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1.
Magmatism in central Anatolia is characterized by petrographically and chemically distinct granitic and syenitic rocks. The granitic magmatism comprises C-type (crustal-derived) and H-type (hybrid) monzogranites and monzonites. Garnet-bearing C-type leucogranites represent the oldest magmatic phase, but younger hornblende ± biotite ± K-feldspar H-type plutons dominate the geology of the Central Anatolian Crystalline Complex (CACC). These typically include mafic microgranular enclaves. The granitic magmatism predates syenitic intrusions, among which quartz-bearing syenites were emplaced prior to feldspathoid-bearing ones.

The nature of magmatism in central Anatolia varies through time from peraluminous to metaluminous to alkaline. These different magma types reflect distinct stages of postcollisional magmatism, in which interaction between crust and mantle varied considerably. The C-type granites of the early stages of postcollisional magmatism were likely derived by partial melting of the lower continental crust induced by mafic magma underplating as a result of lithospheric delamination. The H-type granites and syenites of the mature and advanced stages of postcollisional magmatism indicate a significant contribution from mande-derived magma within a continuous or episodic extensional tectonic regime.  相似文献   

2.
We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension  相似文献   

3.
骆文娟  张招崇  侯通  王萌 《岩石学报》2011,27(10):2947-2962
茨达复式岩体位于中国西南扬子地台西缘的攀西裂谷内,其岩性从基性到酸性连续变化,SiO2含量为40.06％ ～68.54％,但以基性和酸性岩石为主,中性岩石较少,而且非常不均匀,通常具有斑杂构造特征.从基性岩到酸性岩,各岩石样品由轻稀土弱富集型变为较强富集型.微量元素表现为酸性岩中Rb、Th、K、La、Ce、Pb、Nd、Zr、Hf、Sm呈正异常和Ba、Nb、Ta、Sr、P、Ti的负异常;基性岩除Ti负异常和Pb正异常外,其它异常不明显;中性岩具有Ti、Sr负异常和Pb正异常,其它特征介于基性岩和酸性岩石之间.野外和岩相学特征明显指示出中性岩石具有混合特征.酸性端元岩浆准铝质的特征以及相对低的SiO2含量指示其起源于玄武质下地壳的部分熔融,而基性端元岩浆的地球化学特征以及高温特征暗示着其起源于地幔柱源区.锆石U-Pb年龄数据表明,该复式岩体中基性端元LA-MC-ICP-MS U-Pb锆石年龄为243.76±0.77Ma,酸性端元年龄为240.5±0.76Ma,可能代表了峨眉山大火成岩省岩浆活动的尾声阶段.  相似文献   

4.
报道了滇东南个旧超大型锡多金属矿区西区北部白云山碱性岩新的锆石U-Pb年龄、全岩地球化学和Sr-Nd同位素数据。LA-ICP-MS锆石U-Pb定年结果表明,白云山碱性正长岩形成于晚白垩世（80.0±0.6 Ma）,与个旧地区的中基性岩及花岗岩均为同一次构造岩浆事件的产物;碱性正长岩与霞石正长岩具有相似的主微量元素地球化学特征及Sr-Nd同位素组成,暗示二者很可能是源于同一富集地幔源区并经历了不同程度演化的产物。结合已有的元素和同位素组成结果,认为碱性岩、中基性岩和成矿花岗岩很可能分别源自富集的岩石圈地幔、正常的岩石圈地幔和地壳源区。在晚白垩世伸展构造背景控制下,源于不均一岩石圈地幔的碱性和中基性的岩浆底侵,促使中下地壳岩石部分熔融形成花岗质熔体,在上升至近地表过程中引起构造活动带成矿物质的富集,从而形成个旧超大型锡多金属矿床的矿化格局。可以说,源于富集地幔的碱性岩浆在含矿花岗质岩浆的成岩成矿过程中,应不只是提供热量的贡献。  相似文献   

5.
后石湖山杂岩体是与垮塌破火山口有关的碱性环状杂岩体, 主要由呈环形分布的碱性火山岩、环状岩墙(斑状石英正长岩)、嵌套的中心复式岩株(晶洞碱长花岗岩和斑状碱长花岗岩)和锥状岩席(石英正长斑岩和花岗斑岩)组成.LA-ICPMS锆石U-Pb年代学分析表明, 斑状石英正长岩环状岩墙、石英正长斑岩和花岗斑岩锥状岩席的侵位年龄分别为119±3Ma、121±2Ma和121±2Ma.该环状杂岩体火山岩与侵入岩的形成年龄相近, 体现了它作为火山-侵入杂岩体的特征.斑状石英正长岩富碱(Na2O+K2O=10.0%~10.5%), K2O含量较高(5.21%~5.42%), 具正的Eu异常(Eu/Eu*=1.05~1.40).碱长花岗岩和斑岩均具有富碱、高FeOtot/MgO、Ga/Al、Zr、Nb和REE值(Eu除外), 以及低Al2O3、CaO、MgO、Ba、Sr和Eu含量的特征, 都属于A型花岗岩质岩石.其中斑岩为铝质A型花岗岩, 具有高的初始岩浆温度(880~901℃).所有A型花岗质岩石均具有较富集的Nd同位素组成, εNd(t)值变化于-13.9~-12.2之间.斑状石英正长岩是下地壳中-基性麻粒岩和片麻岩部分熔融产生的熔体与幔源玄武质岩浆混合, 后又发生单斜辉石分离结晶的产物; 碱长花岗岩源于上地壳长英质岩石部分熔融产生的熔体与幔源玄武质岩浆混合, 随后经历长石的分离结晶作用而成; 斑岩是受幔源岩浆底侵加热的上地壳长英质岩石的部分熔融产生的熔体, 并经历了长石的分离结晶作用而产生.该环状杂岩体的形成过程可以概括为: (1)火山爆炸性喷发形成大量的碱性火山熔岩和火山碎屑岩; (2)地下岩浆房空虚导致压力下降, 其顶板围岩失稳而沿火山口周围近直立的环状断裂垮塌, 形成塌陷的破火山口.与此同时, 下覆岩浆房的岩浆被动挤入环状断裂而形成斑状石英正长岩环状岩墙; (3)浅部地壳的长英质岩浆房过压, 促使其高温过碱质A型花岗质岩浆上升侵位形成了中心的斑状碱长花岗岩岩株, 这些岩浆的上涌导致上覆围岩产生倾角中-陡的、内倾的锥状裂隙, 为石英正长斑岩锥状岩席侵位提供了空间; (4)浅部岩浆房复活, 高温过碱质A型花岗质岩浆再度上升侵位形成被嵌套的晶洞碱长花岗岩岩株.同样, 这种岩浆的再度上侵导致上覆围岩产生了倾角较陡而内倾的锥状裂隙, 为花岗斑岩锥状岩席提供了侵位空间.后石湖山碱性环状杂岩体的形成是华北东部早白垩世与克拉通破坏相关的伸展构造体制下的产物, 这种构造体制可能与古太平洋板块的俯冲作用有关.   相似文献   

6.
Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial Nd(≈ 0), whereas a few samples trend towards Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.  相似文献   

7.
塔里木巴楚小海子正长岩杂岩体的岩石成因探讨   总被引：6，自引：4，他引：2  
位荀  徐义刚 《岩石学报》2011,27(10):2984-3004
巴楚小海子正长岩杂岩体是二叠纪塔里木大火成岩省的重要组成部分.SIMS锆石U-Pb定年显示其形成于279.7±2.0Ma,与本区辉绿岩脉和石英正长斑岩岩脉近于同时侵位.根据矿物学特征,小海子正长岩体可分为铁橄榄石正长岩和角闪正长岩两类.前者主要由碱性长石、铁橄榄石、单斜辉石、角闪石和少量石英、斜长石组成,后者主要由碱性长石、角闪石、黑云母和少量的石英、斜长石组成.小海子正长岩体为铁质、碱性系列,轻稀土相对富集,重稀土亏损,具有明显的Eu正异常,无Nb、Ta负异常,相对低的(87Sr/86Sr);(0.7033 ～0.7038)和正的εNd(t)值(+3.1～+3.8),暗示它们来自亏损的地幔源区,没有地壳物质的加入.主微量和同位素地球化学分析,暗示巴楚小海子正长岩的母岩浆为碱性的幔源玄武质岩浆经橄榄石、单斜辉石分离结晶后的残余熔体,并且含有堆晶的碱性长石.这种含有碱性长石堆晶的熔体,在相对还原的条件下结晶,形成铁橄榄石正长岩;在相对氧化的条件下结晶,并经过不同程度斜长石的分离结晶形成角闪正长岩.  相似文献   

8.
The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinel inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc.  相似文献   

9.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.  相似文献   

10.
We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30–40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.  相似文献   

11.
广西东南部罗容杂岩体由辉长岩－闪长岩－二长岩－正长岩组成，马山由碱性辉长岩－正长岩－花岗闪长岩－花岗岩组成。它们富K、富大离子亲石元素（LIL），无Nb、Ta负异常等特征表明它们是形成于板内环境的钾玄质侵入岩。罗容杂岩体的各种岩石和马山的基性岩的εNd(T)稍低，为－0．6，此岩石单元是由幔源岩浆加热地壳使之熔融的壳源岩浆形成的，并伴有幔源岩浆的混合或交换。此二杂岩体形成于拉张构造环境。地幔物质上涌导致了中生代华南地壳张裂以及华南大规模花岗岩和相关矿床的形成。  相似文献   

12.
Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.  相似文献   

13.
The Burpala alkaline massif contains rocks with more than 50 minerals rich in Zr,Nb,Ti,Th,Be and rare earth elements(REE).The rocks vary in composition from shonkinite,melanocratic syenite,nepheline and alkali syenites to alaskite and alkali granite and contain up to 10%LILE and HSFE,3.6%of REE and varying amounts of other trace elements(4%Zr,0.5%Y,0.5%Nb,0.5%Th and 0.1%U).Geological and geochemical data suggest that all the rocks in the Burpala massif were derived from alkaline magma enriched in rare earth elements.The extreme products of magma fractionation are REE rich pegmatites,apatite-fiuorite bearing rocks and carbonatites.The Sr and Nd isotope data suggest that the source of primary melt is enriched mantle(EM-Ⅱ).We correlate the massif to mantle plume impact on the active margin of the Siberian continent.  相似文献   

14.
ABSTRACT

Appinite complexes preserve evidence of mantle processes that produce voluminous granitoid batholiths. These plutonic complexes range from ultramafic to felsic in composition, deep to shallow emplacement, and from Neo-Archean to Recent in age. Appinites are a textural family characterized by idiomorphic hornblende in all lithologies, and spectacular textures including coarse-grained mafic pegmatites, fine-grained ‘salt-and-pepper’ gabbros, as well as planar and linear fabrics. Magmas are bimodal (mafic-felsic) in composition; ultramafic rocks are cumulates, intermediate rocks are hybrids. Their geochemistry is profoundly influenced by a mantle wedge extensively metasomatized by fluids/magmas produced by subduction. Melting of spinel peridotite sub-continental lithospheric mantle (SCLM) produces appinites whose geochemistry is indistinguishable from coeval low-K calc-alkalic arc magmatism. Coeval felsic rocks within appinite complexes and adjacent granitoid batholiths are crustal magmas. When subduction terminates, asthenospheric upwelling (e.g. in a slab window, or in the aftermath of slab failure) induces melting of metasomatized garnet SCLM to produce K-rich sho shonitic magmas enriched in large ionic lithophile and light relative to heavy rare earth elements, whose asthenospheric component can be identified by Sm-Nd isotopic signatures. Coeval late-stage Ba-Sr granitoid magmas have a ‘slab failure’ geochemistry, resemble TTG and adakitic suites, and are formed either by fractionation of an enriched (shoshonitic) mafic magma, or high pressure melting of a meta-basaltic protolith either at the base of the crust or along the upper portion of the subducted slab. Appinite complexes may be the crustal representation of mafic magma that underplated the crust for the duration of arc magmatism. They were preferentially emplaced along fault zones around the periphery of the granitoid batholiths (where their ascent is not blocked by overlying felsic magma), and as enclaves within granitoid batholiths. When subduction ceases, appinite complexes with a more pronounced asthenospheric component are preferentially emplaced along active faults that bound the periphery of the batholiths.  相似文献   

15.
云南个旧碱性杂岩体由边缘相碱长正长岩和中心相霞石正长岩组成。全岩地球化学分析表明,该碱性杂岩体具有高碱、富钾、富铁、低镁、高分异的碱性-过碱性岩石特征,晚期更富集碱金属元素; LREE/HREE值为20～59,(La/Sm)N=8～50,(Sm/Yb)N=1.2～5.0,富集轻稀土元素,轻稀土元素较重稀土元素分馏程度高,具Eu负异常,亏损Ti、Nb、P、K、Sr等元素,富集Zr、Hf、Th、La、Ce、Nd、U、Rb等元素,岩浆来源与幔源物质有关;碱长正长岩和霞石正长岩具有相似的微量元素和稀土元素特征,具有同源岩浆分异演化的特点; Rb/Sr、Nb/Ta、Zr/Hf等比值均高于或接近于原始地幔的相应值; CIPW标准矿物计算结果表明边缘相碱长正长岩中出现紫苏辉石、锥辉石、橄榄石,中心相霞石正长岩中出现橄榄石。结合(Th/Nb)N和Nb/La值特征以及前人Sr-Nd同位素研究成果,认为个旧碱性杂岩体的岩浆来源于遭受交代作用的富集地幔部分熔融,同时受有限的地壳混染作用而成,形成于后碰撞的伸展环境。碱性岩浆演化晚期更加富碱、经历了更高程度的结晶分异作用是稀土元素、Nb、Ga和Zr元素超常富集的重要原因。  相似文献   

16.
Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust. Received: 26 September 1995 / Accepted: 5 June 1996  相似文献   

17.
The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments in large-ion lithophile elements relative to high field strength elements generally characteristic of subduction influence. We suggest that this signature is inherited from partial melting of a lithospheric mantle source previously affected by subduction during assembly of the Arabian-Nubian Shield. Little evidence of the early evolution of the suite is preserved; there are no associated mafic rocks. We therefore restrict our attention to a petrogenetic model that can explain the relations among the observed felsic composition. The REE patterns of all samples are enriched in light REE and fractionated, but it is notable that there are small positive Eu anomalies in the alkali-feldspar syenites contrasting with small negative Eu anomalies in the quartz syenites. Positive Eu anomalies suggest a cumulate nature for the alkali-feldspar syenites; there are also breaks in the slopes of most variation trends between the alkali-feldspar syenites and the quartz syenites. The general trends in all major oxides and trace elements within the suite can be modeled by fractional crystallization of feldspars—with smaller roles for pyroxene, biotite, apatite, and Fe-Ti oxides—from an intermediate liquid to form the quartz syenites and by assimilation of the near-liquidus phases into the same starting liquid to form the alkali feldspar syenites. The geothermobarometry of pyroxenes and amphiboles suggests shallow emplacement (<10 km depth) and crystallization temperatures ranging from 1100 °C down to 800 °C.  相似文献   

18.
The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.  相似文献   

19.
An early Cretaceous alkaline ultramafic-mafic complex is emplaced within the Proterozoic rocks of Shillong plateau at Jasra, Karbi Anglong district of Assam. It is associated to the fracture system of Barapani-Tyrsad shear zone, Kopali faults, and Um Ngot lineaments and mainly comprises pyroxenite, gabbro and nepheline syenite. Few small mafic dykes, emplaced within pyroxenitic and granitic plutons, are also reported. No such dyke is reported to cut gabbros or nepheline syenites. Nepheline syenites occur either in the form of small dykes in pyroxenites or as differentiated bodies in the gabbros. Mineralogical and chemical composition of pyroxenite and gabbro clearly indicate their affinity to the alkaline magmatism. Syenitic samples show miaskitic character (agpaitic index <1), also indicates affinity with alkaline-carbonatite magmatism. Calcite is encountered in a number of pyroxenite samples. From the presented petrological and geochemical data it is difficult to establish any significant genetic relationship through simple differentiation process between these rocks. These data probably suggest that these rocks are derived from a primary carbonatite magma, generated by the low-degree melting of a metasomatized mantle peridotite. CO2 released by this process also progressively metasomatizes the lherzolite to an alkaline wehrlite and melts derived from alkaline wehrlite (ultrabasic alkaline silicate magma) may be responsible for crystallization of Jasra alkaline ultramafic-mafic rocks.  相似文献   

20.
Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai. Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma) in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenite subvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry of these rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enriched in TiO2, Fe2O3, MgO, CaO, Sr, Ba and depleted in SiO2, Nb, Y, and Rb. The G. Tarbush alkaline suite most likely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) from a common quartz syenite parental magma, which formed via partial melting of middle crustal rocks of ANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting. The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation could explain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late- to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related to crustal uplift during the late-collision stage could account for the lack or infrequent occurrence of older lithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.  相似文献   

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