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1.
New geochemical and isotopic data for post-collisional Early Eocene and Late Miocene adakitic rocks from the eastern part of the Sakarya Zone, Turkey, indicate that slab and lower crustal melting, respectively, played key roles in the petrogenesis of these rocks. The Early Eocene Yoncal?k dacite (54.4 Ma) exhibits high Sr/Y and La/Yb ratios, low Y and HREE concentrations, moderate Mg# (44–65), and relatively high εNd and low ISr values, similar to adakites formed by slab melting associated with subduction. Geochemical composition of the Yoncal?k dacite cannot be explained by simple crystal fractionation and/or crustal contamination of andesitic parent magma, but is consistent with the participation of different proportions of melts derived from subducted basalt and sediments. Sr/Y correlates horizontally with Rb/Y, and Pb/Nd correlates vertically with Nd isotopic composition, indicating that Sr and Pb budgets are strongly controlled by melt addition from the subducting slab, whereas positive correlations between Th/Nd and Pb/Nd, and Rb/Y and Nb/Y point to some contribution of sediment melt. In addition to low concentrations of heavy rare earth elements (~2–3 times chondrite), a systematic decrease in their concentrations and Nb/Ta ratios with increasing SiO2 contents suggests that slab partial melting occurred in the garnet stability field and that these elements were mobilized by fluid flux. These geochemical and isotopic signatures are best explained by slab breakoff and fusion shortly after the initiation of collision. Although the Late Micone Tavda?? rhyolite (8.75 Ma) has some geochemical features identical to adakites, such as high Sr/Y and La/Yb ratios, low Y and HREE concentrations, other requirements, such as sodic andesite and/or dacite with relatively high MgO and Mg# (>50), relatively high Ni and Cr, low K2O/Na2O (<0.4), high Sr (>400 ppm), for slab-derived adakites are not provided. It is sodic in composition and shows no traces of fractionation from dacitic parent magma. Low Nd and high Sr isotope ratios suggest derivation by partial fusion of calc-alkaline, juvenile crust with high Sr/Y and La/Yb ratios.  相似文献   

2.
Post-collisional (23–8 Ma), potassium-rich (including ultrapotassic and potassic) mafic magmatic rocks occur within the north–south-trending Xuruco lake–Dangre Yongcuo lake (XDY) rift in the Lhasa terrane of the southern Tibetan Plateau, forming an approximately 130-km-long semi-continuous magmatic belt. They include both extrusive and intrusive facies. Major and trace element and Sr–Nd–Pb isotopic data are presented for all of the known exposures within the XDY rift. The potassium-rich, mafic igneous rocks are characterized by high MgO (5.9–10.8 wt.%), K2O (4.81–10.68 wt.%), Ba (1,782–5,618 ppm) and Th (81.3–327.4 ppm) contents, and relatively high SiO2 (52.76–58.32 wt.%) and Al2O3 (11.10–13.67 wt.%). Initial Sr isotopic compositions are extremely radiogenic (0.712600–0.736157), combined with low (206Pb/204Pb) i (18.28–18.96) and (143Nd/144Nd) i (0.511781–0.512046). Chondrite-normalized rare earth element patterns display relatively weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements relative to high-field-strength elements and display strongly negative Ta–Nb–Ti anomalies. The combined major and trace element and Sr–Nd–Pb isotopic characteristics of the K-rich igneous rocks suggest that the primitive magmas were produced by 1–10 % partial melting of an asthenospheric mantle source enriched by both fluids and partial melts derived from Indian passive continental margin sediments subducted into the shallow mantle as a consequence of the northward underthrusting of the Indian continental lithosphere beneath Tibet since the India–Asia collision at ~55 Ma. The best-fit model results indicate that a melt with trace element characteristics similar to those of the K-rich rocks could be generated by 8–10 % partial melting of a metasomatized mantle source in the south and 1–2 % melting in the north of the XDY rift. Trace element and Sr–Nd–Pb isotopic modeling indicate that the proportion of fluid derived from the subducted sediments, for which we use as a proxy the Higher Himalayan Crystalline Sequence (HHCS), in the mantle source region increases from north (rear-arc) to south (front-arc), ranging from 0 to 5 %, respectively. Correspondingly, the proportion of the melt derived from the subducted HHCS in the source increases from north (2 %) to south (15 %). The increasing proportion of the fluid and melt component in the mantle source from north to south, together with a southward decreasing trend in the age of the K-rich magmatism within the XDY rift, is inferred to reflect rollback of the subducted Indian lithospheric mantle slab during the period 25–8 Ma. Slab rollback may be linked to a decreasing convergence rate between India and Asia. As a consequence of slab rollback at 25 Ma beneath the Lhasa terrane, its geodynamic setting was transformed from a convergent (55–25 Ma) to an extensional (25–8 Ma) regime. The occurrence of K-rich magmatism during the period 25–8 Ma is a consequence of the decompression melting of an enriched mantle source, which may signal the onset of extension in the southern Tibetan Plateau and provide a petrological record of the extension process.  相似文献   

3.
江西会昌盆地晚白垩世喷发的站塘安山岩,其形成与晚中生代岩石圈伸展和玄武质岩浆的底侵作用有关。这些岩石是高Na(6.59%～8.46%Na2O)、高Al的奥长花岗质岩石,具有与埃达克岩相似的高Sr和Ba、低Y和HREE、高Sr/Y和La/Yb比等特点。与埃达克岩相比,它们的Na2O较高而CaO较低,其εNd(t)值-2.3～-3.8和87Sr/86Sr初始比值0.707～0.708也与具大洋同位素组成特征的埃达克岩有明显区别。这些化学的和同位素特征,表明它们并非消减板片部分熔融的产物。站塘埃达克质岩浆可能源自底侵玄武质下地壳,其异常的高Na和低Ca特点反映其源岩成分的特殊性。因其成分与由实验产生的富碱玄武岩部分熔融体组成相当,因此,我们认为站塘高钠安山质岩浆可能是富碱玄武质岩石部分熔融的产物。  相似文献   

4.
《International Geology Review》2012,54(13):1755-1771
The tectonic setting of the West Qinling orogenic belt (QOB) during the Middle–Late Triassic remains a subject of debate. Petrogenesis of adakitic granodiorite plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. The Tadong adakitic granodiorite pluton in the western part of the West QOB is an important element of this system. Its petrogenesis can place severe constraints on the nature of the lower continental crust and mantle dynamics during the formation of the orogenic belt. U–Pb dates obtained through zircon laser-ablation inductively coupled mass spectrometry indicate that the Tadong pluton was emplaced at 220.2 ± 2.5 Ma, coeval with abundant magmatic rocks in the region. This indicates a prominent magmatic event in the western part of West Qinling during the Late Triassic. Geochemically the granodiorites are metaluminous to peraluminous high-K calc-alkalic and characterized by relatively high SiO2 (63.84–67.91 wt.%), Al2O3 (15.39–16.54 wt.%), and Sr (435.08–521.64 ppm), and low MgO (1.16–1.88 wt.%; Mg# = 38–46), Y (5.49–8.84 ppm) and Yb (0.34–0.91 ppm) contents, variable Eu anomalies (Eu/Eu* = 0.87–1.1), and high Sr/Y (51.72–84.45) ratios. These are compositional features of adakites that are commonly assumed to have been produced through partial melting of subducted oceanic basalt. In addition, the adakitic rocks are relatively enriched in light rare earth elements, large ion lithophile elements (Rb, Ba, Sr, Th, and K), and depleted in high field strength elements. However, petrological, geochronological, and geochemical characteristics indicate that the adakitic rocks were most likely formed by partial melting of a thickened mafic lower crust. Therefore, we suggest that the Tadong adakitic granodiorites were produced in a syn-collisional regime and associated with asthenospheric upwelling triggered by slab break-off or gravitational instability. This mechanism was responsible for generating the Late Triassic magmatism of West Qinling.  相似文献   

5.
ABSTRACT

Several alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to ~20° S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (~0.47 My, 40Ar/39Ar dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P–TLiquidus using PELE software show temperatures of 1045°C and 818°C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm3 and 2.26 g/cm3 for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The 87Sr/86Sr (~ 0.703800) and 143Nd/144Nd (~ 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03–0.04) with 2.5 wt. % of CO2.  相似文献   

6.
青藏高原新生代埃达克质岩的厘定及其意义   总被引：34，自引：2，他引：34  
赖绍聪 《地学前缘》2003,10(4):407-415
常量、微量及Sr-Nd-Pb同位素研究表明,青藏高原藏北石水河—浩波湖—多格错仁北部分布的一套新近纪(9.4～26.9 Ma)安山质-英安质-流纹质火山岩具有埃达克岩的地球化学特征。岩石ω(SiO_2)>58％,ω(Sr)>350×10~(-6),低Y和Yb,高La/Yb比,无铕异常。岩石N(~(87)Sr)/N(~(86)Sr)=0.706 365～0.708 156,N(~(208)Pb)/N(~(204)Pb)=38.955～39.052,N(~(207)Pb)/N(~(204)Pb)=15.651～15.672,N(~(206)Pb)/N(~(204)Pb)=18.679～18.839,N(~(143)Nd)/N(~(144)Nd)=0.512 411～0.512 535,ε(Nd)=—2.01～—4.43,充分表明它们为一套典型的壳源中酸性火山岩系,源自高原加厚陆壳下部的一个榴辉岩质源区的部分熔融。  相似文献   

7.
俯冲陆壳部分熔融形成埃达克质岩浆   总被引：4，自引：0，他引：4  
在岛弧背景,埃达克质岩浆形成于俯冲洋壳板片的部分熔融已得到共识,但在大陆碰撞背景,埃达克质岩浆是否形成于俯冲陆壳的部分熔融尚未有研究报导。对祁连山东南部关山花岗岩（229 Ma）的地球化学和岩石成因研究提供了俯冲陆壳部分熔融形成埃达克质岩浆的一个实例。关山花岗岩以高K（K2O=4.12%~5.16%,K2O/Na2O=0.97~1.64）、高Sr/Y比值（13.6~84.1）、低Y （6.8×10-6 ~15.7×10-6 ）和低HREE（eg. Yb=0.62×10-6~1.31×10-6）为特征,并具有强分异的稀土元素组成模式[（La/Yb）N=17.5~41.6]和演化的Sr-Nd同位素组成[初始87Sr/86Sr=0.70587~0.70714, εNd（t）=-10.9~-5.16, tDM=1.10~1.49 Ga]。这些地球化学特征表明关山花岗岩属于大陆型（C型）埃达克质岩石,而明显不同于俯冲洋壳板片或底侵玄武质下地壳部分熔融形成的埃达克岩。关山花岗岩Pb-Sr-Nd同位素组成与商丹断裂北侧的祁连山前寒武纪基底岩石、早古生代火山岩和花岗岩类存在显著差异,但类似于商丹断裂南侧秦岭早中生代花岗岩类的Pb-Sr-Nd同位素组成,由此认为具有埃达克质的关山花岗岩的岩浆来自于南部俯冲陆壳物质的部分熔融,并提出了大陆碰撞背景中埃达克质岩浆产生的一个新的地质模型。  相似文献   

8.
Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit), constitute vast outcrops (up to 250 km2) of high-Mg basaltic andesites to dacites that are associated with high-Nb hawaiites and mugearites. Whole-rock 40Ar/39Ar ages show a restricted range of 24.1 ± 0.4–22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of 87Sr/86Sr and 143Nd/144Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in the high-Mg basaltic andesites–dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27–100) and the isotopic composition of the subalkaline high-Mg basaltic andesites–dacites indicate derivation from a mantle modified by slab and sediment partial melts. Evidence such as reverse zoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate that magma mixing with mafic melts and concurrent fractional crystallization lead to the compositional evolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodic alkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and a wide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr–Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from a mantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggested to have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatized by slab-derived melts.  相似文献   

9.
ABSTRACT

Tongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of ?3.00 to ?4.52 and initial 87Sr/86Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.  相似文献   

10.
Zircon dating, geochemical and Nd-Sr isotopic analyses have been determined for samples from two granitic intrusions in the Talate mining district, Chinese Altay. Our data suggest that these intrusions were emplaced from 462.5 Ma to 457.8 Ma. These rocks have strong affinity to peralumious S-type granite and are characterized by prominent negative Eu anomalies(δEu=0.20–0.35), strong depletion in Ba, Sr, P, Ti, Nb, Ta and positive anomalies in Rb, Th, U, K, La, Nd, Zr, Hf. Nd-Sr isotopic compositions of the whole rock show negative εNd(t) values(-1.21 to-0.08) and Mesoproterozoic Nd model ages(T2 DM=1.20–1.30 Ga). Their precursor magmas were likely derived from the partial dehydration melting of Mesoproterozoic mica-rich pelitic sources and mixed with minor mantle-derived components, under relatively low P(≤1 kbar) and high T(746–796°C) conditions. A ridge subduction model may account for the early Paleozoic geodynamic process with mantle-derived magmas caused by Ordovician ridge subduction and the opening of a slab window underplated and/or intraplated in the middle–upper crust, which triggered extensive partial melting of the shallow crust to generate diverse igneous rocks, and provided the heat for the crustal melting and juvenile materials for crustal growth.  相似文献   

11.
Hadi Omrani 《Petrology》2018,26(1):96-113
Cretaceous to Eocene plutonic and volcanic rocks of the Sabzevar zone have an adakite characteristic with high Sr/Y ratio, depleted HFSE and enriched LILE features. Most of the Sabzevar adakites are high silica adakites with low Ni, Cr and Co contents. LREE/HREE ratio is high, while K2O content is low to intermediate. Adakites in the Sabzevar zone are exposed in two areas, which are named southern and northern adakites here. The combination of Sr, Nd and Pb isotopic data with major and trace elements indicates that the adakitic rocks are formed by partial melting of the Sabzevar oceanic slab. Nb/Ta content of the samples indicates that the adakitic magmas were generated at different depth in the subduction system. Dy/Yb ratios of adakitic samples indicate positive, negative and roughly flat patterns for different samples, suggesting garnet and amphibole as residual phases during slab-derived adakitic magma formation. Sabzevar adakites emplaced during late to post-kinematic events. Sabzevar oceanic basin demised during a northward subduction by central Iranian micro-continents (CIM) and Eurasia plate convergence.  相似文献   

12.
Abstract. The late Jurassic Tongshankou and Yinzu plutons in southeast Hubei have been investigated for their contrasting metal mineralization features. The former is closely associated with porphyry Cu‐Mo mineralization, while the latter is barren of metal mineralization, althouth both are located very close to each other. The Tongshankou granodiorite porphyries and the Yinzu granodiorites are geochemically similar to adakites, e.g., high Al2O3 and Sr contents and La/Yb and Sr/Y ratios, enriched in Na2O, depleted in Y and Yb, very weak Eu anomalies and positive Sr anomalies. However, different geochemi‐cal characteristics exist between the two plutons: the Tongshankou adakitic rocks (1) are relatively enriched in SiO2, K2O, MgO, Cr, Ni, and Sr and depleted in Y and Yb; (2) have higher degree REE differentiation; (3) have positive Eu anomalies in contrast with very weak negative or unclear Eu anomalies in the Yinzu rocks; and (4) isotopically have relatively higher ePNd(t) values (‐5.19 to ‐5.38) and lower initial 87Sr/86Sr ratios (0.7060 to 0.7062), while the Yinzu adakitic rocks have relatively lower ePNd(t) values (‐7.22 to ‐8.67) and higher initial 87Sr/86Sr ratios (0.7065 to 0.7074). The trace element and isotopic data demonstrate that the Tongshankou adakitic rocks were most probably originated from partial melting of delaminated lower crust with garnet being the main residual mineral whereas little or no plagioclase in the source. On the contrary, the Yinzu adakitic rocks were likely derived from partial melting of thickened lower crust, with residual garnet and a small quantity of plagioclase and hornblende in the source. Interactions between the adakitic magmas and mantle peridotites possibly took place during the ascent of the Tongshankou adakitic magmas through the mantle, considering that MgO, Cr, and Ni contents and ePNd(t) values of the adakitic magmas were possibly elevated and initial 87Sr/86Sr ratios were possibly lowered due to the contamination of mantle peridotites. In addition, the Fe2O3 of the adakitic magmas was likely released into the mantle and the oxygen fugacities (?o2) of the latter were obviously possibly raised, which made metallic sulfide in the mantle oxidized and the chalcophile elements such as Cu were incorporated into the adakitic magmas. The ascent of the adakitic magmas enriched in Cu and Mo will lead to the formation of porphyry Cu‐Mo deposit. Nevertheless, the Yinzu adakitic magmas were possibly lack of metallogenetic materials due to not interacting with mantle peridotite, and thus unfavorable to metal mineralization.  相似文献   

13.
埃达克质岩石是高Na、Al和Sr、低Y和HREE以及Nb、Ta亏损的钠质花岗质岩石,奥长花岗岩-英云闪长岩-花岗闪长岩(TTG)是早期(太古宙)大陆壳主要组分,成分与埃达克质岩石相似,这些成分独特的岩石总体上认为是俯冲洋壳、下地壳和拆沉的下地壳中变质玄武岩部分熔融的产物。文中综述我们近年来在变质玄武岩体系相平衡和矿物-熔体微量元素分配实验研究成果:相平衡实验和熔体微量元素特征研究表明,变质玄武岩部分熔融过程中金红石是导致TTG/埃达克岩浆Nb、Ta亏损的必要残留矿物,从而否定了前人“TTG由无金红石的角闪岩熔融产生”的观点;证实金红石仅仅在压力1.5GPa以上才能稳定存在,从而限定TTG/埃达克岩熔体必定产生在大约50km以上,表明TTG/埃达克岩是在相对较深的含金红石榴辉岩相条件下熔融产生的。矿物(石榴子石、角闪石,单斜辉石和金红石)-熔体微量元素分配系数测定和部分熔融模拟结果进一步限定俯冲洋壳和下地壳起源的TTG/埃达克岩浆由含金红石角闪榴辉岩熔融产生,而拆沉下地壳起源的埃达克岩浆的产生要求软流圈地幔高温,由无水或含有少量含水矿物的榴辉岩熔融产生。  相似文献   

14.
We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H2O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.  相似文献   

15.
This study reports new geochemical and Sr and Nd isotope data for 11 samples of hynormative late Miocene (～6.5 Ma) basalt, basaltic andesite, and rhyolitic volcanic rocks from Meseta Rio San Juan, located in the states of Hidalgo and Queretaro, Mexico, in the north-central part of the Mexican Volcanic Belt (MVB). The in situ growth-corrected initial isotopic ratios of these rocks are as follows: 87Sr/86Sr 0.703400-0.709431 and 143Nd/144Nd 0.512524-0.512835. For comparison, the isotopic ratios of basaltic rocks from this area show very narrow ranges as follows: 87Sr/86Sr 0.703400-0.703540 and 143Nd/144Nd 0.512794-0.512835. The available geological, geochemical, and isotopic evidence does not support the generation of the basic and intermediate magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The basaltic magmas instead could have been generated by partial melting of the upper mantle. The evolved basaltic andesite magmas could have originated from such basaltic magmas through assimilation coupled with fractional crystallization. Rhyolitic magmas might represent partial melting of different parts of the underlying heterogeneous crust. Their formation and eruption probably was facilitated by extensional tectonics and upwelling of the underlying mantle. The different petrogenetic processes proposed here for basaltic and basaltic andesite magmas on one hand and rhyolitic magmas on the other might explain the bimodal nature of Meseta Rio San Juan volcanism. Finally, predictions by the author about the behavior of Sr and Nd isotopic compositions for subduction-related magmas is confirmed by published data for the Central American Volcanic Arc (CAVA).  相似文献   

16.
Massive mafic sheets were recently recognized intruding the Neoproterozoic strata in Fuyang area, eastern Jiangnan orogen. Geochronological, geochemical, and isotopic studies were carried out to understand their mantle source, crust–mantle interaction, and tectonic setting. LA-ICP-MS U-Pb zircon data indicate that the sheets were generated at 808 ± 7 Ma. The mafic sheets consist of two groups: high Ti and low Ti. They are enriched in light rare earth elements (LREE; 3.3–5.3 ppm) and show negligible Eu anomalies (δEu = 0.77–1.12). They also have strong large-ion lithophile element (LILE; Sr, K, Rb, Ba) enrichment, moderately strong high-field-strength element (HFSE) enrichment (except for Nb-Ta depletion), and positive εNd(t) (5.1–9.1). Geochemical and isotopic data indicate that the mafic sheets were generated from a depleted asthenospheric mantle source. The high-Ti mafic sheets have higher HFSE contents and less Nb-Ta depletion than the low Ti series, indicating a lower degree of partial melting and crustal contamination. The mafic sheets grew in a within-plate setting, concurrent with the ~820–750 Ma rifting events in the eastern Jiangnan orogen. They are likely related to the breakup of the Rodinia supercontinent.  相似文献   

17.
In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1 wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.  相似文献   

18.
The origin of microgranitoid enclaves in granitic plutons has long been debated (hybrid magma blobs vs. refractory restites or cognate fragments). This article presents detailed petrography, SHRIMP zircon U–Pb chronology, bulk-rock major and trace element analyses, and Sr–Nd isotope and in situ zircon Hf isotopic geochemistry for microgranitoid enclaves within two Late Triassic granitic plutons in the Qinling orogen. Zircon U–Pb dating shows that the enclaves formed during the Carnian (222.5 ± 2.1 to 220.7 ± 1.9 Ma) coeval with their host granitoids (220.0 ± 2.0 to 218.7 ± 2.4 Ma). Field and petrological observations (e.g. double enclaves, xenocrysts, acicular apatite, and poikilitic K-feldspar or quartz) suggest that the enclaves are globules of a mantle-derived more mafic magma that was injected into and mingled with the host magma. The enclaves are mainly ultrapotassic, distinct from the host granitoids that have high-K calc-alkaline bulk-rock compositions. Although the enclaves have closely similar bulk-rock Sr–Nd isotope [initial 87Sr/86Sr?=?0.7046–0.7056, ?Nd (T)?=?–0.3 to –5.0] and in situ zircon Hf isotope [?Hf (T)?=?–1.5 to?+2.9] ratios as the granitoids [initial 87Sr/86Sr?=?0.7042–0.7059, ?Nd (T)?=?–0.6 to –6.3, ?Hf (T)?=?–2.2 to?+1.6], chemical relationships including very different bulk-rock compositions at a given SiO2 content lead us to interpret the isotopic similarities as reflecting similar but separate isotopic source rocks. Detailed elemental and isotopic data suggest that the enclaves and the host granitoids were emplaced in a continental arc environment coupled with northward subduction of the Palaeo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with melts interacting with the overlying mantle wedge, formed high-K calc-alkaline granitic magmas, whereas partial melting of diapiric phlogopite-pyroxenites, solidified products of the same subducting sediment-derived melts, generated ultrapotassic magmas of the microgranitoid enclaves. Our new data further confirm that in the Late Triassic time the Qinling terrane was an active continental margin rather than a post-collisional regime, giving new insights into the tectonic evolution of this orogen.  相似文献   

19.
ABSTRACT

The magmatic generation for the Late Triassic–Early Jurassic (~215–200 Ma) and Early Cretaceous–Late Cretaceous (~108–79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean 206Pb/238U age of 199.8 ± 2.5 Ma, aplite granite of 108.2 ± 2.3 Ma, monzogranite porphyry of 80.8 ± 1.0 Ma, and diorite enclaves of 79.2 ± 0.9 Ma and 77.9 ± 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO2 contents, high Mg# values, high zircon δ18O values, and negative whole-rock ?Nd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO2 contents, high (Na2O + K2O)/CaO ratios, high zircon δ18O values, and enriched whole-rock Sr–Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO2 contents, high Mg# values, and high zircon δ18O values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic–Early Jurassic and Early Cretaceous–Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.  相似文献   

20.
南祁连地区化石沟花岗闪长岩位于化石沟铜矿附近,东距阿克塞县城约120 km。花岗闪长岩LA-ICP-MS锆石U-Pb年龄为261.1±3.8 Ma,形成于中二叠世晚期。岩石属钙碱性系列,具有埃达克岩的地球化学特征,SiO_2含量在68.35%~69.14%之间,高铝(Al_2O_3=15.83%~16.06%)、Sr(367×10~(–6)~381×10~(–6))、低Y(15.12×10~(–6)~19×10~(–6))和Yb(1.34×10~(–6)~1.85×10~(–6)),富Na贫K(Na_2O=4.3%~4.47%,K_2O=2.22%~2.46%,Na_2O/K2O=1.75~2.01),MgO介于0.79%~0.89%之间,Mg~#为0.35左右。岩石富集强不相容元素Ba、Rb、Sr、Th、U和LREE,强烈亏损高场强元素Nb、Ta、Ti以及HREE,(La/Yb)_N=8.3~11.77,具轻微的Eu负异常(δEu=0.76~0.86),高Rb/Sr(≈0.2)值。化石沟花岗闪长岩的(~(87)Sr/~(86)Sr)i、ε_(Nd)(t)、(~(176)Hf/~(177)Hf)_i、δ~(18)O_(V-SMOW)分别为0.7065~0.7068、–1.45~–0.78、0.282765、12.1‰~12.5‰。综合以上特征,认为化石沟花岗闪长岩源于新生地壳物质在玄武岩浆底侵作用下发生的部分熔融,在岩浆作用过程中可能发生过轻微的分离结晶作用,形成于板内背景,处于造山挤压向后造山伸展的构造体制转变阶段。花岗闪长岩的Nd和Hf模式年龄(分别为1.1 Ga和0.9 Ga)指示玄武质岩浆上侵的时间应为中元古代晚期。化石沟埃达克质岩显示了良好的斑岩型Cu、Au成矿潜力,寻找与埃达克岩有关的斑岩型Cu、Au矿应是下一步找矿的重点方向。  相似文献   

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