Alkaline Volcanism and Xenoliths of Sikhote-Alin

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

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

Late Miocene-Pliocene tholeiite-alkaline volcanism of the Sikhote-Alin was extension- related and with fissure-type
eruptions. Alkaline basalts (together with basanites and hawaiites) are more late and form independent volcanic structures.
They include Ne-normative rocks and transitional basalts with TiO2 (1.7-2.4%), Zr (~180ppm), V (100-150 ppm), Rb/Sr (0.073-0.114), Zr/Nb (6.2-7.4), Y/Nb (0.9-1.1). REE distribution reveals the increased contents of LREE, absence of Eu-
anomalies and coincides with the model distribution at 2% partial melting. La/Yb varies from 15 to 28 (Lan 67-153,
Ybn 5.6-7.1). It is possible to state that generation of this alkaline series was controlled by differences in partial melting degrees, PT-conditions, and, partly, olivine fractionation. The alkaline basalts of the Eastern Sikhote-Alin carry out abundant mantle and crustal xenoliths. The mantle xenoliths include: 1) "black" series (pyroxenites, olivine pyroxenites); 2) "green" series (spinel lherzolites, olivine websterites, harzburgites, olivine enstatites, enstatitites, wehrlites and websterites. Crustal xenoliths include basic and ultrabasic granulites. Pyroxenite xenoliths contain Al-augite, olivine, spinel, magnetite and feldspar. Mg# varies from 54 to 75, and Ni/Co 1.7-6.2. Pyroxenites have a steady Ne-normative composition. "Green" series magnesian xenoliths have a steady mineral association: olivine + Cr-diopside + enstatite + spinel. Olivine Mg# varies from 90.4 to 87.1, NiO 0.268-0.362%. Diopside Mg# is 89-92, Cr varies from 0.02 to 0.09. Enstatites have 3.5-6.5% Al2O3, <0.3% TiO2, Mg# 89-91. Spinel Mg# varies from 70 to 80, there are wide variations in content of Cr, Al and Fe. Variations in composition of the abundant xenoliths indicate to the regional mantle heterogeneity. The absence of garnet-bearing peridotite xenoliths and plagioclase peridotite ones indicates that the depth of alkaline basalts generation is restricted by levels of the spinel facies.

High-Magnesian Magmas of the Eastern Sikhote-Alin

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


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

Oligocene-early Miocene calc-alkaline suite of the Eastern Sikhote-Alin includes the subduction-related high-aluminous lavas and subordinary high-magnesian basalts and andesites. The latter contain phenocrysts of plagioclase (olivine, or hornblende) mainly. Compatible elements content is high compared to that of high-Al lavas: Ni (50-100 ppm), Cr (80-85 ppm), V (150-180 ppm), Sc (10-28 ppm); major elements are K2O (1-1.7%), P2O5 (0.16-0.3%), TiO2 (0.65- 0.95%). At spider-diagram high-Mg magmas reveal negative Th, Ta, Nb anomalies and positive Sr, P, Zr peaks. They are less enriched in LREE and more depleted in HREE. The REE distribution is relatively smooth with a more marked HREE depletion. In cross-sections high-Mg lavas are less voluminous than abundant high-aluminous ones and more late by eruptions, besides, their Al2O3 content is relatively high (16-18%). These peculiarities support the following model of high-Mg magmas generation. High-Al magmas are produced by melting of the slab eclogites. Generation of high-Mg takes place due to hybridization of high-Al magmas with the mantle wedge and lithosphere during an uplift in intermediate chambers. Such processes may be facilitated by the presence of structural and thermal heterogeneities in lithosphere (keels) which cause the appearance of deep-seated magmatic chambers. Thus, at experimental Pl-Ol-Q diagram high-Mg lavas show the 8 kbar pressure (while high-Al ones - only 5 kbar). High-Mg andesites of the Eastern Sikhote-Alin refer to adakites (distinguished by M.Defant in the Aleutians) by composition. They
are produced from high-Mg basalts with AFC processes. Lower crustal contamination is supported by increase of 87Sr/86Sr (0.7033-0.7042) and Sr/Y (55-85) values. The material of the regional lower crust is represented partly by granulites carried out by alkaline basalts. They are depleted in Rb (1.6-2.4 ppm), Zr (8-30 ppm), Y (5-18 ppm). Thus, their capture by high-Mg basalts may explain the geochemistry of andesites.