Lidong Zhu

Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

ABSTRACT We report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas Type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than Type 1 ones. Type 2 eclogites are significantly more enriched in TiO2, P2O5, and light rare-earth elements, but more depleted in Al2O3 contents and Mg numbers. P–T estimates for both types of eclogites are consistent with minimum pressures of 23–25 kbar at 830–920°C, with the retrograde amphibolite facies assemblage of 15.2–17.5 kbar and 500–732°C. Zircon U–Pb dating indicates that the protolith of Type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at ~260 Ma, whereas the protolith of Type 2 eclogites, geochemically familiar with oceanic-island basalt (OIB), erupted at ~242 Ma. 40Ar/39Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize eclogite-bearing high-pressure rocks were exhumed at ~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of ~85 km along the southern margin of the Qiangtang block during the Late Triassic.

Zircon U-Pb age and geochemical characteristics of granodiorites from the western Qilian block

Qilian Block is located in between the South China Craton and the North China Craton and the Tarim Craton (Fig. 1a), which is one of the key area to study the tectonic evolution of China. The Phanerozoic tectonic framework of Qilian Block has become increasingly clear, however, the understanding about the Precambrian tectonic framework has still a great deal of controversy (Wan et al., 2003; Gehrels et al., 2003; Tung et al., 2007; Song et al., 2012; Tung et al., 2012; Tung et al., 2013; Yu et al., 2013). The Liuhuangkuang granodiorite is located in the western Qilian block, and intrudes into Qingbaikouan weakly Metamorphic terrigenous clastic rocks and carbonate rocks (Fig. 1b). For a long time, due to the lack of accurate chronological data constraints, its age have different understanding, such as some researchers thought it was the product of the late Paleozoic magmatism (BGMRGP, 1972), but others believe that its age was Precambrian (BGMRQP, 1991). Therefore, a detailed research on geochronology and geochemistry has important significance for constraining the tectonic setting and petrogenesis of the granodiorites.

Zircon U-Pb dating, Geochemical Features of the Halunwusu Composite Granites at the Northern Edge of Southern Qilian, Qinghai

2.2 Geochemical Analysis The early adamellite to granodiorite is characterized by the mass fraction of SiO2(62.73%~67.73%)and Al2O3(14.96%~16.48%),the value of A/CNK is 0.87~1.07. The late granite is characterized by the mass fraction of SiO2(69.99%~73.88%)and Al2O3(13.53%~ 14.59%),the value of A/CNK is 1.00~1.22.The total REE contents range from 106.68×10 to 329.81×10, the ratio of LREE/HREE is between11.87 and 28.71, and show weak anomaly of Eu(δEu=0.53~0.92),representing the fractional crystallization of the Plagioclase and K-Feldspar during magmatic evokution.With the similar trace element distribution,the enrichment of LILE(Rb,Th,K,Pb) and depletion of HSFE(Nb,Ta,P,Ti) imply the Halunwusu Composite Granites mainly derived