Reply to Zhang et al.’s comment on “Geochronology and geochemistry of deep-drill-core samples from the basement of the central Tarim basin”

Abstract

We welcome Zhang et al.’s comment on our recentpaper dealing with the significance of the first geochemicaland geochronological analyses of deep drill-core samplescollected from the basement of the central Tarim basin(Guo et al., 2005). In their comments, Zhang et al. raisedthree questions with regard to our paper (1) the compatibil-ity of our model with regional geology, (2) our interpreta-tion that the central Tarim granitoid was derived from amagmatic arc, and (3) if the timing of our inferred arc mag-matism and suturing event across the central Tarim basin isconsistent with recently radiometric ages of mafic dikes andNeoproterozoic rift sequences sequences around the Tarimbasin. We address them below.1. Regional geologyIn our paper we point out clearly the potential compli-cation of Cenozoic deformation in the southern Tian Shanin modifying the original tectonic position of the Aksublueschist belt in the northern margin of the Tarim basin(Yin et al., 1998; Allen et al., 1999; Burchfiel et al., 1999).In addition, large-scale late Paleozoic strike-slip faultingin the Tian Shan could also displace Precambrian magmat-ic arcs (e.g., Yin and Nie, 1996). Thus, the lack of a mag-matic arc in the southern Tian Shan cited by Zhang et al.as evidence against our tectonic could have resulted fromthese processes. However, a more plausible explanation isthat the arc is covered by the late Proterozoic–early Paleo-zoic cover sequence extensively exposed along the southernmargin of the Tian Shan and northern Tarim basin (Xinji-ang BGMR, 1993). Regardless of the detailed and certainlynon-unique interpretation of the where-about of ourinferred arc in the southern Tian Shan, the most importantfact our mode relies on is the existence of a blueschist in thesouthern Tian Shan, which cannot be produced by Precam-brian rift as would be required by the Zhang et al.’s alter-native interpretation, but instead must have beenassociated with oceanic subduction as seen in the Francis-can Complex in California or continental collision as seenin the Dabie Shan of east-central China.2. Geochemistry and geochronologyWe did not suggest that our samples are ‘‘calc-alkalinerocks’’ as quoted by Zhang et al. in their comments.Instead, we discuss in our paper the difference in geochem-istry between our sample and typical calc-alkaline igneousrocks. We are not aware of any existing work that pre-cludes the occurrence of syenodiorite during arc magma-tism, which was suggested by Zhang et al.’s comment. Aclear and nearby case can be made in the CarboniferousTian Shan, where trachyandesites can be demonstrated tohave occurred in an island arc setting (Zhu et al., 2005).We also find that the criticism of Zhang et al. about notciting some of the recent geochronologic results in ourpaper around the Tarim basin unfair. Our manuscriptwas in the process of printing since 2003 and we clearlydid not have the chance to read papers by Zhang et al.(2005) and Xu et al. (2005). Nevertheless, we commentbelow on our work in the context of these new radiometricages around the Tarim basin. In Zhang et al. (2005), theyreport ages of mafic dikes near Aksu to be 807 ± 12 and