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1.
In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K2O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.  相似文献   

2.
Mafic dikes of mainly Early Cretaceous age (130–110 Ma) are widely developed on the Jiaodong Peninsula, China. Previous studies of the dikes, which have focused mainly on occurrences in the Jiaobei uplift and in the Sulu orogenic belt, have thoroughly examined their petrogenesis and geodynamic setting. This study identified four previously unknown mafic dikes (dolerite and lamprophyre) in southeastern Jiaolai basin (near Haiyang city), Jiaodong Peninsula. Detailed geochemical and geochronological analyses were conducted to determine the petrogenesis of the dikes and to infer their geodynamic setting. Zircon U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) indicates that the dikes were emplaced at ~126 Ma. The dikes are characterized by low SiO2 contents (44.3–52.3 wt.%), high contents of MgO (7.28–10.13 wt.%), Cr (267–652 ppm), and Ni (93–335 ppm), and high Mg# values (63–73); they are enriched in large ion lithophile elements (LILEs; Ba, K, and Sr), depleted in high field strength elements (HFSEs; Nb, Ta, P, and Ti), and are characterized by high (87Sr/86Sr)i isotope ratios (0.707226–0.708222), low εNd(t) values (?12.3 to ?13.6), and zircon εHf(t) values (?15.6 to ?23.6). These features suggest that the dikes were derived from enriched subcontinent lithospheric mantle (SCLM). The wide range of Rb/Sr (0.04–0.18) and Ba/Rb (5–34) ratios, and the low and limited range of Dy/Yb (1.93–2.52) and K/Yb (5.71–11.99) ratios of the dikes indicate that the magmas originated from a low degree of partial melting of an amphibole- and phlogopite-bearing lherzolite in the spinel–garnet transition zone. The parental magma might have experienced fractionation of olivine and clinopyroxene during its ascent without significant crustal contamination. Evident depletion of Nb–Ta and Zr–Hf, low and limited range of Th/Yb ratios, elevated Ba/La ratios, constant chondritic Zr/Hf ratios, and a large range of Hf/Sm ratios further indicate that the mantle sources of the dikes were altered by carbonate-related metasomatism from subducted slab-derived fluids, which were most likely related to subduction of the Palaeo-Pacific plate during the Mesozoic. The mafic dikes in the southeastern Jiaolai basin resemble the arc-like mafic dikes in the Jiaobei terrain and the Sulu orogenic belt, and possibly indicate lithospheric thinning induced by slab rollback of the Palaeo-Pacific plate.  相似文献   

3.
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   

4.
The development of Early Cretaceous mafic dikes in northern and southern Jiangxi allows an understanding of the geodynamic setting and characteristics of the mantle in southeast China in the Cretaceous. Geological and geochemical characteristics for the mafic dikes from the Wushan copper deposit and No. 640 uranium deposit are given in order to constrain the nature of source mantle, genesis and tectonic implications. According to the mineral composition,the mafic dikes in northern Jiangxi can be divided into spessartite and olive odinite types, which belong to slightly potassium-rich calc-alkaline lamprophyre characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), large depletion in high strength field elements (HSFE) and with negative Nb, Ta and Ti anomalies, as well as 87Sr/86Sr ratios varying from 0.7055 to 0.7095 and 143Nd/r44Nd ratios varying from 0.5119 to 0.5122.All features indicate that the magma responsible for the mafic dikes was derived mainly from metasomatic lithosphere mantle related to dehydration and/or upper crust melting during subduction. Differences in geochemical characteristics between the mafic dikes in northern Jiangxi and the Dajishan area, southern Jiangxi were also studied and they are attributed to differences in regional lithospheric mantle components and/or magma emplacement depth. Combining geological and geochemical characteristics with regional geological history, we argue that southeast China was dominated by an extensional tectonic setting in the Early Cretaceous, and the nature of the mantle source area was related to enrichment induced by asthenosphere upwelling and infiltration of upper crust-derived fluids responding to Pacific Plate subduction.  相似文献   

5.
Pillow lavas in Bompoka island of the Andaman–Nicobar islands, forming a part of Sunda–Burmese forearc, are composed of plagioclase and clinopyroxene microphenocrysts in a fine-grained ferruginous groundmass along with glass. They are also characterized by several quench plagioclase and clinopyroxene morphologies. Zr/TiO2 versus Nb/Y relationship of these pillow lavas show that these are tholeiitic basalts in composition. These basalts have low MgO (5.19–6.12 wt%), Ni (84–118 ppm), and Cr (144–175 ppm) abundance and high FeO(T)/MgO (1.71–1.92) ratios, reflecting their fractionated nature. In Th/Yb versus Nb/Yb and Ti/Yb versus Nb/Yb binary diagrams, they show N-MORB affinity. However, La/Nb–Y and Ce/Nb–Th/Nb relationships along with a slight LREE depleted (LaN/YbN = 0.75–0.82) pattern and high Ba/Zr (0.28–0.40) ratios and LILE (K, Rb, Ba, Sr and Th) enrichment relative to N-MORB, suggest their back-arc basin basalt affinity. It is inferred that these pillow basalts have been derived from a metasomatised N-MORB-like mantle source in a trench-distal (wider) back-arc basin, probably near the leading edge of the Eurasian continent during Early to Late Cretaceous times, prior to the currently active Andaman–Java subduction system.  相似文献   

6.
The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive εNd(t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures (εNd(t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd–Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high εNd(t) values (+6.3 to +6.8) and low ISr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic.  相似文献   

7.
新疆哈拉奇地区出露的辉绿岩岩脉为研究西南天山地区的区域大地构造演化提供了重要信息。哈拉奇地区辉绿岩岩脉亏损高场强元素Nb、Ta和重稀土元素Yb、Y,富集大离子亲石元素Ba、Sr、K和轻稀土元素La、Ce,具亏损地幔的元素地球化学特征。可以理解为亏损的地幔在板块俯冲时受到板块俯冲改造,表明在晚石炭-早二叠纪时工作区应处于岛弧环境。  相似文献   

8.
The Aqishan-Yamansu belt in the Eastern Tianshan (NW China) contains many intermediate to felsic intrusive rocks and spatially and temporally associated Fe (-Cu) deposits. Zircon U-Pb dating of the Bailingshan granitoids, including diorite enclaves (in granodiorite), diorite, monzogranite and granodiorite, and andesitic tuff from the Shuanglong Fe-Cu deposit area yielded ages of 329.3 ± 2.1 Ma, 323.4 ± 2.6 Ma, 313.0 ± 2.0 Ma, 307.5 ± 1.7 Ma and 318.0 ± 2.0 Ma, respectively. These new ages, in combination with published data can be used to subdivide magmatism of the Bailingshan intrusive complex into three phases at ca. 329–323 Ma, ca. 318–313 Ma and ca. 308–297 Ma. Of the analyzed rocks of this study, the Shuanglong diorite enclave, diorite and andesitic tuff show calc-alkaline affinities, exhibiting LILE enrichment and HFSE depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg# values, with depleted εHf(t) and positive εNd(t) values, similar crustal-derived Nb/Ta and Y/Nb ratios, low Th/Yb and Th/Nb, and high Ba/La ratios, which are consistent with them being sourced from a depleted mantle wedge metasomatized by slab-derived fluids and crustal contamination. However, the monzogranite and granodiorite are metaluminous with characteristics of low- to high-K calc-alkaline I-type granites. The granitic rocks are enriched in LILE, depleted in HFSE and have significant Eu anomalies, with high Y contents and low Sr/Y ratios, resembling typical of normal arc magmas. Depleted εHf(t) and positive εNd(t) values with corresponding young TDMC ages of zircons, as well as Nb/Ta, Y/Nb, Th/U and La/Yb ratios suggest that the granitic rocks were probably formed by re-melting of juvenile lower crust or pre-existing mantle-derived mafic–intermediate igneous rocks. Integrating published data, we conclude that the Bailingshan granitoids (excluding the Shuanglong diorite and diorite enclave) were derived from re-melting of juvenile lower crust and mantle-derived mafic–intermediate igneous rocks, with mantle components playing a more prominent role in the formation of the younger and more felsic rocks. A comprehensive review, including our new data, suggests that the Aqishan-Yamansu belt formed as a fore-arc basin during the Carboniferous (ca. 350–300 Ma) when the Kangguer oceanic slab subducted beneath the Yili-Central Tianshan block. The ongoing southward subduction of the slab resulted in the closure of the Aqishan-Yamansu fore-arc basin (ca. 320–300 Ma), due to slab steepening and rollback followed by slab breakoff and rebound. During the Aqishan-Yamansu fore-arc basin inversion, the main phase of the Bailingshan granitoids emplaced in the Aqishan-Yamansu belt, accompanied by contemporary Fe and Fe-Cu mineralization.  相似文献   

9.
基性岩墙,与层状、环状基性杂岩体和高Ti、低Ti玄武岩共同组成了峨眉山大火成岩省岩石组合.为进一步确定大火成岩省及相关生物灭绝事件的时间联系,及更深化研究大火成岩省的成因,对分布于贵州省南部的基性岩墙进行了主、微量元素、Sr-Nd同位素测定和锆石SHRIMP U-Pb年代学研究.黔南基性岩墙∑REE=135.66×10-6~280.59×10-6,LREE/HREE为6.42~7.54,（La/Yb）N为7.94~9.85,轻重稀土分异明显,δEu为1.0~1.3,具有Ba、Sr、K等LILE富集,Nb、Ta、Zr、Hf等HFSE亏损特征,显示与峨眉山高钛玄武岩相似的地球化学特征.Th/Ta（1.80~1.94）、Nb/U（30.8~39.88）、Th/La（0.08~0.10）、Nb/Th（7.89~8.40）比值与原始地幔相似,较低的初始（87Sr/86Sr）i比值（0.705 278~0.706 052）、εNd（t）（-0.5~+1.6）、以及Th/Ta比值（< 2.13）显示岩浆无明显的地壳混染,岩浆可能形成于受地幔柱作用的富集石榴石地幔源区10%~12%的部分熔融.SHRIMP锆石206Pb/238U加权平均年龄为261.2±2.6 Ma,反映峨眉山大火成岩的喷发时间可能集中在260 Ma左右,并可能与瓜德鲁普末期的生物灭绝有关.   相似文献   

10.
Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.  相似文献   

11.
The Singhbhum Orissa craton, eastern India contains rocks as old as 3.6 Ga. The Newer Dolerites occur in two distinct orientations (NE/SW and NW/SE) in the Singhbhum Granitoid Complex (SBGC). These dikes are mostly tholeiites and quartz-normative dolerites associated with subordinate norites. We recognize three geochemical groups of the Newer Dolerites that were emplaced in the SBGC. Group I dikes contain lower SiO2 ( < 53.29%) and higher Mg #, Ni and Cr than group II dikes. Group III dikes have higher SiO2 than groups I and II. A few investigated samples show boninitic geochemical features. They have high-MgO (>8%), high-SiO2 (>52%) and low-TiO2 ( ≤ 0.5%) bulk-rock compositions. The main feature of the Newer Dolerite spidergrams is enrichment in the large-ion lithophile elements (LILE, e.g. Rb, K and Ba) relative to high field-strength elements (HFSE), resulting in high LILE/HFSE ratios. These geochemical characteristics suggest that the Newer Dolerites are subduction related. High La/Ta ratios (21–66) support a non-plume source. Therefore, we conclude that the Newer Dolerites show geochemical signatures similar to those of back-arc basalts.  相似文献   

12.
This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (Lan/Smn – 0.83 to 4.58) and (Nbn/Lan – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Ybn = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.  相似文献   

13.
The magmatic arc of the Fuegian Andes is composed mostly of Upper Mesozoic to Cenozoic calc-alkaline plutons and subordinated lavas. To the rear arc, however, isolated mid-Cretaceous monzonitic plutons and small calc-alkaline dykes and sills crop out. This calc-alkaline unit (the Ushuaia Peninsula Andesites, UPA) includes hornblende-rich, porphyritic quartz meladiorites, granodiorites, andesites, dacites and lamprophyres. Radiometric dating and cross-cutting relationships indicate that UPA is younger than the monzonitic suite. The geochemistry of UPA is medium to high K, with high LILE (Ba 500–2000 ppm, Sr 800–1400 ppm), HFSE (Th 7–23 ppm, Nb 7–13 ppm, Ta 0.5–1.1 ppm) and LREE (La 16–51 ppm) contents, along with relatively low HREE (Yb 1.7–1.3 ppm) and Y (9–19 ppm). The similar mineralogy and geochemistry of all UPA rocks suggest they evolved from a common parental magma, by low pressure crystal fractionation, without significant crustal assimilation. A pure Rayleigh fractionation model indicates that 60–65% of crystal fractionation of 60% hornblende + 34% plagioclase + 4% clinopyroxene + 1% Fe-Ti oxide, apatite and sphene (a paragenesis similar of UPA mafic rocks) can explain evolution from lamprophyres to dacites. The UPA has higher LILE, HFSE and LREE, and lower HREE and Y than the calc-alkaline plutons and lavas of the volcanic front. The HREE and Y are lower than in the potassic plutons as well. High concentrations of Th, Nb, Ta, Zr, Hf, LREE and Ce/Pb, and low U/Th, Ba/Th ratios in UPA, even in the least differentiated samples, suggest contributions from subducted sediments to the mantle source. On the other hand, relatively low HREE and Y, high LREE/HREE (La/Yb 11–38) ratios and Nb-Ta contents can be interpreted as mantle metasomatism by partial melts of either subducted garnetiferous oceanic sediment or basalt as well. Additionally, high LILE content in UPA, similar to the potassic plutons, suggests also a mantle wedge previously metasomatized by potassic parental magmas in their route to crustal levels. Therefore, UPA represents a unique suite in the Fuegian arc generated in a multiple hybridized source. UPA generation is related to a transition from normal to flat subduction which additionally caused the widening and landward migration of the magmatic arc, as well as crustal deformation. Rear-arc magmatism endured ca. 22 m.y.; afterwards, calc-alkaline magmatism remained at the volcanic front.  相似文献   

14.
湘东北中生代基性岩脉岩石地球化学及构造意义   总被引：8，自引：1，他引：8  
湘东北中生代发育以辉绿岩类和煌斑岩类为代表的基性岩脉,属陆内拉斑玄武岩系,部分煌斑岩属于碱性系列。岩石富集LREE,δEu负异常不明显,其形成主要受软流圈地幔部分熔融作用制约。煌斑岩类微量元素总体上具有洋岛玄武岩(OIB)岩浆源区特征,富集Nd、P、Cs而K、Rb、Sr、U、Th等富集程度不明显,Ta、Nb略有富集。辉绿岩类表现出Ta、Nb、Ti亏损,但LILE并不富集,反映地壳混染程度的增强。基性岩脉形成于陆内拉张带的构造环境,岩浆活动未受到中生代大洋板块俯冲的影响。基性岩脉在时、空及物质组成上与湘东南玄武质岩石基本一致,属于整个湘东南岩石圈拉张-减薄带的一部分。  相似文献   

15.
Mafic dike–granite associations are common in extensional tectonic settings and important and pivotal in reconstructing crust–mantle geodynamic processes. We report results of zircon U–Pb and hornblende 40Ar-39Ar ages and major-element and trace-element data for mafic dike–granite association from the northern West Junggar, in order to constrain their ages, petrogenesis, and geodynamic process. The mafic dike–granite association was emplaced in the early Devonian. The Xiemisitai monzogranites have high SiO2 contents and low MgO, Cr, and Ni concentrations, suggesting that they were mainly derived from crustal sources and were probably generated by partial melt of the juvenile mid-lower crust. The mafic dikes have low Mg# and Cr and Ni abundances, suggesting that they have experienced significant fractional crystallization. The Xiemisitai mafic dikes contain hornblende and biotite and display negative Nb–Ta–Ti anomalies, enrichment of LREEs and LILEs, and depletion of HREEs and HFSEs, consistent with an origin from a lithospheric mantle metasomatized by subducted slab-derived fluids. In addition, the Xiemisitai mafic dikes are plotted within melting trends with little to no garnet (Cpx: Grt = 6:1) in their source. The La/Yb versus Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai mafic dikes. Therefore, it can be inferred that the Xiemisitai mafic dikes were generated at a correspondingly shallow depth, mostly within the spinel stability field. The Xiemisitai mafic dikes were most probably generated by the partial melting of the metasomatized lithospheric mantle at relatively shallow depths (<80 km). The Xiemisitai mafic dike–granite association could have been triggered by asthenospheric upwelling as a result of the rollback of the subducted Irtysh–Zaysan oceanic lithosphere.  相似文献   

16.
Geochemical studies on the arnphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The arnphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N=0.33-0.55, (La/Sm)N= 0.45-0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N-MORB. The second group of rocks have (La/Yb)N=0.63-0.95, (La/ Sm)N = 0.69--0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N-MORB and E-MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in th  相似文献   

17.
Singhbhum granitoid complex has been intruded by numerous mafic dykes trending in different directions. The studied dykes were originated from subalkaline magma, ranging in composition from basalt through basaltic-andesite to andesite. In the present work, the studied dykes are divided into two groups on the bases of abundance of SiO2, MgO, Al2O3, Ni, and Cr. The first one has higher Mg# than that of group II. These dykes have enriched incompatible trace element patterns. These are particularly enriched in the light rare earth elements and large ion lithophile elements with depleted high field strength elements (Nb, P, Ti). High Ba/Nb and Sr/P ratios of present mafic dykes are the indications of subduction signature. The geochemical characters of the back arc extension basalt tectonic setting is suggested for the studied dykes. Higher PM-normalized Th/Nb, Rb/Nb, and Ba/Nb ratios of studied samples support their non-plume source.  相似文献   

18.
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   

19.
《Gondwana Research》2016,29(4):1466-1481
Early Carboniferous volcanic rocks in the Batamayineishan Formation overlie unconformably the molasse deposits and the ophiolitic mélanges and are restricted in narrow zones along both sides of the Kalamaili orogenic belt in North Xinjiang, southern Central Asian Orogenic Belt. These rocks demonstrate the post-collisional setting in East Junggar commenced in Tournaisian and also mark an important transitional period from the final amalgamation to late Paleozoic voluminous juvenile granitoids in East Junggar. The volcanic rocks are composed of basalt, basaltic andesite, andesite, trachyte and rhyolite. Both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements, light rare earth elements and depletion in Nb and Ta, low initial 87Sr/86Sr and high, positive ɛNd(t). Three groups of mafic rocks have been identified: Shoshonitic group 1 has the highest MgO, CaO, Ni and Cr and the lowest Na2O, Al2O3, La, Ba, La/Yb and Ba/Th with primary magma features; group 2 calc-alkaline and high-K calc-alkaline mafic rocks have the lowest K2O, P2O5, Th and Th/Nb, and the highest TiO2; and group 3 (shoshonitic to potassic alkaline) has the highest K2O, P2O5, La, Ba, La/Yb and Th/Nb, and the lowest TiO2. The A-type-like felsic rocks were derived from the differentiation of the mafic magma. Geological and geochemical evidences indicate that the Batamayineishan Formation was generated from the process of slab breakoff (detachment). Group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet (50:50) lherzolite which has been enriched by overlying metasomatized lithosphere during ascent. Group 2 is derived from 5–10% partial melting of shallower spinel-bearing lithospheric mantle induced by the hot rising asthenosphere, where the contribution of slab-derived fluid is predominant. Low partial melting (3–5%) of the mantle wedge and/or thickened lithospheric mantle enriched by slab-derived components generates group 3. Slab breakoff as an important geodynamic process accounts for the post-collisional magmatism between 343.5 Ma–330 Ma, providing a model for post-collisional crust–mantle interaction in the CAOB.  相似文献   

20.
红柳河地区下二叠统拉斑玄武岩,主要表现为低Mg和全碱(Na2O+K2O)以及高Fe和Ti的特征,微量元素Rb,K,Sr和Ba等大离子亲石元素富集,轻稀土元素(LREE)相对富集。La／Yb=10．61—16．87,Th/Ta=1．03—1．99,Fe/Ti=5．30—6．39,Ti／P=5．61—7．30,Ti/Zr=38．83～46．30;Zr=(254～328)×10^-6,Zr／Y=5—10,Y／Nb=1．52—2．45。这些地球化学特征表明它们主要为板内玄武岩,其岩浆主要源于亏损地幔(DM)和富集地幔(EM)组分的混合,其中富集地幔(EM)为主要端元,且部分熔融程度低,受很低的地壳混染作用。  相似文献   

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