Lower Crustal Material Research Articles

Age and origin of the Cannon Point syenite, Essex County, New York: southernmost expression of Monteregian Hills magmatism?

Two syenite sills intrude the local Paleozoic strata of eastern New York State and are exposed along the western shore of Lake Champlain. The sills are fine-grained, alkali feldspar syenites and quartz syenites, with phenocrysts of sanidine and albite. The two sills are compositionally distinct, with crossing rare earth element profiles and different incompatible element ratios, which eliminates the possibility of a simple petrogenetic relationship. Zircon extracted from the upper sill yields a U–Pb age of 131.1 ± 1.7 Ma, making the sills the youngest known igneous rocks in New York State. This age is similar to that of the earliest intrusions in the Monteregian Hills of Quebec, >100 km to the north. Sr and Nd radiogenic isotope ratios are also similar to those observed in some of the syenitic rocks of the eastern Monteregian Hills. The Cannon Point syenites have compositions typical of A-type, within-plate granitoids. They exhibit unusually high Ta and Nb concentrations, resulting in distinct trace element signatures that are similar to those of the silicic rocks of the Valles Caldera, a large, rift-related magmatic system. We suggest that the Cannon Point syenites were melts derived primarily by anatexis of old, primitive, lower crustal material in response to Mesozoic rifting and to the intrusion of mantle-derived magmas. The sills indicate that the effects of continental rifting were spatially and temporally extensive, resulting in the reactivation of basement faults in the Lake Champlain Valley hundreds of kilometers west of the active rift boundary, and crustal melting >50 Ma after the initiation of rifting.

Plume-induced continental rifting and break-up in ultra-slow extension context: Insights from 3D numerical modeling

Breaking the lithosphere in extension without exceeding the driving far-field forces available on Earth is a tough quantitative modeling problem. One can tear it apart by propagation of an existing oceanic basin or weakness zone or one must assist rifting with magmatic processes, which drop the effective stress and weakens locally the lithosphere. While previous 3D models have demonstrated that non-cylindrical plumes produce almost cylindrical rift structures in a lithosphere under slight far-field loading, our contribution goes one step further by producing models of complete continental break-up. We investigate in details how the rheological stratification of the continental lithosphere interacting with active mantle plume influences the geometry and dynamics of rifting to continental break-up in 3D. We find that, irrespective of the rheological stratification, a plume-induced rifting process always occurs in two stages: an early crustal rifting stage and a late lithospheric necking (breakup) stage. In case of a rheologically decoupled lithosphere, initial brittle deformation is concentrated in the upper crust and strongly localized due to compensating ductile flow of lower-crustal material (core complex extension mode). On the contrary, rheological coupling between upper crust and lithospheric mantle results in highly distributed brittle deformation in the crust above mantle plume head (wide rift mode). Both core complex-like and wide rifting are followed by an abrupt transition to narrow rift stage when a localized ascent of mantle plume material focuses high strain along faults zones breaking through the entire lithosphere. The Main Ethiopian Rift, the Basin and Range province, and the East Shetland Basin may be natural examples of regions that have passed through these two stages of extension. Across-strike and along-strike asymmetry of break-up patterns arising spontaneously within initially symmetrical and laterally homogenous environment seems to be an intrinsic characteristic of plume-induced rifting.

The origin of the Palaeoproterozoic AMCG complexes in the Ukrainian shield: New U-Pb ages and Hf isotopes in zircon

The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815Ma and continued until 1743Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.

東北日本弧，男鹿半島の寒風山に産する角閃石含有同源集積岩と輝石グラニュライトの岩石学的，同位体的特徴：高圧下における角閃石の結晶作用と下部地殻物質の交代作用

東北日本弧，男鹿半島の寒風山に産する角閃石含有同源集積岩と輝石グラニュライトの岩石学的，同位体的特徴：高圧下における角閃石の結晶作用と下部地殻物質の交代作用

Geophysical imaging of metacratonizaton in the northern edge of the Congo craton in Cameroon

We used the World Gravity Map (WGM 2012) data to investigate the Archean Congo craton and the Oubanguides orogenic belt in Cameroon. The Oubanguides orogenic belt constitutes, from northwest to southeast, the Neoproterozoic West Cameroon domain, the Paleoproterozoic-Neoproterozoic Adamawa-Yade domain, and the dominantly Neoproterozoic Yaoundé domain (the crustal expression of the suture zone between the Congo craton and the orogenic terranes). We analyzed the WGM 2012 data to identify different gravity anomalies. We also applied the two-dimensional (2D) radially-averaged power spectral analysis to the WGM 2012 data to estimate the Moho depth. Additionally, we developed a 2D forward gravity model along a NS profile to image the lithospheric structure of the Precambrian entities. We found that: (1) the Congo craton, the Yaoundé domain, the southeastern part of the West Cameroon domain, and the northern part of the Adamawa-Yade domain are characterized by low gravity anomaly. (2) the southern part of the Adamawa-Yade domain is marked by a pronounced E-W trending high gravity anomaly. (3) the crust is thicker beneath the Congo craton, the Yaoundé domain and the southern part of the Adamawa-Yade domain. (4) the presence of a denser lower crust material beneath the southern part of the Adamawa-Yade domain. We propose that this denser crustal material is an under-thrusted portion of the Congo craton that has been densified through metacratonization processes that accompanied collision between the craton and the orogenic terranes. This is in good agreement with geological and geochemical observations indicating that the northern edge of the Congo craton and the Adamawa-Yade domain had undergone metacratonization during the Neoproterozoic. Our suggestion is also in good agreement with observations which show that the margins of many cratons worldwide have been decratonized due to subduction processes. Our work highlights the importance of potential field geophysical data in mapping the metacratonized margins of cratons.

Sr–Nd–Pb isotopic geochemistry of phenocrysts in pumice from the central Okinawa Trough

AbstractPumice is the most widely distributed magmatic rock in the Okinawa Trough. Numerous scholars have conducted extensive research on the mineralogy, petrochemistry and whole‐rock isotopic characteristics of pumice. However, even with extensive research, some information about pumice is still unknown, including the initial provenance of magma, influences from subducting plates and the possibility of crustal material (including sediments) mixing in during pumice formation. As pumice is loose and porous, seawater re‐modification and injection of sediments may occur during the condensation and rock‐forming process after magma erupts from the seafloor, or in the seawater immersion process after pumice formation. In order to get a better understanding of the isotopic characteristics of magmatism phase, phenocrysts were first separated from the pumice and divided into both light and heavy minerals to analyse their Sr–Nd–Pb isotopic composition. This information was combined with existing isotopic data of volcanic rocks from the Okinawa Trough and its adjacent areas to assess the magma source composition, influence from subduction components and mantle properties in the magmatic source. The research results indicated light minerals had an average 87Sr/86Sr ratio of 0.70454, while heavy minerals averaged 0.70546. The magma, formed pumice, was a product of the crystal fractionation evolution of basic magma, with the additional injection of crustal material. The magma source in the central trough is a mixture of depleted MORB mantle (DMM) and enriched mantle (EMI‐like and EMII‐like) end members, where the EMI‐like end member is the residual subcontinental lithosphere or lower crustal material and the EMII‐like end member is subducted oceanic sediments. The Pb isotopic composition indicated that the magma source was affected by subduction components (sediments) and that pumice magma was mixed with more sediment than basalt magma material in the same area. The mantle of the magma source in the Okinawa Trough showed signatures of the Indian Ocean‐type mantle. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis of Mineral Compositions and Crater Morphology on the Moon surface using Moon Orbital Satellite Data

Abstract Analysing vertical and lateral distribution of minerals within an impact crater on lunar surface would aid in understanding the crustal compositions to a larger extent and provides clue about geological evolution of the Moon. The Chandrayaan-1 Moon Mineralogy Mapper (M3) and Lunar Reconnaissance Orbiter Camera (LROC) data have high spectral and spatial resolutions, which help in identifying the mineral compositions and morphological features of impact crater. Here we analyse mineral compositions and their correlations with crater morphology using M3 and LROC satellite data of Eijkman impact cater in SouthPole Atiken (SPA) basin. The result shows that low-Ca pyroxene (LCP) dominant rocks are identified on Central Peak (CP), Crater Floor (CF), Crater Wall (CW) and Crater Rim (CR). An olivine dominant rock is detected on the CW. Fe-Mg-spinel lithological unit is observed on the CF. The results implicate that, (i) Low-Ca pyroxene minerals could be from the lower crust during SPA main event; (ii) Presence of olivine and Fe-Mg-spinel lithology on the surface could be a later stage mafic intrusions or the lower-crustal material exposed on the surface due to major impacts.

Electrical resistivity structures of the Kunlun–Qaidam–Qilian system at the northern Tibet and their tectonic implications

The northern Tibetan Plateau is characterized by northwest–southeast trending basin-range systems such as the Kunlun Shan Ranges–Qaidam Basin–Qilian Shan Ranges. The Cenozoic evolution and deformation of the Qaidam Basin and its neighboring ranges are important indications for the northward growth of the Tibetan Plateau. The latest magnetotelluric data were collected along a profile in N16.5°E from the Songpan-Ganzi Block, across the Kunlun Shan Ranges, east section of the Qaidam Basin, and ending in the Qilian Shan Ranges. Both two-dimensional and three-dimensional inversions of the data along the profile were performed and the results provided new evidence for lithospheric structures in the northern Tibetan Plateau. With three-dimensional isotropic inversion codes, we are able to model the Phase Roll Out of Quadrant (PROQ) magnetotelluric data, which exist at low frequencies in a number of continuous sites at the northern Qaidam Basin. Sensitivity studies of the three-dimensional models indicate that the Qaidam Basin with low-resistivity upper crust is the major contributor for the PROQ effect, while a south–north-striking low-resistivity gap intersecting with the Qaidam Basin at its northern part affects the range of the PROQ effect. In the magnetotelluric resistivity models, the Qaidam Basin is not in symmetric structure at upper-crust level, with its depocenter near the Northern Qaidam Fault. At mid-lower crust level, the compositions of both the Kunlun Shan Ranges and the southern Qaidam Basin are of generally high-resistivity, which does not support large-scale lower-crust materials of the Songpan-Ganzi Block flow below the Kunlun Shan Ranges or the Qaidam Basin. Instead, the unsymmetrical Qaidam Basin and the shovel-shape high-resistivity anomalies in crust below the Kunlun Shan Ranges and the southern Qiadam Basin indicate the regional northward push from the Songpan-Ganzi Block. The south-deepening low-resistivity anomalies in the crust of the Qilian Shan Ranges support the existence of an intracrustal detachment, which controls the thrusts in northern Qaidam Basin and the Qilian Shan Ranges. The crustal deformation in northern Tibetan Plateau is consistent with mantle convergence or collision between the Kunlun–Qaidam lithospheric mantle and the Asian lithospheric mantle.

Geochronology and geochemistry of the major host rock of the Dong’an gold deposit, Lesser Khingan Range: Implications for petrogenesis and metallogenic setting during the Early–Middle Jurassic in northeast China

The Dong’an gold deposit is a large-sized epithermal gold deposit recently discovered in the Lesser Khingan Range, NE China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of a medium–coarse grained alkali-feldspar granite, the major host rock of the Dong’an gold deposit. The LA–ICP–MS zircon U–Pb dating of the medium–coarse grained alkali-feldspar granite yields an early Jurassic age of 176.3±1.1Ma (MSWD=0.62). The whole-rock geochemical data indicate that the samples are felsic, ferroan, alkali-calcic and peraluminous with relatively high alkali (K2O+Na2O) content. They are enriched in LREEs and LILEs (e.g., Rb, Ba, K), but are depleted in HFSEs (e.g., Nb, Ta, P, Ti), especially in P and Ti, showing characteristics of volcanic arc magmas and similarities with the Early–Middle Jurassic granitic rocks in Xing’an Mongolian orogenic belt. Meanwhile, the negative Eu, Nb, Ta, Ti, and P anomalies are consistent with fractional crystallization of plagioclase, Ti-bearing phases (rutile, ilmenite, titanite, etc.) and apatite during magma evolution. The samples have low Nb/Ta ratios (8.65–14.91) and low Mg# values (18–36), which are indicative of crustal derived magmas and no interaction between source magmas and the mantle. In-situ Hf isotopic analyses of the zircons from the medium–coarse grained alkali-feldspar granite yield εHf(t) values of +3.38–+5.68 and two-stage model ages (TDM2) of 772–900 Ma, indicating the magmas formed this intrusion were generated by partial melting of Neoproterozoic basaltic materials in the young lower crust, and the magma source could be derived from a depleted mantle. The medium–coarse grained alkali-feldspar granite most likely formed in the late stage of Toarcian subduction of the Pacific plate, which can be identified on the tectonic setting discrimination diagrams, and the formation of this intrusion was associated with underplating of mantle-derived magmas, which provided heat for crustal partial melting. Similar to the medium–coarse grained alkali-feldspar granite, large amounts of granitic rocks and a series of nonferrous metal hydrothermal deposits (Mo, Cu, Au) formed in northeast China as results of magmatic activities triggered by subduction of the Pacific plate during the Early–Middle Jurassic.

Crustal structure and composition beneath the northeastern Tibetan plateau from receiver function analysis

We analyzed hundreds of P-wave receiver function data recorded by 53 portable seismic stations to investigate the crustal structure beneath the NE Tibetan plateau. The Moho depths identified by this study reveal that the crustal thickness decreases laterally from the Qilian Orogen (∼63km) to east Kunlun fault (EKLF) area (∼44km). The Moho depth becomes obviously deeper northeast of Songpan–Ganzi terrane and beneath the Qilian Orogen. A remarkable contrast is observed in the measured Poisson’s ratio between the Qilian Orogen and EKLF area. The Qilian Orogen is characterized by lower Poisson’s ratio. In general, the higher Poisson’s ratios (υ>0.30) in the EKLF area can be considered as the evidence for dominantly mafic rocks in the crust. The measured low Poisson’s ratio and the negative correlations between VP/VS and the crustal thickness beneath the Qilian Orogen suggest dominantly felsic crust beneath the Qilian Orogen, which is in contrast with the mid-to-lower low-velocity crustal model beneath the NE margin of the Tibetan plateau indicating that the low velocity zone terminates beneath the east Kunlun area. The seismic evidence suggests that the thickened crust is probably resulted from superposition of the successive intracrustal thrusts. Our results reveal the lateral inhomogeneity of crustal structure in this area and are inconsistent with the scenario of an inflated crust due to extrusion of lower crust material from the central Tibetan plateau.

Geochronology, geochemistry, and Sr–Nd–Pb isotopes of Cretaceous granitoids from western Tibet: petrogenesis and tectonic implications for the evolution of the Bangong Meso-Tethys

New zircon U–Pb age data, whole-rock major and trace elemental data, and Sr–Nd–Pb isotopic data of granitoids from the Woruo and Bailong areas in the southern margin of the Qiangtang Block (western Tibet, China) provide important evidence for northward subduction of the Bangong Meso-Tethyan oceanic lithosphere. Five granitoid samples are dated at 120.7–112.7 Ma. These granitoids mostly belong to calc-alkaline and high-K calc-alkaline series, and show strong enrichments in large-ion lithophile elements (e.g. Rb, Ba, and Th), depletions in Nb and Ta, and negative correlations between P2O5, TiO2, MgO, and SiO2 contents, consistent with the evolution trend of I-type magmas. The samples are characterized by a wide range of Sr–Nd–Pb isotopic compositions (initial εNd values from −0.06 to 5.84, initial 87Sr/86Sr from 0.704458 to 0.705596, (206Pb/204Pb)0 from 18.5436 to 19.2465, (207Pb/204Pb)0 from 15.6115 to 15.6712, and (208Pb/204Pb)0 from 38.6427 to 39.6942). The trace elemental and Sr–Nd–Pb isotopic data suggest that these samples are derived from a mixture of lower crust and mantle materials. Together with the published data, these Early Cretaceous magmatic rocks represent a continental arc setting formed by the northward subduction of the Bangong Meso-Tethys during the Early Cretaceous (120–112 Ma).

Geology and ore genesis of the Yu'erya gold deposit, eastern Hebei Province, China

The large Yu'erya gold deposit (65t of contained gold averaging 2.3g/t Au) in the eastern part of the Hebei Province of China is spatially associated with the Yu'erya Granite, and a group of NE- and NNE-trending faults. The alteration associated with mineralization is characterized by the assemblage pyrite, quartz, sericite, albite, and carbonate. Four stages of mineralization, in chronological order, are (1) quartz and medium- to coarse-grained pyrite; (2) quartz, fine-grained pyrite, and gold; (3) quartz, polymetallic sulfide, tellurobismuthite, and gold; and (4) quartz, pyrite, and carbonate. Most of the gold was deposited during the second and third stages of alteration from mesothermal fluids. These fluids were relatively rich in H2O, CO2, K+, Ca2+, Cl, and S, and low salinity. The H–O and sulfur isotope ratios determined for the mineralized samples indicate a magmatic source, and the Pb isotope data indicate that the Au mineralization originated from the mantle and lower crustal materials. Geochronological data indicate that the gold mineralization event was restricted to 200–163Ma whereas the associated magmatism occurred between 200 and 150Ma. This Mesozoic gold mineralization is related to the subduction of the Mongolia–Okhotsk and Paleo-Pacific oceans along the edges of the North China Craton.

北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

We analyzed the Sr-Nd isotopic and geochemical compositions of 112 late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan. The volcanic rocks are divided, on the basis of Nd isotopic compositions, into a high-Nd group (HND) and a low-Nd group (LND). The HND has higher Zr/Nb and Hf/Nb ratios than the LND volcanic rocks. The isotopic ratios and high field strength element (HFSE) ratios indicate that the HND volcanic rocks were derived from a relatively depleted mantle compared with the LND rocks. The relationship between the HND and LND rocks, in terms of their isotopic and Zr/Nb, Hf/Nb ratios, is similar to that observed between Quaternary rear-arc and frontal-arc volcanic rocks of the NE Japan arc. The rear-arc rocks from NE Japan have higher Nd isotopic and HFSE ratios than the frontal-arc rocks, indicating the injection of depleted asthenospheric mantle beneath the rear-arc, associated with Miocene opening of the Sea of Japan. The HND rocks occur in the central part of the Fossa Magna region in the form of a NW-trending wedge. This wedge shape of the HND area is consistent with the wedge-shaped rifting that occurred perpendicular to the strike of the arc and that opened the Fossa Magna contemporaneously with the opening of the Sea of Japan. The Sr and Nd isotopic compositions of the HND and LND rocks fall within the mantle array and extend into the high-Sr isotopic field. Some of the LND rocks contain lower-crustal mafic xenoliths with Sr-Nd isotopic compositions that overlap those of the LND rocks. In the Sr-Nd isotope diagram, the trend of the HND rocks can be extrapolated to the most radiogenic Sr isotopic compositions obtained for the xenoliths. This result suggests that the lower-crustal materials, which have the most radiogenic Sr isotopic compositions, contributed to the Sr isotopic enrichment of the LND and HND magmas.

Oscillatory Sr isotopic signature in plagioclase megacrysts from the Damiao anorthosite complex, North China: Implication for petrogenesis of massif-type anorthosite

Formation of Proterozoic massif-type anorthosites is known to be related to polybaric process involving early high-pressure crystallization of plagioclase and high-Al orthopyroxene megacrysts at mantle–crust boundary, followed by emplacement of plagioclase-dominated mushes to shallow-level crust. Therefore, the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites. The ~1.74-Ga Damiao complex, North China, comprises >90vol.% anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high-Al orthopyroxene megacrysts (HAOMs). The plagioclase megacrysts are generally euhedral to subhedral (mostly 1 to 30cm in diameter), and some of them contain very fine lamellae of orthoclase and undefined Fe–Ti-rich minerals (1–5μm). The HAOMs occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts. They contain abundant thin, regular lamellae of exsolved plagioclase (15–20vol.%), indicative of originally high-Al features (originally 6.0–7.3wt.% Al2O3). Based on the Al-in-orthopyroxene geobarometry, the HAOMs and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9.4–11.2kbar (33–36km), indicative of their crystallization at lower crust depths or crust–mantle boundary. In contrast, orthopyroxene grains from late differentiation phases such as oxide-apatite gabbronorites do not contain plagioclase lamellae, and have much lower Al2O3 contents (1.5 to 1.7wt.%), indicating final crystallization at <5kbar (<15km). Four plagioclase megacrysts analyzed in this study have slightly different compositions, but collectively they all display comparable, oscillatory variations of An values (43–55), Sr (1100–1800ppm), Ba (800–1400ppm), La (2 to <7ppm) and 87Sr/86Sr ratios (0.70283–0.70466) from center to rim. Although Sr content is also likely related to pressures, our work suggests that the oscillatory chemical and Sr isotopic signature of the plagioclase megacrysts was mainly controlled by the compositions of the magmas, which temporally changed in the deep magma chamber. A mixing-assimilation modeling, based on Sr contents and initial 87Sr/86Sr ratios of synchronous mantle-derived mafic dykes and ancient lower crustal xenoliths in North China, suggests that the parental magma was initially a depleted mantle-derived basaltic magma that assimilated with ~30% lower crustal materials (Al2O3=15–24wt.%; Sr=800–2000ppm) or partial melts of the lower crust when ponding at the base of lower crust. We consider that an increasing of Al2O3 in the basaltic magma due to assimilation may have triggered saturation of plagioclase, because the oscillatory chemical and isotopic patterns are consistent from center to rim in all plagioclase megacrysts, indicative of initiation of the assimilation processes at the very beginning of plagioclase crystallization. Our study supports the model that the parental magmas were derived from partial melting of the depleted mantle combined with all-important crustal contamination in deep magma chambers or during rising of crystal mushes, and this may also account for variable isotopic signature for different phases in many anorthosite suites.

New isotopic data for Mid-Atlantic Ridge basalts from the Arkhangelsk-Sierra Leone fracture zone (central Atlantic)

This study presents Sr, Nd, and Pb isotopic data for basalts collected from the MAR axis between the Arkhangelsk transform fault and 5° N. The isotopic data for basalts from the MAR segment between the Arkhangelsk and Vernadsky faults indicate the presence of N-MORB depleted tholeiites, which are similar to some basalts from the MAR segment between the Cape Verde and Marathon faults, at the location of the 14° N geochemical anomaly. Broad positive along-axis gradients in isotopic values (and negative for 143Nd/144Nd) are observed from south to north, superimposed by isotopic variations in the 9.6° N anomalous basalts. A north-south increase in (La/Sm)n suggests a smaller degree of partial melting. The range of isotopic variations in basalts south of the Bogdanov fault is similar to that of basalts recovered at the more southerly 1.7° N anomaly. P-MORB compositions with a large proportion of the HIMU-like component are widely distributed in the vicinity of the 5.4° N rise. The isotopic ratios decrease gradually (with increasing 143Nd/144Nd) farther to the north toward the Bogdanov transform fault. A similar trend is observed for (La/Sm)n. Therefore, the basalts recovered to the south and north of Bogdanov fault have different sources of radiogenic isotope enrichment: 5.4° N and 9.6° N rises. Basalts recovered to the north of the Vernadsky transform fault have higher 143Nd/144Nd and lower 206Pb/204Pb at similar 87Sr/86Sr ratios. We conclude that the localized manifestations of enriched basalts with isotopic characteristics close to the HIMU-like plume-derived component, including the 14° N and 1.7° N geochemical anomalies, may have been formed by microplumes entrained at the base of the upper mantle by asthenospheric mantle flows arranged into a system of cylindrical convective cells, 450–500 km in diameter. Northward of the Bogdanov transform, the upper mantle is contaminated with lower continental crust material, which was entrained during tectonic erosion of the lithosphere beneath the equatorial segment of Gondwanaland during its late stage opening.

Geochemical signature of the granitoids in the Chalukou giant porphyry Mo deposit in the Heilongjiang Province, NE China

The Chalukou porphyry Mo deposit, located in the northern Great Xing'an Range, is the largest Mo deposit in the Xing'an–Mongolia orogenic belt. Its ore bodies are mainly hosted in the intermediate-felsic complex and Jurassic volcanic sedimentary rocks, of which Late Jurassic granite porphyry, quartz porphyry and fine-grained granite are closely associated with Mo mineralization. The Middle Jurassic monzogranite belongs to shoshonite series, with SiO2 and Al2O3 contents of 69.48 to 74.98% and 12.35 to 14.48%, respectively. The total alkali (K2O+Na2O) content ranges from 7.67 to 10.42%, with K2O/Na2O ratios between 1.07 and 2.81. These rocks are strongly enriched in Rb and K but are depleted in Ta, Nb, P and Ti, with negative Eu anomalies and positive εHf(t). The Late Jurassic granite porphyry, quartz porphyry and fine-grained granite are shoshonite or high-K calc-alkaline series. Their SiO2 and Al2O3 contents range from 73.87 to 78.95% and 10.35 to 13.47%, respectively. The total alkali (K2O+Na2O) contents range from 8.06 to 10.02%, with K2O/Na2O ratios from 1.03 to 8.20. These rocks are strongly enriched in Rb, K and Th, but are depleted in P, Ti, Ba and Sr, indicating clear negative Eu anomalies and positive εHf(t). The Middle–Late Jurassic granitoids in the Chalukou deposit are highly fractionated I-type granitoids, and derived from juvenile lower crust materials that originated from the Neoproterozoic depleted mantle. These granitoids formed in the extension tectonic setting of the post-collision stage of the Mongol–Okhotsk orogenic belt, probably superposed by the back-arc extension related to the subduction of the Paleo-Pacific Plate.

Tectonics of the Musandam Peninsula and northern Oman Mountains: From ophiolite obduction to continental collision

ABSTRACT The tectonics of the Musandam Peninsula in northern Oman shows a transition between the Late Cretaceous ophiolite emplacement related tectonics recorded along the Oman Mountains and Dibba Zone to the SE and the Late Cenozoic continent-continent collision tectonics along the Zagros Mountains in Iran to the northwest. Three stages in the continental collision process have been recognized. Stage one involves the emplacement of the Semail Ophiolite from NE to SW onto the Mid-Permian–Mesozoic passive continental margin of Arabia. The Semail Ophiolite shows a lower ocean ridge axis suite of gabbros, tonalites, trondhjemites and lavas (Geotimes V1 unit) dated by U-Pb zircon between 96.4–95.4 Ma overlain by a post-ridge suite including island-arc related volcanics including boninites formed between 95.4–94.7 Ma (Lasail, V2 unit). The ophiolite obduction process began at 96 Ma with subduction of Triassic–Jurassic oceanic crust to depths of &amp;gt; 40 km to form the amphibolite/granulite facies metamorphic sole along an ENE-dipping subduction zone. U-Pb ages of partial melts in the sole amphibolites (95.6– 94.5 Ma) overlap precisely in age with the ophiolite crustal sequence, implying that subduction was occurring at the same time as the ophiolite was forming. The ophiolite, together with the underlying Haybi and Hawasina thrust sheets, were thrust southwest on top of the Permian–Mesozoic shelf carbonate sequence during the Late Cenomanian–Campanian. Subduction ended as unsubductable cherts and limestones (Oman Exotics) jammed at depths of 25–30 km. The Bani Hamid quartzites and calc-silicates associated with amphibolites derived from alkali basalt show high-temperature granulite facies mineral assemblages and represent lower crust material exhumed by late-stage out-of-sequence thrusting. Ophiolite obduction ended at ca. 70 Ma (Maastrichtian) with deposition of shallow-marine limestones transgressing all underlying thrust sheets. Stable shallow-marine conditions followed for at least 30 million years (from 65–35 Ma) along the WSW and ENE flanks of the mountain belt. Stage two occurred during the Late Oligocene–Early Miocene when a second phase of compression occurred in Musandam as the Arabian Plate began to collide with the Iran-western Makran continental margin. The Middle Permian to Cenomanian shelf carbonates, up to 4 km thick, together with pre-Permian basement rocks were thrust westwards along the Hagab Thrust for a minimum of 15 km. Early Miocene out-of-sequence thrusts cut through the shelf carbonates and overlying Pabdeh foreland basin in the subsurface offshore Ras al Khaimah and Musandam. This phase of crustal compression followed deposition of the Eocene Dammam and Oligocene Asmari formations in the United Arab Emirates (UAE), but ended by the mid-Miocene as thrust tip lines are all truncated along a regional unconformity at the base of the Upper Miocene Mishan Formation. The Oligocene–Early Miocene culmination of Musandam and late Cenozoic folding along the UAE foreland marks the initiation of the collision of Arabia with Central Iran in the Strait of Hormuz region. Stage three involved collision of Arabia and the Central Iran Plate during the Pliocene, with ca. 50 km of NE-SW shortening across the Zagros Fold Belt. Related deformation in the Musandam Peninsula is largely limited to north and eastward tilting of the peninsula to create a deeply indented coastline of drowned valleys (rias).

Spatial variations of seismic attenuation and heterogeneity in the Pyrenees: Coda Q and peak delay time analysis

Abstract Lateral variations of seismic attenuation in the Pyrenees are explored from the analysis of local earthquake records. Scattering loss and intrinsic absorption both control the propagation of short period S waves through the crust. The role of intrinsic and scattering attenuation is analyzed in two steps. Firstly, the coda quality factor Q c , which quantifies the energy decay of coda waves, is estimated at large lapse time in five frequency bands and interpreted as intrinsic absorption. Next, we systematically measure the peak delay time defined as the time lag from the direct S-wave onset to the maximum amplitude arrival. This parameter quantifies the strength of multiple forward scattering due to random inhomogeneities along the seismic ray path. Comparison of coda- Q and peak delay time measurements allows a qualitative interpretation of the origin of seismic attenuation (scattering/absorption) in the Pyrenean crust. At low frequency, coda- Q variations mainly depend on the tectonic units of the Pyrenees, with stronger absorption in sedimentary basins and smaller absorption in Paleozoic basements. At high frequency, coda- Q is low at the location of Neogene structures in the Eastern Pyrenees. A more enigmatic low- Q c anomaly is also observed at the location of the Maladeta Massif in the Central Pyrenees. In all frequency bands, peak delay time measurements systematically show stronger scattering in the Western Pyrenees. In the Labourd–Mauleon area, absorption and scattering are both important at low frequency. The Western Pyrenees also correspond to a high-velocity/density anomaly revealed from tomography and gravity data analysis. This suggests that the high level of inhomogeneities and absorption may be related to intrusion of mantle and/or lower-crustal materials. In the Eastern Pyrenees, absorption appears dominant over scattering at high frequency. We hypothesize that the strong absorption observed in this area may be related to volcanic structures.

Formation of Wulong Mafic Dikes via Mixing of Enriched Lithospheric Mantle-Derived and Crustal Magmas, Eastern North China Craton

Zircon U-Pb age and geochemical data are reported for the Wulong dike swarms from the Liaodong Peninsula, NE China. The studied dikes include diorite, lamprophyre, and dolerite. Together with previous dating, the diverse rocks have a peak age of ca. 120–125 Ma. They are marked by inversely zoned phenocrysts with Mg- and Cr-rich rims, and they exhibit mantle-like signatures (i.e., high Mg-Cr-Ni) associated with prominent subduction-type element enrichments. The early diorites have relatively radiogenic Sr (–0.7143) and less radiogenic Nd ( to −11.0) isotopes coupled with high and variable MgO (Cr, Ni) contents, indicating that they were mainly derived from ancient lower crust with subordinate contributions of enriched lithospheric mantle. The transitional lamprophyres are characterized by high Rb/Sr ratios (0.22–0.54) but low initial 87Sr/86Sr ratios (0.7069–0.7093), moderate negative εNd(t) values (−14.4 to −12.6), and low promantle elements (such as Mg-Cr-Ni), reflecting that juvenile lower crustal components with a short residence time were significantly added to their source as well. The late dolerites possess extremely high Mg, Cr, and Ni concentrations, less radiogenic Sr (–0.7082) and much radiogenic Nd ( to −2.4) isotopes, suggesting that they mainly originated from an enriched mantle source with minor involvement of old lower crustal materials. Petrographic and chemical evidence points to an origin of these rocks by mixing of enriched mantle-derived and lower crustal magmas. The rapid progressive change of source compositions first indicates a complex, multistage hybridization process, and second, it shows an extensional tectonic regime during magma generation.

Petrogenesis of the Wudaogou intermediate–mafic complex, NW China: zircon U–Pb dating, whole-rock and isotope geochemistry, and geological implications

The Wudaogou plutonic complex is located in the eastern Yanbian area of Jilin Province and consists of hornblende gabbros, gabbroic diorites, and quartz diorites that contain abundant dioritic microgranular xenoliths. Zircon U–Pb dating of gabbroic and quartz diorites yielded weighted mean 206Pb/238Pb ages of 263.5 ± 5.1 Ma (N = 12, mean squared weighted deviation (MSWD) = 0.78, probability = 0.66) and 262.0 ± 5.6 Ma (N = 10, MSWD = 0.50, probability = 0.87), respectively. These units are characterized by high Na2O/K2O (0.33–0.77) ratios and Al2O3 (15.05–18.91 wt%) concentrations and are large ion lithophile element (LILE) (light rare earth element (LREE), Rb, Ba, K, etc.) enriched and high field strength element (HFSE) (Nb, Ta, P, Ti) depleted. They also have initial 87Sr/86Sr values of 0.70192–0.70420 and ϵNd(t) values of +1.9 to +4.7 with two-stage model ages (TDM2) of 653–878 Ma. These characteristics indicate that these rocks formed from calc–alkaline magmas derived from partial melting of a mixture of juvenile crust formed attending the Neoproterozoic subduction of the Palaeo-Asian oceanic crust and lower crustal material. The dioritic xenoliths have whole-rock compositions that are similar to their host rocks, but with negative ϵNd(t) values (−1.6 to −4.3) and older TDM2 ages (1166–1382 Ma), further indicating that this magmatic event involved older crustal material. Combining these data with existing knowledge of the crustal evolution of this area, we conclude that this complex formed in a post-collisional extensional setting during closure of the Palaeo-Asian Ocean.