Cenozoic Magmatism Research Articles

Early cenozoic magmatism in the continental margin of Kamchatka

The paper presents isotopic-geochemical features of magmatic rocks that were produced at the continental margin of Kamchatka during its various evolutionary stages. Continental-margin magmatism in Kamchatka is demonstrated to have evolved from the Paleocene until the present time. The Paleocene and Middle-Late Eocene magmatic complexes show features of suprasubduction magmatism. The magmatic melts were derived from isotopically heterogeneous (depleted and variably enriched, perhaps, as a consequence of mixing with within-plate melts) mantle sources and were likely contaminated with quartz-feldspathic sialic sediments. The Miocene preaccretion stage differs from the Paleogene-Eocene one in having a different geochemical and isotopic composition of the mantle magma sources: the magmatic sources of the Miocene suprasubduction magmas contained no compositions depleted in radiogenic Nd isotopes, whereas the sources of the within-plate magmas were enriched in HFSE. The Late Pliocene-Quaternary postaccretion magmas of the Eastern Kamchatka Belt are noted for the absence of a within-plate OIB-like component.

Tectonic and magmatic evolution of the northwestern Basin and Range and its transition to unextended volcanic plateaus: Black Rock Range, Nevada

The seismically active eastern and western margins of the northern Basin and Range have been extensively studied, yet the northwestern margin of the province remains incompletely understood. The Black Rock Range of northwestern Nevada straddles the transition from the Basin and Range province to the south and east, and flat-lying volcanic plateaus to the west. This poorly understood range preserves a remarkably complete record of Cenozoic magmatism and provides an important window into the pre-Miocene history of the unextended volcanic plateaus of northeastern California and southern Oregon. Geologic mapping and 40Ar/39Ar geochronology from the northern Black Rock Range document three significant episodes of Eocene to middle Miocene volcanism. Eocene (35 Ma) basalts directly overlie Mesozoic granites and arc-related volcanic and sedimentary rocks. Locally erupted Oligocene to early Miocene (27–21 Ma) bimodal volcanic rocks comprise the bulk of the Cenozoic section and conformably overlie the Eocene basalt flows. These bimodal units include rhyolitic lavas, variably welded rhyolitic ash flows, unwelded ash-fall deposits, and thin basalt flows. In the neighboring Pine Forest Range ∼20 km to the north, similar Oligocene to early Miocene units are overlain by more than 500 m of ca. 16.4 Ma Steens-equivalent basalt flows and are capped by ca. 16 Ma rhyolitic ash-flow tuffs. In the northern Black Rock Range, the ca. 16.4 Ma middle Miocene basalts are absent from the section, and a 16.2 Ma rhyolitic ash-flow tuff directly overlies the early Miocene flows. Basaltic and rhyolitic volcanic products in the northern Black Rock Range span 35–16 Ma, with many of the Oligocene volcanic units derived from local vents and dikes. Despite the map-scale complexities of locally derived lava flows, the Cenozoic section is broadly conformable and dips gently (∼5°–10°) to the northwest. The region experi enced no significant tilting between 35 and 16 Ma, with moderate tilting (∼5°–10°) and concomitant uplift occurring after 16 Ma. This tectonic history is consistent with that of the nearby Pine Forest and Santa Rosa Ranges, where low-temperature thermochronology documents footwall exhumation along the range-bounding normal faults after 12 Ma. The velocity structure of the crust beneath the northern Black Rock Range is constrained by a recent geophysical survey (seismic reflection, refraction, and gravity) and contains gradients that correspond to basin depths predicted by our geologic mapping. Together with recently completed geological and geophysical studies from the surrounding region, our results suggest that the evolution of the northwestern margin of the Basin and Range was characterized by long-lived and voluminous volcanism without significant tectonism, followed by low-magnitude (≤20%) extension along high-angle normal faults.

A tephra chronostratigraphic framework for the Frontier Mountain blue-ice field (northern Victoria Land, Antarctica)

Englacial tephra provide chronostratigraphic markers in the Antarctic ice sheets. Structural, mineralogical, geochemical and geochronological data on selected samples allowed the reconstruction of a chronostratigraphic framework for the Frontier Mountain blue-ice field—an important meteorite trap on the southeastern flank of Talos Dome in northern Victoria Land. The stratigraphic thickness of the blue-ice succession is ∼1150 m. The 40Ar– 39Ar age of one layer close to the stratigraphic bottom of the ice succession is 100±5 ka and constrains the maximum age of the bulk of Frontier Mountain blue ice. The 49±11 ka age of a second layer at a depth of ∼950 m in the stratigraphic succession indicates that >90% of the ice is younger than this value. These ages agree well with the terrestrial ages of meteorites found on the blue ice (up to 140±30 ka), suggesting a mechanism of exhumation of meteorites by ablation after englacial transport. Particle size (up to several tens of microns) and the alkaline compositional character of 22 layers allow correlation with source volcanoes within the Cenozoic magmatism associated with the West Antarctic Rift System. The proximal Mount Melbourne, Mount Rittman and The Pleiades (within a radius of ∼250 km) are the best candidate source volcanoes. The reconstructed chronostratigraphic framework thus lays the foundations for a detailed investigation of ∼100 ka of explosive volcanism in northern Victoria Land. Furthermore, in light of the ongoing ice core drilling project at Talos Dome, the Frontier Mountain ice succession may become important for establishing regional correlations, for sampling and dating key tephra layers, and for selecting ice successions for high-resolution studies of past atmospheric chemistry and fallout.

Sources and evolution of the Cenozoic suprasubduction magmatism of the Olyutorsky tectonic block, southern Koryak highland

This paper reports the results of an investigation of the geochemical and isotopic compositions of rocks formed during the Eocene suprasubduction magmatism in the Olyutorsky tectonic block. The contribution of various suprasubduction components to the formation of magmatic melts was estimated; the characteristics of the Eocene and Miocene-Quaternary suprasubduction magmatism of the Olyutorsky tectonic block were compared; and relations of the Cenozoic magmatism to the tectonic development of the block were evaluated. The Eocene-early Oligocene suprasubduction magmas were derived from geochemically and isotopically heterogeneous garnet lherzolites in a mantle wedge. The initially depleted lherzolites of the mantle wedge were probably locally and variably enriched by OIB-type mantle melts before the generation of island-arc magmas and then again depleted below the MORB level by the extraction of magmatic materials from them. In the Eocene, a considerable amount of quartz-feldspar sediments enriched in radiogenic Nd was consumed in the subduction zone, which resulted in a strong contamination of magmas derived from the garnet lherzolites of the mantle wedge. The later stages of subduction were accompanied by active generation of adakite magmas with depleted Nd isotope signatures and HFSE-rich melts showing no evidence for their contamination by sialic sediments. It was supposed that the Late Cenozoic subduction zone plunged northward beneath the Olyutorsky tectonic block. It was shown that the established characteristics of the suprasubduction magmatism of the Olyutorsky tectonic block could be related to Cenozoic spreading processes in the proto-Komandorsky basin of the Bering Sea.

The 40Ar/39Ar chronology and eruption rates of Cenozoic volcanism in the eastern Bering Sea Volcanic Province, Alaska

The eastern sector of the Bering Sea Volcanic Province, Alaska, consists of a number of large late Cenozoic volcanic fields that occur in a broad region inboard from the Aleutian arc front to the Arctic Circle. We estimate that about 750 km3 of magma were erupted in the volcanic centers that we have studied, all within the past 6.0 Ma. Several discrete eruption episodes have been identified with the new 40Ar/39Ar data at circa 6.0 Ma (Imuruk volcanic field), circa 3.5 Ma (Teller volcanic field), circa 2.5–1.5 Ma (St. George Island), circa 1.0 Ma (St. Lawrence Island), and the youngest activity which started at circa 0.7 Ma and continued throughout region until historic times. Combining age information with volume estimates reveals that the intensity of volcanic activity in the Bering Sea Volcanic Province has increased through time, with only about 15% of lava erupted before 3 Ma and about 30% of all late Cenozoic magma erupted within the last 500 ka. Eruption rates also increase toward more recent times, from the 6 Ma Imuruk basalts, which erupted at the rate of ∼70 ± 15 m3 km−2 yr−1, to the ≤0.7 Ma Nunivak Island and St. Michael volcanic field basalts, which erupted at a rate of ∼165 ± 25 m3 km−2 yr−1. High volumes of erupted lava accompanied by high eruption rates for the youngest volcanic rocks suggest significant changes in the melting rates, which are best explained by either an evolving tectonic regime or significant changes in the melting processes.

Partial Melting of Thickened Tibetan Crust: Geochemical Evidence from Cenozoic Adakitic Volcanic Rocks

Major and trace elements and Sr-Nd-Pb isotopic data are presented for the Late Paleogene to Neogene age (9.4-26.9 Ma) adakitic volcanic rocks from the northern Qiangtang region, Tibetan Plateau. Lacking Eu depletion, this group of rocks exhibits high SiO2 (>58%), Sr (>350 ppm) and La/Yb (>22) ratios, and low Y (<18 ppm) and Yb (<1.5 ppm). Samples plot within the adakite field on the Sr/Y versus Y diagram. Isotopic compositions of this group rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.706365 to 0.708156; 208Pb/204Pb = 38.955 to 39.052; 207Pb/204Pb = 15.651 to 15.672; 206Pb/204Pb = 18.679 to 18.839), and non-radiogenic Nd (143Nd/144Nd = 0.512411 to 0.512535; εNd = -2.01 to -4.43). These geochemical features suggest magma generation by partial melting of an eclogitic mafic lower crust beneath the Tibetan Plateau. On the basis of Mg#, the Qiangtang "adakites" can be divided into two groups—a high-Mg group (Mg# ≥0.45) and a subordinate, low-Mg group (Mg# ≤0.45). These groups are attributed to convective thinning of the enriched lithospheric mantle beneath the Qiangtang region and southward intracontinental subduction of the Kunlun-Qaidam block along the Jinshajiang suture (JRS) zone, respectively. Our study indicated that both convective thinning of the mantle lithosphere and intracontinental subduction played important roles in the Cenozoic magmatism of northern Tibet.

Lithospheric control on late Cenozoic magmatism at the boundary of the Tuva-Mongolian Massif, Khubsugul area, northern Mongolia

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic tra- chyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melt- ing processes demonstrates that the basaltic magmas were produced mainly by 0.5-5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor control- ling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimen- tal data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km ( 24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.

Mesozoic to Cenozoic magmatism, Lorne Basin, NSW

SUMMARY The he Basin on the mid north coast region of New South Wales is a small circular-shaped basin within the southern margin of the New England Fold Belt. It comprises an Early Triassic terrestrial sedimentary sequence (the Camden Haven Group) overlain by rhyolitic volcanics which appear to be co-genetic with the Late Triassic granitoids (the Brothers) which intrude the Lorne Basin sedimentary sequence. Over the last few years, we have conducted fieldwork in the Lome Basin, mainly investigating the magmatic rocks, as part of a larger project on the volcanic sequences of eastern Australia. Initial work resulted in the identification of a number of rhyolitic dykes intruding the Brothers granitoids, previously unrecorded. As a result of this, we then undertook detailed geochronological and geochemical investigations of the magmatic sequences throughout the basin to determine their relationship to each other, and other peripheral sequences, outside of the basin. Our results show that the Lome Basin had a more complex and protracted magmatic history than previously thought, beginning with Late Triassic rhyolitic volcanics, followed by co-genetic rhyolitic volcanics and granitoids, then Late Jurassic and Early Cretaceous rhyolitic volcanics, terminating in Mid Miocene mafic volcanism.

Neogene volcanic activity of western Syria and its relationship with Arabian plate kinematics

The Cenozoic (mostly Neogene) volcanic activity in Syria is part of the extensive magmatism that took place in the Mashrek Region, Middle East, from upper Eocene to Holocene (∼40–0.0005 Ma). Samples in western Syria are mostly high TiO2 (TiO2 ∼1.8–3.7 wt.%) alkaline mafic rocks (basanites, hawaiites and alkali basalts) plus rare transitional/tholeiitic basalts and basaltic andesites) with within-plate-like trace element signature. On the basis of incompatible trace element content, the volcanic activity in Syria has been divided into two stages: the first lasting from ∼25 to ∼5 Ma and the second from ∼5 to recent times. Indeed, the Syrian lavas show incompatible trace element content increasing with decreasing age from ∼25 to ∼5 Ma, followed by an abrupt decrease to low values roughly at the Miocene–Pliocene boundary. This temporal shift in composition is related to major tectonic re-organization occurred during upper Miocene. The proposed petrogenetic model invokes three steps: (a) passive upwelling of the shallow asthenosphere during the development of the Dead Sea transform fault system. Different degrees of partial melting were followed by variable extents of fractional crystallization and limited upper crustal contamination; (b) the Miocene–Pliocene boundary tectonic change enhanced passive decompression of the same sources and a consequent increase in degree of partial melting resulting in low incompatible trace element content of the relatively high-volume liquids; (c) after this phase, the incompatible trace element content in the basaltic magmas increased as consequence of fractional crystallization processes. Major and trace element content similarities with the rest of the circum-Mediterranean igneous rocks are consistent with a common relatively shallow origin for the Cenozoic anorogenic magmatism of the entire circum-Mediterranean area (the so-called Common Magmatic Reservoir). Because much of the igneous activity in the studied area is concentrated near the Dead Sea fault, the origin of Cenozoic magmatism in Syria (and in the rest of the circum-Mediterranean area) reflects a strong lithospheric control on the loci of partial melting. Mantle plumes from lower mantle and/or north-westward channelling of the Afar mantle plume is not needed to explain volcanic activity in Syria and the Mashrek area.

K-Ar age and Sr-Nd characteristics of subalkali basalts in the Central Georgian Neovolcanic region (Greater Caucasus)

Late Cenozoic magmatism in the Greater Caucasus during the last 8.5 Ma was manifested in the Elbrus, Kazbek, and Central Georgian neovolcanic regions. In our work, we present K‐Ar dates obtained for young rocks of Central Georgia, which, in turn, allowed us to determine the age of volcanism in this region and specify its role in the Neogene‐Quaternary magmatic history of the Greater Caucasus. In addition, based on new Sr‐Nd isotopic‐geochemical and petrochemical data, we consider the petrogenesis of volcanics in this region and the problem of material source for young igneous rocks in the Greater Caucasus as a whole. The present work continues our recent geochronological investigations in this region [1‐5]. The Central Georgian neovolcanic region is located

Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.

The temporal and spatial distribution of volcanism in the South China Sea region

Cenozoic magmatism in the South China Sea, the associated continental margins, coastal South China and Indochina is reviewed. Volcanism contemporaneous with rifting and sea floor spreading in the South China Sea was very weak in the margins and adjacent areas. There was only very limited magmatic activity, most probably after the cessation of sea floor spreading. The South China Sea has non-volcanic margins. Large basement relief caused by extension formed major basins and a continental-oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting. The paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the South China Sea, which was supposedly linked to the escape of Indochina induced by Indo-Eurasia collision.

Thermal plume models and melt generation in East Africa: A dynamic modeling approach

The hypothesis that thermal plumes contribute to the Cenozoic magmatism in East Africa is now widely accepted. The controversy lies on how many plumes exist and where they may be located. In this study we show numerical experiments of mantle convection models for a number of thermal plume models and discuss the implications for the melt generation in East Africa. We investigate how the plume(s), the Tanzania craton, and the African lithospheric structure may interplay to result in the magmatism distribution in East Africa since the Eocene. Our results demonstrate that the variable thickness of the lithosphere modulates melt generation. A single-plume model cannot reproduce the observations consistently. Double-plume models with plumes located at Afar and Kenya regions are viable with reasonable physical properties. The distribution of the plume material, however, is sensitive to the angle at which the Tanzania craton and regions of thick lithosphere approach the plume, as the African plate moves. Models that have present-day location of the second plume (Kenya plume) under the Eastern rift or the interior of the Tanzania craton can best match the basalt distribution. Our model results suggest that the basaltic eruptions associated with the Afar plume tap a relatively deep source of the plume body in general, whereas melting occurs at shallower depths for the Kenya plume except for the Eocene episode. The magmatism is derived from a more depleted mantle source in the low-Ti basalt province of northwestern Ethiopia. Our experiments indicate the thermal influence of the Afar plume but predict an absence of plume-derived melts, suggesting the melt generation within lithosphere triggered by thermal influence of Afar plume in this region. Our model results suggest that plume plays an active role on the initiation of the rifting process in East Africa.

δ11B as tracer of slab dehydration and mantle evolution in Western Anatolia Cenozoic Magmatism

AbstractBoron isotope data are presented for Cenozoic Western Anatolia rocks, which define two main associations: (i) calc‐alkaline, shoshonitic and ultra‐potassic rocks (Early to Middle Miocene); and (ii) Late Miocene–Quaternary intraplate alkali basalts. Boron data, together with Sr–Nd isotope and other trace elements, are consistent with a progressive dehydration of the slab, producing fluid phases gradually depleted in B (and 11B). These fluids were added to the supraslab mantle, triggering a partial melting that gave rise to orogenic magmatism. The stretching and tearing of the slab caused by the faster convergence of Greece over Africa with respect to Anatolia facilitated an interaction of the upwelling subslab asthenosphere with residual slab‐fluids during the Late Miocene followed by production of typical intraplate magmas during the Pleistocene–Holocene, whose relatively high δ11B (approximately −2‰) is considered representative of the local asthenosphere not affected by subduction contamination.

Episodes of Cenozoic Gold Mineralization on the Eastern Margin of the Qinghai-Tibet Plateau: 40Ar/39Ar Dating and Implication for Geodynamic Events

Abstract A lot of new gold deposits have been found on the eastern margin of the Qinghai-Tibet Plateau during the past two decades. Among them, three main types of gold deposits have been recognized, including quartz-vein-type, shearzone-type and porphyry-type. The former two types of gold deposits are mainly hosted within metamorphic rocks, while the latter is related to Cenozoic magmatism. Although all of these gold deposits are believed to have been formed during the uplift process of the Qinghai-Tibet Plateau in the Cenozoic era (Wang et al., 2002b), precise isotopic age constraints have still been lacking until quite recently. This paper presents new 40Ar/39Ar data of some gold deposits on the eastern margin of the Qinghai-Tibet Plateau, which indicate that gold mineralization in the region occurred in response to the episodic stages of the orogenies. Recently obtained 40Ar/39Ar data on quartz and feldspars from several gold deposits, such as the Sandiao deposit, the Baijintaizi deposit, the Pusagang deposits, provide new constraints on gold mineralization on the eastern margin of the Qinghai-Tibet Plateau. Geochronological studies of gold deposits along the Daduhe River indicate that there are three stages of gold mineralization. The early two stages occurred as early as 65.1 Ma in the Shuibaiyang deposit and 58.95 Ma in the Ruoji deposit, while the latter stage occurred as late as 25.35 Ma in Baijintaizi and 24.70 Ma in Sandiao. Isotopic dating of three plagioclases from the Beiya deposit, Zhifanggou deposit and Luobodi deposit and a K-feldspar from the Jinchangqing deposit in Yunnan Province indicates that these deposits were formed at two stages. The Zhifanggou and Jinchangqing deposits have early stage records as old as 58.82 Ma in Zhifanggou and 55.49 Ma in Jinchangqing, but all of the above four deposits in Yunnan have late stage records of 23.18 Ma in Jinchangqing, 24.54 Ma in Zhifanggou, 24.60 Ma in Luobodi and 24.56 Ma in Hongnitang. The above results suggest that the gold deposits on the eastern margin of the Qinghai-Tibet Plateau were formed concentratedly at two main episodes, i.e. the end of the Paleocene (about 58 Ma) and the boundary between the Paleogene and the Neogene (about 25 Ma). The later episode appears to be looks like more important and was coupled with the Sichuan movement, which was extensively activated at that period. The beginning of the Cenozoic Era (about 65 Ma) might be another episode of gold mineralization, but only one deposit (Shuibaiyang) in this study has been proved to have been be formed at this stage and might be earlier than the initial collision between the Indian Plate and the Eurasia Plate. In view of geology, the above three episodes of gold mineralization are associated with three events of tectonic-magmatism and/or fluid events. Even though the gold deposits (for example, the Shuibaiyang deposit, Ruoji deposit and Pusagang deposit) were formed at different episodes, all of them are genetically related to tectonic movements in large-scale shear zones. It looks like theat tectonic events (including large-scale strike-slip) between Paleogene and Neogene had a wide influence upon gold mineralization, with new deposits formed and old deposits enriched or superimposed to be a higher grade by new stage of mineralization. The above data suggest that gold deposits were not only concentrated in some areas, but also formed mainly at different boundaries of geological times, indicating that there existed some peak stages of gold mineralization (metallogenic episodes), and that the gold deposits were formed mainly by episodic mineralization.

A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin

Common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long‐lived, low‐volume, diffuse alkaline magmatic province (DAMP) encompassing the easternmost part of the Indo‐Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, in contrast to large igneous provinces where mantle temperatures are presumed to be high. The SW Pacific DAMP magmatism has been conjecturally linked to rifting, strike‐slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. We suggest instead that sudden detachment and sinking of subducted slabs in the late Cretaceous induced Rayleigh‐Taylor instabilities along the former Gondwana margin that in turn triggered lateral and vertical flow of warm Pacific mantle. The interaction of the warm mantle with metasomatized subcontinental lithosphere that characterizes much of the SW Pacific DAMP concentrates magmatism along zones of weakness. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline magmatism, where the oceanic areas are characterized primarily, but not exclusively, by short‐lived hot spot tracks not readily explained by conventional mantle plume theory. This proposed south Pacific DAMP is much larger and longer‐lived than previously considered.

PYROXENITES AND MEGACRYSTS IN ALKALINE BASALTIC MAGMAS FROM NORTHERN VICTORIA LAND (ANTARCTICA): CONSTRAINT ON THE THERMAL EVOLUTION OF SUB-CONTINENTAL LITHOSPHERE

Cenozoic alkaline basaltic magmas of northern Victoria Land, Antarctica yield a wide variety of ultramafic xenoliths including clinopyroxene-rich xenoliths and augite and kaersutite megacrysts. Pyroxenites and megacrysts were collected in Mt Melbourne Volcanic Province, respectively at Browning Pass, Baker Rocks (both sites are near the coast of the Ross Sea) and at the base of Mt. Overlord, about 80 km inland. Sixteen samples have been petrographycally examined and chemically analyzed, including trace element analyses of minerals by ion microprobe. The dominant assemblage of pyroxenites consists of clinopyroxene + olivine ± spinel ± amphibole with strongly variable modal proportions of minerals. Rare interstitial plagioclase is found in Browning Pass samples. All pyroxenites belong to Al-augite Group (Frey and Prinz, 1978); the collection includes wehrlites, clinopyroxenites and rare dunites and olivinewebsterite. Moreover, Baker Rocks xenoliths are sometimes composite showing wehrlite and depleted lherzolite lithologies in sharp contact. Their typical texture is cumulitic and in only few cases deformation fabrics and polygonal texture may be found. Mt Overlord samples show widespread metasomatic effects, evidenced by clinopyroxene-amphibole replacement and by variably re-crystallized melt pools. In the Browning Pass pyroxenites the metasomatic effects are incipient as evidenced by rare amphibole lamellae in substitution of the clinopyroxene. In few samples from Baker Rocks and Mt. Overlord clinopyroxenes show orthopyroxene exsolutions. Major and trace element compositions of bulk rock and minerals indicate that pyroxenites and megacrysts are related to Cenozoic alkaline magmas and that formed through polybaric fractionation processes affecting McMurdo alkaline magmas. We evaluated the P and T of segregation of clinopyroxenes from parent basaltic melts. The obtained data depict a P-T gradient of 15°C/kbar for the whole area in contrast with widespread volcanic activity. If we hypothesize that pyroxenite veins represent the walls of the paths followed by adiabatically rising basaltic melts, we can assess the temperature of the ambient mantle, if we know the mantle potential temperature (Tp) at some depth. From the onset of crystallization in primitive melts we get Tp =1300-1350°C at P= 2- 2.5GPa. In this way we obtain, for the present day geothermal gradient of the area, a more realistic value of 30°C/kbar. This value is considerably higher than the 8°C/kbar obtained from the equilibration coarse crystals of lherzolite nodules in the same area and allows to clearly identify the heating of the mantle due to Cenozoic magmatism. The different thermal profile obtained from the pyroxenites with respect to those obtained by spinel-peridotite xenoliths, thus seems to be linked to the geodynamic evolution in this area. In particular the computed geothermal gradient fits with the heat-flow values of this area that range between 66 e 114 mW/m2 (Blackman et al., 1987; Della Vedova et al., 1992). The gradient that we propose seems to be compatible with a static rift geotherm of 90mW/m2 . See figure.

LITHOSPHERIC MANTLE EVOLUTION IN THE WESTERN-CENTRAL MEDITERRANEAN AREA: EVIDENCE FROM WITHIN-PLATE CENOZOIC VOLCANISM AND ASSOCIATED MANTLE XENOLITHS

Extension-related within-plate Cenozoic volcanism in the central-western Mediterranean mostly occurs in three provinces: Veneto, Adria plate, i.e. African northern promontory; Iblei (Sicily), African lithosphere; and Sardinia, a drifted fragment of the European lithosphere. In Veneto (Paleogene) and Iblei (Neogene-Quaternary), transtensional rift volcanism developed as foreland reaction to collisional processes along the Alpine and Maghrebian chains respectively, generating prevailing basic magmas - from tholeiites to Na-alkali basalts and nephelinites. In Sardinia, the Pliocene-Quaternary volcanism - related to general tensional tectonics of the central Mediterranean – produced comparatively more potassic magmas from subalkaline basalts, alkali basalts/trachybasalts to basanites, locally associated with rhyolitic and phonolitic differentiates. Integrated petrogenetic study, based on incompatible element and Sr-Nd-Pb isotope systematics for the three volcanic provinces, leads to the following constraints: 1) the primary magmas, from tholeiites, alkali basalts to basanites and nephelinites, were generated by decreasing melting degrees of progressively deeper lithospheric mantle sources (ca. 30 to 100 km depth); 2) extensive and multiple enrichment processes by alkali-silicate metasomatizing components widely affected all mantle sources, which invariably require 5 – 10% metasomatic volatile-bearing phases, mainly amphibole for Veneto and Iblei, and phlogopite + amphibole for Sardinia; 3) a previously depleted lithospheric mantle (DM) is enriched by prevailing HIMU (with subordinate EMII) geochemical components in both Veneto and Iblean Provinces, whereas in Sardinia distinctly more potassic EMI signature predominates, in addition to HIMU; 4) in all the investigated provinces, the more alkaline deeper basic magmas (compared to sub-alkaline basalts) show relatively more marked HIMU signatures, also with involvement of carbonatitic components in the lowest lithospheric sources where nephelinites were generated. Mantle xenoliths associated to alkaline lavas of the three volcanic provinces, generally represent shallow portions of the lithospheric mantle column (< 40-50 km depth, i.e. in the Mechanical Boundary Layer, MBL) according to thermobarometric estimates and rheologic characteristics (e.g. grain size). Petrological data invariably indicate that these mantle xenoliths underwent a complex compositional evolution, characterized by depletions due to extraction of basic magmas (mostly occurring in the pre-Palaeozoic), and multiple metasomatic enrichments reflected in compositional variations of the constituent mineral phases, particularly pyroxenes, and the variable occurrence of pyrometamorphic textures and new phases, including glass + feldspar + amphibole + phlogopite. The resulting isotopic signatures conform well to those of their host magmas, being dependent on the variable contributions of the DM, HIMU, and EM components: a prevailing HIMU imprint is recorded for both Veneto and Iblean mantle materials, whereas a predominant EMI component, in addition to HIMU, is observed for the Sardinian mantle. A comparison at a circum-Mediterranean scale reveals that the geochemical features of Sardinia are also observed in the Cenozoic anorogenic magmas and associated mantle xenoliths located within the Variscan basement of centralwestern Europe. Peridotite massifs, tectonically emplaced during the Alpine orogeny, are similarly characterized (in addition to HIMU) by a prevalent EMI signature, which could thus have been active at least since the pre-Mid Mesozoic. By contrast, EMI signature is lacking in Veneto and Iblei, as well as in the mantle xenoliths and host alkaline lavas from other occurrences of the North African domain, where the HIMU component predominates. The widespread presence of HIMU in the European/North African domains may be related to the existence of a Cenozoic plume region extending from the eastern Atlantic to Central Europe and Central-Western Mediterranean, also suggested by the seismic tomographic data. Accordingly, the more recent HIMU metasomatizing agents rising from the convecting mantle appear to have been effectively accumulated in the lower lithospheric portions (i.e. Thermal Boundary Layer, TBL), whereas older metasomatic components (e.g. EMI for Europe) may have been better preserved in the upper, more rigid lithospheric mantle (i.e. MBL).

エチオピア北西部Bure地域の新生代マグマ活動：プルーム-リソスフェア相互作用

エチオピア北西部Bure地域の新生代マグマ活動：プルーム-リソスフェア相互作用

後期新生代，東北本州弧における火成活動史と地殻・マントル構造

東北本州弧の後期新生代火山岩類の地質学的・岩石学的研究により,火成活動の時間的発展が詳細に明らかになってきた.この時間的発展は,島弧の構造地質学的発展と密接に関連している.火山活動には明らかに3つのステージが識別され,それらは大陸縁辺ステージ,背弧海盆ステージ,そして島弧ステージに区分される.おのおののステージの火山岩類は,それぞれ特徴的な島弧横断方向の化学組成変化を示す.これは,おそらく背弧海盆拡大期から島弧形成期にかけてのマントルの温度構造変化に伴って起こったものであろう.島弧ステージはさらに4つのサブステージに区分される.それらは海底火山活動期(13.5～8Ma),後期中新世カルデラ形成期(8～5Ma),鮮新世カルデラ形成期(5～1.7Ma),そして第四紀成層火山形成期(1.7～0Ma)である.火山噴火様式の変化は,地殻内応力場の変動によって起こったものと解釈され,それはプレート運動に規制されていると考えられる.近年の地震波トモグラフィーの発展によって,東北本州弧の地殻・マントル構造を,詳しく可視化することができるようになった.地質学と岩石学をあわせ考えると,地震波イメージは地殻・マントルの温度構造を明らかにするための有効な道具となる.地質学と岩石学に基づいて推定される現在のマントルウェッジの温度構造は,マントルウェッジの地震波速度構造とよく対応している.後期中新世～鮮新世カルデラの地下で起こったであろうマグマの貫入は,地殻内に大規模なマグマ溜りを形成し,地殻内温度構造に影響を与えたと考えられる.この温度構造擾乱の名残は,現在も地震学的に検出できる.東北本州弧の地殻・マントル温度構造は火成島弧の発展と関連して発達してきたものと考えられる.