Geochemical and Nd, Sr Isotopic Study of the Post‐Orogenic Granites in the Yidun Arc Belt of Northern Sanjiang Region, Southwestern China

Abstract

Abstract:In the arc (basin)-back area of the Yidun arc belt in the north segment of the Sanjiang tectonic zone, southwestern China, there occurs a post-orogenic granite belt extending for more than 300 km in NNW direction. It strides across two different tectonic units of the arc (basin)-back area and the subduction area, and is accompanied by extensive Ag-Sn polymetallic mineralizations. More than ten granite bodies have very similar geochemical characteristics: high SiO 2 (73.8~76.3 wt%) and K 2 O+Na 2 O (7.16~8.41 %), and low Al 2 O 3 (11.9~13.6 %), CaO (0.46~1.54 %) and MgO (0.16~0.61 %), as well as high enrichment of Nb, Ta, Ga and Y, and strong depletion of Sr and Eu. Most of these features are peculiar to A-type granite. Rb-Sr and 40 Ar/ 39 Ar isotopic dating results indicate that the formation ages of the granites decrease from 103.7 Ma of the north end to 75.2 Ma near the south end, and that the magmatism became younger from north to south. The tectonic environment analysis clearly reveals that they were formed in post-orogenic within-plate extension settings. The magma genesis was controlled by a united crustal extension regime after the arc-continent collision. The granites have low ε Nd values ranging from -4.96 to -8.40, whereas the ε Sr values vary greatly ranging from -31.7 to 296, reflecting that the source composition transited from mantle -differentiated igneous rocks in the north to basement -dominated metamorphosed sedimentary rocks in the south. Under high temperature and water-absent conditions, the anatexes of the crustal rocks made a great amount of plagioclase separated from melts and left in magma sources. Through this mechanism, the post-orogenic granites took geochemical characteristics such as low Al 2 O 3 and CaO, and strong depletion of Sr and Eu.

Thermochronology of the Yidun Arc, central eastern Tibetan Plateau: constraints from 40Ar/39Ar K-feldspar and apatite fission track data

“…1) and are otherwise absent from eastern Tibet (Chen et al, 1987). A belt of Cretaceous granites intrudes the central Yidun Arc (Qu et al, 2002) (Fig. 1), although it is uncertain whether these Cretaceous granites are related to intra-continental extension as a possible precursor to the Indian collision with Asia, or possibly from Jurassic and Cretaceous subduction-accretion that occurred along the southeastern (Pacific) margin of the Yangtze/South China Craton (e.g.…”

Late Cretaceous Adakitic Granites of the Southeastern Tibetan Plateau: Garnet Fractional Crystallization of Arc‐Like Magmas at the Thickened Neo‐Tethyan Continental Margin

“…While the Jurassic-Paleocene magmatic rocks in the Lhasa block and the western margins of the Baoshan and Tengchong blocks have commonly been interpreted as resulting from the Neo-Tethyan sunduction (Xin et al, 2018, and references therein), similar rocks in other blocks are not dynamically related to the Neo-Tethyan subduction. For example, Qu et al (2002) argued that the Late Cretaceous magmatic rocks of the Yidun region were formed through orogenic collapse after collision of the Yidun arc and the Yangtze block (see also Hou et al, 2004). However, the Yidun-Yangtze collision took place before ca 180 Ma (Yang et al, 2012), and no collisional orogeny can remain stable for more than 100 Myr.…”

“…However, the petrogenesis and tectonic setting of the widespread Cretaceous granitoids in the southeastern Tibetan Plateau remain debated(e.g., Liu et al, 2006;Ying et al, 2006;Reid et al, 2007;Li et al, 2014;Wang et al, 2014a, b;Yang et al, 2016Yang et al, , 2017aGao et al, 2017). These issues are of particular significance because the Cretaceous granitoids of the southeastern Tibetan Plateau host numerous largescale Cu-Mo-W-Sn-Ag deposits (Hou et al, 2001;Qu et al, 2002;Liu et al, 2006;Ying et al, 2006;Wang et al, 2014a;Yang et al, 2015).…”

Zircon geochronology and Hf isotopes of Mesozoic intrusive rocks from the Yidun terrane, Eastern Tibetan Plateau: Petrogenesis and their bearings with Cu mineralization

“…Numerous sulfide deposits have been found to be hosted in these volcanic rocks (Hou et al, 2001, such as the Gacun Ag-polymetallic VMS-type deposit (Hou et al, 2001;Pan et al, 2003). Jurassic and Cretaceous sediments are absent from the whole Yidun terrane, although some Cretaceous granites intrude the eastern Yidun terrane (Qu et al, 2002;Reid et al, 2007).…”