Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia)     

P. Barbey  D. Gasquet  C. Pin  A.L. Bourgeix   《Lithos》2008,104(1-4):147-163

This study deals with the origin of igneous layering in plutons, and, especially, the extent layering is related to mafic–silicic magma interactions. The Budduso pluton (Sardinia) shows three main scales of organization.(i) Large scale lithological variations correspond to three main magmatic units, with differentiation increasing from the Outer (hornblende-bearing biotite granodiorite/monzogranite) to the Middle (biotite monzogranite) and the Inner (leucomonzogranite) units. The striking homogeneity of ⁸⁷Sr/⁸⁶Sr initial ratios (0.7090 ± 4) and ε_Nd(t) values (− 5.6 ± 0.1) strongly suggests that magma isotopic equilibration was achieved prior to emplacement, whereas mixing/mingling structures observed within the pluton reflect second-stage processes involving broadly cogenetic components.(ii) Metre to decametre-scale igneous layering may be isomodal or modally-graded, locally with cross-layering. Biotite and plagioclase compositions are similar in both biotite-rich and quartzofeldspathic layers, as are the trace-element patterns which differ only by relative abundances. This precludes an origin by fractional crystallization. A penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation.(iii) Composite layering and schlieren are commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones or disrupted synplutonic dykes. In that case, there is a progressive shift in biotite X_Fe values from the core of enclave ( 0.65) to the host monzogranite ( 0.72): schlieren in the monzogranite show biotite X_Fe values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower X_Fe values ( 0.69) towards those of enclave rims.These features can be ascribed to three main processes: (i) assembly of differentiated (± mixed/mingled) magmatic pulses; (ii) local hydrodynamic sorting related to density currents in a mush, and segregation of residual melt; (iii) mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics. None of these processes affect the whole pluton but they are limited to specific magmatic units. Therefore, pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes.  相似文献   

Geochemistry of the mafic dykes in the Prakasam Alkaline Province of Eastern Ghats Belt, India: Implications for the genesis of continental rift-zone magmatism     

K. Vijaya Kumar  K. Rathna 《Lithos》2008,104(1-4):306-326

Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.  相似文献   

Post-collisional plutonism with adakitic signatures： The Triassic Yangba granodiorite （Bikou terrane,northern Yangtze Block）     

QIN Jiangfeng  LAI Shaocong  and LI Yongfei State Key Laboratory of Continental Dynamics  Department of Geology  Northwest University  Xi’an  China 《中国地球化学学报》2008,27(1):72-81

The post-collisional Yangba granodiorite intruded into the Bikou metasedimentary-volcanic group, southern Mianlue Suture, central China. The host granodiorites contain many mafic microgranular enclaves which have acicular apatite, phenocrysts of host granodiorites, implying that the enclaves have been incorporated as magma globules into host granodioritic magma and undergone rapid cooling. The variation trends of major and trace elements between enclaves and host rocks suggest a mixing and mingling process with respect to their petrogenesis. The mafic microgranular enclaves are characterized by shoshonite with SiO2≤〈63%, σ （4.54-6.18）〉3.3, high K2O content （4.22%-6.04%）, K2O/Na2O〉1; in the K2O-SiO2 diagram, all the samples plot in the shoshonite field, which are enriched in LILE and LREE, with obvious Nb, Ta negative anomalies, indicating a subducting fluid-metasomatised mantle source. Zircon LA-ICP-MS dating of the granodiorites yielded an age of 215.4±8.3 Ma, indicating they were formed during the late-orogenic or post-collisional stage （≤242±21 Ma） of the South Qinling Mountain Belt. The host granodiorites have many close compositional similarities to high-silica adakites from supra-subduction zone setting, but tend to have a higher concentration of K2O （3.22%-3.84%） and Mg^#. Chondrite-normalized rare-earth element patterns are characterized by high ratios of （La/Yb）N, the extreme HREE depletion and a lack of significant Eu anomalies. In conjunction with the high abundances of Ba and Sr as well as the low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionation mineral assemblage. High Mg^# values demonstrate that the host granodiorites were contaminated by enclave magma. On a whole, integrated geological and geochemical studies suggested the Yangba granodiorites and their mafic microgranular enclaves resulted from mixing of enriched mantle-derived shoshonitic magma and thickened lower crust-derived felsic magma. In combination w  相似文献   

Microgranitoid enclave swarms in granitic plutons, central Sierra Nevada, California   总被引：4，自引：0，他引：4  

O.T. Tobisch  B.A. McNulty  R.H. Vernon 《Lithos》1997,40(2-4):321-339

Microgranitoid enclaves are common in granitic plutons worldwide, occurring individually and in homogeneous or heterogeneous swarms. Three plutons in the central Sierra Nevada batholith contain swarms with mostly heterogeneous suites of enclaves in the intermediate composition range, and occur in a number of two-dimensional shapes, specifically as dikes, small rafts, lenses, pipe/vortices and large massive shapes. Swarms are characterized by various features, including the nature of their boundary with the host, their planar or non-planar character, internal geometry, density of enclave packing, presence or absence of schlieren and crystal aggregates, and axial ratios and degree of preferred alignment of enclaves. We propose that heterogeneous enclave swarms form by one, or some combination of, the following mechanisms: (1) velocity-gradient sorting parallel or normal to the flow, (2) gravitational sorting or (3) break-up of heterogeneous dikes. Common sites where enclave swarms form include pluton margins or internal viscosity walls, within fractures, and near the pluton roof.  相似文献   

广西大宁花岗闪长岩体中暗色微粒包体的地球化学及成因模式     

沙连堃  袁奎荣 《地球科学》1991,(4)

本文通过对广西大宁花岗闪长岩体及其中的暗色微粒包体的岩石学、微量元素及稀土元素地球化学、Rb—Sr同位素及微量元素初始比值等研究,认为该岩体中的暗色微粒包体属于幔源物质,同时提出了包体与主体花岗岩成因的上地幔—地壳相互作用模式,强调了花岗岩浆形成过程中系统的开放性及上地幔基性岩浆参与的重要性。  相似文献   

陕西商县秦王山镁铁质岩体的特征及成因   总被引：1，自引：0，他引：1  

张稳胜  安三元 《岩石学报》1990,6(4):59-66

秦王山镁铁质岩体是一在动荡的造山环境里形成的同构造异地岩块。岩体的普通铅模式年龄为407±3Ma,锶同位素初始比为0.7045,δ~(18)O=7.92,稀土配分式样为比球粒陨石富集的右倾型。岩石是在T=939℃,P=2.5×10~8Pa下结晶的。  相似文献   

Living the good life? The rise of urban mixed-use enclaves in Metro Manila     

Jana Maria Kleibert  Lisa Kippers 《Urban geography》2016,37(3):373-395

Gated communities and other forms of privatized enclaves have become features of many cities. The rise of urban mixed-use enclaves, which integrate functions of work, entertainment and living, is a relatively new and understudied phenomenon that is expanding rapidly throughout the Global South. In this article, we advance the concept of enclave urbanism through a case study of Eastwood City in Metro Manila, the Philippines. We trace the enabling factors for mixed-use enclave creation and assess how the different functions are marketed and integrated in practice into one “live–work–play” development. Based on extensive qualitative research, we argue that the global capital accumulation strategy of real estate developers leads to a strong focus on production and entertainment functions, but neglects questions of liveability. We reflect on the implications of our findings for mega-urban regions and urban studies research.  相似文献   

Petrology,phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex,North China Craton          下载免费PDF全文

L. Tang  M. Santosh  T. Tsunogae  T. Koizumi  X.‐K. Hu  X.‐M. Teng 《Journal of Metamorphic Geology》2017,35(5):517-540

The Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H₂O contents from P/T–M(H₂O) diagrams define the retrograde P–T conditions of <6.1 kbar, <795 °C (15FP‐02), 5.6–5.8 kbar, <795 °C (15FP‐03), and <9 kbar, <865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.  相似文献   

Multi-stage pseudomorphic replacement of garnet during polymetamorphism: 1. Microstructures and their interpretation     

Cesare 《Journal of Metamorphic Geology》1999,17(6):723-734

Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high- P /low- T  metamorphism overprinted high- T  assemblages that contained sillimanite or andalusite.  相似文献   

Early Paleozoic lithospheric extension law,dynamic mechanism,origin of mafic dykes in Ziyang,South Qinling Block,ChinaSCIEI北大核心CSCD     

刘燊  冯彩霞  陈晓青  赵慧博  徐静妍  范焱 《岩石学报》2023,(3):938-962

南秦岭地块紫阳地区广泛出露早古生代基性岩墙群,其研究具有重要的地球动力学意义。虽然如此,已有相关研究目前主要集中于志留纪基性岩墙群方面,而对早古生代(如,寒武纪、奥陶纪)基性岩墙群的研究仍相对薄弱。从而制约了对南秦岭早古生代岩石圈伸展过程相关问题(如,时空分布规律、地幔性质、动力学机制和相关成矿作用)的总体把握。鉴于尚存的科学问题,本研究选取康家坪、梨树梁、大竹坪、清明寨、苟家山、庙梁上、曾家山和毛坝村的基性岩墙群为研究对象,开展了矿物学、岩石学、锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素方面的研究,对其精细时代格局、成因及成岩动力学背景进行了系统探讨。研究结果显示,紫阳早古生代基性岩墙以辉绿岩和辉绿玢岩为主,形成时代为奥陶纪(478.8-486Ma),属碱性系列岩石(K_(2)O+Na_(2)O变化范围为4.10%-5.14%)。另外,基性岩墙群具有富集LREE、Rb、Ba、Sr、Nb、Ta、Zr、Hf和Eu(Eu/Eu*=1.13-1.35),亏损HREE、U、Pb和Ti的典型地球化学特征。(87Sr/86Sr)i=0.7044-0.7050、εNd(t)=3.1-3.6、εHf(t)=4.4-12.1,176Hf/177Hf=0.282634-0.282846,暗示基性岩墙为地幔柱作用过程亏损岩石圈或软流圈地幔部分熔融作用的产物。另外,在辉绿岩成岩过程经历了橄榄石和单斜辉石的分离结晶,但地壳混染的影响不明显。  相似文献