Geochronology of the Hannan intrusive complex to adjoin the Qinling orogen and its rapid cooling reason

The geochronological works for the Hannan intrusive complex, cosisting of the gabbro-quartz dio-rite-plagiogranite bodies, in the north border of the Yangtze craton block, adjoining the Qinling orogen, are reported, because its age and genesis are of great importance to research of the tectonic evalutional history for the Qinling orogen and of the rifted processes for the Late Proterozoic supercontinent Rodinia. A good isochron of t=(837±26) Ma (2σ), corresponding to INd=0.51165±2(2σ), εNd(t)=+1.9, MSWD1.02, was difined by the Nd isotopic analytical results for the 21 whole-rock samples from the varied lithological complex bodies, but for the Rb-Sr isotopic aralytical data there is no isochron. The 40Ar/39Ar plateau age of the biotite, sorted from plagiogranites, is (796±20) Ma(2σ), and yet for the Rb-Sr isotopic data of the plagiogranite whole-rock samples (WR) and the sorted biotite (Bio), plagioclase (Plag) and apatite (Apt) mineral samples from the plagiogranitewhole-rock samples, an isochron of t=(824.86±3.8) Ma (2σ) with ISr=0.70393±14(2σ), MSWD2.44 is given. The U-Pb isotopic results for the single zircon, sorted from pla-giogranite samples, yielded an upper and a lower intercept ages of -876 Ma and -273 Ma. The Nd isotope data of complex indicate that it is probably the products of crystallization differentiation from the magma with 3333Nd>0 when the rapid uplift in 837-800 Ma took place in the northern border of the Yangtze craton block.     

Geologic relationships and geochronology of the Cenozoic volcanoes and interbedded weathered mantles of Yulinshan in Qiangtang, North Tibet

The Yulinshan accumulations of volcanic rocks and interbedded weathered mantles provide a unique chance for studying the processes and environment of the formation of the planation surface in central Tibetan Plateau. Geochemical,⁴⁰Ar/³⁹Ar and K/Ar geochronologic investigations of the weathered mantles and high-potassium volcanic rocks reveal that the volcano-weathered mantle accumulations formed under arid or semi-arid subtropic environment in 30–24 Ma. The sole planation surface of the Qiangtang region finally formed in about 20 Ma.     

辽东青城子矿田煌斑岩锆石U-Pb年龄及其地质意义

为了深入了解辽东青城子矿集区煌斑岩岩石成因、形成的大地构造环境和地质意义,通过单颗粒锆石LA-ICP-MS U-Pb定年系统和X射线荧光光谱仪,对青城子矿集区煌斑岩进行锆石U-Pb年代学和主量元素地球化学方面研究.结果表明,煌斑岩具有低硅、富镁铁的特点,煌斑岩侵位年龄为(226.0±1.0)M a~(227.7±1.3)M a,属于晚三叠世岩浆活动的产物.结合区域构造背景和前人研究,推测煌斑岩是扬子克拉通-华北克拉通深俯冲过程的板片断离的产物,扬子克拉通的深俯冲和陆-陆碰撞可能是华北克拉通东部在中生代破坏的主要原因.     

Zircon SHRIMP geochronology of the Xinkailing-Kele complex in the northwestern Lesser Xing＇an Range, and its geological implications

Located in the eastern portion of the Xing'an-Mongolian Orogenic Belt (XMOB), the Xinkailing-Kele complex has previously been considered to be Precambrian metamorphic rocks, mainly according to its relatively high metamorphic grade. Our filed observation, however, revealed that the complex is composed mainly of metamorphic rocks (Kele complex), tectono-schists ("Xinkailing Group"), and granitoids (Xinkailing granitic complex). Dating on these rocks using advanced SHRIMP zircon U-Pb technique indicates that: (1) Biotite-plagioclase gneiss from the Kele complex has a protolith age of 337±7 Ma (2σ) and a metamorphic age of 216±3 Ma (2σ); (2) the tectono-schist of the "Xinkailing Group" gave a magmatic age of 292±6 Ma (2σ), indicative of felsic volcanic protolith of the schist formed in late Paleozoic time; and (3) the Menluhedingzi and Lengchuan granites of the Xinkailing granitic complex were emplaced at 167±4 (2σ) and 164±4 Ma (2σ), respectively. These results suggest that the Xinkailing-Kele complex is not Precambrian metamorphic rocks and the so-called Precambrian "Nenji-ang Block" does essentially not exist. In combination with regional geological data, we propose that the Kele metamorphic complex is likely related to a collisional tectonism that took place in Triassic time, as indicted by its metamorphic age of 216±3 Ma. The Xinkailing granitic complex was em-placed along the collisional zone during Mid-Jurassic time, likely in a post-orogenic or anorogenic setting.     

SHRIMP U-Pb geochronology of the detrital zircons from the Longshoushan Group and its tectonic significance

Sixty-two geologically meaningful U-Pb dates were obtained by using SHRIMP technique for the detrital zircons in three metasedimentary rocks from stratigraphically uppermost parts of the Longshoushan Group in the present study. Eighty percents of these dates range from 1.7 Ga to 2.2 Ga with a peak at 1.8-2.0 Ga and twenty percents from 2.3 Ga to 2.7 Ga. The youngest detrital zircon is dated at 1724±19 Ma which is interpreted as the maximum depositional age of the metasedimentary rocks. Therefore, the age for the diagenesis and lithification of the original sedimentary rocks of the Longshoushan Group before the metamorphism must be younger than 1724±19 Ma. Comparison of the age histograms of these detrital zircons with the ages of the igneous rocks on the surrounding older massifs suggests that the sediments of the Longshoushan Group were most likely derived from the Alaxa Block and Tarim Craton. This implies that the affinity between Alaxa Block and Tarim Craton was strong and that they might have been a unified craton during middle-early Proterozoic time.     

Zircon SHRIMP U-Pb geochronology and petrogenesis of the plagiogranites from the Lagkor Lake ophiolite, Gerze, Tibet, China

Field relationship, along with petrographical, petrological, and geochemical characteristics, and zircon populations, indicates that the plagiogranites from the Lagkor Lake ophiolite in the Bangong-Nujiang suture zone (~ 20 km south of Gerze County, central Tibet, China), derived from anatexis of hydrated gabbros by ductile shearing during transport of the oceanic crust. Therefore, this kind of plagiogranite is coeval to or little later than the spreading of the oceanic crust. Zircon SHRIMP age of 166 ± 2.5 Ma for the plagiogranites indicates that an oceanic basin existed in the western part of the Bangong-Nujiang zone in the Middle Jurassic.     

^40Ar-^39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43--43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.     

Precise timing of the Early Paleozoic metamorphism and thrust deformation in the Eastern Kunlun Orogen

In Dulan County, Qinghai Province NW China, the arc volcanic sequences in the northern side of the Central Fault of the East Kunlun were metamorphosed progressively from upper greenschist facies in the south to epidote-amphibolite facies in the north. High-angle thrust deformation was developed synchronously with the peak metamor-phim and superimposed with later low-angle striking-slip deformation. Zircon U-Pb dating yields a concordant age of(448 ± 4) Ma for the metavolcanics. Syn-kinematic horn-blende and muscovite separated from the high-angle thrusting belt give ⁴⁰Ar-³⁹Ar plateau age of (427 ± 4) Ma and 408 Ma, respectively. These results precisely constrain the timing of the closure of early Paleozoic volcanic basin (Proto-Tethys) over the eastern portion of the East Kunlun Orogen, and the thrust tectonic slice had a cool rate of ca. 9℃/Ma. Keywords: East Kunlun Mountains, metamorphic and deformation event, geochronology.     

20世纪末激光显微探针定年把K-Ar年代学推向了新的里程碑

已有50年历史的K-Ar定年法,由于过剩Ar和Ar丢失的普遍发现,使其最广泛的应用面临着严重挑战。在20世纪的最后几年,以激光显微探针⁴⁰Ar/³⁹Ar定年方法的完善和精度的提高为标志,把K-Ar年代学研究推向了一个新高度。微区微量高精度高分辨定年,把定年时限扩展到人类历史范畴,精细的分析技术拓宽了年代学的应用范围,使之解决的地质问题更广泛。     

西秦岭光头山花岗岩锆石U-Pb年代学及其地质意义

光头山花岗岩体出露于勉略缝合带北侧, 主要由英云闪长岩和二长花岗岩组成。英云闪长岩表现为片麻状构造, 局部英云闪长岩糜棱岩化形成花岗质糜棱岩。而二长花岗岩在糜棱岩带形成之后侵位, 含有少量的石榴石,弱的片麻状到块状构造。LA-ICPMS 锆石原位U-Pb同位素定年结果表明, 光头山岩体为两个阶段侵位, 糜棱岩化英云闪长岩( 样品GT18-01)的侵位结晶年龄是221±6Ma, 而二长花岗岩(样品GT11-01)的结晶年龄是199±4Ma,代表了晚期二长花岗岩形成的时代。结合区域构造背景和前人研究的地球化学特征, 早期的英云闪长岩可能在勉略洋盆闭合前的岛弧发育阶段侵位, 代表了洋壳俯冲的弧岩浆活动的产物。然后扬子地台与秦岭微陆块拼合, 形成勉略缝合带。约199Ma秦岭主造山期同碰撞岩浆活动形成了晚期(石榴石)二长花岗岩。因此, 勉略洋盆闭合和勉略缝合带形成时期大约为221～199Ma 。