Metamorphic and Tectonic Evolution of the Hwacheon Granulite Complex, Central Korea: Composite P-T Path Resulting from Two Distinct Crustal-Thickening Events

The Hwacheon granulite complex (HGC), occupying the northeasternmargin of the Gyeonggi massif, consists mainly of garnetiferousleucocratic gneiss and leucogranite together with minor kyanite–garnetgneiss, aluminous gneiss, mafic granulite and garnet amphibolite.Mineral assemblages and reaction textures in various rock typesof the HGC document five distinct metamorphic stages: pre- (M₁)and peak (M₂) granulite-facies metamorphism; lower temperature,high (M₃) and low (M₄) pressure upper amphibolite-facies metamorphism;and local retrogression (M₅) producing andalusite-bearing assemblages.Each metamorphic stage can be integrated to give a compositeP–T path consisting of two distinct trajectories, characterizedby clockwise P–T loops at relatively high and low temperatures,respectively. The first P–T trajectory (M₁–M₃) correspondsto a Palaeoproterozoic tectonometamorphic event responsiblefor the formation of the granulite complex at     

Constraining the prograde and retrograde P-T paths of granulites using decomposition of initially zoned garnets: an example from the Central Indian Tectonic Zone

The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (M_o) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M₁) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M₂) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (X_An=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinel_ss {X_Mg (Mg/Mg+Fe²⁺) = 0.23–0.26}, X_Al (Al/Al+Fe³⁺)=0.71–0.81}-plagioclase (X_An=0.91)-cordierite {X_Mg (Mg/Mg+Fe²⁺) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M₃), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M₂ spinel_ss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).Editorial responsibility: W. Schreyer     

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continentalshelf deposits in the Saih Hatat tectonic window of northeasternOman, where they structurally underlie the Semail ophiolite.These eclogites and blueschists constitute part of a coherenthigh-P/T metamorphic terrain comprised of interlayered metabasites,metapelites, and quartzofeldspathic and calcareous mica schists.The metabasites record a continuous regional trend of increasingP and T from west to east, where crossite-epidote schists incentral and western Saih Hatat grade into blueschists and eclogitesat As-Sifah, allowing for the subdivision of this terrain intothree metamorphic zones. P-T conditions range from 4?5 to 5?5kb atT <340?C for zone A to >10?2 kb at 500–580?Cfor zone C. Zoning patterns of garnets, pyroxenes, and phengite, and thecompositions of pyroxene and amphibole inclusions in garnet,indicate that the eclogites and enclosing high-grade blueschistsof zone C followed a ‘clockwise’ P-T rpath of increasingP and T followed by increasing T and decreasing P and a finalstage of retrogression. Garnets and pyroxenes crystallized atdifferent stages of P-T evolution of the terrain in the variouseclogite pods and boudins. This conclusion and the contrastingmineralogies of the eclogites and the enclosing blueschistsare attributed to differences in bulk-rock chemistry, fluidavailability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a resultof A-type subduction and crustal thickening of the Oman continentalmargin along an east-northeast-dipping thrust or ‘subductionzone’ which was initiated by the change in plate motionbetween Africa and Eurasia at 131 Ma. High P/T metamorphismcontinued until the final stages of ophiolite emplacement inthe late Cretaceous, as indicated by the metamorphism of thethrust sheets overlying the coherent terrain.     

Petrology, Geochemistry and Zircon U-Pb Geochronology of the Xiangshan Group in the Eastern Hexi Corridor Belt: Implications for Provenance and Tectonic Evolution

The eastern Hexi Corridor Belt (HCB) is located in the transitional belt among the Alxa Block, the Qilian Orogenic Belt and the North China Block. Because of its unique tectonic location, the tectonic setting, provenance, and even the age of the sedimentary strata in the eastern HCB during the Early Paleozoic remain controversial. This study analyzes the provenance of the poorly studied Xiangshan Group, discusses its age of development and tectonic setting in the eastern HCB using a combination of petrological, geochemical and LA-ICP-MS U-Pb zircon dating methods. Based on the youngest age peaks and the fossil evidence, we suggest that the Xiangshan Group is Middle Cambrian to Late Ordovician in age. The complexity of the geochemical characteristics and associated diagrams suggests that the early stage of the Xiangshan Group developed in a passive continental margin environment, late in the back-arc basin of the eastern HCB. Based on the sandstone detrital composition, whole-rock geochemistry and detrital zircon ages, we conclude that the Xiangshan Group had an early provenance that was mainly from the Qilian Block and a late provenance from the Qilian Block and the western Alxa Block. The eastern HCB and its northern and southern blocks have similar palaeontology, lithology and basement age characteristics to the South China Block. This indicates that the eastern HCB might not have formed in the intra-continental aulacogen of the North China Block during the Early Paleozoic but has a close affinity to eastern Gondwana.     

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M₁) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling P–T history (M₂). M₁ caused pervasive biotite-dehydrationmelting, producing garnet–orthopyroxene and garnet–rutileand sapphirine–spinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M₂,garnet–corundum and later spinel–sillimanite–biotiteassemblages were produced by reacting sapphirine–spinel–sillimaniteand rehydration of garnet–corundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M₁ garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 2040–2090Ma. Based on the deep crustal heating–cooling P–Ttrajectory, the authors infer an overall counterclockwise P–Tpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M₃ and subsequentnear-isothermal loading to 8 kbar during M₄. In the felsic granulites,the former is marked by decomposition of M₁ garnet to orthopyroxene–plagioclasesymplectites. During M₄, there was renewed growth of garnet–quartzsymplectites in the felsic granulites, replacing the M₃ mineralassemblage and also the appearance of coronal garnet–quartz–clinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M₄ metamorphism is dated at 1525–1450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M₃–M₄ stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise P–T path; monazite chemical dating     

青海南山黑马河岩体岩浆混合成因的岩石地球化学和年代学证据及其构造意义

黑马河岩体位于西秦岭北缘青海南山构造带内,岩体中发育大量闪长质包体,但对其成因却有一定的争议.从岩石学、矿物学、地球化学和年代学等方面对黑马河岩体花岗闪长岩及其中的闪长质包体进行了详细研究.包体的野外产出状态、形态、结构构造和矿物学特征均显示出,它们是基性岩浆注入到中酸性岩浆中快速冷凝结晶的产物.在主量元素协变图解中,花岗闪长岩和闪长质包体显示出壳幔岩浆混合作用的趋势.另外,两者稀土总量和相似的稀土元素配分模式及Eu负异常程度,也显示二者具有岩浆混合的特征.LA-ICPMS锆石U-Pb定年结果显示,花岗闪长岩形成于246 Ma,闪长质包体形成于245 Ma,两者具有几乎一致的结晶年龄,排除了包体为源区难熔残余或围岩捕掳体的可能性,也排除了基性岩浆在花岗质岩浆固结后再侵入的可能性.结合区域地质资料,认为黑马河岩体形成于西秦岭北缘有限小洋盆向南的俯冲阶段,青海南山构造带与天峻南山一带具有相同的构造岩浆演化历史.      

Petrological and geochemical studies of paleoproterozoic mafic dykes from the Chitrangi Region,Mahakoshal Supracrustal Belt,Central Indian Tectonic Zone: Petrogenetic and tectonic significance

A number of Paleoproterozoic mafic dykes are reported to intrude volcano-sedimentary sequences of the Mahakoshal supracrustal belt. They are medium to coarse-grained and mostly trend in ENE-WSW to E-W. Petrographically they are metadolerite and metabasite. Geochemical compositions classify them as sub-alkaline basalts to andesites with high-iron tholeiitic nature. Both groups, i.e. metabasites and metadolerites, show distinct geochemical characteristics; high-field strength elements are relatively higher in metadolerites than metabasites. This suggests their derivation from different mantle melts. Chemistry does not support any possibility of crustal contamination. Trace element modeling advocates that metabasite dykes are derived from a melt originated through ∼20% melting of a depleted mantle source, whereas metadolerite dykes are probably derived from a tholeiitic magma generated through <10% melting of a enriched mantle source. Chemistry also reveals that the studied samples are derived from deep mantle sources. HFSE based discrimination diagrams suggest that metabasite dykes are emplaced in tectonic environment similar to the N-type mid-oceanic ridge basalts (N-MORB) and the metadolerite dykes exhibit tectonic setting observed for the within-plate basalts. These inferences show agreement with the available tectonic model presented for the Mahakoshal supracrustal belt. The Chitrangi region experienced N-MORB type mafic magmatism around 2.5 Ga (metabasite dykes) and within-plate mafic magmatism around 1.5–1.8 Ga (metadolerite dykes and probably other alkaline and carbonatite magmatic rocks).     

An Exposed Hercynian Deep Crustal Section in the Sila Massif of Northern Calabria: Mineral Chemistry, Petrology and a P-T Path of Granulite-facies Metapelitic Migmatites and Metabasites

In the Sila massif of northern Calabria a continuous sectionis exposed through a segment of a Hercynian deep continentalcrust, which has been interpreted by previous workers as a stackof basement nappes (‘Monte Gariglione Complex’).The section consists essentially of metapelitic migmatites andsubordinate metabasites and marbles, which were metamorphosedat medium-pressure–high-temperature granulite-facies conditions.A continuous metamorphic gradient through the exposed segmentcan be deduced from the systematic change in the compositionsof ferromagnesian minerals in divariant metapelitic assemblages.This gradient is partly supported by conventional geothermobarometryand by applying the TWEEQU method. However, peak-metamorphicconditions are better defined by dehydration melting reactions,which reveal     

胶-辽-吉活动带中段古元古代早期构造演化:来自牧牛河和大房身岩体的证据

辽吉花岗岩是胶-辽-吉活动带的重要组成部分,它们是限定胶-辽-吉活动带早期构造属性的重要依据.由于辽吉花岗岩的属性一直存在争议,所以胶-辽-吉带的早期构造环境也一直没有定论.通过对牧牛河-大房身地区的辽吉花岗岩(牧牛河岩体和大房身岩体)进行年代学和地球化学分析,以探讨该地区辽吉花岗岩成因和地质意义.锆石U-Pb定年结果...     

Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).     

Ultra high temperature-metamorphism and its significance in the Central Indian Tectonic Zone

In the present study from the southern margin of the Central Indian Tectonic Zone, it is demonstrated how the metamorphic P–T path of ultrahigh-temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later FMAS solid–solid reaction. The extreme metamorphism in these rocks caused two-stage biotite melting, resulting in initial porphyroblastic garnet₁ and later sapphirine–spinel₁ incongruent solid mineral assemblages. During this process, the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica-oversaturated to a later silica-undersaturated domain. In the melanocratic layer, this allowed localized concentration of sapphirine-spinel₁ and residual sillimanite₁, producing an extremely restitic assemblage, at the culmination of peak metamorphism, BM₁. BM₁ is constrained at  1000 °C at relatively deep crustal levels (P  9 kbar) from the stability of ferroaugite in a co-metamorphosed Iron Formation granulite. During subsequent metamorphism (BM₂), the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine, spinel₁ and sillimanite to produce coronal garnet₂–corundum assemblage via a FMAS univariant reaction. In the final stages of reaction history, biotite₂–sillimanite₂–spinel₂ assemblage was produced after garnet₂–corundum due to localized melt–crystal interaction. The metamorphic sequence, when interpreted with the help of a newly constructed, qualitative KFMASH petrogenetic grid, reveals successive stages of heating, increasing pressure and cooling around the KFMASH invariant point, [Opx,Crd], which is consistent with a counterclockwise metamorphic P–T path. The near isobaric nature of post-peak cooling (ΔT  250–300 °C) is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the Iron Formation granulites. This study provides the first tight constraint for ultrahigh-T metamorphism along a counter clockwise P–T trajectory in the Central Indian Tectonic zone, and has important bearing for terrane correlations in this part of East Gondwanaland. In addition, the new KFMASH grid allows evaluation of metamorphic phase relations in ultrahigh-T, corundum-bearing and corundum-absent aluminous granulites.     

印度次大陆中央构造带沉积—变质型锰矿的矿物学和地球化学

对印度中央构造带sauaar带锰矿及其围岩的31个样品进行了显微镜仔细的观察和鉴定,并分析了其主量元素、微量元素和稀土元素的地球化学特征,发现该带的锰矿主要是软锰矿和硬锰矿,锰矿主要富集在锰榴石英岩内.含锰岩系样品具非常高的Mn/Fe比值,说明热液运输和成矿作用期间锰和铁的强烈分异;MnO/TiO2为67.98-1417.52,说明有大量陆源物质经风化、搬运加入并混合沉积而成.用太古界平均澳大利亚页岩（PAAS）标准化后,稀土元素含量中δEu,δCe都有明显的正异常,说明该带锰矿床是在较强氧化海洋环境和有大量陆源物质加入的条件下沉积,并经过热液的变质交代作用而成的沉积一变质型锰矿床.该沉积—变质型锰矿的发现和研究对我国南方扬子克拉通中元古代浅变质岩系的锰矿寻找具有一定的借鉴意义.     

中天山巴仑台地区花岗质岩石的Hf同位素研究：对构造演化及大陆生长的约束

中国中天山地块位于天山造山带的核心部位,对研究中亚造山带西南段构造演化、块体起源和陆壳生长等科学问题具有重要意义。本次研究围绕中天山地块西段巴仑台一带出露的古生代花岗质岩石展开,报道了部分代表性岩体的LA-ICP-MS锆石U-Pb年龄和MC-ICP-MS锆石Lu-Hf同位素特征。同时还系统总结前人发表的研究区内花岗质岩石的锆石U-Pb年龄和Lu-Hf同位素资料。本次研究的7个花岗质侵入体的锆石LA-ICPMS年龄分别为435±5 Ma、432±4 Ma、359±4 Ma、312±4 Ma、346±2 Ma、306±5 Ma和362±5 Ma。它们在成因类型上均属于I型花岗岩,起源于深部不同性质地壳物质的部分熔融,部分岩体具有壳幔混合成因。研究表明,巴仑台一带的岩浆事件主要发生在早志留世—早泥盆世(435~408 Ma)、晚泥盆世—早石炭世(370~339 Ma)和晚石炭世—中二叠世(313~263 Ma)三个阶段,第一期岩浆事件是古天山洋闭合形成的后碰撞体制以及古南天山洋俯冲导致的大陆岛弧环境共同作用的结果。而古南天山洋俯冲活动的暂停和新一轮的洋盆扩张是导致区内中、晚泥盆世岩浆活动暂停的原因。第二期花岗质侵入体是古南天山洋第二期北向俯冲所导致的大陆岛弧岩浆作用的产物,而第三期花岗质侵入体形成于古南天山洋最终闭合之后的后碰撞环境中。通过总结区内古生代花岗质岩体的锆石的Hf同位素特征,我们认为巴伦台地区前寒武基底主要形成于三个阶段:1~2.47Ga至~2.26Ga,为研究区对全球陆核生长事件的响应;2~1.47Ga至~1.25Ga,对应于哥伦比亚超大陆的裂解;3~1.20Ga至~0.90Ga,很可能是研究区对导致罗迪尼亚超大陆聚合的"Grenville运动"的响应。     

Xiba Granitic Pluton in the Qinling Orogenic Belt,Central China: Its Petrogenesis and Tectonic Implications

Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U–Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite (218±1 Ma). The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with (La/Yb)N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 (mean= 0.97). Most of these samples from Xiba granitic pluton exhibit εNd(t) values of ?8.79 to ?5.38, depleted mantle Nd model ages (TDM) between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios (87Sr/86Sr)i from 0.7061 to 0.7082, indicating a possible Meso- to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher (87Sr/86Sr)i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.     

Petrology and Geochronology of Granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and Implications for the Evolution of the Prydz Belt

A combined petrological and geochronological study was carriedout on mafic granulites and associated felsic gneisses fromthe McKaskle Hills, eastern Amery Ice Shelf, East Antarctica.Garnet-bearing mafic granulites exhibit reaction textures andexsolution textures that indicate two-stage metamorphic evolution.Thermobarometric estimates from matrix and symplectite assemblagesyield peak and retrograde P–T conditions of 9·0–9·5kbar and 880–950°C and 6·6–7·2kbar and 700–750°C, respectively. Similar but slightlyscattered peak P–T estimates of 7·9–10·1kbar and 820–980°C are obtained from the core compositionsof minerals from felsic para- and orthogneisses. Evidence forthe prograde history is provided by muscovite inclusions ingarnet from a paragneiss. Sensitive high-resolution ion microprobeU–Pb zircon dating reveals an evolutionary history forthe granulites, including a mafic and felsic igneous intrusionat 1174–1019 Ma, sedimentation after 932–916 Ma,and a high-grade metamorphism at 533–529 Ma. In contrast,Sm–Nd mineral–whole-rock dating mainly yields asingle age population at 500 Ma. This suggests that the McKaskleHills form part of the Prydz Belt, and that the relatively highpeak P–T conditions and a decompression-dominated P–Tpath for the rocks resulted from a single Cambrian metamorphiccycle, rather than two distinct metamorphic events as formerlyinferred for the granulites from Prydz Bay. The age data alsoindicate that the Precambrian history of the McKaskle Hillsis not only distinct from that of the early Neoproterozoic terranein the northern Prince Charles Mountains, but also differentfrom that of other parts of the Prydz Belt. The existence ofmultiple basement terranes, together with considerable crustalthickening followed by tectonic uplift and unroofing indicatedby the clockwise P–T–t evolution, suggests thatthe Prydz Belt may represent a collisional orogen that resultedin the assembly of Gondwana during the Cambrian period. KEY WORDS: Mesoproterozoic basement; Cambrian metamorphism; P–T path; Prydz Belt; East Antarctica     

Petrology, Geochemistry and Tectonic Significance of the Metamorphic Peridotites from Longmuco–Shuanghu Ophiolitic Melange belt, Tibet

Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ) was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole–rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ–type peridotites. They exhibit typically U–shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two–stage evolution history is considered: Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%–20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo–Tethys, forming at middle Ordivician–upper Cambrian, and they may be the direct evidences for spreading of palo–Tethys.     

Precambrian Terranes of the Central Asian Orogenic Belt: Comparative Characteristics,Types, and Peculiarities of Tectonic Evolution

Geotectonics - The structure and peculiarities of the tectonic evolution of Precambrian terranes included in the structure of Paleozoides in different parts of the Central Asian orogenic belt are...     

Early Paleozoic Ocean Plate Stratigraphy of the Beishan Orogenic Zone,NW China: Implications for Regional Tectonic Evolution

The Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well-enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Pa...