Metamorphic P-T-t Paths Of High-Pressure(HP) And Ultrahigh-Temperature(UHT) Pelitic Granulites In The Helanshan,and Their Tectonic Implications

Metamorphic supracrustal rocks（such as metapelitic rocks）within collisional orogenic belts usually experienced the whole process of the formation and evolution of the orogenic belt,including the subduction of continental lithosphere after ocean closure,continental collision,uplift and exhumation of continental crust and collapse of orogenic belt after slab breakoff.The information of temperature,pressure and age of metamorphic supracrustal rocks in the processes of burial depth and uplift could be well recorded in metamorphic P–T–t evolution paths,and it is essential for undersatanding the orogenic processes.The Khondalite Belt of the North China Craton is a typical Paleoproterozoic collision orogenic belt,and a large number of khondalite series that have experienced upper amphibolite to granulite facies metamorphism are exposed in its interior.The Helanshan Complex is located in the western part of the Khondalite Belt,where both high-pressure（HP）and ultrahigh-temperature（UHT）pelitic granulites are exposed,which are important to the tectonic evolution of the Paleoproterozoic Khondalite Belt.In this paper,we have carried to a combined study of petrography,mineral chemistry,metamorphic PT calculation,and monazite and zircon U–Pb dating on the HP（kyanite/sillimanite-garnet gneisses）and UHT（spinel-bearing sillimanite-garnet gneisses）pelitic granulites,in order to construct their complete P–T–t evolution paths.Based on these results,the genetic relationship of the two types of granulites and the tectonic setting of their formation are determined,which provides an important constraint for limiting the orogenic evolution of the Khondalite Belt,and also provides an important reference for exploring the Paleoproterozoic collisional orogeny process.The main results are summarized as follow:1.Based on petrographic observations,the metamorphic evolution stages of Helanshan HP pelitic granulites can be divided into four stages（M₁–M₄）,and the mineral inclusions and edge reaction structures in the garnet record the complete metamorphic evolution process.The peak metamorphic stage（M₂）is characterized by kyanite/sillimanite+rutile.Garnet diffusion and growth modeling suggest that high-temperature（850°C）diffusion had relatively little effect on the zoning of trace-elements in the garnets,and that the zoning can therefore be interpreted as initial growth zoning which can records the prograde metamorphism information from the earliest metamorphic stage to gradually growing to peak or near peak stage.The combination of garnet trace element thermobarometer,phase equilibrium modeling,ternary feldspar thermometer and biotite Ti thermometer was used to study the Helanshan HP pelitic granulites,and to determine the PT conditions of different metamorphic stages.The metamorphic conditions at the M_2-1 stage（kyanite stable domain）of the peak metamorphic stage were 794–811°C/9.8–10.6 kbar,while the metamorphic conditions at the M_2-2 stage（sillimanite stability domain）were 800–831°C/8.5–10.2 kbar.Combined with the occurrence of monazite and zircon,internal mineral inclusions,trace elements and U–Pb dating,it is determined that the peak metamorphic age of the HP pelitic granulites is 1.96–1.95 Ga,the retrograde and cooling age is 1.93–1.92 Ga,and the age of 1.89–1.87 Ga may represent a metamorphic era of cooling to near the solidus.Finally,the complete clockwise P–T–t evolution path of the HP pelitic granulites is constructed,involving the heating and pressure rising during the prograde metamorphic stage,and the isothermal decompression and near isobaric cooling after peak metamorphic stage.2.The prograde geothermal gradient of the HP pelitic granulites in the Helanshan Complex was determined to be～16°C/km by inversion of the garnet trace element thermobarometer,which is higher than that of the pelitic schists in the Himalayan Orogen（～12°C/km）,indicating that the buried geothermal gradient in the Paleoproterozoic orogen is higher than that in present-day orogen.3.Based on petrographic observations,the metamorphic evolution stages of Helanshan spinel-bearing UHT pelitic granulites can be divided into three stages（M₁–M₃）,and the peak metamorphic stage（M₂）is characterized by the occurrence of spinel.The combination of phase equilibrium modeling and ternary feldspar thermometer was used to study the granulites,and to determine the PT conditions of different metamorphic stages,of which the peak metamorphic condition is 950–1050°C/6–8 kbar.Combined with the occurrence of monazite and zircon,internal mineral inclusions,trace elements and U–Pb dating,it is determined that the peak or near peak metamorphic age is 1.93–1.92 Ga.Finally,the clockwise P–T–t evolution path of the UHT pelitic granulites is constructed,involving the decompression–heating and near isobaric cooling.In addition,the"corundum+chlorite"inclusions within the spinel in the spinel-bearing UHT pelitic granulites indicate the presence of sapphirine in the peak metamorphic stage.4.The Helanshan HP and spinel-bearing UHT pelitic granulites were experienced the same metamorphic evolution process in the early metamorphic stages.The difference is that the UHT pelitic granulites experienced superposition of UHT metamorphism during the decompression process,and its formation be related to the underplating or emplacement of local mantle-derived magmas in a post-collisional extensional setting.5.The formation and evolution of the Khondalite Belt include:（1）The stage before the collision between the Yinshan and Ordos Terrane（2.0–1.96 Ga）,the protolith of the khondalite series was deposited on the active continental margin at this stage;（2）syn-collision stage（1.96–1.95 Ga）,the crustal thickening led to the burial of the protolith of khondalite series to the depth of the lower crust and undergoing HP granulite facies metamorphism,and the melts formed during partial melting were separated and aggregated to form syn-collisional S-type granites;（3）Then the salsb breakoff caused tectonic extension and rapid crustal uplift,and the khondalite series was uplifted to the level of the middle crust at 1.93–1.92 Ga.During this process,local mantle-derived magmas underplating or emplacement,which lead to the khondalite series that have previously undergone granulite facies metamorphism to undergo decompression and/or heating and melting,and finally form M-HP and UHT pelitic granulites,and post-collisional S-type granites.