Relationships between O isotope equilibrium,mineral alteration and Rb–Sr chronometric validity in granitoids: implications for determination of cooling rate

A combined study of mineral O and Rb–Sr isotopes was carried out for a number of Mesozoic granitoids in China in order to compare the degree of O isotope equilibrium between coexisting minerals, with the validity of mineral Rb–Sr isochrons for granitoids. A scrutiny of both O isotope geothermometry and Rb–Sr internal isochron dating for corresponding minerals indicates that equilibrium O isotope fractionation between Rb–Sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in ⁸⁷Rb/⁸⁶Sr ratio is big enough to provide reasonably small uncertainties in age. Significant deviation of the Rb–Sr isochron age from the actual age appears to depend on the difference in Sr isotopic composition between an external fluid and the igneous minerals. As a result, O isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration. Post-magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock. The O isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature. Both Rb and Sr concentrations and the Sr isotope ratios of isochron minerals also changed due to the mixing of different Sr reservoirs. Nevertheless, the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed. This provides an insight into the effect of internal and external fluids on the validity of mineral Rb–Sr chronometry. In addition, an alternative approach is proposed to construct the cooling curve by a combined use of O isotope temperature and mineral isotope age for the granitoids of interest. Comparing with the traditional method using the empirical closure temperature for Rb–Sr chronometry, the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way.     

Tangent Vectors of Isochron Rays and Velocity Rays Expressed in Global Cartesian Coordinates

This paper provides Cartesian expressions for tangent vectors of isochron rays and velocity rays previously derived in so-called isochron-orthonormal coordinates. The Cartesian expressions are simpler and easier to implement than the expressions in isochron-orthonormal coordinates. It is shown that the expressions in both coordinate systems are equivalent.     

Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D₁/M₁, D₂/M₂ and D₃/M₃). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D₁ garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D₁/pre-D₂ decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D₂ garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D₃, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D₁/M₁ and D₂/M₂ episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D₃/M₃ episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D₂ decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D₁/pre-D₂ granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D₁/pre-D₂ amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M₃ metamorphism.     

Palaeomagnetism and Rb-Sr ages of the Ratagan and Comrie intrusions

Summary. Study of the palaeomagnetism of two complexes from the Newer Granite Suite in Scotland, at Ratagan (NW Highlands) and Comrie (central Highlands), reveals the variable nature of the natural remanence encountered in granodioritic intrusions and the surrounding metamorphic country rock. Forty-eight specimens from Ratagan, dated at 415 ± 5 Ma, gave a mean direction: D = 8°, I =−32°, and a palaeomagnetic south pole: 15°S, 346°E (δ p = 5°, δ m = 9°). Twenty-eight specimens from Comrie, dated at 408±5 Ma, gave a mean direction: D = 75°, I =−30°, and a palaeomagnetic south pole: 6°S, 287°E (δ p = 4°, δ m = 7°). These results have been compared with the established apparent polar wander path (APWP) for Britain. The Ratagan pole improves the reliability of the APWP but doubt remains as to whether the primary magnetization from Comrie represents a true late Silurian direction or whether it has been affected by post-cooling rotation, possibly associated with the nearby Highland Boundary Fault.     

Development of high precision Rb-Sr phlogopite and biotite geochronology; an alternative to Ar/Ar tri-octahedral mica dating

Potassium-Ar and Rb-Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K-Ar and, in particular, ⁴⁰Ar/³⁹Ar dating of micas is still widely applied, Rb-Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb-Sr ages than the K-Ar or ⁴⁰Ar/³⁹Ar system. Moreover, a reduction of uncertainties typically reported for Rb-Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb-Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% (n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield ⁴⁰Ar/³⁹Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb-Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb-Sr and ⁴⁰Ar/³⁹Ar ages demonstrates the robustness of the Rb-Sr system in tri-octahedral micas and cautions against the sole use of ⁴⁰Ar/³⁹Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb-Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb-Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma.Combining the mica Rb-Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb-Sr system in 1-2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.     

Rb—Sr Isotopic Chronology of the Au—Bearing Quartz Veins in Gold Depostits in the Eastern Part of North China Platform

In the eastern part of Chian there had occurred large-scale gold mineralization during the Mesozoic,resulting in a large number of inprotant gold ore-concentrated areas.In this paper we have selected some isotope data(including four gold deposits previously studied and two gold deposis in this work)of Au-bearing quartz veins of the representative gold deposits in six impor-tant gold ore-concentrated areas in the periphery of the North China Platform and calculated their metallogenic ages using the method of Ludwig(2.90 version).The results show that the repre-sentative gold deposits in the six gold ore-concentrated areas were formed during the Mesozoic.     

Isotope Geochemistry of the Dongchuan Copper Deposit,Yunnan,SW China： Stratigraphic Chronology and Application of Lead Isotopes in Geochemical Exploration

In this paper the Pb-Pb isochron method was used to determine the isotopic ages of the Luoxue Formation dolomites (1716 ±56 Ma), the Heishan Formation carbonaceous slates ( 1607 ±128 Ma) of the middle sub-group of the Kunyang Group and the Dayingpan Formation carbonaceous slates (1258 ±70Ma) of the upper sub-group of the Kunyang Group. The age of the middle sub-group of the Kunyang Group is about 1800 – 1600 Ma, belonging to the Late Paleo-Proterozoic, and that of the Dayingpan Formation in the upper sub-group of the Kunyang Group is about 1200 – 1300 Ma, belonging to the Middle Proterozoic. In addition, the Pb isotope chemical exploration techniques were used to study systematically the Pb isotopic compositions of rocks and ores across the No. 14 vein in the middle segment of the No. 1 pit of the Tangdan copper deposit and the result showed that the Pb isotope V₁ values are negatively correlated with the copper contents. It is the first trial to use the systematic section method to conduct Pb isotope chemical exploration. This research project was financially supported jointly by the National Natural Science Foundation of China (Grant Nos. 49702022, 40073001) and the State 973 Program (No.G1999043215).     

~(36)Ar/~(39)Ar-~(40)Ar/~(39)Ar等时线方法的应用——以新疆查汗萨拉锑、银矿带为例

等时线方法在K Ar年代学中得到广泛应用 ,本文利用一种新的3 6Ar 3 9Ar 4 0 Ar 3 9Ar等时线方法 ,计算查南疆汗萨拉锑、银矿带中石英脉的40 Ar 3 9Ar实验数据 ,并与其他等时线方法进行对比 ,得到非常一致的结果。     

内蒙古包头—白云鄂博地区金矿的同位素研究

从内蒙古包头至白云鄂博地区,横跨高、中、低级变质岩区,都发现有金矿及金矿化点。对三个主要金矿床的Pb、S、Rb—Sr、K—Ar等同位素的研究表明,成矿物质来自变质岩和花岗岩,成矿时代与古生代晚期华北陆台活化的岩浆作用有关。铅的混合模型说明在不同变质岩区变质岩和花岗岩对矿源的贡献不同。     

福建沿海牛头山玄武岩的铷,锶和稀土元素地球化学

牛头山玄武岩分为亚碱性、过渡性和碱性三类。微量和稀土元素及同位素组成表明它们是活动大陆边缘的钠质玄武岩类。从亚碱性到碱性玄武岩,M值减小,K、Ce、Rb、Sr增加。~(37)Sr/~(36)Sr和1/~(36)Sr相关分析表明本区母岩浆及分离结晶产物均未遭受壳层放射成因锶的混染。定量模拟计算表明本区橄榄拉斑玄武岩母岩浆是上地幔石榴石二辉橄榄岩7％部分熔融的产物。