| PDF: | |
| Time: | 2010 |
| Journal: | Acta Petrologica Sinica |
| Volumn: | 26 |
| Issue: | 1 |
| Pages: | 274-282 |
| Type: | SCI |
| Author: | Weihua Bian,Pujun Wang,Xiaomeng Sun,Huafeng Tang |
Abstract: | In Mohoershan Formation (is an element of M-2), Middle Cambrian, 3 mafic sills formed during Late Permian to Middle Triassic have been detailed lithologically logged and sampled for geological dating and geochemical analysis in Kuruktag, Tarim basin, Xinjiang, NW China. The mafic sills were composed of tower diabase sill (with thickness of 3m), middle gabbro sill (with thickness of 20m) and upper dolerite sill (with thickness of 2.5m) which intruded into Middle Cambrian strata along the interface of neighboring sedimentary beds. The geochemical characteristics of these sills show great similarities, for instance, the high absolute concentration of total rare earth element (Sigma REE = 210 X 10(-6) similar to 297 x 10(-6)) the enrichment of LREE ((La/Yb)(N) = 13.54 similar to 17.96), weak Eu anomaly(delta Eu = 0.97 similar to 1.21) weak positive Cc anomaly (delta Ce = 1.11 similar to 1.25), enrichment of large ion lithophile element (LILE), significant positive anomaly of Sr and negative anomaly for Ta, Nb and HE Taking into account the geochemical features as well as the geological settings, these sills derived from same magma source. The magma was formed within the upper mantle and partially contaminated by the lower continental crust according to the standard mineral forming pressure, the REE pattern and trace earth element spider plot. The low MgO content (4% similar to 6%), Mg-# (37.6 similar to 55.6) and Sl (18.7 similar to 29.3) hinted the primary magma had undergone differentiation. The K-Ar age of dolerite sill is about 255.2 +/- 7.3 Ma, the K-Ar age of diabase sill is about 242.8 +/- 5.7 Ma and the Ar-Ar age of gabbro sill is about 229.7 +/- 3.7 Ma. The geological dating results indicated that the magma intrusion took place after the close of south Tianshan ocean, the paleo-tectonic environment is extensional settings of post-collision. |