Zircon U-Pb Dating of Yushugou Terrain of High-Pressure Granulite Facies in Southern Tianshan Mountain and Its Geological Significance

The Yushugou terrain of high\|pressure granulite facies in southern Tianshan Mountain is composed mainly of an ophiolite suite. Most selected zircons are round or elliptical in shape, and some are of tetragonal prism with round edges. The granulometric analyses show that they are well sorted in sedimentation. ZrO\-2/HfO\-2 ratios in zircons range from 45 to 57. These characters, together with the petrologic and geochemical characters of plagioclase\|garnet\|orthopyroxenite bearing zircons, indicate that the protolith of plagioclase\|garnet\|orthopyroxenite may be derived mainly from volcanic base surge sedimentary debris in oceanic islands and from clays formed by seafloor weathering. Zircons are simply of pyroclastic debris. The ophiolite formation age of (440±18) Ma and the first\|stage metamorphic age (amphibolite or granulite facies) of (364±5) Ma were obtained with a method of multiple grains in different groups and a method of concordia plot. These ages provide important information on the temporal and spatial occurrence of southern Paleozoic Tianshan Ocean, the subduction rate of the oceanic crust and the formation mechanism of ophiolite of granulite facies.

Garnet-bearing Granulite Facies Rock Xenoliths from Late Mesozoic Volcaniclastic Breccia, Xinyang, Henan Province

This paper presents the primary results of petrologic, mineralogical and petrochemical studies of garnet-bearing granulite facies rock xenoliths from Xinyang, Henan Province. These xenoliths, which are found in a pipe of late Mesozoic volcaniclastic breccia, are of high density (3.13–3.30 g/cm3) and high seismic velocity (Vp = 7.04–7.31 km/s), being products of underplating of basaltic magmas and had experienced granulite facies metamorphism. The underplating and metamorphism took place before the eruption of the host rock. Petrographical studies and equilibrium T-P calculations show that these xenoliths were captured at a 49 km depth and experienced at least a 16 km uplift before they were captured. The dynamics of the uplift could be related to the continent-continent collision between the North China plate and the Yangtze plate during the Triassic.

Granulite-facies Middle-Lower Crustal Xenoliths from Nushan Alkali Basalt in Northeastern Anhui Province,China

Metamorphic xenoliths within the Nushan alkali basalt of northeastern Anhui (NEA),China ,are from the middle-lower crust.They could be divided into two end-members:basic and acid.Interme-diate xenoliths are scarcely found.Basic two-pyroxene granulites(pyriclasites) were formed at 720-810℃ and 7-8kb.Petrological and geochemical studies indicate that the primary magma of the protoliths of basic granulites was derived from the metasomatized upper mantle, while the pa-rental magma of the acid end-member was probably produced by partial melting of the basic rocks. The protoliths of charnockites and grey gneisses represent respectively the early and late crystallization products of the granitic magma.The Nushan granulites are much different in many aspects from the granulites exposed in the northern part of North China ,which implies the inhomogeneity regarding to the early evolution of the North China terranc.

Geochronological Study of Caledonian Granulite and High-Pressure Gneiss in the Dabie Mountains

: Using the single—zircon evaporation technique and U—Pb method, the authors have conducted an isotope geochonological study of the Huilanshan granulite and Shima garnet-bearing plagioclase gneiss (“country rocks” of the Shima eclogite) in the Dabie Mountains. The study shows that these rocks have peak metamorphic ages of 443–455 Ma, which are essentially consistent with that of the Caledonian high—ultrahigh pressure eclogites. This indicates the existence of the Caledonian collisional orogeny in the Dabie Mountains.

LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207^ pb/206^Pb age in the range of （2 49±54 ） -- （2 500±180） Ma. The magmatic genesis-derived detrltal zircon domain gives a 207^pb/ 206^Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206^ pb/ 238^U age of （2 790 ± 150） Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a diesordia with an upper intercept age of （2 044. 7 ± 29.3 ） Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. -2.8 Ga magmatlsm and -2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at （2.04±0.03） Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT （ultrahigh temperature） metamorphosed slab.

Experimental Study of the Melting Reaction and Genetic Mechanism of Mineral Phase Transformation in Granulite Facies Metamorphism

The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy + Ilm + Mt, and garnet, into the hypersthene (Hy) + spinel (Sp) + cordierite (Crd) assemblage. The melt proportion and its evolutionary characteristics are mainly controlled by dehydration-melting of hydrous minerals and partial melting of felsic minerals besides P-T conditiops: In addition to the traditional solid 4- solid (or fluid) reaction and dehydration-melting reaction, the metamorphic reaction involving melts (reaction between unmelted minerals and melts) is one of the most important reactions in granulite facies metamorphism and its attendant remelting (or regional migmatization). This experiment may provide dependable experimental data for an in-depth study of the genetic mechanism of mineral assemblage evolution and its geological dynamic significance in granulite facies metamorphism of the studied area.

The Metamorphism of High-Pressure Basic Granulite in Laixi District, East Shandong, China

A high pressure basic granulite ultramafics belt is found in the late Archaean gneisses of Laixi Laiyang Qixia district, east Shandong. This belt has previously been considered as another eclogite belt in Sulu ultra high pressure(UHP) terrain. The typical high P granulite is characterized by mineral assemblages of three generations formed in different metamorphic episode respectively. The mineral assemblage of first episode is Grt(core)+Cpx+Pl, that of the second episode is Grt(rim)+Cpx+Opx+Pl+Amp+Q+Mt and that of the third episode is Cpx+Pl+Amp+Q+Mt. In first assemblage, the garnet porphyroblast is rich in grossular and pyrope component, and the coexisting Cpx is Al rich. Using mineral thermobarometer and Berman’s (1991) TWQ program, the P T conditions of three episodes are established, i.e. T=840℃～860℃, P>1.4GPa for first episode; T=720℃～780℃,P=0.8～1.2GPa for second episode and T=600℃～800℃,P=0.5～0.7GPa for the third episode.Considering the reaction texture, mineral Chemistry and PT estmations, a clockwise PTt path can be constructed, which shows post peak isothermal decompression at first and followed by a process of pressure and temperature decreasing. The mineral wholerock Sm Nd isochron age of the granulite is 1752Ma, which indicates its last thermal event during early middle Proterozoic stage. Therefore, it is concluded that the high pressure granulite and ultramafics of this belt represents the lower continental curst of the North China Craton during early Precambrian without any relationship to tectonic process of the Sulu UHP terrain.

The Activity of H_2O, Fugacity of O_2 and Variation of Their Chemical Potential During Metamorphic Peak of the Granulite Complex Near Taipingzhai-Louzishan Region, East Hebei Province

The thermodynamic calculation of dehydration reacton suggests very low activity of H\-2O during metamorphic peak of the Archaean granulite complex in the region studied.The a\-\{H\-2O\} values for Al\|rich gneiss and hypersthene biotite gneiss\|granulite in the Taipingzhai region are usually between 0.10 and \{0.20\},and those in the Louzishan region are \{0.15\}-\{0.25\}. The fugacity of O\-2 in terms of lgf\-\{O\-2\} in the whole region ranges from -8 to -14. The average coefficients of (μ\-\{H\-2O\}/X\+\{Bt\}\-\{Mg\}) and (μ\-\{O\-2\}/X\+\{Bt\}\-\{Mg\}) in the Taipingzhai region are \{-0.293\} and \{-1.60\} respectively,and those in the Louzishan region are \{-0.364\} and \{-1.420\}. The activity of H\-2O is very low in the whole region, but its values and other data mentioned above are considerably constant from place to place within a given region, even in rocks of different lithological characters. However, they show a certain gradient between different regions. Such characteristics are compatible with the genetic mechanism known as \!carbonic metamorphism" put forward by Newton et al.,i.e., the a\-\{H\-2O\} during the peak stage is controlled by permeation of pervasive CO\-2 influx of the mantle source,and shows features of external buffering.

Variscan terrane of deep-crustal granulite facies in Yushugou area, southern Tianshan

The Yushugou terrane of deepcrustal granulite facies in southern Tianshan consists of two parts, granulite and metaperidotite. The whole terrane is a metamorphism of (high-pressure) grunulite facies, and typical mineral associations are: Gt-Cpx-P1-Tit-Ilm (±Qz) (silica-saturated and oversaturated mafic rocks), Gt-Ky (pseudomorph)-P1-Ru-Ilm± Qz (metapelitic rocks) and Spi-Opx-Cpx-01 (meta-ultramafic rocks). The peak-stage P-T conditions are 795— 964°, 0.97—1.42 GPa, which are obtained with mineral chemistry, assemblage analyses and P-T estimation. The Sm-Nd isochron age of peak-stage metamorphic minerals is (315 ± 3.62) Ma. All of these indicate that the terrane is a deep-crustal body, which subducts to the depth of 40—50 km in the middle late-Paleozoic, undergoing metamorphism of (high-pressure) granulite facies, and exhumed again to the surface by tectonic uplifting.

Discovery of High-Pressure Basic Granulite Terrain in North China Archaean Craton and Preliminary Study

High-pressure basic granulite terrain of 2.65 Ga from Jiouhuai’an in the junction of the Hebei and Shanxi provinces was discovered and reported recently. The three metamorphic mineral assemblages and their mineral reaction texture of the basic granulites have been identified, which represent three main metamorphic episodes of 1.4—1.5 Gpa, 800℃; 0.7—0.9 Gpa, 820℃ and 0.5—0.6 Gpa, 700℃. Their steep decompressional clockwise P-T path implies that a late Archaean continental collision and sole thrusting event perhaps occurred in North China Craton. This is important to study petrogenesis of high-pressure granulites, composition and texture of deep crust in North China Craton, and early Precambrian collision tectonism.

Study and implication of the high-pressure felsic granulite in the Qinling complex of East Qinling

There is a typical assemblage of garnet + kyanite + microperthite + quartz + rutile in high-pressure (HP) felsic granulite of Qinling complex in Songshugou area. East Qinling. The HP granulite was formed at 800 -900℃ and 1.3 -1.6GPa and has experienced two stages of retrograde metamorphism at 600- 650 ℃, 0.8-1.0GPa and 500-600℃, 0.3-0.6GPa, forming two retrograde metamorphic assemblages of margarite + plagiodase (PlI)+quartz and sillimanite + biotite + plagioclase(PlII) + microdine+quartz, respectively. They construct a two-stage clockwise P-T path which shows down-pressure cooling in both early and late stage.

Discovery and study of high-pressure basic granulites in Songshugou area of Shangnan, East Qinling

1 Geological occurrence The ultramafic rocks in the Songshugou area of the North Qinling are structurally emplaced in middle-late Proterozoic amphibolites (fig. 1) which are regarded as an external rootless ophiolite sheet with emplacement age of (983±140) Ma. The lenticular diopsidegarnet-amphibolites, named garnet-amphibolte by others, are exposed in the amphibolits. The high-pressure basic granulites discussed in this note were found in the lenticular garnet-

Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline

We report preliminary results of a geochemical study on banded iron formations （BIFs） in the Zhaojiayangpo （赵家阳坡） area from the Kongling （崆岭） Group in the northern Huangling （黄陵） anticline, on the northern margin of the Yangtze craton. The CL （cathodoluminescence） images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim tex-tures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660-808 ℃ （714 ℃C in aver-age）, all indicating that the BIFs underwent granulite facies metamorphism. The age of zir-cons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indi-cating that there was a significant granulite fa-cies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the as-sembly of the Columbia supercontinent with South China. Moreover, the REE pattern ischaracterized by depletion in LREE while relatively flat in HREE, LaN/YbN=0.26, with a positive Eu anomaly （Eu/Eu^＊---1.59）, which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.

High-pressure granulite from Western Kunlun,northwestern China:Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication

High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were obtained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic condition of these high-pressure granulites is about 760–820°C, 1.0–1.2 GPa and the retrograde metamorphic condition is about 620–720°C, 0.7–0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening, subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important implications for the research on Tethys and Paleo-Asian tectonic zone.

Discovery of high-pressure basic granulite in Namjagbarwa area, Tibet, China

Recently, a series of basic granulite enclosures are found in the granulite gneisses along the right side of the Yarlung Zangbo River, lying west of the Namjagbarwa Mountain. Es-pecially, some high-pressure granulite enclosures have been discovered in eastern gully of the Zhibei village. To the northwest of the high-pressure granulite belt there occur Yarlung Zangbo ophiolite zone. Southeastward the granulites change into amphibolite and green-schist facies rocks, which constitute a complete lower crust section.

Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications

Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tianshan have been investigated by cathodoluminescence, LAICPMS and ion microprobe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. Theweighted mean ages of these two samples are 392±7 and 390±11 Ma, respectively, representing the granulite-facies metamorphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian.

Early Paleozoic Granulite-Facies Metamorphism and Magmatism in the Northern Wulan Terrane of the Quanji Massif:Implications for the Evolution of the Proto-Tethys Ocean in Northwestern China

The nature and evolution of the Proto-Tethys Ocean originated from the breakup of the supercontinent Rodinia remain controversial. Early Paleozoic magmatism and metamorphism can pro- vide important constraints on the closure of the Proto-Tethys Ocean. This paper reports on a set of geological, petrographical, geochronological, mineralogical and geochemical data for Early Paleozoic granite, gabbro, granulite and granitic leucosome in the northern Wulan terrane of the Quanji Massif. Zircon LA-ICP-MS U-Pb dating reveals two episodes of magmatism, with the emplacement of a gran- itic pluton at 476.7±2.8 Ma and a gabbroic dike at 423±2 Ma. Whole-rock geochemistry suggests an arc affinity for the magma of the granitic pluton but a post-collisional extension setting for the gabbroic dike. Zircon LA-ICP-MS U-Pb dating also shows that the peak granulite-facies metamorphism and anatexis occurred at --475 Ma, coeval with the formation of the granitic pluton in the Quanji Massif as well as the early lawsonite-bearing eclogites in the North Qaidam high-pressure and ultrahigh-pressure （HP-UHP） metamorphic belt to the south. The granulite-facies metamorphism with peak P-T condi- tions at 718-729 ℃ and 0.46-0.53 GPa is characterized by an anticlockwise P-T path. Our data provide compelling evidence for Early Paleozoic paired metamorphic belts with HP-UHP metamorphism in the North Qaidam to the south and low PIT metamorphism in the Quanji Massif as a continental arc to the north, hence suggesting a northward subduction polarity for the Proto-Tethys oceanic plate. The intrusion of the post-collisional gabbroic dike supports for the closure of the Proto-Tethys Ocean in north- western China before 423 Ma.

Origin and geological significance of the 1.81 Ga hyalophane-rich pegmatite veins from the high-pressure granulite terrain in the Central Zone of North China Craton

Hyalophane-rich pegmatites are identified from the Manjinggou high-pressure granulite terrain in the Central Zone of North China Craton. Based on field investigation, mineral assemblage and mineral geochemistry, two types of pegmatites can be defined, i.e., hyalophane pegmatite and hyalophane-rich pegmatite. The hyalophane pegmatite is composed of pure hyalophane with 18.7 mol%-19.4 mol% celsian, whereas the hyalophane-rich pegmatite consists of clinopyroxene + titanite + epidote + hyalophane with 11.9 mol%-12.5 mol% celsian. Hyalophane-rich pegmatite has typical magmatic zircons with oscillatory zoning and high Th/U ratios, implying that this type of pegmatite crystallized from special melt similar to magma. SIMS (Cameca 1280) zircon U-Pb dating shows that the crystallization age of the hyalophane-rich pegmatite is 1812±5 Ma, younger than the regional metamorphic age (peak of ca. 1.85 Ga). Zircon δ18O (8.0 ‰-9.3 ‰) and ?Hf (-7.0 to-2.7) values measured by SIMS suggest that the high-pressure granulite terrain was the source of these veins. Therefore, the hyalophane-rich pegmatite veins were likely to be generated by melting of the high-pressure granulite terrain during post collisional uplift. A quick tectonic uplifting process with a velocity of 0.4 to 0.6 mm/a has been estimated for the high-pressure granulite terrane from the Central Zone of North China Craton.

柴北缘欧龙布鲁克地块中元古代晚期麻粒岩相变质作用——来自石榴夕线堇青石片麻岩的岩石学、相平衡模拟和U-Pb年代学的制约

在欧龙布鲁克地块乌兰北部地区察汗河岩群中识别出麻粒岩相石榴夕线堇青石片麻岩,其矿物组合为石榴子石、夕线石、堇青石、黑云母、斜长石、钛铁矿和少量钾长石等。岩相学观察显示,M1阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英,M2阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英+钛铁矿+黑云母,M3阶段矿物组合有堇青石+黑云母+钛铁矿+石英+石榴子石+斜长石±钾长石。相平衡模拟计算结果显示,该岩石的峰期温压条件为p=0.92~1.08 GPa,峰期温度t>790℃,峰期之后经历升温降压的p-T演化轨迹。锆石和独居石LA-ICP-MS U-Pb年代学研究获得的变质年龄分别为1133±14 Ma和1125±37 Ma,1133~1125 Ma应代表了该期麻粒岩相变质作用的时代。结合区域地质资料和已有的研究成果,我们认为乌兰北部察汗河岩群的石榴夕线堇青石片麻岩可能形成于大洋俯冲作用下的弧或弧后构造环境,乌兰北部的岩浆-变质杂岩带经历了从中元古代晚期-新元古代早期俯冲增生到碰撞造山的演化过程,是全球Rodinia超大陆汇聚过程的响应。

柴达木盆地南缘金水口群的早古生代构造热事件:锆石U-Pb SHRIMP年龄证据

通过柴达木盆地南缘(东昆仑北缘)原定为金水口群麻粒岩相片麻岩和花岗质岩石中锆石的SHRIMP测定,确定其麻粒岩相变质时代为460Ma±8Ma,而具有深熔成因特征的花岗质岩石深熔作用时间为402Ma±6Ma。这些年龄数据表明,金水口群经历了早古生代与麻粒岩相变质和深熔作用有关的构造热事件,原认为是柴达木地块的前寒武纪变质基底明显在早古生代造山过程中发生了活化作用。花岗质岩石的继承锆石给出了少量太古宙和大量1600～1800Ma之间的年龄,代表了其锆石的主要源区物质年龄,这与祁连—柴达木地区以及扬子地块的地壳形成年龄基本一致,反映柴南缘(东昆仑北缘)的变质基底与扬子克拉通具有明显的亲缘性。