首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Metamorphic units of the Khavyven Highland that crop out in the northern portion of the Khavyven Uplift of the basement structures of the Central Kamchatka Trough are formed by rocks of the Khavyven Formation, which are metamorphosed in the green-schist facies. The formation comprises two strata: the lower part that consists of amphibole-micaceous ± garnet, epidote-micaceous ± garnet crystalline schists, and micaceous ± garnet quartzite schists has a total thickness of some 500 m, and the upper part, which is formed by epidote-amphibole and phengite-amphibole green schists and overlying epidote-amphibole-micaceous quartzites, with a visible thickness of some 750 m. The isotopic ratios of Sr, Nd, and Pb were determined in the examined rocks of the Khavyven Formation for the first time. The high 87Sr/86Sr and low 143Nd/144Nd ratios and the high K/La, Ba/Th, Th/Ta, and La/Nb ratios in combination with a deep Ta-Nb minimum indicate that the original volcanites of the crystalline schists of the lower rock mass had a subduction nature. The green schist of the upper rock mass, whose composition corresponds to that of spilitic basalts, have elevated 87Sr/86Sr and 143Nd/144Nd ratios, thus combining indications of depleted melts of the N-MORB and E-MORB types and those of subduction melts, which explains the deep Ta-Nb minimum and the low (La/Yb) N ratios. The isotopic signatures of lead in rocks of the lower and the upper strata are similar. The composition points of the crystalline schists and the green schists are located near the trend of isotopic evolution of lead in the depleted mantle, which indicates that the rocks are closely related to this mantle source.  相似文献   

2.
Siberian Traps Large Igneous Province (STLIP) is one of the most voluminous volcanic provinces on Earth. The dominant erupted rocks are low-Ti basalts, which make up 80% by volume of the classical Noril’sk lava sequence. In the west Siberian basin and Maymecha-Kotuy area, the low-Ti basalts make up about 99% and 50% by volume, respectively. Dolerite sills in the Angara-Taseevskaya Syncline at the southeastern STLIP exhibit trace element patterns and Sr isotope ratios typical of the low-Ti basalts of the Noril’sk sequence. The most Mg-rich (MgO 9.5–11 wt%) and hence least differentiated dolerites are characterized by trace element patterns with Ta-Nb depletion, low Ce/Pb and high Sr/Pr. These trace element features are similar to water-saturated, mantle wedge-derived island arc basalts. These imply an important role of subduction fluid-derived trace elements in the source of melting beneath the Angara-Taseevskaya Syncline and other regions of the STLIP. Less magnesium rocks (MgO 3.8–6.1 wt%) with less prominent Ta-Nb depletion, higher Ce/Pb and lower Sr/Pr could be produced via olivine-plagioclase fractionation of primary high-magnesium melts.  相似文献   

3.
The metamorphic complexes of eastern Kamchatka exposed on the Khavyven Highland and Karaginsky Island, as well as on the Kamchatka and Ozernoi peninsulas, compose large (up to 1.5 km) elongated blocks spatially associated with ophiolitic peridotite and gabbroic rock bodies (the Khavyven Highland and Karaginsky Island) or make up isolated fragments and blocks among serpentinite melange (the Ozernoi and Kamchatka peninsulas). The degree of metamorphism of the primary rocks varies from the greenschist/amphibolite boundary facies (Karaginsky Island and the Khavyven Highland) to the high-pressure amphibolite facies (the Ozernoi and Kamchatka peninsulas).  相似文献   

4.
This paper presents detailed mineralogical and geochemical data on the metamorphosed ferromanganese rocks observed as rare lenticular bodies among the metavolcanics of the Khavyven Highland and Ganal Ridge in Eastern Kamchatka. They are specific porphyry-type banded Mn-garnet and amphibole rocks with large (up to 1–5 mm) magnetite porphyroblasts. In the Khavyven Highland, ilmenohematite is associated with magnetite in the metamorphosed ferromanganese rocks of the greenschist facies. In the Ganal Ridge, magnetite in metamorphosed ferromanganese rocks of the amphibolite facies shows decay structures and contains Mn-ilmenite lamellae surrounded by tiny crystals of ferroaluminous Zn-spinel. Comparison of Fe-Ti oxides in the ferromanganese rocks of the Khavyven Highland and Ganal Ridge shows that metamorphism of these structures differed not only in temperature and pressure levels but also in the oxygen fugacity. The rocks of the Khavyven Highland were metamorphosed under greater oxidizing conditions relative to the analogous rocks of the Ganal Ridge. The garnets in the studied rocks demonstrate progressive zonality from the center to the rims of crystals: the Fe, Mg, and Ca concentrations are increased, while the Mn concentration is decreased. Amphibolites in the ferromanganese rocks of the Khavyven Highland are also characterized by progressive zonality: the crystal cores are composed of Mn-actinolite, whereas the marginal zones consist of Ca-Na amphibole (winchite and less common barroisite), testifying to an increase of temperature, pressure, and oxygen fugacity during metamorphism. The higher-temperature rocks of the Ganal Ridge usually contain Mg-hornblende and tschermakitic hornblende.  相似文献   

5.
内蒙古达茂旗北部奥陶纪埃达克岩类的识别及其意义   总被引：19，自引：0，他引：19  
内蒙古达茂旗北部奥陶纪侵入岩分布于槽台边界断裂(乌兰布拉格—哥舍深大断裂)以北,岩石类型主要有闪长岩、石英闪长岩、英云闪长岩和花岗闪长岩。该套侵入岩具高w(Sr)、低w(Y)和w(Yb)的特征,具高的w(Sr)/w(Y)和w(La)/w(Yb)比值;轻稀土富集,重稀土亏损,具正铕异常或微弱的负铕异常;微量元素蛛网图中具明显的Sr峰和Nb、Ti低谷,与典型的埃达克岩(adakite)特征一致。通过与典型adakite、太古代TTD(G)及岛弧和大陆边缘弧ADR的对比,表明本区奥陶纪侵入岩以具轻、重稀土强烈分馏型而与岛弧和大陆边缘弧ADR区别明显,而与adakite和TTD(G)相似,同时又以具明显的Sr正异常而区别于太古代的TTD(G)。地球物理、重力及航磁资料显示槽台边界断裂两侧有完全不同的地壳结构。北侧可能是在晚古生代早期拼贴到华北陆块的一个小微地块,拼贴过程中二者之间的加里东洋壳向微地块之下俯冲消减,俯冲板片在榴辉岩相或角闪岩相—榴辉岩相过渡相发生部分熔融形成adakite岩浆,熔融残留物主要为石榴石。  相似文献   

6.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time.  相似文献   

7.
New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites.  相似文献   

8.
The Urumieh-Dokhtar magmatic arc (UDMA) of Central Iran has been formed during Neotethyan Ocean subduction underneath Eurasia. The Rabor-Lalehzar magmatic complex (RLMC), covers an area ～1000?km2 in the Kerman magmatic belt (KMB), SE of UDMA. RLMC magmatic rocks include both granitoids and volcanic rocks with calc-alkaline and adakitic signatures but with different ages.Miocene adakitic rocks are characterd by relatively enrichmented in incompatible elements, high (Sr/Y)(N) (>40), and (La/Yb)(N) (>10) ratios with slightly negative Eu anomalies (EuN/Eu*≈ 0.9), depletion in HFSEs, and relatively non-radiogenic Sr isotope signatures (87Sr/86Sr?=?0.7048–0.7049). In contrast, the Oligocene granitoids exhibit low Sr/Y (<20) and La/Yb (<9) ratios, negative Eu anomalies (EuN/Eu*?≈?0.5), and enrichment in HFSEs and radiogenic Sr isotope signatures (87Sr/86Sr?=?0.7050–0.7052), showing affinity to the island arc rocks. Eocene volcanic rocks which crusscut the younger granitoid rocks comprise andesites and dacites. Geochemically, lavas show calc-alkaline character without any Eu anomaly (EuN/Eu*?≈?1.0). Based on the geochemical and isotopic data we propose that melt source for both calc-alkaline and adakitic rocks from the RLMC can be related to the melting of a sub-continental lithospheric mantle (SCLM). Basaltic melts derived from a metasomatized mantle wedge might be emplaced at the mantle-crust boundary and formed the juvenile mafic lower crust. However, some melts fractionated in the shallow magma chambers and continued to rise forming the volcanic intermediate-mafic rocks at the surface. On the other hand, the assimilation and fractional crystallization in the shallow magma chambers of may have been responsible for the development of Oligocene granitoids with calc-alkaline affinity. In the mid-Late Miocene, following the collision between Afro-Arabia and Iranian block the juvenile mafic crust of UDMA underwent thickening and metamorphosed into garnet-amphibolites. Subsequent upwelling of a hot asthenosphere during Miocene was responsible for partial melting of thickened juvenile crust of the SE UDMA (RLM complex). The adakitic melts ascended to the shallow crust to form the adakitic rocks in the KMB.  相似文献   

9.
俯冲型和碰撞型含矿斑岩地球化学组成的差异   总被引：2，自引：2，他引：0  
Cu-Mo-Au含矿斑岩不仅可以形成于与洋壳俯冲相联系的弧环境,而且也产于碰撞造山带内。通过对比俯冲型和碰撞型含矿斑岩的地球化学特征,发现它们特别在微量元素上具有较大差别,暗示它们有着不同的物源区组成或形成机制。同冈底斯带碰撞型含矿斑岩相比,太平洋东岸俯冲型含矿斑岩有着明显高的HREE和Y含量,低的Sr/Y、(La/Yb)N以及(Dy/Yb)N比值,表明其物质源区不含或含有少量的石榴子石并可能以角闪石组成为主。统计发现这些俯冲型含矿斑岩部分样品具有埃达克岩地球化学特征,但大部分样品却显示出具有与正常岛弧系列火山岩相似的特征,它们很可能是板片释放流体交代地幔楔形成的熔体并在后期经历MASH过程的产物。冈底斯带碰撞型含矿斑岩具有典型埃达克岩地球化学特征,指示其形成条件达到了石榴子石相变,可能形成于增厚的下地壳,其物质源区很可能与前期的洋壳俯冲有着密切的联系。普朗-雪鸡坪含矿斑岩具有与俯冲型含矿斑岩十分相似的地球化学特征,它们有可能是西向俯冲的甘孜-理塘洋发生断离,进而诱发前期俯冲流体交代的富集地幔楔发生部分熔融的产物,而并非是俯冲洋壳直接发生部分熔融的产物。  相似文献   

10.
Andreas Stracke  Ernst Hegner 《Lithos》1998,45(1-4):545-560
The Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain occurs in a zone of lithospheric extension superimposed on a post-collisonal tectonic setting along the Pacific and Indo-Australian plates northeast of Papua New Guinea. We present geochemical and Sr, Nd, and Pb isotope data for volcanic rocks from these islands and three recently discovered seamounts located at Lihir island. Major element data document an alkalic affinity of the sample suite and trachybasalts as the predominant rock type. Negative Nb-anomalies in extended trace element patterns, enrichment of the light rare earth elements, and Ce/Pb ratios of about 4 are typical of the values in calc alkaline island arc volcanics and support an origin from subduction-modified mantle. 87Sr/86Sr ratios of 0.7037 to 0.7044 and Nd values of +5.6 to +6.8 indicate that the upper mantle evolved with a time-integrated depletion in LREE, however, not as severe as that recorded in basalts from the East Pacific Rise. Variable 87Sr/86Sr ratios at less variable 143Nd/144Nd ratios suggest that 87Sr/86Sr ratios of the melts were modified by secondary processes, such as assimilation of seawater Sr from crustal rocks. The Pb isotope ratios are uniform, moderately radiogenic (206Pb/204Pb ca. 18.7 to 18.8), and similar to those reported for the active Mariana arc. Elevated 207Pb/204Pb ratios relative to Pacific MORB suggest melting of small amounts of subducted sediments (ca. 1–2 wt.%). An important control of subducted sediment on the chemistry of the melts can also be inferred from the ratios of highly incompatible trace elements (e.g., Th, U, Pb, La, and Nb). Additional mantle enrichment by subduction derived fluids is reflected in high values of highly incompatible trace element ratios between fluid mobile (e.g., Ba) and fluid immobile elements (e.g., Th, Nb). The results of this study document that the chemical composition of igneous rocks from post-collisional tectonic settings are strongly influenced by previous plate tectonics. This conclusion implies that the information conveyed by tectonic discrimination diagrams for these rocks must be interpreted with care.  相似文献   

11.
Mineral chemistry, major and trace elements, and 87Sr/86Sr ratios are presented for 29 igneous rocks dredged from the northern portion of the Izu-Ogasawara arc. These rocks are compositionally bimodal. Basement gabbro and trondhjemite from the arc are extremely poor in K2O (0.05–0.19%) and Rb (0.48–0.62 ppm), and their REE patterns and Sr isotope ratios indicate that there are island arc tholeiites. Quaternary volcanic rocks from the present volcanic front (Shichito Ridge; active arc), back-arc seamounts (east side; inactive arc) and Torishima knoll between the two back-arc depressions (incipient back-arc basins) behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in K and Rb. Rhyolite pumice from the backarc depression is also the depleted island arc tholeiite, whereas basalts from the depression have compositions that are transitional between MORB and island arc tholeiites in trace element (Ti, Ni, Cr, V, Y and Zr) and mineral chemistries. The back-arc depression basalts have relatively high BaN/CeN(0.66–1.24), Cen/YbN(1.1–1.9) and K/Ba(45–105) and low 87Sr/86Sr (0.70302–0.70332) and Ba/Sr (0.1–0.2), which are similar to other back-arc basin basalts and E-type MORB, but are quite unlike the depleted island arc tholeiites. The diverse trace element and Sr isotope compositions of basalt-andesite from the back-arc depressions imply the interplay between E-type MORB and island arc tholeiite. These chemical characteristics and the relationships of (Ce/Yb)N vs (Ba/Ce)N and (Ce/Yb)N vs 87Sr/86Sr suggest that the back-arc depression magmas are generated by mixing of E-type MORB and depleted island arc tholeiite magmas. Geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust.  相似文献   

12.
We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.  相似文献   

13.
The Langdu high-K calc-alkaline intrusions are located in the Zhongdian area, which is the southern part of the Yidun island arc. These intrusive rocks consist mainly of monzonite porphyry, granodiorite, and diorite porphyry. The K2O content of majority of these rocks is greater than 3%, and, in the K2O-SiO2 diagram, all the samples fall into the high-K calc-alkaline to shoshonitic fields. They are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs; LaN/YbN = 14.3-21.2), and show slightly negative Eu anomalies (δEu = 0.77-1.00). These rocks have high K, Rb, Sr, and Ba contents; moderate to high enrichment of compatible elements (Cr = 36.7-79.9 ppm, Co = 9.6-16.4 ppm, and MgO = 2.2%-3.4%); low Nb, Ta, and Ti contents, and characteristic of low high field strength elements(HFSEs) versus incompatible elements ratios (Nb/Th = 0.75, Nb/La = 0.34) and incompatible elements ratios (Nb/U = 3.0 and Ce/Pb = 5.1, Ba/Rb = 12.0). These rocks exhibit restricted Sr and Nd isotopic compositions, with (87Sr/86Sr) i values ranging from 0.7044 to 0.7069 and εNd(t) values from -2.8 to -2.2. The Sr-Nd isotope systematic and specific trace element ratios suggest that Langdu high-K calc-alkaline intrusive rocks derived from a metasomatized mantle source. The unique geochemical feature of intrusive rocks can be modeled successfully using different members of a slightly enriched mantle, a slab-derived fluid, and terrigenous sediments. It can be inferred that the degree of partial melting and the presence of specific components are temporally related to the tectonic evolution of the Zhongdian island arc. Formation of these rocks can be explained by the various degrees of melting within an ascending region of the slightly enriched mantle, triggered by the subduction of the Garzê-Litang ocean, and an interaction between the slab-derived fluid and the terrigenous sediments.  相似文献   

14.
We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of LaN, YbN and(La/Sm)N with Eu* [Eu* = 2 x EuN/(SmN + GdN), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)Nwith Eu*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclase–garnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 25–30 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middle–upper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middle–upper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the ‘arc paradox’ of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth  相似文献   

15.
Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial 87Sr/86Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial 87Sr/86Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.  相似文献   

16.
The Banda arc of eastern Indonesia manifests the collision of a continent and an intra-oceanic island arc. The presently active arc is located on what appears to be oceanic crust whereas the associated subduction trench is underlain by continental crust.Recent lavas from the Banda arc are predominantly andesitic and range from tholeiitic in the north through calc-alkaline to high-K calc-alkaline varieties in the southern islands. Defining this regular geochemical variation are significant increases in the abundances of K (2,600–21,000 ppm), Rb (10–90 ppm), Cs (0.5–7.0 ppm), and Ba (100–1,000 ppm) from tholeiitic to high-K calc-alkaline lavas. 87Sr/86Sr ratios in the tholeiites are relatively low, from 0.7045 to 0.7047. In the calc-alkaline lavas, 87Sr/86Sr ratios range from 0.7052 to 0.7095, and in the high-K calc-alkaline lavas from 0.7065 to 0.7080. There is no correlation between 87Sr/86Sr and major and trace element abundances, even among lavas from the same volcano. Late Cenozoic cordierite — bearing lavas from Ambon, north of the presently active arc, are highly enriched in K, Rb and Cs, which together with 87Sr/86Sr ratios of approximately 0.715 is consistent with their derivation from partial melting of pelitic material in the locally — thick crust.The high 87Sr/86Sr ratios in the Recent calc-alkaline lavas are interpreted to result from mixing of a sialic component with a mantle derived component. The most likely cause is subduction and subsequent melting of either sea-floor sediments or continental crust. However, it is probably unrealistic to model this type of deep contamination by simple two-component mixing. Such contamination implies that the volcanic rocks from the Banda arc are at least partly a manifestation of melting at or near the Benioff seismic zone. Temperatures of at least 750–800 ° C at the top of the subducted lithospheric slab at depths of approximately 150 km are also implied; temperatures very close to the solidus of hydrous basalt (eclogite) at such pressure. It is concluded that partial melting of the crustal component of the subducted lithospheric slab may play a significant role in island arc petrogenesis.This paper is the result of a cooperative project with the Geological Survey of Indonesia, Ministry of Mines and Energy  相似文献   

17.
The Jurassic ophiolites in the South Apuseni Mountains represent remnants of the Neotethys Ocean and belong to the East Vardar ophiolites that contain ophiolite fragments as well as granitoids and volcanics with island-arc affinity. New U–Pb zircon ages, and Sr and Nd isotope ratios give insights into their tectono-magmatic history. The ophiolite lithologies show tholeiitic MOR-type affinities, but are occasionally slightly enriched in Th and U, and depleted in Nb, which indicates that they probably formed in a marginal or back-arc basin. These ophiolites are associated with calc-alkaline granitoids and volcanics, which show trace element signatures characteristic for subduction-enrichment (high LILE, low HFSE). Low 87Sr/86Sr ratios (0.703836–0.704550) and high 143Nd/144Nd ratios (0.512599–0.512616) of the calc-alkaline series overlap with the ratios measured in the ophiolitic rocks (0.703863–0.704303 and 0.512496–0.512673), and hence show no contamination with continental crust. This excludes a collisional to post-collisional origin of the granitoids and is consistent with the previously proposed intra-oceanic island arc setting. The new U–Pb ages of the ophiolite lithologies (158.9–155.9 Ma, Oxfordian to Early Kimmeridgian) and granitoids (158.6–152.9 Ma, latest Oxfordian to Late Kimmeridgian) indicate that the two distinct magmatic series evolved within a narrow time range. It is proposed that the ophiolites and island arc granitoids formed above a long-lived NE-dipping subduction zone. A sudden flip in subduction polarity led to collision between island arc and continental margin, immediately followed by obduction of the ophiolites and granitoids on top of the continental margin of the Dacia Mega-Unit. Since the granitoids lack crustal input, they must have intruded the Apuseni ophiolites before both magmatic sequences were obducted onto the continental margin. The age of the youngest granitoid (~153 Ma, Late Kimmeridgian) yields an estimate for the maximum age of emplacement of the South Apuseni ophiolites and associated granitoids onto the Dacia Mega-Unit.  相似文献   

18.
Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.  相似文献   

19.
The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044?0.7047, ?Nd(T) values of –1.5 to?+0.6, and ?Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.  相似文献   

20.
北天山东段阿奇山组火山岩的地球化学特征及锆石U-Pb年龄   总被引：13，自引：7，他引：6  
阿奇山组分布于北天山东段觉罗塔格构造带内部的雅满苏岛弧带中,属于原雅满苏组下部的火山岩部分,而上部火山岩则属于土古土布拉克组。在阿奇山地区,阿奇山组由中-酸性火山岩、火山碎屑岩和火山碎屑沉积岩组成。岩石地球化学特征显示为钙碱性岛弧火山岩系列,火山岩主体具有高Sr、Na2O、Al2O3、SiO2, 低MgO、Y、Yb,及明显亏损HFSE、Nb、Ta等特征,与经典埃达克岩地球化学特征吻合,Nb、Sr、Sr/Y值显示为俯冲型埃达克岩。地球化学相关图解等表明岩浆同化混染作用弱、经历了辉石、斜长石、钛铁氧化物和磷灰石的分离结晶作用、且部分熔融发生在石榴石稳定域。与东部土屋-雅满苏地区阿奇山组火山岩的对比研究表明,东部地区的火山岩不具埃达克岩特征,说明埃达克岩分布的局限性。锆石U-Pb SHRIMP 谐和年龄为341.7±2.7Ma,与首次在该组中采集的化石资料完全一致,代表了火山岩的形成年龄。与其北部大南湖岛弧带中的小热泉子组、企鹅山组形成年龄（325.1±3.2Ma、322.6±2.0Ma）对比研究暗示,准噶尔板块南缘石炭纪存在构造-岩浆活动随时间由南向北迁移。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号