Metamorphic And Deformation Analysis Of Hornblende Rocks From Caohekou-Jiguanshan Area,Eastern Liaoning Province

Amphibolites from South Liaohe Group at Caohekou-Jiguanshan area are widespread as layers,blocks and lenses within Palaeoproterozoic metamorphicsedimentary rocks and granite,which are the products of retrograde metamorphism of diopside-amphibolite and experienced two-stage retrograde metamorphism.Amphibolites with different deformation characteristics from research area can be divided into four types(massive,weak-gneissic,medium-gneissic,strong-gneissic).Affected by varied temperature and pressure,amphibolites with varied deformation degrees show different characteristics in microstructure,rock fabric and deformation mechanism: the massive amphibolites(P=0.39 Gpa,T=639 ?)formed before the orogeny and display no or weak deformation;Amphiboles from Weak-gneissic amphibolites(P=0.52 Gpa,T=753?)deformed by recrystallization growth under the stress in the same direction and display LPO of(100)planes;Plastic deformation are commonly observed from amphiboles of medium-gneissic amphibolite(P=0.61 Gpa,T=771?)with the LPO of both(100)planes and [001] axis and dislocation slip is considered as the main deformation mechanism.Strong-gneissic amphibolites(P=0.77 GPa,T=686?)display long-columnar amphiboles subgrains and twins and both of them show the same direction as the main crystal with the strongest LPO of(100)[001] controlled by dynamic recrystallization,twinning and twinning nucleate recrystallization.Besides,during the process of deformation,the greater varity of pressure(?0.38Gpa)could be related to the thickening crust due to the orogeny.Zircon U/Pb analysis show that amphibolites emplaced at 2150 Ma,then experienced retrogressive metamorphism under amphibolite facies at 1893 Ma and migmatization at1852Ma(second phase of retrogressive metamorphism under amphibolite facies),in addition,rocks are reformed by sheer deformation from 1850 Ma to 1900 Ma.All the process above developed amphibolites in research area with different deformation/metamorphic characteristics.In the hornblende rocks,the hornblende is often considered as a passive “weak”phase,but under certain conditions,it can also be transformed into a “strong” phase of active deformation.The transformation of “strong” and “weak” phases support relationship is actually a relatively concentrated process of the same mineral composition,which is common in the middle-lower crust rheology.The deformation in such case not only causes the inhomogeneity of physical and chemical properties of the rocks in the deep crust,but also reflects the heterogeneity of the rocks in the middlelower crust and the differences in the rheological properties of the rocks.