矿床地球化学国家重点实验室知识库

Paleoproterozoic granites, divided into alkali-feldspar granite and albite granite, were newly discovered in the Changyi area of the Jiao-Liao-Ji Belt from the Eastern Block of the North China Craton. LA-ICP-MS zircon U-Pb dating shows that the alkali-feldspar granite was emplaced at 2193 Ma, whereas the albite granite intruded at 2171 Ma. These granites have high SiO2, Na2O + K2O, Zr, Nb, Ga, Y and HREE contents and show remarkably low CaO, Eu, Ba and Sr concentrations, strongly suggesting their A-type affinities. Zircon saturation thermometry and Al-T-in-biotite geobarometer indicate that the alkali-feldspar granite crystallized at temperatures of 823-856 degrees C (averaging at 839 degrees C) and pressures of 1.54-2.73 kbar (averaging at 2.05 kbar) while the albite granite was cooled at the temperatures of 851-853 degrees C and the pressures of 2.34-2.64 kbar (averaging at 2.45 kbar). Combined with such high temperatures and low pressures, the crustal-like Nd-Hf isotopic compositions (epsilon(Nd)(t)= -5.3 to -4.4, epsilon(Hf)(t) = -7.0 to -3.8.) support their derivation from the shallow melting of Archean crustal rocks under high heat flow. However, the contrasted K2O and Na2O compositions between the alkali-feldspar granite (K2O = 4.76-5.14 wt%, Na2O = 4.76-5.14 wt.%) and the albite granite (K2O = 0.29-0.44 wt.%, Na2O = 6.15-6.56 wt.%) imply that they may have been derived from distinct source rocks or have undergone different magmatic processes. Genetic modeling suggests that the alkali-feldspar granite was originated from melting of granodioritic rocks. The albite granite, however, was most likely derived from the reworking of plagioclase-rich layers after the extraction of A-type melts. Coupled with other studies on sedimentary and volcanic successions in the Jiao-Liao-Ji Belt, the unusual association of the A-type alkali-feldspar granite and albite granite indicates that the southern segment of the Jiao-Liao-Ji Belt experienced continental rifting during 2.1-2.2 Ga.