Initial Pb Isotope Ratios Research Articles

The Triassic Bilugangan deposit: Geological constrains on the genesis of one of the oldest Mo deposits in Inner Mongolia, China

The large Bilugangan porphyry Mo deposit is one of the oldest Mo deposit in eastern part of the Central Asian Orogen in Inner Mongolia. The orogen is currently regarded as the most important Phanerozoic region with widespread crustal growth between the Siberian Craton to the north, and North China Block to the south. The Bilugangan Mo mineralisation is hosted by a porphyritic monzogranite and along the contact with the Late Permian Linxi Formation. The mineralisation is disseminated in places and includes various types of hydrothermal veins. The mineralising stages of the deposit includes the pre-ore quartz–K-feldspar(–biotite) pegmatite succeeded by quartz–K-feldspar–molybdenite–muscovite–sericite(–pyrite) veins, quartz–molybdenite–chalcopyrite–perthite–muscovite–sericite(–pyrite–sphalerite), and post-ore quartz–fluorite–calcite (–sericite–muscovite–) veins. Seven molybdenite samples from the mineralisation yield a Re-Os Mo weighted mean date of 238 ± 1 Ma, which is the same, within error, as the Re-Os isochron age of 238 ± 2 Ma, and the ca. 240 Ma age of the porphyritic monzogranite. This is the oldest porphyry Mo – type deposit in the eastern part of the orogen, and the only large –sized Mo deposit in the region, which shows that the east CAO is prospective for Triassic mineralisation. Thirty-three samples of sulfide from the deposit have a narrow δ34SVCDT(‰) range of 1.5 to 4.3‰, indicating that the sulfur the sulfur has primarily a magmatic source. The sulfides from the Mo-bearing veins have relatively concentrated Pb isotopic compositions with 206Pb/204Pb ratios between 18.295 and 19.576, 207Pb/204Pb ratios between 15.535 and 15.662, and 208Pb/204Pb ratios between 38.066 and 38.653. These values are consistent with those of the initial Pb isotope ratios for whole rock samples from the porphyritic monzogranite and hornfels. Fifteen gangue quartz samples from various veins define a range of δ18Ofluid values from 0.5 to 5.2‰ with δDfluid values ranging from −115 to −60‰, indicating that the onset of the mineralising fluid was generated from in-situ degassing of a magmatic source, followed by an input of meteoric water during the late ore-forming stage. It is proposed that the deposit is a collision-related Dabie-type deposit, which is related to the closure of the Paleo-Asian Ocean between the North China Block and Siberian Craton.

Petrogenesis and tectonic implications of Late Cretaceous highly fractionated I-type granites from the Qiangtang block, central Tibet

The tectonic evolution of the Lhasa and Qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central Tibet. In this study, we present zircon U–Pb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopic data for the newly discovered Chuburi pluton in the southern Qiangtang block. Zircon U–Pb data reveal that the Chuburi pluton was emplaced during the Late Cretaceous (ca. 73 Ma). Bulk-rock analyses show that this intrusion is characterized by high SiO2, Na2O and K2O, but low MgO, CaO and P2O5. The granites are enriched with large-ion lithophile elements and light rare earth elements, with marked Eu anomalies, and there are few heavy rare earth elements and high field strength elements. These geochemical features indicate that the Chuburi pluton is a highly fractionated I-type granite. The nine granitic samples show high (87Sr/86Sr)i ratios (0.7064–0.7089), negative εNd(t) values (−1.94 to −5.59) and have Nd isotopic model ages ranging from 0.76 to 0.97 Ga. The initial Pb isotopic ratios of the Chuburi pluton is uniform: (206Pb/204Pb)t = 18.774–18.884; (207Pb/204Pb)t = 15.695–15.713; (208Pb/204Pb)t = 39.362–39.613. Based on geochemical characteristics of the Chuburi pluton, and consideration of the region geological setting, we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust, and then undergoing extensive fractional crystallization with removal of K-feldspar, plagioclase, mafic mineral and minor accessory mineral. The crust beneath the central Tibet may have been significantly thickened following the final Lhasa-Qiangtang amalgamation. We argue the generation of the Chuburi pluton can be interpreted as a result of regional lithospheric delamination.

Geochronology, Geochemistry and Sr-Nd-Pb Isotopic Study of the Wulong Flower-Like Glomerophyric Diorite Porphyry (Central China): Implications for Tectonic Evolution of Eastern Qinling

The Wulong diorite porphyry displays a very unusual flower-like glomerophyric texture and is thus well-known as a gemstone type of “Luoyang peony stone” for non-geologists. Hereby, we present a comprehensive study in terms of petrography, zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope. The rock exposed as small-scale dykes in the field, intruding in the Archean migmatized biotiteplagioclase genesis at the southern margin of the North China Craton. It exhibits typical porphyrite texture with plagioclase and amphibole as dominant and subdominant phenocrysts, respectively. The diorite porphyry rocks contain 51 wt.%–59 wt.% SiO2, belonging to medium-K calc-alkaline rock series. They are characterized by enrichment of large ion lithophile elements (LILEs, e.g., Ba) and depletion of high field-strength elements (HFSEs, e.g., Nb, Ta, Zr and Hf), resembling island arc magmatic rocks. They show high initial Sr isotopic ratios (0.710 7–0.715 8), low ɛNd(t) values (-9.9 to -13.3) and variable initial Pb isotopic ratios (206Pb/204Pb vary from 17.3 to 19.3), similar to the coeval intermediate and mafic magmatic rocks of adjacent region. These geochemical features indicate that the Wulong diorite porphyry is likely to originate from an enriched mantle with contamination of crustal materials. Zircon U-Pb dating results reveal a magma crystallization age of 480±3 Ma, consistent with the summit of the magmatic events of the Qinling orogenic belt and reflecting the northernmost Paleozoic magmatic activity of the belt. This may indicate the influence of northward subduction of Paleotethyan oceanic slab can extend a distance of ca. 200 km to the north and reach Luoyang-Songshan region in the East Qinling Orogen.

Crust-mantle interaction inferred from the petrology and Sr-Nd-Pb isotope geochemistry of Eocene arc lavas from the Kahrizak Mountains, north-central Iran

The Eocene volcanic rocks from the Kahrizak Mountains in north-central Iran are part of the Urumieh-Dokhtar magmatic arc, which runs parallel to the Main Zagros Thrust segment of the Neo-Tethys suture. These volcanic rocks, similar to those from eastern Pontides and northern Anatolia, Turkey, were mainly produced during the Eocene magmatic flare-up associated with the Arabia-Eurasia convergence. The rock suite includes basalt, trachyandesite/andesite and trachydacite/rhyolite lavas and pyroclastic deposits that evolved compositionally from calc-alkalic to shoshonitic. Their normalized trace element concentration patterns are moderately enriched in light rare earth element and depleted in high field-strength elements (HFSE; e.g., Nb, Ta, Ti). They have narrow ranges of initial Pb isotopic ratios and 143Nd/144Ndi, but highly variable 87Sr/86Sri. The new analyses indicate that the parental magmas of the volcanic rocks were derived from a mantle source that had been enriched by fluids released from a subducted oceanic slab. The fluids introduced significant amounts of large ion lithophile elements, but negligible HFSE to the source. The parental magmas underwent fractional crystallization and assimilation of upper crustal materials to produce the range of volcanic rocks. Integration of new analyses with regional data suggests that the Eocene volcanic rocks from north-central Iran, together with ~coeval volcanic rocks in eastern Pontides and northern Anatolia, were most probably derived from a lithospheric mantle source that had been previously metasomatized by fluids derived from a subducted slab before and during the Arabia-Eurasia collision.

Genesis of the South Zhuguang Uranium Ore Field, South China: Pb Isotopic Compositions and Mineralization Ages

AbstractU–Pb isotopic analyses indicate that ores from the South Zhuguang uranium ore field, south China, have high common (non‐radiogenic) Pb contents, with variable and relatively radiogenic initial Pb contents. The U–Pb isochron method was used to date these ores, with plots of208Pb/204Pb and207Pb/204Pb versus206Pb/204Pb being used to identify sample suites with similar initial Pb isotopic ratios and to normalize variable initial Pb isotopic ratios. The resulting U–Pb isochrons indicate two substages of uranium mineralization at ~57 and 52 Ma, with a later hydrothermal reformation at ~49 Ma, which homogenized Pb isotopic compositions. Initial Pb isotopic systematics indicate that the ore‐forming fluid was characterized by high206Pb/204Pb and207Pb/204Pb ratios and low208Pb/204Pb ratios, suggesting that the ore‐forming fluid was sourced from Cretaceous–Paleogene red‐bed basins, rather than from magma or the mantle, with consideration of mineralization ages.

Jurassic magmatism related Pb-Zn-W-Mo polymetallic mineralization in the central Nanling Range, South China: Geochronologic, geochemical, and isotopic evidence from the Huangshaping deposit

The Nanling Range in South China is characterized by extensive Mesozoic magmatism and coeval nonferrous and rare metal mineralization. Huangshaping is a world-class Pb-Zn-W-Mo polymetallic skarn deposit in the central Nanling Range. Magmatic rocks occurring in this ore district include quartz porphyry, granite porphyry, granophyre, dacite porphyry, and aplite, with only the first three granitoids genetically associated with polymetallic mineralization. Most of the orebodies are constrained within the contact zones as skarn and veins between these granitic stocks and the carbonate wall rocks.Since the age of the quartz porphyry is still controversial, and studies of the dacite porphyry and aplite are absent, we focus on these magmatic rocks first. LA-ICP-MS zircon U-Pb dating suggests that the crystallization ages of the quartz porphyry, dacite porphyry, and aplite are 154.3±1.9Ma, 158.1±0.8Ma, and 148.4±3.4Ma, respectively. Combined with previously published age data, we infer the evolutionary sequence of magmatic rocks should be dacite porphyry→quartz porphyry→granite porphyry (granophyre)→aplite. The quartz porphyry, dacite porphyry, and aplite yield high contents of high field strength elements (Zr+Nb+Ce+Y=255–440ppm), high ratios of 10,000×Ga/Al (2.6–3.2), and prominent depletions in Ba, Sr, Eu, P, and Ti, indicating their crustal affinities to A-type granites. They have negative εNd(t) values (−9.4 to −7.0) and high initial Pb isotopic ratios (206Pb/204Pbi=18.307–18.644, 207Pb/204Pbi=15.689–15.742, 208Pb/204Pbi=38.589–38.986), suggesting that they were probably derived by partial melting of ancient granulitic crustal materials.The sulfide minerals exhibit a wide range of δ34SV-CDT values from −22.6 to 24.2‰, with 206Pb/204Pb of 17.669–19.708, 207Pb/204Pb of 15.492–15.714, and 208Pb/204Pb of 37.880–39.789, indicating that sulfur, lead, and other associated metals were derived from a mixture of magmatic components and the Carboniferous wall rocks. Fluid inclusions in pyrrhotite, sphalerite, and marmatite samples have 3He/4He ratios of 0.12 to 1.53 Ra, with calculated mantle helium proportions of 1.3 to 18.9%, indicating a predominantly crustal origin for the ore fluids, with minor inputs from the mantle. The Huangshaping deposit is a typical example of the genetic relationship both spatially and temporally between Jurassic magmatism and polymetallic metallogeny in the Nanling Range.

Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica

The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian−Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution−thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4−177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3−173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706−0.708), unradiogenic 143Nd/144Nd (eNd < −5), and Pb isotopes consistent with a low-µ source (where μ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5−1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica.

The calc-alkaline and adakitic volcanism of the Sabzevar structural zone (NE Iran): Implications for the Eocene magmatic flare-up in Central Iran

A major magmatic flare-up is documented along the Bitlis–Zagros suture zone in Eocene–Oligocene times. The Cenozoic magmatism of intraplate Central Iran is an integrant part of this tectono-magmatic scenario. The Cenozoic magmatism of the Sabzevar structural zone consists of mostly intermediate to felsic intrusions and volcanic products. These igneous rocks have calc-alkaline and adakitic geochemical signatures, with nearly coincident zircon U–Pb and mica Ar–Ar ages of ca. 45Ma. Adakitic rocks have quite low HREE and high Sr/Y ratio, but share most of their geochemical features with the calc-alkaline rocks. The Sabzevar volcanic rocks have similar initial Sr, Nd and Pb isotope ratios, showing their cogenetic nature. Nd model ages cluster tightly around ~0.2–0.3Ga. The geochemistry of the Sabzevar volcanic rocks, along with their isotopic signatures, might strangle that an upper mantle source, metasomatized by slab-derived melts was involved in generating the Sabzevar calc-alkaline rocks. A bulk rock trace element modeling suggests that amphibole–plagioclase–titanite-dominated replenishment-fractional crystallization (RFC) is further responsible for the formation of the middle Eocene Sabzevar adakitic rocks. Extensional tectonics accompanied by lithospheric delamination, possibly assisted by slab break-off and melting at depth was responsible for the Eocene formation of the Sabzevar magmatic rocks and, more in general, for the magmatic “flare-up” in Iran.

Geochronology and geochemistry of Grenville-aged (1063 ± 16 Ma) metabasalts in the Shennongjia district, Yangtze block: implications for tectonic evolution of the South China Craton

To better understand the evolution of the South China Craton (SCC), we have determined the geochronological and geochemical compositions of newly recognized Grenville-aged metabasalts in the Shennongjia region of the northern Yangtze block. LA-ICP-MS U-Pb dating of zircons indicates that the metabasalts formed at 1063 ± 16 Ma. The rocks are calc-alkaline, are characterized by SiO2 contents (50.50 to 55.62 wt.%), and have moderate-to-high MgO contents (7.25–9.60 wt.%). They display light rare earth element enrichment ((La/Yb)N = 7.0–9.8) with slightly negative Eu anomalies (Eu/Eu* = 0.82–0.90) and have pronounced depletion in high-field strength elements as well as positive Pb anomalies in the primitive mantle-normalized trace element pattern. They possess high initial Sr isotopic ratios of 0.7092–0.7107, large negative εNd(t) values of −12.1 to −11.0, and a relatively narrow range of initial Pb isotope ratios (206Pb/204Pb = 16.503–17.019, 207Pb/204Pb = 15.259–15.452, and 208Pb/204Pb = 36.169–36.994). These isotopic characteristics are typical of basalts derived from an EM2 source region and suggest a subcontinental lithospheric mantle source that was metasomatized by subducted components (fluids and melts). Integrating our new data with documented igneous and metamorphic events during late Mesoproterozoic to early Neoproterozoic time in the region and western segments of the Yangtze block, we suggest that the Shennongjia area might be a microcontinent that was independent of the continental nucleus of the Yangtze block and was accreted to the Yangtze block at the end of Mesoproterozoic time. This study thus argues against the traditional view that the SCC was formed simply by Yangtze–Cathaysia collision and supports a hypothesis in which the Yangtze block was a collage of microcontinents accreted during the Grenvillian period accompanying the assembly of the Rodinia supercontinent.

Plume-lithosphere interaction in the generation of the Tarim large igneous province, NW China: Geochronological and geochemical constraints

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined ⁴⁰Ar/³⁹Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO₂ = 44.0-47.9 wt.%, Na₂O + K₂O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO₂ (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)_N = 6.27-7.71; (Dy/Yb)_N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform ε_Nd(t) (−2.3 to −3.8) and low (²⁰⁶Pb/²⁰⁴Pb)_i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither ε_Nd(t) nor (²⁰⁶Pb/²⁰⁴Pb)_i correlates with SiO₂; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO₂ (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)_N = 10.1-14.0; (Dy/Yb)_N = 1.79-1.99]. They have variable isotope compositions [ε_Nd(t) = −0.3-4.8, (²⁰⁶Pb/²⁰⁴Pb)_i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low ε_Nd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.

Petrogenesis of the Bao'anzhai granite and associated Mo mineralization, western Dabie orogen, east-central China: Constraints from zircon U–Pb and molybdenite Re–Os dating, whole-rock geochemistry, and Sr–Nd–Pb–Hf isotopes

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie–Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon 238U–206Pb age of 123.2 ± 1.1 Ma and a molybdenite Re–Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial 87Sr/86Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [(206Pb/204Pb)i, 16.423–16.699; (207Pb/204Pb)i, 15.285–15.345; (208Pb/204Pb)i, 37.335–37.633], and is characterized by low ϵNd(t) and ϵHf(t) values (−14.92 to −14.22 and −21.67 to −19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated I-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (206Pb/204Pb, 16.592–17.674; 207Pb/204Pb, 15.300–15.476; 208Pb/204Pb, 37.419–37.911) and low Re content in molybdenite (5.693–10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.

In-situ Pb isotope analysis of Fe–Ni–Cu sulphides by laser ablation multi-collector ICPMS: New insights into ore formation in the Sudbury impact melt sheet

Laser-ablation (LA) multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) is ideally suited to in situ determination of isotope ratios in sulphide minerals. Using samples of magmatic sulphide ore from the Sudbury impact structure, we test LA-MC-ICPMS analytical protocols that aim to meet a range of analytical challenges in the analysis of Pb isotopes. These include: potential matrix sensitive isotopic fractionation; interferences on Pb isotopes; low melting points of many sulphide minerals; the availability of standards.Magmatic sulphides of wide ranging mineralogy (pyrrhotite, pentlandite, chalcopyrite, pyrite and sphalerite) were analysed for Pb isotopic composition, using the silicate glass NIST SRM 610 as an external standard to correct for instrumental mass-fractionation. Despite matrix sensitive melting and re-deposition around ablation pits, several lines of evidence indicate that all analyses are accurate, within typical analytical uncertainties of 0.003–2% (2σ), and that the defined approach is insensitive to compositional diversity in sample matrix: (a) laser ablation and dissolution based measurements of sulphide powders are in agreement; (b) analyses from each sample define isochron ages within uncertainty of the known crystallization age (1850Ma); (c) the results of sulphide measurements by laser ablation are consistent with age-corrected feldspar analyses from the same samples.The results have important implications for ore formation in Sudbury. The Pb isotope data regressions are consistent with age corrected feldspar analyses from each respective sample, which together with time integrated Th/U ratios that match whole rock values (3.1, 4.0 and 6.1 for the Worthington, Copper Cliff and Parkin Offset Dykes, respectively) indicate chemical equilibrium between the silicate and sulphide systems during ore formation. The sulphides within each respective sample have indistinguishable model initial Pb isotope ratios (207Pb/204Pbm), irrespective of mineralogy or texture, indicating a common origin for ores within each of three different Offset Dykes. Furthermore, variations between Offset Dykes (e.g., 207Pb/204Pbm=15.514±0.012, 15.399±0.009 and 15.275±0.003) show that the ores have differing crustal sources on previously unrecognized scales. Mass balance considerations, particularly for MgO, Ni and Cu, indicate that the spatial distribution of mafic target rocks played a significant role in controlling the mineralization potential in different parts of the melt sheet.

Northwestern Junggar Basin, Xiemisitai Mountains, China: A geochemical and geochronological approach

We report SIMS U–Pb zircon, elemental and Nd–Sr–Pb isotopic data for the volcanic rocks and their subvolcanic units from the Xiemisitai Mountains in the West Junggar Region (Xinjiang, China), aiming to determine their ages, source regions and tectonic setting. Zircon U–Pb dating results of the volcanic rocks (422.5±1.9Ma–411.2±2.9Ma) indicate that the volcanic strata occurred in the Xiemisitai Mountains at Late Silurian–Early Devonian time rather than Middle Devonian as suggested previously. They are probably the eastern extension of the early Paleozoic (Cambrian–Silurian) Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.The Xiemisitai volcanic rocks include andesite, rhyolite, and their pyroclastic equivalents. The subvolcanic units are felsite and granite porphyry. Andesite is moderately LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment. Rhyolite, felsite and granite porphyry are enriched in LREE and Th and depleted Nb. The initial Sr isotope values of the Xiemisitai volcanic rocks and their subvolcanic units range from 0.702294 to 0.704456, εNd(420Ma) from +0.19 to +1.88 with Nd model ages ranging from 600 to 803Ma. Initial Pb isotope ratios (206Pb/204Pb) are 17.77–17.96, (207Pb/204Pb) are 15.49–15.54 and (208Pb/204Pb) are 37.56–37.80. These features suggest that the Xiemisitai volcanic rocks and their subvolcanic units derived from a mantle wedge significant mixed by subducted material (EMI) in a subduction zone. They also indicate significant involvement of Neoproterozoic primitive crust in the formation of the Xiemisitai magmatic arc. These rocks are associated with a Late Silurian–Early Devonian southward subduction of the oceanic lithosphere in the northern West Junggar Region. This gives rise to an EW-trending Boshchekul–Chingiz volcanic arc and associated metallogenic belt in the northern West Junggar Region and adjacent Kazakhstan.

Late Triassic subduction-related ultramafic–mafic magmatism in the Amasya region (eastern Pontides, N. Turkey): Implications for the ophiolite conundrum in Eastern Mediterranean

The eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the geology, petrology, geochemistry, Nd–Sr–Pb isotopes and U–Pb zircon ages from a ultramafic–mafic complex in Aksalur which forms part of a series of Alaskan-type ultramafic–mafic bodies in the Amasya region located at the south-western corner of the eastern Pontides orogenic belt. These ultramafic–mafic intrusions occur as minor (30–300 m in diameter), scattered, roughly circular bodies within the low-grade metamorphic rocks of the Tokat metamorphic massif. Petrographic studies characterise the rock types as gabbro and wehrlite, with the latter showing typical cumulate texture and mainly comprising serpentinized olivine, clinopyroxene, hornblende, mica and opaque minerals. The gabbroic rocks are characterized by relatively high initial ratios of 143Nd/ 144Nd (0.512618–0.512633) and low initial ratios of 87Sr/ 86Sr (0.70420–0.70435). Their initial Pb isotope ratios range from 17.306 to 17.771. The gabbros and cumulate wehrlites display geochemical distribution patterns analogous to the Alaskan-type intrusions that were derived from a subduction-related, high-alumina, hydrous basaltic parental magma in a convergent plate setting. LA-ICPMS analyses of zircons separated from the gabbros show high Th/U ratios (1.35–5.26) typical of magmatic origin. The U–Pb isotopic data define a weighted mean 206Pb/ 238U age of 203.3 ± 4.9 Ma. Detailed evaluation of the magma tectonics suggests that the Alaskan-type Aksalur ultramafic–mafic intrusions were accumulated in magma chamber(s) located at relatively shallow crustal levels. Two distinct magmatic pulses are identified: the first phase produced the gabbroic intrusions and the second phase was characterized by numerous small-scale werhlitic cumulate intrusions. In contrast to the previous models which assigned a mid-oceanic ridge-derived ophiolite affinity for ultramafic–mafic complexes in the eastern Pontides orogenic belt, our study reveals that these intrusions were derived from subduction-related high-alumina hydrous basaltic magmas transferred into the overlying relatively thin continental crust along deep-seated fractures.

SOURCES OF THE MEALY MOUNTAINS AND ATIKONAK RIVER ANORTHOSITE-GRANITOID COMPLEXES, GRENVILLE PROVINCE, CANADA

We report geochemical and isotopic data for anorthosite – mangerite – granite samples of the ~1.65 Ga Mealy Mountains Intrusive Suite (MMIS) and the ~1.13 Ga Atikonak River Complex (ARC), located in the northeastern Grenville Province of Canada. Initial Pb isotopic ratios for feldspar samples from the ARC indicate assimilation of an old residual lower crustal component by all rock types. Monzodiorite and some granites with initial e Nd in the range − 2.0 to − 3.8 are interpreted as products of fractional crystallization and mixing in a lower crustal sill of mixed melts derived from depleted mantle and early Paleoproterozoic crust (tDM = 2.4 Ga). For the anorthositic rocks, with systematically higher initial e Nd of +2.0 to − 1.3 than in the granitoids, we suggest a parental melt comprising assimilated plagioclase–pyroxene cumulates and mantle-derived melt. The inferred plagioclase cumulates are hot remnants of the collapsed magma-chambers after granite extraction. If the ferrodiorite sample of the ARC represents a residual melt after anorthosite separation, additional contamination of the magma by older material is required. Anorthositic and granitic rocks of the MMIS show mostly depleted-mantle e Nd values up +3.6 that overlap the composition of the juvenile Labradorian basement. Enriched 87Sr/86Sr values in the samples were probably inherited from mantle sources modified during Labradorian plate subduction. Trace-element data support incompatible-element-enriched garnet-bearing mantle sources for the inferred parental basaltic melt. Crustal contamination of the mantle-derived melts is well documented by distinct Pb isotopic compositions, similar to those for the Labradorian basement. High Th and U concentrations relative to La and HREE depletion in the granite (s.s.) samples support melting of fertile garnet-bearing lower crust. Low abundances of Th and U in the monzodiorite and monzonite samples can be reconciled with a component derived from residual lower crust.

Sources and geodynamics of the Late Cenozoic volcanism of Central Mongolia: Evidence from isotope-geochemical studies

In the Late Cenozoic, the volcanism of the South Khangai Volcanic Region (SKhVR) spanned the Khangai Range and its framing. Geochronological, petrochemical, geochemical, and isotope studies were performed for volcanic rocks of this region, which are represented by high-K basic and intermediate rocks of OIB affinity. Initial Sr, Nd, and Pb isotope ratios in the volcanic rocks of the SKhVR are close to those of the volcanic rocks of Pitcairn Island and form trends between PREMA, EMI, and EMII sources.

Isotopic heterogeneity in the Sudbury impact melt sheet

A unique terrestrial large impact melt sheet is preserved in the 1850 Ma Sudbury Structure, Ontario. We have undertaken a Pb isotope investigation of the southern limb of the melt sheet, termed the South Range Main Mass. The model initial Pb isotope ratios ( 207Pb/ 204Pb m) vary stratigraphically through the predominantly quartz monzogabbroic Lower Unit, varying from 15.40 to 15.45 at the base to ca. 15.35 at the top of the sequence. Lateral variations of similar range occur in basal Lower Unit samples over scales of less than 5 km. The range of these variations is similar to those of locally exposed upper crustal target rocks, and it is evident that the melt sheet has efficiently preserved inherited variability. During the violent phases of crater formation superheated impact melts are expected to be well-mixed mechanically, therefore significant post-impact melting of target rocks, fallback material and entrained clasts is required to explain such heterogeneity. The Sudbury Structure hosts world class Ni-Cu-PGE sulphide ore deposits. Systematic variation in 207Pb/ 204Pb m occurs throughout sulphide ores within the Creighton Embayment, from massive (15.42–15.45) to interstitial (ca. 15.40–15.41) and disseminated (ca. 15.39) sulphide. Linking the Pb isotope composition of these ores to the immediately overlying Lower Unit stratigraphy, a protracted sulphide segregation history is apparent. Massive sulphides segregated early, prior to or during initial silicate crystallisation, although the total time involved in sulphide accumulation spanned much of the crystallisation of the Lower Unit. It is also shown that lateral variations in Ni depletion throughout the Main Mass correlate with Pb isotopes. Those segments with the strongest chalcophile element depletion signatures, reflecting the accumulation of significant basal sulphides, have high initial Pb isotope values, consistent with early sulphide segregation. The characterisation of Pb isotopic heterogeneity has therefore provided insights into the evolution and scales of mixing of the melt sheet, with the identified chemical variability between melt cells having a significant influence on ore forming processes.

First alkaline magmatism during Iberia–Newfoundland rifting

AbstractThe age and origin of alkaline rocks emplaced into the sediments of the rift‐related continental Lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between Iberia and Newfoundland. The U–Pb titanite ages are 146.5 ± 1.6 (2σSTERR), 145.3 ± 1.4 and 142.3 ± 1.0 Ma, and initial Pb isotopic ratios of feldspars lie at 18.418–18.978 for 206Pb/204Pb, at 15.594–15.925 for 207Pb/204Pb and at 37.105–39.216 for 208Pb/204Pb. Initial 87Sr/86Sr ratios measured in the same feldspar fractions lie at 0.705409–0.706462. This episode of magmatic activity lasting for at least 4.2 ± 2.6 Myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface. Such zones are preferentially re‐activated Palaeozoic faults of the Iberian plate. The isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental Iberian mantle.

Common Pb of UHP metamorphic rocks from the CCSD project (100–5000 m) suggesting decoupling between the slices within subducting continental crust and multiple thin slab exhumation

In order to understand the vertical structure of the Dabie–Sulu ultrahigh-pressure metamorphic (UHPM) belt, common Pb isotopic compositions of omphacites in eclogites and feldspars in gneisses from the Chinese Continental Scientific Drilling (CCSD) project (100–5000 m) have been investigated in this study. Samples from 0 to 800 m (unit 1) in the drilling core have moderately high radiogenic Pb isotopes with small variations of 206Pb/ 204Pb (16.82–17.38), 207Pb/ 204Pb (15.37–15.49), and 208Pb/ 204Pb (37.21–37.72), indicating either high µ ( 238U/ 204Pb) or high initial Pb isotope ratios of their protoliths. In contrast, the samples from 1600 to 2040 m (unit 3) and most of samples from 3200 to 5000 m (unit 5) have moderately or very unradiogenic Pb (unit 3: 206Pb/ 204Pb from 16.05 to 16.46, 207Pb/ 204Pb from 15.22 to 15.29, and 208Pb/ 204Pb from 36.68 to 37.48; unit 5: 206Pb/ 204Pb from 15.52 to 15.69, 207Pb/ 204Pb from 15.15 to 15.27, and 208Pb/ 204Pb from 36.48 to 37.20), indicating either low µ or low initial Pb isotope ratios of their protoliths. Pb isotopes of samples from 800 to 1600 m (unit 2) and from 2040 to 3200 m (unit 4) in the drilling core with abundant ductile shear zones are intermediate between those of units 1 and 3 or 5 and display larger variations. Pb isotopes combined with the published oxygen isotope data of the CCSD samples reveal the original positions of the five units before the Triassic continental subduction. Units 1, 3, and 5 as three UHPM rock slabs could be derived from the subducted upper continental crust, upper–middle continental crust and lower–middle continental crust, respectively. The ductile shearing zones in units 2 and 4 could be the interfaces where the detachment and decoupling took place between the upper, upper–middle and lower–middle continental crusts. The detachment between the upper slab and subducting continental lithosphere probably occurred during continental subduction, and the upper slab (unit 1) was uplifted to a shallow depth along the detachment surface by thrusting. Units 3 and 5 may be detached later from the subducted middle and lower crust and uplifted to a shallow level underneath unit 1. The low δ 18O values (− 4.0 to − 7.4‰) [Xiao, Y.-L., Zhang, Z.-M., Hoefs, J., Kerkhof, A., 2006. Ultrahigh-pressure Metamorphic Rocks from the Chinese Continental Drilling Project-II Oxygen Isotope and Fluid Inclusion Distributions through Vertical Sections. Contribution Mineral Petrology 152, 443–458.; Zhang, Z.-M., Xiao, Y.-L., Zhao, X.-D., Shi, C., 2006. Fluid-rock interaction during the continental deep subduction: oxygen isotopic profile of the main hole of the CCSD project. Acta Petrologica Sinica 22 (7), 1941–1951.] in units 2 and 4 suggest that the detachment interfaces could be developed along an ancient fault zones which were the channels of meteoric water activity during the Neoproterozoic.

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 33°S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U–Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28·8–5·2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the Nazca–Farallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)N between 3·6 and 7·0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18·358–19·023 for 206Pb/ 204Pb, 15·567–15·700 for 207Pb/ 204Pb and 38·249–39·084 for 208Pb/ 204Pb. Initial 87Sr/ 86Sr are mostly in the range of 0·70369–0·70505, with two more radiogenic values at 0·7066. Initial Hf isotopic compositions of zircons yield exclusively positive εHfi ranging between + 6·9 and + 9·6. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile element–high field strengh element depletion and high 238U/ 204Pb. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantle–continental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of ≈0·039 and μ ≈ 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (>90%) basaltic–gabbroic to andesitic–dioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet- and spinel-peridotite stability fields. The genesis of more differentiated magmas reaching rhyolitic–granitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material.