Magmatism and Metamorphism of the Proterozoic in the Northeastern Part of Korea : Petrogenetic and Geochemical Characteristics of the Okbang Amphibolites

한국(韓國) 북동부지역(北東部地域) 원생대(原生代)의 화성활동(火成活動)과 변성작용(變成作用) : 옥방(玉房) 앰피볼라이트의 암석성인(岩石成因)과 지구화학적(地球化學的) 특징(特徵)

  • Chang, Ho-Wan (Department of Geological Science, Seoul National University) ;
  • Lee, Dong-Hwa (Department of Geological Science, Seoul National University) ;
  • Park, Kye-Hun (Korea Basic Science Center)
  • Received : 1993.11.16
  • Published : 1993.08.31

Abstract

The Okbang amphibolites occurring as sill-shaped bodies within the Precambrian Wonnam Group have been studied in terms of geochemical characteristics for their tectonomagmatic environments. The amphibolites fall in the ortho-amphibolite fields in Ni and Cr versus Cu diagrams. They belong to subalkaline and tholeiitic series in total alkali versus silica and ternary AFM diagrams, respectively. They show the compositional variation corresponding to the differentiation trend of tholeiitic suites. In discrimination diagrams using high-field-strength elements such as Ti, Zr, Nb and Y, the amphibolites show geochemical affinities to both of volcanic-arc tholeiites and normal (depleted) mid-oceanic ridge tholeiites. The REE patterns of the amphibolites are nearly flat and extremely similar to those of back-arc tholeiites. $(La/Yb)_{CN}$ ratios vary from 0.89 to 2.02 with an average value of 1.23. Such low light-REE abundances in the amphibolites suggest that they were derived from the upper mantle source depleted in these elements. In view of geochemical characteristics showing strong enrichments of incompatible elements such as K and Rb, distinctive negative Nb anomalies, depletions of light-REE observed also in normal (depleted) mid-oceanic ridge tholeiites, and unfractionated immobile elements such as Y and Yb, the tholeiitic magmas, from which the parent rocks of the amphibolites were formed, would be generated from a depleted upper mantle source and contaminated by continental crustal materials en route to surface. Tectonomagmatic environment for the amphibolites can be assumed to be continental back-arc basin.

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