Rb-Sr Isotopic Data Research Articles

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe, Arkhangelsk province, Russia: Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite, from mantle-derived xenoliths (garnet peridotite, eclogite and clinopyroxene–phlogopite rocks) and for megacryst, macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite. Based on the analysed xenoliths, phlogopite is characterized by several generations. The first generation (Phl1) occurs as coarse, discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene–phlogopite xenoliths. The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate, serpentine, chlorite and spinel. In garnet peridotite xenoliths, phlogopite occurs as overgrowths surrounding garnet porphyroblasts, within which phlogopite is associated with Cr-spinel and minor carbonate. In eclogite xenoliths, phlogopite occasionally associates with carbonate bearing veinlet networks. Phlogopite, from the kimberlite, occurs as megacrysts, macrocrysts, microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts. Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains, which indicates that they are the disintegrated fragments of previously larger grains.Phl1, within the garnet peridotite and clinopyroxene–phlogopite xenoliths, is characterised by low Ti and Cr contents (TiO2 < 1 wt.%, Cr2O3 < 1 wt.% and Mg# = 100 × Mg/(Mg + Fe) > 92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences. They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene–phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas. One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H2O. Rb–Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma. Compared with peridotite xenoliths, eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr2O3 despite a wider range of TiO2 concentrations. The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite. Phl2 has high Ti and Cr concentrations (TiO2 > 2 wt.%, Cr2O3 > 1 wt.% and Mg# = 100 × Mg/(Mg + Fe) < 92) and compositionally overlaps with phlogopite from polymict breccia xenoliths that occur in global kimberlite formations. These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phl1 grains or crystallized directly from stalled batches of kimberlitic magmas. Megacrysts, most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths. Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts. Based on the results of this study, we propose a schematic model of SCLM metasomatism involving phlogopite crystallization, megacryst formation, and genesis of kimberlite magmas as recorded by the Grib pipe.

The Rb-Sr isotopic dating of sulfides and geological significance of the Lushi polymetallic ore-concentrated area in southern margin of the North China Craton

The Rb-Sr isotopic dating of sulfides and geological significance of the Lushi polymetallic ore-concentrated area in southern margin of the North China Craton

Geological, geochemical and Rb–Sr isotopic studies on tungsten mineralised Sewariya–Govindgarh granites of Delhi Fold Belt, Rajasthan, NW India

Neoproterozoic granites are widespread in the Delhi Fold Belt of the Aravalli craton, some of which are associated with tungsten mineralisation. In one such instance, the volcano-sedimentary sequence of Barotiya Group in the South Delhi Fold Belt is intruded by a pluton of biotite granite gneiss known as Sewariya Granite (SG) and later by stocks and dyke swarm of tourmaline leucogranite known as Govindgarh Granite (GG). GG magmatism was associated with wolframite mineralisation in hydrothermal quartz veins occurring along the sheared contact between SG pluton and Barotiya mica schist. SG pluton shows the evidence of ductile and brittle deformations, whereas GG is by and large undeformed. Apart from quartz and feldspars, SG contains biotite and muscovite, and GG contains muscovite, tourmaline and garnet. Although both SG and GG are peraluminous, SG has a wide range of \(\hbox {SiO}_{2}\) and narrow range of alkalis, and GG has a narrow range of \(\hbox {SiO}_{2}\) and a wide range of alkalis. REE (rare Earth elements) modelling shows that the parent magma of SG and GG was derived from partial melting at different crustal levels. Rb–Sr isotope data of GG yield a mineral isochron age of \(860\pm 7.4\,\hbox {Ma}\) which represent the time of igneous crystallisation and cooling of the granite to less than 400\({^{\circ }}\)C.

Dating agpaitic rocks: A multi-system (U/Pb, Sm/Nd, Rb/Sr and 40Ar/39Ar) isotopic study of layered nepheline syenites from the Ilímaussaq complex, Greenland

The Ilímaussaq complex in southern Greenland is a shallow crustal composite intrusion comprising augite syenite, peralkaline granite and volumetrically dominant agpaitic nepheline syenites. Previous studies indicated a baddeleyite U-Pb age of 1160 ± 5 Ma for the augite syenite, the earliest intrusive unit of the complex. A similar crystallization age, within error, is inferred for the main sequence of agpaitic nepheline syenites. However, direct age determination of these units has been challenging because agpaitic rocks characteristically lack robust phases for in situ U-Pb dating (e.g. zircon/baddeleyite). An additional challenge is the pervasive subsolidus alteration, of which the isotopic effects are poorly constrained. Here we present new U-Pb, Sm-Nd and Rb-Sr isotopic data from whole rocks and mineral separates and a 40Ar/39Ar amphibole age of three co-genetic agpaitic nepheline syenites (kakortokite) from the lowermost exposed part of the complex. Using a multi-system geochronological approach for mineral separates and whole rocks, we explore the effects of late-stage alteration for each isotopic system. Assuming a closed-system evolution for the hydrothermal fluids (i.e. isotopically similar to the melts) and cooling within a relatively short time-frame (<0.8 Ma), we evaluate whether traditional mineral-whole rock isochron methods can provide useful age constraints for agpaitic rocks. We compare our data with those in the literature, corrected for the most recent decay constants.Single-crystal40Ar/39Ar step-heating experiments yield an amphibole plateau age of 1156.6 ± 1.4 Ma (MSWD = 1.5, external error ± 7.7 Ma), which we put forward as the most precise crystallization age for the agpaitic units to date. Kakortokite whole rock and mineral separates (amphibole, eudialyte, feldspar) yield a 206Pb-207Pb isochron age of 1159 ± 17 Ma (MSWD = 0.96) and a 235U-207Pb isochron age of 1168.5 ± 8.8 Ma (MSWD = 0.82). These are within error of the baddeleyite and zircon U-Pb ages from the augite syenite and alkali granite, as well as the new plateau age, if we take into account the external error of 7.7 Ma (i.e. accuracy). The 235U-207Pb age thus far provides the best non-single mineral age estimate for the agpaitic suite. Sm-Nd isotopes for the same whole rock and mineral separates yield an isochron age of 1156 ± 53 Ma (MSWD = 0.23) with εNdi = −0.8 ± 0.8, with significantly less scatter than Nd data for the rest of the complex. Rb-Sr isotopes yield errorchron ages that are either unrealistically young (3-point feldspar errorchron: 1106 ± 9 Ma, suggesting partial 87Sr loss), or old (WR, amphibole and eudialyte: 1237 ± 21 Ma, n = 9). The data demonstrate that the U-Pb and Sm-Nd systems are relatively insensitive to late-magmatic alteration and re-equilibration during cooling. In contrast, the Rb-Sr system records significant disturbance, reflecting the highly mobile nature of Rb and Sr in peralkaline systems. This warrants careful reconsideration of previously published Rb-Sr isochron data, and caution in interpreting Rb-Sr data for other peralkaline complexes. Initial isotopic compositions for the kakortokite support petrogenetic models that describe Ilímaussaq melt evolution towards strongly radiogenic Sr values at relatively constant εNdi, with progressive evolution from the early augite syenite to the most fractionated agpaitic melts. The melts experienced variable but minor degrees of lower crustal assimilation and preferential leaching of radiogenic Sr from the Proterozoic granitic country-rock.

Intense silicification of Ordovician carbonates in the Tarim Basin: Constraints from fluid inclusion Rb–Sr isotope dating and geochemistry of quartz

AbstractAn unconventional porous silicified carbonate reservoir was unveiled in the Middle Ordovician fractured limestone within a strike‐slip fault zone in the Tarim Basin, NW China. The silicified carbonates are mainly composed of fine‐ to medium‐crystalline fascicular quartz matrix and coarse crystalline bladed to columnar quartz cement. Fluid inclusion leachate‐residue paired Rb–Sr isotopic dating of the quartz cement yields an isochron age of 167 ± 15 Ma, attesting to a causal link to the Jurassic (Yanshanian) orogeny. Pervasive replacement of matrix quartz and subsequent precipitation of cement quartz through nonisothermal effervescence probably induced by episodic influxes of the hot silica‐ and 87Sr‐rich basinal brines from the neighbouring deeper Manjiaer depression. This study thus signifies the usefulness of using Rb–Sr isotope dating method in constraining the timing of carbonate alteration, enabling to establish the deterministic relationship of basinal fluid evolution with the major tectonic activity.

Rb–Sr dating of sphalerite and S–Pb isotopic studies of the Xinxing crypto-explosive breccia Pb–Zn–(Ag) deposit in the southeastern segment of the Lesser Xing’an–Zhangguangcai metallogenic belt, NE China

The Xinxing Pb–Zn–(Ag) deposit is located in the southeastern segment of the Lesser Xing’an–Zhangguangcai metallogenic belt, NE China. It is a small-scale deposit (Pb + Zn < 50,000 tonnes) that occurs in a crypto-explosive breccia pipe. Ore bodies are hosted in Permian granodiorite, structurally controlled by three sets of faults. This paper reports results of Rb–Sr isotopic dating of sphalerite, which yields an isochron age of 182.8 ± 3.1 Ma (MSWD = 1.02). The concentrations of Rb and Sr in sphalerite range from 0.0839 × 10−6 to 0.7124 × 10−6 and 0.4775 × 10−6 to 7.4810 × 10−6, respectively. The initial 87Sr/86Sr ratio of sphalerite is 0.710954 ± 0.000061, indicating that the ore-forming components were derived from a mixed crust–mantle source. Sulfur isotopic values of the sulfides vary in a narrow range from −4.4% to −2.0% (av. of −3.28%), indicating a magmatic source for the sulfur. Lead isotopic compositions of the sulfide minerals (206Pb/204Pb = 18.277–18.323, 207Pb/204Pb = 15.514–15.571, 208Pb/204Pb = 38.016–38.199) are relatively consistent and cluster together between the mantle and orogenic belt lines, but are closer to the latter, reflecting the mixing of mantle-derived and minor crust-derived materials. These results, combined with the region geology, indicate that the Xinxing Pb–Zn–(Ag) deposit resulted from regional tectono-magmatic metallogenic processes that were related to the westward subduction of the Pacific plate underneath Eurasia in the Early Jurassic.

Age of Plutonic Rocks from the Vema Fracture Zone (Central Atlantic) and Nature of Their Mantle Sources

This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov. The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and 39Ar–40Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (eNd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.

Age and genesis of the Gangcha gold deposit, western Qinling orogen, China

The Gangcha gold deposit in Hezuo, Gansu, China, is a newly discovered medium‐scale (19 tons) epithermal deposit occurring within the western Qinling orogenic belt. The Daguanshan Formation, represented by metamorphosed volcanic–clastic rock suite, is the main ore‐bearing strata. Gold occurs mainly as submicroscopic inclusions in sulphides and quartz. The hydrothermal alteration is characterized by mid‐ to low‐temperature mineral assemblages such as pyrite, arsenopyrite, and fluorspar. We report results from Rb–Sr isotopic dating of pyrites in the main orebody, which yield an isochron age of 225.3 ± 3.4 to 229.9 ± 4.7 Ma suggesting that the mineral deposit formed during the middle Triassic within a collisional orogenic setting. The initial 87Sr/86Sr value of pyrite (0.710326 to 0.710349) is lower than that of the continental crust and higher than that of the mantle. The S isotope data of pyrites from the deposit show a narrow range from 0.6‰ to 1.3‰ with an average of 0.975‰. Their Pb isotopes exhibit characteristics of a crust–mantle mixed source with a μ (238U/204Pb) of 9.39 to 9.53 and a ω (232Th/204Pb) of 36.81 to 38.14. The S and Pb isotopic composition indicates that ore‐forming materials were derived from a magma sourced in the lower crust, which also incorporated mantle components.

Timing of formation of the Hongdonggou Pb-Zn polymetallic ore deposit, Henan Province, China: Evidence from Rb-Sr isotopic dating of sphalerites

The Hongdonggou Pb-Zn polymetallic ore deposit, located in the southwestern part of the Luanchuan Mo-W-Pb-Zn-Ag polymetallic ore mineralization in Henan Province, China, is an important part of the East Qinling metallogenic belt. The orebodies in the deposit, which are vein, bedded and lenticular, are mainly hosted in the syenite porphyry, and formed within the carbonate and clastic rocks of the Yuku and Qiumugou formations partially. The genesis of the deposit has previously been argued to be of hydrothermal-vein type or of skarn-hydrothermal type. In this study, we report the results of Rb-Sr isotopic dating based on sphalerites from the main orebody of the Hongdonggou Pb-Zn polymetallic ore deposit, which yield an isochron age of 135.7 ± 3.2 Ma, constraining the timing of mineralization as early Cretaceous. The age is close to those reported for the Pb-Zn deposits in the Luanchuan ore belt. The (87Sr/86Sr)i values of the sphalerites (0.71127 ± 0.00010) are lower than that of terrigenous silicates (0.720) and higher than the mantle (0.707), suggesting that the metallogenic components were mainly derived through crust-mantle mixing. Combining the results from this study with those from previous work, we propose that the Hongdonggou Pb-Zn polymetallic ore deposit is a hydrothermal-vein deposit associated with the early Cretaceous tectonothermal event, and the mineralization is controlled by NW- and near EW-trending faults in the Luanchuan Mo-W-Pb-Zn-Ag polymetallic ore concentration belt.

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

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

Rb-Sr Isotopic Dating and Metamorphism of a Suite of Migmatite and Granites from Sandmata Granulite Complex, Rajasthan: An Evidence of Mesoproterozoic Thermal Event

Abstract: Sandmata Granulite complex (SGC) of Rajasthan of Western India is a multiply deformed, metamorphosed and migmatised granulite units representing a Mesoproterozoic collisional orogen. It consists of migmatised metapelites, calcsilicate granulites, pyroxene granulites and charnockitic intrusives at 1.7 Ga. The granulites mostly occur as dismembered, isolated enclaves in garnet bearing migmatites and quartzofeldspathic gneiss. Intrusion of small scale post-tectonic granite in granulite documents the last thermal activity in the area. At the eastern part of SGC, greenschist to lower amphibolite facies rocks of Manglawar Complex (MC) are exposed. The P-T conditions of garnet formation during metamorphism in migmatite, leptynite and quartzo-feldspathic gneiss are 780-850°C and 7–8 kbar. Retrograded biotite forming reactions occur at much lower temperature of 500 to 600°C and pressure of 3.5-4.5 kbar. The intrusive granite is weakly deformed, calc-alkaline to sub-alkaline, peraluminous in nature, having trace element signature of volcanic-arc to syn-collisional granite. The chondrite-normalised REE patterns indicate a garnet-free source. The whole-rock Rb-Sr age of 1715 ± 24 Ma deduced for the granite mark its emplacement age. Intrusive nature of granite suggests that the age of granulite metamorphism and migmatization should be &amp;gt;1715 Ma, which is much closer to the whole-rock Rb-Sr age of migmatite and quartzofeldspathic gneiss. The whole-rock Rb-Sr analysis of migmatite gives an age of 1735±150 Ma with an initial 87Sr/86Sr ratio of 0.704 which is interpreted as an age when the temperature of migmatization passed below the blocking temperature of Rb and Sr for biotite and plagioclase; an upper age limit of migmatization. SGC represents a major 1.9-1.7 Ga Palaeoproterozoic, tectonothermal event. The granulite facies metamorphism is succeeded by 1.7 Ga migmatisation, intrusion of calc-alkaline granite and emplacement of charnockite protolith. The whole granulite complex is subsequently cooled and later exhumed, and became juxtaposed with MC during a late major orogeny.

Modern problems of geochemical and U-Pb geochronological studies of zircon in oceanic rocks

We present results of zircon LA-ICP-MS U–Pb, Lu–Hf, and trace-element study in combination with whole-rock Sm-Nd and Rb-Sr isotope data on the magmatic rocks of the Markov Deep and Ashadze hydrothermal field (Mid-Atlantic Ridge). Zircon from three gabbronorite samples in the Markov Deep defined an U–Pb ages between 0.90 ± 0.02 and 2.00 ± 0.05 Ma, with the youngest age found in the deepest sample. Zircons from four samples of gabbros and trondhjemites of the Ashadze Field have identical ages: from 1.04 ± 0.07 to 1.12 ± 0.09 Ma. Plagioclase troctolite from the Markov Deep (sample I-1069/19) contains exotic zircon grains with ages widely ranging from 90 Ma to 3.2 Ga, which is inconsistent with age of the rocks in the Mid-Atlantic Ridge. Several hypotheses are discussed to explain the origin of such exotic grains, in particular, their formation at mantle depths, or reaching these depths with subducted crust, and others. Experimental study of zirconium solubility shows that the mafic and ultramafic melts could be oversaturated with respect to zirconium only at unrealistically high contents, which usually do not occur in the corresponding rocks. Entrapped xenogenic zircon must be dissolved in the mafic and ultramafic melts and its finds in these rocks presumably indicate its disequilibrium precipitation. Zircon could be formed in the intrusive mafic rocks at the final stages of fractional crystallization, which explains the presence of own zircon in gabbroids. Zircon is very stable in crustal magmatic processes, especially at lowered activity of alkalis, but almost instantly (on geological scale) loses radiogenic lead by diffusion way under upper mantle conditions (1300–1500°C). While applying REE distribution for interpreting zircon origin, as many as possible elements should be analyzed to discriminate between intrinsic zircon element distribution and anomalies caused by defects in its structure.

Sources and geodynamic setting of petrogenesis of the Middle Cambrian Upper Petropavlovka alkaline basic pluton (Kuznetsk Alatau, Siberia)

Early Paleozoic alkaline basic magmatism in the Kuznetsk Alatau is manifested in the Upper Petropavlovka pluton of gabbro, feldspathoid rocks (theralites, mafic foidolites, and nepheline syenites), and Ca-carbonatites. According to Sm–Nd and Rb–Sr isotope data, the pluton formed in the Middle Cambrian (509 ± 10 Ma). The silicate igneous rocks correspond in the contents of silica, alumina, and alkalies to derivates of a K–Na alkaline basic association. The Ca-carbonatites are characterized by a high-temperature (600–900 °C) paragenesis of apatite, clinopyroxene, ferromonticellite, phlogopite, and magnetite. They are enriched in P2O5 (up to 6.4 wt.%), Sr (up to 3000–4500 ppm; Sr/Ba ~ 5–7), and REE + Y (up to 800 ppm) and show evidence for liquation genesis. The predominant magmatic source (εNd(T) = 5–7) was moderately depleted PREMA, possibly combined with E-MORB and EM. According to the isotopic data ((87Sr/86Sr)T ~ 0.7024–0.7065; δ18O ~ 6.3–15.5‰; δ18C ~ –3.5 to –2.0‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the mafic rocks testifies to the participation of a substance similar to the substrata of the parental magmas of MORB, IAB, and OIB in the magma generation. This suggests intrusion in the geodynamic setting of interaction between the active continental margin and an ascending mantle diapir. Most likely, the intrusion led to the mixing of material from different sources, including the components of PREMA, enriched suprasubduction lithospheric mantle (EM), and continental crust. The assumption is made that the complexes of highly alkaline rocks and carbonatites in the western Central Asian Fold Belt are of plume origin and belong to an Early Paleozoic large igneous province.

Timing of the formation of the Changba–Lijiagou Pb–Zn ore deposit, Gansu Province, China: Evidence from Rb–Sr isotopic dating of sulfides

The giant Changba–Lijiagou Pb–Zn deposit is located in the north of the Xihe–Chengxian (abbreviated as “Xicheng”) ore cluster in Gansu Province, China. The orebodies in the deposit are mainly hosted in the marble, dolomitic marble, and biotite–calcite–quartz schist of the Middle Devonian Anjiacha Formation. The genesis of the deposit has previously been argued to be of SEDEX type (sedimentary exhalative type) or of epigenetic hydrothermal type. This paper reports results of Rb–Sr isotopic dating on sphalerite and pyrite taken from the main orebody, which yield an isochron age of 222.3±2.2Ma for eight sphalerite samples, and 222.0±3.0Ma for the eight sphalerite samples combined with four pyrite samples, indicating that the deposit formed during the Late Triassic. The (87Sr/86Sri) value of the sphalerite is 0.71370±0.00013, and that of the sphalerite and pyrite is 0.71371±0.00014, which are identical within experimental error, suggesting that the ore metals are derived mainly from the continental crust. By integrating the present results with the regional geology, we propose that the Changba–Lijiagou Pb–Zn deposit is a product of regional hydrothermal mineralization processes, forced by tectono-magmatic activities, which took place in the Xicheng ore cluster during Triassic orogenic processes.

Origin and geodynamic significance of fault-hosted massive sulfide gold deposits from the Guocheng–Liaoshang metallogenic belt, eastern Jiaodong Peninsula: Rb–Sr dating, and H–O–S–Pb isotopic constraints

More recent explorations (from 2009 to 2013) indicate a minimum of 27tonnes Au of additional resources in the Guocheng–Liaoshang gold belt, northeastern Jiaolai basin, Jiaodong gold province. The ores are hosted in faults/fractures in Paleoproterozoic metamorphic and granitic rocks, and are marked by massive sulfide type with sulfides of up to 40 to 95vol.%. Rb–Sr isotopic dating of mineralized quartz yielded an isochron age of 116.2±2.4Ma (MSWD=0.36). The ore-related quartz samples have δD values of −86 to −69‰ (mean −78‰), with calculated δ18OH2O values of 0.6 to 7.7‰ (average 3.6‰). The δ34S values of ore sulfides vary from 8.5 to 12.7‰ with an average of 9.9‰. Lead isotope signatures recorded in sulfides are much less radiogenic (206Pb/204Pb=17.039–17.862) relative to that of Phanerozoic convecting mantle (206Pb/204Pb=18.179–18.384). These results show that the gold mineralization style in this belt is different from the traditionally identified three types of gold deposits in the Jiaodong Peninsula (namely, Linglong, Jiaojia and Pengjiakuang type). They also suggest deep-seated, most likely mafic dike magmatic and mantle-derived, sources for the ore fluids, sulfur and metals in the hydrothermal system, despite the fact that wall rocks might provide some sulfur to satisfy the enriched 34S signature. The inferred mantle reservoir was probably formed by subduction-related fluid metasomatism during the Paleoproterozoic (ca. 1.85Ga) Jiao-Liao-Ji mobile belt assembly (suture within Columbia supercontinent). Extensional tectonic inversion during early Cretaceous, caused by subduction of the Kula–Pacific plate rather than postcollisional process of the Qinling–Dabie–Sulu Orogen, triggered partial melting of this mantle domain, subsequent magma mixing and final exsolution of ore-forming fluids.

Zircon U‐Pb age, Lu‐Hf Isotopic Characteristics and Origin of the Banshanping Granitoid Rocks in East Qinling Orogenic Belt

AbstractThe Banshanping granitoid rocks distribute in the east of the North Qinling orogenic belt. It is a diorite‐quartz diorite‐granodiorite‐granite series, spreading in a NW‐SE direction, and intrudes into the Erlangping Group. The SiO2 content ranges from 57.04% to 76.56%, Na2O from 2.05% to 4.65%, K2O from 0.84% to 3.40%. Major element characteristics indicate that Banshanping granitoid rocks have properties of I type granotoids. SREE ranges from 36.51 ppm to 473.25 ppm, and LREE/HREE ratios lie between 3.95 and 22.18. Negative Eu anomalies are not obvious in most samples, though there are obvious Nb, P and Ti positive anomalies. The zircon LA‐ICP‐MS ages of Banshangping granitoid rocks are 496.0±8.1 Ma–486.9±9.3 Ma. Hf isotope shows that 176Hf/177Hf ratios range from 0.282721 to 0.282876, εHf(t) values from 8.5 to 14, all positive, and corresponding modal ages (TDM2) range from 559 Ma to 908 Ma. Based on Hf isotope characteristics and existing Sm‐Nd and Rb‐Sr isotope data, we consider that the Banshanping granitoid rocks originate from mantle‐derived material, i.e. the igneous rocks that formed in Neoproterozoic, and there may be a certain amount of crust‐derived material during the formation of Banshanping granitoid rocks.

Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian CRATON, Baltic Shield

Manifestations of the main types of metasomatites (beresite, propylite, listwaenite, aceite, and gumbeite) were identified in the shear zones of the Fenno-Karelian craton on the basis of the previously proposed systematics of metasomatic facies. These metasomatites were formed in shear stress environments, which determined their morphological features, in particular, finely banded texture. Comparatively low-temperature conditions of infiltration process and salting out effect (reduction of CO2 solubility with increasing salt content) lead to the heterogenization of fluid into two phases: aqueous salt solution and almost pure CO2. This results in more aggressive and mobile behavior of the fluid, and, correspondingly, more intense differentiation of the matter and contrast in metasomatic banding. Relations between metasomatic parageneses indicate an evolution trend of the processes from propylite, beresite and listwaenite to alkaline varieties and their repeated manifestation in the same shear zones. The results of Rb-Sr isochron dating of ore metasomatites from eight deposits and occurrences of the Fenno-Karelian craton (more than 100 samples of rocks and minerals) confirm previous assumptions. In general obtained data show that the shear zones controlling the distribution of the studied occurrences operated as fluid pathways during a long time period, up to 200 Ma, after the Svecofennian orogeny completion and did not show any correlation with Paleoproterozoic and Neoarchean magmatism. Rb-Sr isotope data on the metasomatites indicate three peaks of the post-Svecofennian metallogenic activity: 1700–1780, 1600–1650, and 1400–1500 Ma. Since the studied ore deposits were formed within tectonic structures, which evolved on the Archean crust and have a long prehistory, and fluid flows were subjected to intensive contamination by ancient crustal material, a relatively high initial Sr isotope ratios of formed ore-metasomatic systems were developed. High variablity of this value in the studied rocks ((87Sr/86Sr)0 from 0.706 to 0.750) is related to the heterogeneity of crustal protolith and to the relative storage and manifestation of the juvenile component of the fluid, which was responsible for the metasomatic transformation of the Archean and Paleoproterozoic rocks and ore-deposit formation.

East Scandinavian and Noril’sk plume mafic large igneous provinces of Pd-Pt ores: Geological and metallogenic comparison

This paper compares the geological, geophysical, and isotopic geochemical data on the Paleoproterozoic East Scandinavian Pd-Pt province in the Baltic Shield and the Late Paleozoic Noril’sk Pd-Pt province in the Siberian Craton. Both provinces contain large magmatic PGE deposits: low-sulfide in the Baltic Shield and high-sulfide in the Siberian Craton. Multidisciplinary evidence shows that the East Scandinavian mafic large igneous province, which has a plume nature, is intracratonic and was not subjected to the crucial effect of subduction-related and other contamination processes, whereas the Noril’sk province is pericratonic with substantial crustal contamination of the intrusive processes. Low-sulfide Pd-Pt deposits dominate in the East Scandinavian province, while high-sulfide Ni-Cu-PGE deposits play the leading role in the Noril’sk province. The U-Pb, Sm-Nd, and Rb-Sr isotopic data indicate multistage and long-term (tens of millions of years) geological history of mafic large igneous provinces. The plume magmatism with specific geochemistry and metallogeny is probably related to lower mantle sources.

Petrogenetic association of the oldest lunar basalts: Combined Rb–Sr isotopic and trace element constraints

We report a new radiometric Rb–Sr age determination of 4.03±0.04Ga for high-Al basaltic clast 14321,1353, which is a Group B Apollo 14 high-Al basalt based on its whole-rock incompatible-trace-element chemistry. In addition, available literature Rb–Sr isotope data for Apollo 14 high-Al basalts were reprocessed using the same decay constant and data reduction protocol as used for sample 14321,1353. The Rb–Sr isotopic ages of high-Al basalts range from 4.31±0.17 to 3.93±0.06Ga. The trace element and Rb–Sr isotopic compositions of high-Al basalts are distinct from those of Apollo 14 aluminous impact melts that crystallized at 3.86±0.01Ga. The Rb–Sr isotope systematics, coupled with incompatible-trace-element data are consistent with the parental melts of Apollo 14 high-Al basalts being derived from sources consisting of primitive crystallization products of the lunar magma ocean that were variably metasomatized by late-stage KREEP-rich fluids. Our new Rb–Sr isotopic age for 14321,1353 suggests that the KREEP metasomatism must occur in the lunar mantle no later than 4.03±0.04Ga. Collectively, the Apollo 14 high-Al basalts represent the earliest known volcanic products from the Moon and place a time constraint on the formation of the high-Al basalt source through the crystallization of the lunar magma ocean.

Zircon U–Pb and K-feldspar megacryst Rb–Sr isotopic ages and Sr–Hf isotopic composition of the Mesozoic Heyu pluton, eastern Qingling orogen, China

Mesozoic granitoids are ubiquitous in China and have attracted the interests of many geologists not only because of their mineralization potential but also because they carry information on source material, emplacement mechanisms and crustal architecture. Most of the plutons were created by multi-stage magma emplacement and offer particular challenge for unraveling the emplacement and cooling history. A multi-isotope approach combined with single grain dating analyses provides a means to understand magma geneses even in complex magmatic settings. This study presents Rb–Sr isotope data for K-feldspar megacrysts and U–Pb ages and Hf isotopic composition for zircons from the composite Heyu pluton exposed in eastern Qinling orogen, central China. Zircon U–Pb concordia ages of 150 and 140Ma and single K-feldspar Rb–Sr isochron ages between 133 and 121Ma were obtained from early and late magmatic stages, respectively, consistent with slow cooling or isotopic re-equilibration of the Rb–Sr K-feldspar system. The distinct K-feldspar morphology of early and late intrusive phases is controlled largely by the emplacement depth, likely to be associated with a gravity-driven sinking model. Corresponding zircon εHf(t) values are −21.8 to −18.0 and −17.2 to −10.3 for early and late intrusive phases, respectively, indicating different magma sources.