The Metamorphic Evolution and Tectonic Significance of the Sumdo High and Ultrahigh Pressure Metamorphic Terrane, Central-South Lhasa Block, Tibet

Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

The Dabie metamorphic terrane is tectonically located within the intersection area of the WNW trending east Qinling orogenic belt with the NNE trending west Pacific domain. The Punctuated multistage convergent movement and the emplacemeat of YanShanian granitoid intrusions account for the complication of its tectonic framework. The Dabie metamorphic complex is composed of 3 units: supracrustals of early Precambrian age, granite gneiss and metamorphic magic rocks of lats Precambrian age. Three major metsmorphic Stages can be traced, i. e. granulite factes, amphibolite facies of Precambrian and the HP/ LT epidote amphibolite and greenschist facies of Phanerozoic. The Hong'an Group underlying the Dabie complex shows epidote amphibolite facies which is probably synchronous with the amphibolite facies of the Dabie complex. The Protolith of the HP and UHP eclogiter are diversided. The amphibole, epidote and other mineral inclusions within the eclogite minerals testify to the earlier history of Pre-eclogitic metamorphism. The peak metamorphic assemblage coesite-omphacite-garnet is superimposed by amphibolite, epidote amphibolite and greenschist facies of retrograde metamorphism. In many regions the eclogites appear as pods, lenses and layers within ductile shear zones having synkinematic greenschist or epidote amphibolite facies of retrograde metamorphism. The Sm-Nd isochron of whole rock and garnet combined with Ph-Ph dating of zircon crystals from Mifengjian eclogite block in Hubei suggest that the UHI, metamorphism probably took Place nearly 480 Ma Which evideoces one of the most important tertono-thermal events of the punctuated evolution in the Dabie terrane.

Metamorphism of the northern Liaoning Complex:Implications for the tectonic evolution of Neoarchean basement of the Eastern Block,North China Craton

As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North China Craton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic （TTG） gneisses, massive granitoids and supracrustal rocks. The supracrustal rocks, named the Qingyuan Group, consist of interbedded amphibolite, hornblende granulite, biotite granulite and BIF. Petrological evidence indicates that the amphibolites experienced the early prograde （M1）, peak （M2） and post-peak （M3） metamorphism. The early prograde assemblage （M1） is preserved as mineral inclusions, represented by actinotite ＋ hornblende - plagioclase ＋ epidote ＋ quartz 4- sphene, within garnet porphyroblasts. The peak assemblage （M2） is indicated by garnet ＋ clinopyroxene ＋ hornblende ＋ plagioclase ＋ quartz ＋ ilmenite, which occur as major mineral phases in the rock. The post-peak assemblage （M3） is characterized by the garnet 4- quartz symplectite. The P-T pseudosections in the NCFMASHTO system constructed by using THERMOCALC define the P-T conditions of M1, M2 and M3 at 490-550 C＋（4.5 kbar, 780 810 C/7.65- 8.40 kbar and 630-670 ＋C]8.15-9.40 kbar, respectively. As a result, an anticlockwise P-T path involving isobaric cooling is inferred for the metamorphic evolution of the amphibolites. Such a P-T path suggests that the late Archean metamorphism of the northern Liaoning Complex was related to the intrusion and underplating of mantle-derived magmas. The underplating of voluminous mantle-derived magmas leading to metamorphism with an anticlockwise P-T path involving isobaric cooling may have occurred in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift envi- ronments. A mantle plume model is favored because this model can reasonably interpret many other geological features of late Archean basement rocks from the northern Liaoning Complex in the Eastern Block of the North China Craton as well as their anticlockwise P-T paths involving isobaric cooling.

The Metamorphism and Its Tectonic Implications of Indosinian High Pressure Granulites from the Badu Complex of the Cathaysia Block, Southwestern Zhejiang Province, South China

The Badu Complex is the oldest metamorphic rock in Cathaysia Block which experienced several episodes of metamorphism Especially indosinian metamorphic reworking in the southwestern Zhejiang Province, South China. The degree of indosinian metamorphism reaches granulite facies. However, there is still insufficient understanding of the characteristics of the Indosinian granulite metamorphism in the Cathaysia and many interpretations of its tectonic significance. Therefore, we present detailed petrology, mineral chemistry and LA-ICP-MS zircon U-Pb age in this paper from pelitic granulites of the Badu Complex, which is composed of "sillimanite + garnet + cordierite + spinel + biotite + k-feldspar" assemblage and garnet pyroxenite with garnet amphibolite which is consists of "garnet + clinopyroxene + orthopyroxene + amphibole + plagioclase". By comprehensive study we get following new findings: Pelitic granulites record four stages of metamorphic mineral assemblages, including prograde(M1), pressure peak(M2), Peak(M3) and post-peak decompressional and then cooling(M4) stages. The prograde M1 assemblage consists of garnet1(core) + staurolite + kyanite + biotite + quartz ± rutile ± chlorite;The pressure peak M2 assemblage consists of garnet1(mantle) + sudoite + rutile + kyanite + corundum + biotite + quartz;The peak M3 have garnet2(rim-mantle) + biotite + sillimanite + quartz ± K-feldspar ± plagioclase ± ilmenite assemblag;the M4 stage is consist of garnet + cordierite + biotite + sillimanite + quartz + ilmenite ± spine ± K-feldspar. The garnet pyroxenite and garnet amphibolites have experienced three stages of metamorphic evolution. Peak high-pressure granulite facies stage M2 consists of garnet + sahlite ± ilmenite ± quartz;Post-peak near isothermal decompression medium granulite facies stage M3 is characterized by typical decompression reaction textures and assemblage of orthopyroxene + plagioclase(An=90–92);amphibolites facies retrograde metamorphic stage M4 is characterized by amphibole + plagioclase(An=33–35) + ilmenite ± sahlite ± quartz mineral assemblage. By means of phase equilibrium simulation and traditional thermobarometer, P-T conditions of 785–820 ℃ and 8.9–9.9 kbar for M3 stage, 780–860 ℃ and 5.7–6.2 kbar for decompressional M4 stage, 705–720 ℃ and 4.5–4.7 kbar for cooling M4 stage in pelitic granulites were obtained. And also 11.6–12.5 kbar and 780–840 ℃ for M2 stage, 7.4–8.2 kbar and 800–880 ℃ for M3 stage, 6.6–7.5 kbar and 500–560 ℃ for M4 stage were obtained in garnet pyroxenite and garnet amphibolite. A clockwise P-T path is confirmed in the two type rocks of the Badu Complex which reflected a near-isothermal decompressional metamorphic process. The peak metamorphism can reach highpressure granulite facies. In addition, the mineral assemblage of garnet + rutile + kyanite + corundum in the peak metamorphic stage of pelitic granulite indicates that it may underwent ultra-high-pressure metamorphism, and the acidic plagioclase exsolution of clinopyroxene in garnet pyroxenite also suggests that it may be retrograded eclogites, which indicates that the deeper Cathaysian block may have eclogite metamorphism. Analyses of LA-ICP-MS zircon U-Pb dating indicate that the metamorphic age of pelitic granulite is 233.5 Ma–subduction/collision followed by rapid exhumation and cooling events. The events may relate with the amalgamation of the Indochina BlockSouth China Block North China Block in the paleo-Tethyan domain.