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Geochemical Characteristics and Tectonic Implications of the HP-UHP Metabasites from the Habutengsu Area, Southwestern Tianshan, Xinjiang
详细信息   
摘要
The high-pressure ( HP) and ultrahigh-pressure ( UHP) metabasites including eclogite and blueschist are exposed in garnet phengite schists mainly as rock slices or variable tectonic lenses in the Habutengsu area,southwestern Tianshan,Xinjiang. Detailed petrology,mineralogy and geochemisitry studies have been carried out to put constrains on their protolith and regional tectonic evolution. Geochemical analyses indicate that compositions of the metabasites are mainly tholeiites with minor alkali basalts. Based on trace element and REE features,the eclogites can be subdivided into three types: N-MORB type,E-MORB type,and 0IB type,while all the blueschists are 0IB type. The first type eclogites are characterized by low Ti ( TiO2 = 1. 00%～1. 08% ) ,P ( P20 5 = 0. 09%～0. 15% ) and ∑REE (35. 98 × 10 - 6 ～43. 51 × 10 - 6 ) contents,and low LREE/HREE ( 1.35 ～1.43) and ( La/Yb) N ( 0. 63 ～0. 71) ratios,which are similar to those of typical N-MORB basalts,and are considered to be products of magmaderived from the depleted mantle sources in an mid-ocean ridge setting. The second type eclogites are characterized by high Ti ( TiO2 = 1.35% ～2.25% ) ,P ( P20 5 = 0. 05%～0.20% ) and ∑REE ( 60. 1 × 10 - 6～77.35 × 10 - 6 ) contents,and high LREE/HREE ( 1.25 ～2.43) and ( La/ Yb) N ( 0. 52 ～1. 52) ratios,which are similar to those of typical E-MORB basalts,and are considered to be products of magma from the enriched mantle sources. The third type eclogites are characterized by higher Ti ( TiO2 = 1. 16% ～2. 86% ) ,Zr/Y ( 5.32～7. 78) ,∑REE ( 79. 12 × 10 - 6～192. 1 × 10 - 6 ) contents,and higher LREE/HREE (4. 15～6. 54) and ( La/Yb) N (3. 77～9.44) ratios,which are similar to those of typical0IB basalts,indicative of enriched mantle sources. Geochemical characters of the0IB type blueschists are similar to the0IB type eclogites,characterized by high Ti ( TiO2 = 1.39% ～2. 86% ) ,∑REE ( 88. 18 × 10 - 6 ～227. 8 × 10 - 6 ) contents and high ( La/Yb) N ( 5. 03 ～9. 84) ,Zr/Y (4. 93 ～9. 55) ratios,indicating the same magmatic sources. Combined with the previous data from this region,the presence of such MORB and 0IB rock assemblages suggests the existence of anocean basin in the Early Paleozoic era from the Habutengsu area,southwestern Tianshan,Xinjiang. The evolution process may be also impacted by the crystallizationdifferentiation anddifferent sources magma mixing,which produced the elementgeochemistry affinity of MORB and 0IB in the metabasites. It is suggested that protolith of the metabasites were formed under the tectonic setting of mid-ocean ridge and adjacentocean island,and the formation of the blueschists and eclogites might be related to HP-UHP metamorphism and later tectonic exhumation.

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