Abstract:Eight aspects related to the study of Early Precambrian metamorphic stratigraphy are dealt with in this paper: general situation of the study; complexity of the stratigraphy and difficulties of the study; thoughts and methods of the study; division of rock suite, rock group and complex rock; researches on important regional discordances; types and characteristics of protolith and recovery of palaeoenvironment; correct differentiation of the metamorphic strata, TTG rock system and other metamorphic plutonic rocks; determination of the ages of the metamorphic strata. In addition, three suggestions are put forward for future work.

Abstract:There were at least four microcontinents on the eastern margin of the Tarim paleocontinent prior to Early Paleozoic. They are, from north to south, Longshoushan, Qilian, Oulongbuluke and Qaidam, which have somewhat similar yet different Early Precambrian metamorphosed basements and Mesoproterozoic meta_sediments. Quite complex geological evolution was detected along the margins of these microcontinents, especially along the margin of Qaidam, where a Neoproterozoic granite gneiss belt over 700 km long is developed. In addition, Cambrian and Ordovician volcanic arc and ophiolite melange were superimposed upon the microcontinental margins. From north to south, the north Qilian ophiolite melange,the south Qilian ophiolite melange, the Shaliuhe_Yuqiahe high_pressure and super high_pressure meta_morphic belt and the Qingshuiquan ophiolite melange in central Kunlun Mountains are developed between two adjacent microcontinents, representing suture belts between two microcontinents. The suture belts are from Cambrian to Ordovician in age. Geological characteristics of these microcontinents, especially the characteristics since Neoproterozoic, are generally similar to features of the Tarim paleocontinent. However, the time and location of amalgamation between the Tarim and the microcontinents require further investigation.

Abstract:The northern margin the NS-extending ancient continental block of Archean gran- ulite facies in northern Hebei Province is intersected by the WE-stretching Early Proterozoic orogenic belt. Lots of residual oceanic crusts such as serpentinized harzburgites, spinifex picrite, tremolitite, troctolite, rodingite, spilite, retrograded eclogite, pillow basic lava and plagiogranite are found in the continental marginal sedimentary-accreting zone. These crusts and continental marginal sedimentary rocks make up a typical ophiolitic mélange belt. It can be con- cluded that this ophiolitic mélange belt was formed progressively from south to north between Early Proterozoic and the end of Proterozoic.

Abstract:Similar to metamorphism, the sequence of geological events can also be used to trace the pTt paths of an orogenic process. The two independent approaches used for inverse modeling of pTt paths are complementary in understanding the dynamic evolution of an orogenic belt. The eventuality of geological processes, the CW-pTt and CCW-pTt paths of geological events and their dynamic significance are discussed in this paper.

Abstract:Recent studies reveal records of weak metamorphic-magmatic events of 1 300~ 1 000 Ma and 800~650 Ma in the North China Block (NCB), which probably correspond to collision events (Sibao orogenic event) and breakup events in the South China Block(SCB).The Sibao-age ophiolitic complex in NCB is represented only by Neoproterozoic North Qinling orogenic belt. Some granitic intrusive rocks were recognized along the North Qinling belt,which demonstrate properties of collisional and post-collisional granites. There are some volcanic-sedimentary rocks distributed along the northern margin of NCB, most of which do not show characteristics of the complex in the divergent margin, but probably reflect extensional process of the continent and continental margin. The rocks of 800~650 Ma are mainly basic dikes in the enriched mantle and sedimentary successions in continental rift, most likely related to the breakup of Rodinian supercontinent. The characteristics of sedimentology and palaeontology suggest that NCB was quite different from SCB in Proterozoic, but similar to the Siberia block. Based on the above discussion, the authors hold that, being a part of Rodinia supercontinent and located at its edge, NCB seems not to connect with the SCB but is quite close to the Siberia block.

Abstract:On the basis of garnet zonation, a pT path is inversely modeled for medium_pressure metamorphism of the southern Qinling orogenic belt. The path indicates that the garnet zonation recorded the pT increasing process before the metamorphic peak, and that the maximum temperature was coincident with the maximum pressure in the process, probably corresponding to a continuous crustal thickening process. Following the metamorphic peak, a rapid uplift accompanied by tectonic processes took place, resulting in a pT path of ITD type. The present study is helpful to understanding the medium_pressure metamorphic process of the continental collision orogenic belt.

Abstract:The orthoamphibole association is a special rock type developed mostly in Precambrian metamorphic terranes. It is rich in Mg and Al but depleted in Ca and K. Its major mineral constituents include cordierite, orthoamphibole, Al2SiO5 minerals, staurolite, garnet, plagioclase, micas and clinoamphiboles. Associations of this kind are distributed in a lot of ancient metamorphic complexes throughout the world, with some of them closely related to sulfide deposits. According to metamorphic conditions, the orthoamphibole association can be tentatively divided into two types: the lower pressure type and the high pressure type. The former type is characterized by the cordierite- anthophyllite-cummingtonite association, while the latter type is Al2SiO5 minerals-gedrite association, sometimes with garnet. The Al2SiO5 in high_pressure terranes is kyanite. Cordierite and staurolite mainly formed corona textures surrounding kyanite or garnet, indicating the nearly adiabatic uplift of a metamorphic terrane. The present paper deals with the mineral paragenesis, the baric types, the related metamorphic reactions and the conditions under which these rocks were formed.

Abstract:Ultra-high pressure metamorphic minerals were found in three types of eclogite in western Tianshan Mountains, inclusions of coesite pseudomorphs in garnets were detected in type I eclogites interlayered with mafic blueschists and carbonate eclogites, quartz exsolution lamellae in omphacites were recognized in typeⅡeclogites with pillow structure,and magnesite of metamorphic origin was discovered in typeⅢlenticular calcite/dolomite eclogites within Based on the study of mineralogy and petrology, the metamorphic evolution of typeⅠeclogites can be divided into three stages, i.e. pre-peak ultra-high pressure (UHP) metamor- phicstage (356~433℃, 0.8~1.0 GPa), peak UHP eclogite-facies stage (496~598℃, 2.572~2.666 GPa) and retrograde epidote blueschist_facies stage (500~530℃, 1.0~1.2 GPa). The authors have set up phase equilibrium in NCMASCH system (Na2O-CaO-MgO -Al2O3-SiO2-CO2-H2O), and the calculation shows that the peak metamorphism of mag- nesite_bearing glaucophane eclogites in western Tianshan Mountains occurred at 2.7~2.8 GPa and 525~607℃, XCO2<0.006. Consequently, the eclogites from western Tianshan Mountains have obviously undergone UHP metamorphism, and southern Tianshan orogenic belt might be the largest UHP metamorphic belt formed by subduction of oceanic crust in the world.

Abstract:According to data of fluid inclusions within minerals and thermochemical estimation of dehydration reaction during granulite-facies metamorphism of Jining Group and Qianxi Group, the fluids existent at the peak stage must have been carbonic, withx(CO2+CH4)≈70%~80% andx(H2O)≈10%~20%, the density is generally 1.0~1.1 g/cm3. In the Jining Group, components of the metamorphic fluids and activity of H2O seem to have been spatially inhomogeneous, and were apparently controlled by lithology. TheαH2Ovalues of Al_rich gneisses, biotite hypersthene gneisses and basic granulites are 0.1~0.2, 0.2~0.4 and 0.4~0.6 respectively. Such inhomogeneity indicates that the fluids might have experienced internal buffering, and did not permeate pervasively through rocks. They were probably developed as a result of various kinds of devolatilization and oxidation of the organic materials (now as crystalline graphite) extensively distributed in rocks, both of which must have occurred at the progressive metamorphic stage. Moreover, H2O could escape from the system more easily than CO2, and this might be partly related to their carbonic affinity. On the contrary, the components and physical habits of fluids during metamorphic peak of the Qianxi Group were rather homogeneous in space, being independent to lithology. In Taipingzhai district, for example, theαH 2Ovalues of rocks of all the three types are 0.13~0.25 with XH2O≈0.1~0.2. These features suggest that the fluids in this region were probably of external_buffering, and had pervasively permeated through rocks. CO2in fluids might mostly have come from underplating mantle_source mafic magma and its crystallization process.

Abstract:According to the observation of mineral compositions and assemblages of the amphibolites along the China-Pakistan highway profile in southern West Kunlun Mountains, it is held that the components of the volatiles directly affect the assemblages and compositions of amphibole, biotite, scapolite and titanite. Someone once believed that scapolite did not experience exolution, but this opinion seems not to stand. Pyrite rods are actually exsolved parallel to the {100} crystal faces, henceforth the state of sulfur in scapolite is most likely S2-, not SO4 2- . Scapolite is the reservoir of such volatiles as Cl, CO3 2- and S2- in the early metamorphic process. In the late shear deformation period, cooling and/or decompression result in the release of the volatiles, and the associated hydrothermal process might form metal sulfides and even lead to mineralization.

Abstract:SHRIMP analyses of zircons from the Dawaliang granite define a crystallisation age of 2176 Ma for the granite, which was affected by the thermal event aged 2107 Ma.

Abstract:Neoproterozoic Jinningian deformed granites were recognized from the formerly-defined Alax Group. The ages of zircons from two deformed granite samples determined by step-evaporation technique are 971 Ma and 845 Ma, respectively, implying that they were formed by Jinningian magmatic event. The deformed granites are mainly augen gneisses, characterized by high content of SiO2, alkali(Na 2O+K 2O) and REE, low content of MgO, TFeO and CaO, K2O>Na2O, and intense negative Eu anomaly. Geochemical characteristics and tourmaline_ and muscovite-bearing rock associations suggest that Jinningian deformed granites in Alax area were formed in the syn-collisional setting. The discovery of Jinningia deformed granites demonstrates that magmatic activity of early Neoproterozoic affected the northwest margin of North China platform.

Abstract:Anatexis resulted in the formation of leucosomes of various shapes and sizes in leucoleptites of Neoarchaean Fuping Group. The anatectic derivatives have lower REE content and HFS elements than their melting parent rocks. In most cases, however, they show similar element distribution patterns. The derivatives formed at the early stage of anatexis are different from the parent rocks in concentrations and distribution patterns of elements. With the development of anatexis, the difference between the derivatives and the parent rocks in Nd isotopic composition became smaller and smaller until approaching to disappearance at last. The accessory minerals, especially zircon and apatite, played an important role in the variation of elements and Nd isotopic compositions of the anatectic derivatives.

Abstract:Carbonatite dykes and alkaline basic dykes are widely distributed in Bayan Obo. A study on their petrology, mineralogy, petrochemistry, REE and trace elements indicates that carbonatite dykes are igneous carbonatite, and REE content of some dykes is 14.675%, similar to that of dolomite carbonatite, implying that the two kinds of carbonatite are both REE-rich rocks. K2O+Na2O content of basic dykes in the ore district is higher than that of general basic rocks, suggesting that these dykes are alkaline rocks and belong to alkaline basic dykes. Carbonatite dykes are similar to alkaline basic dykes in REE and trace element patterns, and they also have similar Sr, Nd and Pb isotopic characteristics. These features indicate that the two kinds of dykes were formed in a special and similar source area, thus genetically related to each other.