Mantle Contamination Research Articles

Chronological and Mineralogical Records of the Langqi Pluton, Fuzhou: Constraints on the Magma Mixing Process

The mafic microgranular enclaves (MMEs) from Mesozoic intermediate-acid magmatic rocks, widely developed along the Fujian coast, are considered to be the results of large-scale crust–mantle interaction by magma mixing. This paper is based on zircon U-Pb chronology, along with zircon Hf isotope and mineral analyses for the host granite and MMEs from Langqi Island, in order to investigate the magma mixing mechanism of the Langqi pluton in Fuzhou, Southeast China. The results indicate that the MMEs were emplaced during the late Early Cretaceous (98.9 ± 2.2 Ma), identical to the age of the granite (100.1 ± 1.1 Ma) within the error range. The zircon εHf(t) values for the granite and MMEs are in the ranges of −2.1~0.0 and −1.7~+1.1. The zircon Hf isotope data indicate that both the granite and MMEs were predominantly derived from the ancient crustal basement of Cathaysia, with a partial mantle-derived contribution. The An values of plagioclase phenocrysts with oscillatory zoning patterns in the MMEs show oscillatory changes from the core to the rim, indicating multiple mixing events between the two magmas with different compositions. Amphiboles in the MMEs show characteristics of crust–mantle contamination, and the Ti migrated from the mafic magma with high concentration to the felsic magma with low concentration during the magma mixing process. Biotites in the host rock and MMEs belong to primary biotite, and they have relatively high MgO contents (ave. 12.78 wt.%) and relatively low FeOT/(MgO + FeOT) ratios (ave. 0.56), showing characteristics of crust–mantle contamination. The crust–mantle magma interaction in a crystal, mushy state played a significant role in controlling the formation and evolution of the Langqi pluton. The magmatism was predominantly sourced from mixing between the mantle-derived mafic magma and the crust-derived felsic magma during the subduction of the Paleo-Pacific Plate, resulting in the formation of the Langqi doleritic veins, granites, and MMEs.

Assessing crustal contamination in the 28 to 18 Ma Dulce dike swarm with Nd-Sr isotopic data, southwestern Colorado

Alkaline to subalkaline mafic dikes in the 28 to 18 Ma Dulce swarm were emplaced in a zone of incipient extension from southern Colorado into northern New Mexico on the northeastern boundary of the San Juan Basin. The 87Sr/86Sr ratios for the dikes are 0.70503 to 0.70584, akin to most post-28 Ma mafic rocks across the northern San Juan Basin. These data are consistent with melting of metasomatized subcontinental lithospheric mantle with little to no crustal contribution as revealed by the geochemical and Sr-Nd isotopic signatures of most 28–0.6 Ma mafic rocks in the region. Time-corrected εNd(t) values of −4.1 to −7.4 for rocks in the Dulce swarm, however, indicate that magma production involved the crust. A previous hypothesis for Dulce magmas was contamination of lithospheric mantle melts with up to 45% mafic lower crust ± 0.5% upper crust. In this investigation, six new whole-rock Sr and Nd isotopic analyses were combined with published data to further investigate the contamination of lithospheric mantle melts with different crustal reservoirs. The Nd isotope signatures of the Dulce swarm offer evidence for the long-term involvement (∼10 Ma) of lower crust in the production of rift-related mantle magmas. Isotopic mixing curves support previous hypotheses for the contamination of lithospheric mantle melts with 10 to 40 percent lower mafic crust. This provides further insight into regional variations in mantle magmas produced after 28 Ma in the Four Corners region that likely triggered crustal melting related to caldera complexes in the western San Juan Mountains.

Petrology of Granites of the Tommot Rare-Earth Ore Field (Verkhoyansk–Kolyma Orogenic Belt)

The article presents the results of studying the aegirine–arfvedsonite granites of the Somnitelnyi massif within the Tommot ore field located in the Verkhoyansk–Kolyma orogenic belt (NE Asia). Along with the crustal signatures, the rocks display features of mantle contamination at their origin. Their affinity for A-type granites characteristic of continental rifts and hot spots is shown. The associated Tommot REE deposit is the only one discovered in NE Russia. New data are presented for the previously studied Tommot massif within the same ore field, with a wide compositional range from alkaline-ultrabasic rocks to alkaline syenites. It is established that despite a common geochemical enrichment of both massifs’ rocks with REEs, the Somnitelnyi massif granites cannot be interpreted as the final phase of the Tommot massif emplacement. Specific REE mineralization and high crystallization temperatures (up to 1045 °C) of the Somnitelnyi granites may be explained by the existence within the study area of an undepleted mantle source (“hot spot”), whose maximum activity occurred during the granitic melt generation. The ore bodies of the Tommot deposit consist of fenitized albitites, granite gneisses, and, more rarely, the cross-cutting pegmatite veins. They are confined mostly to exocontacts of the Somnitelnyi massif, are less often in its endocontacts, and are not found in the host rocks and in the inner part of the massif away from the contacts. Principal ore minerals are chevkinite, yttrialite, gadolinite, and fergusonite. Based on the data obtained, the deposit is classified as a metasomatic complex Ce–Y–Nb–Zr deposit associated with the alkaline granites.

Nature of the Kergelen Plateau and Its Place in the Structural Plan of the Southern Sector of the Indian Ocean

— The morphostructural characteristics of the main taxa of southern Indian Ocean, such as the Del Cano-Crozet, Conrad, and Kerguelen plateaus, are given based on structural-tectonic analysis. A depth map of the acoustic basement of the eastern part of the Afro-Antarctic megadepression is first built using the digitized NGDS database on the thickness of the sedimentary cover of the Indian Ocean, which is available on the Internet. A critical analysis of deep seismic sounding (DSS) data on the Kerguelen plateu hs been performed. A geochemical analysis of more than 100 samples of basalts, acidic rocks, and rocks of sedimentary origin, collected on the islands of the Kerguelen archipelago and raised by dredging and deep-sea drilling from the Kerguelen and Conrad plateaus, is carried out. The study has shown that the plateaus resemble the Madagascar Ridge in terms of morphostructure and a number of other features. It is concluded that these flat-topped horst massifs were the part of the “structural bridge” that connected Madagascar and the eastern part of mainland Antarctica no later than the Paleogene. A proposed alternative version of the geological interpretation of the deep seismic data states that the plateau has a relatively homogeneous structure along the strike, including a thick (at least 15-km) layer of continental crust. An analysis of the depth map of the acoustic basement made it possible to identify the depocenters of the relatively isolated Enderby, Shackleton and Labuan basins framing the Kerguelen Plateau on the southern side. The paragenetic connections of the Kerguelen Plateau with the basins indicate that the isolation of the Kerguelen Plateau from East Antarctica and the formation of the modern structure of the region began in the Late Cretaceous and ended with the last phase of tectonic activation in the Late Miocene–Pliocene. The results of geochemical analysis show that the geochemical and petrological anomalies of the plateau and islands are associated with the crust, and not with the mantle contamination, as well as with the continental nature of the Kerguelen Plateau.

Petrogenesis of the Cenozoic volcanic rocks in Baekdu volcano in northeastern Asia and the expected depth of the magma chamber based on geochemistry, mineral chemistry, and Sr-Nd-Pb isotope chemistry

Baekdu volcano is a Cenozoic composite volcano in northeastern Asia that consists of a basaltic plateau, a basaltic shield, and a trachytic to comenditic stratovolcano. In this study, the petrogenesis and tectonic setting of Baekdu volcano are studied based on the whole-rock geochemistry and Sr-Nd-Pb isotope compositions of the Baekdu volcanic rocks. The K–Ar age and mineral chemistry are also investigated to estimate the evolution of the magma chamber depth beneath Baekdu volcano. The Toudao and Baishan basalts formed ca. 2.5 Ma and 1.1 Ma and erupted from magma chambers located at depths of 11.5 ± 2.2 km and 24.9 ± 3.8 km, respectively. The Xiaobaishan and Middle trachytes and the comendites formed during the Pleistocene from magma chambers located at depths of 2.9 ± 0.2 km, 5.4 ± 0.45 km, and 4.4 ± 0.3 km, respectively. These data suggest that the trachytes and comendites erupted from magma chambers located at depths of approximately 3–5 km. The Baishan basalts have geochemical characteristics comparable with those of ocean island basalt (OIB), whereas the Toudao basalts show enriched mid-ocean ridge basalt (E-MORB) or OIB characteristics. The trends of several components, such as the CaO/Al2O3 ratio and the TiO2 and Sc concentrations, are different between older basalts (>2 Ma, including the Toudao basalts) and younger basalts (<1.5 Ma, including the Baishan basalts), indicating that the magma-forming process changed ca. 2–1.5 Ma. The enrichment in Th/Yb ratios in the Toudao basalts indicates that they experienced crustal contamination. Trachytes and comendites are considered to be products of the fractional crystallization of the Baishan basalt based on the similar geochemical trends of the two rocks. Crustal assimilation (up to 20%) also occurred with fractional crystallization (more than 70%) during the formation of peralkaline silicic volcanic rocks. All volcanic rocks in Baekdu volcano have Sr-Nd-Pb isotope compositions comparable with the enriched mantle (EM1)-like signature. This EM1-like signature may have originated from the contamination of asthenospheric mantle with Paleoproterozoic lithosphere that sank into the asthenospheric mantle due to delamination caused by regional extensional events in northeastern Asia between the Cretaceous and the Cenozoic. The characteristics of trace element compositions indicate a within-plate tectonic setting for Baekdu volcano. These data imply that Baekdu volcano origin may have been related to a plume of EM1-type upper mantle under the North China Craton.

Intracontinental extension and geodynamic evolution of the Paleoproterozoic Jiao-Liao-Ji belt, North China craton: Insights from coeval A-type granitic and mafic magmatism in eastern Liaoning Province

Abstract To better understand the origin and tectonic evolution of the Paleoproterozoic Jiao-Liao-Ji belt, North China craton, which have long been debated, we have conducted whole-rock geochemical and zircon U-Pb and Hf isotope analyses on Paleoproterozoic monzo(syeno)granitic gneisses and metamafic rocks from eastern Liaoning Province. The results indicate that these monzo(syeno)granitic gneisses and metamafic rocks formed concurrently at ca. 2.2–2.1 Ga. The monzo(syeno)granitic gneisses show geochemical features of A-type granites and were derived from dehydration melting of Archean tonalite-trondhjemite-granodiorite (TTG) with minor involvement of coeval mantle-derived magma in an extensional setting. The metamafic rocks are geochemically similar to mid-ocean-ridge basalt; they were generated by partial melting of isotope-depleted asthenospheric mantle in an intracontinental extensional setting and experienced crustal and continental lithospheric mantle contamination and fractional crystallization. Based on this study and published geological evidence, we propose the following tectonic model for the origin and geodynamic evolution of the Jiao-Liao-Ji belt: (1) Generation of high-density eclogitized lower crust accompanied by formation of voluminous Archean TTGs in the Eastern block; (2) intracontinental extension driven by upwelling of an asthenospheric hotspot along the Jiao-Liao-Ji belt in period of 2.20–2.0 Ga; (3) convergent orogenic processes between the Longgang block and Nangrim block governed by gravity-driven subduction of high-density eclogitized lower crust at ca. 1.95–1.85 Ga; and (4) postorogenic extension caused by delamination of the orogenic root and subsequent hot asthenospheric upwelling. This tectonic model can well explain the origin and geodynamic evolution of the Paleoproterozoic Jiao-Liao-Ji belt.

Neoproterozoic Amdo and Jiayuqiao microblocks in the Tibetan Plateau: Implications for Rodinia reconstruction

Abstract The Tibetan Plateau is composed of several microblocks, the tectonic affinity and paleogeographic correlations of which remain enigmatic. We investigated the Amdo and Jiayuqiao microblocks in central Tibet Plateau with a view to understand their tectonic setting and paleogeographic position within the Neoproterozoic supercontinent Rodinia. We present zircon U-Pb and Lu-Hf isotope, and whole-rock geochemical data on Neoproterozoic granitic gneisses from these microblocks. Zircon grains from the Jiayuqiao granitic gneiss yielded an age of 857 ± 9 Ma with variable εHf(t) values (–8.9 to 4.0). The Amdo granitic gneisses yielded ages of 893 ± 5 Ma, 807 ± 5 Ma, and 767 ± 11 Ma, with εHf(t) values in the range of –4.9 to 3.5. Geochemically, the granitoids belong to high-K calc-alkaline series, with the protolith derived from partial melting of ancient crustal components. The ascending parental magma of the Amdo granitoids experienced significant mantle contamination as compared to the less contaminated magmas that generated the Jiayuqiao intrusions. In contrast to the Lhasa, Himalaya, South China, and Tarim blocks, we suggest that the Amdo and Jiayuqiao microblocks probably formed a unified block during the Neoproterozoic and were located adjacent to the southwestern part of South China craton. The Neoproterozoic magmatism was probably associated with the subduction of the peripheral ocean under the South China craton and the delamination of lithospheric mantle beneath the Jiangnan orogen.

Constraining Mantle Heterogeneity beneath the South China Sea: A New Perspective on Magma Water Content

The nature of upper mantle is important to understand the evolution of the South China Sea (SCS); thus, we need better constrains on its mantle heterogeneity. Magma water concentration is a good indicator, but few data have been reported. However, the rarity of glass and melt inclusions and the special genesis for phenocrysts in SCS basalts present challenges to analyzing magmatic water content. Therefore, it is possible to estimate the water variations through the characteristics of partial melting and magma crystallization. We evaluated variations in Fe depletion, degree of melt fractions, and mantle source composition along the fossil spreading ridge (FSR) using SCS basalt data from published papers. We found that lava from the FSR 116.2° E, FSR 117.7° E, and non-FSR regions can be considered normal lava with normal water content; in contrast, lava from the FSR 117° E-carbonatite and 114.9–115.0° E basalts have higher water content and show evidence of strong Fe depletion during the fractional crystallization after elimination of the effects of plagioclase oversaturation. The enriched water in the 117° E-carbonatite basalts is contained in carbonated silicate melts, and that in the 114.9–115.0° E basalts results from mantle contamination with the lower continental crust. The lava from the 117° E-normal basalt has much lower water content because of the lesser influence of the Hainan plume. Therefore, there must be a mantle source compositional transition area between the southwestern and eastern sub-basins of the SCS, which have different mantle evolution histories. The mantle in the west is more affected by contamination with continental materials, while that in the east is more affected by the Hainan mantle plume.

Melt inclusions track melt evolution and degassing of Etnean magmas in the last 15 ka

We present major elements compositions and volatiles contents of olivine-hosted melt inclusions from Etna volcano (Italy), which extend the existing database with the aim of interpreting the chemical variability of Etnean magmas over the last 15 ka. Olivine phenocrysts were selected from the most primitive Fall Stratified (FS) eruptive products of picritic composition (Mg# = 67–70, Fo89–91), the Mt. Spagnolo eccentric lavas (Mg# = 52–64, Fo82–88) and among the more recent 2002–2013 eruptive products (Mg# = 33–53, Fo68–83). Crystal fractionation and degassing processes were modeled at temperatures of 1050–1300 °C, pressures <500 MPa, and oxygen fugacity between 1 and 2 log units above the nickel-nickel oxide buffer, in order to interpret melt inclusions data.Melt inclusions show a great variability in major elements chemistry (e.g., 44–57 wt% SiO2, 3–16 wt% CaO, 4–13 wt% FeO, 2–12 wt% MgO, 1–6 wt% K2O), designating a continuous differentiation trend from FS toward 2013 entrapped melts, which is mostly reproduced by the fractional crystallization of olivine + spinel + clinopyroxene ± plagioclase, in order of appearance.Volatile contents in the glass inclusions are also extremely variable, with maxima up to 6 wt% H2O and 0.6 wt% CO2 in FS melt inclusions, and up to 0.43 wt% S in Mt. Spagnolo inclusions. H2O and CO2 contents in the melt inclusions suggest minimum entrapment depths of 4–19 km (below crater level) for FS inclusions and <10 km for the 2002–2013 trachybasalts.Petrological arguments coupled to the modeling of fractional crystallization and degassing processes concur to suggest that magmas from Mt. Spagnolo and the recent eruptions may be produced by differentiation from the most primitive volatile-rich FS magma along variable P-T paths, occasionally accompanied by secondary processes as crustal assimilation, mixing, and CO2 flushing. We do not exclude the occurrence of source processes at Etna, e.g., variable degrees of mantle melting and/or variable degrees of mantle contamination, already proposed by previous authors. Our data, nevertheless, suggest that the first-order features of the Etnean magmas erupted in the last 15 ka can be modeled by differentiation through fractional crystallization and degassing.

Early Cretaceous Kalakan Magmatic Area (Vitim Region, Northern Transbaikalia): Stages of Formation, Magmatic Sources, and Tectonic Setting

The Kalakan volcanic area in the Vitim region was formed in three stages: 137–121, 119–117, and 110 Ma. The first two stages were responsible for the formation of the mild-alkaline basaltic lavas of the Vitim Group and a complex of minor intrusions of the dolerite–granite porphyry series. The Early Cretaceous sequence is completed by a terrigenous sequence, which is intruded by a differentiated teschenite sill. The differences between the teschenite and the earlier volcanic rocks are best expressed in the distribution of incompatible elements. In the teschenite, their content steadily increases with SiO2 increase (crystallization differentiation trend). The early volcanic rocks show no such trend, evolving chaotically, which can be explained by the contamination of mantle melts with crust and/or anatectic melts. The differences in both groups of the Kalakan magmatic area are caused by a change of magmatic sources prior to the late Early Cretaceous.

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~ 5.3‰) and are apparently at odds with the mantle-like eNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like eHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (> 90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Reconciling the shadow of a subduction signature with rift geochemistry and tectonic environment in Eastern Marie Byrd Land, Antarctica

Basalt-trachyte volcanoes in the Marie Byrd Land (MBL) Cenozoic province lie along the Amundsen Sea coast on the north flank of the West Antarctic rift. Basalts here are characterized by OIB-like geochemistry, restricted ranges of 87Sr/86Sr (0.702535–0.703284) and 143Nd/144Nd (0.512839–0.513008) and a wide range of 206Pb/204Pb (19.357–20.934).Basalts at three MBL volcanoes display two anomalies compared with the above and with all other basalts in West Antarctica. They include 143Nd/144Nd (0.512778–0.512789) values at Mt. Takahe and Mt. Siple that are 2σ lower than other West Antarctic basalts, and Ba/Nb, Ba/La, and Ba/Th values at Mt. Murphy and Mt. Takahe that are 3–8 times higher than normal OIB. Isotope and trace element data do not support crustal and lithospheric mantle contamination, or the presence of residual mantle amphibole or phlogopite as explanations of these anomalies.The apparent coincidence of these anomalies with the site of a pre-Cenozoic convergence zone along the Gondwanaland margin suggests a subduction influence. Major episodes of subduction and granitic plutonism took place in MBL during the Devonian, Permian, and Late Cretaceous. Relicts in the source region, of components from these subducted slabs, provide a credible explanation for the uncoupling of Ba from other large ion lithophile elements (LILE), for its erratic distribution, and for the anomalously low 143Nd/144Nd at Mt. Takahe.The last episode of subduction ended ~85Ma, and was followed by continental break-up, rifting and lithospheric attenuation that produced the West Antarctic rift as we know it today. Thus, the enigmatic geochemical signatures in these three volcanoes seem to have been preserved roughly 61–85m.y. after subduction ended. New calculations of source melting depth and a new determination of lithospheric thickness suggest that the source of the anomalies resides in a fossil mélange diapir that rose from the Cretaceous subducting slab, became attached to the base of the lithosphere at 80–100km depth, and remained there during the subsequent plate motion and source remobilization history of this region.

The Upper Miocene magmatism of the Island of Elba (Central Italy): compositional characteristics, petrogenesis and implications for the origin of the Tuscany Magmatic Province

Late Miocene intrusive magmatism of the Island of Elba, Tuscany (central Italy), consists of stocks, laccoliths, sills, and dikes showing dominant monzogranite and granodiorite compositions, with minor leucogranitic dike-sill complexes, aplites and pegmatites. A few mafic rocks occur as dikes, and as microgranular enclaves hosted inside the main intrusions. The Elba magmatism belongs to the Tuscan Magmatic Province, an 8.5 to 0.3 Ma old association of mafic to felsic rocks, of mantle and crustal origin, cropping out in Tuscany and northern Latium. Major and trace element abundances of Elba rocks are extremely variable, testifying to complex origin and evolutionary history for magmas. 87Sr/86Sr (~ 0.708-0.723) and 143Nd/144Nd (~0.5121-0.5124) are close or within the field of upper continental crust, with mafic dikes showing the lowest Sr- and the highest Nd-isotope ratios. Petrological, geochemical and textural data of Elba igneous rocks are better explained by invoking a leading role for multiple mixing processes between crust-derived felsic magmas and mafic-intermediate melts of ultimate mantle origin, accompanied by fractional crystallisation. Proxies of crustal anatectic melts are represented by some highly radiogenic-Sr rocks from northern Monte Capanne pluton. Crustal magmas were formed by melting of sedimentary rocks, likely metagreywakes, at pressures exceeding 0.3 GPa. Mafic-intermediate magmas have calcalkaline to shoshonitic compositions and originated in an anomalous mantle, moderately contaminated by siliceous sediments. Selective enrichments in Sr, Ba and LREE are shown by some intermediate rocks (Orano dikes), revealing the occurrence of a distinct magma type at Elba. Similar compositions are also observed at Capraia island, San Vincenzo and Campiglia (southern Tuscany), suggesting a regional relevance for this magma type. Sr-Ba-LREE-rich rocks do not show obvious genetic relationships with other Tuscany magmas and may represent a distinct end-member whose origin deserves further studies to be understood. Petrogenetic processes occurred at Elba allow a better understanding of magma genesis at the regional scale. Some of the first-order petrological characteristics of the Tuscany Magmatic Province are comprised inside the space defined by three main end-member compositions, represented by acidic peraluminous rocks, calcalkaline-shoshonitic basalts, and mafic ultrapotassic lamproites. Mixing of various proportions of end-members generated a wide spectrum of hybrid compositions. These, in turn, were affected by evolution processes, such as fractional crystallisation and crustal assimilation, further complicating the petrological puzzle of the Tuscany Magmatic Province. The lamproitic and calcalkaline magmas were respectively generated by melting of phlogopite-bearing harzburgitic and lherzolitic mantle rocks, which were contaminated by subducted upper crustal siliceous materials. Lithospheric mantle contamination occurred during the Eocene, forming harzburgite-sediment melange bodies. These were dismembered by ascent of lherzolitic asthenosphere behind the eastward migrating Northern Apennine subduction front, during Miocene to Quaternary backarc extension. Asthenospheric upwelling generated an increase in temperature, dehydration and melting of dispersed harburgite-sediment melange megaliths, with release of fluids that contaminated the surrounding newly emplaced lherzolites. Melting of such a heterogeneous mantle gave the wide range of mafic magmas observed in Tuscany. Backarc mantle doming and uprise of hot mafic magmas prompted anatexis within the continental crust of the overriding plate.

Cambro-Ordovician magmatism in the Araçuaí Belt (SE Brazil): Snapshots from a post-collisional event

The focus of the present work is the rebound of the magmatism at the late stages of orogenic collapse in the Araçuaí Belt, in Brazil. At the end of the orogen bimodal inversely zoned plutons were emplaced along a tectonic corridor following the Atlantic coast of Brazil. This event culminates around 500 Ma. We review geology, petrography, geochemistry and geochronology of these plutonic structures in the region of Espírito Santo. New geochemical modeling of trace element data together with new whole rock Sm–Nd and Rb–Sr isotopic data depict a gradual process of mantle contamination during different stages of the orogen. In the post-orogenic stage contamination reaches extreme values. Basic rocks reach εNd(0.5Ga) values below −10. Gabbros to peridotites are more enriched in Ba, Zr, LREE and Sr than the surrounding metasediments. Four new U–Pb data sets from determinations in zircon and monazite crystals, (central northern part of the belt) confirm peak magma production around 500 ± 15 Ma, ranging up to 525 ± 3 Ma in one of the structures. Based on the pictured data, we discuss the significance of this magmatism and present a possible model: a process of gradational delamination of the subcontinental lithospheric mantle and adjacent deep continental crust through underplating. As a consequence, an already enriched mantle was dramatically further contaminated. At the end of this event the intrusion of alkaline melts, with less negative εNd (−5) and further enrichment in incompatible elements, point towards a new input of deeper juvenile mantle magma. Our data suggest the possible onset of a hotspot due to the destabilization of the asthenospheric mantle after orogenic collapse.

Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

Continental flood basalts (CFBs) represent large-scale melting events in the Earth’s upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental lithosphere. Large-scale partial melting of the cold subcontinental lithospheric mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental lithosphere, however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land, Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of lithospheric and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean ridge basalts, and thus help to constrain asthenospheric parental melt compositions and (2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial e Nd from −16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using (1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wall rock and (2) assimilation-fractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1–15 wt%) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.

Geochronology and Geochemistry of the Tinggong Porphyry Copper Ore Deposit, Tibet

AbstractWe have determined the ages of the ore‐bearing Tinggong porphyries and the Eocene granites using the LA‐ICPMS zircon U‐Pb method. Zircons from one adamellite porphyry and two diorite porphyries yield ages of 15.54±0.28 Ma, 15.02±0.25 Ma and 14.74±0.22 Ma, respectively. The ages of two granites are 50.48±0.71 Ma and 50.16±0.48 Ma. Light Rare Earth Elements (LREE) are enriched in the ore‐bearing adamellite porphyries, which are high‐K calc‐alkaline and metaluminous, while Heavy Rare Earth Elements (HREE) and Y are strongly depleted, indicating an adakitic affinity. The Large Ion Lithophile Elements (LILE) of the adamellite porphyries are highly enriched, whereas some High Field Strength Elements (HFSE) are depleted. The diorite porphyry in this study is chemically similar to the adamellite porphyries, except that the Mg# of the diorite porphyry is a little higher, demonstrating more mantle contamination. Four samples from different rocks are selected for in situ zircon Hf isotopic analyses. The samples show positive εHf(t) values and young Hf model ages, indicating their derivation from juvenile crust However, the adamellite porphyry and diorite porphyry formed in the Miocene exhibit more heterogeneous Hf isotopic ratios, with lower εHf (t) values than the granites formed in the Eocene, suggesting the involvement of old Indian continent crust in their petrogenesis. The geochronology and geochemistry of the adamellite porphyries and the diorite porphyries indicate that they formed from the same source region in a post‐collisional environment, but contaminated by crust and mantle materials in different ratios. The metallic minerals formed mainly during the older adamellite porphyry stage, but they were recycled and reactivated by the diorite porphyry intrusion.

The Pan-African Kekem gabbro-norite (West-Cameroon), U–Pb zircon age, geochemistry and Sr–Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt

The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U–Pb zircon analyses provide an age of 576±4Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ).The REE patterns display enriched LREE (LaN/YbN=14.2–23.5) while HREE present a nearly flat profile (DyN/YbN=1.3–1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb–Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6–10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068–0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6–1.8Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust.The ca. 576Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.

Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate

Abstract:On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units: high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high‐grade metamorphism, but the latter is less than 1% and crops out in the upper part with low‐grade metamorphism. The high‐TiO2 unit is dominated by tholeiitic lavas showing high rare earth element (REE) contents (ΣREE = 83.4–180.8 μg/g), high light/heavy REE (LREE/HREE) ratios (LREE/HREE=2.17–5.85) and weak negative Eu anomaly (Eu=0.79–1.01). Its trace element patterns display weak Nb‐Ta anomalies with respect to Th, K, La, Ce, showing within‐plate basalt affinities. In contrast, the low‐TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb‐Ta anomalies, indicating typical arc or continental arc signature. Chondrite‐normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid‐oceanic basalt. Therefore, we suggest that these lavas should be formed in a back‐arc basin setting. Sr‐Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ∼1144 Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic‐island basalt‐like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U‐Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid‐oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift‐related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.