Metamorphism of the Amdo metamorphic complex,Tibet: implications for the Jurassic tectonic evolution of the Bangong suture zone

Tibet consists of several terranes that progressively collided with the southern margin of Asia during the Mesozoic following the closure of intervening ocean basins. This Mesozoic amalgamation history, as well as the extent to which it may have contributed to crustal thickening prior to the Cenozoic Indo‐Asia collision, remains poorly constrained and strongly debated. Here, we present a metamorphic petrological and U‐Pb zircon geochronological study of the Amdo metamorphic complex, one of the few exposures of high‐grade metamorphic rocks in central Tibet, located along the Bangong suture between the Qiangtang terrane to the north and the Lhasa terrane to the south. U‐Pb ages of metamorphic zircon in gneiss constrain the timing of peak metamorphism at c. 178 Ma, prior to the Early Cretaceous collision between the two terranes. Peak P–T conditions of gneiss within the metamorphic complex are constrained by conventional as well as multi‐equilibrium (THERMOCALC v.3.21 and v.3.33) geothermobarometry of two samples of garnet‐amphibolite. Whereas THERMOCALC v.3.21 yields similar results as conventional geothermobarometry, THERMOCALC v. 3.33 yields dramatically lower pressures, mostly due to the change in the amphibole activity model used. Using THERMOCALC v.3.21, the two garnet‐amphibolite samples yield similar P–T conditions of 0.83 ± 0.06 GPa at 646 ± 33 °C and 0.97 ± 0.06 GPa at 704 ± 35 °C. Plagioclase coronas on the garnet‐amphibolite sample with lower peak P–T conditions indicate a period of isothermal decompression. Additional geothermometry on two garnet‐free amphibolites yielded similar temperatures of 700–750 °C and suggests similar P–T conditions across most of the complex. However, two exposures of garnet‐kyanite schist located along the southern edge of the metamorphic complex yielded slightly lower peak conditions of 0.75–0.85 GPa and 550–610 °C. Petrographic and field relations suggest the difference in metamorphic grade between the schist and gneiss is due to an intervening thrust fault. The existence of the thrust fault indicates that at least part of the exhumation of the complex was due to contractional deformation, possibly during the Lhasa‐Qiangtang collision. Our P–T–t results indicate the occurrence of a significant Early Jurassic tectonothermal event along the southern, active margin of the Qiangtang terrane that deeply buried the Amdo rocks. We suggest that the metamorphism is a result of arc‐related tectonism that may have been regionally extensive along the southern Qiangtang terrane; geological records of this tectonism may be rarely exposed along strike because of a lack of exhumation or subsequent depositional and structural burial.     

保山地体新元古代——早古生代沉积岩碎屑锆石年代学及其构造意义

滇西潞西地区位于青藏高原东南缘,大地构造位置上属于保山地体。由于新生代强烈的陆内变形作用,保山地体与青藏高原腹地体的对应关系难以确定。野外观察及LA-ICP-MS锆石U-Pb测年结果表明,潞西新元古代—早古生代地层(震旦系—寒武系蒲满哨群及下奥陶统大矿山组)大部分碎屑锆石Th/U0.1,说明其大多为岩浆成因。U-Pb年龄跨度较大,太古宙—早古生代都有分布,且具有明显的562Ma、892Ma及2265Ma年龄峰,以及较弱的1680Ma和2550Ma年龄峰。保山地体潞西地区沉积岩碎屑锆石年龄分布特征与特提斯喜马拉雅、南羌塘沉积地层碎屑锆石年龄分布特征相似,说明其具有相同的物源——冈瓦纳大陆北部的印度大陆。在新元古代晚期—早古生代,保山地体位于印度大陆北缘,与南羌塘、喜马拉雅地体相邻。伴随着俯冲相关的增生造山过程,保山地体形成相应的新元古代末期—早古生代沉积地层。     

Heavy clastic minerals from the Far East island-arc complexes

The results of the study of heavy clastic minerals from the Cretaceous-Paleogene terrigenous complexes of Sikhote-Alin and Kamchatka, as well as from the Cenozoic sediments of the deepwater Vanuatu Trench, are summarized. The data obtained have been interpreted on the basis of their comparison with heavy mineral assemblages of recent sediments deposited in known geodynamic settings. It is shown that the heavy clastic minerals of sedimentary rocks, their relative quantities, and chemical compositions may serve as reliable indicators of different island-arc settings and magmatic processes; these indicators may also be used for identification of such settings in paleobasins of orogenic regions.     

大别山前寒武纪变质地体基本组成

本文以新城－圻春断裂为界将大别山前寒武纪变质地体划分为华北陆块南缘和场子陆块北缘两个次级变质地体，两个次级地体不仅在地球物理，构造变形方面明显不同，而且在物质成分上有显著差异，它们有各自独立的变质地层系统，遭受了不同类型的变质作用，有完全不同的岩浆活动图象，上述差异均可指示华北，扬子两古陆碰掸对接时扬子陆块北缘向北俯冲至华北陆块南缘之下，这可能包括两次合作用，从元古代开始至中生代最终结束的长期复杂     

喀喇昆仑地体甜水海地区102Ma辉长岩的发现及其对区域中生代构造演化的约束

喀喇昆仑地体是青藏高原的重要构造单元。近来,我们对其中的林济塘中生代盆地地层进行了调查,并在喀喇昆仑地体甜水海地区褶皱的中侏罗统龙山组灰岩中发现了辉长岩侵入体。辉长岩具有高Al2O3(15.07%~15.32%)与TiO2(1.31%~1.34%)、贫P2O5(0.19%~0.21%)和低碱(Na2O+K2O=3.49%~3.72%)的特征,明显富集轻稀土(La/Yb=6.17~7.39),富集Th、La、Nd与Tb,亏损Rb、Sr、P与Y,辉长岩Sr-Nd同位素表现出富集的特征(εNd(t)值分别为-5.8与-5.9)。LA-ICP-MS锆石U-Pb测年结果显示辉长岩岩浆锆石206Pb/238U年龄为102±1Ma(MSWD=0.92;n=14),指示岩体形成于早白垩世晚期。辉长岩中含有大量继承锆石,38颗继承锆石年龄分布在2450~232Ma之间,其中16颗为前寒武纪年龄锆石,意味着该地区可能存在前寒武纪基底。这些特征意味着辉长岩在形成的过程中经历壳源物质的强烈混染(Nb/U、Ta/U、Ce/Pb分别为26.6~29.1、1.69~1.76、9.72~11.3)。这种经历壳源物质的强烈混染岩浆的形成可能与软流圈上涌有关,即102Ma辉长岩形成时甜水海地区处于伸展背景。结合区域地质特征,指出102Ma时研究区进入新的伸展与海盆发育阶段,并建立了喀喇昆仑地体中生代三阶段构造演化模型,即侏罗纪-早白垩世早期的正常俯冲与弧盆演化、早白垩世的平板俯冲与造山阶段及晚白垩世的正常俯冲与弧盆演化。     

福建长乐-东山构造带的变形特征及动力学研究

福建长乐—东山构造带是我国东南沿海中生代NE向线性构造带的一部分。从西向东分为三个亚带:福清—云霄钙碱性火山岩带(J_3);长乐—东山NE向韧性剪切带(K_1); 平潭—东山变质地体(P_z—J_3)。火山岩带以浅层构造、脆性挤压变形为特征。变质地体为一外来体,以深层构造、多期(即前J,J,K_1)叠加变形为特征,并于J_3末增生到欧亚大陆边缘。韧性剪切带是叠加在变质地体增生带上,为一左行平移简单剪切带。     

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO₂‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X_Mg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative ε_Nd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).     

三维古地貌重建技术在中生代羌塘地体古地理演化分析中的应用

近年来,从古地貌-沉积组合与油藏关系的角度进行油气远景评价成为研究热点。针对古地貌的复原研究,提出一种建立三维古地貌模型的方法,将二维平面古地理图通过对资料的整理、挖掘与转换,并结合GIS技术生成了三维立体模型,补充了区域地形地貌的相关信息。利用该方法,建立了青藏高原油气资源潜力最大的区域——羌塘地体在三叠纪和侏罗纪的一系列三维古地貌模型,进而对其古环境和演化过程进行分析,这对于该区域的沉积矿产资源的预测与评价有重要的参考价值。     

Cretaceous–Tertiary geology of the Gangdese Arc in the Linzhou area, southern Tibet

Knowledge of the Cretaceous–Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous–lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located  30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic–Jurassic strata and a granite intrusion in the hanging wall. U–Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at  52 Ma, coeval with dike emplacement into footwall Cretaceous strata. ⁴⁰Ar/³⁹Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at  42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab.     

东北亚跃进山拼贴带及邻域岩石圈结构特征的地电学证据

为进一步查清东北亚完达山地体与两侧的佳木斯地块、兴凯地块间接触关系以及对跃进山拼贴带域沉积盆地油气条件的控制作用,布设5条MT剖面,计长1104 km.结果显示:(1)研究区地电学结构主要特征为佳木斯地块从中部向南北两方向,这一稳定地块的“高阻核”减薄,兴凯地块的高阻块体自东而西逐渐加深增厚,完达山地体在南部具有较多的高阻块体,向北渐少.(2)完达山地体西南端呈一“牛角”形状范围把佳木斯地块和兴凯地块分开;与原认识的完达山地体、佳木斯地块在同江-宝清-当壁一带贴合的位置相比,从宝清开始向南移至麻山-桦林一带.(3)完达山地体构造成因包括两部分,一是俯冲引起的拼贴,二是软流圈物质的热上涌;佳木斯地块内所存在的“高阻核”边界可能是该地块东界.(4)4个沉积盆地除勃利盆地外,另外3个盆地的基底间或存在规模不大的高阻块,大多范围分布着高导带,对盆地的成烃环境是有利的.