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PALEOPROTEROZOIC CRUSTALLY DERIVED CARBONATE-RICH MAGMATIC ROCKS FROM THE DAQINSHAN AREA, NORTH CHINA CRATON GEOLOGICAL, PETROGRAPHICAL, GEOCHRONOLOGICAL AND GEOCHEMICAL (Hf, Nd, O AND C) EVIDENCE [查看] YUSHENGWAN、DUNYILIU、ZHONGYUANXU、CHUNYANDONG、ZHEJIUWANG、HONGYINGZHOU、ZHENSHENGYANG、ZHENGHONGLIU、JIASHANWU

ABSTRACT. Most carbonate-rich magmatic rocks are mantle-derived, namely car-bonatite(s), with a minority of them being contaminated by crustal rocks. It is debated whether there are also carbonate-rich magmatic rocks derived solely from crustal sources. In this contribution, we report crustally derived carbonate-rich magmaticrocks, named herecrustal carbonatite(s) , in the Daqingshan area, Western Block of the North China Craton. The Daqingshan crustal carbonatites were previously considered to be metasedimentary marbles. However, they cut adjacent rocks and contain some enclaves that are irregular in shape and show unoriented distribution of lithologies that cannot be found in the local wall rocks. Zircons from the crustal carbonatites show clear oscillatory zoning, and contain calcite, quartz, feldspar, diopside and CO2 inclusions. The zircons have correlated U and Th abundances and give a U/Pb age of 1951±5 Ma, being the same age as metamorphic zircon cores and rims from a diopsidite enclave (1954±27 Ma of core and 1944±40 Ma of rim). The zircon from the crustal carbonatite has tDM(Hf) and dHf(t) of 2353 to 2457 Ma and –3.2 to 0.7, whereas the tDM(Hf) and dHf(t) of the core and rim zircons from diopsidite range from 2228 to 2160 Ma and 3.0 to 4.8 and 2219 to 2057 Ma and 3.2 to 7.4, respectively. The δ18O(‰)Zircon-V-PDB of zircon from the crustal carbonatite ranges from –21.5 to –19.6, with O isotope equilibrium temperature being 555°C to 635 °C. The crustal carbonatite shows a large variation in chemical composition, with SiO2 δ10.2–37.3% and Total REE δ48 –267 ppm. The tDM(Nd) age and dNd(t) are 2.5 Ga and –2.9 to –2.4. Theδ18O(‰)Rock-V-PDB andδ13C(‰)Rock-V -PDB vary from –19.5 to –15.2 and from –5.2 to –2.4, being distributed between primary carbonatite field and the field of dolomitic marble from the study area inδ18O(‰) vs.δ13C(‰) diagram. Combined with previous studies, we drew the conclusion that some of the carbonate-rich rocks in the study area are magmatic in origin, by anatexis of impure marble plus common contamination by calc-silicates and other materials. This is consistent with the high-P-T experiments of CaO-CO2-H2O system (Wyllie and Tuttle, 1960) and MgO-CaO-CO2-H2O system (Fanelli and others, 1986), which indicate that partial melting of limestone will happen when temperature is> 700°C and when water is also present.

NEW U-Pb AND Hf ISOTOPIC DATA CONFIRM ANSHAN AS THE OLDEST PRESERVED SEGMENT OF THE NORTH CHINA CRATON [查看] DUNYILIU、SIMONA.WILDE、YUSHENGWAN、JIASHANWU、HONGYINZHOU、CHUNYANDONG、XIAOYANYIN

ABSTRACT. The Anshan area in the northeastern part of the North China Craton contains the oldest known rocks in China. Rocks with ages of 3.8 Ga are present in three trondhjemite-dominated gneiss complexes, referred to as the Baijiafen, Dongs-han and Shengoushi complexes. The oldest material identified in these complexes has weighted mean 207Pb/206Pb ages of 3800 5 Ma, 3794 4 Ma and 3777 13 Ma,respectively. In addition, younger geological events at 3680 19 Ma, 3620 23 Ma, and 3573 21 Ma have been identified, recording the emplacement of several generations of trondhjemite. A biotite schist of possible supracrustal origin contains a zircon population with a weighted mean 207Pb/206Pb age of 3723 17 Ma and is cut by a 3620 Ma trondhjemite, implying formation prior to3.62 Ga. TDM(Hf) model ages for the zircons range from 3256 to 4264 Ma, but are mostly 3.9 Ga, indicating that significantly older crust was not present in the source region of the trondhjemitic magmas.

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