Phase Equilibria Modelling And P-T Evolution Of The Deep Seated Xenoliths From The Southeastern Margin Of The North China Craton

A variety of deep-seated xenoliths in the Mesozoic dioritic porphyry is located at the southeastern margin of the North China Craton(NCC)and most of them belong to the Precambrian lower-crustal(metamorphic basement)rocks and a part of them are mantle-derived rocks.They are mainly exposed in the Xuzhou-Suzhou region.The main rock types of lower-crustal xenoliths comprise a variety of lithologenesis,including garnet amphibolite,garnet granulite,garnet-bearing plagioclase-amphibole gneiss,granitic gneiss and so on.Mantle-derived rocks are spinel-bearing garnet clinopyroxenite,phlogopite clinopyroxenite and spinel pyroxenite.Detailed field investigation,petrologic study and electron probe analysis of minerals are carried for the lower-crustal xenoliths(garnet amphibolites and garnet granulites)and mantle-derived rocks(spinel-bearing garnet clinopyroxenite and phlogopite clinopyroxenite)to reconstruct different stages of the metamorphic process.In this study,in view of multistage metamorphic evolution and reworking that would affect the using of conventional geothermobarometers.So,we use d conventional geothermobarometers to estimate P-T conditions and phase equilibria modelling was for the first time used to better constrain accurate peak P-T conditions of the mafic lower-crustal xenoliths.Conventional geothermobarometer was applied for the deep-seated xenoliths reverse modelling of metamorphic evolutions,while phase equilibria modelling were first used for the lower-crustal xenoliths forward modelling to verify and supplement the inversion results.Combined with the previous zircon U-Pb dating,it is indicated that the deep-seated xenoliths experienced a clockwise P-T-t evolution with near-isothermal decompressional process,futher suggestive of the late Paleoproterozoic subduction or arc-continent collisional setting.In this regard,the studied region altogether with Jiao-Liao-Ji belt is further documented to to make up a whole Paleoproterozoic collisional orogen in the Eastern Block of the NCC.Garnets of the mafic lower-crustal xenoliths have a weak zonal structure in composition with homogeneous cores and were surrounded by thin rims with an increase in XMg and a decrease in XCa(or XMg).Garnet amphibolites have increased in XMg from 0.25 to 0.32,while XCa decreases from 0.27 to 0.19.However,in garnet granulite XCa and XMg decrease gradually from the core to the margin,which is respectively 0.25-0.19 and 0.28-0.17.Clinopyroxene contain varying amounts of Na2O and Al2O3 as well as amphibolc of Ti02,which of them occur as inclusions and core of matrix porphyroblasts have higher sodium and aluminum content(Na2O is 0.83-1.54 Wt%and Al2O3 is 2.74-7.01 Wt%)and margin of matrix porphyroblasts have relatively low sodium and aluminum content(Na2O is 0.24-0.33 Wt%,Al2O3 is 0.68-2.67 Wt%).Plagioclases are different in calcium contents.The plagioclases exist in the form of inclusions with high Ca contents and its composition is An27Ab70Or3(garnet amphibolite)and An29Ab69Or3(garnet granulite),while those produced by matrix porphyroblasts and symplectic had lower calcium content(An5-36Ab62-95Or0-2 and An6-48Ab50-94Or0-1 in garnet amphibolites,An5-51Ab48-94Or1-2 and An32-79Ab21-68Or0-3 in garnet granulites).Different types of amphiboles also have a large difference in titanium content.The brown amphiboles have a high content of titanium,with the content of TiO2 being 0.53-1.44 Wt%,whereas the green amphiboles have very low or almost no titanium content and the Ti02 content is 0.03-0.38 Wt%.The results show that the Jiagou mafic xenoliths underwent high-pressure granulite facies,low-pressure granulite to high-amphibolite facies,amphibolite facies and greenschist facies metamorphic processes.Their P-T conditions have been estimated by using traditional thermobarometry,the range of high-pressure granulite facies metamorphism is 824-844 ?/1.5-1.9 GPa;the range of low-pressure granulite to high amphibolite facies is 667-730 ?/1.0-1.3 GPa and the range of amphibolite facies metamorphism is 609-646 ?/0.4-0.8 GPa.Peak metamorphic P-T conditions are also calculated by the smallest garnet X(g)(Fe2+/(Fe2+Mg))contours and the smallest plagioclase Ca(pl)(Ca/(Ca+Na))contours in NCFMASHTO(Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3)system,which are consistent with those estimated by conventional geo thermobarometers.The new results show that the peak and decompressional P-T conditions for the rocks are 850-900 ?/1.4-1.6 GPa and 820-850 ?/0.9-1.3 GPa,respectively,suggestive of high and middle-low pressure granulite facies metamorphism.The LA-ICP-MS zircon ages of garnet amphibolite and garnet granulite can be categorized into three groups of?1.91 Ga,?1.83 Ga and?1.75 Ga,corresponding to the time of high-pressure granulite facies,mid-low pressure granulite facies and amphibolite facies metamorphism.The mantle-derived rocks have a typical character porphyroblastic structure of inequigranular.They are mainly composed of clinopyroxene,garnet and spinel,with minor amphibole and biotite or phlogopite.The exsolution structure in clinopyroxene is a result of the slow cooling of the upper mantle under high temperature conditions.Garnet corona around spinel indicates that the mantle-derived rocks are formed under the spinel-garnet transition facies,with possible temperature and pressure conditions of 1.6-2 GPa and 900-1100 ?.The extremely high Mg#values(62-77)support the upper mantle origin.The presence of amphibole and phlogopite may reflect the original rock composition(water-bearing)and the formation of tectonic backgrounds of upper mantle,which are consistent with their production on the edge of the North China Craton.