Genesis of zircon megacrysts in Cenozoic alkali basalts and the heterogeneity of subcontinental lithospheric mantle, eastern China

Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown–maroon, and some of them are up to 16 mm long. U–Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are 19.2?±?0.7 Ma, 1.2?±?0.1 Ma and 4.1?±?0.2 Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05–18.87 Ma, 0.9–2.2 Ma, 3 Ma). ε_Hf(t) values of zircon megacrysts are 9.02?±?0.49, 6.83?±?0.47, 4.46?±?0.48 for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.     

Vendian tectono-magmatic events in mantle ophiolitic complexes of the polar Urals: U-Pb dating of zircon from chromitite

Geological and isotopic evidence of Late Vendian magmatic events in restitic ultramafic mantle rocks of the Voikar-Syn’ya ophiolitic massif are considered and correlated with events at the eastern margin of the East European Platform. The geological and isotopic data show that the ophiolitic complexes of the Polar Urals were formed during several stages. The percolation of melts through peridotites was recorded in the newly formed mineral assemblages, for example, olivine + chromite ± zircon. Zircon crystallized from the residual fraction of the evolved basic melt that impregnated peridotite. The active interaction of hot restitic harzburgite with the migrating melt resumed repeatedly and could have led to the formation of several generations of chromite-bearing dunite. An important geological inference can be made from this suggestion: There is a high probability that isotopic markers of different age have been retained in restitic mantle complexes of ophiolites. The U-Pb dating of zircons with a SHRIMP-2 ion microprobe has shown that the isotopic age of seven grains is 585.3 ± 6 Ma (MSWD is 0.036 and the probability of concordance is 0.85). The obtained age of zircon from chromitite marks a Vendian tectonomagmatic event that occurred in the upper mantle of the transitional zone between the East European Plate and the oceanic basin. The island-arc complexes of the Polar Urals developed on the tectonically juxtaposed fragments of the Early Paleozoic and pre-Paleozoic oceanic crust. These crustal rocks were reworked during younger magmatic events related to the origin of the Middle Paleozoic island arcs. As a result, the rocks that formed in different geological epochs were locally retained in the restitic mantle complexes of a spatially indivisible ophiolitic association.     

Important role of hornblende fractionation in generating the adakitic magmas in Tongling,Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

ABSTRACT

Tongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of ?3.00 to ?4.52 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.     

Devonian rodingite from the northern margin of the North China Craton: mantle wedge metasomatism during ocean–continent convergence

The northern margin of the North China Craton (NCC) was an active convergent margin during Palaeozoic and preserves important imprints of magmatic and metasomatic processes associated with oceanic plate subduction. Here, we investigate the mafic–ultramafic rocks in the Xiahabaqin–Sandaogou complexes from the northern NCC including pyroxenite, hornblendites, hornblende gabbro, and their rodingitized counterparts within a serpentinite domain. We present petrological, zircon U–Pb geochronological, and geochemical data to constrain the nature and timing of the magmatic and metasomatic processes in the subduction zone mantle wedge. The rock suites investigated in this study are characterized by low contents of SiO₂, Na₂O, and K₂O, with high CaO, FeO, Fe₂O₃, and MgO. The rodingitized rocks show markedly high CaO and lower MgO compared to their ultramafic protolith, suggesting extensive post-magmatic infiltration of Ca-rich, Si-poor fluids derived by serpentinization of mantle peridotite. The enrichment of large ion lithophile and light rare earth elements such as Ba, Sr, K, La, and Ce with relative depletion of high field strength elements like Nb, Ta, Zr, and Hf in the ultramafic rocks collectively suggest metasomatism of a fore-arc mantle wedge by fluids released through dehydration of subducted oceanic slab and subduction-derived sediments. Dehydration and decarbonation leading to metasomatic fluid influx and serpentinization of mantle wedge peridotite account for the enriched geochemical signatures for the rodingitized rocks. The zircon grains in these rocks show textures indicating magmatic crystallization followed by fluid-controlled dissolution–precipitation. Magmatic zircons from altered pyroxenite, hornblendite, and rodingitized pyroxenite in Xiahabaqin yield protolith crystallization ages peaks at 396 Ma and 392 Ma and metasomatic grains show ages of 386 Ma, 378 Ma, and 348 Ma. The zircons from hornblendite and basaltic trachyandesite indicate protolith emplacement during 402–388 Ma. Metasomatic zircon grains from rodingitized hornblende gabbro in Sandaogou complex show a wide range of ages as 412 Ma, 398 Ma, 383 Ma, and 380 Ma. The common magmatic zircon ages peaks at 398–388 Ma in most of the rocks suggest a similar time for magma crystallization in the Xiahabaqin and Baiqi during Middle Devonian. Subsequently, repeated pulses fluids and melts resulted in metasomatic reactions in mantle wedge until early Permian. The Lu–Hf analysis of the zircon grains from these rocks display markedly negative εHf(t) values ranging from ?22.4 to ?7.7, suggesting magma derivation from an enriched, hydrated lithospheric mantle through fluid–rock interaction and mantle wedge metasomatism. Rodingitization processes are associated with exhumation of ultramafic mantle wedge rocks within a serpentinized subduction channel close to the subducted slab in response to slab roll back in a long-lasting subduction regime. This study offers insights into magmatic and metasomatic processes of ultramafic rocks in the fore-arc mantle wedge which were exhumed and accreted to an active continental margin during the southward subduction of the Palaeo-Asian oceanic lithosphere beneath the NCC.     

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

     

New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps,Italy)

Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.     

河北汉诺坝玄武岩中深源包体和巨晶分布的动力学机理

汉诺坝碱性玄武岩是深源包体和巨晶的主要寄主岩,同时,也有不含包体的碱性玄武岩和含包体的拉斑玄武岩。本文基于理论计算,探讨了包体和巨晶分布的动力学机理。     

Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these ¹⁸O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average ¹⁸O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average ¹⁸O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, ¹⁸O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low ¹⁸O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of significant quantities of magma in the sub-continental mantle that was regionally variable in ¹⁸O and this information restricts theories about the nature of ancient subduction. Received: 8 August 1997 / Accepted: 6 May 1998     

Geochemistry and zircon U–Pb–Hf isotopes of the 780 Ma I-type granites in the western Yangtze Block: petrogenesis and crustal evolution

ABSTRACT

Neoproterozoic I-type granites could provide vital insights into the crust–mantle interaction and the crustal evolution along the western Yangtze Block, South China. This paper presents new zircon U–Pb ages, bulk-rock geochemistry, and in situ zircon Lu–Hf isotope on the Dalu I-type granites from the southwestern Yangtze Block. Zircon U–Pb dating show the crystallization ages of 781.1 ± 2.8 Ma for granodiorites and 779.8 ± 2.0 Ma for granites, respectively. The Dalu granodiorites are Na-rich, calc-alkaline, metaluminous to slightly peraluminous (A/CNK = 0.94–1.08). Zircons from granodiorite have positive εHf(t) values (+2.16 to +7.39) with crustal model ages of 1.21–1.54 Ga, indicating juvenile mafic lower crust source. The Dalu granites are high-K calc-alkaline, peraluminous rocks. They have variable zircon εHf(t) values (?4.65 to +5.80) with crustal model ages of 1.31–1.97 Ga, suggesting that they were derived from the mature metasediment-derived melts by the mixing of newly formed mafic lower crust-derived melts. The geochemical variations in Dalu pluton is dominated not only by the different source rocks but also by the different melting temperatures. Combining with the geochemistry and isotopic compositions of I-type granitoids and tectonic setting in the western Yangtze Block, we propose that the Dalu I-type granodiorites–granites associations are the magmatic response from different crustal levels, which were induced by the heat anomaly due to the asthenosphere upwelling in the subduction-related setting.     

大陆板片-地幔相互作用:来自大别造山带碰撞后安山质火山岩的地球化学证据

俯冲到地幔深度的地壳物质不可避免地在板片-地幔界面与地幔楔发生相互作用,由此形成的超镁铁质交代岩就是造山带镁铁质火成岩的地幔源区.因此,造山带镁铁质火成岩为研究俯冲地壳物质再循环和壳-幔相互作用提供了重要研究对象.为了揭示俯冲陆壳物质再循环的机制和过程,对大别造山带碰撞后安山质火山岩开展了元素和同位素地球化学研究.这些安山质火山岩的SIMS锆石U-Pb年龄为124±3~130±2 Ma,表明其形成于早白垩世.此外,残留锆石的U-Pb年龄为中新元古代和三叠纪,分别对应于大别-苏鲁造山带超高压变火成岩的原岩年龄和变质年龄.它们具有岛弧型微量元素特征、富集的Sr-Nd-Hf同位素组成,以及变化的且大多不同于正常地幔的锆石δ18O值.这些元素和同位素特征指示,这些安山质火山岩是交代富集的造山带岩石圈地幔部分熔融的产物.在三叠纪华南陆块俯冲于华北陆块之下的过程中,俯冲华南陆壳来源的长英质熔体交代了上覆华北岩石圈地幔楔橄榄岩,大陆俯冲隧道内的熔体-橄榄岩反应产生了富沃、富集的镁铁质地幔交代岩.这种地幔交代岩在早白垩世发生部分熔融,就形成了所观察到的安山质火山岩.因此,碰撞造山带镁铁质岩浆岩的地幔源区是通过大陆俯冲隧道内板片-地幔相互作用形成的,而加入地幔楔中长英质熔体的比例决定了这些镁铁质岩浆岩的岩石化学和地球化学成分.      

Zircon trace element characteristics and ages in granulite xenoliths: a key to understanding the age and origin of the lower crust,Arkhangelsk kimberlite province,Russia

Garnet granulite and pyroxenite xenoliths from the Grib kimberlite pipe (Arkhangelsk, NW Russia) represent the lower crust beneath Russian platform in close vicinity to the cratonic region of the north-eastern Baltic (Fennoscandian) Shield. Many of the xenoliths have experienced strong interaction with the kimberlite host, but in others some primary granulite-facies minerals are preserved. Calculated bulk compositions for the granulites suggest that their protoliths were basic to intermediate igneous rocks; pyroxenites were ultrabasic to basic cumulates. A few samples are probably metasedimentary in origin. Zircons are abundant in the xenoliths; they exhibit complex zoning in cathodoluminescence with relic cores and various metamorphic rims. Cores include oscillatory zircon crystallized in magmatic protoliths, and metamorphic and magmatic sector-zoned zircons. Recrystallization of older zircons led to the formation of bright homogeneous rims. In some samples, homogeneous shells are surrounded by darker convoluted overgrowths that were formed by subsolidus growth when a change in mineral association occurred. The source of Zr was a phase consumed during a reaction, which produced garnet. Late-generation zircons in all xenoliths show concordant U–Pb ages of 1.81–1.84 Ga (1,826 ± 11 Ma), interpreted as the age of last granulite-facies metamorphism. This event completely resets most zircon cores. An earlier metamorphic event at 1.96–1.94 Ga is recorded by some rare cores, and a few magmatic oscillatory zircons have retained a Neoarchaean age of 2,719 ± 14 Ma. The assemblage of metaigneous and metasedimentary rocks was probably formed before the event at 1.96 Ga. Inherited magmatic zircons indicate the existence of continental crust by the time of intrusion of magmatic protoliths in the Late Archaean. The U–Pb zircon ages correspond to major events recorded in upper crustal rocks of the region: collisional metamorphism and magmatism 2.7 Ga ago and reworking of Archaean rocks at around 1.95–1.75 Ga. However, formation of the granulitic paragenesis in lower crustal rocks occurred significantly later than the last granulite-facies event seen in the upper crust and correlates instead with retrograde metamorphism and small-volume magmatism in the upper crust.     

Origin of clinopyroxene megacrysts in volcanic rocks from the North China Craton: a comparison study with megacrysts worldwide

ABSTRACT

Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface.     

东昆仑夏日哈木铜镍矿镁铁质超镁铁质岩体岩相学、锆石U-Pb年代学、地球化学及其构造意义

文章报道了东昆仑夏日哈木铜镍矿成矿岩体的岩相学、锆石U-Pb年代学、全岩地球化学以及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及其形成的构造环境。夏日哈木Ⅰ号镁铁质超镁铁质岩体位于昆中基底隆起花岗岩带中段,北侧靠近昆北断裂。岩体走向NEE,剖面呈平缓的“岩盆状”,地表出露面积约0.7 km2。该杂岩体主要由辉长苏长岩、斜方辉石岩、橄榄辉石岩、斜长二辉橄榄岩和方辉橄榄岩组成,橄榄岩相和辉石岩相是主要的Cu、Ni赋矿岩相。镁铁质超镁铁质岩体主量元素具有低硅(w(SiO₂)=36.68%~52.58%)、低钛(w(TiO2)=0.13%~0.47%)、高镁(w(MgO)=10.91%~35.81%)、贫碱(w(K₂O+Na₂O)=0.26%~1.95%)的特征,属亚碱性系列岩石,m/f为3.88~6.29,属铁质超基性岩类(m/f=2~6.5)。岩石稀土元素球粒陨石标准化配分模式为轻稀土富集型,(La/Yb)N=1.44~2.98,Eu异常不明显,相似的稀土元素配分模式说明岩体的同源性。岩石富集大离子亲石元素(Rb、Th、U、K),相对亏损高场强元素(Nb、Ta)。岩体中存在新元古代花岗岩的捕虏体以及La/Yb、Ce/Yb、Th/Yb、Nb/La、La/Sm比值显示岩体经历了有限的地壳混染。辉长苏长岩锆石的LA-MC ICPMS、U-Pb年代学研究表明,岩体形成年龄为(423±1) Ma,MSWD=0.14,属晚志留世。锆石的176Hf/177Hf比值为0.282 628~0.282 833,相应的εHf(t)均为正值(4.0~10.9),Lu-Hf的单阶段模式年龄(tDM1)为610~875 Ma,平均值为788 Ma,大于锆石U-Pb年龄。研究认为,岩体的岩浆源区主要为亏损地幔,可能有早期流体交代的富集岩石圈地幔组分的加入和地壳物质的混染。结合区域构造演化,文章认为岩体形成于碰撞后伸展的构造环境,可能与俯冲板片的断离作用有关。岩浆演化过程中橄榄石和斜方辉石的分离结晶作用和地壳中硫的加入可能是促使岩浆体系达到硫饱和的主要机制。     

Crystallization of Cr-poor and Cr-rich megacryst suites from the host kimberlite magma: implications for mantle structure and the generation of kimberlite magmas

Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.     

Fission track thermochronology of the Beni Bousera peridotite massif (Internal Rif,Morocco) and the exhumation of ultramafic rocks in the Gibraltar Arc

The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene–Upper Oligocene time (～22–24 Ma). The zircon FT analyses give an average cooling—i.e., below 320 °C—age of ～19.5 Ma. In addition, the apatite FT data give an average cooling—i.e., below 110 °C—age of ～15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 °C/Ma. This cooling rate constrains a denudation rate of about ～2 mm year^?1 from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (～100 °C) has reheated the massif during the Late Miocene before the Pliocene–Quaternary tectonic uplift.     

The origin of K-feldspar megacrysts hosted in alkaline potassic rocks from central Italy: a track for low-pressure processes in mafic magmas

In situ Sr-isotope and microchemical studies were used to determine the provenance of K-feldspar megacrysts hosted in mafic alkaline potassic, ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central Italy.

At Monte Cimino volcanic complex, mafic leucite-free ultrapotassic megacryst-bearing rocks of olivine latitic composition are associated with evolved latite and trachyte. Here, latites and trachytes straddle the sub-alkaline field. Age-corrected ⁸⁷Sr/⁸⁶Sr values (Sr_i) of the analysed Cimino olivine latites vary from 0.71330 and 0.71578 and strongly increase at constant Mg value. Latite and trachyte have lower Sr_i than olivine latites ranging between 0.71331 and 0.71361. Sr_i of K-feldspar megacrysts from olivine latites are between 0.71352 and 0.71397, but core and rim ⁸⁷Sr/⁸⁶Sr ratios within individual megacryst are indistinguishable. In all the mafic rocks, the megacrysts are not in isotopic equilibrium with the hosts. K-feldspar megacrysts from both the latite and trachyte have similar Sr-isotope compositions (Sr_i=0.71357–0.71401) to those in the olivine latites. However, Sr_i of megacryst in the trachyte vary significantly from core to rim (Sr_i from 0.71401 to 0.71383). As with the olivine latites, the K-feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte.

At Vico volcano, megacryst-bearing rocks are mafic leucite-free potassic rocks, mafic leucite-bearing ultrapotassic rocks and old trachytic rocks. The mafic leucite-bearing and leucite-free rocks are a tephri-phonolite and an olivine latite, respectively. A megacryst in Vico trachyte is isotopically homogeneous (Sr_i CORE=0.71129, RIM=0.71128) and in equilibrium with the host rock (Sr_i bulk ROCK=0.71125). Sr_i of megacryst from tephri-phonolite is clearly not in isotopic equilibrium with its host (Sr_i bulk ROCK=0.71158), and it increases from core (Sr_i=0.71063) to rim (Sr_i=0.71077). A megacryst in Vico olivine latite is isotopically homogeneous (Sr_i CORE=0.71066, RIM=0.71065), but not in equilibrium with the host rock (Sr_i bulk ROCK=0.71013).

The Sr isotope microdrilling technique reveals that Cimino megacrysts were crystallised in a Cimino trachytic magma and were subsequently incorporated by mixing/mingling processes in the latitic and olivine latitic melts. A model invoking the presence of a mafic sub-alkaline magma, which was mixed with the olivine latite, is proposed to justify the lack of simple geochemical mixing relation between Cimino trachytes and olivine latites. This magmatological model is able to explain the geochemical characteristics of Cimino olivine latites, otherwise ascribed to mantle heterogeneity.

The similarity of core Sr_i of megacrysts hosted in Vico tephri-phonolite and olivine latite suggests that the K-feldspar megacrysts are co-genetic. Isotopic equilibrium between megacryst and Vico host trachyte indicates that the trachyte is the parent of this megacryst. On the contrary, the megacrysts hosted in tephri-phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim Sr isotope increase of the xenocryst hosted in the tephri-phonolite. The megacrysts hosted in the Vico mafic rocks might derive from a trachytic melt similar in composition to the old Vico trachytes.     

Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic–ultramafic bodies from the Sulu UHP terrane, China

Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ¹⁸O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ¹⁸O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.     

A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds

A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.     

Pb−Sr−Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif,Western Alps,Italy-II: what is the age of the ultrahigh-pressure metamorphism?

Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young. Dedicated to Professor Borwin Grauert on the occasion of his sixtieth birthday     

Genesis of garnet peridotites in the Sulu UHP belt: Examples from the Chinese continental scientific drilling project-main hole, PP1 and PP3 drillholes

The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo_79-89) and other geochemical features. Zoned garnet and omphacite (with 4-5 wt.% Na₂O) are typical metamorphic minerals in these rocks, and, along with P-T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U-Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346-461 Ma), and a Mesozoic UHP metamorphic age (ca. 220-240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90-92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92-93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U-Pb isotope dating of zircon from these rocks yielded two age groups: 726 ± 56 Ma for relic magmatic zircon grains and 240 ± 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie-Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent-continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus, the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block.