Anatexis And Shear Deformation Of Migmatites And Granulite Lens In The Weihai Area,Sulu Orogen

Granulite-facies metamorphism（including HT-UHT metamorphism）and anatexis,which are closely related to orogenic evolutions,play a key role in revealing orogenic processes,thermal structure and rheological properties.Anatexis can forming melts which strongly affect the rheological properties,trigger crust deformation and promote the exhumation of deeply subducted crust.So,identifying the features and periods of anatexis and deformation is crucial to understanding the geological processes in the subduction channel.As one of the largest and well-exposed metamorphic terranes in the world,the Dabie-Sulu orogen preserves multiple-staged partial melting and tectonic evolution among various rocks,which has become a natural laboratory for studying anatexis and shear deformation in the subducted crust.A combined study of high precision structure-petrology mapping,detailed petrology,microstructure,various accessory minerals（zircons,monazites and titanites）geochronology and trace element analyses were carried out to study the leucosome veins,migmatites and pelitic granulites lenses in the Weihai area,Sulu orogen.This study identifies multiple-staged partial melting and deformation in macro-to micro-scales within these rocks and preciously constrain the period of the last shear deformation in the studied region.Crustal anatexis can happen during the orogenic processes as well as post-orogenic collapse episode,and is often accompanied by crumple deformation in some cases,which makes it difficult to link the accurate period with the metamorphic processes.Titanites of the deformed leucosome veins and migmatites in the Weihai area provide an ideal window to solve the problem.Samples within the ductile shear zone record at least two stages of deformation:the early fold deformation with axial NWW-SEE direction was overlapped by the later NNW-SSE ductile shear deformation.In the macroscopic scale,the leucosome veins shows as folds with thickened hinge zone and thinner limbs.The fold limbs are separated into lenses or augens during the shear deformation.The migmatites mainly occur as secondary S-Z folds near the deformed leucosome veins.In petrographic aspect,some quartz,plagioclase,biotite and titanite grains exhibit undulatory extinction.Titanite grains mainly grow oriented along the direction of melt belt,some of which occur as rotational augens.In the microscopic scale,EBSD results indicates that titanite grains are deformed with low-angle boundaries（2～10°）,deformation micro-twins and dynamically recrystallization.The maximum degree of misorientation is up to～16°.Triple junctions of coarse-grained K-feldspar and plagioclase,embayed quartz and leucosome pockets composed of plagioclase and quartz indicates obvious partial melting in micro-scale.Three titanite grains in the leucosome veins and migmatites are clarified as two groups:early remnant magmatic titanite and late anatectic titanite.The magmatic titanite are mainly in the cores of titanite,showing bright BSE,low Al and Y,and high Ca,Ti,Th,REE contents with high Th/U ratios.The anatectic titanite display dark BSE,opposite trace element compositions and mainly occurred as the rims of titanite with cuspate and patchy zoning.Rutile,quartz and plagioclase were common paragenesis mineral assemblage within or near the anatectic titanite.Combined with previous studies,we propose that the isotopic and trace elements were redistributed during anatexis and shear deformation.The anatectic titanite provides directed information on deformation and fluid activity during the anatexis,and yields a consistent lower intercept age（207～208Ma）,indicating the latest tectonism in the studied area.This is the first time to link intragranular plastic deformed titanite grains with in-situ U-Pb chronology to accurately constrain the period of deformation in the Sulu orogen,which provide a new thought to discuss the stages and periods of anatexis and deformation in the subduction channel.Apart from widely distributed migmatites and leucosome veins,some pelitic granulites also occur as lenses in the Weihai area.The pelitic granulite lens is gradually changed in lithology from the core to the margin:the undeformed coarse-grained granulite,the foliated fine-grained granulite,and the migmatized granulite.The undeformed coarse-grained granulite occurs in massive structure and preserves a peak granulite-facies mineral assemblages of garnet+sillimanite+plagioclase including antiperthite+quartz+rutile+biotite.The fine-grained granulite exhibits a weak foliated structure with similar mineral assemblages of the undeformed granulite.The migmatized granulite showed obvious gneissic structure,domino and rotational augen features,displaying ductile shear deformation.The leucosome layers of the migmatized granulite comprise plagioclase+K-feldspar+quartz,and the melanosome layers are consisted of garnet+sillimanite+plagioclase+quartz+biotite.Both field and petrographic features indicate that the granulite lens is increasingly foliated,fine-grained,partially melted and shear-deformed from the core to the margin.Both zircon and monazite in all samples recorded a consistent Paleoproterozoic age（ca 1.84 Ga）.The trace element composition（e.g.,low Th/U,high Hf/Y,Eu anomaly and depleted HREE in zircon,and high Gd _N/Lu _N with strongly depleted HREE in monazite）and the internal structure suggest that these Paleoproterozoic zircon and monazite have been grown under granulite-facies metamorphic conditions.Ti-in-zircon thermometers for the Paleoproterozoic zircons give very high temperatures（mostly>900°C）,suggesting that the samples have been undergone a UHT granulite-facies metamorphism during Paleoproterozoic.The Paleoproterozoic zircons in all rocks have consistentε_Hf（t=1840 Ma）values（ca-5.5）and two-stage(t _DM2)“crust”Hf model ages（ca 2.8 Ga）.Coupled with the previous studies on Paleoproterozoic granulite-facies rocks in the Jiao-Liao-Ji metamorphic belt（JLJB）,this study suggest that the studied pelitic granulite lens has a tectonic affinity with the North China Craton.Furthermore,both zircon and monazite grains in the margin of the granulite lens record the overgrowth from Triassic（229-210 Ma）anatectic melt.The Triassic new growth monazite widely exist in the foliated and migmatized granulites,whereas,only a few anatectic zircon grains were overgrown or recrystallized in the migmatized granulite.Triassic anatectic zircon grains give a low-temperature condition（687 to755℃）.This suggests that monazite is more sensitive than zircon in recording metamorphic events at low temperature.Compared with the Paleoproterozoic zircon grains,the Triassic anatectic zircon grains have deeply negativeε_Hf（t=230 Ma）values（Mean=-13.7±3.5）with greatly younger t _DM2 ages（ca 2.1 Ga）.Similarly,the new growth monazite grains have lower Th,higher Y and HREE contents with lower Th/U ratios than the metamorphic monazite grains,suggesting the fluid infiltration during the anatexis and deformation processes.This study proposes a tectonic model for the granulite lens in the Weihai area as follows.（1）The Paleoproterozoic（～1.84Ga）granulite-facies rocks as a part of the JLJB took part in the global-scale supercontinent evolution of Columbia.（2）During Triassic（229～210Ma）,they were mechanically dragged into the subducted crust of the Yangtze Block（YB）once again and involved in the continent-continent collision between YB the and the NCC.（3）At the exhumation stage,the“exotic”rock slice followed the HP-UHP rocks to be exhumated.In this period,it was remoulded by anatexis,shear deformation and fluid infiltration in the subduction channel.This thesis provides a case study that during continental collisions,crust from the upper plate also may be drawn into the subduction channel,and subsequently exhumed in association with anatexis and deformation.In conclusion,the leucosome veins,the migmatites and the granulite lenses in the Weihai area all preserve anatexis and deformation features during the exhumation stage of the Sulu orogen but behave differently.In the subduction channel,the leucosome veins and migmatites from the underlying plate exhibit strong partial melting and fold-shear deformation.The granulite lenses from the upper plate only show limited anatexis and deformation.This research suggests that during the exhumation stage,rocks in the subduction channel may have been undergone multiple periods of metamorphism and deformation,however,chronological information on deformation can only be recorded during the late amphibolite facies.