Geochronology And Geochemistry Of The Paleoproterozoic A-type Granites In The Southern Margin Of The Central North China Craton

This thesis reports a comprehensive study of mineralogical,petrological,LA-ICP-MS zircon U-Pb geochronology,whole-rock geochemistry,whole-rock Sr-Nd isotopic and zircon in situ Lu-Hf isotope for the Shangdian and Dengfeng A-type granites in the southern margin of the Central North China Craton?CNCC?.These studies have determined their formation age,illustrated the nature of magma source,and revealed their petrogenesis and tectonic background,which provide a constraint on the further study of tectonic evolution and and deep dynamic process of southern margin of CNCC.1.The formation age of the Paleoproterozoic granites in the southern margin of CNCC The LA-ICP-MS zircon U-Pb geochronology studies have been determined for the Paleoproterozoic A-type granites in the southern margin of CNCC.The dating results for Shangdian and Dengfeng granites yield ²⁰⁷Pb/206 Pb ages of 1870-1790 Ma,1868-1771 Ma and 1810-1785 Ma,with weighted mean ²⁰⁶Pb/238 U ages of 1801±14 Ma?n=17?,1801±15 Ma?n=22?and 1795±11 Ma?n=20?,respectively,indicating they all formed in the late Paleoproterozoic?circa 1.80 Ga?.2.Geochemistry of the Paleoproterozoic granites in the southern margin of CNCC The Paleoproterozoic Shangdian granite porphyry and Dengfeng syenogranite in the southern margin of CNCC have similar geochemical characteristics.They have high contents of Si O2?70.30%-82.06%?and total-alkali?K2O+Na2O=6.79%-11.01%?,and low amounts of Ca O?0.10%-0.36%?,P2O5?0.001%-0.15%?,and Mg O?0.02%-0.20%?,belonging to the quasi-weakly peraluminous calc-alkaline rocks.They have high REEs contents?32-798×10-6?.Additionally,they are characterized by enrichment in LREEs and LILEs?Cs,Rb,Ba,and K?,and depletion in HREEs and HFSEs?Nb,Ta,Zr,and Hf?.They exhibit prominent negative Eu anomalies??Eu=0.44-0.59?and are depleted in Sr,P and Ti,which are similar to the characteristics of the acidic volcanic rocks from Xiong'er Group in the southern margin of the North China Craton.3.The Sr-Nd-Hf isotopic compositions of the Paleoproterozoic granites in the southern margin of CNCC The initial ⁸⁷Sr/86 Sr ratios of the Paleoproterozoic granites in the southern margin of CNCC is relatively dispersed due to high Rb/Sr ratios,ranging from 0.801822 to 0.893654.In contrast,the Sm-Nd isotopic composition is more homogeneous,the ¹⁴³Nd/144 Nd ratios are between 0.511090 and 0.511222,and the 147Sm/144 Nd ratios range from 0.0977 to 0.1156.The whole-rock ?_Nd?t?and two-stage model ages(T_DM2)values of Shangdian granites range from-6.87 to-8.94 and from 3243 Ma to 3049 Ma,respectively.Zircons from Shangdian granites have 176Hf/177 Hf ratios ranging from 0.281274 to 0.281480 and ?_Hf?t?values ranging from-6.5 to-14.1,and the corresponding T_DM2 range from 3030 Ma to 2655 Ma.The in situ Lu-Hf isotopic compositions are all below the chondrite evolution line.4.The genetic type of the Paleoproterozoic granites in the southern margin of CNCC The Paleoproterozoic granites in the southern margin of CNCC have high contents of Si O2 and total-alkali,show enrichment in LILEs and depletion in HFSEs,and have strong negative anomalies of Sr,P and Ti.In addition,they have higher 10000Ga/Al ratios ranging from 2.64 to 3.74?the average value is 3.21?and Rb/Sr ratios ranging from 2.26 to 5.41?the average is 3.90?.Combined with the higher zircon saturation temperature?755 to 915 oC?and the Ti saturation temperature?695 to 1166 oC?of Shangdian granite porphyry and Dengfeng syenogranite,they all show the characteristics of the A-type granite.The Paleoproterozoic Shangdian and Dengfeng A-type granites in the southern margin of CNCC have a uniform whole-rock Sr-Nd composition with ?_Nd?t?values ranging from-6.87 to-8.94,which are lower than that of the late Paleoproterozoic mafic dikes(?_Nd?t?=-0.6 to-5.5)derived from the mantle materials in the study area,indicating there is no mantle magma mixing in their source region.The low contents of Al2O3 and Pb,and the Neoarchaean Hf-Nd model age suggest that they were derived from partial melting of the ancient crust caused by underplating of mantle-derived magma.The Neoarchaean?3.0-2.5 Ga?basement in the southern margin of CNCC may be the source composition of Shangdian granite porphyry and Dengfeng syenogranite.5.Petrogenesis of the Paleoproterozoic A-type granites in the southern margin of CNCC6.The tectonic setting of the Paleoproterozoic A-type granites in the southern margin of CNCC The Paleoproterozoic Shangdian and Dengfeng A-type granites are located in the southern margin of CNCC.Combined with the geochemical characteristics of intraplate granites,the Paleoproterozoic A-type granites were formed in an extensional setting after the continental collision of Eastern Block and Western Block of NCC in circa 1.80 Ga.The above results imply that Shangdian and Dengfeng granites were dominantly derived from partial melting of Meso-Neoarchaean crystalline basement materials caused by the underplating of mafic magmas,and indicate that the continental collision of Eastern Block and Western Block of NCC is over at least in circa 1.80 Ga.