Hadong-Sancheong area in the Jirisan gneiss complex, the southwestern part of the Yeongnam massif, is characterized by the widespread occurrence of the north-south trending Paleoproterozoic anorthosite, charnockite and quartz-feldspathic, migmatitic, porphyroblastic and granitic gneisses. The protolithes of gneisses and charnockite were formed during the post-collision stage by partial melting of continental crust or lithospheric mantle, which experienced continental arc stage before collision. On the other hand, anorthosite may be formed during the post-collisional extension stage by the partial melting of uplifting asthenospheric mantle.
The porphyroblastic gneiss had experienced prograde metamorphism from 605-670℃/6.0-8.5 kbar to 790-870℃/8.5-9.9 kbar and then was retrograded into 420-440℃/1-2 kbar. The temperature and pressure conditions for quartz-feldspathic gneiss measured from the weakly and strongly retrograded rims, are 755-775℃/8.2-9 kbar and 555-590℃/3.2-4.5 kbar, respectively, indicating that the peak metamorphic conditions may be higher than 755-775℃/8.2-9 kbar. From charnockite, 815-825℃/9.2-9.6 kbar are obtained as peak metamorphic conditions, indicating that orthopyroxene-bearing mineral assemblage in charnockite is a metamorphic product. Different from other gneisses, the granitic gneiss gives very low metamorphic conditions (515-550℃, 440-450℃/1-2 kbar), which are similar to the retrograde P-T conditions of other gneisses. In this study, Sensitive high resolution ion microprobe (SHRIMP) age dating has been done on anorthosite, massive and foliated charnockites and granitic gneiss. The intrusion age of 1860 Ma is obtained from the anorthosite. Both massive and foliated charnockites have 1880-1871 Ma intrusion ages and 1869-1859 Ma metamorphic age.
This study combined with the previous studies suggests the following tectonic evolution. In Hadong-Sancheong area in the Jirisan gneiss complex, orthogneisses had intruded in the post collision tectonic setting at ages between 1880-1871 Ma and then underwent regional granulite facies peak metamorphism (790-870℃/8.2-9.9 kbar) at times between 1869 Ma and the intrusion ages of anorhosite. The anorthosite intruded at 1862-1860 Ma and the study area experienced migmatitization and retrograde metamorphism during its uplift to the depth of 1.0-4.5 kbar.