Potassic volcanic rocks and adakitic intrusions in southern Tibet: Insights into mantle–crust interaction and mass transfer from Indian plate

Abstract

Elucidating geodynamic processes at depth relies on a correct interpretation of petrological and geochemical features in magmatic records. In southern Tibet, both potassic volcanic rocks and adakitic intrusions exhibit high Sr/Y and La/Yb, and low Y and Yb concentrations. But these two rocks types have contrasting temporal-spatial distributions and isotopic variations. Here we present a systematic study on the postcollisional potassic and adakitic rocks in order to investigate their petrogenetic links with the coeval mantle-derived ultrapotassic rocks and shed light on the potential input from underthrusted Indian continental crust. We found that adakitic intrusions with higher K 2 O/Na 2 O tend to display lower Y and higher SiO 2 , suggesting that the mantle-derived ultrapotassic melts, showing relatively high Y and Yb concentrations, only played a minor role in adakitic magmatism. Therefore, unradiogenic 143 Nd/ 144 Nd and the dramatic decrease of zircon ε Hf (t) values since ~35 Ma shown by postcollisional adakites should be interpreted as reflecting the crust input from Indian continental crust. Unlike adakitic intrusions in southern Lhasa subterrane, potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks, and their isotopic discrepancy is diminishing with volcanic activity becomes younger and migrates eastward. Evidence from whole-rock Pb and zircon Hf isotopes further indicates that potassic volcanic rocks are more likely to originate from partial melting of the overthickened and isotopically heterogeneous Lhasa terrane crust rather than the underthrusted Indian crust. The elevated Rb/Sr and varying Sr/CaO in potassic volcanic rocks provide an argument for sanidine + plagioclase + clinopyroxene as the major fractionating phases during magmatic differentiation. These findings not only highlight the significance of potassic and adakitic rocks in providing constraints on the geodynamic processes beneath southern Tibet, but also imply that special caution is needed if we attempt to probe into the nature of mantle lithosphere using isotopic tracers from the Tibetan ultrapotassic rocks.

Miocene potassic and adakitic intrusions in eastern central Lhasa terrane, Tibet: Implications for origin and tectonic of postcollisional magmatism

“…Data sources of the postcollisional potassic and adakitic rocks in southern Tibet are as in Figure . Data sources of the ultrapotassic rocks in southern Tibet are from Miller et al (), Zhao et al (), Liu et al (, ), and Guo et al (). [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Following the continuous convergence between India and Eurasia, postcollisional ultrapotassic and potassic volcanic rocks and potassic and adakitic intrusions were formed in the western central and southern Lhasa terranes. Previous studies proposed that the postcollisional ultrapotassic and potassic volcanic rocks are mainly distributed in the western central Lhasa terrane (Miller et al, ; Ding et al, ; Zhao et al, ; Chen et al, ; Liu et al, , ), whereas the postcollisional adakitic rocks are mainly distributed in the southern Lhasa terrane (Chung et al, ; Guo et al, ; Hou et al, ; Hou et al, ; Li et al, ; Xu et al, ). High‐quality zircon U–Pb dating results indicate that most potassic and adakitic rocks were emplaced in the Miocene (<23 Ma; Miller et al, ; Ding et al, ; Chung et al, ; Guo et al, ; Zhao et al, ; Xu et al, ; Chen et al, ; Li et al, ; Hou et al, ; Hou et al, ; Liu et al, , ; Guo et al, ; Li et al, ).…”

“…Miocene postcollisional potassic and adakitic rocks in southern Tibet are generally thought to be crucial to comprehensively understand the geological evolution of the Tibetan plateau (Chung et al, ; Ding, Kapp, Zhong, & Deng, ; Miller, Schuster, Klötzli, Frank, & Purtscheller, ). However, these postcollisional potassic and adakitic rocks widely distributed in southern Tibet have been recognized for almost 20 years (Figure a,b), their origins and geodynamics are still controversial (Miller et al, ; Ding et al, ; Chung et al, ; Guo, Wilson, & Liu, ; Guo, Wilson, Zhang, Cheng, & Zhang, ; Zhao et al, ; Xu, Zhang, Guo, & Yuan, ; Chen et al, ; Li et al, ; Hou et al, ; Liu et al, , ; Li et al, ). So far, several models have been proposed to interpret the origin of Miocene postcollisional potassic rocks.…”

“…So far, several models have been proposed to interpret the origin of Miocene postcollisional potassic rocks. These include (a) partial melting of the heterogeneous subcontinental lithospheric mantle (SCLM) metasomatized by fluids derived from dehydration of the subducted Indian lower crust (Ding et al, ; Miller et al, ; Tian et al, ), (b) partial melting of a mafic lower‐crustal source (Chen et al, ), (c) partial melting of the overthickened and isotopically heterogeneous Lhasa terrane crust (Liu et al, , ), and (d) partial melting of mélanges associated with the subducted Indian continent crust and mantle wedge materials in a continental subduction channel (Zhang, Guo, Zhang, Cheng, & Sun, ). Similarly, several models have been proposed for the formation mechanism of Miocene postcollisional adakitic rocks.…”

“…Major outcrops of postcollisional potassic rocks in the Lhasa terrane include Yare (23–17 Ma; Liu et al, ), Xungba‐Bangba (25–17 Ma; Miller et al, ; Chen et al, ; Liu et al, ), Sailipu (17–16 Ma; Sun et al, ), Maiga (18–16 Ma; Zhao et al, ), Zabuye (16–15 Ma; Nomade, Renne, Mo, Zhao, & Zhou, ), Tangra Yum Co‐KonglongXiang (23–17 Ma; Ding et al, ; Chen, Xu, Wang, Kang, & Jie, ), Yangying (11–9 Ma; Nomade et al, ; Liu et al, ), Nanmulin (15–12 Ma; Zhao et al, ; Spicer et al, ; Williams, Turner, Pearce, Kelley, & Harris, ), Chazi (13–9 Ma; Liu et al, ), and Dajia co (17 Ma; Liu et al, 2017). Major outcrops of postcollisional adakitic rocks in the Lhasa terrane include Jiama (17–15 Ma; Hou, Gao, Qu, Rui, & Mo, ), Qulong (18–17 Ma; Hou et al, ), Lakang'e (14–11 Ma; Qu, Hou, & Li, ), Nanmu (16–12 Ma; Hou et al, ; Chen et al, ), Maquiang (15–10 Ma; Turner et al, ), Xigaze (18–15 Ma; Chung et al, ), Dajia co (15–10 Ma; Williams et al, ; Liu et al, ), Zhongba (17 Ma; Li et al, ), Bangba (17 Ma; Chen et al, ), and Shiduo (16 Ma; Cai et al, ). BNS = Bangong‐Nujiang suture zone; SNMZ = Shiquan river‐Nam Tso mélange zone; LMF = Luobadui‐Milashan Fault; KF = Karakorum Fault; IYZS = Indus‐Yarlung Zangbo suture zone [Colour figure can be viewed at wileyonlinelibrary.com]…”