Geochronology, mineralogy and geochemistry of alkali-feldspar granite and albite granite association from the Changyi area of Jiao-Liao-Ji Belt: Implications for Paleoproterozoic rifting of eastern North China Craton

Abstract

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 AlT-in-biotite geobarometer indicate that the alkali-feldspar granite crystallized at temperatures of 823–856°C (averaging at 839°C) and pressures of 1.54–2.73kbar (averaging at 2.05kbar) while the albite granite was cooled at the temperatures of 851–853°C and the pressures of 2.34–2.64kbar (averaging at 2.45kbar). Combined with such high temperatures and low pressures, the crustal-like Nd–Hf isotopic compositions (ɛNd(t)=−5.3 to −4.4, ɛHf(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=4.76–5.14wt.%, Na2O=4.76–5.14wt.%) and the albite granite (K2O=0.29–0.44wt.%, Na2O=6.15–6.56wt.%) 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.