Mineral phase equilibria and zircon geochronology constraining the P-T-t path of granulite-facies metapelites of the Mashan Complex in the Shangsanyang area, Eastern Heilongjiang Province, China

The Mashan Complex is the oldest metamorphic rock series in the Jiamusi Massif, which consists mainly of amphibolite to granulite facies metamorphic rocks accompanied by various granitic rocks. Herein, we investigate the pelitic granulites from this complex as well as associated granites and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M-1) is recorded by the core of the garnet and inclusions within: Grt(core)-Bi-Sil-Qz. The peak granulite facies metamorphism (M-2) is distinguished by the garnet mantle and the related mineral assemblages in the matrix: Bi-Kfs-Grt(mantle)-Ilm-Sil-Qz. The post-peak decompression stage (M-3) is characterized by the formation of cordierite corona and minerals in the cordierite of the matrix: Bi-Liq-Crd-Pl-Kfs-Grt-Ilm-Sil-Qz-Mt. The retrograde stage (M-4) is featured by the garnet rim and the nearby assemblages: Bi-Crd-Pl-Kfs-Grtrim-Ilm-Sil-Qz-Mt. According to the phase equilibria modeling, the metamorphic P-T conditions from M-2 to M-4 were estimated as 6.5-8.5 kbar/815-825 degrees C, 5.3-5.7 kbar/780-795 degrees C, and 4.5-5.3 kbar/730-775 degrees C, respectively, which together define a clockwise P-T path. LA-ICP-MS U-Pb dating for zircons from four metamorphic rocks shows that most of the detrital material came from middle Paleoproterozoic, middle Mesoproterozoic and early Neoproterozoic source rocks. In addition, metamorphic zircon grains in the metamorphic rocks yield two groups of ages at ca. 530 Ma and ca. 500 Ma, which are regarded as the peak of granulite-facies metamorphic (M-2) and the retrograde cooling metamorphic (M-4) ages, respectively.

Automated summary

The study of pelitic granulites and associated granites from the Mashan Complex in the Jiamusi Massif identified four distinct mineral assemblages corresponding to different metamorphic stages, with estimated metamorphic P-T conditions through phase equilibria modeling. Zircon U-Pb dating revealed detrital material from different geological periods, and metamorphic zircon grains showed ages at ca. 530 Ma and ca. 500 Ma, corresponding to peak granulite-facies metamorphism and retrograde cooling metamorphism, respectively.