The Stages of Formation of the Main Batholite Belt,Northeast Russia: U–Th–Pb SIMS and Ar–Ar Geochronological Data

Doklady Earth Sciences - The original and published geochronological data and the geological position of different granitoid complexes of the Main (Kolyma) Batholith Belt of Northeast Asia have...     

The Late Neoproterozoic Age of Differentiated Volcanic Complexes of the Ulutau Massif: Results of U–Th–Pb (SIMS) Dating

Doklady Earth Sciences - The metamorphosed differentiated volcanogenic strata of the Aralbai Group have been studied in the eastern part of the Precambrian Ulutau massif of Central Kazakhstan. The...     

New Data on the Age of Neoproterozoic Volcanic Rocks of the Isakovka Terrane,Sayan–Yenisei Accretion Belt (U–Pb,Zircon)

Doklady Earth Sciences - A Late Neoproterozoic U–Pb zircon age is established for the first time for arc metadacites (691 ± 8.8 Ma) and basalts (572 ± 6.5 Ma) from the...     

Zircon U／Pb Dating of Cretaceous Adakitic Volcanic Rocks in the Eastern Part of North Dabie Mountains

1IntroductionMostgeologistshaveacceptedthattheQinling Dabieorogenicbeltistheproductofcontinentaldeepsubduction (Okayetal.,1993;Cong ,B .etal.,1995 ;Hackeretal.,1995 ;DongShuwenetal.,1993) .Butasthebiggest scaleUHPmetamorphicbeltintheworld ,thereactionbetweenthecrustandthemantleatthepost orogenicstageandtheexhumation ,upliftinganddenudationoftheorogenicbeltarethecomplexdynamicprocesses .ItwasreportedpreviouslythatsomeCretaceousvolcanicrocksexistinthemiddleoftheDabieMountains (GuanYuncaiet…     

Early Ordovician Age of Suprasubduction Ophiolites in the Northeastern Part of Central Kazakhstan: U–Th–Pb (SIMS) Dating of Plagiogranites

Doklady Earth Sciences - Suprasubduction zone ophiolites in the Bayanaul and Maikain–Kyzyltas zones in the northeastern part of Central Kazakhstan were studied. U–Pb dating of...     

Zircon U–Pb Geochronology and Geochemistry of the Early Cretaceous Volcanic Rocks from the Manitu Formation in the Hongol Area,Northeastern Inner Mongolia

We undertook zircon U-Pb dating and geochemical analyses of volcanic rocks from the Manitu Formation in the Hongol area,northeastern Inner Mongolia,to determine their age,petrogenesis and sources,which are important for understanding the Late Mesozoic tectonic evolution of the Great Xing'an Range.The volcanic rocks of the Manitu Formation from the Hongol area consist primarily of trachyandesite,based on their chemical compositions.The zircons from two of these trachyandesites are euhedral-subhedral in shape,display clear oscillatory growth zoning and have high Th/U ratios(0.31-1.15),indicating a magmatic origin.The results of LA-ICP-MS zircon U-Pb dating indicate that the volcanic rocks from the Manitu Formation in the Hongol area formed during the early Early Cretaceous with ages of 138.9-140.5 Ma.The volcanic rocks are high in alkali(Na_2O + K_2O = 6.22-8.26 wt%),potassium(K_2O = 2.49-4.58 wt%) and aluminium(Al_2O_3 = 14.27-15.88 wt%),whereas they are low in iron(total Fe_2O_3 = 3.76-6.53 wt%) and titanium(TiO_2 = 1.02-1.61 wt%).These volcanic rocks are obviously enriched in large ion lithophile elements,such as Rb,Ba,Th and U,and light rare earth elements,and are depleted in high field strength elements,such as Nb,Ta and Ti with pronounced negative anomalies.Their Sr-Nd-Pb isotopic compositions show positive ε_(Nd)(t)(+0.16‰ to+1.64‰) and low T_(DM)(t)(694-767 Ma).The geochemical characteristics of these volcanic rocks suggest that they belong to a shoshonitic series and were likely generated from the partial melting of an enriched lithospheric mantle that was metasomatised by fluids released from a subducted slab during the closure of the MongolOkhotsk Ocean.Elemental and isotopic features reveal that fractional crystallization with the removal of ferromagnesian minerals,plagioclase,ilmenite,magnetite and apatite played an important role during the evolution of the magma.These shoshonitic rocks were produced by the partial melting of the enriched lithospheric mantle in an extensional regime,which resulted from the gravitational collapse following the final closure of the Mongol-Okhotsk Ocean in the Middle-Late Jurassic.     

First Results of U–Th–Pb (SIMS) Geochronological Study of Zircon from Serpentinized Ultramafic Rocks of the Tekturmas Ophiolite Zone (Central Kazakhstan)

Doklady Earth Sciences - Accessory zircon extracted from serpentinized dunite and harzburgite of the Tekturmas Zone of Central Kazakhstan, which is one of the largest ophiolite zones in the western...     

Geochronology and Geochemistry of Small Granitoid Intrusions in the Western Yana–Kolyma Gold Belt (Northeast Asia)

Doklady Earth Sciences - Data on the petrographic–geochemical and U–Th–Pb geochronological studies of granitoids of the Bukeschensky and Samyrsky massifs of small intrusions...     

Sr,Nd,Pb Isotope Geochemistry and Magma Evolution of the Potassic Volcanic Rocks,Wudalianchi,Northeast China

Wudalianchi volcanic rocks are the most typical Cenozoic potassic volcanic rocks in easten China.Compositional comparisons between whole rocks and glasses of various occurrences indicate that the magma tends to become rich in silica and alkalis as a result of crystal differentiation in the course of evolu-tion.They are unique in isotopic composition with more radiogenic Sr but less radiogenic Pb.^87Sr/^86Sr is higher and ^143Nd/^144Nd is lower than the undifferentiated global values.In comparison to continental pot-ash volcanic rocks,Pb isotopes are apparently lower.These various threads of evidence indicate that the rocks were derived from a primary enriched mantle which had not been subjected to reworking and shows no sign of incorporation of crustal material.The correlation between Pb and Sr suggests the regional heterogeneity in the upper mantle in terms of chemical composition.     

Early Neoproterozoic (~920 Ma) Granite–Gneiss of the Junggar Alataw,South Kazakhstan: Age Substantiation Based on the Results of U–Th–Pb (SIMS) Dating

Doklady Earth Sciences - Based on the results of U–Th–Pb (SIMS) dating, an Early Neoproterozoic (924 ± 4 Ma) age was obtained for granite–gneiss of the Sarychabyn Complex,...     

Middle Devonian Age of Metamorphism of Garnet Amphibolite at the Sole of the Kempirsai Ophiolite Allochthon (Southern Urals): Results of U–Th–Pb (SIMS) Dating

Doklady Earth Sciences - Metagabbroid garnet amphibolite formed after high-pressure granulite with an estimated P–T peak of 12–16 kbar at 700–790°C occurs at the sole of the...     

Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagzê and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations, we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41 %-50.4 % and 64 %-69 %, belonging to calc-alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content, 0.66 %-1.01 %, much lower than those of continental tholeiite. The ΣREE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb*＝0.54-1.17 averaging 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*＝0.74-1.06 averaging 0.86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone. The spinal lherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.     

Early Paleozoic Protoliths of Gneisses and Granitic Gneisses in the Eastern South Urals: Results of U–Th–Pb (SIMS) Geochronological Studies

Doklady Earth Sciences - The Early Paleozoic age of the protolith for gneisses in the East Uralian megazone (South Urals) is proved by zircon dating. Two metamorphic complexes have been identified...     

Spinel–Sapphirine Reaction Structures in the Garnet Metaultramafic Rocks of the Omolon Massif: Petrogenesis and Geological Interpretation (Northeast Asia)

Spinel–sapphirine reaction structures are studied in detail in the sapphirine gedritites which form a small segregation in one of the garnet metaultramafic bodies framing the Aulandzha charnockitic dome (the pre-Riphean basement of the Omolon Massif). It is established that the sapphirine gedritites resulted from the retrograde evolution of the garnet–spinel metaultramafic rocks, the formation temperature of which may have exceeded 900°C at a probable pressure no more than 7 kbar. It is shown that the spinel–sapphirine reaction structures were formed under conditions of elevated O₂ potential close to the magnetite–hematite buffer. Subsequent diaphthoresis of the metaultramafic rocks was accompanied by an increase in the H₂O potential and decrease in temperature to at least 760°C. This explains the formation of another group of diaphthorites, garnet gedritites, the index minerals of which are sodium gedrite and calcic plagioclase with strong inverse zoning. A comprehensive analysis of the new petrological data and published materials on the isotope–geochronological study of the pre-Riphean basement of the Omolon Massif makes it possible to attribute the formation of the sapphirine gedritites to 1.9 Ga (middle of the second half of the Early Proterozoic, according to the General Stratigraphic Scale of Russia). The unusually high value of the O₂ potential calculated for the spinel–sapphirine reaction structures; the unique magnesian–alumina composition of the gedritites; and the extremely high contents of Zr, Ba, Rb, and Hf allow O.V. Avchenko to hypothesize that the protoliths of these scarce rocks were products of weathering crust after orthomagmatic ultramafic rocks. In this case, the calculated parameters for the formation of the spinel–sapphirine reaction structures may indicate that the value of the O₂ potential on the Earth’s surface in the Paleoproterozoic corresponded to the magnetite–hematite buffer.     

Formation Conditions of Basic Granulites and High-Alumina Gneiss of the Baikal–Muya Belt (Northern Baikal Area)

The Kichera zone of the Baikal–Muya Belt consists of alternating tectonic plates with rocks of different metamorphic facies. Garnet–cordierite–sillimanite gneiss from the tectonic fragment in granite–gneiss of the Baikal massif with an age of 755 ± 15 Ma from the Goremyka plate and two-pyroxene schist of the granulite complex with an age of 617 ± 5 Ma from the Boguchan plate were studied. Thermobarometric studies of these key metamorphic rocks were carried out using the avPT (THERMOCALC) and TWEEQU (TWQ 2.01) methods. The P–T parameters estimated for the cordierite?sillimanite gneiss of the Goremyka plate correspond to the boundary between the amphibolite and granulite facies. Granulites of the Boguchan plate belong to the HT–LP type. Exhumation of metamorphic rocks could be caused by extension upon the evolution of the Late Baikal rifting.     

Formation Conditions and Distribution Regularities of Oil—gas Pools in Tertiary Volcanic Rocks in Western Huimin Depression,Shandong Province

The formation conditions and distribution regularities of oil-gas pools in volcanic rocks in western Huimin Depression have been studied in terms of geolgic,sesmic and well logging information,This paper discusses the types and lithofacies,development and distribution of Tertiary volcanic rocks in the area.The results demonstrate that volcanic activity occurred mainly during the period from the Sha-4 stage to the Guantao episode,i.e.,before the oil-generating period(before the end of the Guantao episode and the Minghuazhen episode).The activity did not destroy oil and gas formation and accumulation,but was favourable for the concentration of organic matter and its conversion to hydrocarbons;besides,volcanic rocks can serve as reservoir rocks and cap rocks,playing a role very similar to that of a syndepositional anticline,The volcanic rocks are distributed near the margins of the oil-generating depression;there are many secondary interstices in the rocks,which are connected with each other.These are the leading conditions for the formation of oil-generating period and their self-sealing or good combination with other cap rocks are important factors for forming volcanic rock-hosted oil and gas pools.The oil-gas pools associated with volcanic rocks in western Huimin are mainly distributed around the deep oil-generating depression,in the central up lift or the high structural levels on the margins of the depression.In particular,the sites where several faults cross are usually locatons where hith-yielding oil-gas pools in volcanic rocks are concentrated.     

Zircon U–Pb Geochronology and Petrogenesis of Early–Midlle Permian Arc–Related Volcanic Rocks in Central Jilin: Implications for the Tectonic Evolution of the Eastern Segment of Central Asian Orogenic Belt

This paper presents age and geochemical data of a recently identified Late Paleozoic volcanic sequence in central Jilin Province, with aims to discuss the petrogenesis and to constrain the tectonic evolution of the Central Asian Orogenic Belt in this area. Firstly, the volcanic rocks have zircon U-Pb ages of 290–270 Ma. Secondly, they are characterized by(a) ranging in composition from the low-K tholeiite series to high-K calc-alkaline series;(b) enrichment in light rare earth elements and deple...     

SHRIMP U–Pb and Ar–Ar geochronology of major porphyry and skarn Cu deposits in the Balkhash Metallogenic Belt,Central Asia,and geological implications

The Balkhash Metallogenic Belt (BMB) in Kazakhstan, Central Asia, with the occurrence of the super-large Kounrad and Aktogai, the large Borly porphyry Cu–Mo deposits, and the large Sayak skarn polymetallic ore-field, is one of the central regions of the Paleozoic Central Asian metallogenic domain and orogenic belt. In this study, newly obtained SHRIMP zircon U–Pb ages of nine samples and ⁴⁰Ar/³⁹Ar ages of six mineral samples (inclding hornblende, biotite and K-feldspar) give more detailed constraints on the timing of the granitic intrusions and their metallogeny. Porphyritic monzonite granite and tonalite porphyry from the Kounrad deposit yield U–Pb zircon SHRIMP ages of 327.3 ± 2.1 Ma and 308.7 ± 2.2 Ma, respectively. Quartz diorite and porphyritic granodiorite from the Aktogai deposit yield U–Pb SHRIMP ages of 335.7 ± 1.3 Ma and 327.5 ± 1.9 Ma, respectively. Porphyritic granodiorite and granodiorite from the Borly deposit yield U–Pb SHRIMP ages of 316.3 ± 0.8 Ma and 305 ± 3 Ma, respectively. Diorite, granodiorite, and monzonite from the Sayak ore-field yield U–Pb SHRIMP ages of 335 ± 2 Ma, 308 ± 10 Ma, and 297 ± 3 Ma, respectively. Hornblende, biotite, and K-feldspar from the Aktogai deposit yield ⁴⁰Ar/³⁹Ar cooling ages of 310.6 Ma, 271.5 Ma, and 274.9 Ma, respectively. Hornblende, biotite, and K-feldspar from the Sayak ore-field yield ⁴⁰Ar/³⁹Ar cooling ages of 287.3 ± 2.8 Ma, 307.9 ± 1.8 Ma, and 249.8 ± 1.6 Ma, respectively. The new ages constrain the timing of Late Paleozoic felsic magmatism to ∼336 to ∼297 Ma. Skarn mineralization in the Sayak ore-field formed at ∼335 and ∼308 Ma. Porphyry Cu–Mo mineralization in the Kounrad deposit and the Aktogai deposit formed at ∼327 Ma, and in the Borly deposit at ∼316 Ma. The Late Paleozoic regional cooling in the temperature range of ∼600 °C to ∼150 °C occurred from ∼307 to ∼257 Ma.     

U–Pb and Th–Pb dating of apatite by LA-ICPMS

Apatite is a common U- and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but widespread detrital component in clastic sedimentary rocks. U–Pb and Th–Pb dating of apatite has potential application in sedimentary provenance studies, as it likely represents first cycle detritus compared to the polycyclic behavior of zircon. However, low U, Th and radiogenic Pb concentrations, elevated common Pb and the lack of a U–Th–Pb apatite standard remain significant challenges in dating apatite by LA-ICPMS, and consequently in developing the chronometer as a provenance tool.This study has determined U–Pb and Th–Pb ages for seven well known apatite occurrences (Durango, Emerald Lake, Kovdor, Mineville, Mud Tank, Otter Lake and Slyudyanka) by LA-ICPMS. Analytical procedures involved rastering a 10 μm spot over a 40 × 40 μm square to a depth of 10 μm using a Geolas 193 nm ArF excimer laser coupled to a Thermo ElementXR single-collector ICPMS. These raster conditions minimized laser-induced inter-element fractionation, which was corrected for using the back-calculated intercept of the time-resolved signal. A Tl–U–Bi–Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias. External standards (Ple?ovice and 91500 zircon, NIST SRM 610 and 612 silicate glasses and STDP5 phosphate glass) along with Kovdor apatite were analyzed to monitor U–Pb, Th–Pb, U–Th and Pb–Pb ratiosCommon Pb correction employed the ²⁰⁷Pb method, and also a ²⁰⁸Pb correction method for samples with low Th/U. The ²⁰⁷Pb and ²⁰⁸Pb corrections employed either the initial Pb isotopic composition or the Stacey and Kramers model and propagated conservative uncertainties in the initial Pb isotopic composition. Common Pb correction using the Stacey and Kramers (1975) model employed an initial Pb isotopic composition calculated from either the estimated U–Pb age of the sample or an iterative approach. The age difference between these two methods is typically less than 2%, suggesting that the iterative approach works well for samples where there are no constraints on the initial Pb composition, such as a detrital sample. No ²⁰⁴Pb correction was undertaken because of low ²⁰⁴Pb counts on single collector instruments and ²⁰⁴Pb interference by ²⁰⁴Hg in the argon gas supply.Age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common Pb-corrected ages, a Tera–Wasserburg Concordia intercept age and a Tera–Wasserburg Concordia intercept age anchored through common Pb. The samples in general yield ages consistent (at the 2σ level) with independent estimates of the U–Pb apatite age, which demonstrates the suitability of the analytical protocol employed. Weighted mean age uncertainties are as low as 1–2% for U- and/or Th-rich Palaeozoic–Neoproterozoic samples; the uncertainty on the youngest sample, the Cenozoic (31.44 Ma) Durango apatite, ranges from 3.7–7.6% according to the common Pb correction method employed. The accurate and relatively precise common Pb-corrected ages demonstrate the U–Pb and Th–Pb apatite chronometers are suitable as sedimentary provenance tools. The Kovdor carbonatite apatite is recommended as a potential U–Pb and Th–Pb apatite standard as it yields precise and reproducible ²⁰⁷Pb-corrected, ²³²Th–²⁰⁸Pb, and common Pb-anchored Tera–Wasserburg Concordia intercept ages.