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Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea
Author(s) -
Kim CheongBin,
Rajesh V. J.,
Santosh M.
Publication year - 2008
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.2007.00597.x
Subject(s) - geology , geochronology , zircon , geochemistry , continental arc , chondrite , rhyolite , partial melting , volcanic rock , primitive mantle , magmatism , mantle (geology) , volcano , tectonics , meteorite , paleontology , physics , astronomy
Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO 2 contents ranging from ∼72 to 73 wt%, K 2 O/Na 2 O ratios of 1.04–1.23 and low MgO/FeO t values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al 2 O 3 /Na 2 O + K 2 O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb) N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial 87 Sr/ 86 Sr isotopic ratios of 0.7104–0.7106 and identical 143 Nd/ 144 Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of −12. The depleted mantle model ages (T DM ) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are 206 Pb/ 204 Pb = 18.522–18.552, 207 Pb/ 204 Pb = 15.642–15.680 and 208 Pb/ 204 Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.