Rb-Sr Isochron Age Research Articles

Geochronometry of the Middle Paleozoic volcanics of the Omolon Massif in Northeast Asia: A comparison of the K-Ar, Rb-Sr, and U-Pb data and their geological interpretation

The results of the K-Ar, Rb-Sr, and U-Pb (SHRIMP zircon method) dating of the Middle Paleozoic volcanogenic rocks of the Omolon Massif are summarized. It was concluded that they are principally consistent with each other, as well as with the geological data. The formation of the Kedon Group, which makes up the main volume of the Middle Paleozoic volcanics, began at the Early-Middle Devonian boundary about 400 Ma ago (U-Pb dates of 400.5 ± 4.4 and 387 ± 6.4; Rb-Sr isochron age of 402 ± 6 Ma). The isotopic age of the upper boundary of the Kedon Group remains unclear due to disagreements concerning its stratigraphic assignment. The histogram based on the 111 K-Ar dates of the volcanic rocks from the Kedon Group gives a polymodal distribution, which indicates that the K-Ar isotopic system was disturbed by thermal events, which occurred 310–290 (terminal Carboniferous—beginning of the Permian) and 240–220 (Middle-beginning of the Late Triassic) Ma ago. Both thermal events were associated with mantle (ultrabasic-basic) magmatism, which spanned a significantly wider territory than the distribution area of the Kedon Group

The age of carbonatites and mafic rocks (SHRIMP II and Rb-Sr dating) from the Oshurkovo apatite-bearing pluton ( western Transbaikalia)

The paper presents geochronological data on silicate and endogenic carbonate rocks from the Oshurkovo apatite-bearing pluton, namely, the U-Pb and Rb-Sr isochron ages of gabbros and carbonatites. The U-Pb age of the carbonatites from this pluton (126.55 ± 0.85 Ma) is close to that of the other carbonatite occurrences in southwestern Transbaikalia. Also, its similarity to the age of the gabbro (125.4 ± 2 Ma), which makes up the bulk of the pluton, suggests that these rocks are comagmatic.

Crustal accretion and metamorphism of Mesoarchean granulites in Palghat-Cauvery Shear Zone, Southern India

This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛTCHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛTCHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.

Sm–Nd and Rb–Sr studies of lherzolitic shergottite Yamato 984028

The distribution pattern of the trace elements Rb, Sr, Nd and Sm for Yamato 984028 (Y984028) is consistent with its classification as a lherzolitic shergottite. The Sm–Nd mineral isochron of this lherzolitic shergottite defines its age to be 170 ± 10 Ma for an initial ɛ Nd = +11.6 ± 0.2. The corresponding Rb–Sr mineral isochron yields an identical age of 170 ± 9 Ma and an initial 87Sr/ 86Sr = 0.710389 ± 0.000029. The concordant Sm–Nd and Rb–Sr isochron ages suggest that Y984028 crystallized 170 ± 7 Ma ago contemporaneously with five other lherzolitic shergottites and ten enriched basaltic and olivine-phyric shergottites. The age, Sr- and Nd- isotopic signatures further suggest that Y984028 and Y-793605, and also probably Y000097 could come from a single magmatic body. Using a two-stage evolution model, the time-averaged 87Rb/ 86Sr-ratio for the mantle source of the parent magma of Y984028 is ∼0.182, within the range of 0.178–0.182 that has been reported for other lherzolitic shergottites. The corresponding time-averaged 147Sm/ 144Nd-ratio for the source mantle of its parent magma is super-chondritic at ∼0.217, implying its source was a depleted mafic part of the Martian mantle similar to that of diabasic shergottite Northwest Africa (NWA) 1460. Rb, Sr, Sm and Nd distributions in Y984028 are likely produced by pyroxene and olivine accumulation, probably from a NWA 1460-like parental melt, in an intrusive magma body.

Mesozoic collision-related magmatism and crust–mantle interaction along the Anhui segment of the Yangtze River Valley, east-central China

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J3) to Early Cretaceous (K1). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ∼145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO2, 13.12–17.73 wt.% Al2O3, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeOT, and 4.71–7.87 wt.% total alkalis (Na2O + K2O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ∼130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO2, 17.67–20.65 wt.% Al2O3, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeOT, and 9.24–13.92 wt.% total alkalis (Na2O + K2O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35. The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.

The Shoshonite-Absarokite-Picrite Karashoho Pipe, Uzbekistan: An Unusual Diamond Deposit in an Atypical Tectonic Environment

The diamondiferous Karashoho pipe was discovered in the Bukantau Mountains, Uzbekistan, in the 1990s and evaluated from 2002 to 2007. It is an elongate, twinned, subvertical-dipping body with an approximate surface expression of 530 × 65–155 m (~45,600 m2). Its northern part is composed of eruptive breccias and shoshonite, whereas the southern part is composed of micaceous shoshonite-absarokite-picritic rocks and shonkinite. The pipe is located within the southern Tian-Shan Hercynian fold system enclosed between the Karakum-Tajik Massif in the south and the middle Tian-Shan system to the north. The Rb-Sr isochron age of the pipe is 353 ± 13 Ma. By petrographic, mineralogical, and chemical compositions the Karashoho rocks belong to a potassic shoshonite-absarokite-picrite rock series, similar to Permian shoshonitic series rocks from the Tian-Shan volcanic belt in north Xinjang, China. However, they differ significantly from common shoshonites and absarokites because of the high magnesian index of rock-forming minerals in the pipe and the presence of some high-pressure mineral phases, such as chromian spinel (including high-chromian varieties of the diamond association) and manganoilmenite. This indicates that the rocks originated under high-pressure conditions. Bulk sampling of the Karashoho rocks yielded a diamond grade in the range of 0.1 to 5 cpht (carats per hundred tons), with the weighted-mean for different rock varieties ranging from 0.87 to 2.55 cpht. The highest grades are in eruptive breccias of the first generation and shoshonitic-absarokitic rocks. The Karashoho pipe is the first example of diamond potential in a shoshonite-absarokite-picrite rock series; and the first primary diamond deposit located within a Phanerozoic fold system.

Geochemistry of peridotite xenoliths in Early Cretaceous high-Mg# diorites from the Central Orogenic Block of the North China Craton: The nature of Mesozoic lithospheric mantle and constraints on lithospheric thinning

The North China Craton is the best example of an Archean craton that has lost its thick lithospheric keel. Although removal of the Archean keel is generally considered to have occurred in the Meosozoic–Cenozoic in the Eastern Block and Paleoproterozoic in the Central Orogenic Block, the exact timing and areal extent of the removal is debated, partly due to lack of knowledge about the nature of the Meosozoic lithospheric mantle. Here we report mineralogical and geochemical data on rare peridotite xenoliths from the Early Cretaceous high-Mg# diorites from Fushan in the Trans-North China Orogen (TNCO) of the North China Craton (NCC). These xenoliths provide insights into the nature of the lithospheric mantle underlying the central NCC during the Mesozoic. The peridotite xenoliths are dominated by spinel harzburgite and clinopyroxene (Cpx)-poor spinel lherzolite, with minor chromite-bearing dunite. The harzburgite and Cpx-poor lherzolite have average forsterite contents of 92.3 and are depleted in CaO (0.59–1.06 wt.%) and Al 2O 3 (0.15–1.47 wt.%). These features are similar to those of Archean cratonic lithospheric mantle, suggesting the presence of Archean cratonic mantle beneath the Central Orogenic Block of the NCC in the Early Cretaceous. Dunites make up ∼ 5% of the xenolith population and are characterized by relatively low Mg# (90.1) and the presence of disseminated chromite (Cr# = 73–85). The olivines in the dunites have high Ca (320–770 ppm) and Ti (18–29 ppm) concentrations as well as low Ni abundances (2000–2690 ppm) compared to those from harzburgite and Cpx-poor lherzolite (Ca = 40–80 ppm; Ti = 0.23–8.1 ppm; Ni = 2970–3440 ppm), suggesting that the dunites were produced through an interaction between the Archean lithosphere and siliceous melts. The presence of secondary phlogopite and amphibole in the harzburgite and Cpx-poor lherzolites as well as veined orthopyroxene in the foliated dunite is coupled with light rare earth element enrichments. Together with low Ca/Al (5–18) and La N/Yb N ratios (0.41–2.99) and high Ti/Eu ratios (526–1474) of clinopyroxenes, these metasomatic minerals indicate that the Archean lithospheric mantle had been overprinted by a volatile-bearing silicate melt. The Rb–Sr isochron age (111 ± 23 Ma) of the harzburgite and Cpx-poor lherzolite xenoliths shows that the metasomatic overprinting occurred in the Early Cretaceous, in agreement near the time of the emplacement of the host magma and coeval voluminous igneous activities in the NCC. This section of Mesozoic lithospheric mantle is markedly different from the contemporaneous lithospheric mantle in the eastern NCC, implying that the destruction of the NCC lithosphere was confined to the eastern NCC and that the Archean mantle was preserved in the central NCC.

A new, high precision U–Pb date from the oldest known rocks in southern Britain

AbstractA new high precision U–Pb zircon age of 710.8 ± 1.5 Ma for granophyric granitic rock from the Stanner Hanter Complex of the Welsh Borderland lies just within error of an older Rb–Sr isochron age. ϵNd values of −0.3 and −1.2 combined withTDMof 1394 Ma and 1468 Ma indicate that the magma incorporated an older crustal source component. The Nd data highlight differences with western Avalonia, the widely considered Late Neoproterozoic north American counterpart to southern Britain, and point toward a closer similarity with other Peri-Gondwanan terranes that incorporate older, cratonic source material.

Geochronology, petrogenesis and tectonic implications of granites from the Fuxin area, Western Liaoning, NE China

New geochemical and Sr–Nd isotopic data from the Ping'andi granitic pluton in the Fuxin area, which straddles the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) border, are presented here to discuss the geochronology, petrogenesis and tectonic implications. Petrographically, the Ping'andi granites contain monzogranites to minor granodiorites. They are high-K calc-alkaline series, silica-rich (SiO 2 = 66–78 wt.%), peraluminous (A/CNK = 0.98–1.10), with enrichment in LREE (La n/Yb n = 6.51–8.53), moderately negative Eu anomalies (Eu/Eu* = 0.33–0.90), with enrichment in LILE (Rb and K) and with depletion in HFSE (Nb, Ti and P). They also have slightly negative and positive ε Nd(t) (− 1.1–0.4), low initial I Sr (0.70587–0.70620) and young depleted-mantle model ages ( T DM2 = 911–1027 Ma). The emplacement of the Ping'andi granites took place at 150 Ma as revealed by whole rock Rb–Sr isochron age. The petrography, geochemistry and isotopic compositions indicate that they are I-type and were the result of partial melting of mixed sources composed of a juvenile component and a pre-existing crust, coupled with plagioclase-dominated fractionation, and that there was a significant crustal growth in the Phanerozoic in the northern margin of the NCC. The formation of the Ping'andi granites was related to the tectonic transformation from the convergent tectonic regime of Paleo-Asian Ocean to that of the Paleo-Pacific Ocean, which also induced the huge NNE-trending Jurassic granite belt and Mo–Cu metallogenic belt in eastern China.

Geology, age, and fluid inclusions of the Tanjianshan gold deposit, western China: Two orogenies and two gold mineralizing events

The Tanjianshan gold deposit, located at the tectonic boundary between the Oulongbuluk block (OB) and North Qaidam orogenic belt (NQO) on the northern margin of the Qaidam basin (NMQB), western China, underwent Early Palaeozoic and Late Palaeozoic to Early Mesozoic multiple accretions and collisional orogenies. It is a typical example of a deposit resulting from two gold mineralizing events related to two collisional orogenies. Two phases of ore-controlling NW-trending shear zones have been identified in the deposit: dextral brittle–ductile shear deformation during the late Early Palaeozoic collisional orogeny and sinistral strike-slip shear deformation during the Late Palaeozoic–Early Mesozoic orogeny. N–S-trending folds developed in the NW-trending shear zone and resulting from sinistral strike-slip motion of the shear zone are nose-shaped in plan view and control most gold orebodies in the deposit. The major gold orebodies trend NNE and are concentrated in the hinge zones of these N–S-trending folds, occurring along the fault zones parallel to both limbs of these folds. These gold orebodies, composed of altered mylonite schist type ore and altered dyke type gold ore, occur as saddle-shaped bodies, beds and lenses mainly within altered carbonaceous quartz–sericite mylonite schist of a highly strained zone and locally in altered igneous dykes of the highly strained zone. Alteration related to gold mineralization includes silicification, pyritization and sericitization. A progressive increase of SiO 2, S, Au, Ag, As and Sb from unaltered or weakly altered rocks through altered rocks to gold ore is consistent with progressively enhanced silicification and pyritization, suggesting the flow of mineralizing fluids along the NW-trending shear zone and N–S-trending folds. Native gold and electrum are present as very fine grains, mainly enclosed in disseminated pyrite or occurring in fractures within the pyrite. Three generations of pyrite and associated hydrothermal quartz are observed. The fabrics of gold ore, generations of auriferous pyrite, field relationship between multiple ore-controlling structures and gold mineralization, and the bimodal distribution of ore elements in the gold district suggest the occurrence of two main hydrothermal and mineralizing events. The first event, marked by a mineral assemblage of first-generation quartz (qz 1) and first-generation pyrite (py 1), was restricted to the carbonaceous mylonite schist within the NW-trending shear zone. Sericite from the ore-controlling NW-trending shear zone gave a 40Ar– 39Ar age of 409.4 Ma, which is consistent with the age of the regional late Early Palaeozoic collisional orogeny in the region. The second event, marked by a mineral assemblage of second-generation quartz (qz 2) and second-generation pyrite (py 2), occurred within the N–S-trending folds and was related to late Palaeozoic orogeny in the region. We have obtained a K–Ar age of 268.9 ± 4.3 Ma and an Rb–Sr isochron age of 288 ± 9.7 Ma for hydrothermal minerals from gold ores. The latter age is closer to the age of this event of gold mineralization. Mineralization of this event was superimposed onto gold mineralization of the early phase and formed most of the gold orebodies in the deposit. The distribution and microthermometric data of fluid inclusions indicate that late Early Palaeozoic ore fluids belong to the H 2O–CO 2–CH 4–NaCl fluid system formed at 186 to 250 °C, whereas Late Palaeozoic fluids comprise two kinds of immiscible fluid: a H 2O–NaCl fluid with salinity of 1.6 to 10.8 wt.% NaCl equiv., which probably formed at temperatures of 381 to 449 °C and, and H 2O–CO 2–NaCl fluids with salinities of 1.8 to 7.9 wt.% NaCl equiv., which are genetically the product of evolution of former immiscible magmatic fluids, and which formed at temperatures of 274 to 410 °C.

Rb-Sr Geochronology of Chalcopyrite from the Chehugou Porphyry Mo-Cu Deposit (Northeast China) and Geochemical Constraints on the Origin of Hosting Granites

The Chehugou porphyry Mo-Cu deposit is located ca. 300 km north of Beijing in the boundary zone between the Archean North China craton and the Neoproterozoic-Mesozoic Central Asian orogenic belt. Using hydrothermal chalcopyrite from this deposit, we are able to show for the first time that the Rb-Sr isotope system in massive, nonfractured chalcopyrite grains can provide reliable age constraints for Mo-Cu deposits. We analyzed two chalcopyrite samples from slightly different petrogenetic “environments” showing contrasting texture and deformation history. Five 2- to 5-mg fragments of a single nonfractured chalcopyrite grain containing spatially isolated inclusions of K-mica, K-feldspar, and quartz yielded an Rb-Sr isochron age of 256 ± 7 Ma (2σ including λ uncertainty). For this sample, the dense chalcopyrite “matrix” apparently inhibited diffusion of Rb and Sr after chalcopyrite crystallization, and the age should therefore reflect the time of crystallization of chalcopyrite from the Mo-Cu mineralizing fluids. The evidence for Mo-Cu mineralization during the late Permian requires a revision of the previous notion that the Chehugou deposit was formed during the Yanshanian orogeny lasting from 200 to 60 Ma. Fragments of a second chalcopyrite sample that, in contrast to the first, had experienced deformation as indicated by intragranular fractures and alignment of K-mica and K-feldspar inclusions along these fractures. This sample yielded a clearly younger Rb-Sr isochron age of 207 ± 15 Ma (2σ). This age obviously reflects an isotopically open system due to fluid movement and/or prolonged diffusion of Rb and Sr along the fractures. Single biotite crystals from an associated monzogranite yielded an Rb-Sr isochron age of 250 Ma, which is interpreted as a lower age limit for the Mo-Cu mineralization, and the time when the granite cooled below 400° to 300°C. Considering the field evidence for a broadly contemporaneous origin of the Mo-Cu mineralization and the hosting granitoids, we suggest that the granitoids are not much older than ca. 260 Ma. The Mo-Cu–hosting granitoids range in composition from monzogranite to highly evolved syenogranite with high La/Yb and Sr/Y ratios. Initial eNd values of ca. –21 to –23 (Nd crustal residence times of the sources 1.9 to 2.7 Ga) indicate melting of the predominantly Archean North China craton, and a subordinate juvenile mantlederived component. Much older U-depleted lower crustal sources for the granitoids are supported by high initial 208Pb/204Pb ratios. Lithospheric delamination and underplating of hot mantle-derived material may have facilitated crustal melting in a late orogenic to postcollision.

39Ar– 40Ar ages of martian nakhlites

We report 39Ar– 40Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39Ar recoil and variable amounts of trapped 40Ar in the samples. Thus, we examine possible Ar–Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar–Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2 σ errors). For NWA 998 and MIL 03346 the Ar–Ar ages are within uncertainties of preliminary Rb–Sr isochron ages reported in the literature. These Ar–Ar ages for Y-000593 and Nakhla are several Ma older than Sm–Nd ages reported in the literature. We conclude that the major factor in producing Ar–Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40Ar on weathered grain surfaces that was degassed along with the first several percent of 39Ar. A total K– 40Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2 σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar–Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.

Geochronological framework and Pb, Sr isotope geochemistry of the Qingchengzi Pb–Zn–Ag–Au orefield, Northeastern China

The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi ( 206Pb/ 204Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores ( 206Pb/ 204Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios ( 206Pb/ 204Pb = 17.12 to 17.41, 207Pb/ 204Pb = 15.47 to 15.54, 208Pb/ 204Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios ( 206Pb/ 204Pb = 18.20 to 24.28 and 18.32 to 20.06, 207Pb/ 204Pb = 15.69 to 16.44 and 15.66 to 15.98, 208Pb/ 204Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively). Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi. Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.

Petrogenesis of the Nesryin gabbroic intrusion in SW Sinai, Egypt: new contributions from mineralogy, geochemistry, Nd and Sr isotopes

The Wadi Nesryin gabbroic intrusion is part of the Neoproterozoic Pan-African basement cropping out in southern Western Sinai of Egypt. The intrusion comprises hornblende gabbro, pyroxene–hornblende gabbro, diorite and appinitic varieties. It exhibits chilled margins against the older rocks represented by fine-grained gabbro and dolerite and belongs to what is known throughout Egypt as the “younger gabbro suite”. Mineralogy, mineral chemistry and whole rock geochemistry indicate that these rocks were derived from tholeiitic magmas with minor calc-alkaline affinity. They have chemical signatures of subduction related arc rocks formed at an active convergent plate margin. They were formed by 15–30% of partial melting of a garnet lherzolite and to a minor extent of spinel-garnet lherzolite sources, modified by fluids related to a subducting slab. Pressure estimates using the amphibole geobarometer indicate that the gabbroic rocks crystallized at pressures between 2.8 and 5.6 kbar (average = 4.3 kbar). Diorites record lower formation pressures between 1.8 and 3.7 kbar (average = 3.0 kbar). The temperature estimates calculated by several geothermometers yielded crystallization temperatures ranging from 674°C to 961°C, with an average of about 817°C. The whole rock Rb–Sr isochron age of the Nesryin gabbroic intrusion is 617 ± 19 Ma with initial 87Sr/86Sr = 0.70322 ± 0.00004. This age indicates that the mafic–ultramafic plutons in the Pan-African belt in southern Sinai belong to the Egyptian younger gabbros and not to the older metagabbro–diorite complexes or ophiolitic suites. The rocks have low 87Sr/86Sr initial ratios ranging from 0.703141 to 0.703338 and negative ɛ Sr ranging from −6.34 to −9.14. The initial 143Nd/144Nd ratios range from 0.511944 to 0.512145 with positive and high ɛ Nd values (1.93 to 5.86) reflecting a mantle contribution in their petrogenesis.

New U–Pb and Rb–Sr constraints on pre-Acadian tectonism in North Wales

A new U–Pb date of 615.2 ± 1.3 (2σ) Ma for the Twt Hill Granite, North Wales, contrasts with an Rb–Sr isochron age of 491 ± 12 (2σ) Ma from the same body. The latter age is thought to result from isotope resetting during regional low-grade metamorphism or fault reactivation. The Rb–Sr age also coincides with the onset of latest Cambrian to Early Tremadoc regression and is taken to reflect tectonic uplift prior to the Arenig overstep at around 478 Ma. It is proposed that this in turn reflects plate-scale processes along the contemporary peri-Gondwanan continental margin.

A deformed alkaline igneous rock–carbonatite complex from the Western Sierras Pampeanas, Argentina: Evidence for late Neoproterozoic opening of the Clymene Ocean?

A deformed ca. 570 Ma syenite–carbonatite body is reported from a Grenville-age (1.0–1.2 Ga) terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite–calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb– 238U ages between 433 and 612 Ma, with a prominent peak at 560–580 Ma defined by homogeneous zircon areas. Textural interpretation of the zircon data, combined with the constraint of the Rb–Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U–Pb system took place at 525 ± 7 Ma (Pampean orogeny) and at ca. 430–440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly unradiogenic 87Sr/ 86Sr values in calcites (0.70275–0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield ɛNd 570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust.

Isotope dating of ultrapotassic magmatism in the central Chukchi region: Rb-Sr age and geochemistry of monzonites from the Linkor stock

Isotope dating of ultrapotassic magmatism in the central Chukchi region was carried out. Based on the Rb-Sr isochron age of monzonites from the Linkor stock (107±2.2. Ma) and using earlier obtained geochronological isotope data, we have estimated the time of one of the stages of ultrapotassic magmatism, 107–113 Ma. Igneous rocks of this age occur throughout the central Chikchi region, which suggests their genetic relationship with some great geodynamic event there, likely, the accretion of the Mainit island arc to the Chukchi microcontinent. The results obtained permit the reconstruction of the geologic history of the region and can be used to construct more detailed geodynamic models.

The Shanggong gold deposit, Eastern Qinling Orogen, China: Isotope geochemistry and implications for ore genesis

The Shanggong Au deposit in the Xiong’er Terrane, East Qinling, China, has resources of about 30 ton Au, making it one of the largest orogenic-mesothermal Au deposits hosted in volcanic rocks of the Mesoproterozoic Xiong’er Group. Three stages of hydrothermal activity are recognized (early, middle and late), of which two (early and middle) were ore producing and characterized by quartz–pyrite and polymetallic sulfides, respectively. The third and late stage is represented by a carbonate–quartz assemblage. Hydrogen, oxygen and carbon isotope systematics of the Shanggong deposit from a previous work suggest that the early stage fluids were derived from magmatic and/or metamorphic devolatilization of sedimentary rocks at depth. This is supported by new C, S and published Sr and Pb isotopic data, presented in this paper. These new data, δ 13C values ranging from 1.5 ‰ for early stage ankerite to −2.2 ‰ for late stage ankerite, negative δ 34S values for sulfides from the middle stage (–19.2 to –6.3 ‰), suggest a contribution from organic matter and that the ore fluid evolved from deeply sourced to shallowly sourced, with those of the middle stage representing a mixture of these two fluid systems. The comparison of the hydrogen–oxygen–carbon–sulfur–lead–strontium isotope systematics between the Shanggong deposit and the main lithologies in the Xiong’er Terrane, shows that neither these nor the underlying lower crust and mantle, or combinations thereof, could be considered as the source of ore fluids for the Shanggong Au deposit. A likely source was a carbonaceous carbonate, sandstone, shale, chert sequence in the underthrusted Guandaokou and Luanchuan Groups, exposed south of the Xiong’er Terrane. Ar–Ar and Rb–Sr isochron ages for mineral phases of the early, middle and late stages, together with geological field data, constrain the timing of the hydrothermal activity and Au metallogenesis at 242 ± 10, 167 ± 7 and 112 ± 7 Ma, respectively. This metallogenesis and associated granitic magmatism, can be related to the continental collision between the Yangtze and North China Cratons that resulted in the formation of the Qinling Orogen, led to the different hydrothermal systems that were responsible for the three stages that formed the Shanggong Au deposit, over a period of about 130 Myrs.

The basement of Calabria (southern Italy) within the context of the Southern European Variscides: LA-ICPMS and SIMS U–Pb zircon study

U–Pb spot zircon ages from metagabbros and metasediments of the Variscan lower crust in the Serre massif (southern Calabria) have been determined using LA-ICPMS and SIMS. The results, together with the literature data deriving from conventional and spot zircon ages and from Rb–Sr whole-rock isochrons, allow the geological history of the crustal block to be more tightly constrained from the Late Neoproterozoic to the Triassic, with particular emphasis on Variscan evolution. In a peri-Gondwana Pan-African/Cadomian basement, preserving memory of recycled older terrains with West African Craton affinities, mafic and felsic magmas emplaced at ca. 584 Ma (protoliths of metagabbros) and in the range 575–537 Ma (protoliths of felsic orthogneisses). Partially or completely recrystallized domains of zircons from metaigneous rocks yield ages ranging from 500 to 440 Ma. These ages (consistent with those of recrystallized cores of magmatic zircon from felsic orthogneisses and with Rb–Sr isochron data for the lower crust metasediments) suggest tectonothermal Ordovician activity, which is likely to be connected to the opening of Palaeozoic basin(s) separating the future Variscan blocks from Gondwana. Most zircon ages in the metasediments cluster in the 340–260 Ma range, which probably corresponds to two major events: (i) crustal thickening between 340 and 300 Ma (Barrowian metamorphic event) and (ii) lower-P/high-T metamorphism, Bt-dehydration melting, crustal extension between 300 and 260 Ma. Triassic ages ranging from 249 ± 4 Ma to 231 ± 5 Ma, together with whole-rock Rb–Sr isochron age at 234 Ma for the lowermost metapelites of the section, suggest tectonothermal activity, which prepared the opening of the Jurassic Neotethys. Similarities and differences relative to some circum-Mediterranean Variscan lower crust sections are also discussed, which contribute to the knowledge of south European Variscides evolution.

Isotopic dating of the Chengjiang Fauna-bearing horizon in Central Yunnan Province, China

Twenty black shale samples, which are free from the influence of weathering, were collected from the Chengjiang Fauna-bearing horizon, central Yunnan Province, yielding a Pb-Pb isochron age of 534±60 Ma. Although this age is younger than both the Rb-Sr isochron age and 40Ar-39Ar age, it should represent the lower isotopic age limit of the Chengjiang Fauna.