New Nd and Pb isotope data on the ophiolite complexes of the Kronotskii paleoarc, Eastern Kamchatka

Abstract

The problem of typification of tectonically disintegrated ophiolite complexes in the accretionary framing of the northwestern Pacific remains a matter of debate. In addition to detailed geological, mineralogical, and geochemical data, the isotopic characteristics of the rocks of ophiolite complexes play an important role in solving this problem. We studied ophiolites of the Eastern peninsulas of Kamchatka (Kamchatskii Mys and Kronotskii peninsulas), which are the fragments of the Late Cretaceous‐Eocene Kronotskii paleoarc (Fig. 1). In order to obtain the isotopic characteristics of the ophiolite complexes, we studied Pb and Nd isotopic ratios in the least altered rocks. This paper presents new data on the isotopic composition of ophiolites of the Kronotskii paleoarc. The 143 Nd/ 144 Nd, 208 Pb/ 204 Pb, 206 Pb/ 204 Pb, and 207 Pb/ 204 Pb isotopic ratios were analyzed on a Triton TI solid phase multichannel mass spectrometer at the Center of Isotopic Research (Karpinskii All-Russia Research Institute of Geology, St. Petersburg) using the standard technique. The average reproducibility of analyses during determination of the Nd isotopic composition was no worse than 0.005% (2 σ ) for the 143 Nd/ 144 Nd ratio. The isotopic composition of the Pb standard measured during the analytical works was as follows: 143 Nd/ 144 Nd = 0.512112 ± 0.000002 for JNdi-1 standard; 206 Pb/ 204 Pb = 16.937 ± 0.011, 207 Pb/ 204 Pb = 15.492 ± 0.017, 208 Pb/ 204 Pb = 36.722 ± 0.017 for NIST-981 standard. We united ophiolite fragments in the Kamchatskii Mys Peninsula into three ophiolite complexes developed in the Afrikan block [1]: (1) Aptian‐Cenomanian oceanic complex represented by tholeiitic basalts, jaspers, and limestones of the Afrikan Complex; (2) Late Cretaceous suprasubduction complex including ultrabasic rocks of Mt. Soldatskaya and Ol’khovsk gabbro‐ plagiogranite massif; (3) Paleocene‐Eocene intra-arc complex including basalts, jaspers, and mudstones of the Kamensk Complex and gabbroids of the Olenegorsk Complex. The Stolbovskii block of the Kamchatskii Mys Peninsula contains Maastrichtian islandarc tholeiites of the Tarkhovka Subformation (Stolbovskaya Group), which are likely complementary to suprasubduction ophiolites of Mt. Soldatskaya. Similar island-arc volcanic rocks occur as tectonic blocks in the