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1.
The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10–100 m scale metaluminous to peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites, which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite Moho. Compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a H2O and pressures of 3 to 5 kbar. The LILE (Sr, Rb and Ba) covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments. Initial 87Sr/86Sr ratios of analysed dykes vary between 0.710 and 0.706 at initial ɛNd values of between −6.3 and −0.5. Cogenetic units of a composite sill from Ra's Dadnah yield a Sm-Nd isochron age of 98.8 ± 9.5 Ma (MSWD = 1.18). Geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill (Haybi complex) sediments, preserved as greenschist (Asimah area) to granulite facies (Bani Hamid area) ophiolitic metamorphic sole terranes. The Sr-Nd isotope systematics suggest that hybrid granitic melts were derived from pre-magmatic mixing of two contrasting subduction zone sources. Received: 17 December 1998 / Accepted: 19 July 1999  相似文献   

2.
Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper, the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock ε Nd(t) values ranging from −17.4 to −9.3 and zircon ε Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher ε Nd(t) values from −11.1 to −9.3 and ε Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower ε Nd(t) value of −17.4 and zircon ε Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic.  相似文献   

3.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time.  相似文献   

4.
《地学前缘(英文版)》2020,11(6):2271-2286
In this study we present new mineral chemistry, whole-rock geochemical and zircon U–Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet (China). Modal analyses of these dykes gave averages of ~40%–65% plagioclase and ~35%–60% amphibole and small amounts of (igneous) clinopyroxene, epidote and opaque minerals. This mineral assemblage resembles that of typical orthoamphibolites. Nevertheless, due to the absence of foliation the investigated rocks are described as metamafic lithologies. These rocks have primitive mantle (PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak, negative Ti anomalies. In addition, they have initial 87Sr/86Sr ratios [(87Sr/86Sr)i] of 0.702844–0.703581, initial 143Nd/144Nd ratios [(143Nd/144Nd)i] of 0.512891–0.512959 and high εNd(t) values (+7.9 to +9.3). Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at ~119.0–118.5 ​Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process, from (epidote-)amphibolite facies (~470–610 ​°C, ~1.9–4.3 ​kbar) and to prehnite-pumpellyite facies (≤280 ​°C, ≤ 3 ​kbar), active within a rift-produced oceanic lithosphere. Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros. Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle (DM) source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab. The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone (YZSZ) in the literature (~129–123 ​Ma). We, therefore, suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction re-initiation (Aptian). Our tectonomagmatic model provides insights into the igneous accretion and post-solidification evolution of the oceanic lithosphere in South Tibet.  相似文献   

5.
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.  相似文献   

6.
Described half a century ago, the Galiléia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Araçuaí-Ribeira orogenic system (southeastern Brazil). In the 1990's, analytical studies on the Galiléia tonalite finally revealed the existence of a Late Neoproterozoic calc-alkaline magmatic arc in the Araçuaí orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Araçuaí-Ribeira boundary. After those pioneer studies, the calc-alkaline plutons showing a pre-collisional volcanic arc signature and age between 630 Ma and 585 Ma have been grouped in the G1 supersuite, corresponding to the Rio Doce arc infrastructure. Here, we revisit the Rio Doce arc with our solid field knowledge of the region and a robust analytical database (277 lithochemical analyses, and 47 U–Pb, 53 Sm–Nd, 25 87Sr/86Sr and 7 Lu–Hf datasets). The G1 supersuite consists of regionally deformed, tonalitic to granodioritic batholiths and stocks, generally rich in melanocratic to mesocratic enclaves and minor gabbroic to dioritic plutons. Gabbroic to dioritic enclaves show evidence of magma mixing processes. The lithochemical and isotopic signatures clearly reveal a volcanic arc formed on a continental margin setting. Melts from a Rhyacian basement form the bulk of the magma produced, whilst gabbroic plutons and enclaves record involvement of mantle magmas in the arc development. Tonalitic stocks (U–Pb age: 618–575 Ma, εNd(t): −5.7 to −7.8, Nd TDM ages: 1.28–1.68 Ga, 87Sr/86Sr(t): 0.7059–0.7118, and εHf(t): −5.2 to −11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Araçuaí orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galiléia, Muniz Freire, São Vítor) record a long-lasting magmatic history (632–580 Ma; εNd(t): −5.6 to −13.3; Nd TDM age: 1.35–1.80 Ga; 87Sr/86Sr(t): 0.7091–0.7123). At arc western border, the magmatic evolution started with gabbro-dioritic and tonalitic plutons (e.g., Chaves pluton, U–Pb age: 599 ± 15 Ma, εNd(t): −4.8 to −6.8, Nd TDM ages: 1.48–1.68 Ga, 87Sr/86Sr(t): 0.7062–0.7068, and εHf(t): −4.3 to −9.7; and Brasilândia pluton, U–Pb age: 581 ± 11 Ma, εNd(t): −8.2 to −10.2, Nd TDM ages: 1.63–1.68 Ga, 87Sr/86Sr(t): 0.7088–0.7112, εHf(t): −12.3 to −14.9), followed by late granodioritic intrusions (e.g., Guarataia pluton, U–Pb age: 576 ± 9 Ma, εNd(t): −12.52 to −13.11, Nd TDM age: 1.74–2.06 Ga, 87Sr/86Sr(t): 0.7104–0.7110, εHf(t): −12.9 to −21.6). The Muriaé batholith (U–Pb age: 620–592 Ma, εNd(t): −8.2 to −13.6, Nd TDM age: 1.41–1.88 Ga) and the Conceição da Boa Vista (586 ± 7 Ma) and Serra do Valentim (605 ± 8 Ma) stocks represent a segment of the Rio Doce arc correlated to the Serra da Bolívia and Marceleza complexes, making the link between the Araçuaí and Ribeira orogenic domains. We suggest three phases of arc development: i) eastward migration of arc front (632–605 Ma), ii) widespread magma production in the whole arc (605–585 Ma), and iii) late plutonism in the western arc region (585–575 Ma). Usual processes of volcanic arc development, like subduction of oceanic lithosphere under a continental margin, followed by asthenosphere ascent related to slab retreating and break-off may explain the Rio Doce arc evolution.  相似文献   

7.
The Tuwu–Yandong porphyry copper belt lies in the eastern Tianshan mountains, eastern section of the Central Asian orogenic belt. The copper mineralization is mainly hosted in plagiogranite porphyries intruded into early Carboniferous volcanic rocks of the Paleozoic Dananhu island arc between the Tarim and Siberian plates. The plagiogranite porphyries have contents of 65–73 wt% SiO2, 14–17 wt% Al2O3, 0.9–2.2 wt% MgO, 3–16 ppm Y, 0.4–1.40 ppm Yb, 347–920 ppm Sr, and positive Eu anomalies. The rocks also exhibit positive ɛ Nd(t) values (+5.0 to +9.4) and low initial 87Sr/86Sr values (0.70316–0.70378). Such features are similar to those of adakites derived from partial melting of a subduction-related oceanic slab. The mineralization age is early Carboniferous (350–320 Ma), which is close to that of the porphyries. The close relationship between the Cu mineralization and the porphyry is also indicated by their similar Sr-Nd-Pb isotopic compositions. We suggest that the copper porphyry (magma) system in the Dananhu island arc was formed by direct melting of an obliquely subducting early Carboniferous oceanic slab.  相似文献   

8.
The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km2) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks.The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend (Nd/(87Sr/86Sr)i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (Nd/87Sr/86Sr)i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust.The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.  相似文献   

9.
The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = 238U/204Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, 206Pb/204Pb ca. 19.5–20.5, 207Pb/204Pb ca. 15.63–15.73, 208Pb/204Pb ca. 39–40 and 87Sr/86Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

10.
Sensitive high-resolution ion microprobe (SHRIMP) U–Pb dating, laser-ablation multi-collector ICPMS Hf isotope and electron microprobe element analyses of inherited/antecrystal and magmatic zircons from five granitoid intrusions of Linxi area, in the southern segment of the Great Xing’an Range of China were integrated to solve continental crustal growth mechanisms. These intrusions were divided into two suites. Suites 1 and 2 are mainly granodiorite and syenogranite and correspond to magnesian and ferroan granites, respectively. SHRIMP dating establishes an Early Cretaceous (135–125 Ma) age for most Linxi granitoids and a time of ∼146 Ma when their source rocks were generated or re-melted. However, some granitoids were generated in Early Triassic (241 Ma) and Late Jurassic (146 Ma), after their source rock experienced precursory melting episodes at 263 Ma and 165 Ma, respectively. All zircon 206Pb/238U ages (<300 Ma, n = 100), and high positive zircon εHf(t) values (n = 175) suggest juvenile source materials with an absence of Precambrian basement. Hf–Nd isotopic decoupling of Linxi granitoids suggests a source component of pelagic sediments, i.e. Paleozoic subduction accretion complexes. Zircon εHf(t) values (t = 263–165 Ma) form a trend sub-parallel to the depleted mantle Hf isotope evolution curve, whilst those with t = 146–125 Ma fall markedly below the latter. The first trend indicates a provenance from essentially subducted oceanic slabs. However, the abrupt εHf(t) decrease, together with extensive Early Cretaceous magmatism, is interpreted as reflecting mantle upwelling and resultant underplating, and exhumation of subducted oceanic slabs. Suite 1 granitoids derive mainly from subducted oceanic slabs or Paleozoic subduction accretion complex, whereas Suite 2 from underplated mafic rock and, subordinately, Paleozoic subduction accretion complex. Compositions of Suites 1 and 2 depend on the hydrous, oxidized or relatively anhydrous, reduced nature of source rocks. Among each of these five intrusions, magmatic zircons have systematically lower 176Hf/177Hf than inherited/antecrystal zircons. Hf isotopic and substituting element profiles through inherited/antecrystal zircons (t = 263 to ∼146 Ma) indicate repeated low melt-fraction melting in the source region. In contrast, profiles through inherited/antecrystal and magmatic zircons (t = 146–125 Ma) reveal melting region expansion with a widening range of source compositions and increasing melt fractions. These results lead to the conclusion that continental growth in this region involved a three-step process. This included subduction accretion and repeated underplating, intermediary differentiation of juvenile rocks, and granitoid production from these differentiated rocks.  相似文献   

11.
The Francistown plutonic rocks at the south-western margin of the Zimbabwe craton consist of three igneous suites: Sanukitoid, Tonalite–Trondhjemite–Granite (TTG) suites and High-K granites. The TTG suite is subdivided into High Aluminum-TTG (HA-TTG) and Low Aluminum-TTG (LA-TTG) sub-suites. Their Rb–Sr isotope systems were partially homogenized by post-crystallization thermo–tectonic events, in which hydrothermal solutions and migmatization played an important role. Therefore, the Rb–Sr isochron age of 2427±54 Ma can only be regarded as a lower limit to the Francistown plutonic rock age. The large errors in the Sm–Nd isochron dates of Francistown granitoids indicate that these dates are not really constrained. In this study we compared the rock types of Francistown and adjacent areas, adopting the precise U, Th–Pb single zircon SHRIMP ages from the Vumba area as references. For TTG and Sanukitoid suites, the age we adopted is ca. 2.7 Ga, which is close to their depleted-mantle Sm–Nd model ages (T DM). For High-K granites, the age adopted is ca. 2.65 Ga, which is also close to their Sm–Nd isochron age. The highest ε Nd t values of Sanukitoids and TTG are +2.1 and +2.3, respectively. The positive ε Nd t values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of Sanukitoid and the TTG suites respectively. The lowest ε Nd t values of Sanukitoids and TTGs are −1.0 and −1.1, respectively, indicating contamination by continental crust, up to 10 and 14%, respectively. The ε Nd t values of TTG decrease with decreasing Al2O3 and Sr contents and increasing Eu negative anomalies (Eu*–Eu), suggesting that the TTG magmas underwent a coupled fractionation crystallization and crustal contamination, and that the LA-TTG was the product of the fractionation and contamination of the HA-TTG sub-suite. In contrast, negative ε Nd t values for the High-K granites (from −0.4 to −3.5) indicate the involvement of LA-TTG and some materials from an old continental crust in their genesis. The products of partial melting of both oceanic and continental crusts at the south-western margin of the Zimbabwe craton occurred within a short time interval (from 2.7 to 2.65 Ga ago) suggesting that the Francistown plutonic rocks were formed in a active continental margin environment, where a young ocean plate (Limpopo oceanic plate) subducted underneath an old continental plate (Zimbabwe craton).  相似文献   

12.
The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N–S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U–Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860–850 and 795–785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive εNd (4.5–7.0) and low initial 87Sr/86Sr (0.7029 ± 0.0002) and a mean T DM model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~830–690 Ma).  相似文献   

13.
Whole rock elemental and Sr–Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810–580 Ma), and is dominated by crust-derived relatively low-T (850–750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (−8 to −10) and highest mg# (30–40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96–1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595–580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (−12 to −18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0–2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714–0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd–Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites.  相似文献   

14.
An early Cretaceous (135 ± 2 Ma) felsic volcanic suite of dacite and rhyolite from Huolinhe, NE China is characterized by large ion lithophile element and light REE enrichment and high field strength element (HFSE, e.g., Nb and Ta) and Ti–P depletion, and bulk silicate earth-like Sr [87Sr/86Sr(i) = 0.70409–0.70481], quite radiogenic Nd [ε Nd(t) = +3.98 to +5.88], Pb [e.g., 206Pb/204Pb(i) = 18.46–18.55] and Hf [ε Hf(t) ~+9.2] isotope compositions. Compared with contemporaneous mafic rocks in the region, these felsic rocks have even higher Nd and Hf isotopic ratios, precluding an origin through differentiation of coeval mantle-derived magmas. Isotope calculation results suggest that these magmas were derived from a preexistent mixture composed of mainly juvenile crust (70–80%), and a subordinate recycled crustal component (20–30%) having highly radiogenic Sr and Pb and unradiogenic Nd and Hf. About 25–30% melting of such a mixed source produced the primary dacitic magma. The rhyolites, which have relatively low MgO, FeO*, Al2O3, CaO, TiO2, P2O5, Na2O, Ba, Sr, REE, HFSE and Y, were differentiates of the dacites after removal of a fractional assemblage of hornblende + plagioclase + K-feldspar + apatite + zircon. Considering the prolonged events (from 262 to 130 Ma) that produced such highly positive ε Nd felsic igneous rocks in the region, we prefer a pre-Mesozoic crustal growth model related to arc accretion associated with the Paleo-Asian Ocean subduction.  相似文献   

15.
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, ε Nd = +6.6 to +7.0, and ε Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε Nd = −8.1 to +2.5, and ε Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.  相似文献   

16.
 Peridotites, dykes and gabbros from the 470–420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity Ophiolite, which covers an area of ≈1600 km2, consists of three distinct units: (1) a ∼2–4 km-thick sheet of plastically deformed peridotites, including various ultrabasic lithologies (plagioclase and spinel lherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle lithosphere of oceanic affinity; (2) a series of small (∼1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geochemical and isotopic variations, with ɛNd(T) values ranging from +10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model ɛNd(T) values is reduced to 7 ɛNd units: 0 to +7. As a general rule, peridotites, gabbros and dykes with ɛNd(T) values around 0 or +3 give less depleted L(light)REE patterns than do those with ɛNd(T) values in the range +7 to +10. In the peridotites, the Nd isotopic variations take place over very short distances, with jumps as large as 7 ɛNd units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with ɛNd(T)≈+10 could be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with Sm-Nd results demonstrate that the 10 ɛNd units variation displayed by the Trinity Peridotite is a primary feature and not an artefact due to REE mobility during seawater interaction. The variable Nd isotopic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle. Field, petrographic, geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 Ma proto-peridotite with ɛNd(T)=+10 and younger (420 Ma) basaltic melts with lower ɛNd(T) values (i.e. the gabbroic massifs and the dykes). The gabbros and basic dykes of the Trinity Complex have geochemical and isotopic compositions similar to the arc basalts from the adjacent Copley Formation, so it is proposed that the younger melts are related to arc magmatism. Received: 13 January 1995/Accepted 5 May 1995  相似文献   

17.
The Dharwar Craton is a composite Archean cratonic collage that preserves important records of crustal evolution on the early Earth. Here we present results from a multidisciplinary study involving field investigations, petrology, zircon SHRIMP U–Pb geochronology with in-situ Hf isotope analyses, and whole-rock geochemistry, including Nd isotope data on migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses, dark grey banded gneisses, calc-alkaline and anatectic granitoids, together with synplutonic mafic dykes along a wide Northwest – Southeast corridor forming a wide time window in the Central and Eastern blocks of the Dharwar Craton. The dark grey banded gneisses are transitional between TTGs and calc-alkaline granitoids, and are referred to as ‘transitional TTGs’, whereas the calc-alkaline granitoids show sanukitoid affinity. Our zircon U–Pb data, together with published results, reveal four major periods of crustal growth (ca. 3360-3200 Ma, 3000-2960 Ma, 2700-2600 Ma and 2570-2520 Ma) in this region. The first two periods correspond to TTG generation and accretion that is confined to the western part of the corridor, whereas widespread 2670-2600 Ma transitional TTG, together with a major outburst of 2570–2520 Ma juvenile calc-alkaline magmatism of sanukitoid affinity contributed to peak continental growth. The transitional TTGs were preceded by greenstone volcanism between 2746 Ma and 2700 Ma, whereas the calc-alkaline magmatism was contemporaneous with 2570–2545 Ma felsic volcanism. The terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca. 3200 Ma, 2960 Ma, 2620 Ma and 2520 Ma. Elemental ratios [(La/Yb)N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data suggest that the magmatic protoliths of the TTGs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase + amphibole ± garnet + titanite/ilmenite in the source residue, whereas the elemental (Ba–Sr, [(La/Yb)N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data [εHf(T) = −0.67 to 5.61; εNd(T) = 0.52 to 4.23; ] of the transitional TTGs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole + garnet + clinopyroxene ± plagioclase + ilmenite in the residue. The highly incompatible and compatible element contents (REE, K–Ba–Sr, Mg, Ni, Cr), together with Hf and Nd isotope data [εHf(T) = 4.5 to −3.2; εNd(T) = 1.93 to −1.26; ], of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination. Field, elemental and isotope data [εHf(T) = −4.3 to −15.0; εNd(T) = −0.5 to −7.0] of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust. Secular increase in incompatible as well as compatible element contents in the transitional TTGs to sanukitoids imply progressive enrichment of Neoarchean mantle reservoirs, possibly through melting of continent-derived detritus in a subduction zone setting, resulting in the establishment of a sizable continental mass by 2700 Ma, which in turn is linked to the evolving Earth. The Neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere, with continued convergence resulted in the assembly of micro-blocks, with eventual slab break-off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust. This led to lateral flow of hot ductile crust and 3D mass distribution and formation of an orogenic plateaux with subdued topography, as indicated by strain fabric data and strong seismic reflectivity along an E-W crustal profile in the Central and Eastern blocks of the Dharwar Craton.  相似文献   

18.
This paper presents new SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotopic data of two Early Paleozoic granitic plutons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U–Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513?±?7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6?±?6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial 87Sr/86Sr ratios of 0.7072–0.7096, εNd (T) of ?0.2 to ?1.6 and εHf (T) (in-situ zircon) of ?1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A1 subtype granite, with initial 87Sr/86Sr ratios of 0.7077–0.7101, εNd (T) of ?3.5 to ?4.0 and εHf (T) (in-situ zircon) of ?3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (～513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (～421 Ma), most probably in Middle Silurian.  相似文献   

19.
Magnesian andesites in north Xinjiang,China   总被引：1，自引：0，他引：1  
Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan and west- and east-Tianshan in the north Xinjiang region. These MAs are andesitic rocks with 53–65% SiO2,<1% (0.21–1.08%; average of 0.72%) TiO2, and ≥50 Mg#. Magnesian dacites and diorites, with 52.38–66.91% SiO2, <0.30% TiO2 and ≥42 Mg# commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na. They have characteristic flat chondrite-normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65–1.15), low (La/Yb)N (0.98–6.4, mostly 4±) and low total REE contents (15–95 ppm). They also have high contents of compatible elements Cr and Ni (72–790 and 29–276 ppm, respectively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams. If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high Y contents (>15 ppm), low Sr/Y ratios (4.4–6.2), low (La/Yb)N, and high Mg# of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle wedge peridotite. New petrographic, chemical and isotopic [(143Nd/144Nd)I = 0.51221–0.51255 (εNd(t) +0.28 to +7.2); (87Sr/86Sr)I = 0.7029–0.7065] data suggest that the petrogenesis of the MAs in the north Xinjiang region may have involved: (1) multiple source materials including subducted oceanic slab, juvenile crustal materials (mainly volcanic-volcanoclassic rocks with low maturity and clear mantle geochemical signatures) coming from the forearc accretionary prism and mantle wedge peridotite; (2) a combination of different petrogenetic processes including partial melting of subducted oceanic slab and juvenile crustal materials, followed by interaction of slab melts with the mantle wedge peridotite; (3) high geothermal gradient creating a high temperature (>1,000°C) environment in a volatile-rich source region; (4) unique tectonic settings including oblique subduction, slab break off resulting in slab window formation and asthenosphere upwelling, and subduction erosion resulting in transfer of forearc accretionary materials into the source region of MA magma.  相似文献   

20.
The western Anatolian volcanic province formed during Eocene to Recent times is one of the major volcanic belts in the Aegean–western Anatolian region. We present new chemical (whole-rock major and trace elements, and Sr, Nd, Pb and O isotopes) and new Ar/Ar age data from the Miocene volcanic rocks in the NE–SW-trending Neogene basins that formed on the northern part of the Menderes Massif during its exhumation as a core complex. The early-middle Miocene volcanic rocks are classified as high-K calc-alkaline (HKVR), shoshonitic (SHVR) and ultrapotassic (UKVR), with the Late Miocene basalts being transitional between the early-middle Miocene volcanics and the Na-alkaline Quaternary Kula volcanics (QKV). The early-middle Miocene volcanic rocks are strongly enriched in large ion lithophile elements (LILE), have high 87Sr/86Sr(i) (0.70631–0.71001), low 143Nd/144Nd(i) (0.512145–0.512488) and high Pb isotope ratios (206Pb/204Pb = 18.838–19.148; 207Pb/204Pb = 15.672–15.725; 208Pb/204Pb = 38.904–39.172). The high field strength element (HFSE) ratios of the most primitive early-middle Miocene volcanic rocks indicate that they were derived from a mantle source with a primitive mantle (PM)-like composition. The HFSE ratios of the late Miocene basalts and QKV, on the other hand, indicate an OIB-like mantle origin—a hypothesis that is supported by their trace element patterns and isotopic compositions. The HFSE ratios of the early-middle Miocene volcanic rocks also indicate that their mantle source was distinct from those of the Eocene volcanic rocks located further north, and of the other volcanic provinces in the region. The mantle source of the SHVR and UKVR was influenced by (1) trace element and isotopic enrichment by subduction-related metasomatic events and (2) trace element enrichment by “multi-stage melting and melt percolation” processes in the lithospheric mantle. The contemporaneous SHVR and UKVR show little effect of upper crustal contamination. Trace element ratios of the HKVR indicate that they were derived mainly from lower continental crustal melts which then mixed with mantle-derived lavas (~20–40%). The HKVR then underwent differentiation from andesites to rhyolites via nearly pure fractional crystallization processes in the upper crust, such that have undergone a two-stage petrogenetic evolution.  相似文献   

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