Early cretaceous ophiolites of the Yarlung Zangbo Suture Zone: insights from dolerites and peridotites from the Baer upper mantle suite, SW Tibet (China)

Melt evolution of upper mantle peridotites and mafic dikes in the northern ophiolite belt of the western Yarlung Zangbo suture zone (southern Tibet)

“…7B). These geochemical features are similar to those of microgabbro and dolerite dike rocks from the North Baer (Liu et al, 2015b;Zheng et al, 2017) and Cuobuzha massif (Liu et al, 2015c) in the NB of the western YZSZ (Fig. 7).…”

“…These dikes are commonly 0.5-1.5 m in thickness and have welldeveloped chilled margins against their ultramafic host rocks. Previous in situ zircon dating of several of these dolerite and microgabbro dikes in the North Baer submassif yielded crystallization ages ranging from 128 Ma to 126 Ma (Liu et al, 2015b;Zheng et al, 2017). However, Liu et al (2015b) reported the zircon U-Pb age of 128.1 ± 2.1 Ma yielded by merely 6 zircon grains with the SHRIMP (sensitive high-resolution ion microprobe) method; Zheng et al (2017) reported 2 laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) zircon ages of 126.3 ± 2.4 and 125.6 ± 2.4 Ma, but their zircon ages span a large range, including 145, 139, 135-119, 117 Ma (Zheng et al, 2017).…”

“…Previous in situ zircon dating of several of these dolerite and microgabbro dikes in the North Baer submassif yielded crystallization ages ranging from 128 Ma to 126 Ma (Liu et al, 2015b;Zheng et al, 2017). However, Liu et al (2015b) reported the zircon U-Pb age of 128.1 ± 2.1 Ma yielded by merely 6 zircon grains with the SHRIMP (sensitive high-resolution ion microprobe) method; Zheng et al (2017) reported 2 laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) zircon ages of 126.3 ± 2.4 and 125.6 ± 2.4 Ma, but their zircon ages span a large range, including 145, 139, 135-119, 117 Ma (Zheng et al, 2017). Moreover, the whole-rock geochemical data from Zheng et al(2017), especially the heavy rare earth element (HREE) contents, are not only much lower than those of samples from the same location in North Baer (12YL61, this study), but all the mafic dikes from Dajiweng (Zhang et al, 2005), North Baer (Liu et al, 2015b), South Baer (this study), Cuobuzha (Liu et al, 2015c; this study) in the NB, and from Purang (Liu et al, 2010;Liu et al, 2011;Liu et al, 2013;Miller et al, 2003) and Xiugugabu (Bezard et al, 2011) in the SB.…”

“…Moreover, the whole-rock geochemical data from Zheng et al(2017), especially the heavy rare earth element (HREE) contents, are not only much lower than those of samples from the same location in North Baer (12YL61, this study), but all the mafic dikes from Dajiweng (Zhang et al, 2005), North Baer (Liu et al, 2015b), South Baer (this study), Cuobuzha (Liu et al, 2015c; this study) in the NB, and from Purang (Liu et al, 2010;Liu et al, 2011;Liu et al, 2013;Miller et al, 2003) and Xiugugabu (Bezard et al, 2011) in the SB. The age and geochemical data from Zheng et al (2017), therefore, are not 100% reliable. In this study we collected additional dike rocks from both the North and South Baer submassifs for new geochemical and isotopic analyses and for U-Pb zircon dating.…”

“…Pillow lavas and sheeted dikes have not been observed in any of the massifs within the NB along the western YZSZ. The upper mantle peridotites are composed predominantly of serpentinized lherzolite and harzburgite in the Baer, Gongzhu, and Zhalai massifs (Lian et al, , 2017Zheng et al, 2017), and clinopyroxene harzburgite and harzburgite with minor dunite-chromitite occurrences in the Cuobuzha and Kazhan massifs (Liu et al, 2015c;Feng et al, 2015;Lian et al, 2016Lian et al, , 2017. Northwest-southeast-striking dolerite, clinopyroxenite, and microgabbro dikes crosscut these peridotites in almost all ophiolite massifs in the NB.…”