Mineral chemistry and crystallization conditions of the Late Cretaceous Mamba pluton from the eastern Gangdese, Southern Tibetan Plateau

Abstract

The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly constrained. Here we present mineralogical data for the Mamba pluton, including host rocks and their mafic microgranular enclaves (MMEs), to provide insights into their overall crystallization conditions and information about magma mixing. All amphiboles in the Mamba pluton are calcic, with B (Ca+Na)>1.5, and Si=6.81-7.42 apfu for the host rocks and Si=6.77-7.35 apfu for the MMEs. The paramount cation substitutions in amphibole include edenite type and tschermakite type. Biotites both in the host rocks and the MMEs collectively have high MgO (13.19 wt.%-13.03 wt.%) contents, but define a narrow range of Al apfu (atoms per formula unit) variations (2.44-2.57). The oxygen fugacity estimates are based on the biotite compositions cluster around the NNO buffer. The calculated pressure ranges from 1.2 to 2.1 kbar according to the aluminum-in-hornblende barometer. The computed pressure varies from 0.9 to 1.3 kbar based on the aluminum-in-biotite barometer which corresponds to an average depth of ca. 3.9 km. Besides, the estimates of crystallization pressures vary from 0.8 to 1.4 kbar based on the amphibole barometer proposed by Ridolfi et al. (2010), which can be equivalent to the depths ranging from 3.1 to 5.2 km. The MMEs have plagioclase oscillatory zonings and quartz aggregates, probably indicating the presence of magma mixing. Besides, core-to-rim element variations (Rb, Sr, Ba, and P) for the K-feldspar megacrysts serve as robust evidence to support magma mixing and crystal fractionation. This indicates the significance of the magma mixing that contributes to the formation of K-feldspar megacryst zonings in the Mamba pluton. KEY WORDS: Mamba pluton, Gangdese terrane, MMEs, K-feldspar megacrysts, magma mixing, P-T conditions, oxygen fugacity, in-situ trace element. INTRODUCTIONPrevious studies have demonstrated that the composition and structure of feldspar, biotite and amphibole in felsic rocks can serve as ideal proxies for physico-chemical conditions of magmas, such as crystallization and emplacement temperature, pressure, oxygen fugacity, water activity and compositional variations (Ayati et al.

Neo‐Tethyanslab tearing constrained by PalaeoceneN‐MORB‐like magmatism in southern Tibet

“…The Neo‐Tethys Ocean was subducted northward beneath the Lhasa Terrane, shaping the formations that formed between India and Asia from the Late Triassic (Dong et al, 2006; Huang, Xu, et al, 2015; Kang et al, 2014) until the Late Cretaceous (Li, Mo, Scheltens, & Guan, 2016; Ma et al, 2015, 2017). Mafic magmatism at ~57 Ma in the southern Lhasa Terrane is small in scale (Huang et al, 2017) compared to the widespread Cenozoic arc‐related intermediate to silicic rocks (Ji et al, 2009; Mo et al, 2008; Zhang et al, 2019).…”

Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust

“…The Neo‐Tethys Ocean was sandwiched between India and Asia, and was subducted northward beneath the Lhasa Terrane from the Late Triassic (Dong et al, ; Huang et al, ; Kang et al, ) until the Late Cretaceous (Li et al, ; Ma et al, ; Zhu et al, ). In this scenario, the fluids and/or melts released from the subducted slab would have metasomatized the continental lithospheric mantle, as shown for the mafic dikes in the eastern part of the southern Lhasa Subterrane (Gao et al, ; Ma et al, ; Yue & Ding, ; Zhao et al, ).…”

“…Mesozoic magmatic and sedimentary rocks are ubiquitous in the central and northern Lhasa subterranes (Figure b) (Coulon et al, ; Q. Wang et al, ; Zhu et al, ). The southern Lhasa Subterrane is mainly composed of Jurassic‐Miocene Gangdese batholiths and the Paleocene‐Eocene Linzizong volcanic succession (Dong et al, ; Huang et al, ; Ji et al, ; Kang et al, ; Lee et al, ; Li et al, ; Mo et al, ; Zhu et al, ), the latter consists of Dianzhong, Nianbo, and Pana formation. The sedimentary cover in the southern Lhasa Subterrane is limited, and is mainly of Late Triassic‐Cretaceous age (Zhu et al, ).…”

Petrogenesis of the Quxu intrusive complex: Implications for Eocene magmatism in the southern Lhasa Terrane, Tibet

“…It is mainly composed of diorite and granodiorite with abundant mafic enclaves and dykes, ranging in age from 210 to 10 Ma (Ji et al, 2009;Ma et al, 2017;Wang et al, 2015;Wen et al, 2008;Zeng et al, 2017;Zhu et al, 2015). The Gangdese Batholith records the subduction of the Neo-Tethyan Oceanic lithosphere and the subsequent India-Asia collision (Chung et al, 2005;Huang et al, 2017Huang et al, , 2019Ji et al, 2009;Li et al, 2016;Wen et al, 2008;Zhu et al, 2011Zhu et al, , 2015. The Quxu Batholith, part of the Gangdese Batholith, is a well-known intrusive complex consisting mainly of granitoids with minor gabbroic rocks.…”