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

摘要

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 magmatic rocks, named here crustal 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 CO_2 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 t_(DM)(Hf) and epsilon_(Hf)(t) of 2353 to 2457 Ma and -3.2 to 0.7, whereas the t_(DM)(Hf) and epsilon_(Hf)(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 delta~(18)O (per thousand)_(Zircon-V-PDB)of zircon from the crustal carbonatite ranges from -21.5 to -19.6, with O isotope equilibrium temperature being 555 deg C to 635 deg C. The crustal carbonatite shows a large variation in chemical composition, with SiO_2=10.2-37.3 percent and Total REE=48-267 ppm. The t_(DM)(Nd) age and epsilon_(Nd)(t) are approx 2.5 Ga and -2.9 to -2.4. The delta~(18)O(per thousand)_(Rock-V-PDB) and delta~(13)C(per thousand)_(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 in delta~(18)O(per thousand)) vs. delta~(13)C(per thousand) 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-CO_2-H_2O system (Wyllie and Tuttle, 1960) and MgO-CaO-CO_2-H_2O system (Fanelli and others, 1986), which indicate that partial melting of limestone will happen when temperature is > 700 deg C and when water is also present.