Although the Permian Songdo ocean-type eclogite in the Lhasa Terrane was identified as the remnant of the Paleo-Tethyan (here named it as the Songdo Tethyan Ocean) oceanic lithosphere for years, the opening timing of this Tethyan ocean remains poorly constrained because of the rare occurrence of related geological records. To explore this issue, this paper reports whole-rock major, trace element, zircon U-Pb dating, and zircon Hf isotope data of the gneissic granitoids from Nang in the southern margin of the Lhasa Terrane. Two gneissic granitoid samples have been dated to be 341 ± 2Ma and 355 ± 2Ma , respectively, indicating that the Nang gneissic granitoids were emplaced at the Early Carboniferous. These rocks are mainly high-K calc-alkaline to shoshonitic , consisting mainly of granodiorite and monzogranite that are characterized by high SiO2(69% ∼76% ) and K2O/Na2O ratios (1. 04 ∼ 3. 04). The presence of muscovite , high aluminum saturation index ( A/CNK = 1. 04 ∼ 1. 52) , and high corundum ( > 1% ) suggest that the Nang gneissic granitoids are of S-type granites. All samples are enriched in light rare earth elements ( LREEs) and large ion lithophile elements (LILEs) , depleted in Ba , Eu , Sr , and high field strength elements (HFSEs). Whole-rock zircon saturation temperatures vary mainly from 780CC to 800T1 (up to 859T1 , with an average of 808T1 ). The occurrence of dioritic enclaves , high zircon saturation temperatures , low Pb and high Ba abundances , and negative zircon εHf (t) values ( -7.2∼ -1.2 , averaging = -4.4), indicate that the gneissic granitoids are of high-temperature S-type granites resulting from the anatexis of shallow mature crustal materials with significant contributions of mantle-derived materials. The nature of the high temperature ( >780°C) and low pressure ( < 5kbar) indicates an extensional setting for the generation of the Nang gneissic granitoids. Considering the Early Carboniferous magmatic arc and Devonian-Carboniferous sedimentary records that are dominated by limestone and fine-grained detritus interbedded with volcanic rocks preserved in the Lhasa Terrane , this paper proposes that the Nang gneissic granitoids were probably emplaced at a back-arc extensional region. Such region may subsequently develop a back-arc basin that ultimately evolved into the Songdo Tethyan Ocean. This means that the lifespan of this ocean did not exceed 80Myr (i. e. 340 ∼260Ma).
|Number of pages||15|
|Journal||Acta Petrologica Sinica|
|Publication status||Published - 2013|
- Back-arc extensional setting
- Early carboniferous granitoids
- Lhasa terrane
- Songdo tethyan ocean