magmas. Petrographic observations and major and trace element variations suggest that diabase melts underwent variable fractionation of clinopyroxene, olivine, and Fe-Ti oxides and minor crustal contamination during the differentiation process. Primitive mantle-normalized multi-element diagrams display enrichment in LILE, such as Rb, Ba, Th, U, and Sr compared to HFSE, as well as negative anomalies of Nb, Ta, P, and Ti, suggesting derivation from subduction-modified mantle. Chondrite-normalized REE plots show moderately LREE enriched patterns (<3.83 LaN/YbN <8.27), and no significant Eu anomalies. Geochemical modelling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of batch melting (~1–3%) of a phlogopite-spinel peridotite source to generate the mafic dikes. The geochemical signatures suggest that the Roshtkhar mafic dikes cannot be related directly to subduction and likely resulted from melting of upper mantle in an extensional setting where the heat flow was provided from deeper levels. These dikes presumably derived the zircon xenocrysts from the assimilation of upper crust of Gondwanian basement. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in NE Iran was triggered by heating due to asthenospheric upwelling in an extensional setting.  相似文献   

20.
The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?Nd(T) values (+0.86 to?+2.65), whereas values for ?Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.  相似文献   

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