The Refined Crustal Structure Beneath The Mid-northern Margin Of The North China Craton: Implications From The Crust-mantle Evolution

The lower continental crust,as the link between the lithospheric mantle and shallow crust,provides the most important information for exploring the earth.The widespread of the Precambrian complex and Phanerozoic deep-seated xenoliths in the North China Craton?NCC?make it to be an ideal area to study the lower crust.In this study,data on zircon U-Pb age and Hf-isotope analyses for felsic granulite xenoliths in Hannuoba,terrain granulites and gneisses to the south of Hannuoba are reported.Additionally,the whole rock geochemistry,isotopic compositions and seismic wave for pyroxenite xenoliths and terrain granulites are gained from the same area.Based on the data,this study is trying to expound the episodic crustal accretion and reworking from the Archean to the late Mesozoic time and build the refined layered architecture of the lower crust beneath the mid-northern edge of the NCC.The high-pressure mafic granulites in Shangyi and ordinary mafic granulites in Mashikou are located in Huai'an complex.The mineral assemblage of the former is Cpx + Pl + Qz ± Opx ± Am ± Grt,the Opx + Pl symplektite around the Grt by isothermal decompression can be observed.In contrast,the felsic granulite band which consisted of Pl + Kfs + Cpx + Qz + Opx can be observed in Mashikou,and the mineral assemblage of Mashikou mafic granulites is Cpx + Opx + Pl ± Kfs ± Am ± Bi.The Huai'an mafic granulites have high Al2O3,the Na2 O + K2 O increases while MgO,FeOT and CaO decrease with the ascending SiO₂.The Cr and Ni get up with the elevated MgO.Those imply the crystal fractionation of Ol and Py of the parent magma.They have depletion in HFSE and Ti,similar to the island arc basalts.The zircon U-Pb age analyses for one Shangyi mafic granulite and Mashikou felsic granulite show the protolith formation time at ².46 Ga.The mafic granulites have depleted ?Nd?t??-0.3 +4.5?and ?Hf?t??+2.1 +11.0?that approach to the depleted mantle at ².5 Ga,implying that the parent magma of the protolith derived from the depleted mantle with rare crustal contamination.The felsic granulite in Mashikou has high SiO₂,Al2O3,K2 O,Na2O and low Fe2O3 T,MgO,it's enriched in LILEs and depleted in HFSEs.The analysis of magmatic zircon show that the protolith of the felsic granulite also formed at ².46 Ga,these zircons have low positive ?Hf?t??+2.9 +5.3?and ancient Hf model age?>2.7 Ga?.The underplating of the mantle derived magma may lead to the remelting of the preexisted ancient?> 2.7 Ga?lower crust,and the mixed magma should be the source of the felsic granulite which has low positive zircon ?Hf?t?from +2.9 to +5.3.The granitic gneisses in Zhuozi and gneisses in Liangcheng are collected from the Jining complex.The former is composed of Pl + Kfs + Qz in major and minor Am,Cpx and Ms,the mineral assemblage of the latter is Pl + Kfs + Qz + Grt ± Bi.The analysis of zircon U-Pb age shows that the protolith of the granitic gneisses formed at ².55 Ga,metamorphism at ².40 Ga and ¹.87 Ga are also recorded.The zircons with age of ².55 Ga and ².40 Ga have positive ?Hf?t?from +4.7 to +7.7 and from +1.2 to +6.6,respectively,implying the juvenile crustal material resulted from the same tectonic thermal event.The metamorphism at ¹.87 Ga is related to the collision between the eastern and western block of the NCC,that resulted in the reworking of the lower crust??Hf?t?=-6.0 +1.0?.The zircon ages in Liangcheng gneisses show that they were deposited mainly at 2.20-1.98 Ga.These zircons have wide range of ?Hf?t??-2.5 +8.1?,indicating the mixed source for the protolith.In addition,there are also metamorphic zircons at ¹.95 Ga with ?Hf?t?of-0.3 +2.0 in the gneisses,related to the collision between the Yinshan and Erdos blocks.The deep-seated xenoliths carried by alkaline basalts in Hannuoba include granulite and pyroxenite.The felsic granulites are granoblastic with mineral assemblage of Pl + Oz + Opx + Cpx.The felsic granulites have complicated zircon U-Pb ages: upper intercepts at 2449 Ma and 1880 Ma,concordant age between 140-73 Ma.They represent the protolith formation time.The zircons altered by metamorphic recrystallization didn't give meaningful ages.The late Mesozoic and 1880 Ma zircons have negative ?Hf?t??-18.9 ^-23.3 and-10.1 ^-3.7?,the Hf model ages are between 2.4-2.6 Ga and 2.8-3.2 Ga,respectively,indicating the reworking of the mid-late Archean crust.The zircons at 2499 Ma have positive ?Hf?t??+7.5 +10.6?,Hf model age peaks at 2.5 Ga,indicating the crustal accretion.The pyroxenite xenoliths include orthopyroxenite,websterite?G1: black,G2: green?and Grt-pyroxenite.The pyroxenites without garnet have cumulate structure,the orthopyroxenite consists of dominated Opx and minor Cpx,the websterites consist of 40-60% Cpx + 50-35% Opx ± Pl.Cpx in G1 is black with Mg# of 77.6-79.3,Cpx in G2 is green with Mg# of 89.8-90.3.The garnet pyroxenites consist of Grt + Opx + Cpx + Ol with minor Rutile and Ilmenite,the gradual contact with peridotite xenolith can be observed in the margin of the garnet pyroxenite.The analysis of zircon U-Pb age for G1 websterite indicates the formation age at 158 Ma.G1 and G2 websterites crystallized at temperature of 865-934 °C and 831-896 °C,respectively,similar to the mafic granulite while lower than the peridotite xenoliths in Hannuoba.We thus deduce that the websterites derived from the lowermost of the crust.The Grt-pyroxenites record high temperature of ¹050 °C and pressure > 1.5 Gpa.The websterite and orthopyroxenite have high MgO,wide range of Mg#?66.1-89.4?and low Ni,suggesting the cumulative origin,consistent with their cumulate structure.The Grt-pyroxenites have high Mg#?89.4-90.3?,and high abundances of LILEs and Ni.We thus interpret it as the reaction product between evolved melt and peridotite.The orthopyroxenite and G1 websterites have more evolved initial 87Sr/86 Sr ratios,?Nd?t??-14.0 ^-18.7?and ?Hf?t??-17.0 ^-29.2?than G2 websterites,the lower crust materials contributed to the precursor sources of the orthopyroxenites and G1 websterites.The Grt-pyroxenites have depleted isotopic composition?87Sr/86 Sr = 0.703576,?Nd?t?= +5.7,?Hf?t?= +20.7?,the reacted melt may come from the asthenosphere or fresh oceanic crust.All the pyroxenites have ²⁰⁶Pb/²⁰⁴ Pb of 16.18-18.82,207Pb/²⁰⁴ Pb of 15.27-15.56,208Pb/²⁰⁴ Pb of 36.28-38.71.The 208Pb/²⁰⁴ Pb are much higher than the NHRL and have been delineated as Dupal anomaly,implying the involvement of recycled oceanic curst in the magma sources.The measured seismic velocities show that the Vp of felsic granulite xenoliths,Huai'an mafic granulites and pyroxenite xenoliths are 5.25-6.09 km/s,6.33-7.44 km/s and 6.80-7.67 km/s,respectively.Although the much lower Vp of felsic granulite xenoliths is resulted from the alteration,all the Vp has positive relationship with SiO₂.In addition to our own research,the published data of mafic granulite xenoliths have been intergrated to build the lithological-age architecture corresponded to the seismic refraction profiles: the upper lower crust?24-33 km?consists dominantly of Archean?².7-2.5 Ga?felsic granulite terrains with subordinate felsic granulite xenoliths that reworked at 140-120 Ma,average SiO₂ is 62.1 wt.%;both Precambrian and late Mesozoic mafic granulites?including terrain and xenolith?are important constituents of the middle lower crust?33-38 km?,average SiO₂ is 48.1 wt.%;major late Mesozoic?140-120 Ma?and less Cenozoic?45-47 Ma?granulite and pyroxenite xenoliths are presented in the lowermost crust?38-42 km?,average SiO₂ is 48.1 wt.%.This model reflects the vertical accretion and reworking of the crust resulted from the magma underpalting that responsible for the lithospheric destruction beneath the mid-north margin of the NCC during late Mesozoic.