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.

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.

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.

Field report:Sailing around the exhumed roots of the Mesozoic Patagonian paleo-accretionary wedge(Diego de Almagro Island,Chile)

We undertook a boat expedition to explore the geological framework of a very remote, lesser-known island, in the Chilean Patagonia: the Diego de Almagro Island(latitude S51°330'). This uninhabited, ca.400 km^2 Island is one of the very rare exposures of the Mesozoic accretionary subduction complex along the Chilean margin. Unstable weather, strong winds, steep topography, and very dense vegetation make an on-land mission difficult. Careful preparation based on high-resolution satellite images is advised to optimize shore access and minimize risks of injury. Despite a relatively important degree of regional reequilibration of metamorphic assemblages due to sluggish exhumation through the forearc crust, our results have shown that the island is composed of a nappe stack of ocean-floor derived slivers of metasedimentary units that exhibit very different pressure-temperature-time paths during burial by subduction under the Chilean margin and subsequent exhumation. These rocks are witness to a complex thermal evolution of the subduction zone between Jurassic and Cretaceous times from granulite facies to blueschist facies conditions as well as multiple episodes of accretion at ca. 35 -40 km in depth for almost100 Ma over the Mesozoic era.

Apatite fission track thermochronology in the Kuluketage and Aksu areas,NW China:Implication for tectonic evolution of the northern Tarim

Tarim Precambrian bedrocks are well exposed in the Kuluketage and Aksu areas, where twenty four samples were taken to reveal the denudation history of the northern Tarim Craton. Apatite fission track dating and thermal history modeling suggest that the northern Tarim experienced multi-stage cooling events which were assumed to be associated with the distant effects of the Cimmerian orogeny and India-Eurasia collision in the past. But the first episode of exhumation in the northern Tarim, occurring in the mid-Permian to Triassic, is here suggested to be induced by docking of the Tarim Craton and final amalgamation of the Central Asian Orogenic Belt. The cooling event at ca. 170 Ma may be triggered by the Qiangtang-Eurasia collision. Widespread Cretaceous exhumation could be linked with docking of the Lhasa terrane in the late Jurassic. Cenozoic reheating and recooling likely occurred because of the northpropagating stress, however, this has not affected the northern Tarim much because the Tarim is characterized by rigid block-like motion.

黑龙江依兰地区黑硬绿泥石片岩岩石学与P-T条件研究

依兰地区黑龙江杂岩中黑硬绿泥石片岩矿物组合为黑硬绿泥+绿泥石+多硅白云母+钠长石+透闪石+钾长石±黑云母。岩石矿物学研究表明黑硬绿泥石片岩形成于250～400℃,6～9kbar的变质条件下。黄褐色黑硬绿泥石呈束状、放射状集合体。黑硬绿泥石片岩形成于佳木斯地块向西与松嫩地块俯冲拼贴过程中蓝片岩变质作用的后期,压力稍有降低而温度略有升高的变质变形环境,是蓝片岩向绿片岩转变的蓝闪绿片岩相变质条件下的产物。

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.

Thermal evolution of an ancient subduction interface revealed by Lue Hf garnet geochronology, Halilba■? Complex(Anatolia)

The thermal structure of subduction zones exerts a major influence on deep-seated mechanical and chemical processes controlling arc magmatism, seismicity, and global element cycles. Accretionary complexes exposed inland may comprise tectonic blocks with contrasting pressureetemperature(Pe T)histories, making it possible to investigate the dynamics and thermal evolution of former subduction interfaces. With this aim, we present new Lue Hf geochronological results for mafic rocks of the Halilbag?Complex(Anatolia) that evolved along different thermal gradients. Samples include a lawsoniteeepidote blueschist, a lawsoniteeepidote eclogite, and an epidote eclogite(all with counter-clockwise Pe T paths),a prograde lawsonite blueschist with a "hairpin"-type Pe T path, and a garnet amphibolite from the overlying sub-ophiolitic metamorphic sole. Equilibrium phase diagrams suggest that the garnet amphibolite formed at w0.6 -0.7 GPa and 800 -850℃, whereas the prograde lawsonite blueschist records burial from 2.1 GPa and 420℃ to 2.6 GPa and 520℃. Well-defined Lue Hf isochrons were obtained for the epidote eclogite(92.38 ± 0.22 Ma) and the lawsoniteeepidote blueschist(90.19 ± 0.54 Ma),suggesting rapid garnet growth. The lawsoniteeepidote eclogite(87.30 ± 0.39 Ma) and the prograde lawsonite blueschist(ca. 86 Ma) are younger, whereas the garnet amphibolite(104.5 ± 3.5 Ma) is older.Our data reveal a consistent trend of progressively decreasing geothermal gradient from granulite-facies conditions at ~104 Ma to the epidote-eclogite facies around 92 Ma, and the lawsonite blueschist-facies between 90 Ma and 86 Ma. Three Lue Hf garnet dates(between 92 Ma and 87 Ma) weighted toward the growth of post-peak rims(as indicated by Lu distribution in garnet) suggest that the HP/LT rocks were exhumed continuously and not episodically. We infer that HP/LT metamorphic rocks within the Halilbag?Complex were subjected to continuous return flow, with "warm" rocks being exhumed during the tectonic burial of "cold" ones. Our results, combined with regional geological constraints, allow us to speculate that subduction started at a transform fault near a mid-oceanic spreading centre. Following its formation, this ancient subduction interface evolved thermally over more than 15 Myr, most likely as a result of heat dissipation rather than crustal underplating.

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.

Metamorphism of the Bikou Group in the Shanxi-Gausu-Sichuan Border Region

The Blkou Group on the Shaanxi-Gansu-Sichuan border is composed of Mid-Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist facies. Three metamorphlc zones, i.e. zones A, B, and C, may be distinguished on the basis of the field distribution of metamorphlc rocks and the variation of b0 values of muscovite. Blueschists are characterized by coexistence of sodic amphiboles and epidote and occur as stripes or relict patches in extensive greenschists of zone A. Studies of metamorphic minerals such as amphiboles, chlorite, epidote and muscovite and their textural relationships indicate that blueschists and greenschists were not formed under the same metamorphic physico-chemical conditions. The blueschist facies was formed at temperatures of 300-400℃ and pressures of 0.5-0.6 GPa. The greenschist facies in zones A and B has similar temperatures but its pressure is only 0.4 GPa or so. The transition from the blueschist to greenschist facies is a nearly isothermal uplift process. The rock and mineral assemblages of the Bikou Group indicate that the blueschist facies metamorphism of the group might be related to crustal thickening or A-subduction accompanying the closure of an intracontinental small ocean basin.

Formation Mechanism and Exhumation Processes for HP-UHP Metamorphic Rocks in Dabie Mountains

The high, ultrahigh pressure metamorphic rocks, widely distributed in Dabie Mountains, were described in terms of the geological setting, the marks of the petrology and the mineralogy of the ultrahigh pressure (UHP) metamorphic rocks. According to the estimated uplifting and denudation of the Dabie Mountains, and to the thermodynamics theory, were assessed the depth and pressure (high pressure autoclave) of the formation setting of the UHP metamorphic rocks. Based on all the information mentioned above, a new explanation is derived from the mechanism of formation and the processes of exhumation of the UHP metamorphic rocks.

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）.

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.

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

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: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.