Catherine Deniel

Crustal generation of the Himalayan leucogranites

Abstract Detailed studies of the Himalayan two-mica leucogranites, such as the Manaslu pluton, indicate that they have very uniform mineralogical, petrological and structural characteristics. One can relate their occurrence to the thickest zones of the underlying Tibetan Slab. In these zones, migmatization attains its greatest development and vertical extension. The granite is emplaced at first along the main disharmonic plane above the Main Central Thrust (MCT), at the top of the Tibetan Slab (infrastructure). Ductile deformation of the granite is variable; the granite being syn-to late-kinematic with regard to the functioning of the MCT. Major elements are very homogeneous (except for Na and K) implying that P-T conditions of melting were relatively uniform. The melted material was of a similar composition over a vast volume, and the percentage of melting was small (10–15%). Trace elements are highly variable. Some are characteristic for very evolved material (Ta, Rb, Cs, U) or show the link with the Tibetan Slab (Ba-Sr), whilst others are problematic (Th, REE). REE and Th abundances, being much less in the granite than in the Tibetan Slab, imply that they have been extracted during one of the main stages of formation, possibly by monazite. Radiogenic (Pb, Sr, Nd) and stable (O) isotopes are consistent with the origin of the granite from the Tibetan Slab. However, the heterogeneous Sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution. H-isotope data indicate that magmatic compositions of the main body of the Manaslu granite have been preserved. Late or post-magmatic alterations are extremely local in the main pluton. To the north, another belt of two-mica granites occurs whose characteristics are very similar to the High Himalaya belt. They were probably generated in a similar way during this recent intracontinental evolution.

Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low (ɛNdin: −13 to −16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.

ISOTOPIC AND TRACE ELEMENT SIGNATURES OF ETHIOPIAN FLOOD BASALTS : EVIDENCE FOR PLUME-LITHOSPHERE INTERACTIONS

Abstract Trace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau. Our aim was to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) basalts display a strong and variably developed lithospheric signature characterized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and Pb compared to oceanic basalts. The High-Ti magma type basalts (HT2) display much more homogeneous compositions and have ocean island basalt-like trace element signatures, whereas HT1 basalts exhibit intermediate compositions between those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variations (87Sr/86Sr = 0.70304–0.70429; 143Nd/144Nd = 0.51271–0.51298; 206Pb/204Pb = 18.00–18.86). Correlations among isotopic and trace element ratios provide evidence for the involvement of various mantle and crustal components in the petrogenesis of these flood basalts. Two distinct mantle components are involved in the genesis of the LT and HT2 extreme magma types. The HT2 basalts were derived from an ocean island basalt-like mantle component (87Sr/86Sr ∼ 0.704; 143Nd/144Nd ∼ 0.51295; 206Pb/204Pb ∼ 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a more depleted mantle component (87Sr/86Sr ∼ 0.7033; 143Nd/144Nd ∼ 0.5130; 206Pb/204Pb ∼ 18.6), either intrinsic to the plume itself or entrained in the Afar plume head during its ascent. Correlations of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature of the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. The effects of crustal contamination are much more evident in the LT basalts because of their much less enriched initial characteristics.

The northwestern Ethiopian Plateau flood basalts: Classification and spatial distribution of magma types

Abstract The extensive, complex, continental flood basalt (CFB) province which occurs in Ethiopia and Yemen consists of Oligocene prerift volcanism related to the Africa–Arabia continental break-up. Basalts from the northwestern Ethiopian Plateau exhibit a particularly large range of compositions and, for the first time in the Afro-Arabian CFB province, low-Ti basalts have been encountered. Major and some trace element data have been used to identify distinct geochemical groups and evaluate the role of differentiation processes. Three magma types have been distinguished: two high-Ti groups (HT1 and HT2) and one low-Ti group (LT). The transitional to tholeiitic LT suite exhibits low TiO2 (1–2.6%), Fe2O3* (10.5–14.8%), CaO/Al2O3 (0.4–0.75), Nb/La (0.55–0.85) and high SiO2 (47–51%). In contrast, the HT2 suite exhibits high TiO2 (2.6–5%), Fe2O3* (13.1–14.7%), CaO/Al2O3 (0.9–1.43), Nb/La (1.1–1.4) and low SiO2 (44–48.3%). The HT1 series is intermediate between the LT and HT2 groups. These three groups of lavas originated from different parental magmas. They display distinct differentiation trends, either controlled by the removal of a shallow level gabbroic (Pl+Ol+Cpx) assemblage (LT and HT1 suites) or by deeper Ol+Cpx fractionation (HT2 suite). Most of this thick continental flood lava pile was emplaced over a short time interval (about 1–2 Ma). The three contrasted magma types do not reflect a temporal evolution of their sources but rather a strong spatial control. Indeed, the northwestern Plateau may be subdivided into two different subprovinces as all the low-Ti basalts are located in the northern part of the plateau, and the high-Ti basalts are exposed in the eastern and southern parts. The LT and HT1 basalts display compositional ranges similar to those of the low- and high-Ti groups from other main CFB provinces (e.g. Parana, Deccan, Karoo, Siberia, …). However, the HT2 group exhibits extreme OIB-like compositions. This unusual geochemical signature suggests the involvement of deep mantle in the genesis of the HT2 magmas. The LT compositions rather reflect the participation of the continental lithosphere, through mantle derived melts and/or crustal contamination.

Temporal evolution of mantle sources during continental rifting : the volcanism of Djibouti (Afar)

Magmatism occurred almost continuously over the past 25 m.y. in the Republic of Djibouti. Lavas are mainly basic to intermediate with some rhyolites. Large chemical and isotopic variations among the volcanic series are interpreted in terms of mantle source heterogeneity. Crustal contribution is only evidenced in the oldest rhyolites emplaced during the initial stages of rifting. Excluding these old rhyolites, a clear evolution through time of the mantle sources is observed in relation to rifting. Three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (87Sr/86Sr ≈ 0.706, 206Pb/204Pb ≈ 17.9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (high U/Pb ratio)-type reservoir, and (3) a depleted mantle. As rifting goes on, there is an increasing contribution of an HIMU-type mantle source. It is attributed to the influence of a mantle diapir (Afar plume) thermally eroding the subcontinental lithosphere. The geochemical characteristics of 9 to 1 Ma old lavas, erupted after the strong increase of spreading rate in Afar, reflect this evolution of mantle sources. The influence of the mantle plume is most prominent in the northern youngest lavas (<1 Ma), particularly Manda, characterized by the strongest HIMU signature (87Sr/86Sr ≈ 0.7035, 206Pb/204Pb ≈ l9.2). The contribution of the depleted mantle component originating from the asthenosphere is best recognized in the young (<4 Ma) lavas, particularly Tadjoura and Asal lavas (3 to 1 Ma). The evolution of Djibouti lava sources through time may be accounted for by the recent models developed for plume structure.

Crustal control in the genesis of Plio-Quaternary bimodal magmatism of the Main Ethiopian Rift (MER): geochemical and isotopic (Sr, Nd, Pb) evidence

Abstract A striking feature of the recent Asela–Ziway magmatism ( N 2 is observed among the most differentiated samples (SiO 2 >68%). A strong negative Eu anomaly is also observed in felsic samples. The Asela–Ziway mafic samples and Chilalo rocks display smooth multi-element patterns with positive Ba and strong negative Cs anomalies. Negative Sr and Ti anomalies appear in Chilalo intermediate lavas and become particularly important in the Asela–Ziway rhyolites, reflecting lower-pressure fractionation of plagioclase, alkali feldspar and titanomagnetite. Strong negative Ba and P anomalies, reflecting alkali feldspar and apatite fractionation, are also observed in these rhyolites. Mafic and felsic samples display a similar range of Nd and Pb isotopic ratios: 0.51265 143 Nd / 144 Nd 206 Pb / 204 Pb 207 Pb / 204 Pb 208 Pb / 204 Pb 87 Sr / 86 Sr (0.70392–0.70510). Differentiation among these rocks is accounted for by fractionation of more primitive melts plus variable degrees of contamination, at variable depths, by heterogeneous Pan-African crust. A two-stage model involving small degrees (∼10%) of melting of the mafic lower crust (underplated magmas?) followed by moderate degrees (∼40%) of low-pressure fractionation (mostly Na-feldspar) is proposed to account for the genesis of the huge volumes of pantelleritic ignimbrites. The high F content of apatite in Chilalo benmoreites as well as in waters coming from felsic and alkaline rocks in this sector of the MER give evidence for anomalously high halogen concentration in these rocks. In this area, melting of the lower crust was induced by the thermal anomaly created by the Afar mantle plume and may have been enhanced by the presence of halogens, either originating from the deep mantle or/and scavenged from the surrounding crust.

The Hasan Dagi stratovolcano (Central Anatolia, Turkey) : evolution from calc-alkaline to alkaline magmatism in a collision zone

Abstract The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Kecikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi ( 87 Sr / 86 Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO2). Meso- and Neovolcano differentiated lavas (60–68% SiO2) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO2) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene.

New 230Th-238U and 14C age determinations from Piton des Neiges volcano, Reunion — A revised chronology for the Differentiated Series

230Th-238U age measurements were carried out on 18 samples of lava from Piton des Neiges. These lavas range from basalts to trachytes. The ages extend from 270,000 to 12,000 a B.P. and cover most of the period in which Differentiated Series magmas were erupted. New 14C ages on charcoal sampled in young pyroclastic deposits are also presented. The late-stage evolution of the volcano is discussed in the light of these new ages and a chronology of the explosive activity is given. The oldest pyroclastic flow (Ste. Suzanne) is more than 220,000 a; other pyroclastic units were emitted between 220,000 and 110,000 a. There were at least two episodes of caldera formation: the first episode is inferred to have occurred about 190,000 a ago; the last occurred between 190,000 and 110,000 a. Although the most recent lava activity was mainly concentrated on the eastern flank of the volcano and around the summit area, it also occurred, about 70,000 a ago, in the southwestern part of the massif. The youngest explosive activity (about 50,000 to 20,000 a) on the eastern flank of the volcano has now been dated by three different methods (14C and Th-U ages on block- and ash-flow tuffs, K-Ar ages on associated domes). Youthful explosive activity (about 35,000 to 12,000 a) on the western flank of the volcano is also demonstrated by 14C dating on charcoal and by a Th-U age of 12,000 a on a trachytic nuee ardente deposit. This last age indicates that activity may have persisted later than previously supposed and implies a high rate of erosion for the Cirque de Mafate.

Magmatic evolution of Pacaya and Cerro Chiquito volcanological complex, Guatemala

Four major phases are distinguished during the building of the Pacaya volcanological complex (Guatemala): (1) the ‘ancestral volcano’, now much eroded, covered by younger deposits and battered by faulting and landslides; (2) the initial cone made up of large lava flows and dated at about 0.5 Ma; (3) andesito-dacitic domes (Cerro Chiquito dome and others) emplaced during an extrusive phase at about 0.16 Ma; and (4) the active Pacaya volcano. Lavas of phases 2 and 4 are basalts and basaltic andesites with almost the same major and trace element compositions. Classical enrichment in LILE and depletion in HFSE are observed. Phase 3 domes show magma-mingling features. The dacitic host rock includes basaltic andestic enclaves, 20 to 30% in volume. According to geochemical and mineralogic data (Mg/Fe ratios of basic minerals higher in dacite, groundmass glasses sodic in dacite and potassic in basaltic andesite), the basaltic andesites and dacites of phase 3 cannot be related by a simple fractional crystallization process. The existence of such differences suggests that magma mingling/mixing processes were involved by a connection between the two magma chambers prior to the extrusion of the andesito-dacitic domes. However, some trace element data clearly suggest that fractional crystallization played a significant role in the differentiation of these lavas. Remelting of amphibole-bearing cumulates from the dacite may also have played a role in the basaltic andesitic liquid genesis. Thermodynamical parameters of each liquid are contrasted. The basaltic andesitic magma, at a high temperature (1037°C) and in relatively small amounts, is embayed in the cooler (905° C) dacitic magma. The former liquid, denser (2.72) and less viscous (103.31 poises for free crystal liquid) may crystallize while the latter, lighter (2.60) and more viscous (104.46 poises), remains still liquid. Isotopic data (0.70383<87Sr/86Sr <0.70400; 0.512785<143Nd/144Nd<0.512908; 18.61<206Pb/204Pb<18.66; 15.56<207Pb/204Pb <15.58; 38.30<208Pb/204Pb<38.40) indicate that all the lavas (from Pacaya as well as from Cerro Chiquito) are cogenetic and derive from the same mantle source. Sr, Nd and Pb isotope ratios are similar to those of OIBs. (230Th/232Th) activity ratios on two historical lavas are respectively 1.2 and 1.3. The Th excess is similar to that of other calcalkaline volcanoes emplaced on a continental crust. These lavas evolved, possibly in separate magma chambers, through processes of fractional crystallization and magma mixing.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Post-collisional potassic-rich volcanism of Gölcük Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Gölcük flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Gölcük lavas were derived from a metasomatized lithospheric mantle source containing phlogopite–amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Gölcük lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte–trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte–trachyandesites of the extracaldera volcanics. The Sr–Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Gölcük volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Gölcük lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19–19.54, 207Pb/204Pb = 15.64–15.67, 208Pb/204Pb = 39.12–39.49) indicate an enriched mantle source region.