Granulite facies lower crustal xenoliths from the Eifel,West Germany: petrological and geochemical aspects

Petrographic, petrological and geochemical data for 16 mafic meta-igneous, granulite facies lower crustal xenoliths from the East Eifel were collected in order to develop a model for the lower crustal history for this region. The xenoliths consist of plagioclase±amphibole±clinopyroxene±garnet±orthopyroxene±scapolite + opaque minerals±apatite±rutile±zircon. Garnet has reacted to a variable extent with plagioclase and clinopyroxene to form a corona of plagioclase_II+ amphibole + orthopyroxene_II. Pyroxenes and plagioclases show complex zoning patterns with regard to Al and Ca which can be interpreted in terms of P, T history. Decreasing temperature and pressure conditions are recorded by decreasing Al in clinopyroxene rims coexisting with increasing anorthite contents in plagioclase rims and the breakdown of garnet. In addition, a young heating event that affected the granulites to different degrees is inferred from the complementary Ca-zoning patterns in clino- and orthopyroxenes. Rare earth element (REE) patterns of whole rocks together with the trends displayed and fractionated liquids. REE analyses of the mineral separates display equilibrium partitioning patterns for amphibole and clinopyroxene, although isotopic data show that amphibole contains externally-derived Sr and Nd components not recognized in other minerals. At least a 4-stage history for the granulites is recorded: (1) intrusion and crystal fractionation of basaltic magmas in the lower crust, probably accompanied by crustal assimilation, (2) granulite facies metamorphism, (3) a decrease in temperature and pressure, and (4) a later heating event. The complicated thermal history is reflected in Sm–Nd mineral isochron ages which range from about 170 Ma down to about 100 Ma and cannot be assigned to distinct geological events. These ages correlate with inferred temperatures; the low ages are measured for xenoliths with the highest temperatures. In some cases the young heating event is likely to be responsible for partial resetting of the mineral isochrons.     

Evolution of pantellerite-trachyte-phonolite volcanoes by fractional crystallization of basanite magma in a continental rift setting,Marie Byrd Land,Antarctica

The Marie Byrd Land province includes 18 large (up to 1,800 km³) central volcanoes distributed across an active volcano-tectonic dome. The typical volcano structure consists of a basal 1,000–5,000 m of basanite surmounted by trachyte and subordinate intermediate rocks, plus phonolite, or pantellerite, or comendite. The volumes of felsic sections are large (~30–700 km³), but these rocks probably make up <10% of volcanic rock in the province. This paper describes pantellerite volcanoes in the Ames and Flood Ranges, which include a large and varied suite of these iron-rich, silica-poor rhyolites. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic range, and the results of modeling, all exclude significant crustal contamination and point to fractional crystallization as the process that controls magmatic evolution. The most unusual feature of these volcanoes is the apparent need to derive pantellerites from basanite, the long interval of fractionation at the base of the lithosphere and crust, involving kaersutite as the key phase in developing pantellerite, and a plumbing system that permitted coeval eruption of pantellerite and phonolite from the same edifice. Peralkalinity most likely developed in upper crustal reservoirs during the final 4–5% of magmatic history, by fractionating a high proportion of plagioclase under low pH₂O. Mantle plume activity appears to drive doming and volcanism. This, a stationary plate, and continental lithospheric structure seem to provide an optimal environment for the evolution of a diverse, large volume suite of felsic rocks by fractional crystallization.     

Geology of the Hubert Miller seamount,Marie Byrd seamounts province,Amundsen Sea,West Antarctic

This work is dedicated to the results of joint Russian-German geodynamic studies carried out in the West Antarctic (areas of the Amundsen Sea, the Southern Ocean, the Marie Byrd seamounts, and the foot of the continental slope of Marie Byrd Land) during cruises 18/5a and 23/4 of the “R/V Polarstern” in 2001 and 2006, respectively. The material collected on the Hubert Miller seamount (Marie Byrd seamount) attests to the relict continental appearance of the rocks. This suggests the heterogeneity of the Amundsen seafloor and its formation through a spatiotemporal combination of the destruction of continental crust, progressive thalassogenesis (oceanization-taphrogenesis), and rifting, as opposed to a spreading origin. The high postconsolidation mobility during the destruction stage led to the areal dismembering and high permeability of the continental crust, as well as tectonomagmatic activation. The main process during the reworking of the continental crust is its magmatic substitution by mantle-derived basic-ultrabasic material with subsequent formation of a secondary oceanic crust and preservation of relics of the continental crust. The endogenic activity of the Earth was driven by transmagmatic fluids, which were supplied from the liquid core and caused transformation of the Earth’s crust and mantle.     

Geochemical and isotopic characteristics of lower crustal xenoliths, San Francisco Volcanic Field, Arizona, U.S.A

A wide variety of xenoliths has been entrained in Miocene-to-Recent alkali olivine and hypersthene-normative basalts in the San Francisco Volcanic Field (SFVF), northern Arizona, U.S.A. Based on petrography, mineralogy, bulk rock chemistry and Sr-Nd isotopic characteristics, SFVF xenoliths can be divided into two major groups: cumulates and granulites. The cumulates are genetically related to the Cenozoic volcanic rocks and represent under- and/or intraplated additions to the crust of the Colorado Plateau. Assemblages are mafic to ultramafic and are dominated by clinopyroxene-orthopyroxene-plagioclase-spinel-amphibole-olivine. The granulites are probably Proterozoic in age, mafic-to-intermediate/felsic in bulk composition, either two pyroxene-plagioclase-spinel or plagioclase-alkali feldspar-quartz-magnetite-amphibole-biotite assemblages. Many of the granulites show evidence of partial melting. Some high SiO₂, very high Rb/Sr glasses are close in composition to erupted rhyolites, and probably represent end-member melts that have interacted with basalt to produce a variety of hybrid intermediate lavas. The major element, trace element and Sr-Nd isotope geochemistry is highly variable in the SFVF xenoliths. Extremely high Ba contents and Ba/Nb of a number of the granulites are equivalent to values characteristics of modern supra-subduction zone magmas. The considerable variation of chemical and isotopic composition depends upon mineral proportions, assemblages and chemistry. Isotopically, three end-members can be identified within the granulites: (i) lowest ⁸⁷Sr/⁸⁶Sr (0.702870) with low ¹⁴³Nd/¹⁴⁴Nd (0.511541, εNd-21.4); (ii) high ⁸⁷Sr/⁸⁶Sr (0.711069) with the lowest ¹⁴³Nd/¹⁴⁴Nd (0.511434, εNd-23.5); (iii) highest ⁸⁷Sr/{86}Sr (0.715306) with low ¹⁴³Nd/¹⁴⁴Nd (0.511793, εNd-16.5). Two important age ranges deduced from the isotopic data probably relate to episodes of crustal-growth beneath the SFVF (1.88 ± 0.33 Ga and Cenozoic). Thermobarometric calculations assuming equilibrium show that the xenoliths are derived from the lower crust (0.6–1.3 GPa, 850–1050 °C). The average SFVF lower crust is mafic in composition. In the absence of partial lithospheric delamination, the lower crust may become mafic with time due to under- and intraplating of continental crust by mafic magmas derived from the mantle.     

Crustal thinning and mafic underplating beneath the Neogene Volcanic Province (Betic Cordillera,SE Spain): evidence from crustal xenoliths

Three metapelitic xenolith suites in the Neogene Volcanic Province (NVP) of SE Spain (from SW to NE: El Hoyazo, Mazarrón and Mar Menor) originated by partial melting at different crustal depths, decreasing from 20–25 km in the SW to 9–12 km in the NE. Peak temperatures reached c. 900 °C. The xenolith source level is equated with the base of a felsic upper crust of high melting potential (‘fertility’). At El Hoyazo, this matches a thin, intracrustal low‐velocity zone recently inferred from seismic studies. Isostatic calculations indicate that this zone increases in thickness from SW to NE. A model of increasing upper crustal thinning from SW to NE in the NVP, accompanied by mafic underplating, is consistent with the 9 Ma petrological data, with current heat flow, seismic data and gravimetry. It is concluded that significant crustal extension occurred in the NVP in the late Miocene, i.e. after the main phase of widespread extension, exhumation of high‐pressure rocks and formation of the Alborán Sea.     

Helium and argon isotopes in lower crustal xenoliths from the belomorian mobile belt

The isotopic systematics of noble gases (He and Ar) were studied in Neoarchean and Paleoproterozoic lower crustal xenoliths from the Belomorian mobile belt. The xenolith suite is dominated by garnet granulites (Grt + Cpx + Pl ± Opx ± Qtz ± Kfs ± Phl ± Hbl) and two-pyroxene or garnet pyroxenites (Cpx + Pl ± Grt ± Opx ± Hbl ± Qtz). The xenoliths and the host Devonian ultramafic lamprophyres forming diatremes and explosion dikes contain fluid with similar He and Ar isotopic compositions. It was found that the fluid was trapped by the rocks and xenolith minerals approximately simultaneously with the formation of the lamprophyres. This conclusion is based on the identical K-Ar ages of the majority of xenoliths and ultramafic lamprophyres. When the xenoliths were transported toward the surface by high-temperature ultrabasic melts, the noble gases occurring in them were partly (Ar) or completely (He) lost. The melts were contaminated by meteoric waters during their emplacement in the upper crust, which resulted in that the Ar isotopic composition of lamprophyres approached the composition of atmospheric Ar. The fluid phase that was liberated during melt crystallization severely affected the xenoliths, diminishing the difference between the isotope compositions of He and Ar in the xenoliths and ultramafic lamprophyres. The He isotope composition includes an admixture of mantle ³He, which is suggested by the high measured ³He concentrations, exceeding the calculated values, and high ³He/³⁶Ar ratios in the xenoliths and their host lamprophyres. The fraction of the mantle component in the fluid trapped by the xenoliths was estimated as ～20%.     

Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (¹⁴⁷Sm/¹⁴⁴Nd = 0.1225–0.1608; ¹⁴³Nd/¹⁴⁴Nd_{t 0} = 0.512000–0.512324; ⁸⁷Rb/⁸⁶Sr = 0.099–0.172; ⁸⁷Sr/⁸⁶Sr_{t 0} = 0.708188–0.7143161) and common lead isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 18.43–18.48; ²⁰⁷Pb/²⁰⁴Pb = 15.62–15.70; ²⁰⁸Pb/²⁰⁴Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower ²⁰⁸Pb/²⁰⁴Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998     

Crustal xenoliths from Calbuco Volcano,Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction

Crustal xenoliths in the 1961 andesite flow of Calbuco Volcano, in the southern Southern Volcanic Zone (SSVZ) of the Andes, consist predominantly of pyroxene granulites and hornblende gabbronorites. The granulites contain plagioclase+pyroxene+magnetite±amphibole, and have pronounced granoblastic textures. Small amounts of relict amphibole surrounded by pyroxene-plagioclase-magnetite-glass symplectites are found in some specimens. These and similar textures in the gabbronorites are interpreted as evidence of dehydration melting. Mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism: a moderate pressure, high temperature stage accompanied by melting and melt extraction from some samples, followed by thermal metamorphism after entrainment in the Calbuco andesite lavas. High _Nd ^T values (+4.0 to +8.6), Nd-isotope model ages of 1.7–2.0 Ga, and trace element characteristics like chondrite normalized La/Yb< and La/Nb1 indicate that the protoliths were oceanic basalts. Similar oceanic metabasalts of greenschist to amphibolite facies are found in the Paleozoic metamorphic belt that underlies the Chilean coastal ranges. Mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate, crystallizing from the basaltic andesite magma at Calbuco. Crystallization pressures for the gabbros based on total Al contents in amphibole are 6–8 kbar. These pressures point to middle to lower crustal storage of the Calbuco magma. Neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of Calbuco andesites, although an ancient sedimentary contaminant is indicated by the lava compositions. The presence of oceanic metabasaltic xenoliths, together with the sedimentary isotopic imprint, suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt, which is an accretionary complex of intercalated basalts and sediments that formed along the Paleozoic Gondwanan margin. If this is the case, the geochemical composition of the lower and middle crust beneath the SSVZ is significantly different from that of most recent SSVZ volcanic rocks.     

西南极利文斯顿岛百耳斯半岛中生代火山岩地球化学特征

西南极利文斯顿岛百耳斯半岛中生代火山岩是晚侏罗纪至晚白垩纪三期火山活动的产物。岩石普遍贫钾,轻稀土元素略为富集,但轻重稀土分馏不强烈;Ｓｒ、Ｋ、Ｒｂ、Ｂａ、Ｔｈ等大离子亲石元素（ＬＩＬ）明显富集,Ｎｂ和Ｔａ明显亏损,过渡金属族元素Ｃｒ和Ｎｉ强烈亏损,表明它们属于火山弧环境中生成的钙碱性系列岩石。白垩纪中期和白垩纪晚期岩石的８７Ｓｒ／８６Ｓｒ比值主要在０．７０３９～０．７０５３之间变化,εＮｄ＞０,８７Ｓｒ／８６Ｓｒ相对１／Ｓｒ、Ｒｂ、Ｋ和ＳｉＯ２等的变化不大,表明其源区岩浆可能直接来自上地幔,很少受到壳源物质的混染。而晚侏罗纪火山岩的Ｓｒ同位素比值较高,说明早期岩浆可能受到少量地壳物质的混染。但在Ｔｈ／Ｔａ－Ｔｈ和Ｔｈ／Ｓｍ－Ｔｈ图解上所有岩石的投影点分布趋势相同,趋势线的斜率＞０。百耳斯半岛中生代火山岩是上地幔部分熔融生成的同源岩浆不同期的产物     

The role of mylonites in the uplift of an oblique lower crustal section, East Antarctica

Abstract Three generations of mylonites discovered in the northern Prince Charles Mountains (nPCM) are associated with episodes of crustal thickening and thinning. First-generation mylonites (MY1) are shallow thrusts which pre-date both folding and peak metamorphic conditions, and formed during early crustal thickening. Second-generation mylonites (MY2) are significant subvertical normal faults that formed at conditions of c. 5 kbar and 700° C, and throughout the nPCM consistently display NW-block uplift. It is argued that MY2 uplift was rapid in the north-west, produced exhumation of approximately 6–7 km, and caused re-equilibration of most nPCM assemblages at lower pressures. It is suggested that features of this terrane may be reconciled with a tectonic model involving simultaneous crustal thickening and lithospheric thinning; MY2 uplift may reflect isostatically induced uplift. In contrast, the adjacent east Lambert Glacier Region (eLGR) was unaffected by MY2 uplift and remained at lower crustal levels. P-T trajectories across this oblique terrane thus reflect a gradual transition in uplift rates: nPCM paths preserve mostly cooling after partial MY2 exhumation, while those in the eLGR are dominated by slower uplift which facilitated the retrograde growth of coronas and symplectites at amphibolite facies conditions. Amphibolite facies third-generation mylonites, MY3, post-date the preserved P-T segments and are low-angle normal faults which indicate consistent easterly transport across the entire terrane. It is proposed that they are related to tectonic collapse.     

The petrology of ultramafic xenoliths from Summit Lake,near Prince George,British Columbia

Approximately 200 upper mantle xenoliths from Summit Lake, near Prince George, British Columbia, were collected from a basanitoid flow of Late Cenozoic (possibly post-glacial) age. The most abundant xenolith is spinel lherzolite (55%), with subordinate wehrlite (22%), clinopyroxenite (10%), olivine websterite (10%), websterite (2%) and dunite (1%). Xenoliths have granular textures and both green chrome diopside-bearing and black aluminous augitebearing xenoliths are present. About 5% of the xenoliths are banded on a cm scale, suggesting that the upper mantle beneath north-central British Columbia is heterogeneous on a scale of cm to meters.Microprobe data on the mineral phases indicate that the xenoliths are generally well equilibrated. Typically in spinel lherzolite, olivines are Fo₈₉, orthopyroxenes are En₉₀ and chrome diopside is Wo₄₅En₅₀Fs₅. Spinels vary in composition from xenolith to xenolith. The evidence for partial melting observed in five xenoliths, may be due to heating during incorporation of the xenoliths within the host magma or to instability caused by decompression as the xenoliths are transported to the surface.Using element partition geothermometers, equilibration temperatures are calculated to be between 1080–1100° C. Pressures, estimated from a Cordilleran geotherm, are between 18–20 kbar. These temperatures are somewhat higher than estimates from xenoliths from other localities in Late Cenozoic alkali basalts in south and central British Columbia. It is concluded, therefore, that either the Summit Lake suite represents samples from a deeper source region in the upper mantle or the Late Cenozoic geotherm varied in time and space.On leave from the Geological Institute, University of Tokyo     

汉诺坝新生代玄武岩中下地壳麻粒岩包体的变形显微构造特征及其地质意义

新生代玄武岩中的下地壳包体,由于从下地壳被快速携带至地表,因此保留了下地壳的直接信息.华北北部汉诺坝新生代玄武岩中除了含有丰富的幔源包体之外,还含有许多下地壳麻粒岩包体.本文的主要目的是通过对该区下地壳麻粒岩包体的变形显微构造和位错亚构造特征的详细研究,探讨下地壳的变形特征和变形机制.光学显微镜下观测表明,下地壳麻粒岩包体的低温(＜800℃)样品中确实发育显微破裂,但变形双晶、变形条带、扭折带也同样发育,动态重结晶作用也开始出现.随着温度、压力的升高,变形双晶、变形条带、变形纹、扭折带和重结晶新晶粒等塑性变形特征占主导地位,而显微破裂则主要表现为由塑性失配引起的显微破裂以及流体包裹体面.而明显不同于Ivrea带地体麻粒岩,在这些包体中未发现与韧性剪切有关的变形显微构造特征.透射电镜观测表明,包体中的斜长石和辉石颗粒普遍发育自由位错、位错列、亚晶界、新晶界、变形双晶、包裹体列和出溶片晶等位错亚构造.上述观测结果表明,下地壳变形作用以塑性变形为主而不是准脆性变形,其变形机制主要为位错的滑移和攀移机制,其中包括机械双晶作用和动态重结晶作用.     

Petrology and U–Pb geochronology of lower crustal xenoliths and the development of a craton, Slave Province, Canada

Lower crustal xenoliths recovered from Eocene to Cambrian kimberlites in the central and southern Slave craton are dominated by mafic granulites (garnet, clinopyroxene, plagioclase±orthopyroxene), with subordinate metatonalite and peraluminous felsic granulites. Geothermobarometry indicates metamorphic conditions of 650–800 °C at pressures of 0.9–1.1 GPa. The metamorphic conditions are consistent with temperatures expected for the lower crust of high-temperature low-pressure (HT-LP) metamorphic belts characteristic of Neoarchean metamorphism in the Slave craton. U–Pb geochronology of zircon, rutile and titanite demonstrate a complex history in the lower crust. Mesoarchean protoliths occur beneath the central Slave supporting models of an east-dipping boundary between Mesoarchean crust in the western and Neoarchean crust in the eastern Slave. At least, two episodes of igneous and metamorphic zircon growth occurred in the interval 2.64–2.58 Ga that correlate with the age of plutonism and metamorphism in the upper crust, indicating magmatic addition to the lower crust and metamorphic reworking during this period. In addition, discrete periods of younger zircon growth at ca. 2.56–2.55 and 2.51 Ga occurred 20–70 my after the cessation of ca. 2.60–2.58 Ga regional HT-LP metamorphism and granitic magmatism in the upper crust. This pattern of younger metamorphic events in the deep crust is characteristic of the Slave as well as other Archean cratons (e.g., Superior). The high temperature of the lower crust immediately following amalgamation of the craton, coupled with evidence for continued metamorphic zircon growth for >70 my after ‘stabilization’ of the upper crust, is difficult to reconcile with a thick (200 km), cool lithospheric mantle root beneath the craton prior to this event. We suggest that thick tectosphere developed synchronously or after these events, most likely by imbrication of mantle beneath the craton at or after ca. 2.6 Ga. The minimum age for establishing a cratonic like geotherm is given by lower crustal rutile ages of ca. 1.8 Ga in the southern Slave. Transient heating and possible magmatic additions to the lower crust continued through the Proterozoic, with possible additional growth of the tectosphere.