Abstract
Miocene felsic magmatic rocks with high Sr/Y ratios are widely distributed throughout the Gangdese belt of southern Tibet. These provide a good opportunity to explore the magmatic process and deep dynamic mechanisms that occurred after collision between the Indo and the Asian plates. In this paper, felsic volcanic rocks from the Zongdangcun Formation in the Wuyu Basin in the central part of the southern Gangdese belt are used to disclose their origin. Zircon U-Pb geochronology analysis shows that the felsic magmatism occurred at ca. 10.3 ± 0.2 Ma, indicating that the Zongdangcun Formation formed during the Miocene. Most of these felsic magmatic rocks plot in the rhyolite area in the TAS diagram. The rhyolite specimens from the Zongdangcun Formation have the characteristics of high SiO2 (>64%), K2O, SiO2, and Sr contents, a low Y content and a high Sr/Y ratio, and the rocks are rich in LREE and depleted in HREE, showing geochemical affinity to adakitic rocks. The rocks have an enriched Sr-Nd isotopic composition (εNd(t) = −6.76 to −6.68, (87Sr/86Sr)i = 0.7082–0.7088), which is similar to the mixed product of the juvenile Lhasa lower continental crust and the ancient Indian crust. The Hf isotopes of zircon define a wide compositional range (εHf(t) = −4.19 to 6.72) with predominant enriched signatures. The Miocene-aged crustal thickness in southern Tibet, calculated on the basis of the Sr/Y and (La/Yb)N ratios was approximately 60–80 km, which is consistent with the thickening of the Qinghai-Tibet Plateau. The origin of Miocene felsic magmatic rocks with high Sr/Y ratios in the middle section of the Gangdese belt likely involved a partial melting of the thickened lower crust, essentially formed by the lower crust of the Lhasa block, with minor contribution from the ancient Indian crust. After comprehensively analyzing the post-collisional high Sr/Y magmatic rocks (33–8 Ma) collected from the southern margin of the Gangdese belt, we propose that the front edge tearing and segmented subduction of the Indian continental slab may be the major factor driving the east-west trending compositional changes of the Miocene adakitic rocks in southern Tibet.
Highlights
The adakitic rocks of the Zongdangcun Formation in the Wuyu Basin are characterized by high Sr, low Y, high Sr/Y, and La/Yb ratios and fall into the adakitic rock field (Figure 7)
Due to the occurrence of Cr, Sc, and Y and HREEs in garnet, these studied samples show that the content of the above elements is low and that the Sc content is almost stable with changes in the Cr content, indicating that there are a lot of garnet residues in the source area and the samples were formed in a deeper depth
We propose that the post-collisional adakitic magmatic rocks in the middle segment of the southern margin of the Gangdese belt are mainly derived from partial melting of the thickened lower crust, with garnet and amphibole remaining in the source area
Summary
It is a giant Cenozoic magmatic belt created by the superposition of Neotethys oceanic lithospheric subduction and the collision of the Indian plate with the Eurasian plate [1]. The Oligocene-Miocene magmatic rocks are the products of the India–Eurasian plate collision. Due to the complex genetic mechanism and magmatic dynamics of adakitic rocks, the magma source and tectonic background of the Oligocene-Miocene adakites in southern Tibet have always been controversial [12,13,14,15,16,17,18,19,20,21]. An in-depth discussion of the genetic mechanism of the Oligocene-Miocene post-collision adakitic rocks in the southern Gangdese belt is beneficial to understanding the dynamic of subduction and accretion between the Indian and Eurasian plates and is helpful to accurately determine the evolution of the Gangdese continental crust in Cenozoic. According to the above scientific issues of the Neogene Zongdangcun Formation felsic magmatic rocks from the Wuyu Basin, Nanmulin area, southern Gangdese region are selected to perform systematic petrology, zircon U-Pb chronology, trace element geochemistry and
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