Investigation On The Rheology And Partial Melting Of Garnet Amphibolites Of The Lower Crust Of The Tibetan Plateau

The Tibet Plateau,as the world's largest continent collision orogenic belt that is still continuously subducting,its geodynamic mechanism and magmatic evolution have always been very concerned by the scientific community.Garnet amphibolite may be the main rock type in the middle-lower crust of the Tibet Plateau.However,there is currently no international experimental research on partial melting and deformation of garnet amphibolite,which largely limits our understanding of the deep crust.To explore the genesis of crustal-derived magmatism on the Tibet Plateau and the rheological strength,deformation mechanism,and seismic anisotropy of garnet amphibole,we take garnet amphibolite as the main research object and carried out the partial melting and deformation experiments of the garnet amphibolite.The main contents are the following four aspects:partial melting experiments on(1)garnet amphibolite and(2)the same garnet amphibolite mixed with 20 wt.%of a primitive Tibetan shoshonite,deformation experiments on garnet amphibolite,deformation mechanisms and seismic anisotropies of amphibole.1)We report direct experimental evidence for the origins of adakite-like potassic rocks with partial melting experiments on(1)garnet amphibolite and(2)the same garnet amphibolite mixed with 20 wt.%of a primitive Tibetan shoshonite.The experiments were conducted at 1.5-2.0 GPa and 800-1000?.The partial melts of garnet amphibolite have typical adakitic signatures and are calc-alkalic but lack the enrichment in potassium and other strongly incompatible elements(Rb,Ba,Th,U)that are characteristic of Tibetan adakite-like rocks.In contrast,all characteristic features of the natural adakite-like rocks are convincingly reproduced by the hybrid experiments.The input of mantle-derived shoshonitic mafic melts to a crustal source can be argued to provide not only the high concentrations of incompatible elements characteristic of adakite-like potassic magmas but also the heat necessary for crustal melting.Our experimental results demonstrate that,in the case of the Tibetan Plateau at least,the production of adakite-like potassic rocks in postcollisional settings can be best explained by such a model.2)We report axial compression experiments on synthetic fine-grained amphibolites with varied mineralogical compositions(100%amp,80%amp+20%grt,and 50%amp+50%pl)at deep crust conditions(750-900? and 1.5-2.4 GPa).Incorporation of plagioclase results in a significant increase in the strength of the amphibolite.The flow law of the 80%amp+20%grt amphibolite can be described by ?=10-6.95?3.2e(-133+7.6*P/RT),where ? is in s-1,? in MPa,P in GPa,and T in Kelvin.To further constrain the effect of plagioclase on strength of amphibolite,we combine the present experimental data to published flow laws for rocks with different plagioclase compositions(An30,An50)and contents.We conclude that amphibolites are 10-20 times weaker than mafic granulites,eclogites,or dry peridotites,depending on the plagioclase and garnet contents and compositions.An amphibole-rich deep crust will thus behave as a weak layer in the continental lithosphere,which supports the "crustal flow" model in Tibet Plateau.3)We have analyzed in detail the microstructural characteristics and intracrystalline deformation characteristics of amphibole from experimental and natural garnet amphibolite samples.The amphibole is mainly deformed by dislocation creep,with dominant activation of the[001](100)slip system and secondary(010)[100],{110}1/2<110>slip systems.Twinning deformation is also found in amphibole from Lhasa terrane.Amphibole displays cry stall ographi cally controlled rotations accommodating intragranular misorientations,which are dominantly[001]and to a lesser extent[010]in amphibole,which is inconsistent with the[001](100)slip system in amphibole.We calculated the GND densities for each of the three slip systems from the EBSD misorientation data.It indicated that although the[001](100)slip system contributed the main deformation,the GND densities of the secondary slip systems was greater than that of[001](100)slip system.To study the deformation mechanisms and CPO evolution of amphibole and assess the twinning effect,we simulated the amphibole deformation process under axial compression and simple shear deformation through the VPSC simulation software.A comparison of the experimental results with natural samples shows that amphibole develops an S-type(foliation type)fabric under axial compression,while under simple shear deformation conditions,amphibole mainly develops an L-type(lineation type)fabric.Meanwhile,the twinning will form a secondary point maximum of[001]with a small angle to the lineation,which can be used as another indicator to identify the twinning.4)We used MTEX software to calculate the seismic properties of experimental deformed amphibolite samples and natural amphibolite samples and discussed the effects of plagioclase and garnet on the seismic properties of amphibolite.The results show that different amphibole fabric types have different seismic properties.For S-type fabric,when the incident wave propagates parallel to the foliation,the seismic anisotropy of amphibole can reach 13.9%,and the fast S-wave polarization direction is perpendicular to the direction of compression,for the L-type fabric,when the incident wave propagates parallel to foliation and vertical to lineation,the seismic anisotropy of amphibole can reach 9.9%,and the polarization direction of the fast S-wave is parallel to the shearing direction.For both types,when the incident wave propagates perpendicular to the foliation,the anisotropy is almost zero.The addition of plagioclase and garnet hardly changes seismic anisotropy patterns,but it will change the value of wave velocity and anisotropy.The addition of plagioclase will reduce the P wave and S wave velocity and anisotropy of amphibolite.The addition of garnet will increase the velocity of the P wave and S wave of amphibolite and significantly reduce its anisotropy.Finally,we calculated the delay time caused by the seismic anisotropy of amphibolite and proposed that the approximately 20-25 km thick amphibolite layer can explain the crustal seismic anisotropy in the central and southeastern margins of Tibet.