Petrogenesis And Geological Significance Of The 2.5-2.2 Ga Intermediate-felsic Intrusions In The Southern North China Craton

Craton is part of ancient and stable continental lithosphere,which preserves evidence of formation and evolution of the continental crust.The North China Craton?NCC?is one of the ancient cratons on Earth,where the Archaean-Paleoproterozoic basement are widespread.The NCC thus provides an important window to study the formation and evolution of the continental crust in the early Earth history.Based on the detailed field investigations,we carried out analyses of new zircon U-Pb ages and Hf isotopes,whole-rock major and trace elements,as well as Nd isotopes for the Neoarchean-Paleoproterozoic intermediate-felsic intrusions from the Linshan Complex and the Taihua Complex in the southern NCC.In combination with the previous studies,several important findings have been achievd:?1?The 2529 Ma TTG-type rocks and 2543-2503 Ma high-K granitoids have been identified from the Linshan Complex.The TTG-type rocks have high SiO₂?62.81wt.%-80.39 wt.%?and Na₂O?3.35 wt.%-6.00 wt.%?,and low K₂O?1.25 wt.%-3.28wt.%?and P₂O₅?0.02 wt.%-0.13 wt.%?with variable Al₂O₃?10.17 wt.%-15.55 wt.%?.According to the An-Ab-Or diagram,the samples plot in the“trondhjemite”field.They have low total REE and display right inclined REE patterns with negative Eu anomalies?Eu/Eu*=0.36-0.96?.Their spider diagram is characterized by enrichment of Rb,K,Th,Pb and depletion of Nb,Ta,P,Ti.The rocks have positive and variable zircon?_Hf?t??+2.8 to+7.0?and whole-rock?_Nd?t??+5.4 and+2.2?with young two-stage model ages(T^Hf_DM2:2.87-2.60 Ga;T^Nd_DM2:2.47 Ga and 2.68 Ga).These lines of evidence suggest that the TTG-type rocks were derived from the juvenile mafic lower crust.The high-K granitoids mainly consist of monzogranite and granodiorite which have high SiO₂?65.86 wt.%-78.08 wt.%?and K₂O?3.29 wt.%-7.62 wt.%?,and low P₂O₅?0.01 wt.%-0.27 wt.%?.They display right inclined REE patterns with negative Eu anomalies?Eu/Eu*=0.20-0.81?.Their spider diagram is characterized by enrichment of Rb,K,Th,U and depletion of Nb,Ta,Zr,Ti.The rocks have positive and variable zircon?_Hf?t??+2.5 to+6.6?and whole-rock?_Nd?t??+0.7 to+4.5?with two-stage model ages of 2.5-2.9 Ga(T^Hf_DM2=2.87-2.64 Ga;T^Nd_DM2=2.77-2.50 Ga),similar to the ages of the Archean TTG-type rocks,amphibolite and diorite in the region.Thus,the high-K granitoids were produced by partial melting of TTG-rocks,amphibolite and diorite in the Linshan Complex.Both the TTG-type rocks and high-K granitoids in the Linshan Complex show high whole-rock zirconium saturation temperatures?684-827?;694-889??and low Sr/Y ratios?1.79-5.19;0.27-21.1?,indicating low pressure partial melting.Combined with other geological evidences,these rocks are suggested to have been formed in a post-collision extensional environment after the arc-continent collision.The plate tectonics in the NCC initiated before².5 Ga,and its cratonization occurred through the assemblages of several micro-blocks.?2?The 2168 Ma Liujiping granitic pluton and the 2176 Ma Muce quartz dioritic pluton have been identified in the Taihua Complex at the Xiong'ershan Area.Rocks from the Liujiping pluton have high SiO₂?70.15 wt.%-74.66 wt.%?and K₂O?5.17wt.%-6.98 wt.%?,low P₂O₅?0.03 wt.%-0.10 wt.%?.They have high total REE and display right inclined REE patterns with negative Eu anomalies?Eu/Eu*=0.23-0.42?.They are rich in Rb,Th,Nb,Ce,Zr and Ga and depleted in Ba,Sr,P,Eu and Ti in the spider diagram.They have high 10⁴×Ga/Al?2.44-3.55?,Zr+Nb+Ce+Y?432 ppm-617ppm?and whole-rock zirconium saturation temperatures?774-920??.These lines of evidence suggest that the rocks from the Liujiping pluton are A-type granites.They show negative?_Hf?t??-10.7 to-8.2?and?_Nd?t??-5.8 to-3.4?with old two-stage model ages(T^Hf_DM2=3.45-3.27 Ga;T^Nd_DM2=2.92-2.76 Ga),indicating they were sourced from Archean crustal materials.The quartz diorites have low SiO₂?55.71 wt.%-58.15 wt.%?,high K₂O?3.85wt.%-4.79 wt.%?,Fe₂O₃^T?6.09 wt.%-7.44 wt.%?and MgO?2.81 wt.%-3.50 wt.%?.They have low total REE and display right inclined REE patterns with weakly negative Eu anomalies?Eu/Eu*=0.78-0.94?.Their spider diagram is similar to those of Liujiping granites.They have negative zircon?_Hf?t??-4.9 to-1.3?and whole-rock?_Nd?t??-3.6 to-2.4?with old two-stage model ages(T^Hf_DM2=3.45-3.27 Ga;T^Nd_DM2=2.92-2.76 Ga).Therefore,the rocks were produced by partial melting of Archean crustal materials,together with the addition of a small amount of mantle-derived material.The granites have low Sr/Y ratios?2.32-10.5?and high whole-rock zircon saturation temperatures?764-810??,indicating low pressure high temperature partial melting related to an extensional environment,in accordance with the character of A-type granites.In combination with the previous studies,the².2 Ga A-type granites and quartz diorites identified in the Taihua Complex?Xiong'ershan area?were formed in an intraplate rift setting.?3?Five episodes of magmatism have been identified in the southern NCC.In order to investigate the secular evolution of the continental crust in the southern NCC,we compiled 1551 concordant zircon U-Pb ages and 1103 Hf isotope data for the Neoarchean igneous rocks from Linshan,Sushui,Taihua,and Dengfeng complexes which cover the whole Archean complexes in the southern NCC.Zircon U-Pb ages show that the magmatism in the region occurred mainly at 2.8-2.7 Ga and 2.6-2.5 Ga.Their two-stage Hf model ages reveal that the major period of the continental crust growth was at ca.2.7 Ga,consistent with the widespread magmatism in the NCC.In the plot of age?_Hf?t?vs.age,nearly all magmatic zircons from the².5 Ga igneous rocks have positive?_Hf?t?values,and majority of them plot under the line of 0.75×?_Hf?t?of the coeval depleted mantle.Their whole-rock Nd isotopes show the same distributions.These lines of geochemical evidence suggest that the².5 Ga magmatism represents major episode of continental crust reworking,with the formation of a small amount of juvenile crust.The².35 Ga,2.2-2.1 Ga and¹.8 Ga magmatism represent reworking of the continental crust,with the addition of a small amount mantle-derived material.?4?The NCC was formed by the amalgamation of several micro-blocks which were welded together along the greenstone belts at².5 Ga.These micro-blocks were surrounded by small ocean basins during the late Neoarchean time,and the greenstone belts probably represent arc-continent collision zones.The subduction and collision mechanisms are similar to those of modern plate tectonic regimes,but in much small scale.The voluminous granitoids may have been formed during the amalgamation of micro-blocks that was followed by the extension in the NCC.The NCC underwent the Paleoproterozoic rifting-subduction-accretion-collision processes,and and eventually formed the stable continent at¹.85 Ga.