Metamorphic Evolution History Of The Yunkai Orogenic Belt,South China

Yunkai orogenic belt lies between Yangtze and Cathaysia block as an important part of southwestern segment of Wuyi-Yunkai orogeny in South China.The petrogenesis study of charnockites and high-temperature granulite enclaves in Gaozhou-Yunlu area is a key to understand middle to lower crustal compositions and to investigate the early Paleozoic tectonic evolution of South China.In this dissertation,we conducted detailed petrographic study,EMP mineral composition analyses and LA-ICP-MS zircon U-Pb dating for the charnockites,Grt-Crd pelitic granulite enclaves,Grt-Opx semi-pelitic granulite enclaves and calc-silicate granulite enclaves.Conventional thermobarometers and average P-T calculations combining phase equilibrium modeling were carried out to find out the P-T evidence preserved in these rocks.By studying the different stages of mineral assemblages in charnockites,we got the P-T conditions of 720?/7.0 kbar for pre-peak stage?M₁?,835⁸10?/6.5⁵.8 kbar for peak metamorphism-anatexis stage?M₂?and 740?/5.6 kbar for retrograde stage?M₃?,respectively.These P-T estimates define a clockwise P-T evolutional path with a pre-peak minor decompression and remarkable heating,and subsequently,a post-peak nearly isobaric cooling?IBC?process.The charnockites had experienced medium-low pressure,high-temprature granulite facies metamorphism-anatexis and then magma crystallization and retrograde processes.Cathodoluminescence?CL?studies showed that zircons from the charnockites display core-rim texture,LA-ICPMS zircon U–Pb dating yielded concordial ages of 431±4 Ma for the cores and 243±2 Ma for the rims,respectively.We also identified two different types of garnets in the thin sections:magma-type?Grt??and metamorphic-type?Grt??.Accumulating evidence suggest that the peak metamorphism and followed formation of magmatic Grt?might occur in the early Paleozoic?Caledonian?,whereas the retrograde metamorphism and contemporaneous formation of metamorphic Grt?probably occurred in the early Mesozoic?Indosinian?.In addition,P-T results for each metamorphic stage of granulites showed that 730⁷65?/5.7⁶.2 kbar?M₁?,835⁸55?/4.6⁵.1 kbar?M₂?,620⁶40?/3.6⁴.0 kbar?M₃?for Grt-Crd pelitic granulites;and 800⁸30?/7.4⁷.9 kbar?M₁?,855⁸60?/5.2⁵.6 kbar?M₂?,770⁸05?/5.0⁵.5 kbar?M₃?for Grt-Opx semi-pelitic granulites.LA-ICP-MS zircon U-Pb dating results show that the metamorphic zircons gave the ²⁰⁶Pb/²³⁸U weighted mean ages of443±5 Ma and 447±5 Ma for pelitic and semi-pelitic granulites,respectively.On the other hand,as for the calc-silicate granulites,conventional thermobarometers and average P-T calculations result in the following P-T conditions:803?/8.4 kbar for peak metamorphism?M₂?and 630?/6.5 kbar for retrograde metamorphism?M₃?,indicating a retrograded P-T path characterized by a post-peak decompression-cooling process.Geochronological study shows that zircon grains in calc-silicate granulites display clear core-rim texture,LA-ICP-MS zircon U-Pb dating results yielded a consistent early Paleozoic age for the rims?⁴35 Ma?and early Neoproterozoic age?⁹70 Ma?for the cores,interpreted as the early Paleozoic granulite-facies metamorphism on the early Neoproterozoic protolith,which could be considered as the products of Caledonian orogeny in South China.The P-T calculations of the pelitic and semi-pelitic granulite enclaves defined the near-isobaric cooling?IBC?clockwise P-T paths which were totally inconsistent with the near-isothermal decompression?ITD?P-T trajectories in Wuyi area.Combined with the dating results and changes among temperature and pressure of the pelitic and semi-pelitic granulites,suggested that it may be related to the ridge-subduction and the upwelling of asthenosphere through a slab window and caused widespread crustal extension;The P-T-t evolution paths of the charnockite and calc-silicate granulites are occurred later than the Grt-Crd granulites and Grt-Opx granulites,indicating a post-collision process of the Early Paleozoic orogeny.Moreover,the charnockite also suffered from overprinting of the Indosinian tectonic thermal events.Furthermore,we also studied the petrogenesis of charnockite and its associated granulite enclaves,they have geochemically complementary characters in the primitive mantle normalized spider diagrams and chondrite normalized REE patterns.The Perple_X thermodynamic modeling and trace element modeling results show that the charnockite and its granulite enclaves are the products of a same source.The former is the product of magma crystallization from the partial melting of protolith?pelitic siltstone?after the peak metamorphism-anatexis,while the granulite enclaves are the residuum after the partial melting.Based on the thermodynamic calculation of dehydration reactions,the granulite enclaves show low activity of water(a_H2O=0.13⁰.16).The Geo-f_O22 software was used for calculating magmatic oxygen fugacity on the basis of oxybarometers and thermobarometers for zircon grains,the results suggest that the charnockites show variable logf_O2 values,the Grt-Crd granulites and Grt-Opx granulites have lower and uniform logf_O2 values,indicating the peak metamorphic stage was possibly controlled by FeO+O₂=2Fe₃O₄?wustite-magnetite?buffer.The above calculation results show that the formation of charnockites and the peak metamorphic stage of granulite facies are closely related to CO₂ fluid.Therefore,these results allow us to propose that the rich CO₂ fluid of the original charnockite magma derived from the deep crust,and the fluid was released during magma crystallization,which caused the significant decrease of a_H2O in charnockite and its associated residual granulite enclaves.Based on previous studies,the triggering mechanism of early Paleozoic magmatic-tectonic thermal events in South China is most likely related to asthenosphere upwelling.In this dissertation,the geophysical simulation was carried out through Perple_X.We challenged the previous model of late-orogenic lithospheric mantle and lower crust delamination in the Wuyi-Yunkai orogen,and our simulation results revealed that the Chayuanshan basalts,which are from the lithosphere mantle,had lower density and P-wave velocity than the normal lithosphere mantle.Therefore,the lithospheric mantle in the Wuyi-Yunkai orogen can not be delaminated during this time.In conclusion,combined with the petrography,mineral chemical analyses,P-T calculations and thermodynamic modeling results,and the features of Caledonian low pressure and high temperature metamorphism,rock association of sanukite,Nb-rich basalt-dacite-high-Mg andesite,A-type charnockite,gabbro plutons,copper deposit and MORB-like basalt in the study area.These results can be reasonably explained by a high-temperature extension of the upwelling of asthenosphere after the ridge subduction and opening of the slab window,and later post-collisional extension mode.Thus,the early Paleozoic Wuyi-Yunkai orogen in South China was an ocean/continent subduction-collision orogenic belt,the“Huanan Ocean”was not completely closed until the early Paleozoic.