GEOCHEMICAL CHARACTERISTICS AND 40AR-39AR AGES OF THE AMPHIBOLITES AND GABBROS IN TARLANG AREA: IMPLICATIONS FOR TECTONIC EVOLUTION OF THE CHINESE ALTAI

Abstract

Amphibolitic and gabbroic rocks occur in Tarlang area in Chinese Altay. Amphibolites are metamorphosed products of mafic magmatic rocks (SiO2 = 45.42% similar to 52.96%) and contain high Al2O3(13.14% similar to 17.92%), Fe2O3T (9.08% similar to 16.88%) and TiO2 (0.82% similar to 3.23%) contents. The amphibolite samples show relatively flat REE patterns, without significant fractionations of REEs (La/Yb-N = 1.53 similar to 1.79). These samples are depleted in HFSEs slightly, with slightly Ti-Nb-Ta negative anomalies, which suggest a subtle relation to subduction-related environment. The gabbroic samples (SiO2 = 46.96% similar to 49.86%) have intermediate TiO2 (0.38% similar to 1.89%) and relatively low K2O contents (K2O = 0.34% similar to 1.03%), belonging to subalkali rocks (Na2O > K2O). They have relatively high Al2O3 (15.0% similar to 20.81%), CaO (10.13% similar to 11.60%) and moderate MgO (7.3% similar to 8.3%) contents. The gabbroic samples show two types of REE patterns. One is characterized by slight fractionation of REEs (La/Yb-N = 1.10 similar to 1.67) and weak Eu anomalies (delta Eu = 1.03 similar to 1.10), while the other is enriched in LREEs (La/Yb-N = 5.97 similar to 6.39), with significantly positive Eu anomalies (delta Eu = 1.31 similar to 1.44), which may reflect accumulation of plagioclase in the magma chamber. These gabbroic rocks all exhibit Ti-Nb-Ta troughs and Pb-Sr spikes, also implying a subduction-related origin. The Ar-40/Ar-39 spectrum of the gabbroic sample is simple and flat (plateau age of 266.9 +/- 4.2Ma; isochron age of 261 +/- 23Ma), indicating that its K-Ar isotopic system was not seriously disturbed by subsequent thermal events and may reflect the intrusion time of the gabbro. However, Ar-40-Ar-39 analysis of the amphibolite give rise to a saddle-like spectrum, and its youngest plateau (297.3 +/ 6.1Ma) probably reflects the last metamorphic event. The difference on age and geochemistry between the amphibolitic and gabbroic rocks implies that they probably represent different mafic magma. The protolith of amphibolites probably formed in a seamount near a subduction zone and subsequently accreated to the Altai. The gabbro exhibits subduction-releted characteristics and was coeval with transtensional movement in the Chinese Altai, probably reflecting a tectonic transition in the Permian.