Geochemistry, metamorphism and geochronology characteristics of the garnet amphibolites in the Baoyintu Group, central-western Inner Mongolia

The Baoyintu Group is distributed in the areas of Langshan, Tugurige and Damaoqi in the central-western Inner Mongolia. It is mainly composed of mica-schist, quartz schist, quartzite, amphibolite and marble. Garnet amphibolites developed in the Geraobao and Tugurige, occur as bedded or lenticular bodies within the mica-schist, and they are characterized by different mineral assemblages. The former mainly contains garnet, hornblende, plagioclase, rutile and ilmenite, and the latter mainly contains garnet, hornblende, epidote, rutile and ilmenite. Geochemical study shows that the garnet amphibolites are characterized by tholeiitic basalt series. The garnet amphibolites show right-dipping REE patterns with Sigma REE = 83.31 x 10(-6) similar to 125.9 x 10(-6), (La/Yb)(N), = 2.17 6.48 and delta Eu = 0.87 similar to 0.98, and they have no obvious negative anomaly in Ta, Nb and Ti, showing an affinity to enriched mid-ocean ridge basalts (E-MORB). Discrimination diagram indicates that the protolith of garnet amphibolites may have formed in an intra-continental rift setting. Phase equilibria modeling shows that samples LS01 from Geraobaogou and LS35 from Tugurige both experienced clockwise P-T paths including prograde, peak and near isothermal decompression stages. The peak P-T condition of sample LS01 is estimated to be similar to 11 kbar/similar to 735 degrees C with the peak mineral assemblage predicted to be garnet + hornblende + plagioclase + biotite + diopside + rutile + melt. However, the P-M (H2O) pseudosection shows that the diopside would not be retained due to the saturated fluid penetration during the post-peak evolution. The peak P-T condition of sample LS35 is estimated to be degrees 8kbar/675 degrees C, and the peak mineral assemblage is predicted to be garnet + hornblende + epidote + rutile. The different peak P-T conditions of the garnet amphibolites are indicative of the progressive metamorphism in the Baoyintu Group. Zircon LA-ICP-MS U-Pb dating indicates that the metamorphic zircons in garnet amphibolite sample LS01 are aged at 394 +/- 8Ma. Consequently, it is inferred that the protolith of these garnet amphibolites may have formed in the Neoproterozoic rifting basins in the northern margin of North China Craton. In the Middle Devonian, the Baoyintu Group as a part of the North China Craton, was involved in the orogenic process associated with the closure of the Paleo-Asian Ocean and experience progressive metamorphism of medium P/T series.