Cenozoic Shoshonites of the Eastern Sikhote-Alin

Tatiana Petrova United Institute of Geology, Geophysics & Mineralogy, Novosibirsk 630090, Russia


Sergey Esin United Institute of Geology, Geophysics & Mineralogy, Novosibirsk 630090, Russia

M. Shimazu Prefectural Niigata Women's College, Japan

Subduction-related calc-alkaline volcanism of the Eastern Sikhote-Alin (Eocene-Early Miocene) has changed by extension-related volcanic activities. Shoshonitic suites erupted with the Japanese Sea basin opening (18-15 Ma). Absarokites, shoshonites and latites associate with less abundant high-Al lavas and more late tholeiitic lavas. The shoshonite suite has been studied from the Uy volcano (near the Sovgavan city). It has a bimodal character with SiO2 compositional gap from 54 to 59%. The rocks contain phenocrysts of plagioclase, olivine, hornblende, magnetite. K2O content changes from 1.5-2.5% (at <54% SiO2) to 3-4.5% (at >59% SiO2), MgO from 5-6% to 0.5-1.5%, Al2O3 varies in 16-18% range, CaO changes abruptly from 7-8.5% to 2-4%, TiO2 from 1.4-1.6% to 0.5-0.9%, P2O5 from 1.8-2% to 0.2-0.4%, Na2O from 2.5% to 4.5%. The sharp decrease in TiO2 and P2O5 is caused by fractionation of Ti-magnetite and apatite phases. These rocks are enriched in K (up to 38000 ppm), Sr (up to 708 ppm), Ba (up to 1708 ppm), La (up to 35 ppm). These peculiarities do not seem to result from fractional crystallization of associating calc-alkaline and tholeiitic magmas. The source of the magmas with shoshonitic affinities is supposed to combine both subduction (slight Ce anomalies) and plume signatures. Ba/La (29-41.4), Sr/Nd (15.6-27.2), (Ce/Yb)n (3.8-5.3) are the major source characteristics. The rocks reveal enrichment in LREE, though not as significant as Andean shoshonites ((La/Yb)n 4.2- 8), together with slight Eu negative anomalies, but the influence of crustal material seems to be unimportant.
By 143Nd/144Nd (0.512843-0.512861) and 87Sr/86Sr (0.70372-0.70367) characteristics the Sikhote-Alin shoshonites are
intermediate between the rocks of Aleutians and Andes. According to the models of batch melting for the Northern Mariana shoshonites (Lin et al., 1989), the REE distribution of the Sikhote-Alin shoshonites indicates to the low per cent melting of the spinel lherzolite.