^(18)O/^(16)O and D/H studies on the interactions between heated meteoric ground waters and igneous intrusions: Western San Juan Mountains, Colorado and the Isle of Skye, Scotland

Detailed oxygen, hydrogen and carbon isotope studies have been carried out on igneous and metamorphic rocks of the Stony Mountain complex, Colorado, and the Isle of Skye, Scotland, in order to better understand the problems of hydrothermal meteoric water-rock interaction.

The Tertiary Stony Mountain stock (~1.3 km in diameter), is composed of an outer diorite, a main mass of biotite gabbro, and an inner diorite. The entire complex and most of the surrounding country rocks have experienced various degrees of ¹⁸O depletion (up to 10 per mil) due to interaction with heated meteoric waters. The inner diorite apparently formed from a low-¹⁸O magma with δ¹⁸O ≃ +2.5, but most of the isotopic effects are a result of exchange between H₂O and solidified igneous rocks. The low-¹⁸O inner diorite magma was probably produced by massive assimilation and/or melting of hydrothermally altered country rocks. The δ¹⁸O values of the rocks generally increase with increasing grain size, except that quartz typically has δ¹⁸O = +6 to +8, and is more resistant to hydrothermal exchange than any other mineral studied. Based on atom % oxygen, the outer diorites, gabbros, and volcanic rocks exhibit integrated water/rock ratios of 0.3 ± 0.2, 0.15 ± 0.1, and 0.2 ± 0.1, respectively. Locally, water/rock ratios attain values greater than 1.0. Hydrogen isotopic analyses of sericites, chlorites, biotites, and amphiboles range from -117 to -150. δD in biotites varies inversely with Fe/Fe+Mg, as predicted by Suzuoki and Epstein (1974), and positively with elevation, over a range of 600 m. The calculated δD of the mid-to-late-Tertiary meteoric waters is about -100. Carbonate δ¹³C values average -5.5 (PDB), within the generally accepted range for deep-seated carbon.

Almost all the rocks within 4 km of the central Tertiary intrusive complex of Skye are depleted in ¹⁸O. Whole-rock δ¹⁸O values of basalts (-7. 1 to +8.4), Mesozoic shales (-0.6 to + 12.4), and Precambrian sandstones (-6.2 to + 10.8) systematically decrease inward towards the center of the complex. The Cuillin gabbro may have formed from a ¹⁸O-depleted magma (depleted by about 2 per mil); δ¹⁸O of plagioclase (-7.1 to + 2.5) and pyroxene (-0.5 to + 3.2) decrease outward toward the margins of the pluton. The Red Hills epigranite plutons have δ¹⁸O quartz (-2.7 to + 7.6) and feldspar (-6.7 to + 6.0) that suggest about 3/4 of the exchange took place at subsolidus temperatures; profound disequilibrium quartz-feldspar fractionations (up to 12) are characteristic. The early epigranites were intruded as low-¹⁸O melts (depletions of up to 3 per mil) with δ¹⁸O of the primary, igneous quartz decreasing progressively with time. The Southern Porphyritic Epigranite was apparently intruded as a low-¹⁸O magma with δ¹⁸O ≃ -2.6. A good correlation exists between grain size and δ¹⁸O for the unique, high-¹⁸O Beinn an Dubhaich granite which intrudes limestone having a δ¹⁸O range of +0.5 to +20.8, and δ¹³C of -4.9 to -1.0. The δD values of sericites (-104 to -107), and amphiboles, chlorites, and biotites (-105 to -128) from the igneous rocks , indicate that Eocene surface waters at Skye had δD ≃ -90. The average water/rock ratio for the Skye hydrothermal system is approximately one; at least 2000 km³ of heated meteoric waters were cycled through these rocks.

Thus these detailed isotopic studies of two widely separated areas indicate that (1) ¹⁸O-depleted magmas are commonly produced in volcanic terranes invaded by epizonal intrusions; (2) most of the ¹⁸O-depletion in such areas are a result of subsolidus exchange (particularly of feldspars); however correlation of δ¹⁸O with grain size is generally preserved only for systems that have undergone relatively minor meteoric hydrothermal exchange; (3) feldspar and calcite are the minerals mos t susceptible to oxygen isotopic exchange, whereas quartz is very resistant to oxygen isotope exchange; biotite, magnetite, and pyroxene have intermediate susceptibilities; and (4) basaltic country rocks are much more permeable to the hydrothermal convective system than shale, sandstone, or the crystalline basement complex.

Chromite-silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism

The Neoarchean layered anorthositic complex at Sittampundi in southern India is known for its chromitite layers that are mostly associated with anorthosite (An(90-100)). The chromitites contain FeAl-rich chromites concentrated in layers between amphibole-rich layers with a dominant mineralogy of amphibole-spinel-plagiocase+/-sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X-ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2O3 and Al2O3 contents varying between 34-40 wt.% and 23-28 wt.%. The chromite compositions are noticeably different from those in layered igneous intrusions of the Bushveld-Stillwater type. The existence of original highly calcic plagioclase, FeAl-rich chromite, and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra-subduction zone arc setting. In terms of mineralogy and field relations, the Sittampundi chromitites are remarkably similar to anorthosite-hosted chromitites in the Neoarchean Fiskensset anorthositic complex, Greenland. We propose that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone. This melting process produced hydrous, aluminous basalt, which fractionated at depth to give rise to a variety of high-alumina basalt compositions from which the anorthositic complex with its cumulate chromite-rich and amphibole-rich layers formed within the magma chamber of a supra-subduction zone arc. (C) 2011 Elsevier B.V. All rights reserved.

Análise estratigráfica do campo de Pirapema na bacia da foz do Amazonas

A Bacia da Foz do Amazonas localiza-se no extremo noroeste da plataforma continental brasileira, mais precisamente na margem equatorial. Esta margem se distingue da margem leste brasileira, principalmente pela existência de esforços transtensivos que culminaram com a criação de falhas transformantes de direção E-W e, consequente, criação de bacias do tipo pull apart. Esta bacia ainda se difere das demais bacias brasileiras devido à existência de um expressivo pacote sedimentar depositado nos últimos 11 Ma., que pode chegar a mais de 10.000 metros de sedimentos. Tal feição, denominada Cone Amazônico, apresenta ainda um arcabouço estratigráfico pouco compreendido. Neste trabalho, buscou-se trazer novas perspectivas acerca do pacote sedimentar da bacia, com ênfase na análise estratigráfica dos ciclos progradacionais característicos de progradações deltaicas, bem como a distribuição de possíveis reservatórios siliciclásticos. A análise integrada de poços com as interpretações sísmicas possibilitou a confecção de detalhadas correlações estratigráficas para região do Cone Amazônico. Foram também realizadas importantes observações para as formações mais antigas que o Cone Amazônico como, por exemplo, a influência de intrusões ígneas nas formações Caciporé e Calçoene, atingindo até mesmo a Formação Limoeiro (sequência pós-rifte), bem como a presença de falhas normais relacionada à intumescência da feição ígnea, atingindo a plataforma carbonática. Esta bacia constitui uma fronteira exploratória, complexa em seus aspectos estruturais e estratigráficos, onde a interação de fatores como taxa de acomodação e variação do aporte sedimentar ainda não está totalmente compreendido.

Mapeamento sísmico de intrusões ígneas na bacia do Amazonas e suas implicações para um modelo não-convencional de geração de hidrocarbonetos

A maioria das bacias paleozóicas brasileiras apresenta matéria orgânica termicamente pouco evoluída nos intervalos correspondentes ao Devoniano. O modelo mais adequado para se entender a geração, migração e acumulação de HC estaria relacionado às fases de intrusão de diabásio. No caso da Bacia do Amazonas, embora tenha havido condições de soterramento suficientes para a geração de hidrocarbonetos, não se deve descartar o modelo não convencional de geração como uma das formas possíveis de dar origem as acumulações comerciais de óleo e gás. Acredita-se que o intervalo mais apropriado para a geração de hidrocarbonetos (HC) inclua apenas as rochas depositadas no intervalo Frasniano, embora as rochas associadas ao intervalo Llandoveriano, também, devam ser observadas com atenção. Com o intuito de compreender melhor o papel da atividade magmática na evolução da Bacia do Amazonas, foi realizado o mapeamento sísmico de soleiras de diabásio e análise de dados geoquímicos de pirólise Rock-Eval e COT. Assim, foi possível avaliar a geração/migração de hidrocarbonetos e a variação dos parâmetros geotérmicos na Bacia do Amazonas, causados pela intrusão das soleiras de diabásio. A análise sismoestratigráfica baseou-se na interpretação de 20 linhas sísmicas 2D pós-stack, na qual foram reconhecidos e mapeados horizontes sísmicos (topos de formações e corpos ígneos intrusivos), utilizando dados de poços e dados da literatura para correlação. As intrusões de soleiras estão presentes nas sucessões de folhelhos/siltitos e anidritas das formações Andirá e Nova Olinda, respectivamente. Observou-se que as soleiras de diabásio podem estar intimamente relacionadas a diques sistematicamente orientados, tendo estes diques a função de alimentadores das soleiras. Extensas soleiras planares com segmentos transgressivos ocorrem nos níveis estratigráficos mais rasos da Bacia do Amazonas, e em maiores volumes nas formações Andirá e Nova Olinda. Em algumas regiões as soleiras desenvolvem morfologias marcantes em forma de pires. Esses corpos possuem espessuras que podem chegar a 500m. Comumente, a geometria em lençol denotada pelo paralelismo dos refletores está presente em toda extensão do mapeamento da bacia. Também foram observadas estruturas em domo. O efeito térmico imposto pelas intrusões dos corpos ígneos, diques e soleiras foi de grande importância, pois sem ele não haveria calor para a transformação da matéria orgânica. Através da análise de pirólise Rock-Eval e teor de carbono orgânico, foi possível avaliar e correlacionar os parâmetros como S2 (potencial de geração), IH (índice de hidrogênio), S1 (hidrocarbonetos livres) e Tmax (evolução térmica) com a profundidade. Foram utilizados dados de 04 poços na qual dois deles foram compilados a partir de artigos e teses publicados. As rochas potencialmente geradoras de petróleo são aquelas que apresentam COT igual ou superior a 1%. Dos quatro poços analisados, dois deles apresentam COT > 1% para a Formação Barreirinhas, mostrando que as rochas sedimentares são potencialmente geradoras de HC. Altos valores Tmax podem ser justificados pelo efeito térmico causado por intrusões de diabásio. Os resultados de índice de hidrogênio (IH) apresentaram valores abaixo de 200mgHC/g COT, indicando o potencial gerador desta bacia para gás.

Análise da evolução termomecânica da Bacia do Amazonas

Os dados geológicos e geofísicos escolhidos para o tema de estudo pertencem a Bacia do Amazonas, na região centro-norte do Brasil. A Bacia do Amazonas é uma bacia intracratônica com cerca de 500.000 km. A mesma está limitada ao norte pelo Escudo das Guianas e ao sul pelo Escudo Brasileiro. O limite oeste com a Bacia do Solimões é marcado pelo Arco de Purus, ao passo que o Arco de Gurupá constitui seu limite leste. Possui características inerentes a uma bacia intracratônica paleozóica, com uma longa história evolutiva, marcada por discordâncias expressivas e com uma cunha sedimentar relativamente rasa se comparada às bacias cretáceas brasileiras, o que levanta controvérsia a respeito da suficiência do soterramento para a eficiência de geração de hidrocarboneto. Podem ser reconhecidas nos 5000 m do preenchimento sedimentar da Bacia do Amazonas, duas seqüências de primeira ordem: uma paleozóica, intrudida por diques e soleiras de diabásio, na passagem do Triássico para o Jurássico, e uma mesozóica-cenozóica que representam um aspecto importante na evolução térmica da matéria orgânica que ocorre na primeira seqüência. Com relação à exploração de petróleo, apesar do fomento exploratório ocorrido nos últimos anos, a bacia ainda é considerada pouco explorada sendo sua maior reserva a da província de Urucu. Um dos fatores que dificultam bastante a exploração desta bacia assim como a bacia do Solimões a oeste é o acesso restrito, pois estão situadas em áreas remotas e florestadas, de difícil acesso, com muitas reservas indígenas e florestais, o que causa restrições logísticas, operacionais e legais. O efeito térmico das intrusões ígneas é considerado como o responsável pelo acréscimo de calor necessário à maturação da matéria orgânica e conseqüente geração de hidrocarbonetos. Este trabalho contribui com a reconstrução da história térmica desta bacia a partir da modelagem das variáveis termais e da história de soterramento. Para isso, foram utilizados modelos consagrados na literatura, que permitem, de forma simples, a estimativa do fluxo térmico através do embasamento e da seqüência sedimentar. Na análise da influência de intrusões ígneas na estrutura térmica da bacia, o modelo bidimensional desenvolvido pelo método de diferenças finitas se mostrou apropriado. Utilizou-se o fluxo térmico basal calculado nas condições de contorno da modelagem da influência térmica das ígneas. Como resultado obteve-se a estruturação térmica da bacia e a historia maturação de suas rochas geradoras

Unraveling an antique subduction process from metamorphic basement around Medellin city, Central Cordillera of Colombian Andes

In the surroundings of Caldas and El Retiro cities (Colombia) metamorphic rocks derived from basic and pelitic protoliths comprise the Caldas amphibole schist and the Ancon schist respectively. Subordinated metamorphosed granite bodies (La Miel gneiss) are associated to these units, and The El Retiro amphibolites, migmatites and granulites crops out eastwards of these units, separated by shear zones. The Caldas amphibole schist and the Ancon schist protoliths could have been formed in a distal marine reduced environment and amalgamated to the South American continent in an apparent Triassic subduction event. The El Retiro rocks are akin to a continental basement and possible include impure metasediments of continental margin, whose metamorphism originated granulite facies rocks and migmatites as a result of the anatexis of quartz-feldspathic rocks. The metamorphism was accompanied by intense deformation, which has juxtaposed both migmatites and granulite blocks. Afterward, heat and fluid circulation associated with the emplacement of minor igneous intrusions resulted in intense fluid-rock interaction, variations in the grain size of the minerals and, especially, intense retrograde metamorphic re-equilibrium. Thermobarometric estimations for the Caldas amphibole schist indicate metamorphism in the Barrovian amphibolite fades. The metamorphic path is counter-clockwise, but retrograde evolution could not be precisely defined. The pressures of the metamorphism in these rocks range from 6.3 to 13.5 kbar, with narrow temperature ranging from 550 to 630 degrees C. For the Ancon schist metapelites the P-T path is also counter-clockwise, with a temperature increase evidenced by the occurrence of sillimanite and the cooling by later kyanite. The progressive metamorphism event occurred at pressures of 7.6-7.2 kbar and temperatures of 645-635 degrees C for one sample and temperature between 500 and 600 degrees C under constant pressure of 6 kbar. The temperature estimated for these rocks varies between 400 and 555 degrees C at pressures of 5-6 kbar in the retrograde metamorphic path. The El Retiro rocks evidence strong decompression with narrow variation in temperature, showing pressure values between 8.7 and 2.7 kbar at temperatures of 740-633 degrees C. These metamorphic fragments of the basement in the Central Cordillera of the Colombian Andes could represent a close relationship with an antique subduction zone. (C) 2011 Elsevier Ltd. All rights reserved.

Inversão gravimétrica usando regularização entrópica

Apresentamos um novo método para inversão gravimétrica da distribuição espacial do contraste de densidade no plano horizontal, baseado na combinação da maximização da entropia de ordem zero com a minimização da entropia de ordem um. O topo e a base das fontes gravimétricas são presumidos ser planos e horizontais e o modelo interpretativo consiste de uma malha de prismas justapostos em ambas às direções horizontais, sendo os contrastes de densidade de cada prisma os parâmetros a serem estimados. A maximização da entropia de ordem zero é similar ao vínculo de suavidade global, enquanto a minimização da entropia de ordem um favorece descontinuidades na distribuição do contraste de densidade. Conseqüentemente a combinação judiciosa de ambas pode levar a soluções apresentando regiões com contrastes de densidade virtualmente constantes (no caso de corpos homogêneos), separadas por descontinuidades abruptas. O método foi aplicado a dados sintéticos simulando a presença de corpos intrusivos em sedimentos. A comparação dos resultados com aqueles obtidos através do método da suavidade global mostra que ambos os métodos localizam as fontes igualmente bem, mas o delineamento de seus contornos é efetuado com maior resolução pela regularização entrópica, mesmo no caso de fontes com 100 m de largura separadas entre si por uma distância de 50 m. No caso em que o topo da fonte causadora não é plano nem horizontal, tanto a regularização entrópica como a suavidade global produzem resultados semelhantes. A metodologia apresentada, bem como a suavidade global foram aplicadas a dois conjuntos de dados reais produzidos por intrusões em rochas metamórficas. O primeiro é proveniente da região de Matsitama, no nordeste de Botswana, centro sul da África. A aplicação das duas metodologias a estes dados produziu resultados similares, indicando que o topo das fontes não é plano nem horizontal. O segundo conjunto provém da região da Cornuália, Inglaterra e produziu uma distribuição estimada de contraste de densidade virtualmente constante para a regularização entrópica e oscilante para a suavidade global, indicando que a fonte gravimétrica apresenta topo aproximadamente plano e horizontal, o que é confirmado pela informação geológica disponível.

Daibase Sill Comanche County, Oklahoma

The mechanics of intrusion and the processes involved are becoming of more and more interest to con­temporary geologists, because most of our mineral deposits are associated with igneous intrusions, and unknown-outcropping deposits are thought to be a thing of the past. If, by studying these pro­cesses, we could understand why some igneous masses are barren, while others of similar composition carry economic mineral deposits, geological prospecting for hidden mineral concentration would see a new era.

Geochemistry of ODP Site 202-1233 and river sediments

Bulk sediment chemistry from three Chilean continental margin Ocean Drilling Program sites constrains regional continental erosion over the past 30,000 years. Sediments from thirteen rivers that drain the (mostly igneous) Andes and the (mostly metamorphic) Coast Range, along with existing rock chemistry datasets, define terrestrial provenance for the continental margin sediments. Andean river sediments have high Mg/Al relative to Coast-Range river sediments. Near 36°S, marine sediments have high-Mg/Al (i.e. more Andean) sources during the last glacial period, and lower-Mg/Al (less Andean) sources during the Holocene. Near 41°S a Ti-rich source, likely from coast-range igneous intrusions, is prevalent during Holocene time, whereas high-Mg/Al Andean sources are more prevalent during the last glacial period. We infer that there is a dominant ice-sheet control of sediment sources. At 36°S, Andean-sourced sediment decreased as Andean mountain glaciers retreated after ~17.6 ka, coincident with local oceanic warming and southward retreat of the Patagonian Forest and, by inference, westerly winds. At 41°S Andean sediment dominance peaks and then rapidly declines at ~19 ka, coincident with local oceanic warming and the earliest deglacial sea-level rise. We hypothesize that this decreased flux of Andean material in the south is related to rapid retreat of the marine-based portion of the Patagonian Ice Sheet in response to global sea-level rise, as the resulting flooding of the southern portion of the Central Valley created a sink for Andean sediments in this region. Reversal of the decreasing deglacial Mg/Al trend at 41°S from 14.5 to 13.0 ka is consistent with a brief re-advance of the Patagonian ice sheet coincident with the Antarctic Cold Reversal.

The geochemistry of the sedimentary rocks of the Harlech Dome, North Wales

Bedrock geochemical analysis, coupled with detailed data analysis, was carried out on some 260 samples taken from two areas of 'the Harlech Dome, near Dolgellau, North Wales. This was done to determine if rocks from mineralised and non-mineralised areas could be distinguished, and to determine mineralisation types and wall rock alterations. The Northern Area, near Talsarnau, has no recorded mineralisation, while the Southern Area, near Bontddu, has been exploited for gold. The rocks sampled, in both areas, were from the Cambrian Gamlan Flags, Clogau Shales, Vigra Flags, later vein materials, and igneous intrusions. All samples were analysed, using a new rapid, atomic absorption spectrophotometric technique, for Si, AI, Fe, Cu, Ni, Zn, Pb, Sr, Hg, and Ba. In addition 60 samples were analysed by X-ray fluorescence for Mn, Ti, Ca, K, Na, P, Cr, Ce, La, S, Y , Rh, and Th. Total CO2 was determined, on selected samples, using a combustion technique. Elemental distributions, for each rock type, in each area, were· plotted, and means, standard deviations, and enrichment indices were calculated. Multivariate statistical analysis on the results distinguished a Cu-type mineralisation in the Northern area, and both Cu and Pb/Zn types in the Southern Area. It also showed the Northern Area to be less strongly mineralised than the Southern one in which both mineralisation types are associated with wall rock alteration. Elemental associations and trends due to sedimentary processes were distinguished from those related to mineralisation. Hg is related to mineralisation, and plots of factor scores, on the sampling grid, produced clusters of mineralisation related factors in areas of known mineralisation. A double Fourier Trend Analysis program, with a wavelength search routine, was developed and used to recognise sedimentary trends for Sr. Y., Rb, and Th. These trends were interpreted to represent areas of low pH and reducing conditions. They also indicate that the supply of sediment remained constant over Gamlan, Clogau, and Vigra times. The trend surface of Hg showed no association with rock type. It is shown that analysis of a small number of samples, for a carefully selected number of elements, with detailed data analysis, can provide more useful information than analysis of a large number of samples for many elements. The mineralisation is suggested to have been the result of water solutions leaching ore metals from the sedimentary rocks and redepositing them in veins.

Pirometamorfismo provocado por intrusões básicas cenozoicas em rochas da Bacia Potiguar, RN: integração de dados geológicos e petrofísicos

The Potiguar Basin is located in the Brazilian Equatorial Margin and presents sedimentary rocks affected by Cenozoic basic igneous intrusions, known as Macau Magmatism. The most prominent effect related to these intrusions is the formation of buchites, pyrometamorphic rocks that occur at very high temperatures and very low pressures in the sanidinite metamorphic facies. Through literature review, field observations, petrographic and petrophysical data, accessing the database of previous studies and results from this research, it was possible to characterize and estimate the effects produced in the thermal aureole of some hypabyssal bodies in the basin. The most relevant features associated with the intrusions are: compactation, hydraulic fracturing, partial melting and recrystallization of country rocks. According to the observed mineral occurrences, temperature of 800 to 1200 °C and pressure below 0,5 kbar were estimated at the contacts of the igneous bodies. The thermal modeling of the São João plug indicates thermal effects extending up to 150 m away from the contact and cooling time of approximately 265,000 years. After the peak of temperature, followed a cooling phase registered by remobilization and precipitation of minerals at low-temperature in faults, fractures and geodes, interpreted as derived from reactions with sedimentary rocks and metasomatic / hydrothermal fluids with abundant carbonatization and silicification.

Pirometamorfismo provocado por intrusões básicas cenozoicas em rochas da Bacia Potiguar, RN: integração de dados geológicos e petrofísicos

The Potiguar Basin is located in the Brazilian Equatorial Margin and presents sedimentary rocks affected by Cenozoic basic igneous intrusions, known as Macau Magmatism. The most prominent effect related to these intrusions is the formation of buchites, pyrometamorphic rocks that occur at very high temperatures and very low pressures in the sanidinite metamorphic facies. Through literature review, field observations, petrographic and petrophysical data, accessing the database of previous studies and results from this research, it was possible to characterize and estimate the effects produced in the thermal aureole of some hypabyssal bodies in the basin. The most relevant features associated with the intrusions are: compactation, hydraulic fracturing, partial melting and recrystallization of country rocks. According to the observed mineral occurrences, temperature of 800 to 1200 °C and pressure below 0,5 kbar were estimated at the contacts of the igneous bodies. The thermal modeling of the São João plug indicates thermal effects extending up to 150 m away from the contact and cooling time of approximately 265,000 years. After the peak of temperature, followed a cooling phase registered by remobilization and precipitation of minerals at low-temperature in faults, fractures and geodes, interpreted as derived from reactions with sedimentary rocks and metasomatic / hydrothermal fluids with abundant carbonatization and silicification.

Timing and source constraints on the relationship between mafic and felsic intrusions in the Emeiashan large igneous province

Several I- and A-type granite, syenite plutons and spatially associated, giant Fe–Ti–V deposit-bearing mafic ultramafic layered intrusions occur in the Pan–Xi(Panzhihua–Xichang) area within the inner zone of the Emeishan large igneous province (ELIP). These complexes are interpreted to be related to the Emeishan mantle plume. We present LA-ICP-MS and SIMS zircon U–Pb ages and Hf–Nd isotopic compositions for the gabbros, syenites and granites from these complexes. The dating shows that the age of the felsic intrusive magmatism (256.2 ± 3.0–259.8 ± 1.6 Ma) is indistinguishable from that of the mafic intrusive magmatism (255.4 ± 3.1–259.5 ± 2.7 Ma) and represents the final phase of a continuous magmatic episode that lasted no more than 10 Myr. The upper gabbros in the mafic–ultramafic intrusions are generally more isotopically enriched (lower eNd and eHf) than the middle and lower gabbros, suggesting that the upper gabbros have experienced a higher level of crustal contamination than the lower gabbros. The significantly positive eHf(t) values of the A-type granites and syenites (+4.9 to +10.8) are higher than those of the upper gabbros of the associated mafic intrusion, which shows that they cannot be derived by fractional crystallization of these bodies. They are however identical to those of the mafic enclaves (+7.0 to +11.4) and middle and lower gabbros, implying that they are cogenetic. We suggest that they were generated by fractionation of large-volume, plume-related basaltic magmas that ponded deep in the crust. The deep-seated magma chamber erupted in two stages: the first near a density minimum in the basaltic fractionation trend and the second during the final stage of fractionation when the magma was a low density Fe-poor, Si-rich felsic magma. The basaltic magmas emplaced in the shallowlevel magma chambers differentiated to form mafic–ultramafic layered intrusions accompanied by a small amount of crustal assimilation through roof melting. Evolved A-type granites (synenites and syenodiorites) were produced dominantly by crystallization in the deep crustal magma chamber. In contrast, the I-type granites have negative eNd(t) [-6.3 to -7.5] and eHf(t) [-1.3 to -6.7] values, with the Nd model ages (T Nd DM2) of 1.63-1.67 Ga and Hf model ages (T Hf DM2) of 1.56-1.58 Ga, suggesting that they were mainly derived from partial melting of Mesoproterozoic crust. In combination with previous studies, this study also shows that plume activity not only gave rise to reworking of ancient crust, but also significant growth of juvenile crust in the center of the ELIP.