Redefinition of Early Mesoproterozoic (1800–1600 Ma) stratigraphy in the northern Kongling area,China: The nucleus of Yangtze Craton and its tectonic significance

The Wujiatai Formation, which is well exposed in Huangjiatai-Xichahe region of the northern Kongling area of central Yangtze Craton, is a suite of epimetamorphic conglomerates to pebbly sandstones to fine sandstone-dolostones deposited in littoral-carbonate platform facies. The formation has angular unconformity contacts with both the overlying Neoproterozoic Nantuo Formation and the underlying Paleoproterozoic Huanglianghe Formation complex. Detrital zircons from metafine sandstones of the lower Wujiatai Formation have ages ranging from 3377–1828 Ma, with the youngest zircons dating to about 1828 Ma. In addition, whole-rock Pb-Pb isochron ages from dolostones in the upper Wujiatai Formation yield an age of 1718±230 Ma. These dates constrain the depositional age of the Wujiatai Formation between 1800 Ma and 1600 Ma. These are the earliest Mesoproterozoic sedimentary records reported in the Kongling region, and fill the gaps in Early Mesoproterozoic stratigraphy in Yangtze Craton. Histograms of detrital zircon ages for the Wujiatai Formation reveal four major peaks at 2039 Ma, 2691 Ma, 2966 Ma and 3377 Ma, which is consistent with the ages of the basement rocks that underlie the center of Yangtze Craton, indicating that sediment provenance is mainly from the Kongling complex. The lower Wujiatai Formation mainly consists of clastic rocks, whereas the upper Wujiatai Formation consists of dolostones. This stratigraphic change implies a deepening sequence in an expanding basin with an initial cratonic rifting tectonic setting, corresponding to the initial breakup of the Columbia supercontinent in Yangtze Craton.© 2019 China Geology Editorial Office.     

江南古陆南缘四堡群同位素地质年代学研究

四堡群浅变质火山-沉积岩系主要出露于桂九万大山地区，是华南最古老的地层单元之一。对坛地区罗布山枕状玄武岩中细粒原生锆石进行单颗粒锆石蒸节-沉积法年龄测定，两组锆石样品获得207Pb/206Pb年龄分别为1374±20（2σ）Ma和1863±7（2σ）Ma。对宝坛地区基性和超基性火山岩进行了Sm-Nd同位素研究，获等时线年龄为1782±820（2σ）Ma，与锆石单颗粒207Pb/206Pb年龄一致，充分说明四堡群形成于中元古代早期。火山岩的εNd（t）为0.6，表明基性-超基性岩浆源自于亏损地幔。这套岩石的     

四顶黑山镁铁质—超镁铁质岩Sm-Nd等时线年龄及其意义

四顶黑山岩体主要由超镁铁质岩和镁铁质岩组成。超镁铁质岩由橄榄岩、角闪橄榄岩、辉橄岩、辉石岩等组成;镁铁质岩由辉长岩、角闪辉长岩、橄榄辉长岩等组成。本次对辉长岩进行了全岩Sm-Nd同位素分析,得到的等时线年龄为327±9.0Ma,初始εNd（t）=＋3.97,表明镁铁质—超镁铁质岩中辉长岩形成于早石炭世末。属于富集地幔向亏损地幔过渡类型。这一年龄的确定,为探讨北山—天山结合部晚古生代镁铁质—超镁铁质岩的成因机制和该区的区域地质演化提供了有效的年代学依据。     

Isotope Geochemistry of Granitoids in South China and Their Metallogeny

Abstract: On the basis of data of initial strontium ratios for over 300 plutons, neodymium isotopes for 240 plutons and oxygen isotopes for more than 200 plutons, the granites of South China are divided into six belts which are different from each other in petrology, geochemistry and mineralization. The similar Nd isotopic composition between the peraluminous granites and Proterozoic crust reveals that there is intermediate to felsic composition crust at mid-crustal depth throughout the South China interior. The six granitic belts mentioned above belong to different lithospheric plates with different composition basement. Differences in the extent and nature of interaction between mantle and crusts in different geodynamic setting and the different extent of differentiation of magma have all controlled the characteristics of the granites and their related mineralization.     

兰坪盆地白秧坪铅锌铜银多金属矿床:成矿年代及区域成矿作用

针对沉积岩容矿铅锌矿床的定年难题,选用闪锌矿的Rb-Sr法和成矿阶段方解石的Sm-Nd法,对白秧坪铅锌铜银多金属矿床3个矿段(吴底厂、李子坪、富隆厂)分别测定,取得了较好的定年效果。测得白秧坪铅锌多金属矿床的铅锌成矿年龄为30~29Ma,而且3个矿段结果可互相印证。进而通过成矿年龄的探讨,认为矿区发生过2次矿化事件,一期为以铜为主的矿化,发生于古新世末-始新世初期,印度板块-欧亚板块强烈挤压后的应力松弛阶段成矿;另一期是以铅锌为主的矿化,发生于30~29Ma,对应于始新世末-渐新世早期挤压后应力松弛阶段。两期矿化事件在上千千米的"三江"成矿带上普遍存在,但在不同部位铅锌成矿时代略有不同,印度板块的东北角首先对接欧亚板块,位于岬角处正碰位置铅锌成矿要稍早于两侧的铅锌矿化事件。     

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。     

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.     

青海玉树东莫扎抓和莫海拉亨铅锌矿床与逆冲推覆构造关系的确定 ——来自粗晶方解石Rb-Sr和Sm-Nd等时线年龄证据

青海玉树地区的东莫扎抓和莫海拉亨铅锌矿床位于青藏高原金沙江缝合带和班公湖-怒江缝合带夹持的羌塘地体东北缘,是"三江"北段铜铅锌多金属成矿带铅锌矿床的典型代表,处于玉树逆冲推覆构造带的前锋带位置,但矿床与逆冲推覆构造之间的关系并不明确。野外研究表明,方解石为乳白色,结晶粗,呈不规则状、囊状,被后期矿化所穿切,形成于挤压(逆冲推覆阶段)到拉伸(成矿阶段)的过渡阶段,是联系逆冲推覆构造与铅锌矿床之间的"纽带"。本文对该阶段方解石开展了Rb-Sr和Sm-Nd等时线方法定年,测定东莫扎抓铅锌矿床过渡阶段的年龄为35.2~35.5 Ma,平均为35.4 Ma,与其成矿时代35 Ma非常接近;测定莫海拉亨铅锌矿床过渡阶段的年龄为34.0~34.6 Ma,平均为34.3 Ma,与其成矿时代33 Ma也非常接近。结合区域成矿地质背景,建立了玉树地区铅锌矿床从逆冲推覆阶段到成矿阶段的构造控矿模式:伴随着印度-亚洲大陆持续碰撞,青藏高原北缘中生代构造岩片向盆地中央推覆叠置,形成逆冲推覆构造(37~40 Ma),之后处于由挤压到拉伸的松弛状态,早期造山带流体、盆地流体混合形成巨晶方解石脉(34~35 Ma),最后,流体从造山带沿拆离滑脱带长距离向前陆盆地方向运移,运移过程中淋滤围岩的金属物质,并与先期造山带流体混合,形成富铅、锌的成矿流体,通过主逆冲断裂垂向沟通,进入浅部的反冲断层形成铅锌矿床(33~35 Ma)。     

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.     

吉林省八家子大型金矿床Rb-Sr同位素测年及同位素地球化学特征

八家子金矿床是华北克拉通北缘夹皮沟金矿带中的重要金矿床之一,赋存于太古宙花岗-绿岩带内。矿体多呈脉状产出,受NW向夹皮沟断裂控制,与中生代正长斑岩有成因联系。本文在八家子矿床选取8件黄铁矿样品开展了Rb-Sr定年,获得Rb-Sr等时线年龄为218.6±1.5Ma(MSWD=1.4),初始Sr同位素比值ISr=0.71047。成矿流体的δ18 OW值为3.2‰~5.1‰,δDW值为-100.7‰~-88‰。成矿流体主要来源于岩浆流体,混合部分变质流体和大气降水。金属硫化物的δ34S值介于-0.2‰~+7.9‰之间,均值+5.7‰,表明硫主要来自深源岩浆。黄铁矿中流体包裹体3 He/4 He值为1.05~1.20Ra,平均值1.13Ra。地幔流体参与成矿作用的比例为11.6%~13.2%,地壳流体占主导地位。八家子金矿的成矿时代为晚三叠世,形成于华北克拉通北缘的碰撞后伸展环境。