Structure and Metamorphism of Markam Gneiss Dome From the Eastern Tibetan Plateau and Its Implications for Crustal Thickening, Metamorphism, and Exhumation

Abstract

Gneiss domes developed in the eastern Songpan‐Ganzi Flysch (SGF) belt, located in the hinterland of the Longmen Shan Thrust belt of the eastern Tibetan Plateau. These domes have similar pressure‐temperature (denoted as P‐T below) paths and formed during Late Triassic‐Early Jurassic. One of them, the Markam Gneiss Dome (MGD), is cored by granite and is mantled by a metaturbidite aureole. Our structural observations show top‐to‐the‐south shearing that transposed an upright S1 foliation into a flat‐lying NW‐SE striking S2 composite foliation in the Triassic metaturbidites. Thin sections reveal that the metamorphic index minerals biotite, garnet, and staurolite grew prior to the transposition into S2, whereas kyanite and sillimanite crystallized syn‐S2 development. P‐T pseudosections for the metaturbidites indicate a temperature peak of approximately 618–632 °C and a pressure peak of approximately 6–8 kbar. The highest P‐T condition was immediately followed by isothermal decompression, which is represented by the sillimanite overgrowing andalusite and biotite. Granite was emplaced at ~10‐ to 15‐km depth, during isothermal decompression. Zircon U‐Pb geochronology of metaturbidites, granite, and pegmatite of the MGD indicates that (1) the metaturbidites are younger than 240 Ma, (2) the granite was mainly sourced from the metaturbidites, and (3) deformation leading to the transposition of S1 into S2 mainly occurred ca. 215–170 Ma. Our new P‐T‐D‐t results from the MGD confirm those of other gneiss domes in the SGF and indicate Late Triassic‐Early Jurassic crustal shortening and thickening that probably reflects coeval low‐viscosity crustal flow of the SGF.

Late Miocene Hinterland Crustal Shortening in the Longmen Shan Thrust Belt, the Eastern Margin of the Tibetan Plateau

“…It is worth noting that late Triassic amphibolite facies metamorphic rocks developed around the Xuelongbao and Tonghua massifs, west of Wenchuan‐Maoxian fault. Peak metamorphic conditions were estimated as 10–11 kbar, 530–600 °C (Airaghi et al, , ; Dirks et al, ; Worley and Wilson, ), implying that these rocks were exhumed from depths >30 km during Mesozoic‐Cenozoic time (Airaghi et al, ; Worley & Wilson, ; Zhao et al, ). Farther west, the Songpan‐Ganze terrane is composed of a thick (>8–10 km) sequence of strongly folded Triassic flysch deposits (Chang, ; Ding et al, ; Roger et al, ), intruded by late Triassic‐Jurassic granitoids and a few Miocene plutons (Roger et al, ).…”

Jurassic post‐collisional extension in the Songpan–Ganze Terrane, eastern Tibetan Plateau: Evidence from weakly peraluminous A‐type granites within the Zheduo–Gongga Massif

“…On the diagram of Rb versus Y + Nb diagram (Figure 10a; Pearce, 1996), all of the samples fall in the post-collisional granites field, which is in agreement with the results shown in Figure 10b and is similar to those of other typical post-collisional A-type granites (e.g., the Nyanbaoyeche (Zhang et al, 2007), Siguniangshan or Rilongguan (Dai et al, 2011;de Sigoyer et al, 2014;Roger et al, 2004;Zhao et al, 2007Zhao et al, , 2020 and Wulaxigranites (Zhou et al, 2014;Li, Dai et al, 2016) in the SGT. Therefore, the Zheduo-Gongga granites investigated in this study are believed to have generated in the post-collisional environment at~181 Ma, when the SGT experienced tectonic extension after compression-induced crustal thickening ended at 230-216 Ma (Figure 11a, Harrowfield & Wilson, 2005;Zhao et al, 2019;Zhou et al, 2002).…”

“…CD, Chuandian Block; CQ, Chuanqing Block; JS, Jinsha Suture; KAS, Kunlun-Anymaqen Suture; LMS, Longmenshan Belt; XSHF, Xianshuihe Fault system. (b) Geological map of the eastern Songpan-Ganze Terrane (modified from de Sigoyer et al, 2014;Roger et al, 2010;Zhao, Du, Liang, Wu, & Liu, 2019; Colour figure is available online) [Colour figure can be viewed at wileyonlinelibrary.com] and Lu-Hf isotopic data indicated that the Miocene granites have Stype affinity and resulted from partial melting of the crustal rocks (Lai & Zhao, 2018;Liu et al, 2006;Roger et al, 1995;Searle et al, 2016;Tian, 2018). No geochemical data of Mesozoic granites within the massif have been reported to manifest the corresponding tectonic context even though it has been proposed that the Mesozoic granites are related to the collision between the Qiangtang and Lhasa terranes (Li et al, 2015) or an Andean-type setting during the closure of the Palaeo-Tethys Ocean (Searle et al, 2016).…”

“…This terrane is generally considered to have evolved from a remnant oceanic or back‐arc basin to an orogenic belt since the Mesozoic due to the Indosinian Orogeny. During the Mesozoic orogeny, a huge Triassic deep marine turbidite sequence was deposited above the Palaeozoic greywackes and shales with Permian basalts, filling the basin (Roger & Calassou, 1997), after which these strata were intensely deformed by folding, shearing and metamorphism (Billerot, Duchene, Vanderhaeghe & de Sigoyer, 2017; Harrofield & Wilson, 2005; Huang, Buick, & Hou, 2003; Huang, Maas, Buick & Williams, 2003; Weller et al, 2013; Zhao et al, 2019). This sequence of meta‐sediments was intruded by large‐scale plutons at 228–153 Ma (e.g., Chen et al, 2017; de Sigoyer et al, 2014; Roger et al, 2004, 2010; Xiao et al, 2007; Yuan et al, 2010; Zhan et al, 2018; Zhang et al, 2006, 2007).…”

Mineralization Epochs of Granitic Rare-Metal Pegmatite Deposits in the Songpan–Ganzê Orogenic Belt and Their Implications for Orogeny

“…During deformation, the temperature increase may have been promoted by shear heating along the decollement [82,83]. Finally, the accumulated heating led to the melting of Triassic metaturbidites and the widely distributed rare metal deposit formations [16][17][18][19]27,[47][48][49][50][51] from the outer areas (Jiajika, Xuebaoding, and Dahongliutan ore fields) to the middle area (the Zhawulong ore field) and then to the central area (Ke'eryin ore field), as illustrated in Figure 11 and Table 1.…”