Phase Equilibria Modeling and P-T Evolution of the Mafic Lower-Crustal Xenoliths from the Southeastern Margin of the North China Craton

Abstract

The Precambrian lower crust rocks at the southeastern margin of the North China Craton (NCC) are mainly exposed as granulite xenoliths hosted by Mesozoic dioritic porphyry and metamorphic terrains in the Xuzhou-Suzhou area. Garnet amphibolites and garnet granulites are two kinds of typical lower-crustal xenoliths and were selected to reconstruct different stages of the metamorphic process. In this study, in view of multistage metamorphic evolution and reworking, phase equilibria modeling was used for the first time to better constrain peak P-T conditions of the xenoliths. Some porphyroblastic garnets have a weak zonal structure in composition with homogeneous cores and were surrounded by thin rims with an increase in XMg and a decrease in XCa (or XMg). Clinopyroxene contain varying amounts of Na2O and Al2O3 as well as amphibole of TiO2, while plagioclases are different in calcium contents. Peak metamorphic P-T conditions are calculated by the smallest garnet x(g) (Fe2+/(Fe2++Mg)) contours and the smallest plagioclase ca(pl) (Ca/(Ca+Na)) contours in NCFMASHTO (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) system, which are consistent with those estimated by conventional geothermobarometry. The new results show that the peak and decompressional P-T conditions for the rocks are 850–900 °C/ 1.4–1.6 GPa and 820–850 °C/0.9–1.3 GPa, respectively, suggestive of high and middle-low pressure granulite-facies metamorphism. Combined with previous zircon U-Pb dating and conventional geothermobarometry data, it is indicated that the xenoliths experienced a clockwise P-T-t evolution with nearisothermal decompressional process, suggestive of the Paleoproterozoic subduction-collision setting. In this regard, the studied region together with Jiao-Liao-Ji belt is further documented to make up a Paleoproterozoic collisional orogen in the eastern block of the NCC.