Evolution Of Mesozoic And Cenozoic Lithospheric Mantle In Eastern North China Craton-Evidence From Igneous Rocks And Their Deep-Seated Xenoliths And Xenocrysts

The thesis takes the Early Cretaceous high-Mg diorites, Late Cretaceous and Paleogene basalts as well as their deep -seated xenoliths and xenocrysts in the eastern North China Craton as its research target. The formation times for these igneous rocks in the eastern North China Craton are determined using K-Ar and LA-ICP-MS zircon U-Pb dating methods. Based on the major-, trace-elements, and Sr–Nd–Pb isotopic data of these basalts and mineral chemical data from the deep-seated xenoliths and xenocrysts, the natures of their magma sources and the spatial-temporal variations are discussed for the Early Cretaceous high-Mg diorites,Late Cretaceous and Paleogene basalts in the eastern North China Craton (NCC). Taken together, the Mesozoic and Cenozoic evolution model of the lithospheric mantle in the eastern North China Craton is set up.1 Formation times of igneous rocks borne in deep-seated xenoliths and xenocrysts in the eastern North China CratonNew dating results for the igneous rocks in the study area, together with the previous dating results, indicate that Mesozoic and Cenozoic of igneous rocks borne in deep-seated xenoliths and xenocrysts in the eastern NCC can be subdivided into four stages i.e., Early Cretaceous (119～132Ma), Late Cretaceous (73～92Ma), Paleogene (58Ma) and Eocene (36～39Ma).The 40Ar/39Ar plateau age of biotite and LA-ICP-MS zircon U-Pb age for pyroxene-diorite from the Tietonggou intrusion are 132.82±0.5Ma and 132Ma, respectively. The K-Ar dating results for Fangcheng and Feixian basalts are 125 Ma (Zhang et al., 2002) and 119Ma, respectively. The K-Ar ages for Fuxin basalts in western Liaoning are 85Ma and 92 Ma. The K-Ar dating result for Qujiatun basalts in southern Liaoning is 82 Ma. The 40Ar/39Ar plateau age of Daxizhuang basalts in eastern Shandong is 73 Ma (Yan et al., 2003). The K-Ar dating result of Luanshishanzi basalts in southern Liaoning is 58 Ma, i.e., Paleogene, whereas K-Ar age of Caiyuanzi basalts in southern Liaoning is 36-39 Ma, i.e., Eocene.2 Magma source and its nature for the Mesozoic and Cenozoic igneous rocks borne in deep-seated xenoliths and xenocrysts2.1 Early Cretaceous2.1.1 Magma source and nature of Tietonggou high-Mg dioritesEarly Cretaceous Tietonggou high-Mg diorites are characterized by high Mg (Mg^#=60～66), enrichment in Na(Na₂O/K₂O=1.3～1.9)and high SiO2 contents. Geochemically, they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and extremely depleted in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), and have high initial 87Sr/86Sr ratios(0.7067～0.7071)and lowεNd(t) values (-7.82～-13.30).The mineral chemical data and the Archean Re-depletion model ages (TRD=2.60-2.68Ga) for most peridotitic xenoliths from Tietonggou high-Mg diorites in western Shandong reveal that they could come from the ancient lithospheric mantle. However, the petrography of the xenoliths such as orthopyroxene metasomatic vein,the zoned orthopyroxene between olivine and chromite, and the existence of clinopyroxene enclosing olivine, and mineral chemical data indicate the interaction between the adakitic melt and mantle peridotite had taken place.Combined with the enrichments in LREEs, Pb and LILEs, as well as high ISr(0.7083～0.7214), lowεNd(t) value(-0.25～-11.61)for these dunite and harzburgite xenoliths, it is suggested that the primary magma for high-Mg diorites could be derived from partial melting of delaminated lower crust, and subsequently experienced an interaction of adakitic melt with the mantle peridotite in the Early Cretaceous.2.1.2 Magma source and nature of Fangcheng and Feixian basaltsFangcheng and Feixian basalts, chemically, are the transitional type between alkaline and subalkaline series and are enriched in LREEs , LILEs (Sr, Ba) and extremely depleted in HFSEs (Nb, Ta, Zr, Hf), as well as exceedingly high in ISr(0.709631～0.710090)and low inεNd(t)(-13.0～-14.2).Fangcheng and Feixian basalts contain olivine, orthopyroxene and clinopyroxene xenocrysts. Chemically, they are similar to those of the same minerals from the mantle-derived peridotite xenoliths in the Cenozoic basalts from the eastern China, implying that their primary magma could be derived from the lithospheric mantle. In addition, Fangcheng and Feixian basalts contain reversely zoned clinopyroxene xenocrysts. Their REE distribution patterns suggest that their cores are similar to those of the clinopyroxenes from the eclogitic xenoliths or pyroxenite xenoliths with cumulate textures or granulite xenoliths. There is much higher Mg^# values(Mg^# =87.0～88.9)in the mantle zones than their cores and rims(Mg^# =65.0～74.2), implying that these xenocrysts might come from the previously delaminated lower continental crust and then experienced the interaction with asthenospheric material. Taken together, these lines of evidence indicate that the primitive magma for Fangcheng and Feixian basalts could be originated from partial melting of the lithospheric mantle intensively modified by the delaminated lower continental crust. Their magma sources could be composed chiefly of garnet-bearing pyroxenite.2.2 Magma source and nature for Late Cretaceous basalts in the eastern NCCLate Cretaceous basalts in Fuxin western Liaoning, Qujiatun basalts in southern Liaoning and Daxizhuang basalts in eastern Shandong belong to alkaline basalt series. They are characterized by relative enrichment in LREEs and LILEs, depletion in HREEs and the lack of Eu anomalies, similar to those of oceanic island basalt. Their IS(r0.709631～0.710090)andεNd(t) values (1.61～7.55) are similar to those of the Cenozoic basalts in eastern North China, implying that their primary magma could be derived from partial melting of a depleted lithospheric mantle.The spinel-bearing harzburgite, spinel-bearing lherzolite, amphibolite–bearing lherzolite and lherzolite xenoliths can be found in Fuxin basalts from western Liaoning, whereas Daxizhuang basalts in eastern Shandong chiefly contain spinel-bearing lherzolites. The olivine, orthopyroxene and clinopyroxene xenocrysts can be also found in them. The chemical compositions of minerals from these xenoliths and xenocrysts are similar to those of kindred minerals in the mantle-derived peridotite xenolihs from the Cenozoic basalts in eastern China. The above mentioned characteristics imply that their primary magma could be derived from partial melting of the lithospheric mantle. The olivine and clinopyroxene xenocrysts enclosed in Qujiatun basalts have similar chemical compositions to kindred minerals from the pyroxenite xenolihs in the Cenozoic basalt of eastern China. Based on REE distribution patterns, clinopyroxenes in the mantle-derived peridotite xenolihs and xenocrysts in the Late Cretaceous basalts from western Liaoningn and eastern Shandong can be subdivided into five types:(1) Clinopyroxenes are extremely depleted in LREEs and enriched in HREEs, which suggests that the ancient lithospheric mantle could exist and the lithospheric mantle could be modified by multi-periods partial melting events. The magma source could be free in garnet.(2) Clinopyroxenes are extremely depleted in LREEs and HREEs, and enriched in MREE(with convex upward REE patterns), implying that an ancient lithospheric mantle modified by multiple partial melting events could exist and that the garnet could exist in magma source.(3) Clinopyroxenes are enriched in LREEs and depleted in HREEs, suggesting that they might represent the characteristics of the primitive mantle.(4) Clinopyroxene xenocrysts are characterized by poor in LREEs and HREEs and enriched in MREE(with convex upward REE patterns)and have much higher REE abundances than the second and third clinopyroxene xenocrysts, which may represent a newly-accretionary lithospheric mantle.(5) Clinopyroxenes are extremely enriched in LREEs and depleted in HREEs, and have high LREE/HREE ratios, which suggests that they may represent a newly-accretionary lithospheric mantle. The decrease of isothermal plane and transformation of the asthenosphere into newly-accreted lithospheric mantle could be main reasons to result in the above-mentioned characteristics, which is also supported by high REE abundance of clinopyroxene xenocrysts.Taken together, it is proposed that the primary magmas for the Late Cretaceous basalts from eastern North China Craton could dominantly originate from partial melting of a newly accretionary depleted lithospheric mantle, that there are relicts of ancient lithospheric mantle, and that the late Cretaceous lithospheric mantle could experience the modifications of multiple thermal events.2.3 Magma source and nature of the Paleogene and Eocene basaltsPaleogene and Eocene basalts in southern Liaoning belong to alkaline series and are characterized by relative enrichment in LREEs,depletion in HREEs and no Eu anomalies. Geochemically, they are similar to oceanic island basalt. But, Caiyuanzi basalts are relatively enriched in Sr,Ba and depleted in Zr, Hf. The Sr-Nd isotope compositions(ISr=0.70453～0.70516,εNd(t) = 0.22～3.69)for Paleogene and Eocene basalts are similar to those of the Cenozoic basalts in eastern China. Eocene basalts in Caiyuanzi have relative high ISr(0.70473～0.70516)and lowεNd(t)(0.22～2.27),which are consistent with the characteristic of relatively enriched in Ba, Sr and depleted in Zr, Hf. These characteristics imply that crust material could be involved in the magma resource.The Mg^# values(88～94)of olivine xenocrysts in the core from Paleogene Luanshishanzi basalt and Eocene Caiyuanzi basalt are similar to those of olivines in the mantle-derived peridotite xenolihs of the Cenozoic basalts in eastern China. Caiyuanzi basalts contain reversely zoned clinopyroxene xenocrysts. Their cores have similar chemical compositions to the clinopyroxenes from granulite xenoliths, which suggests the delaminated lower continental crust could exist in the magma sources. However, clinopyroxene xenocrysts have higher Mg^# value in the mantle zones than in the rim , indicating the existence of reaction between the delaminated lower continental crust and melt with high Mg^#. This reaction is likely to happen in asthenosphere.Clinopyroxene xenocrysts are characterized by depletion in LREEs and HREEs and enrichment in MREE and display a convex REE patterns. Compositional variations of clinopyroxene xenocrysts from core to rim reveal a more complicated evolution history of the lithospheric mantle.Taken together, it is proposed that the Paleogene basaltic magmas could be derived from partial melting of a depleted lithospheric mantle in general, similar to those of the Late Cretaceous basalts. But, from the late Cretaceous to Paleogene, the lithospheric mantle display an evolutional trend from depletion toward the primitive mantle. The magma sources of the Late Cretaceous and Paleogene basalts could be composed of the depleted mantle and the relict of delaminated lower continental crust (rutile+garnet) after the Early Cretaceous melting.3 Deep processes in the lithospheric mantle3.1 Type and nature of metasomatic events3.1.1 A silica-rich melt (liquid) metasomatism The orthopyroxene metasomatic vein can be found in peridotitic xenoliths from Tietonggou high-Mg diorites. Chemically, its composition is similar to one in dunites but lower Mg^# values. The existence of such metasomatic vein implies that metasomatic medium should be SiO2-rich melt (liquid).The metasomatic vein usually consists of the orthopyroxene, phlogopite and minor plagioclase in peridotitic xenoliths from Tietonggou high-Mg diorites. The existences of orthopyroxene and plagioclase also indicate that metasomatic medium should be SiO2-rich melt (liquid). Besides, the zoning orthopyroxene occurred between chromite and olivine in peridotitic xenoliths also suggests that metasomatic medium should be SiO2-rich melt (liquid).Main reactions are as follows:olivine (Ol)+melt I = orthopyroxene (Opx)+melt IIolivine I (Fo=92-93)+ melt I = olivine II (Fo=85-87)+ melt II (Mg-rich)orthopyroxene (Opx)+ melt = olivine (Ol)+ Clinopyroxene (Cpx)spinel (Sp-Cr#=30-50)+ melt I = chromite (Chr-Cr#=65-80) + melt II (melt)3.1.2 A carbonate-rich melt (liquid) metasomatismClinopyroxenes in the wehrlite xenoliths from Tietonggou high-Mg diorites are extremely enriched in LREEs, depleted in HREEs and HFSEs and relatively low Ti/Eu ratios(2177～2872), suggesting that they could be resulted from crystallization of magma. Meanwhile, it also implies that the metasomatism of an important carbonate-rich melt (liquid) might have taken place in the lithosphere mantle. The poikilitic texture(Lw9-23), metasomatic veinsof the phlogopite, dolomite, talcums and magnetites Lw8-57)in the wehrlite and dunite xenoliths further support the existence of a carbonate-rich melt (liquid) metasomatism.3.2 Magma underplatingOlivine xenocrysts in the Paleogene Luanshishanzi basalts display circle cracks and deformation and reactive rim structures, different from the porphyritic crystals in them. Moreover, olivine xenocrysts is chemically not equilibrium with its host magma.Late Cretaceous Qujiatun basalts contain deep-seated olivine, clinopyroxene and amphibole xenocrysts. Olivine and clinopyroxene xenocrysts in Qujiatun basalts are similar to those in the Luanshishanzi basalts. The chemical compositions of the deep-seated xenocrysts in the Paleogene Luanshishanzi basalts and the Late Cretaceous Qujiatun basalts shows that they could come from the early mantle cumulates near the crust-mantle boundary. They represent a newly accretionary crust and imply that an important magmatic underplating event had taken place before Late Cretaceous.3.3 Melt—Peridotite interactionMost olivine and clinopyroxene (Cpx) xenocrysts in the Early Cretaceous, Late Cretaceous, Paleogene igneous rocks in the eastern North China Craton, display positive zonal structure (except for minor reversely zoned clinopyroxene xenocrysts), i.e., the Mg^# values gradually decreasing from core to rim. Mg^# values of their cores are similar to those of olivines and Cpx from the mantle peridotite xenoliths in the Paleozoic kimberlites and the Cenozoic basalt in NE China, whereas Mg^# values of their rims are similar to those of phenocrysts in the host basalt. Taken together with the occurrence of zonal opx and opx veins in the dunites, it is suggested that the melt—peridotite interaction widely exist in the lithospheric mantle in the eastern North China Craton.4 Evolution of the Mesozoic and Cenozoic lithospheric mantle in eastern North China CratonBased on the above mentioned results, the Mesozoic and Cenozoic evolution model of the lithospheric mantle in eastern North China Craton is set up.(1) The thickening of the Early Mesozoic (T1-T2) lower continental crust in eastern North China Craton: The subduction of the Yangzi Craton beneath the North China Craton in NW direction in the early Mesozoic (T1) resulted in the thickening of the lower continental crust in eastern North China Craton and could be a geodynamic background resulting in the destruction of the North China Craton.(2) The break-off of subducted slab and the delamination of the thickened continental lithosphere in the late Triassic—Early Jurassic(T3-J1):Eclogites were formed during the slab subduction (YC) and the thickening of lower continental crust. High density of eclogites lead to delamination, i.e., break-off of subducted slab and delamination of the thickened continental lithosphere. The formation of monzonite-syenite association with ages of 176-225Ma from Jiazishan in erstern Shandong and Tongshi in western Shangdong should result from the delamination. Taken together, it is concluded that the break-off of the subducted slab and the delamination of the thickened lithosphere could be a major mechanism to result in the destruction of the North China Craton.(3) The Early Cretaceous SiO2-rich melt (liquid)—peridotite interaction (K1):In the Early Cretaceous, the subduction of the Paleo-Pacific Plate beneath the Eurasian continent could agitate in the melting of early delaminated lower continental crust. These rising adakitic melts would react with the overlying lithospheric mantle. The early Cretaceous high-Mg diorites in western Shandong could come from the intrusion of these reacted melt. Meanwhile, an EM-II lithospheric mantle formed, i.e., magma source of Fangcheng and Feixian basalts.(4) The Late Cretaceous accretion of lithospheric mantle:In the late Cretaceous, the decrease of geothermal gradient could result in the transformation of primary asthenosphere into the lithospheric mantle, i.e., accretion of lithospheric mantle. The newly accreted lithospheric mantle and minor relicts of ancient lithospheric mantle, together with the relicts (rutile and garnet) of delaminated eclogites after the early Cretaceous melting, make up the magma source of the Late Cretaceous and Paleogene basalts.