Characteristics of Blueschist in Shuangjiang Tectonic Mélange Zone,West Yunnan Province

Glaucophane in Shuangjiang area, West Yunnan Province, supplies a chance for studying south segment of Lancangjiang tectonic zone. But people are at odds as to whether two stage glaucophane exists or not, glaucophane is the result of dynamic metamorphism later, or indicates a high P/T metamorphic belt when Paleozoic Tethys Sea closed. Authors discover in a recent research that there is only one stage glaucophane in Shuangjiang area, and three blueschist belts are distributed near N S tending, and glaucophane in Shuangjiang area is related to the eastward subduction of Changning Menglian basin.

40Ar-39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang, Northern Tibet, Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China （84 30＇ E, 34024＇ N）, provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite- and glaucophane-bearing assemblages equilibrated at 375-400 C and 11 kbar. 4Ar-39Ar analysis on mineral separate from one blueschist sample yielded a well-defined plateau age of 242 Ma. Geochemical studies show the blueschist is metamorphosed within-plate basalts. The high pressure-low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in situ suture between eastern and western Qiangtang.

THE GEOCHEMISTRY CHARACTERISTICS OF BLUESCHIST IN THE GANGMAR AREA, QIANGTANG BLOCK, TIBET

The Qinghai—Xizang (Tibet) Plateau was assembled by the successive accretion of continental and arc terranes to Asia during the Palaeozoic, Mesozoic and early Tertiary. From north to south, the Plateau was divided into four terranes (from north to south): Songpan\|Garze, Qingtang, Lhasa and India terranes. The Qiangtang terrane was divided into two parts by Chabu\|Shuanghu suture: Northern Qiangtang and Southern Qiangtang blocks. It was believed that there is a suture as Palaeotethyan suture between the two blocks from Lungmu lake to Chabu, to Shuanghu, to Langcangjiang, but some geologists opposited this opinion, such as Wang Chengshan et al.(1987) and Deng Wanming et al.(1996). The blueschists that bear the typical glaucophane were found in the Gangmar Area in summer of 1997. The blueschists consist of glaucophane, crossite, phengite, actinolite, epidote, sphene ,rutile, and so on. Blueschists show nappe structures in enclosing rocks, and thrust over Tertiary from north to south.The age of blueschist is 275～285Ma which stand for the metamorphic age of blueschist. The geochemical contents of blueschists are the following: SiO\-2 is 45 37%～53 82%, Na 2O>K 2O, K 2O+Na 2O are 3 72%～7 61%, Na 2O/K 2O is 1 70～19 67, MnO is 0 16%～0 22%, FeO+Fe 2O 3 are 11 12%～14 27%, TiO 2 is 2 34%～4 28%, P 2O 5 is 0 34%～1 03%. From the contents, SiO 2 is inverse relation with K 2O+Na 2O, K 2O and FeO+Fe 2O 3. In the SiO 2\|K 2O+Na 2O diagram, the blueschists fall in the field of Alkali basalt; in the TiO 2\|MnO(10\|P 2O 5×10 triangular diagram, these plot within the Ocean Island Basalts(OIB) field. The whole REE content is (117 03～390 97)×10 -6 , LREE/HREE is 5 05～9 80, the REE diagram shows the feature that LREE is rich and HREE is depleted, this is similar to the OIB. In the spidergram of trace elements show the same characterristics as the OIB. Meanwhile, the trace elements ratios, such as Ba/Nb, La/Nb, Nb/Th, Nb/Y, Zr/Y, Zr/Nb, Hf/Sm, Ta/Hf, Nb/Ta, Nb/U, suggest the tectonic setting is OIB. On the basis of these materials, the blueschists are akin to within\|plate ocean island basalts. This shows that there is a ocean or oceanic basin in the Chabu\|Shuanghu belts and the formation of blueschists is the quicky subduction of the Ocean Island Basalts in Early Permian between the Gondwana and Eurasia continents. It demonstrated, therefore, that the Chabu\|Suanghu suture exists and may extend to the east to Lnagchangjiang, and may extend into Thailand\|Malaysia Peninsula.

The^(40)Ar/^(39)Ar age record of formation and uplift of the blueschists and eclogites in the western Tianshan Mountains

The 40Ar/39Ar ages indicate that the eclogite facies rocks of the Hasiate slice in the western Tianshan Mountains were formed at the early stage of Devonian （401 Ma） and had been uplifted to the greenschist facies tectonic level in the middle stage of Devonian （381 Ma）. The formation and uplift of the blueschists of the Akesayi slice are constrained to the late stage of Devonian （370-364 Ma）. The different tectonic slices in the high-pressure metamorphic belt have experienced the different uplift history.

Petrology and ^(40)Ar/^(39)Ar iso-topic ages of blueschists in Gangmar, central Qiangtang, northern Tibet

The petrology and mineral assemblages of blueschists in the Gangmar, central Qiangtang, northern Tibet were examined, which indicates that the metamorphic condition is high-pressure low-temperature. In this note, we reported the 40Ar/39Ar dating results of glaucophane from two blueschist samples. Their apparent ages are (275.0±1.3) Ma and (287.6±2.3) Ma and similar isochron ages are (275.0±0.9) Ma and (282.4±0.8) Ma, respectively. These iso-topic datings show the high-pressure metamorphism occurring in Lower Permian. The ages also coincide with the evolution of Palaeo-Tethys ocean in Late Palaeozoic.

Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone,Southern Tibet

The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 ℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16 ℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.

The ^(40)Ar/^(39)Ar Ar metamorphic ages of Tangbale blueschists and their geological significance in West Junggar of Xinjiang

SINCE Tangbale blueschist was found in West Junggar of Xinjiang in 1983, its metamorphicage and tectonic significance have been discussed for a long time. However, no exact dat-ing has been done by now. In this note, we present the 40Ar/39Ar data of sodic amphibolesfrom the blueschists and discuss their possible geological significance.

The ^(40) Ar/^(39) Ar age and its significance of the crossite from the blueschists in the mid-Qiangtang Area, Tibet

1 Sampling background and dating means THE blueschists in the mid-Qiangtang area are situated in Guoganjianian Mt., Qoum Mt., Naro, Qiagelela, etc. They are distributed east-westward, up to 300 km long. Their northern flank is the plate suture zone of Longmu Co-Shuanghu. The mother rocks of the blueschists are the late Carboniferous sandstones, siltstones, basalts, few sandwiching limestones, many ice-sea mix-conglomerates, etc.

Discovery of deerite from the Aksu Precambrian blueschist terrane and its geological significance

Deerite [Fe122+ Fe63+ Si12O40（OH）10] was first discovered in the metamorphic magnetite-bearing quartzites which are interlayered with blueschists in Aksu Precambrian blueschist terrane. The deerite-bearing mineral association includes ① deerite+riebeckite+stilpnomelane+ilvaite+magnetite+quartz and ② deerite+stilpnomelane+magnetite +quartz based on the investigation under microscope. The study of mineral chemistry shows that the deerites from Aksu Precambrian blueschist are Mn-poor deerite similar to that from Alps. But the deerite from Aksu is the nearest to the end member of deerite. According to the equilibrium P-T region of deerite determined by Lattard and Breton （1994）, the deerites in metamorphic magnetite-bearing quartzites from Aksu Precambrian blueschists were formed under the conditions of pressure lower than 1.0 Gpa, temperature ranging from 300 to 400℃ and about the 10℃/km geothermal gradient. The deerite in metamorphic magnetite-bearing quartzites from Aksu

^(40)Ar/^(39)Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma.The isochron age obtained from phengite in epidote blueschist is 415±7 Ma.These data,combining with peak metamorphic P-T conditions and regional geological setting,allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca.413-415 Ma,which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone.This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma,which also represents the convergent age between the North China Craton and the Qaidam block.

PETROLOGY OF BLUESCHIST IN WESTERN YUNNAN PROVINCE,CHINA,AND ITS TECTONIC SIGNIFICANCE

The blueschists in western Yunnan Province are distributed in Lancangjiang and Ailaoshan metamorphic belts and occur as interlayer in green schist and mica schist. Their protolith is basaltic and pelitic rocks with a mineral assemblage of Cr (or Win)+Ab+Qz+Chl+Ph+Ep+Sph+Act and Gl+Ph+Tc+Ep+Alm+Qz+Do, respectively. The P-T conditions of blueschist metamorphism range from 170—443℃ at c. 4—6. 7 kbar. The origin of blue schist of the Lancangjiang belt is related to the subduction and the close of the paleo-Tethys oceanic basin. The Ailaoshan blueschist, however, may be related to the continent continent collision and the dynamic metamorphism.

Blueschist from the Toudaoqiao Area, Inner Mongolia, NE China: Evidence for the Suture between the Ergun and the Xing'an Blocks

Blueschist accompanied by pelitic schist expose along the Xinlin-Xiguitu fault in the Toudaoqiao area, northeastern China. In this paper, the blueschist is systematically studied on the petrography and mineral chemistry. The amphiboles in the blueschist are zoned from winchite core to magnesioriebeckite/glaucophane rim to winchite outermost rim. The peak metamorphic conditions are defined by the mineral assemblage of magnesioriebeckite/glaucophane, epidote, high-Si phengite（Si〈7.1）, chlorite, albite, hematite and quartz, indicating an epidote-blueschist facies metamorphism. The P-T conditions are estimated as T=350–400 ℃ and P=10-12 kbar. The occurrence of the blueschist along the Xinlin-Xiguitu fault strongly suggests the fault is the suture between the Ergun and the Xing＇an blocks situated in the eastern portions of the Central Asia Orogenic Belt（CAOB）.

Blueschist:A window into high-pressure/low-temperature metamorphism and subduction zone dynamics

Blueschist is a regional metamorphic rock formed under high-pressure(HP)low-temperature(LT)conditions.It is formed in the subduction zone environment with low geothermal gradients(4-14℃km^(−1)),and is characterized by the presence of HP/LT index minerals like glaucophane,lawsonite,aragonite,jadeite,and deerite.In general,blueschist-facies rocks are stable in subduction zones at depths of 30-60 km,and transform to eclogite-facies rocks at greater depths.The preservation of blueschists requires a fast exhumation rate.Based on protolith and tectonic setting,blueschists can be grouped into Type-A and Type-B,but some metasomatic blueschists also occur.Blueschist belts distribute mainly as bands along the margins of orogenic belts,and their occurrences within cratons are very limited.Precambrian blueschists,of which the oldest ones are about 800-700 Ma in age,are rare;most of the exposed blueschist terranes are of post-Paleozoic metamorphic age.As diagnostic evidence of ancient subduction zones,blueschist plays an important role in understanding plate tectonics.Blueschist-eclogite transition at cold subduction zones involves dehydration reactions and fluid release,which are of great importance in facilitating slab-mantle wedge water and element recycling,generating arc magmatism,and triggering intermediate-depth earthquakes in the subducting slab.Metamorphic P-T paths of blueschists and associated rocks provide key information on constraining the onset of the subduction initiation and subsequent geodynamic evolution.As a cold geothermal indicator,the emergence of blueschist offers robust evidence for the start of modern plate tectonics on the Earth.Blueschist-facies metamorphism still represents an important research direction in metamorphic geology,which requires further investigations on determining the beginning of plate tectonics,constraining phase-transition processes,and constructing a global blueschist database.

早古生代北阿尔金HP/LT混杂岩片--一个“化石”俯冲隧道

北阿尔金高压/低温(HP/LT)变质岩呈构造岩片分布在俯冲-增生杂岩中,主要由强变形的变质沉积岩(泥质片岩、钙质片岩和石英片岩)和少量呈透镜状分布在变沉积岩中的蓝片岩和榴辉岩组成,与相邻的蛇绿混杂岩呈断层接触。榴辉岩主要矿物为绿辉石、石榴子石、多硅白云母、石英、冻蓝闪石,含少量蓝闪石、绿泥石、方解石、钠长石、榍石。蓝片岩主要矿物为蓝闪石、石榴子石、碳酸盐类矿物、阳起石、绿帘石、钠云母、绿泥石、钠长石和石英,偶见多硅白云母,其中在部分蓝片岩的石榴子石中有少量硬柱石和绿辉石包体。本文对蓝片岩(样品A06-16-7)和榴辉岩(样品A03-3-5.3)开展了岩石学和相平衡模拟研究,得到它们形成的压力峰期的温压条件分别是:T=~524℃、P=~2.1GPa和T=~527℃、P=~2.2GPa,并均经历了后期蓝片岩相的退变质叠加。结合区域上已有的研究表明,北阿尔金HP/LT混杂岩片中不同类型岩石可能经历了不同的变质演化历史,反映了古俯冲隧道的不均匀性,并在俯冲隧道的较浅部混杂在一起,共同经历了蓝片岩相或蓝片岩-绿片岩过渡相条件下的透入性变形作用。

台湾中央山脉东部变质作用及热演化:玉里缝合带的俯冲折返作用

中国台湾中央山脉东部出露的玉里变质带作为板块构造的缝合带,拼接了欧亚大陆板块、俯冲的南海板块、未俯冲的弧前基底和菲律宾海板块(吕宋岛弧),是认识台湾造山运动地球动力学重建的关键。玉里变质带出露含蓝片岩相的铁镁质—超铁镁质变质火成岩块体,这些块体在构造上被以绿片岩相为主的多期变形的云母石英片岩所包围。而玉里带东南侧发育了以云母石英片岩、千枚岩为主的初来组地层,初来组地层是否属于玉里带近年来仍然存在争议。为了解决这一问题,本文对台湾玉里带及其周缘地区的构造演化重新研究,针对中央山脉东部玉里带和初来组地层分别采样,利用碳质物质拉曼光谱温度计(RSCM)计算出变质片岩峰值变质温度的均值。结果表明:玉里带的峰值变质温度范围在400℃~550℃之间,比台湾地区的其它次级构造单元温度高;玉里带内,峰值变质温度高于500℃的地区毗邻玉里带的3个最大的高压变质火成岩块体,说明高压变质块体可能存在与围岩之间的交代变质作用;初来组地层的峰值变质温度约为360℃,与玉里带的温差达100℃以上,说明初来组地层与玉里带是两个不同的次级单元,在中央山脉东南缘二者之间很可能是断层接触,在台湾造山带向东的反冲褶皱逆冲带变形之后,这两个单元可能经历了相同的形变历史。本次更新的峰值变质温度集合了台湾地区的所有已发表的RSCM数据,显示出横跨台湾东部中央山脉热演化的系统性空间展布,重新修正了台湾中央山脉东部地质图。

阿克苏地区前震旦纪蓝片岩原岩恢复及地球化学特征

据岩石、矿物组合,首先将地层划分为阳干山岩组绿片岩相带和阴干山岩组绿片-蓝片岩相带;通过地球化学方法,对阿克苏群蓝片岩进行分析,确定绿片岩相带原岩为碎屑岩,蓝片岩相带原岩以基性玄武岩为主,含少量砂岩类碎屑岩。为后期研究相同岩石提供基础地质资料。

滇西南景洪大勐龙地区蓝片岩岩石学、变质作用及其对古特提斯构造响应

【研究目的】蓝片岩为古洋壳的俯冲及洋-陆和陆-陆碰撞拼贴的产物,对深入探索洋壳或陆壳俯冲变质过程以及造山带的形成演化具有十分重要的意义。本文选择以滇西景洪大勐龙地区新发现的蓝片岩为研究对象,探讨其对昌宁—孟连古特提斯洋俯冲-造山背景的制约。【研究方法】通过岩相学与矿物化学分析,根据矿物组合的不同,识别出研究区内两种不同类型的蓝片岩,初步建立变质演化P-T轨迹,并结合区域地质资料分析蓝片岩的成因机制与就位的动力学背景,约束昌宁—孟连古特提斯洋俯冲-造山演化。【研究结果】获得白云母石榴蓝闪石英片岩峰期变质温压条件为T=450~490℃,P=0.90~1.45 GPa,绿泥绿帘阳起蓝闪片岩具有较低的峰期变质温压条件为T=430~470℃,P=0.69~0.73 GPa,并初步建立了两种不同类型蓝片岩变质演化P-T轨迹。【结论】两类峰期变质条件的差异可能为俯冲隧道中不同俯冲深度的蓝片岩,这一研究成果为进一步探索昌宁—孟连古特提斯洋的俯冲消减-闭合的构造演化过程提供重要依据。

锡林浩特杂岩和蓝片岩的锆石U-Pb年代学及其对索仑缝合带演化的意义

报道了内蒙索仑缝合带附近的锡林浩特杂岩和苏左旗混杂带中蓝片岩块的锆石U-Pb年代学数据。锡林浩特杂岩中碎屑锆石来源复杂,从晚太古至晚古生代均有。最年轻的碎屑锆石U-Pb年龄为280～340Ma,与该区最年轻的弧岩浆岩年龄一致,表明该杂岩的原岩最终形成于晚古生代,而不是前寒武陆块残留。锡林浩特杂岩原岩的沉积源区主要是古生代的弧岩浆岩和部分裸露的前寒武陆壳,可能是弧前沉积建造。蓝片岩的锆石U-Pb年龄为318±5Ma(范围338～280Ma),这些锆石显示典型岩浆振荡环带结构,因此该年龄可能代表其原岩形成年龄,而其变质应该在280Ma之后。所以,锡林浩特杂岩和混杂带中蓝片岩块的变质变形作用可能发生在晚二叠,与索仑缝合带强烈的孤陆碰撞有关,暗示索仑缝合带的碰撞缝合时间在晚二叠,而不是许多人坚持的泥盆纪。

西藏中北部双湖地区蓝片岩带及其构造涵义

西藏中北部的蓝片岩呈岩块断续出露长达300 km.东起西亚尔岗双湖及其以西的恰格勒拉,经纳若、角木日、冈塘错、果干加年山、玛依岗日,西至冈玛错一带。除恰格勒拉及果干加年山的蓝片岩早年有简要报导外,其余均为李才等近年的新发现。蓝片岩的原岩主要为冈瓦纳大陆北缘晚石炭世-早二叠世裂谷型(裂陷槽)沉积-火山岩系,包括基性火山岩(或火山碎屑岩)、泥质砂岩、泥质灰岩等,尚有少量基性煌斑岩。高压低温变质矿物主要为青铝闪石,次为镁钠闪石,少量蓝透闪石。缺失典型蓝片岩相的硬玉和硬柱石。根据钠质角闪石类矿物成分所估算的变质压力为0.67～0.75 GPa,变质温度为410～460 C,视为蓝片岩与绿片岩之间的过渡相更为确切。 蓝片岩典型的矿物组合为:(1)绿泥石+绿帘石+青铝闪石+钠长石;(2)绢云母+青铝闪石(±镁钠闪石)-蓝透闪石-斜长石+石英;(3)方解石+青铝闪石-绿帘石:(4)绿泥石-透闪石+蓝透闪石(±单斜辉石残晶)。 蓝闪石的^(46)Ar-^(48)Ar年龄值为222.5±3.7 Ma,故其变质年龄为晚古生代-早中生代。高压变质带及其伴随的“蛇绿混杂岩”很可能是冈瓦纳古陆北缘边缘海内裂陷槽或“初始有限洋盆”从离解到汇聚消亡的地质记录,龙木错-冈玛错-双湖-丁青-澜沧江一带作为冈瓦纳古陆与欧亚古陆分界地带.基本是可取的。

黑龙江杂岩的碎屑锆石年代及其大地构造意义

黑龙江杂岩带位于佳木斯地体西缘,为佳木斯地体向西与松嫩地体之间俯冲、拼贴、碰撞而形成的高压变质带。黑龙江杂岩沿牡丹江断裂分布,其构造-岩石组合、变质变形特征等显示其为佳木斯地体向松嫩地体俯冲拼帖的过程中形成的增生杂岩,目前保存下来的杂岩带应为大规模增生楔仰冲到佳木斯地体之上的残余部分。88颗碎屑锆石的全部样品SHRIMPU-Pb年代学测试结果显示三个主要年龄区间:170～220Ma,峰值年龄为183Ma;240～338Ma,峰值年龄为256Ma;450～520Ma,峰值年龄为470Ma。而28个碎屑锆石谐和年龄的年龄谱为两组:240～338Ma,峰值年龄为256Ma;450～500Ma,峰值年龄为470Ma。碎屑锆石年龄数据分析得到,240～338Ma峰期年龄为256Ma的年龄应代表黑龙江杂岩主体岩石的沉积年龄上限;而450～500Ma的年龄谱对应于佳木斯地体的基底变质岩年龄,显示佳木斯地体的基底变质岩曾为黑龙江杂岩的物源区;而170～210Ma,峰期年龄为183Ma的不谐和年龄应为受印支期-早侏罗世构造热事件的扰动年龄,与该区变质单矿物的Ar-Ar年龄相一致,应代表了该区陆-陆碰撞的时代。上述年龄为黑龙江杂岩的形成与演化提供了重要的地质年代学制约,即黑龙江杂岩的原岩成岩时代上限为早三叠世,佳木斯地体向西的俯冲时代主体为印支期,而陆-陆拼贴及碰撞过程主要为晚印支期并可能持续到早侏罗世。这些结果将为揭示我国东北地区构造演化的年代学格架以及三叠纪古亚洲构造域向环太平洋构造域叠加和转换的动力学背景研究提供新的基本地质事实依据。