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Zircon SHRIMP U-Pb and in-situ Lu–Hf isotope analyses of a tuff from Western Beijing Evidence for missing Late Paleozoic arc volcano eruptions at the northern margin of the North China block
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Shuan-HongZhang
、
YueZhao
、
BiaoSong
、
Yue-HengYang
Zircon SHRIMP U–Pb and in-situ Lu–Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U–Pb zircon dating of the tuff yielded a 206Pb/238U weighted mean age of 296±4 Ma (95% confidence, MSWD=3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline,I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu–Hf analysis results of most zircons from the tuff yielded initial 176Hf/177Hf ratios from 0.282142 to 0.282284 and εHf(t) values from −15.9 to −10.7. These Lu–Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some arc volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic.
The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton Magmatism related to a Paleoproterozoic orogen
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]
Shuan-HongZhang
、
Shu-WenLiu
、
YueZhaoa
、
Jin-HuiYang
、
BiaoSong
、
Xiao-MingLiu
The AMGRS is a suite of anorthosite-mangerite-alkali granitoid-rapakivi granite in the northern North China Craton (NCC).New zircon SHRIMP and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating of the Damiao anorthosite, Changsaoying K-feldspar granite, Lanying anorthosite and quartz syenite and the Gubeikou K-feldspar granite yielded emplacement ages of 1726±9 Ma, 1753±23 Ma, 1739±43 Ma, 1712±15 Ma and 1692±19 Ma, respectively. Combined with previous geochronological data from the Damiao norite and mangerite and Shachang rapakivi granite, it indicates that the whole AMGRS was intruded between 1750 Ma and 1680 Ma. The Damiao anorthosites and norites exhibit a high Mg#, and this decreases from mafic (up to 63) to felsic (<10) end members. The alkali granitoids of the AMGRS are characterized by high contents of alkali (K2O+Na2O), HSFE, TFeO/MgO, molecular (K2O+Na2O)/Al2O3 and Ga/Al values, that are typical features of A-type granites. All of the diverse components of the AMGRS have similar Sr–Nd–Hf isotopic compositions with low whole-rock initial 87Sr/86Sr ratios of 0.703–0.704, εNd(T) values of −5.0 to −6.3 (with TDM model ages of 2.34 to 2.58 Ga), and zircon εHf(T) values ranging from −4.1 to −7.5 (with Hf isotopic model TDM and TCDM ages ranging from 2.32 Ga to 2.43 Ga and 2.70 Ga to 2.88 Ga,respectively).This suggests that these rocks are the fractional crystallization products of one parental magma. Zircon Hf and wholerock Sr–Nd–Pb isotopic systematics combined with high La/Yb (>10) and intermediate Th/Ta ratios, suggest that the parental magma of the AMGRS could have been derived from EM-I lithosphere mantle enriched by recycled Archean continent crustal materials, accompanied by some assimilation of lower crustal components. The mafic to felsic compositions of the AMGRS are different from typical bimodal rocks formed in an anorogenic intra-continent rift setting, but resemble those emplaced during postcollisional/post-orogenic extensional tectonic setting. This is compatible with their age of <1.85 Ga, indicating they postdate the continent–continent collision between the Western and Eastern blocks of the NCC at ∼1.85 Ga.
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