Pb Isotope Composition And Crustal Evolution: A Case Study From The High Grade Metamorphic Suite Of The Taihua Group In The Southern Margin Of The North China Craton

Continental lower crust provide important clues on the interaction between the upper crust and lithospheric mantle, so Ancient lower crust is often regard as subjects of studying how the ancient continental crust formed and evolved. As the Archean lower crust is deep-seated in the lithosphere, insights into the ancient crustal formation and evolution can be gained through the study of the exposed Archean terrane, recording tectono-magmatic-metamorphic events. Archean high grade metamorphic terrain of Taihua Group in Lushan area, one of the most complete and best-exposed succession of the southern part of the NCC, offer the best sample to probe the origin and evolution of the southern NCC. Therefore, we sampled a suite of representative metamorphic rocks of Lushan-Taihua Group (granitoid gneiss, amphibolite, garnet granulite and pyroxene granulite) as targets in this study. Based on the petrological, major and trace elements, and chemical compositions of the feldspars data, combined with the former research data and geochro no logical in this study, e.g., LA-ICP MS zircon U-Pb geochronology, Lu-Hf isotopic compositions and whole rock Sm-Nd isotopic compositions, we apply in-situ micro analysis of Pb isotopes in feldspars from the high grade basement rocks of the Lushan-Taihua Group in the southern segment of the Archean North China Craton (NCC) using Femtosecond laser ablation (fLA)-MC-ICPMS technique, which can help us to understand the heterogeneous Pb isotope composition feature, and discuss the formation times, petrogenesis of different rock types, and provides evidence to further understand ancient crustal growth and evolution of the southern NCC.Our results lead to the following inferences:(1) The results show large variation in Pb isotopic compositions (206Pb/204Pb= 13.8-17.7, 207pb/204pb = 146.157 and 208Pb/204Pb = 33.6-38.1) and heterogeneous Pb isotopic ratios in feldspar of different rock types. The granitoid gneisses collected from the lower Taihua Group show low and variable Pb isotopic compositions (206Pb/204Pb = 13.8-16.8, 207pb/204pb = 146.15.1 and 208pb/204pb= 33.9-36.6) Common Pb in feldspar and the single stage Pb evolution model indicate that granitoid gneisses, especially the lower Taihua Group, was extracted from the mantle at 2.74±0.33 Ga (all uncertainties are 2?), which is close its formation age. So the protolith of granitoid gneisses can represent juvenile crust and preserve the low Pb isotope composition of the Archean NCC crust. While granitoid gneisses collected from the upper Taihua Group show higher 207Pb/204Pb ratios (15.4-15.5) than the lower Taihua Group, which confirmed that granitoid gneisses of the upper Taihua Group is formed by partial melting of the Archean NCC crust.(2) The amphibolites show slightly higher Pb isotopic compositions (206Pb/204Pb= 14.1-17.7,207Pb/204Pb = 14.6-15.6, and 208Pb/204Pb = 33.6-38.1) relative to the gneisses, whereas the lenticular garnet granulites occur as possess the highest Pb isotopic compositions (206Pb/204Pb = 14.4-17.7,207Pb/204Pb = 14.7-15.7 and 208Pb/204Pb = 33.7-37.7). Pb data of plagioclase from both amphibolites and lenticular garnet granulites show positive linear array in 207Pb/204Pb and 208Pb/204Pb vs.206Pb/204Pb diagram and yield the radio metric age of 3.42±0.18 Ga and 3.29±0.31 Ga using the program Isoplot, respectively. These two closely age suggest the extraction time of their protoliths material mainly extracted from the mantle.(3) The garnet granulite dike have relatively high radiogenic Pb isotope (206Pb/204Pb= 15.6-17.7,207Pb/204Pb = 15.1-15.6 and 208Pb/204Pb = 35.5-37.7) while pyroxene granulite have slightly low Pb isotope (206Pb/204Pb= 14.6-17.6,207Pb/204Pb= 15.0-15.4 and 208Pb/204Pb = 34.4-37.3). Both of two rock type are similar in gabbroic palimpsest texture, field occurrence of intruding Archean granitoid gneisses and Pb isotopic composition plot a good linear trend in 207Pb/204Pb and 208Pb/204Pb vs.206Pb/204Pb diagram, so Pb isotope in feldspar of mafic dike yielded a 207Pb-206Pb age of 2.62±0.30 Ga, suggesting that the protolith of garnet granulite dike and pyroxene granulite dike extracted from the mantle at about 2.62 Ga.(4) To further understand the origin and tectonic history of mafic dike, we take the garnet granulite dike, the sample THQ13-05, as example. The U-Pb geochronological, O and Hf isotopic studies on zircons from the mafic dike, have revealed three tectonic-magmatic events: ? Magmatic emplacement happened at the late Neoarchean (2523±8 Ma), which coincided well with an important period of extensive ancient crust reworking with minor crustal growth at the Neoarchean (?2.50 Ga) in the NCC. Magmatic zircons from the dike have positive ?Hf(t) values (2.88-7.16), suggesting that these zircons formed from basaltic magma derived from depleted mantle. The oxygen isotope feature, i.e. slightly higher (6.12‰-7.47‰) than normal mantle ?18O values, indicates that the crustal contamination was insignificant during magma emplacement, which also is identical with 207Pb-206Pb ageof 2.62 Ga; ? The Paleoproterozoic metamorphism (1922±6 Ma) recorded in metamorphic zircons of the granulite dike is in alignment to extensive metamorphism (1.97-1.85 Ga) of the Archean terranes in the TNCO. In addition, metamorphic zircons also have positive ?Hf(1) values (1.61-5.52), implying their Hf isotope heritage from its protolith. The ?18O values (4.85‰-5.76‰), which are slightly lower than their protoliths, could be caused by high temperature hydrothermal alteration during metamorphism; ? Magmatic activity occurred in the Oligocene (concordia age of 31.38±0.15 Ma), which period of magmatism was first discovered in the NCC. Both ??Hf(t) values (-3.03-1.69) which mainly distribute on the Chondritic Uniform Reservior (CHUR) trend and ?18O values (5.78‰-6.62‰) which are close to the primitive mantle reveal that the Oligocene magmatic activity were also derived from the mantle.(5) Importantly, The ubiquitous Pb isotope disequilibrium among different feldspar grains, especially in the overgrowth rims possessing much higher radiogenic Pb isotopes than its core and the small symplectitic grains always have higher Pb isotopes than that of the larger porphyroblastic plagioclases, which suggests input of exotic melt/fluid with high ?. (U/Pb) during retrograde metamorphism. The large present-day Pb isotopic heterogeneity is either possibly due to the initial compositional differences (such as Th/U ratio in whole rock and An content in plagioclase), or the result of different geological conditions experienced by the various rock types.